Zaključno poročilo o rezultatih raziskovalnega projekta - 2012 Oznaka poročila: ARRS-RPROJ-ZP-2012/29 ZAKLJUČNO POROČILO O REZULTATIH RAZISKOVALNEGA PROJEKTA A. PODATKI O RAZISKOVALNEM PROJEKTU 1.Osnovni podatki o raziskovalnem projektu Šifra projekta Z2-2298 Naslov projekta Karakterizacija tribokorozijskih procesov Vodja projekta 22315 Tadeja Kosec Tip projekta Zt Podoktorski projekt - temeljni Obseg raziskovalnih ur 3400 Cenovni razred B Trajanje projekta 05.2009 - 04.2011 Nosilna raziskovalna organizacija 1502 Zavod za gradbeništvo Slovenije Raziskovalne organizacije -soizvajalke Raziskovalno področje po šifrantu ARRS 2 TEHNIKA 2.04 Materiali 2.04.02 Kovinski materiali Družbeno-ekonomski cilj .3 02 Tehnološke vede - RiR financiran iz drugih virov (ne iz 13.02 SUF) 2.Raziskovalno področje po šifrantu FOS1 Šifra 2.05 - Veda 2 Tehniške in tehnološke vede - Področje 2.05 Materiali B. REZULTATI IN DOSEŽKI RAZISKOVALNEGA PROJEKTA 3.Povzetek projekta2 SLO Pri relativnem gibanju kontaktnih površin je tribološki kontakt zelo zapleten proces, saj vključuje simultan proces trenja, deformacije in obrabe. Meja med mehanizmi na makro in mikro nivoju je težko določljiva, saj so med seboj prepleteni in povezani. V kolikor je pri omenjenih procesih prisoten elektrolit (npr. oksidirano mazivo, kondenz, tkivo, slina) se mehanski procesi obrabe kombinirajo s korozijskimi. V splošnem omenjena kombinacija ni vsota temveč sinergija obeh vrst procesov, zato so vplivi posameznih parametrov pri skupnih tribokorozijskih procesih izrazito nelinearni in nestacionarni. Določene zakonitosti pri uporabi posameznih materialov v izbranih okoljih in določenih obremenitvah so sicer poznane, vendar je povezovanje znanj s področja tribologije in korozije materialov še vedno relativno neraziskano področje. Glavna vzroka za to sta težavnost simuliranja realnih pogojev in nestacionarnost procesov. To pomeni, da je uporabnost konvencionalnih elektrokemijskih tehnik (potenciodinamska polarizacija, elektrokemijska impedančna spektroskopija), ki so sicer primerne za napoved splošne korozijske obstojnosti, pri triboloških procesih delno omejena. Zato je ovrednotenje elektrokemijskega vpliva pri sinergijski kombinaciji z mehanskimi procesi problematično, posledično pa je nezanesljivo tudi modeliranje teh procesov. Glavni cilj raziskovalnega projekta je karakterizacija osnovnih tribokorozijskih procesov, oziroma študij medsebojnega vplivanja mehanskih in elektrokemijskih procesov. V ta namen smo nadgradili tranzientne elektrokemijske tehnike (elektrokemijski šum, metoda merjenja delnih tokov z mikroelektrodami), ki smo jih razvili pri študiju drugih lokalnih oblik korozije (napetostno-korozijsko pokanje, špranjska korozija). Z omenjenimi metodami lahko spremljamo časovni razvoj in porazdelitev korozijskih tokov med različnimi stopnjami procesa obrabe. Pri izvedbi eksperimentov in analizi mehanskih procesov smo sodelovali s Centrom za tribologijo in tehnično diagnostiko, Institutom za materiale in tehnologijo ter institutom Jožef Stefan. Raziskave smo izvajali na izbranih materialih, katerih trajnost je problematična: pri pogonskih agregatih in procesni industriji (orodna jekla, siva litina) ter v biomedicini (nerjavna jekla, titanove zlitine). V okviru projekta smo določili osnovne parametre in njihov vpliv na tribokorozijske procese izbranih materialov pri tipičnih pogojih okolja (mehanska obremenitev, elektrolit). Rezultat projekta je tako nabor primernih tehnik, ki lahko služijo kot orodje za napoved tribokorozijskih lastnosti določenih materialov pri realnih problemih: npr. uporaba biogoriv, evalvacija efektivnosti inhibitorjev, trajnost različnih vrst implantantov. ANG Tribological processes, which occur during the relative motion of contact surfaces, present a complex combination of friction, deformation and wear. It is difficult to differentiate between these processes on a micro and macro scale since they are interconnected. Mechanical processes can induce corrosion in the presence of a corrosive environment, i.e. an electrolyte such as oxidative lubricants, condensation, tissues, and saliva. Mechanical and corrosion processes cannot be simply combined as their sum, but as their synergy, which makes the effects of non-linear and non-stationary tribo-corrosion processes difficult to distinguish. The interdependence of mechanical and electrochemical mechanisms has recently received increased attention due to the fact that this is a relatively unknown scientific field. The main reason for this is the complexity of the simulation of the real conditions and the non-stationary processes. Conventional electrochemical techniques (such as potentiodynamic polarization and electrochemical impedance spectroscopy) are a good tool for predicting corrosion processes, but are not very useful in the prediction of tribocorrosion. The evaluation of electrochemical mechanisms in synergy with mechanical processes can be difficult, including the modelling of the processes. The main goal of the research project is the characterization of the basic tribocorrosion processes, and the study of the corresponding mechanical and electrochemical effects. An upgrade of the transient techniques (electrochemical noise, the measurement of partial currents with microelectrodes) was performed. These techniques have been developed in our laboratory in connection with the study of some local corrosion phenomena (stress corrosion cracking and crevice corrosion). The time effect and corrosion currents were monitored during different stages of wear. Some experiments were performed, and mechanical processes analysed, in collaboration with the Centre for Tribology and Technical Diagnostics, Institute for metals and Technologies and Institute Jožef Stefan. The research was carried out on different materials that show problematic durability, such as engine fuel generators, as well as in process technology (steels, grey castings) and biomedicine (stainless steels, titanium alloys). Theoretical modelling together with experiments under well-controlled electrochemical and mechanical conditions provides an insight into the interactions of the different parameters (mechanical forces, electrolytes) which govern the behaviour of tribocorrosion systems. Some of the results was interpreted in collaboration with partners from other countries. Thus, the result of the proposed project is the selection of techniques as tools for the prediction of the tribocorrosion behaviour of certain materials in real systems, e.g. the use of biodiesel, the evaluation of corrosion inhibitors, and the durability of different types of implants. 4.Poročilo o realizaciji predloženega programa dela na raziskovalnem projektu3 V okviru postdoktorskega projekta z naslovom »Karakterizacija tribokorozijskih procesov« sem nadaljevala delo na vseh predlaganih delovnih področjih v projektu. Med izvajanjem podoktorskega projekta sem skladno s predlaganimi aktivnostmi znotraj 1., 2., 3. in 4. sklopa uspešno sledila ciljem projekta: meritve sem izvajala na tribokorozijskem elektrokemijskem sistemu za meritve tribokorozijskih procesov, vpeljevala in preskušala sem konvencionalne in nove metode in načine za tribokorozijsko preskušanje ter karakterizirala tako mehanske kot korozijske lastnosti obrabljenih površin. Poseben poudarek raziskovalnega projekta je bil na četrtem predlaganem sklopu Uporaba razvite metodologije za študij izbranih sistemov. Iz prvega delovnega sklopa Izdelava tribokorozijske naprave sem uporabila vse potrebne parametre za nadaljnjo študijo vpliva tribološke obrabe na elektrokemijski odziv materiala v izbranem korozijskem okolju. Prav tako sem novo pridobljena znanja iz drugega dela delovnega sklopa z naslovom »Razvoj metodologije za karakterizacijo tribokorozijskih procesov« uporabila za študij tribokorozijskih vplivov na izbranih primerih. Poleg tribokorozijskih/tribo-elektrokemijskih preiskav izbranih materialov v korozijskem okolju sem izvajala tudi referenčne meritve tudi na materialih v enakem izbranem korozijskem okolju samo z elektrokemijskim vplivom. Poznavanje elektrokemijskega vpliva ter elektrokemijskih lastnosti na materialu, ki se pod tribološko obrabo obnaša drugače, namreč omogoča primerjavo lastnosti ter njun medsebojni vpliv. Med procesom obrabe sem v korozivnem elektrolitu na kovinskih materialih študirala elektrokemijski odziv z uporabo tako konvencionalnih elektrokemijskih tehnik in elektrokemijski šum kot novo predlagano tehniko za spremljanje tribokorozijskih procesov. Določevala sem naravo procesov med obrabo; merjen signal je vsota dogajanj na stacionarnem delu delovne elektrode kot tudi odziva na obrabljenem delu. Dobljene rezultate sem primerjala s podobnimi sistemi, ki so že predstavljeni v različnih objavah. Z obravnavanimi elektrokemijskimi tehnikami ter izborom primernih tehnik za optimalno tribokorozijsko napoved vedenja materialov v izbranih sistemih sem uspešno proučila različne načine za prepoznavanje tribikorozijskih procesov. Aktivnosti znotraj tretjega delovnega sklopa »Karakterizacija površine, morfološke in mehanske lastnosti« sem izvajala preko vseh faz projekta in raziskav v trajanju projekta v letih 2009-2011. Na trobokorozijskih in referenčnih vzorcih sem pred in po obrabi karakterizirala in določila njihove lastnosti: pregledala sem korozijsko stanje površine, njene morfološke in mehanske lastnosti. Merila sem trdoto in hrapavost pred in po obrabi. S profilometrom sem določila profil obrabne sledi in iz nje preračunala hitrost obrabe. Prav tako sem površino delovne elektrode-kovinskega materiala vizuelno pregledala pod optičnim mikroskopom ter konfokalnim mikroskopom. Primerjala sem oblike profilov, pridobljenih z dvema različnima tehnikama in sicer klasičnim profilometrom (kontaktni način) ter določanjem profila s konfokalnim mikroskopom (nekontaktni način). Študirala sem prednosti in slabosti posameznih tehnik, določila hitrost obrabe in za vsako metodo ocenila velikost napake pri določanju hitrosti obrabe. S pomočjo elektronskega mikroskopa sem pri večjih povečavah določila obliko korozijskih produktov, mesta, kjer so se korozijski produkti odlagali in z analizo EDS določila njihovo sestavo. Podrobneje smo z elektronskim mikroskopom lahko določili širino obrabne sledi, vključke, ki nastanejo v obrabni sledi, opazovali smo deponiranje obrabljene plasti (Third Body Effect). Prav tako smo merili trdoto na izbranih materialih zunaj in v obrabni sledi ter na vključkih v obrabni sledi. Študirali smo vpliv sile obrabe na obrabno pot, obliko ter hitrost obrabe. Na Ramanskem mikroskopu pa sem (laser 632,8 nm) uspešno detektirala različne oblike oksidov na obrabni sledi, ob sledi ter na različno oddaljenih območjih ob sledi. Tudi Ramanska spektroskopija je novost pri karakterizaciji različnih korozijskih produktov po tribokorozijskem preskušanju. V drugem letu podoktorskega projekta je bil poudarek na raziskovalnih aktivnostih znotraj delovnega sklopa »Uporaba razvite metodologije za študij izbranih primerov«. Za študijo smo izbrali tri različne, tehnološko in biomedicinsko pomembne materiale, in sicer: nerjavno jeklo tipa AISI 316 L, Stelitno zlitino Stellite 6-CoCrNiW zlitino ter nikelj-titanovo zlitino, ki se uporablja v biomedicinske namene- NiTi zlitino. Tako nerjavno jeklo kot stelitne zlitine spadajo v pasivirajoče materiale. Posebej stelliti so bili razviti kot materiali z izrazito majhno obrabo. Nerjavno jeklo AISI 316 L in Stellite 6 zlitino sem študirala v 0,5 M raztopini H2SO4, medtem ko je bil za korozijski medij NiTinol zlitine izbrana umetna slina, ki posnema naravno slino. Tribokorozijski sistemi in aktivnosti so bile naslednje: - elektrokemijsko testiranje jekla 316 L in Stellite 6 zlitine in dentalnih žic ter NiTinol folije - elektrokemijska impedančna spektroskopija jekla 316 L in Stellite 6 zlitine ter dentalnih žic in NiTi folije v simulirani raztopini sline (simulirana raztopina sline) - mikrostruktura izbranih kovinskih materialov -vzdolžno in prečno -tribološki eksperimenti za ugotavljanje koeficienta trenja in sile trenja - triboelektrokemijski eksperimenti pri različnih obremenitvah (1 N, 2 N, 5 N in 10 N), hitrostih obrabe, poteh obrabe. - konfokalna mikroskopija, iz katere sem pridobili podatek o hitrosti obrabe (suho-tribološki kontakt, mokro-tribokemijska obraba). Iz teh podatkov smo ocenjevali vrsto in hitrost obrabe s profilometrom (kontaktni način) ter primerjala obe tehniki. Z aktivnostmi v omenjenem sklopu sem dosegla cilje kot so poznavanje tehnološko zanimivih materialov, določitev specifičnih korozijskih razmer za tribokorozijsko preskušanje ter reševanje industrijskih izzivov med samim tehnološkim procesom pridobivanja končnega produkta. Tako sem z primernim pristopom in izborom pravilnih elektrokemijskih tehnik optimirala metode za tribokorozijsko preskušanje, ki bo z novimi znanji pripomoglo k uspešnejšem dizajniranju lastnosti materialov, ki so podvrženi tako koroziji kot obrabi. V objavo smo poslali izvirni znanstveni članek o vplivih mikrostrukture tehnološko izredno pomembnega materiala-zlitine na osnovi niklja in titana-Nitinola. Obravnavane so bile tako tribološke, elektrokemijske in tribokorozijske lastnosti preiskovane zlitine v pravi in simulirani raztopini sline. Poudariti je pomembno tudi sodelovanje z Medicinsko fakulteto UL-odsek za dentalno medicino. Z njimi prav tako pripravljamo strokovni članek o vzrokih porušitve na NiTiNol dentalnih žicah. V sodelovanju z Institutom Jožef Stefan-Odsekom za tanke plasti smo prijavili projekt o študiji in mehanizmih korozijskega obnašanja biokompatibilnih prevlek na kovinskih materialih. Študija vključuje tribokorozijsko preskušanje in vrednotenje. V okviru sodelovanja smo opravili del preiskav na DLC (Diamon Like carbon coating) prevlekah na različnih kovinskih podlagah v Hankovi raztopini. Prav tako smo preskušali možnost detekcije vključkov in napak na prevlekah (kjer se začne in pospešuje korozija) na mikro X-žarkovni tomografiji. Podoktorski projekt je tako finančno omogočil odpiranje novega področja raziskav, kjer se prepletajo različne vede, tako kemija, strojništvo kot fizika. Z novim znanjem in rezultati projekta smo se uspešno predstavili tudi industrijskim partnerjem. Tako smo v letu 2011 po zaključku projekta že načrtovali nov skupni manjši aplikativni projekt, ki ga bo v četrtinskem deležu sofinancirala Hidria-Rotomatika. Skupaj z njimi ter Institutom za materiale in tehnologije smo prijavili aplikativni raziskovalni projekt »Tribokorozija: raziskave mehanizmov, materialov in prenosa v prakso«. Na področju tribokorozijskih raziskav obrabe dentalnih žic smo uspešno sodelovali z Medicinsko fakulteto. Na področju medicinskih ved smo tako prijavili aplikativni raziskovalni projekt z naslovom «Učinkovitost ortodontske obravnave z nesnemnimi ortodontskimi aparati pri različnih površinskih spremembah njihovih kovinskih delov«. S.Ocena stopnje realizacije programa dela na raziskovalnem in zastavljenih raziskovalnih ciljev4 Predlagani cilji v okviru postdoktorskega projekta »Karakteri zaci ja tribokorozijskih procesov« so bili štirje, in sicer: Izdelava tribokorozijske naprave, Razvoj metodologije za karakterizacijo tribokorozijskih procesov, Karakterizacija površine, morfoloških in mehanskih lastnosti ter Uporaba razvite metodologije za študij izbranih procesov. V obsegu prvega sklopa so bile realizirane vse potrebne aktivnosti za začetek uporabe tribokorozijskega sistema kot osnovnega orodja za karakterizacijo tribokorozijskega sistema. Tribokorozijski sistem je dobro razvit in je uporabljen za različne tribološke in tribokorozijske študije. V okviru zastavljenih ciljev znotraj drugega sklopa je bilo opravljeno večji del nalog. Različne elektrokemijske metode in meritve sem v drugem letu podoktorskega projekta izvajala na drugih različnih izbranih kovinskih materialih. Izvedli smo tudi vrsto preiskav na sklopljenih elektrodnih mrežah CMEA (angl. Coupled Multi Array Electrodes). Na sistemu CMEA iz nerjavnega jekla smo v kloridni raztopini optimizirali preiskave za detekcijo tokovnih in napetostnih odzivov ter optimizirali naprave za detekcijo, da smo pridobili ustrezno čistost signala. Ugotovili smo, da lahko uspešno spremljamo depasivacijo in ponovno repasivacijo izbranih kovinskih materialov. Preiskave na mreži elektrod so zelo pomembne, saj podobnih preiskav v svetu še ni. V letu 2012 bomo poskušali objaviti rezultate preskušanj v reviji s faktorjem vplva- Wear ali Tribology International. V tretjem sklopu sem uporabila številne metode za karakterizacijo in pregled lastnosti materialov pred in po obrabi v korozijskem sistemu. V okviru poteka projekta sem uporabljala različne metode za spektroskopsko analizo kovinskih površin, kot je SEM/EDS analiza, kontaktna profilometrija za oceno obrabe ter Ramanska spektroskopija. Dodatno sem vpeljala pregled obrabljene površine s konfokalnim mikroskopom in ugotovila, da nam nudi številne dodatne informacije, ki so na tribokorozijskem področju novost. Poudarek aktivnosti temeljnega podoktorskega projekta je bil na sklopu «Uporaba razvite metodologije za študij izbranih sistemov«. V okviru tega sklopa so bile pridobljene pomembne tribokorozijske študije. Pridobljeno znanje in ugotovitve realnih tribokorozijski h primerov smo predstavili v izvirnih znanstvenih prispevkih. Znanstveni in strokovni prispevki projekta Karakterizacija tribokorozijskega preskušanja so bili predtavljeni na dveh mednarodnih konferencah in enem mednarodnem simpoziju, ter članek z naslovom The tribocorrosion behaviour of NiTi alloy v reviji s faktorjem vpliva (Wear-priponka1). Rezultati in osnove tribokorozijskega preskušanja bodo objavljeni tudi kot strokovni članek v reviji Vakuumist (priponka2). Opravljeno je bilo tudi diplomsko delo z naslovom Vpliv mikrostrukture na korozijo nikljevih zlitin pod mehansko obremenitvijo. 6.Utemeljitev morebitnih sprememb programa raziskovalnega projekta oziroma sprememb, povečanja ali zmanjšanja sestave projektne skupine5 Ni sprememb v izvajanju programa projekta. 7.Najpomembnejši znanstveni rezultati projektne skupine6 Znanstveni dosežek 1. COBISS ID XX Vir: vpis v poročilo Naslov SLO The tribocorrosion behaviour of NiTi alloy ANG Tribokorozijske lastnosti Niti zlitin Nikelj titanove zlitine so znane po dobri korozijski odpornosti. Posebej Opis SLO ANG zaradi lastnosti oblikovnega spomina se uporabljajo tudi v biomedicinske namene. Kljub temu pa je v nekaterih primerih prisotna luknjičasta in špranjska korozija. NiTi žice so se v uporabi v dentalni praksi pokazale kot problematične, saj mnogokrat prihaja do pretrgov zaradi različnih vrst obrabnih procesov. Predvidevamo, da kljub dobrim elektrokemijskim in hkrati korozijskim lastnostim zlitin na osnovi titana mikrostruktura vpliva na kemijske kot tudi na mehanske lastnosti. Preiskave smo izvedli na dveh mikrostrukturno različnih vzorcih in sicer na dentalni žici ter na pločevini iz zlitine NiTi, v dobavljenem in sveže brušenem stanju. Elektrokemijske preiskave so bile merjene v raztopini umetne sline, dodatne preiskave materialov so bile opravljene na tribokorozimetru. Mikrostrukturne lastnosti so bile pregledane v prečnem ter vzdolžnem preseku pločevine ter dentalne žice. Obrabne raze so bile analizirane s SEM/EDS analizo, profilometrom ter merilcem trdote. Rezultati elektrokemijskih raziskav so pokazali, da vzorci pločevine in žice zlitine NiTi brez oksidne plasti kažejo podobne korozijske lastnosti. Večje razlike v korozijski odpornosti so bile vidne na vzorcih z oksidno plastjo. Tribokorozijske preiskave so pokazale, da je celotna obraba zlitine NiTi večja, če sta kombinirani korozijska in mehanska obraba. Večje obremenitve vodijo do večjih sprememb na pasivni površini in večje obrabe materiala. Ugotovili smo, da so pri večji obremenitvi v obrabno razo vključeni večji obrabni delci in da trdota materiala po obrabi poveča. Nickel titanium alloys are known for their good corrosion resistance. NiTi alloy is due to its shape memory properties particularly used in biomedical application. Their corrosion properties are satisfactory (sufficient), but in some circumstances they suffer from pitting and crevice corrosion. Since dental NiTi archwires experience severe failures in applications it is assumed that their performance is affected also by microstructure. Beside corrosion the wear resistance of this alloy is of critical concern in many applications. In the present work, two types of NiTi alloy samples were investigated, NiTi plate and NiTi dental wire with and without surface oxide. The electrochemical investigation was conducted in artificial saliva and was proved by the use of tribocorrosimeter. Microstructural characteristics were studied in cross section and longitudinal direction. The wear scare was investigated by the use of SEM/EDS analysis and profilometer. The hardness of the wear scar was followed as well. The results of electrochemical investigation showed that the dental alloy and NiTi plate without surface oxide exhibit very similar corrosion properties. The variation in corrosion performance in artificial saliva was bigger at specimens covered with surface oxide films. Tribocorrosion studies showed that the total wear of the NiTi alloy is greater when corrosion is combined by the wear. The bigger wear loads lead to greater changes in passive film and wear scar. Greater loads resulted in bigger inclusions in the wear. Also, the hardness in the wear scar is increased after applying the load. It can be concluded that microstructure of the investigated NiTi sample has effect on electrochemical and tribocorrosion properties of the samples as well. Objavljeno v Wear Tipologija 1.01 Izvirni znanstveni članek 2. COBISS ID 1808231 Vir: vpis v poročilo Naslov SLO Vpliv mikrostrukture na tribokorozijske lastnosti dentalnih zlitin ANG The effect of microstructure on tribocorrosion properties of dental alloys Opis SLO Različni mikrostrukturni vzorci NiTi zlitine imajo različne elektrokemijske in triboelektrokemijske lastnosti. Dentalne NiTi žice z oksidno plastjo imajo najslabše lastnosti. Tribokorozijske preiskave so pokazale, da se način obrabe močno spreminja glede na obrabno silo.. Korozijski potencial se med drgnjenjem zniža. Obrabna raza pri tribološkemv raztopini vsebuje manj vključkov, kot pri suhi tribološki obrabi, hkrati je obraba v kraztopini sline-korozijski medij-večja. ANG Different NiTi samples with different morphological properties have also different electrochemical properties. Dental NiTi wires with oxide film exhibits worse electrochemical behaviour. Tribochemical experiments have shown that the wear regime varies at different applied loads. Corrosion potential is decreased the most at the higher applied force. Wear track after tribolelectrochemical wear has lower number of inclusions whereas the wear rate is higher then after dry tribological wear. Objavljeno v Developing solutions for the global challenge : book of abstracts : EUROCORR 2011, The European Corrosion Congress, 4-8 September 2011, Stockholm, Sweden, (European federation of corossion, event no. 325). [S. l.: s. n.], 2011, str. 654. [COBISS.SI-ID 1808231] Tipologija 1.08 Objavljeni znanstveni prispevek na konferenci S.Najpomembnejši družbeno-ekonomsko relevantni rezultati projektne skupine7 Družbenoekonomsko relevantni dosežki 1. COBISS ID 35703301 Vir: COBISS.SI Naslov SLO Vpliv mikrostrukture na korozijo nikljevih zlitin pod mehansko obremenitvijo ANG The effect of microstructure on tribocorrosion properties of nickel alloys Opis SLO Z novo pridoblejnim znanjem na področju tribokorozije smo omogočili prenos znanja iz raziskovalne institucije do univerze in s tem omogočili širitev znanja na področju tribokorozije. Izredno kvalitetno diplomsko delo bo omogočilo razširitev programa korozije in elektrokemije na dodiplomskem in podiplomskem študiju. ANG With the newly achieved knowledge on the tribocorrosion field, the knowledge transfer was enabled from research institution onto Uniersity level. High quality diploma work will enable to widen the program of corrsoion and electrochemistry at for university students as well as post graduate students. Šifra D.10 Pedagoško delo Objavljeno v [P. Močnik]; 2011; 56 f.; Avtorji / Authors: Močnik Petra Tipologija 2.11 Diplomsko delo 2. COBISS ID 1581671 Vir: COBISS.SI Naslov SLO Projektna naloga: ocena poškodb na ceveh ANG Project task: Evaluation of damages on pipelines Opis SLO V projektni nalogi smo pregledali, analizirali in opisali korozijske poškodbe, ki so bile ugotovljene v NEK (jederska elektrarna Krško) na ceveh uparjalnika. Izvedene so bile kompleksne metalografske in mikroskopske preiskave dostavljenih vzorcev. Izvedene so bile tudi elektrokemijske preiskave na izbranih materialih. V rezultatih raziskave smo natančno podal vplive in dejavnike za nastanek korozijskih poškodb ter način nastanka in potek korozijskih poškodb. Na osnovi elektrokemijskih meritev pa so podane ocene za občutljivost preiskovanih materialov v izbranem korozijskem okolju. ANG In the project corrosion damages observed on pressuriser in NEK (Nuclear Power Plant Krško) were inspected, analyzed and evaluated. Complex metallographic and microscopic studies were realized on received specimens. Additionally, electrochemical research on selected materials was performed. In the results the influences and parameters for inspected corrosion damages and the ways of intimation and growth are described. On the basis of electrochemical measurements performed on selected materials the susceptibility of those materials to corrosion in selected environment was estimated. Šifra D.01 Vodenje/koordiniranje (mednarodnih in domačih) projektov Objavljeno v Zavod za gradbeništvo Slovenije; 2009; 64 str.; Avtorji / Authors: Kosec Tadeja, Legat Andraž, Kovač Jaka, Kuhar Viljem, Gartner Nina, Švara Erika, Smirić Sanja Tipologija 2.13 Elaborat, predštudija, študija 3. COBISS ID 1834599 Vir: COBISS.SI Naslov SLO Trajnost materialov vodovodnih sistemov v stavbah ANG Durability of materials i drinking water systems in buildings Opis SLO Izkušnje korozijskih inspekcij so narekovale potrebo po ozaveščanju javnosti o prepotrebnem sodelovanju strokovnjakov in znanstvenikov iz ožjega korozijskega področja in uporabnikov: zdravstveno varstvo in ministrstvo za zdravje. Opisano področje so napeljave za distribucijo pitne vode. Zaradi pomanjkanja znanja je pogosto poroblem uporabe pravilnih materialov, postopkov dezinfekcije za ohranjanje neoporečnosti pitne vode. Ozaveščamo in povezujemo različne institucije, ki bi omogočile zagotavljanje zdravja in dvig kvalitete življenja. ANG Corrosion inspection experiences have led to the need to inform public for the need to use know-how and knowledge in the applied field :drinking water system. The lack of corrosion knowledge in the field highly affects people's health and quality of living. Materialčs used in drinking water system are usually not appropriate according to disinfection procedures. The awareness of the collaboration of scientific laboratories, Goverment and health system is emphasized. Šifra F.18 Posredovanje novih znanj neposrednim uporabnikom (seminarji, forumi, konference) Objavljeno v Tehnis; Gradbenik; 2011; Letn. 15, št. 11; str. 52-54; Avtorji / Authors: Bajt Leban Mirjam, Kuhar Viljem, Kosec Tadeja, Legat Andraž Tipologija 1.04 Strokovni članek 4. COBISS ID YY Vir: vpis v poročilo Naslov SLO Aplikativni projekt Korozija bakra v bentonitu ANG Applicative project Corrosion of copper in bentonite Opis SLO Aplikativni projekt je del raziskav povezanih z odlagališem izrabljenega jedrskega goriva skupaj s švedskim partnerjem.Tri leta in pol smo spremljali korozijsko hitrost čistega bakra z namestitrvijo električnih uporovnih senzorjev v bentonitu, nasičenim s slano podtalnico. V bentonitu smo dokazali oksidativne pogoje. Naredili smo vrsto meritev elektrokemijske impedančne spktroskopije kot tudi meritve zmanjševanja debeline na uporovnih senzorjih iz bakra. EIS meritve so pokazale postopno zmanjševanje korozijske hitrosti bakra, skladno z pričakovanji. Po treh letih in pol izpostave bakrenih senzorjev bentonitu smo izmerili korozijske hitrosti med 0.4 do 0.7 pm/leto, vrednosti pa so primerljive z merjenjem izgube debeline, kjer je bila korozijska hitrost izmerjena 1.0 pm/leto. ANG Applied project is research project on spent nuclear fuel nuclear waste management for disposal in Sweden. Corrosion of copper has been followed by the use of copper electrical resistance sensors that were placed in bentonite for 3 years and a half. Oxic conditions were confirmed by continuous monitoring. Different measurement techniques were applied: thickness measurements and electrochemical impedance spectroscopy. Both measurements showed the decreasing corrosion rate of copper ER sensors. After exposure the corrosion rates estimated by EIS are 0.4-0.7 micro M/year and around 1 micoM/year with thickness measurement. Šifra F.17 Prenos obstoječih tehnologij, znanj, metod in postopkov v prakso Objavljeno v Electrochimica Acta Tipologija 1.01 Izvirni znanstveni članek 9.Drugi pomembni rezultati projetne skupine8 Predstavljamo še druge vrste delovanja vodje projekta: OTMAČIĆ ĆURKOVIĆ, Helena, KOSEC, Tadeja, LEGAT, Andraž, STUPNIŠEK LISAC, Ema. Improvement of corrosion stability of patinated bronze. Corros. eng. sci. technol., 2010, vol. 45, no. 5, str. 327-333, ilustr. [COBISS.SI-ID 1700967] KOSEC, Tadeja, OTMAČIĆ ĆURKOVIĆ, Helena, LEGAT, Andraž. Investigation of the corrosion protection of chemically and electrochemically formed patinas on recent bronze. Electrochim. acta. 2010, vol. 56, issue 2, str. 722-731, [COBISS.SI-ID 1700711] MILOŠEV, Ingrid, KOSEC, Tadeja, BELE, Marjan. The formation of hydrophobic and corrosion resistant surfaces on copper and bronze by treatment in myristic acid. J. Appl. Electrochem., jul. 2010, vol. 40, no. 7, str. 1317-1323, [COBISS.SI-ID 1665639] BAJT LEBAN, Mirjam, KUHAR, Viljem, KOSEC, Tadeja, LEGAT, Andraž. Corrosion investigation of the Prešeren monument in Ljubljana = Korozijska preiskava Prešernovega spomenika v Ljubljani. Mater. tehnol., 2010, letn. 44, št. 5, str. 265-269 [COBISS.SI-ID 829098] KOSEC, Tadeja, LEGAT, Andraž, MILOŠEV, Ingrid. The comparison of organic protective layers on bronze and copper. Prog. org. coat.2010, vol. 69, no. 2, str. 199-206, [COBISS.SI-ID 1677927] Bo Rosborg, Tadeja Kosec, Andrej Kranjc, Jinshan Pan, Andraz Legat, Electrochemical impedance spectroscopy of pure copper exposed in bentonite under oxic conditions, Electrochim. acta. (2011)vol. 56, issue 23, 7862-7870 [COBISS.SI-ID 1756007] Erika Švara, Tadeja Kosec, Viljem Kuhar, Andraž Legat, Corrosion stability of different bronzes in simulated urban rain = Korozijska stabilnost različnih bronov v umetnem kislem dežju. Mater. tehnol., 2011, letn. 45, št. 6, str. 585-591. [COBISS.SI-ID 1844583] lO.Pomen raziskovalnih rezultatov projektne skupine9 10.1.Pomen za razvoj znanosti10 SLO Projekt »Karakterizacija tribokorozijskih procesov« v celoti sovpada s prednostnimi nacionalnimi raziskovalnimi cilji, saj povezuje področje naprednih materialov in nanotehnologije s tehnološkim razvojem za trajnostno gospodarstvo. Novi kovinski materiali imajo namreč izboljšane posamezne mehanske lastnosti in splošno korozijsko obstojnost, ob specifičnih pogojih okolja pa so lahko podvrženi intenzivnim degradacijskim procesom. Kompleksni korozijski procesi so običajno izrazito lokalni, njihov razvoj pa relativno hiter, zato jih težko detektiramo. Takšni procesi so sinergija posameznih mehanskih in elektrokemijskih dogodkov (napetostno-korozijsko pokanje, korozijsko utrujanje, tribokorozijski procesi) in so najbolj izraziti pri kovinskih materialih, ki tvorijo stabilne pasivne filme (titanove zlitine in nerjavna jekla). Glede na to, da so taki materiali v osnovi zelo obstojni in se uporabljajo v najzahtevnejših industrijskih in medicinskih sistemih, je ključno definirati pogoje in parametre, kjer je njihova trajnost lahko bistveno zmanjšana. Nenadna odpoved industrijskih in medicinskih sistemov namreč poleg velike gospodarske škode lahko povzroči tudi ekološke katastrofe in neposredne človeške žrtve. Industrijsko najrazvitejše države intenzivno povezujejo temeljna in tehnološka znanja, pri čemer je ključno tudi povezovanje različnih področij (multidisciplinarnost). V Sloveniji je bilo povezovanje na omenjenih segmentih že večkrat ocenjeno kot pomanjkljivo, zato je tudi eksplicitno omenjeno v Nacionalnem raziskovalnem in razvojnem programu (NRRP) kot slabost, ki jo je potrebno odpraviti. Pri raziskavah uporabljamo in povezujemo znanje iz različnih področij (kemija, fizika, strojništvo, metalurgija, medicina) in na različnih nivojih (raziskovalna sfera, industrijski tehnološko-razvojni centri). Glede na to, da je tribokorozija relativno nova veda, ter na naše dosedanje uspešno povezovanje različnih znanstvenih in tehničnih področij, pričakujemo objave v revijah najvišjega znanstvenega nivoja. S sodelovanjem s strokovnjaki iz drugih institucij in industrije smo zagotovili tudi sprotni prenos naših rezultatov v prakso, oziroma čimprejšnji začetek usmerjenih tehnoloških raziskav za reševanje specifičnih industrijskih problemov. S primerjavo različnih elektrokemijskih in fizikalnih metod bomo določili optimalno metodologijo za ocenjevanje in karakterizacijo tribokorozijskih procesov. Z uporabo omenjenih merilnih metod in ustrezno analizo smo dopolnili temeljno znanje o posameznih fazah procesov korozije in obrabe, ter njihovega medsebojnega vplivanja. Pri izbranih sistemih kovina/elektrolit smo tudi določili kritične parametre omenjenih procesov. Dosedanje intenzivne mednarodne aktivnosti vseh sodelujočih partnerjev so tudi osnova za tesnejše mednarodno sodelovanje na ožjem področju tribokorozije. Na ta način smo v domači prostor uspešno prenesli posamezne mednarodne izkušnje, oziroma dopolnili naše raziskave in pridobljeno znanje. Pričakujemo, da se bo del znanja prelil tudi v učne programe posameznih fakultet: FS, FKKT in MF, UL. ANG Newly developed materials can have better mechanical characteristics and/or corrrosion resistence, but suffer from intensive deterioration process under certain agressive environment (s). Complex corrosion processes are usually localized and quickly initiated, which makes it difficult to detect. In most cases, the processes are combined actions of both mechanical and electrochemical events (stress corrosion cracking and tribocorrosion). It is usually observed in materials that form stable oxide films (stainless steels and titanium alloys). It was of great importance to set the limiting operation conditions and parameters to ensure the longer lifespan of such alloys, because they are important technological and biomedical materials that are used in a wide range of applications. Namely, sudden failure of technological and medical systems can result in vast damages leading to ecological catastrophy and human deaths (victims of circumstances). The optimal methodology for estimation and characterization of tribocorosion processes will be chosen after evaluation of different electrochemical and physical methods. We expect that the collection of proposed techniques will supplement the fundamental knowledge on different stages of corrosion processes, wear, and their synergistic effect. The critical parameters will be determined at the chosen metal/electolyte systems. Tribocorrosion is a relatively new scientific research area. It is expected that our results will be published in journals of high impact factors. Collaborating with scientists and experts from the field at other research and technology institutes, we assured the results to be applied in practice. The rationale being to start the implementation of technological research to resolve problems of high industry impact. There was an intensive international collaboration with the partners (scientific communities) involved in the past and present. The conclusions were for continuous research, dialogue, and collaboration within these communities towards the selected scientific field of tribocorrosion.These experiences were impelmented in our own research field, which lead to a better research and knowledge. We aim to implement knowledge of tribocorrosion as special courses at different faculties; such as Faculty for Civil Engineering, Faculty for Chemistry and Chemical Engineering, and Medical Faculty. 10.2.Pomen za razvoj Slovenije11 SLO V tehnološko najrazvitejših podjetjih se že dalj časa zavedajo pomena pravilne izbire materialov in optimalnih konstrukcijskih rešitev, saj je to pogoj za ustrezno kvaliteto njihovih proizvodov. Eno od glavnih meril je zagotavljanje ustrezne trajnosti pri izbranih pogojih okolja (kombinacija mehanskih, fizikalnih in kemijskih obremenitev), pri čemer korozija velikokrat predstavlja kritični degradacijski proces. Podobni pogoji nastopajo tudi pri kovinskih vsadkih v človeško telo, saj so poleg agresivnega vpliva okoljskega tkiva v večini primerov prisotne tudi relativno velike mehanske obremenitve. Zaradi uporabe novih materialov (posebni kovinski materiali in prevleke, novi načini spajanja materialov, različna maziva, dodajanje inhibitorjev) in višjih mehanskih zahtev, so raziskave na tem kompleksnem področju korozije nujne. Raziskave tribokorozijskih procesov so bile sicer definirane kot temeljni projekt, vendar so rezultati nujni za nadaljnje industrijske in medicinske aplikacije. V kolikor želimo definirati natančne pogoje raziskav za avtomobilsko industrijo (batni in ležajni deli, zavore), sistemi v parnih generatorjih (korozija pod vplivom toka) in potrebne lastnosti vsadkov v človeško telo (dentalni in ortopedski implantati), smo morali definirati osnovne tribološke procese, predvsem pa metodologijo (merilne tehnike in postopke) za njihovo karakterizacijo. Za rezultate naših raziskav so izrazili zanimanje ključni člani ACS (slovenskega avtomobilskega grozda: HIDRIA), Medicinska fakulteta, UL, NEK in Institut za materiale in tehnologije-IMT. Pri naših raziskavah smo povezali z sodelujočimi institucijami (Center za tribologijo in tehnično diagnostiko, Institut za materiale in tehnologije-IMT in Institut Jožef Stefan). Poudariti je prav tako potrebno, da je tribokorozija kot veda še zelo mlada in da se v Evropi trenutno ukvarjajo s to tematiko le maloštevilne skupine. S podporo Agencije za raziskovalno dejavnost smo tako lahko segli in doprinesli slovensko znanje v mednarodni prostor. Slovenska tribološka znanost je zelo močna in prepoznavna, z dodatnim znanjem na področju kemijskega vpliva na korozijsko propadanje materialov med obrabljanjem pa le-ta predstavlja dodano vrednost. ANG An important condition to achieve quality products, leading companies are aware of the importance in choosing qood, quality materials and constructions. One basic criteria is the assurance for sustainable durability of materials at the chosen environment (combination of mechanical, physical and chemical wear) where the corrosion itself represents the critical deterioration process. Similar conditions can be found in the case of biomedical materials used in orthopaedics and dental surgeries. In vivo degradation of metallic biomaterials due to combined wear and corrosion processes results in the formation of particulate and ionic metallic debris, which can lead to failures. Thus, the tribocorrosion research represents an important scientific field due to increased number of new materials (special alloys and coatings, new cojunction systems, use of lubricants and inhibitors) and higher mechanical demands. These particular tribocorrosion studies proposed in this project are basic research studies, which are needed for further technological and biomedical research studies. There are great needs for defining tribocorossion processes as well as the developement of methodology for the testing. The reasons are to define the field of research for certain applications for automotive industry (pistons, bearings, and brakes) and biomedicine (dental screw and hip implants). There is great interest, for the implementation of the research results in some Slovenian automobile industries (HIDRIA) and the Faculty of Medicine. We will collaborate with different research groups at the Centre for Tribology and Technical Diagnostics, Institute for Metals and technologies and Department for Thin Coatings and Surfaces at Jozef Stefan Institute. Tribocorrosion is a relatively young scientific field and there are very few scientific groups in Europe that are involved in this type of research. With the help of Slovenian research Agency there we were able to present international public the Slovenian knowledge. Slovenia is very strong and recognizable in tribology as a science. Combination of tribology and tribocorrosion science thus represents the added value to general knowledge for sustainable corrosion protection of materials. ll.Samo za aplikativne projekte! Označite, katerega od navedenih ciljev ste si zastavili pri aplikativnem projektu, katere konkretne rezultate ste dosegli in v kakšni meri so doseženi rezultati uporabljeni Cilj F.01 Pridobitev novih praktičnih znanj, informacij in veščin Zastavljen cilj Oda One Rezultat 1 d Uporaba rezultatov 1 d F.02 Pridobitev novih znanstvenih spoznanj Zastavljen cilj Oda One Rezultat d Uporaba rezultatov 1 d F.03 Večja usposobljenost raziskovalno-razvojnega osebja Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov d F.04 Dvig tehnološke ravni Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.0S Sposobnost za začetek novega tehnološkega razvoja Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d F.06 Razvoj novega izdelka Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.07 Izboljšanje obstoječega izdelka Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.08 Razvoj in izdelava prototipa Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d F.09 Razvoj novega tehnološkega procesa oz. tehnologije Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.10 Izboljšanje obstoječega tehnološkega procesa oz. tehnologije Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.11 Razvoj nove storitve Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d F.12 Izboljšanje obstoječe storitve Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.13 Razvoj novih proizvodnih metod in instrumentov oz. proizvodnih procesov Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.14 Izboljšanje obstoječih proizvodnih metod in instrumentov oz. proizvodnih procesov Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.15 Razvoj novega informacijskega sistema/podatkovnih baz Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.16 Izboljšanje obstoječega informacijskega sistema/podatkovnih baz Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d F.17 Prenos obstoječih tehnologij, znanj, metod in postopkov v prakso Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.18 Posredovanje novih znanj neposrednim uporabnikom (seminarji, forumi, konference) Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d F.19 Znanje, ki vodi k ustanovitvi novega podjetja ("spin off") Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov d F.20 Ustanovitev novega podjetja ("spin off") Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.21 Razvoj novih zdravstvenih/diagnostičnih metod/postopkov Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d F.22 Izboljšanje obstoječih zdravstvenih/diagnostičnih metod/postopkov Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.23 Razvoj novih sistemskih, normativnih, programskih in metodoloških rešitev Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.24 Izboljšanje obstoječih sistemskih, normativnih, programskih in metodoloških rešitev Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.25 Razvoj novih organizacijskih in upravljavskih rešitev Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.26 Izboljšanje obstoječih organizacijskih in upravljavskih rešitev Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d F.27 Prispevek k ohranjanju/varovanje naravne in kulturne dediščine Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.2S Priprava/organizacija razstave Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.29 Prispevek k razvoju nacionalne kulturne identitete Zastavljen cilj Oda One Rezultat d Uporaba rezultatov 1 d F.30 Strokovna ocena stanja Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d F.31 Razvoj standardov Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.32 Mednarodni patent Zastavljen cilj Oda One Rezultat d Uporaba rezultatov 1 d F.33 Patent v Sloveniji Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov d F.34 Svetovalna dejavnost Zastavljen cilj Oda One Rezultat I d Uporaba rezultatov 1 d F.35 Drugo Zastavljen cilj Oda One Rezultat d Uporaba rezultatov d Komentar 12.Samo za aplikativne projekte! Označite potencialne vplive oziroma učinke vaših rezultatov na navedena področja Vpliv Ni vpliva Majhen vpliv Srednji vpliv Velik vpliv G.01 Razvoj visoko-šolskega izobraževanja G.01.01. Razvoj dodiplomskega izobraževanja O O O o G.01.02. Razvoj podiplomskega izobraževanja o o o o G.01.03. Drugo: o o o o G.02 Gospodarski razvoj G.02.01 Razširitev ponudbe novih izdelkov/storitev na trgu o o o o G.02.02. Širitev obstoječih trgov o o o o G.02.03. Znižanje stroškov proizvodnje O o o o G.02.04. Zmanjšanje porabe materialov in energije o o o o G.02.05. Razširitev področja dejavnosti o o o o G.02.06. Večja konkurenčna sposobnost o o o o G.02.07. Večji delež izvoza o o o o G.02.08. Povečanje dobička o o o o G.02.09. Nova delovna mesta o o o o G.02.10. Dvig izobrazbene strukture zaposlenih o o o o G.02.11. Nov investicijski zagon o o o o G.02.12. Drugo: o o o o G.03 Tehnološki razvoj G.03.01. Tehnološka razširitev/posodobitev dejavnosti o o o o G.03.02. Tehnološko prestrukturiranje dejavnosti o o o o G.03.03. Uvajanje novih tehnologij o o o o G.03.04. Drugo: o o o o G.04 Družbeni razvoj G.04.01 Dvig kvalitete življenja o o o o G.04.02. Izboljšanje vodenja in upravljanja o o o o G.04.03. Izboljšanje delovanja administracije in javne uprave o o o o G.04.04. Razvoj socialnih dejavnosti o o o o G.04.05. Razvoj civilne družbe o o o o G.04.06. Drugo: o o o o G.05. Ohranjanje in razvoj nacionalne naravne in kulturne dediščine in identitete o o o o G.06. Varovanje okolja in trajnostni razvoj o o o o G.07 Razvoj družbene infrastrukture G.07.01. Informacijsko-komunikacijska infrastruktura o o o o G.07.02. Prometna infrastruktura o o o o G.07.03. Energetska infrastruktura o o o o G.07.04. Drugo: o o o o G.08. Varovanje zdravja in razvoj zdravstvenega varstva o o o o G.09. Drugo: o o o o Komentar 13.Pomen raziskovanja za sofinancerje12 Sofinancer 1. Naziv Naslov Vrednost sofinanciranja za celotno obdobje trajanja projekta je znašala: EUR Odstotek od utemeljenih stroškov projekta: % Najpomembnejši rezultati raziskovanja za sofinancerja Šifra 1. 2. 3. 4. 5. Komentar Ocena C. IZJAVE Podpisani izjavljam/o, da: • so vsi podatki, ki jih navajamo v poročilu, resnični in točni • se strinjamo z obdelavo podatkov v skladu z zakonodajo o varstvu osebnih podatkov za potrebe ocenjevanja ter obdelavo teh podatkov za evidence ARRS • so vsi podatki v obrazcu v elektronski obliki identični podatkom v obrazcu v pisni obliki • so z vsebino zaključnega poročila seznanjeni in se strinjajo vsi soizvajalci projekta Podpisi: zastopnik oz. pooblaščena oseba raziskovalne organizacije: Zavod za gradbeništvo Slovenije in vodja raziskovalnega projekta: Tadeja Kosec Kraj in datum: Ljubljana ZIG 9.3.2012 Oznaka prijave: ARRS-RPROJ-ZP-2012/29 1 Zaradi spremembe klasifikacije je potrebno v poročilu opredeliti raziskovalno področje po novi klasifikaciji FOS 2007 (Fields of Science). Prevajalna tabela med raziskovalnimi področji po klasifikaciji ARRS ter po klasifikaciji FOS 2007 (Fields of Science) s kategorijami WOS (Web of Science) kot podpodročji je dostopna na spletni strani agencije (http://www.arrs.gov.si/sl/gradivo/sifranti/preslik-vpp-fos-wos.asp). Nazaj 2 Napišite povzetek raziskovalnega projekta (največ 3.000 znakov v slovenskem in angleškem jeziku) Nazaj 3 Napišite kratko vsebinsko poročilo, kjer boste predstavili raziskovalno hipotezo in opis raziskovanja. Navedite ključne ugotovitve, znanstvena spoznanja, rezultate in učinke raziskovalnega projekta in njihovo uporabo ter sodelovanje s tujimi partnerji. Največ 12.000 znakov vključno s presledki (približno dve strani, velikosti pisave 11). Nazaj 4 Realizacija raziskovalne hipoteze. Največ 3.000 znakov vključno s presledki (približno pol strani, velikosti pisave 11) Nazaj 5 V primeru bistvenih odstopanj in sprememb od predvidenega programa raziskovalnega projekta, kot je bil zapisan v predlogu raziskovalnega projekta oziroma v primeru sprememb, povečanja ali zmanjšanja sestave projektne skupine v zadnjem letu izvajanja projekta (obrazložitev). V primeru, da sprememb ni bilo, to navedite. Največ 6.000 znakov vključno s presledki (približno ena stran, velikosti pisave 11). Nazaj 6 Znanstveni in družbeno-ekonomski dosežki v programu in projektu so lahko enaki, saj se projekna vsebina praviloma nanaša na širšo problematiko raziskovalnega programa, zato pričakujemo, da bo večina izjemnih dosežkov raziskovalnih programov dokumentirana tudi med izjemnimi dosežki različnih raziskovalnih projektov. Raziskovalni dosežek iz obdobja izvajanja projekta (do oddaje zaključnega poročila) vpišete tako, da izpolnite COBISS kodo dosežka - sistem nato sam izpolni naslov objave, naziv, IF in srednjo vrednost revije, naziv FOS področja ter podatek, ali je dosežek uvrščen v A'' ali A'. Nazaj 7 Znanstveni in družbeno-ekonomski dosežki v programu in projektu so lahko enaki, saj se projekna vsebina praviloma nanaša na širšo problematiko raziskovalnega programa, zato pričakujemo, da bo večina izjemnih dosežkov raziskovalnih programov dokumentirana tudi med izjemnimi dosežki različnih raziskovalnih projektov. Družbeno-ekonomski rezultat iz obdobja izvajanja projekta (do oddaje zaključnega poročila) vpišete tako, da izpolnite COBISS kodo dosežka - sistem nato sam izpolni naslov objave, naziv, IF in srednjo vrednost revije, naziv FOS področja ter podatek, ali je dosežek uvrščen v A'' ali A'. Družbenoekonomski dosežek je po svoji strukturi drugačen, kot znanstveni dosežek. Povzetek znanstvenega dosežka je praviloma povzetek bibliografske enote (članka, knjige), v kateri je dosežek objavljen. Povzetek družbeno ekonomsko relevantnega dosežka praviloma ni povzetek bibliografske enote, ki ta dosežek dokumentira, ker je dosežek sklop več rezultatov raziskovanja, ki je lahko dokumentiran v različnih bibliografskih enotah. COBISS ID zato ni enoznačen izjemoma pa ga lahko tudi ni (npr. v preteklem letu vodja meni, da je izjemen dosežek to, da sta se dva mlajša sodelavca zaposlila v gospodarstvu na pomembnih raziskovalnih nalogah, ali ustanovila svoje podjetje, ki je rezultat prejšnjega dela _ - v obeh primerih ni COBISS ID). Nazaj 8 Navedite rezultate raziskovalnega projekta iz obdobja izvajanja projekta (do oddaje zaključnega poročila) v primeru, da katerega od rezultatov ni mogoče navesti v točkah 7 in 8 (npr. ker se ga v sistemu COBISS ne vodi). Največ 2.000 znakov vključno s presledki. Nazaj 9 Pomen raziskovalnih rezultatov za razvoj znanosti in za razvoj Slovenije bo objavljen na spletni strani: http://sicris.izum.si/ za posamezen projekt, ki je predmet poročanja Nazaj 10 Največ 4.000 znakov vključno s presledki Nazaj 11 Največ 4.000 znakov vključno s presledki Nazaj 12 Rubrike izpolnite / prepišite skladno z obrazcem "izjava sofinancerja" http://www.arrs.gov.si/sl/progproj/rproj/gradivo/, ki ga mora izpolniti sofinancer. Podpisan obrazec "Izjava sofinancerja" pridobi in hrani nosilna raziskovalna organizacija - izvajalka projekta. Nazaj Obrazec: ARRS-RPROJ-ZP/2012 v1.00 65-CA-04-BE-FF-65-72-0F-FC-81-83-46-12-AD-3E-C6-7D-D2-B7-5E Elsevier Editorial System(tm) for Wear Manuscript Draft Manuscript Number: Title: The tribocorrosion behaviour of NiTi alloy Article Type: Full-Length Article Keywords: NiTi, microstructure, simulated saliva, tribo-corrosion, passive film Corresponding Author: Dr Tadeja Kosec, Corresponding Author's Institution: . First Author: Tadeja Kosec, PhD Order of Authors: Tadeja Kosec, PhD; Petra Močnik; Andraž Legat, assoc. Prof, PhD Abstract: NiTi alloys as important technological material finds its wide application in different fields. Due to shape memory and superelastic properties it is widely used in biomedical applications. Generally, nickel titanium alloys are known for their good corrosion resistance, but in some circumstances they suffer from pitting and crevice corrosion. Beside corrosion, the wear resistance of this alloy is of critical concern in many applications. Since dental NiTi archwires experience severe failures in applications, it is assumed that their performance is affected also by their microstructure. In our study, the microstructural properties of NiTi alloy samples were investigated: NiTi plate and NiTi dental wire with and without surface oxide in as-received state, were compared. The electrochemical properties of NiTi wires and sheets was compared by the use of different electrochemical techniques. The tribocorrosion properties were studied in artificial saliva. Wear rate was determined as well as its chemical and tribological contribution. The variation of corrosion performance in artificial saliva was greater for specimens covered with surface oxide films. Tribocorrosion studies showed that the total wear of the NiTi alloy is greater when corrosion is combined with the wear. It was confirmed that microstructure has an influential effect on electrochemical and triboelectrochemical properties. As a result, it is noteworthy to distinguish that electrochemical properties of the exposed working surface varies in dependence of microstructural properties of the studied alloy material. Confirmation of Authorship WEAR Confirmation of Authorship Please save a copy of this MS Word file, complete and upload as the "Confirmation of Authorship" file. As corresponding author, Tadeja Kosec, hereby confirm on behalf of all authors that: 1) The authors have obtained the necessary authority for publication. 2) The paper has not been published previously, that it is not under consideration for publication elsewhere, and that if accepted it will not be published elsewhere in the same form, in English or in any other language, without the written consent of the publisher. 3) The paper does not contain material which has been published previously, by the current authors or by others, of which the source is not explicitly cited in the paper. Upon acceptance of an article by the journal, the author(s) will be asked to transfer the copyright of the article to the publisher. This transfer will ensure the widest possible dissemination of information. *Novelty Statement Novelty statement Hereby, as a corresponding and l^rst author, I confirm that the paper entitled "The tribocorrosion behaviour of NiTi alloy" by the authors Tadeja Kosec, Petra Močnik, Andraž Legat represents significance and novelty of the work presented in the present manuscript. Tadeja Kosec *Manuscript Click here to view linked References 1 2 3 4 5 The tribocorrosion behaviour of NiTi alloy 6 7 8 * 9 Tadeja Kosec , Petra Močnik, Andraž Legat 10 11 12 Slovenian National Building and Civil Engineering Institute, 13 14 Dimičeva 12, SI - 1000 Ljubljana, Slovenia 15 16 17 18 19 20 21 60 61 62 63 64 65 Abstract NiTi alloys as important technological material finds its wide application in different fields. Due to shape 22 memory and superelastic properties it is widely used in biomedical applications. Generally, nickel titanium 23 24 alloys are known for their good corrosion resistance, but in some circumstances they suffer from pitting and 25 26 27 applications. Since dental NiTi archwires experience severe failures in applications, it is assumed that their 28 29 performance is affected also by their microstructure. 30 31 32 dental wire with and without surface oxide in as-received state, were compared. The electrochemical 33 34 properties of NiTi wires and sheets was compared by the use of different electrochemical techniques. The crevice corrosion. Beside corrosion, the wear resistance of this alloy is of critical concern in many In our study, the microstructural properties of NiTi alloy samples were investigated: NiTi plate and NiTi tribocorrosion properties were studied in artificial saliva. Wear rate was determined as well as its chemical alloy is greater when corrosion is combined with the wear. It was confirmed that microstructure has an microstructural properties of the studied alloy material. 35 36 37 and tribological contribution. The variation of corrosion performance in artificial saliva was greater for 38 39 specimens covered with surface oxide films. Tribocorrosion studies showed that the total wear of the NiTi 40 41 42 influential effect on electrochemical and triboelectrochemical properties. As a result, it is noteworthy to 43 44 distinguish that electrochemical properties of the exposed working surface varies in dependence of 45 46 47 48 49 Keywords: NiTi, microstructure, simulated saliva, tribo-corrosion, passive film 50 - 51 52 1. Introduction 53 54 55 56 57 the shape memory alloy that is most used material in practice. The studies can be roughly divided into 58 59 different areas of the interest: basic corrosion and electrochemical studies [1-6], mechanical and material There have been numerous studies conducted on nickel titanium alloy as the most representative example of property studies [7-11] and surface treatment studies [12-18]. 4 Among extensive corrosion studies on nickel titanium alloy [1-6], Rondelli made a comparison of NiTi with 6 other implant materials in body simulating fluids [1]. It was shown that passive films on NiTi are inferior to those formed on TiAlV alloys, but are comparable to those formed on stainless steels [1]. The pitting potentials lower than 250 mV were found on NiTi in simulated body fluid at pH 37 °C, and they were 7 9 10 11 dependant on surface modification procedures, with which it was possible to increase the pitting potential to 12 13 values exceeding 800 mV [3]. Figueira et al. have recently studied NiTi, and compared it to Ti, Al, 316L in Hank's solution and in Eagle's minimum essential medium, also at 37 °C [4]. It was found that NiTi is 16 subjected to crevice corrosion, and that the passive films are thinner in amino acid containing environment [4]. Among different drawbacks of the use of NiTinol such as its subjection to pitting corrosion [3], failures 19 are reported like stress corrosion cracking in orthodontic NiTi wires [8] and extensive Ni release [7]. Since 21 nickel can cause hypersensitive reactions in the human body, a vast choice of surface treatments have been 22 23 proposed and implemented in order to reduce the content of nickel in surface oxide film [12], among which 24 ion implantation [12], mechanical polishing, chemical etching and selective oxidation [13] were proposed 2 6 and investigated. It was found that corrosion properties were improved by chemical etching that provided 27 2 8 depletion of nickel in surface compact oxide film, whereas oxidation at 530 °C as a part of shape setting 20 procedure negatively affected the corrosion performance [13]. Higher corrosion resistance of austenitic 31 grown oxide film was determined [5]. Also alloying with Nb and Co exchanging the content of Ni in NiTi 32 33 alloy was studied. Corrosion properties were improved as well as hardness of the alloy [14]. 34 35 36 coatings like TiN [15,16,19]. Corrosion resistance of NiTi was significantly improved by almost 60 times 37 38 39 40 41 surface also enhanced corrosion resistance of such oxide film [18]. Surface of NiTi was modified by 42 43 implementation of N+ and/or Ar+ ion to produce Nickel free surface on NiTi to improve biocompatibility, 44 45 46 Concerning tribological behaviour of NiTi alloys, there are several studies done in dry conditions [9-11], 47 48 but there is a huge lack in tribocorrosion studies on NiTi alloys [6]. Tribological behaviour of NiTi alloy 49 50 51 where superelasticy and shape memory effect might be lost [9]. The reduction of wear rate on NiTi/steel 52 53 contact at elevated temperature was observed. It was attributed to tribological layer on the wear surface of 54 55 56 resistance as compared to martensitic state, where also higher coefficient of friction was measured as a 57 58 result of lower strength of martensitic state [10]. Also, sliding wear of superelastic NiTi was studied in dry 59 60 61 62 63 Many studies involve evaluation of different coatings on NiTi alloy, from hydroxyapatite [17] to hard by electrodeposition of the hydroxyapatite/zirconia composite coating. The drawback of such system is that it can not be used in load bearing applications [17]. Water plasma immersion ion implantation on NiTi as evaluated by cell culture experiments [11]. against steel and WC at elevated temperatures was studied in order to get data of tribological behaviour NiTi [9Abedini]. Another study by the same authors showed that NiTi in austenitic state had higher wear conditions [11]. It was shown that wear resistance of superelastic TiNi alloy is about 30 and 10 times higher 4 than pure Ti and Ni, respectively. The dominant wear mechanisms observed for TiNi were abrasion and 6 delamination of subsurface cracks as a result of cyclic loading. Moreover, dry tribological tests were done on plasma source ion implantated NiTi samples in austenitic and martensitic state, as well [6]. It was shown that the wear was reduced by 38 % for austenitic state in dry conditions and down to 79 % in the lubricated 7 9 10 11 conditions. The closest study to tribocorrosion investigation of NiTi alloy was done by Rondelli [1]. The 12 13 localized corrosion tests where the passive film was abruptly damaged showed that the characteristics of the 15 passive film on NiTi were inferior to those of TiAlV and in some cases even to stainless steels [1]. 16 Since most tribological studies on NiTi alloy was done in dry conditions and only effect of mechanical wear was studied, there is a great demand in recognizing properties of NiTi alloy at simultaneous process of wear 19 and corrosion in aggressive media. The aim of the present study is to evaluate and compare electrochemical 21 and triboelectrochemical properties of nickel titanium alloys of two different microstructural shapes in 22 23 simulated saliva: dental archwire and superelastic Nickel titanium sheet. 24 Polarization curves were measured to evaluate electrochemical behaviour in a simulated saliva. 25 2 6 Tribocorrosion tests were carried out at open circuit potential. Wear rate of the two microstructural alloys 27 2 8 (archwire and sheet) and the two different surface pre-treatments step (oxidized as received form and 20 polished) was studied. Optical and microscopic investigation of surface modifications after wear tests was 31 investigated as well. 32 33 34 35 36 37 38 39 40 41 42 1) Superelastic, alloy BB NiTi 2 mm sheet, flat annealed, surface oxide free (pickled), Memry 43 44 45 46 2) NiTinol orthodontic wire (3M), superelastic, 0.019x0.025 in (0.48x0.64 mm), Orthoform III 47 Ovoid Upper. 48 2. Experimental 2.1. Preparation of the samples Two samples were chosen for the study, namely: GMBH alloy, Samples (i.e. working electrodes) for electrochemical tests were cut from the sheet in the form of discs with mm. The electrical contact was attached and the back and the sides of the sample as well as electrical 49 50 51 a diameter of 15 mm and from the archwire at the straight part in the full length of 5 cm. Samples for 52 53 triboelectrochemical tests were cut from the sheet with electro discharge machining in the shape of 50x 10 54 55 56 contact were protected by epoxy coating, so that the exposed surface measured 4.5 cm2. Samples for 58 triboelectrochemical tests from archwire were prepared from the flat part of the archwire in the total lenghth 59 60 61 62 63 4 of 40 mm, moulded in an acrylate mould with the electrical contact attached. It was subsequently grinded 6 and polished in order to get surface free oxide film of NiTi dental wire. 7 Two different surface finishes were studied: the first were samples without oxide film, which were prepared by grinding and polishing in order to investigate the effect of microstructure. The second surface finish was oxide film in as received state (with the suppliers' finish) in order to investigate the actual sample 3 min and then well dried. 2.2. Micro-structural examination HNO3 and 50 vol. % H2O for 2 min. Shortly after, the optical microscopy study was conducted at different 9 10 11 12 13 that comes into use in realistic environment (i.e. dental archwire in saliva). For oxide film removal the 14 15 samples were abraded with 800 and 1000-grid SiC paper. Samples were ultrasonically cleaned in ethanol for 16 17 18 19 20 21 Samples for metallographic investigation were etched in a solution of 10 vol. % HF, 40 vol. % 22 23 24 magnifications. 25 27 2.3. Electrochemical measurements 28 29 The choice of adequate saliva solution was made by testing Nickel titanium alloy in various solution 30 31 32 similarly as the tested choice of salivas by Duffo et al. [20]. Thus, the electrochemical measurements were 34 performed in a solution of 0.60 g/L NaCl, 0.72 g/L KCl, 0.22 g/L Caa2-2 H2O, 0.68 g/L KH2PO4, 0.856 g/L 35 Na2HPO4l2 H2O, 0.060 g/L KSCN, 1.5 g/L KHCO3 and 0.03 g/L citric acid, pH=6.5 in order to simulate 36 37 saliva (this solution was designated: "simulated saliva"), as proposed by Duffo et al [20]. 38 39 In the case of flat disc electrode, a three-electrode corrosion cell was used, with a volume of 350 cm3. The 40 2 41 working electrode was embedded in a Teflon holder, and had an exposed area of 0.785 cm . In the case of 42 43 44 expanded with Nova software 6 was used. 45 compositions. The one that most closely represents natural saliva was chosen for further experiments, the wire electrodes, the area exposed to the solution was 1.0 cm2. An Autolab PGStat 100, Floating version expanded with Nova software 6 was used. Electrochemical testing was executed in the following order: 2-hour stabilization at open circuit potential 46 47 48 (OCP), linear polarization measurements at ± 20 mV vs. OCP at a scan rate 0.1 mV/s were performed. 49 50 Potentiodynamic measurements were then performed starting from -0.25 V vs. OCP, and progressing in the anodic direction up to +2.0 V at a scan rate of 1 mV/s. For cyclic polarization measurement the reverse reported with respect to the SCE scale. 51 52 53 potential was chosen at the potential 1.4 V in order to provide the definite unknown to all of the tested 54 2 55 samples. Average current densities at potential of 1.4 V varied from 0.4 to 1 mA cm . All potentials are 56 57 58 59 60 61 62 63 64 65 4 At least three measurements were performed to fulfil electrochemical testing statistical requirement [21]. 6 After estimating the mean values of logarithm results of corrosion resistance, the measurement that had the 7 closest value to the mean value from the set, was chosen to be presented. 9 10 11 2.4 Triboelectrochemical measurements 12 13 The wear tests were performed on a reciprocal tribometer (Tribotechnic, pin on disc + reciprocating 15 tribometer, 2009, France) with a 6-mm Al2O3 ball as a counter body. 16 The test parameters that were evaluated for triboelectrochemical experiments are denoted in each figure. The normal loads in tests varied from 1 N to 5 N at average sliding speed 5 mm/s. The wear length was 10 19 mm. In all of the experiments the friction force and friction coefficient were followed by time. Ball counter 21 body with a 6 mm diameter was made from Al2O3. Triboelectrochemical three-electrode cell, made of 22 23 Teflon, consisted of: working electrode with electrical contact, platinum wire counter electrode and 24 Ag/AgCl reference electrode. 2 6 Electrochemical and tribocorrosion experiments were executed at room temperature. The reason for that is 27 2 8 that tribocorrosion experiments do not enable test conditions at elevated temperatures. Moreover, 29 electrochemical experiments were also executed at elevated temperatures and no in the pitting potential was 31 detected. 32 33 34 35 2.5. SEM/EDS, profilometer and hardness analysis 36 37 A low-vacuum JEOL 5500 LV, JEOL, JAPAN (Japan) scanning electron microscope, equipped with 38 39 energy dispersive spectroscopy (EDX) Oxford Inca (Oxford Instrument Analytical, UK), was used to 40 41 42 43 A Taylor Hobson profilometer Taylor Hobson precision, Surtronic 25 (France, 2010) was used for wear scar 44 analysis. 45 46 The Vikers Hardness was determined by tester Frank Finotest 38542 using an indenter at loads of 9.8 N at 47 48 microscopic enlargement of 40. 49 50 51 3. Results and discussion 52 53 54 3.1 Metallographic examination 5 6 Both samples, NiTi sheet and NiTi archwire in cross section and longitudinal direction were 57 58 59 structure. The not well defined crystal grains are of 40-50 ^m in cross-sectional and longitudinal view. 60 61 62 63 observe the surface products formed using an accelerating voltage of 20 kV. metalographically examined. They are presented in Figure 1. NiTi sheet (Figure 1 a and b) has martensitic 4 Grains are uniformly oriented in all directions due to thermal procedures. There are several bigger 6 inclusions of diameter 2-5 ^m found in cross section view (Fig 1a). There is smaller number of inclusions in longitudinal direction (Fig 1b, the exposed working surface at electrochemical and tribocorrosion experiments). 7 9 10 11 The martensitic microstructure of NiTi archwire in cross section view and longitudinal view was very 12 13 difficult to reveal (Fig 1c and d). Crystal grain size in cross-section (Fig 1c) are relatively small (less than 40 ^m) being elongated in longitudinal direction (Fig 1d, exposed surface for electrochemical testing) due 16 to technological procedure that includes cold drawing. There are many small inclusions (less than 1 ^m) in the microstructure of the wire that occupy 8.8 % of the total area in cross-section and 4.3 % of the total area 19 in longitudinal view. The inclusions are elongated in the pulling direction. They measure from 10-200 ^m 21 in length. Unfortunately, it was impossible to exactly locate area of inclusions in SEM microscope, so their 22 23 consistency could not be determined. It was thus reported that the inclusions are both TiC and Ti2NiOx [22]. 24 25 2 6 Fig. 1. Microstructural examination of NiTinol superelastic sheet a) cross-section and b) longitudinal view 27 2 8 and NiTi archwire c) cross-section and d) longitudinal view. 29 30 31 32 3.2 Potentiodynamic measurements 33 34 35 In order to estimate pitting susceptibility, cyclic polarization measurements at a scan rate 1 mV/s were 36 37 executed for polished NiTi sheet and wire, as well as for NiTi sheet and wire with surface oxide films in as- 38 39 40 41 42 Fig. 2. Cyclic polarization scans for NiTi sheets and wires: a) without surface oxide film, b) with oxide film, 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 received state. Repassivation properties were determined, both for NiTi sheet and wire. in simulated saliva at a scan rate 1 mV/s. 4 NiTi sheet without surface oxide film evidently showed that NiTi was not susceptible to pitting corrosion as 6 negative hysteresis was observed for NiTi sheet in reversed anodic scan in simulated saliva. The corrosion 7 current density is 74 nA/cm2. The forward scan, in which a low passive current density of approximately 3 ^A/cm2 was measured, was consistent with a passive behaviour. A sharp increase in current density marked 9 10 11 the breakdown potential at 1.10 V in the forward scan. The potential applied to the electrode was reversed 12 13 and scanned in reverse direction at potential 1.4 V, where approximate current of 1 mA/cm2 was reached. The reverse scan evidenced currents, lower than currents in the passive forward scan. Under conditions 16 studied, the polished NiTi electrode could successfully repassivate. 17 18 For a NiTi dental wire without oxide film a shape of CP curve is similar to NiTi sheet, while the forward 19 2 2 0 corrosion current density is lower (45.9 nA/cm ) and Ecorr is more positive (-0.267 V) than the one for NiTi 21 sheet. The passive current in forward scan is similar to the NiTi sheet sample, while in the reversed scan it is 22 23 smaller for NiTi wire. The breakdown potential was evidenced at 1.2 V (Table 1). The repassivation 24 25 potentials for both samples were relatively high, 0.9 V for a sheet and 0.86 V for a wire sample. Polarization 26 resistance, as a good measure for corrosion susceptibility, since the scan rates are lower, showed that NiTi 2 8 sheet without surface oxide film is smaller, 0.824 MQ cm2 than the one for NiTi dental wire without surface 29 30 oxide film, 1.19 MQ cm2. 3 2 Comparing the two different samples without surface oxide film it can be concluded that micro-structural 33 characteristics of dental wire exhibited better corrosion properties than NiTi sheet sample. This can be 34 35 attributed to the fact that the exposed metal surface of dental wire has fewer inclusions 4.3 %, relatively 3 7 small number of elongated crystal grains in nominal area. It is also microstructurally less susceptible to 38 corrosion attack that could experience its counterpart of dental wire- cross section with 8.8 % of inclusions 39 40 and much smaller crystal grains. 4 2 This fact is important when interpreting and comparing results with previously published ones. 43 Aziz Kerrzo studied cross section of the Ti-45Ni surface, where the pitting potential was as low as 250 mV 44 45 (vs. SCE) in phosphate buffer solution at 37 °C [3]. In another study, pitting potentials of 800 mV were 4 7 found by Rondelly in dearated physiological solution for NiTi flat specimen [1]. Cross sections of two 48 different dimensions of the rod were tested (2 and 8 mm), where it was found that low diameter electrode 49 50 experienced onset of crevice corrosion at potentials 250 mV vs. SCE, while for larger specimen at 1000 mV 51 52 53 there is probably the effect of microstructure that plays a major role in different pitting susceptibility 54 55 properties of NiTi alloy. It is believed that dental wire in cross-section would show the worst corrosion 57 characteristics. 58 For a realistic evaluation of the corrosion properties of the metal alloy it is important to study their 59 60 characteristics in the "as received" state, with a surface oxide film as achieved from technological 61 62 63 in Hank's solution at 37 °C. Beside susceptibility of crevice corrosion for the samples mounted in resins, 4 procedure. The electrochemical properties of NiTi sheet with surface oxide film are better than NiTi wire 6 with surface oxide film (Figure 1, Table 1). The corrosion potential for NiTi sheet with surface oxide film is more negative (-0.207 V) than the one for NiTi wire with oxide film (-0.143 V), both were more positive than in the case of without oxide film, as described in previous case. Corrosion current density for NiTi 7 9 10 2 11 sheet with surface oxide is as low a s 15.4 nA/cm (see values in Table 1). Passive current densities are 12 13 comparable, while in the case of NiTi sheet with surface oxide film, the breakdown potential is much higher, at 1.27 V. Scanning in the reverse direction showed negative hysteresis pointing at good 16 repassivation properties of investigated samples in the environment and conditions studied. Repassivation potentials in this case are lower (0.32 V for NiTi sheet and 0.06 for NiTi wire) when compared to samples 19 without oxide film. The reverse current density for wire exhibits numerous oscillations in anodic current 20 21 path, pointing at possible metastable passive state. With the surface oxide film present it can be concluded 22 23 that NiTi sheet exhibited better corrosion properties when compared to NiTi dental wire samples. 25 Polarization resistance measurements prove that observation, Rp of NiTi sheet with surface oxide film is 2 6 2.57 MQ cm2 and 1.38 MQ cm2 for NiTi dental wire with surface oxide film. 27 28 29 Higher values for anodic current densities and positive hysteresis in the reverse scan were reported for NiTi 30 31 32 difference are the following: higher experiment temperatures, at body temperature, 37 °C [3], higher content 33 34 of chlorides in phosphate buffer solution (PBS, pH 7.4 contains 8.77 g/L NaCl, 1.42 g/L Na2HPO4 and 2.72 35 36 37 reported by Aziz Kerrzo et al. [3], surface modification techniques did not influence the repassivation 38 39 kinetics- which is in contradiction with the results obtained in our experiments. But comparing the content 40 41 42 suggested by Duffo and Castillo [20], it can be expected that the difference arises from chloride content, 43 44 thus, 0.15 M chloride content induces the pitting corrosion in simulated body fluids but showed 45 46 47 wire and polished sheet. Also, the cross-section in comparison to exposure of the whole longitudinal surface 48 49 of the archwire might have given a rise to pitting susceptibility and lower corrosion properties of cross 50 51 52 in the case of steel wires in pore water [23]. In the previous case, the delicate edges of microstructural grains 53 54 that were exposed as cross-section are more intense corrosion-wise than the elongated grains and its surface 55 56 57 58 59 times in simulated saliva solution at a scan rate 1 mV/s. 60 61 62 63 wire electrode which is in contradiction with our observation. The possible explanations for such a g/L KH2PO4) and the cross sectioned part of the exposed polished NiTi archwire. Namely, as it was of total chloride in PBS solution, used by Aziz Kerrzo et al. [3], and chloride in simulated saliva, as repassivation characteristics in artificial saliva solutions (chloride content 0.123 M), both for polished NiTi sectioned NiTi sample. The microstructure might have such a big in^uence, as already reported by authors when the sides of the wire are exposed. Fig. 3. Potentiodynamic curves for NiTi dental wire a) and NiTi sheet b), at different long term exposure 1 2 3 4 5 6 Long term exposure to corrosive environment of simulated saliva gave insight to possible different 8 behaviour of the studied samples with and without oxide film and are presented in Figure 3. The time of 9 exposure varied in the length, from 48 h to 72 h. 10 2 11 For NiTinol dental wire the corrosion current of the freshly polished wire was the smallest, 16n A/cm in comparison with as-received wire, where corrosion current density was slightly higher 81 nA/cm2. 12 13 15 Corrosion potential of freshly polished wire was more positive (Ecorr= -0.17 V) than for as received wire 16 (Ecorr= -0.54 V). Freshly polished surface of NiTinol wire exhibited well defined passive region with 18 approximate current at around 40 ^A/cm2, where the breakdown potential in simulated saliva was at 0.98 V. 19 For as received wire the passive range extended through wider range with some current fluctuation in 21 voltage range from 0.7 V to 1.4 V vs. SCE. Breakdown potential was estimated at around 1.3 V. Anodic 22 23 current for as received wire was higher than for freshly polished wire, it drifted from 6 ^A/cm2 to 80 24 uA/cm2. 25 2 6 However, electrochemical behaviour of NiTi sheet of as received surface finish and polished electrode was 27 2 8 similar. Corrosion current densities and corrosion potentials for as received surface finish and polished NiTi 30 sheet electrode are similar, reaching value of 61 uA/cm2 and 13 uA/cm2 and -0.18V and -0.12 V, 31 respectively. Anodic currents are similar for as received NiTi sheets and freshly polished electrode. 32 33 Breakdown potentials for as received and polished samples were 1.2 V and 1.15 V, respectively. 34 To conclude, electrochemical behaviour of NiTi sheet in two different surface finish is more similar than 35 36 electrochemical behaviour of as received wire and polished wire. Electrochemical behaviour of as received 37 38 39 40 41 corrosion properties of NiTinol in studied environment. 42 43 44 45 46 electrochemical properties of wire without oxide film has better electrochemical properties than NiTi sheet 47 48 without oxide film. However, there is some discrepancy noticed between the two experiments for the 49 50 51 observed from lower corrosion current densities for the wire, but after CP tests, NiTi sheet exhibited lower 52 53 corrosion current densities and higher Rp than NiTi wire with surface oxide film present. Thus, it is 54 55 56 finish, length of the corrosion tests and nature of experiment. 57 58 59 60 61 62 63 wire thus, is very different from NiTi sheets in both forms and polished dental NiTi wire, respectively. Microstructural characteristics of alloy as well as surface finish had a great effect on electrochemical and When comparing cyclic polarization tests and tests after long term exposure (48 h), it can be concluded that samples with its as received oxide film: after 48 h NiTi wire exhibits better properties than NiTi sheet as important, which microstructural plane is studied, what sample (microstructural characteristics), surface 4 3.3 Tribocorrosion tests 5 6 Since NiTi alloy undergoes different modes of wear during its operation, it is very important to study 7 8 corrosion properties of the alloy simultaneously with its wear. In situ information of the surface state of the NiTi alloy at open circuit potential can be followed when combining wear in simulated saliva solution. The 11 mechanical and chemical data aquired before after and during sliding tests is very useful for getting a better 12 13 understanding of the tribocorrosion processes that take place on samples with and without oxide film. Coefficient of friction (COF), corrosion potential measurement (Ecorr), versus time curves for for NiTi sheet 9 10 14 15 16 with and without surface oxide film and for dental archwire without oxide film at speed 5 mm/s and 17 18 different normal loads in simulated saliva are shown in Fig. 4. The normal load was varied from 1 N, 2 N to 5 N. Each measurement was repeated at least twice and very repetitive measurements were obtained. 19 20 21 For tribocorrosion experiments on NiTi sheet without surface oxide film (Figure 4 a), the normal load 22 23 affected the changes in corrosion potential: the higher the force, the more negative changes in corrosion 24 25 potentials during rubbing were observed. The variation of the corrosion potential value during rubbing is 26 higher for higher applied loads. It was previously reported that during wear (sliding), lowering of the open 2 8 circuit potential is frequently experienced, as well as with increasing normal loads and/or sliding velocity 29 30 [24]. Also, it was observed, that the higher the load, the quicker was the run-in period to the stable 31 coefficient of friction for NiTi without surface oxide film in simulated saliva solution. COF was as high as 32 33 0.80 for the load 1 N and 0.7 for the loads 2 an 5 N, respectively. The repasivation is dependent on the wear 34 35 regime in the wear track. It was 30 s at the load 1 N, 60 s for 2 N and 100 s for the normal load 5 N. The 3 7 different repassivation times are though affected by the exposed, newly uncovered surface after wear. 38 In general, the coefficient of friction curves for all tested materials show similar overall shapes (Fig. 4a). 39 40 COF curves were characterized by two friction regimes; a run-in period, represented by a rapid increase in 41 the value of the coefficient of friction followed by a steady state where the coefficient of friction became 42 43 independent of time. 44 45 Figure 4b shows the decrease of measured potentials on NiTi sheet with oxide film once the rubbing had 4 7 started. The higher the force, the lower the measured corrosion potential during rubbing. Initially lower 48 measured OCP potential for the sample worn with 1 N force, has decreased for approximately 200 mV, but 49 50 relatively high fluctuations in measured potential were observed. It is assumed that the normal force 1 N 51 52 53 stopped, the open circuit potential returned to the initial equilibrium electrochemical potential in 20 s. It was 54 55 observed that the initial OCP potential was reached more quickly for less worn surface, thus it was the 56 57 58 Coefficient of friction was very high, as high as 0.7 for the normal loads 2 N and 5 N. The run in period is 59 60 61 62 63 64 65 was that low that did not damaged the surface film through entire rubbing period. Once rubbing had stopped, the open circuit potential returned to the initial equilibrium electrochemical potential in 20 s. It wa observed that the initial OCP potential was reached more quickly for less worn surface, thus it was th quickest for sample worn with 1 N (20 s) force, then 2 N (70 s) and for sample worn with 5 N force (100 s). Coefficient of friction was very high, as high as 0.7 for the normal loads 2 N and 5 N. The run in period ii smaller for NiTi sheet samples with the surface oxide film when compared to samples without surface oxide 4 film (Figure 4 b). COF increased in the first minutes of rubbing for smaller normal forces as there is evident 6 difference in coefficient of friction on oxide film and base underlying alloy. Once the alloy matrix is reached, the coefficient of friction stayed the same during the entire rubbing period. Special behaviour was observed when a low normal force of 1 N was used. The coefficient of friction 7 counterbody at different applied normal forces in simulated saliva 9 10 11 increased in the first cycles of rubbing, it was constant for 500 s, than it was decreased again after 600 s of 12 13 rubbing. At the same time, the increase of the measured corrosion potential was detected. Repetitive 15 measurements always showed similar characteristics. Namely, the applied normal force is low enough not to 16 damage surface oxide film on the wear track. As it was not destructive, the surface film was quickly 18 repaired, as could be observed from the decrease in coefficient of friction and from the increase in ^O electrochemical potential during the rubbing in simulated saliva with 1 N normal force. 21 22 23 Fig. 4. OCP potential, friction coefficient and friction force vs. time for NiTi sheet without surface oxide 24 25 film (a), with surface oxide film (b) and dental wire without surface oxide film (c) against Al2O3 ball 26 27 28 20 The fixation of the sample was a difficult so that the experiment for dental wire enabled investigation only 31 for dental wire without surface oxide film with two different applied loads, 1 N and 2 N. The tribocorrosion 32 33 study is presented on Figure 4 c. During the rubbing, the coefficient of friction is as high as 0.8 for load 1 N 34 35 36 applied load in the case of dental wire. The electrochemical potential lowered to a great extent for 0.25 V 37 38 39 40 41 and the potential reached the initial value. 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 and 0.7 for load 2 N. It varied more for the 1 N applied load. The run in period is shorter for the higher for 1 N and 0.30 V for the load 2 N. The variation of potential during rubbing is very smooth when comparing to sheet samples without oxide film. Passive film recovered in 30 s after stopping the rubbing 4 3.4. Wear track analysis 6 7 31 32 The wear tracks were observed by SEM and the wear volume loss of the samples was determined by measuring the cross section of the wear tracks by contact profilometry (Figure 5 and 6). 9 10 11 It can be seen that the wear tracks were different in dependence of the load used, the type of the sample and 12 13 14 15 16 17 type of media (ambient air, saliva). Fig. 5. Wear track of NiTi sheet without oxide film after tribological wear (Fn= 5 N) in ambient conditions 18 a) and in simulated saliva b) 19 20 21 22 Fig. 6. Wear track of NiTi sheet with oxide film (as received) after tribological wear (Fn= 5 N) in ambient 23 conditions a) and in simulated saliva b) 24 25 26 27 28 pointed at two possible wear mechanisms, one being abrasion and the other delamination. The wear track 29 30 consisted of parallel grooves as a result of sliding of Al2O3 ball against the investigated sample. The material was displaced to the edges forming topography observed as extruded ridges (Figure 6). General A wear track width and wear track depth increases with the applied normal force. Wear track observation 33 observation of the investigated samples showed that the amount of wear debris increases with the applied 34 35 normal force. Wear debris sizes of delamination plates are of 20-150 ^m for the samples where the applied 36 37 38 39 40 unstable and sheet like wear debris is generated. The number of wear debris for tribocorrosion tests are load was 5 N (Figure 6). Such delamination is a result of harder asperity on soft surface which induces plastic deformation. At repeated loading, microcracks eventually propagate to critical length, become smaller (Figure 6b) than the number for the wear in atmospheric conditions (Figure 6a). Also, the sizes of 41 42 43 delamination particles became larger with the applied normal force. Similar wear mechanisms and type of 44 45 damage was observed for TiNi alloys, investigated by sliding wear experiments in dry conditions [11]. 46 47 EDS analysis outside the track, in the track and on particular inclusions showed different atomic structures 48 (Table 2). Only results for tribocorrosion wear for NiTi sheet with and without surface oxide film in 49 50 simulatedsaliva are presented and discussed. 51 52 53 54 55 56 57 58 59 60 61 62 63 1 2 3 4 5 The composition of the passive film on the samples with surface oxide film outside the track (area No. 1, 6 7 Figure 5a) is as follows: Ti and Ni almost at atomic equilibrium and 24.0 % of oxygen. Composition 8 9 changes in the wear track. It becomes richer in oxygen and also aluminium is detected in small percentage 11 (2.46 %) as a residue of wear with Al2O3 ball counterpart (area No. 2, Fig 5 a). Inclusion consists of Ti and 12 Ni at equilibrium and a relatively high percentage of oxygen and aluminium (Table 2). Similar observations 13 14 were found for polished samples (without oxide film). The composition outside the track gave values for 15 16 17 and some traces of aluminium were found, while on the biggest inclusions the amount of oxygen and 18 19 aluminium were very high pointing at high abrasive wear of Al2O3 counterpart body against NiTi sample. 20 21 22 different loads applied. 23 24 The hardness was also measured outside and inside the wear track, as well as on the inclusions. 25 26 27 normal load, in simulated saliva showed that the hardness was lower outside the wear track (311 HV) and 28 29 was increased in the wear track to 339 HV. This increase in surface hardening is a result of rubbing, and has 30 31 32 hardening, induced internal stresses during oxide film growth, stress release due to dissolution or other 33 34 changes during corrosion processes. Since the hardness in the wear track after tribological wear in ambient 35 36 37 in corrosive environment affects hardness to different extent than in ambient air. The hardness, measured on 38 39 inclusions was as high as 395 HV on inclusions measured on samples without surface film and 430 HV on 45 46 50 51 58 59 60 61 62 63 64 65 NiTi composition of the alloy with no detected oxygen. In the track the amount of oxygen was increased EDS analysis showed similar composition at the comparable positions on NiTi wear track in dependence of After tribocorrosion experiments, the results for NiTi sheet without surface oxide film, loaded with 2 N been observed previously on other materials [25]. Hardness in the wear track thus is a result of work air was lower (331 HV) than the one after tribocorrosion wear in the solution, it could be observed that wear samples with surface oxide film. The high hardness of inclusions are worn debris from Al2O3 counterbody 40 41 42 that immersed into wear track during tribocorrosion wear. The hardness was measured also on NiTi 43 44 archwire without surface oxide film (345 HV) and it was higher than on NiTi sheet (315 HV). The higher hardness resulted in shallower wear profile (not shown) on dental archwires. Also, the inclusions were not 47 detected in the wear track of NiTi archwire. The mechanical resistance of dental archwire is greater and 48 49 wear is lower for these harder polished archwire samples. The wear rate was determined according to mechanistic approach by measuring the wear volume [26]. The 52 total wear is a sum of mechanical and corrosion wear: 53 54 55 Vtot = Vmech + Vchem Eq. (1) 56 57 Wear volumes were determined from profiles and the lengths of wear tracks, where the wear volume is defined as volume wear in a definite measure of time. The wear rates are presented in Table 3, where total 4 wear is defined by tribocorrosion experiments, mechanical wear is a contribution due to tribological wear in 6 ambient air, whereas chemical wear is deduced from the equation above. At lower load forces (1 N), the total chemical wear could not be defined, since the wear rate in dry conditions is higher than wear rate in simulated saliva. Wear rate was higher for NiTi sheet with surface 7 9 10 11 oxide film present. Similarly was found previously, where the wear rate was more severe on annealed 316L 12 13 samples compared to polished samples [27]. It was explained that large accumulation of strain occurred at 15 passive potentials which then faciliated crack formation at microstructural defects [27]. At conditions 16 studied, the chemical wear presents a smaller contribution to the total wear rate than mechanical wear, 18 probably due to very good repassivating properties of the studied alloy. The highest contribution of 19 chemical wear was detected at loads of 2 N, where chemical wear contribution was around 33 % (Table 3). 20 21 Thus, at higher loads, the mechanical wear dominates the chemical wear.Jt was found with our experiments 22 23 and in published literature [27], that for passive materials, the wear rate is increased at passive potentials 24 when compared to cathodic potentials. NiTi alloy at open circuit potentials represent passive potentials, 2 6 where repassivation is enabled and thus wear rate of such material is of great concern. 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 the two microstructurally different NiTi alloys with and without surface oxide film in order to study the 1 2 3 4 5 6 7 4. Conclusions 8 9 Metallographic, electrochemical, tribocorrosion experiments, as well as surface analysis were performed on 10 11 12 effect of microstructure on chemical and physical properties in artificial saliva. 13 14 16 NiTi sheet in cross section and longitudinal plane is microstructurally different from NiTi dental archwire. 17 It is important whether the sample investigated is longitudinal plane of the sheet alloy or cross section or 18 19 longitudinal plane of the wire (rod). 20 21 Cyclic polarization tests showed that it is very important to know the state and the microstructural plane of 22 the sample that is being studied, especially when results are compared to previously reported ones. 24 - Our experiments showed that electrochemical parameters, such as corrosion potential, corrosion current 25 26 density, breakdown potential and repassivation potentials of NiTi sheet samples and longitudinal exposed 27 surface of NiTi archwire without surface oxide films are very similar, but with distinguishing polarization 2 9 resistances. With surface oxide film in as received state, the NiTi sheet samples showed better corrosion 30 31 properties. 3 3 - Electrochemical results in simulated saliva showed good repassivation properties of the investigated 34 microstructural planes and surface states of NiTi alloy. 36 - However, after long term exposure the electrochemical properties of samples without oxide film are 37 38 39 than as-received NiTi sheet, which means that in long term performance, oxide film formed on wire is more 40 41 corrosion resistant than one on NiTi sheet. 42 43 44 as higher wear rate of NiTi alloy in simulated saliva compared to dry tribological wear. 45 46 - All investigated samples showed very good repassivation properties after tribological wear in simulated 47 saliva. 48 49 - During tribological wear in simulated saliva plastic deformation in wear track occurs with many inclusions 50 51 of the worn surface film and counterbody parts in the wear track. The hardness of the wear track is 52 53 54 friction increases in the first cycles of wear and then stays stable for all tested loads. 55 56 57 58 59 sheet sample and longitudinal surface of the wire) do not enable to differentiate between microstructural 60 61 62 63 64 65 comparable to cyclic polarization results but wire in as received form exhibited better corrosion properties -Tribocorrosion experiments confirmed synergistic effect of combined wear and chemical process observed increased while coefficient of friction vary in dependence of the hardness of the material. The coefficient of As an overall conclusion it can be said that tribocorrosion studies on exposed surfaces (longitudinal plane of 4 effect on tribocorrosion properties. Corrosion contribution to total wear is bigger for samples with oxide 6 film. Thus, in the case of possible damage when handling dental wire, cross-section of the wire with many inclusions would show the weaker corrosion properties, leading to quicker corrosion and mechanical failure. These findings have to be confirmed in future studies. 7 9 10 11 12 Acknowledgements 13 14 This work has been sponsored by Slovenian Research Agency grant No Z2-2298. Help of laboratory 16 staff-Viljem Kuhar and Monika Prosenc is gratefully acknowledged. 17 18 References 19 20 21 22 [2] S. Rosi, F. Deflorian, A. Pegoretti, D. D'Orazio, S. Gialanella, Chemical and mechanical treatments to 23 24 improve the surface properties of shape memory NiTi wires, Surf. Coat. Technol. 202 (2008) 2214-2222. 25 [3] M. Aziz-Kerrzo, K. G. Conroy, A. M. Fenelon, S. T. Farrell, C. B. Breslin, Biomaterials 22 (2001) 26 27 1531-1539. 28 29 [4] N. Figueira, T.M. Silva, M.J. Carmezim, J.C.S. Fernandes, Corrosion behaviour of NiTi alloy, [1] G. 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Farhat, Sliding wear of superelastic TiNi alloy, Wear 267 (2009) 9394-9400. 53 54 [12] N. Shevchenko, M-T. Pham, M.F. Maitz, Studies of surface modified NiTi alloy, Appl. Surf. Sci. 235 55 56 57 [13] D. Vojtěch, J.Fojt, L. Joska, P. Novák, Surface treatmentof NiTi shape memory alloy and its influence 58 59 on corrosion behaviour, Surf. Coat.Technol. 204 (2010) 3895-3901. 60 61 62 63 64 65 short-time heat treated NiTi shape memory alloy, Appl. Surf. Sci. 257 (2010) 1573-1582. [9] M. Abedini, H.M. Ghasemi, M. Nili Ahmadabadi, Tribological behaviour of NiTi alloy against 52100 and austenitic states, Materials and Design 30 (2009) 4493-4497. (2004) 126-131. 4 [14] K.W. Ng, H.C. Man, T.M. Yue, Characterization and corrosion study f NiTi laser surface alloyed with 6 Nb or Co, Appl. Surf. Sci. 257 (2011) 3269-3274. [15] D. Starosvetsky, I. Gotman, TiN coating improves the corrosion behavior of superelastic NiTi surgical Alloy, Surface and Coatings Technology 148 (2001) 268-276. 7 9 10 11 [16] A. Kumar, D. Kaur, Nanoidentation and corrosion studies of TiN/NiTi thin films for biomedical 12 13 15 [17] D, Oui, A.Wang, Y.Yin, Characterization and corrosion behaviour of hydroxyapatite/zirconia 16 composite coating on NiTi fabricated by electrochemical deposition, Appl. Surf. Sci. 257 (2010) 177417 18 applications, Surf. Coat. Technol. 204 (2009) 1132-1136. 1778. ^0 [18] X.M. Liu, S.L. Wu, Paul K. Chu, C.Y. Chung, C.L. Chu, K.W.K. Yeung , W.W. Lu, K.M.C. Cheung, 21 K.D.K. Luk, Effects of water plasma immersion ion implantation on surface electrochemical behaviour 22 23 ofNiTi shape memory alloys in simulated body fluids, Appl. Surf. Sci. 253 (2007) 3154-3159 25 [19] L. Neelakantan, S. Swaminathan, M. Spiegel, G. Eggeler, A.W. Hassel, Selective surface oxidation and 2 6 nitridation of NiTi shape memory alloys by reduction annealing, Corros. Sci. 51 (2009) 635-641. 27 2 8 [20] G.S. Duffo, Q. Castillo, Development of artificial saliva solution for studying the corrosion behaviour 30 of dental alloys, Corrosion 60 (2004) 594-602. 31 [21] W. Stephen Tait, An Introduction to electrochemical corrosion testing for practicing Engineers and 32 33 scientists, 1994, USA. 34 35 36 [23] J. Kovač, M. Bajt Leban, A. Legat, Detection of SCC on prestressing steel wire by the simultaneous 37 38 39 40 41 electrochemical and mechanical reactivity of surfaces, Wear 261 (2006) 939-946. 42 43 [25] S. Mishler, A.Spiegel, M. Stemp, D. Landolt, Influence of passivity on the tribocorrosion of carbon 44 45 46 [26] S. Mischler, Triboelectrochemical techniques and interpretation methods in tribocorrosion: A 47 48 comparative evaluation, Tribology International 41 (2008) 573-583. [22] http://en.wikipedia.org/wiki/Nickel titanium (11.2.2012) use of electrochemical noise and acoustic emission measurements. Electrochim. acta. 52 (2007) 7607-7616. [24] J-P- Celis, P. Ponthiaux, F. Wenger, Tribo-corrosion of materials: Interplay between chemical, steel in aqueous solutions, Wear 251 (2001) 1295-1307. [27] A. Bidiville , M. Favero, P. Stadelmann , S. Mischler, Effect of surface chemistry on the mechanical 49 50 51 response of metals in sliding tribocorrosion systems, Wear 263 (2007) 207-217. 52 53 54 55 56 57 58 59 60 61 62 63 64 65 45 Figure Captions 6 7 Fig. 1: Microstructural examination of NiTinol superelastic sheet a) cross-section and b) longitudinal view and NiTi archwire c) cross-section and d) longitudinal view. 9 10 11 Fig. 2: Cyclic polarization scans for NiTi sheets and wires: a) without surface oxide film, b) with oxide film, in simulated saliva at a scan rate 1 mV/s. 12 13 15 Fig. 3: Potentiodynamic curves for NiTi dental wire a) and NiTi sheet b), at different long term exposure 16 times in simulated saliva solution at a scan rate 1 mV/s. 18 Fig. 4: OCP potential, friction coefficient and friction force vs. time for NiTi sheet without surface oxide 19 film (a), with surface oxide film (b) and dental wire without surface oxide film (c) against Al2O3 ball 20 21 counterbody at different applied normal forces in simulated saliva. 22 23 Fig. 5: Wear track of NiTi sheet without oxide film after tribological wear (Fn= 5 N) in ambient conditions 24 a) and in simulated saliva b). 25 2 6 Fig. 6: Wear track of NiTi sheet with oxide film after tribological wear (Fn= 5 N) in ambient conditions a) 27 28 and in simulated saliva b). 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Table 1 : Electrochemical parameters for NiTi dental wire and sheet (Econ and jcorr estimated from CP curves, sheet wire NiTi sheet as received NiTi wire With surface oxide film Ecorr [V] -0.291 -0.267 -0.207 -0.143 ./corr [A/cm2] 74.0 10-9 45.910-9 15.410-9 44.210-9 Rp [MQcm2] 0.824 1.19 2.57 1.38 Eb [V] 1.10 1.20 1.27 1.05 Erp[V] 0.9 0.86 0.32 0.06 Table 2: EDS analysis at different spots after tribocorrosion testing in simulated saliva, load force 5 N Ti Ni O Al NiTi sheet without surface oxide film Out of the track 50. 50 In the track 42.5 43.0 13.8 0.7 inclusion 18.8 18.7 52.5 10.0 NiTi sheet with surface oxide film Out of the track 38.8 37.2 24.0 In the track 33.6 33.8 30.1 2.5 inclusion 19.0 17.9 54.1 9.0 Table 3: Wear rate [cm3/s] during tribocorrosion and tribological wear of NiTi sheet samples NiTi sheet without surface oxide film NiTi sheet with oxide film Total wear Mechanical wear Corrosion wear Total wear Mechanical wear Corrosion wear 1 N 0.045 10-8 0.1810-8 0.14 • 10-8 0.4110-8 2 N 0.6110-8 0.4010-8 0.2110-8 (34 %) 0.99 • 10-8 0.66 10-8 0.33^10-8 (33 %) 5 N 1.6310-8 1.43 10-8 0.2010-8 (12 %) 1.75 • 10-8 1.6710-8 0.0810-8 (4.6 %) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 2: 1.5 UJ O D.5 m > > UJ 0.0 : a) -Sheet -Wire ^^^^^^^ 1 F? / - . 0.5 10-10 10-" 10"' 10' 10-" 10= 10-' 10 j / A cm"' jI Acm" 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 20 ■ ■ as received 60h ■ polished 72ti 1.0 0-5 0.0 ■O.S - lO'" 10-= 10® 10-' 10-= 10-= lO-* !0-= lO-^' 10-' 10' 10-« 10-^ 10-= 10-= 10-' 10-' 10'= 10' j / A cm ^^ j / A cm ■2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1000 2000 3000 4000 5000 6000 5000 6000 Figure 4: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 0.4 0.6 0. Width / mm 0.4 0.6 0.8 Width / mm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 0.4 0.6 Width / mm 0.4 0.6 0.8 Width / mm 'Suggested Reviewers Possible reviewer: Stefano Mischler, Ecole Polytechnique Federale de Lausanne (EPFL), Laboratory for Metallurgical Chemistry, CH-1015 Lausanne, Switzerland Tel.: +41 21 693 2954; fax: +41 21 693 3946. Stefano.mishler@epfl.ch *Highlights Highlights • Tribocorrosion of NiTi alloys is investigated in simulated saliva. • Two microstructurally different alloys are investigated: dental archwire and NiTi sheet. • Synergistic effect of combined wear and chemical process observed as higher wear rate of NiTi alloy is reported. Tribokorozija in tribokorozijsko preskušanje Tadeja Kosec Zavod za gradbeništvo Slovenije, Dimičeva 11, 1000 Ljubljana Povzetek V prispevku so predstavljene osnove tribokorozijskih procesov. Tribokorozijski proces je korozijski proces, ki nastane ob sočasnem drgnjenju ali obrabi materiala. Zaradi odstranjevanja oksidnih plasti je korozijski proces zato največkrat pospešen. Za spremljanje korozijskih procesov potrebujemo posebno preskuševališče - tribometer, predelan za možnost spremljanja elektrokemijskih procesov v troelektrodni korozijski celici. Seznanimo se z različnimi možnostmi tribokorozijskega preskušanja, možnimi metodami za tribokorozijsko spremljanje procesov ter predstavimo tipične primere tribokorozijskega preskušanja. Abstract Basic introduction to tribocorrosion processes is given. Tribocorrosion process is a corrosion process which results from simultanious rubbing, frettening of metals. Corrosion process is in most cases accelerated process due to removal of oxide films. For tribocorrosion study a special tribocorrosion system is needed-it consists of tribometer with specially constructed three-electrode electrochemical cell. Different setups for possible tribocorrosion study are presented as well as different electrochemical techniques that enable to study tribocorrosion processes. Different case studies are presented as well. 1. Splošno o tribokoroziji Pri relativnem gibanju kontaktnih površin je tribološki kontakt zelo zapleten proces, saj vključuje simultan proces trenja, deformacije in obrabe. Meja med mehanizmi na makro in mikro nivoju je težko določljiva, saj so med seboj prepleteni in povezani. V kolikor je pri procesih prisoten elektrolit (npr. oksidirano mazivo, kondenz, tkivo, slina) se mehanski procesi obrabe kombinirajo s korozijskimi. Tribološki proces vključuje tako spremembo materiala in spremembe, ki nastanejo kot rezultat skupnega delovanja obrabe in korozije. V splošnem omenjena kombinacija ni vsota, temveč sinergija obeh vrst procesov, zato so vplivi posameznih parametrov pri skupnih tribokorozijskih procesih izrazito nelinearni in nestacionarni. Za uspešno poznavanje tribokorozijskega obnašanja materiala ni dovolj zgolj poznavanje tribološkega delovanja brez upoštevanja korozivnih razmer, kot tudi ni dovolj poznavanje kemijskega vedenja brez poznavanja vplivov obrabe. Zato tribokorozija kot veda zahteva interdisciplinarno obravnavo. V zadnjem času je povečano zanimanje za razumevanje uporabe različnih materialov in njihovih kombinacij, kar sili tako raziskovalce kot inženirje k sodelovanju zaradi znanstvenih, ekonomskih in praktičnih interesov. Pridobivanje temeljitega vpogleda v mehanizme in določevanje kritičnih mehanskih in kemijskih vplivov na tribološki proces ima zelo velik pomen. Določene zakonitosti pri uporabi posameznih materialov v izbranih okoljih in določenih obremenitvah so sicer poznane, vendar je povezovanje znanj s področja tribologije in korozije materialov še vedno relativno neraziskano področje. Glavna vzroka za to sta težavnost simuliranja realnih pogojev in nestacionarnost procesov. 1.1 Opredelitev problema Večina korozijskih procesov je po naravi elektrokemijskih. To vključuje prenos elektronov. Oksidacija kovine poteče na sledeč način: M ^ Mn+ + n e" (1) kjer je M kovina (angl. Metal) in n število izmenjanih elektronov na atom kovine. Na anodi se kovina raztaplja (reakcija 1) in prehaja iz oksidacijskega stanja 0 v n+ (tj. aktivna kovina). Če pa se v elektrolitu tvori oksid (tj. pasivna kovina), poteče reakcija po enačbi (2) M + n H20^ MOn + 2 n H+ + 2 e (2) Partnerska katodna reakcija je lahko sproščanje vodika, ki je favorizirana v kislem okolju, ter redukcija kisika, ki je favorizirana v bazičnem okolju in zračenih raztopinah. Pri tem potečejo reakcije po (3)-(5): 2 H2O + 2 e- ^ 2 OH- + H2 (3) O2 + H2O + 4 e- OH- (4) 2 H+ + 2 e-^ H2 (5) Pri običajnih korozijskih razmerah je reakcija (1) v ravnotežju z eno ali več redukcijskih reakcij po enačbi (3)-(5). Ko sta katodni in anodni tok enaka, govorimo o korozijskem potencialu, Ekor . Tako je bilo razvitih nekaj vrst elektrokemijskih tehnik za spremljanje in kontroliranje korozijskega potenciala. Prav tako so jih uporabili v tribokorozijskih eksperimentih za spremljanje procesov med drgnjenjem. Največkrat uporabljene elektrokemijske tehnike so meritve korozijskega potenciala, potenciostatske ter potenciodinamske meritve. To pomeni, da je uporabnost konvencionalnih elektrokemijskih tehnik, ki so sicer primerne za napoved splošne korozijske obstojnosti, pri triboloških procesih delno omejena zaradi nestacionarnih razmer med tribološko obrabo. Zato je ovrednotenje elektrokemijskega vpliva pri sinergijski kombinaciji z mehanskimi procesi problematično, posledično pa je nezanesljivo tudi modeliranje teh procesov. Za spremljanje triboelektrokemijskih procesov je potrebno razviti in vključiti tudi druge elektrokemijske tehnike. 2. Korozijski in mehanski procesi Drgnjenje materiala lahko vodi k lokalnem odstranjevanju pasivne plasti. Posledično lahko nastopi korozijska obraba, ki v kontaktnem področju spremeni lastnosti materiala. Pri tem se lahko v kontaktnem področju nalagajo korozijski produkti (prisotnost tretjega telesa, angl. Third Body), ki vplivajo na nadaljnji proces obrabe. neobremenjena anoda katoda / obremenjena katoda neobremenjena b) katoda anoda neobremenjena anoda M obremenjena neobremenjena Slika 1: Grafični prikaz dveh različnih razmer pri drgnjenju, ko obremenjeni material deluje kot a) anoda ali kot b) katoda. Mehanska obremenitev in hitrost drgnjenja vplivata različno na različne materiale (nepasivni materiali kot je baker in pasivni materiali kot nerjavno jeklo). V večina primerov obrabljen material deluje kot anoda (slika 1 a) in redkeje kot katoda (slika 1 b). To lahko spremljamo z ocenjevanjem procesa depasivacije. Glavno zanimanje v tribokorozijskih študijah je ravno izbira pravih korozijskih razmer, saj drgnjenje ustvarja nestacionarne elektrokemijske razmere v kontaktnem področju. Omejitve pri določanju katodnih in anodnih mest (slika 1) lahko zaobidemo s tehnikami in kombinacijo metod. Elektrokemijske tehnike v splošnem zahtevajo stacionarne pogoje med potekom meritve. Dinamične in tranzientne razmere v kontaktnem področju vplivajo na dobljene rezultate. Iskanje metodologij za karakterizacijo osnovnih tribokorozijskih procesov oziroma študij medsebojnega vplivanja mehanskih in elektrokemijskih procesov predstavlja velike izzive za raziskovalce iz različnih področij. Poseben izziv predstavlja nadgradnja tranzientnih elektrokemijskih tehnik (elektrokemijski šum z akustično emisijo, metoda merjenja delnih tokov z mikroelektrodami), ki so bile v preteklosti razvite pri študiju drugih lokalnih oblik korozije (napetostno-korozijsko pokanje, špranjska korozija, korozija jekla v betonu) [1-3]. Tribokorozijske raziskave zaobjemajo tista področja, kjer lahko v agresivnih okoljih pride do korozijskih procesov, hkrati pa so vsi mehanski stiki tudi obrabno obremenjeni. To so materiali, katerih trajnost je problematična: pogonski agregati in materiali v procesni industriji (orodna jekla, siva litina) ter v biomedicini (nerjavna jekla, titanove zlitine in različni steliti). V splošnem je potrebno poznati osnovne parametre in njihov vpliv na tribokorozijske procese problematičnih kovinskih materialov pri tipičnih pogojih okolja (mehanska obremenitev, elektrolit). Potrebno je izdelati in proučiti primerne tehnike, ki bodo v bodoče lahko služile kot orodje za napoved tribokorozijskih lastnosti določenih materialov pri realnih problemih: npr. uporaba biogoriv, lubrikantov, evalvacija učinkovitosti inhibitorjev, trajnost različnih vrst vsadkov. 3. »State of the art« Tribološka obraba, ki poteče v prevodnem elektrolitu ob kontroliranih elektrokemijskih razmerah, je definirana kot triboelektrokemijski eksperiment. Definicija tribokorozije je bila prvič prikazana in omenjena v klasični tribološki knjigi med rezultati, ki jih je predstavil Barker [4]. Prvi standard, ki upošteva tako obrabo kot korozijo, je standard ASTM G119 - Standardna navodila za določanje sinergije med obrabo in korozijo, ki je izšel leta 1995 in bil kasneje dopoljnjevan [5]. Mehanizem obrabe, sinergistični učinek med obrabo in korozijo na tribokorozijske procese ter na drugi strani fenomen tretjega telesa priteguje v zadnjem času precej zanimanja [6-9]. Tako je Stack s sodelavci izdelala različne režime za prepoznavanje prevladujočega mehanizma med tribokorozijskim procesom. Za boljše razumevanje sinergističnega delovanja je določila koeficient K^IK^ ter opredelila različne načine obrabe in korozije. Koeficient K^IK», pri čemer je K^, obraba zaradi korozije in Kw tribološka obraba, služi kot kriterij za ocenjevanje medsebojne odvisnosti korozije in obrabe [6,7]. Če je koeficient KcIKw manjši od 0,01, je prevladujoč mehanizem obraba, če pa je koeficient manjši od KcIK» >10, proces narekuje korozija. Med potekom drgnjenja prihaja do akomuliranja delcev, ki se sproščajo zaradi obrabe teles med kontaktom. Obrabljata se lahko en ali oba materiala v stiku. Landolt je s sodelavci poskušal opisati vplive delcev, nastalih zaradi obrabe (angl. Third Body), katerih vpliv je preiskoval z elektrokemijskimi eksperimenti [9,10]. Opazili so, da imajo delci, nastali zaradi obrabe, velik vpliv na tribokorozijske procese. Različne tribološke obrabe, od ponavljajočih gibanj, enosmernih premikov, drgnjenja ter spinninga so študirali z različnimi tribolelektrokemijskimi tehnikami. Odvisno od načina obrabe in kontaktne geometrije je lahko pričakovati različne odzive oz. načine obnašanja. In-situ analize tribološke obrabe v področju dentalne protetike so pokazale, da je obremenitev v kombinaciji s premiki majhne frekvence zelo zahteven process [11]. Študije so zelo raznolike, s tribološkega stališča so preiskovali sistem jezikInebo [12]. Zelo kompleksen način kombinacije obrabe in korozijskega procesa lahko povzroča tvorbo obrabnih delcev in raztopljenih kovin, kar lahko povzroči krajšo življenjsko dobo kolčne proteze [13]. Med materiali za temeljne študije je najbolj pogosto izbran material nerjavno jeklo (tip 304L in 316L skupaj z sestavnimi kovinami, Cr, Ni) [14-16], biokompatibilni materiali, kot so kobaltove zlitine (Stelliti) [17,18] in titanove zlitine, med katerimi prevladuje TiAlV [13, 19-21]. V zadnjem času se je povečalo zanimanje po tribokorozijskem obnašanju tankih prevlek [22-24]. Bay on je s sodelavci proučeval PVD prevleke v prestavnih sistemih [22]. Prav tako je veliko objav s področja obrabe [23], upoštevajoč korozijo, ki pa ni povezana z njeno elektrokemijsko naravo. Med procesom obrabe in trenja so kemijske in strukturne spremembe, ki se dogajajo na DLC prevlekah med obrabo v prisotnosti mineralnih olj študirali z Ramansko in X-žarkovno spektroskopijo [23]. Druga študija obravnava vplive trdote, debeline in strukture TiCO tankih plasti na tribokorozijsko vedenje [24]. Ocenjena sta tako posamezni kot medsebojni vpliv obrabe in korozije. Prav tako, kot so študije razdeljene po različnih obravnavanih materialih, lahko razdelimo tribokorozijske študije glede na njihovo uporabo; nekatere so temeljne študije, druge so študije tribokorozije v biomedicinske namene kot tudi študije trih tankih in proti obrabi odpornih prevlek. Nekatere tehnike in metode so pogosto uporabljene, medtem ko so nekatere poredko. Med zadnjimi objavami S. Mischlerja se lahko spoznamo s kritično obravnavo glavnih elektrokemijskih tehnik ter metod za razumevanje tribokorozijskih procesov s posebnim poudarkom na procese drsenja in trenja pri študiju pasivnih kovin [25]. V skladu s prej omenjeno pregledno študijo [25] lahko tehnike razdelimo na tehnike spremljanja korozijskega potenciala, galvanske celice ter potenciodinamske tehnike. Na podlagi potenciostatskih tehnik so nekateri avtorji razvili modele za napoved elektrokemijskega odziva pasivnih kovin na tribokorozijski process. Landolt je s sodelavci razvil model za pasivne kovine, ki se obrabljajo, in opisal tok, ki se spreminja med drgnjenjem kot funkcija hitrosti depasivacije in repasivacije [26]. Hitrost depasivacije je odvisna od obremenitve, hitrosti, trdote kovine, površinske topografije in kontaktne geometrije [26]. Tudi Olsson in Stemp sta izdelala model za napoved repasivacijske kinetike med drgnjenjem [27]. Pontiaux poudarja, da je glavni problem pri tribokorozijskih procesih ta, da pri dvosmernem sistemu obrabe, obrabna konica ustvarja nestalne (angl. non-steady) elektrokemijske razmere v področju kontakta [15]. Jemmely s sodel. je ugotovil, da se je vrednost toka med časom, ko material ni bil v procesu obrabe, zmanjševal zaradi repasivacije in se ponovno povečal, ko je obrabna konica potovala nazaj v primeru uporabe ponavljajočega dvosmernega tribometra [10]. Mnoge elektrokemijske tehnike pa zahtevajo stacionarne razmere med samo meritvijo. Dinamične in prehodne razmere v področju kontakta tako lahko drastično vplivajo na dobljene rezultate [15]. Le nekaj je objav, te so zelo redke, ki kot možno tehniko za proučevanje tribokorozije uporabljajo elektrokemijski šum in elektrokemijsko impedančno spektroskopijo [15, 16]. Elektrokemijsko impedančno spektroskopijo so uporabljali le kot dodatno tehniko pred procesom obrabe in po njem [24]. Monticelli s sodelavci [20] pa je ugotovil, da je splošna stabilnost TiAlV zlitine v korozijskih razmerah dovolj stabilna, da so lahko izmerili impedančne spektre. Impedanca depasiviranega področja namreč prevlada nad celotno impedanco elektrode. Tudi merjenje z mrežo galvansko povezanih elektrod še ni bilo uporabljeno v triboelektrokemijskih študijah. Metoda merjenja delnih tokov z mikroelektrodami smo razvili študiju drugih lokalnih oblik korozije, kjer razlike med anodnimi in katodnimi mesti vplivajo na potek korozije [3]. Kot je poudaril Cellis s sodel., obremenjeno področje materiala, ki je podvržen drgnjenju, lahko deluje kot anoda ali kot katoda [28]. Tako uporaba mreže galvansko povezanih elektrod z uporabo elektrokemijskega šuma in akustične emisije predstavlja velik izziv v znanju na področju tribokorozije. Poudariti je potrebno, da obstaja velika potreba po temeljnih raziskavah in kritičnemu ovrednotenju posameznih elektrokemijskih tehnik, ki se uporabljajo za tribokorozijske preiskave. Če povzamemo, večina temeljnih študij uporablja poteciostatsko tehniko v 80 % izbranih literaturnih virih. Druga najbolj uporabljena tehnika je bila merjenje korozijskega potenciala v 50 % primerih. V 10 % preiskane literature so uporabili potenciodinamsko tehniko kot orodje za spremljanje elektrokemijskih procesov, ta pa je problematična za uspešno interpretacijo rezultatov zaradi nestacionarnih pogojev, ki jih ta tehnika zahteva. Prav tako je nekajkrat uporabljena tehnika galvanskega člena. Večina raziskav uporablja eno ali dve tehniki med opisanimi najpogosteje uporabljenimi tehnikami za spremljanje tribokorozijskih eksperimentov. Kot je bilo že bilo poudarjeno, le nekaj raziskav uporablja elektrokemijski šum in impedančno elektrokemijsko spektroskopijo pri študiju tribokorozijskih lastnosti, pri čemer mreža elektrod in akustična emisija kot dodatna tehnika k elektrokemijskemu šumu, še ni bila uporabljena. Tako lahko ugotovimo, da je na področju nadaljnjih in temeljnih študij možnih tehnik za študij tribokorozijskih procesov še veliko odprtih problemov in izzivov. 4. Tribokorozijsko preskuševališče Tribokorozijsko preskuševališče je sestavljeno iz tribometra ter posebej za korozijske meritve predelano korozijsko celico. Prav slednja predstavlja velik problem za dejansko realizacijo tribokorozijskih preiskav. Na tržišču so dostopni različni tribometri (CSM instruments, Phoenix instruments) za raznovrstne tribološke študije trenja in obrabe materialov z ali brez prisotnosti lubrikantov. Obstajajo tudi bolj specializirane različice za testiranje pri nižjih ali višjih temperaturah ali v vakuumu. Aparat za tribokorozijske meritve mora biti ustrezno predelan ali pa posebno izdelan. Prirejen tribometer za elektrokemijske preiskave mora imeti nekatere njemu slične karakteristike, ki imajo prednost pred komercialnimi aparati. Te so: obremenitev na vzorec za natančno določeno silo, kontrolo parametrov, kot so hitrost drgnjenja, frekvenca in čas ter razpon sile nekaj N do nekaj mN ter materiali v stiku s korozijsko raztopino. Materiali morajo biti električno neprevodni: nosilec za obrabno telo, ohišje korozijske celice, nosilci za pritrditev vzorca. Najpomembnejša značilnost je elektrokemijska celica za spremljanje tribokorozijskih procesov, kjer predstavljajo kontakti za delovno elektrodo (preiskovan material), referenčno elektrodo (ponavadi nameščena na posebnem nosilcu) ter proti-elektrodo) poseben tehničen izziv. Delovna elektroda Slika 2: Slika linearnega recipročnega tribometra z elektrokemijsko celico ter shema tipičnega tribokorozijskega preskuševališča Na sliki 2 je prikazan linearni recipročni tribometer z elektrokemijsko celico ter shema tipičnega tribokorozijskega preskuševališča. Na spodnji sliki 3 pa sta predstavljena najpogostejša načina tribokorozijskega eksperimenta, glede na uporabljeno vrsto tribometra: pin on disk ali pa dvosmerni linearni tribometer. Slika 3: Prikaz različnih obrab in načinov gibanja na tribokorozijskih sistemih: a) pin-on disc tribokorozijska obraba in b) linearna dvosmerna (angl. Reciprocating) obraba 5. Elektrokemijske metode za tribokorozijsko preskušanje Pri tribokorozijskih eksperimentih se med procesom obrabe posamezno uporabljajo naslednje elektrokemijske tehnike: merjenje korozijskega potenciala, galvanske celice, potenciostatske ter potenciodinamske meritve. Največkrat uporabljeni tehniki sta potenciostatska in tehnika s spremljanjem korozijskega potenciala. Merjenje korozijskega potenciala med tribokorozijsko obrabo je zelo preprosta tehnika. Z njo pridobimo informacijo o stanju površine med drgnjenjem. Korozijski potencial, Ekor merimo med delovno in referenčno elektrodo (Slika 4). Ko se med obrabo odkriva kovina (obrabljena površina), je neto potencial elektrode spremenjen. Med tribološko obrabo potencial pade na nižje vrednosti, temu pravimo katodna sprememba (angl. cathodic shift). Po končani tribološki obrabi se potencial zopet pomakne na višje začetne vrednosti. Na obrabljeni površini se v odvisnosti od narave kovinskega materiala različno hitro lahko tvori pasivna plast. Slabost te tehnike je, da pri njeni uporabi ne dobimo nikakršnih podatkov o kinetiki reakcij, ki nastopajo med drgnjenjem oz. obrabo. Delovna elektroda Slika 4: Primer elektrokemijskega spremljanja korozijskega potenciala a) shema eksperimenta, b) primer tribokorozijskega preskušanja nerjavnega jekla 316 L v 0,5 M H2SO4 Potentiostatski tribokorozijski test je test, pri katerim delovno elektrodo držimo na izbranem potencialu v troelektrodni elektrokemijski celici. Elektrode so povezane s potencistatom, ki vzdržuje potencial med delovno in referenčno elektrodo. Pri stalni napetosti merimo tok v odvisnosti od časa in s tem spremljamo kinetiko reakcij, ki potekajo na elektrodi (Slika 5). Delovna elektroda 200 150 < 100 50 tribološka obraba 20 mm/s 10 mm/s 50 mm/s 1000 2000 3000 4000 t / s 5000 Slika 5: Primer potentiostatskega tribokorozijskega testa 0 0 a) shema eksperimenta, b) primer tribokorozijskega preskušanja nerjavnega jekla 316 L v 0,5 M H2SO4 pri različnih hitrostih obrabe Enaka kovina Slika 6: Primer tribokorozijskega preskušanja z galvansko celico oz. elektrokemijskim šumom shema eksperimenta a), primer tribokorozijskega preskušanja nerjavnega jekla 316 L v 0,5 M H2SO4 b) Tok med elektrodama, Icell merimo z nizko ohmskim predočajevalcem (angl. zero-resistance ammeter, ZRA), priključen na preiskovan kovinski material (delovna elektroda), pri čemer je protielektroda iz istega materiala kot delovna elektroda s podobno izpostavljeno površino. Tem delu eksperimenta rečemo merjenje galvanske celice. Glavna prednost opisane metode EŠ pred drugimi klasičnimi elektrokemijskimi metodami je dejstvo, da meritev elektrokemijskega šuma poteka v prostokorodirajočih sistemih brez od zunaj vzbujenih signalov. To nam omogoča, da z metodo EŠ zaznavamo naravni (nemoten) razvoj korozijskih procesov in tako na podlagi hitrih sprememb (tranzientov) v merjenem toku in napetosti detektiramo iniciacijo in razvoj korozijskih procesov. Korozijski dogodki na posamezni elektrodi se odražajo v izmerjenem toku in napetosti. Če so vse tri elektrode enake, med delovno in referenčnima elektrodama ni toka in napetosti. Kadar pa se lokalna korozijska poškodba anodnega značaja (poškodba pasivnega filma, nastanek nove korozijske jamice, pojavi na delovni elektrodi, se to pozna tako na tokovni kot tudi na napetostni krivulji kot premik (zasuk) signala v t.i. anodno smer. Pri tem nastali elektroni potujejo iz delovne na referenčni elektrodi. Če se lokalna korozijska poškodba anodnega značaja pojavi na tokovni referenčni elektrodi, se to odraža kot premik toka v nasprotno smer - katodna smer (elektroni potujejo iz tokovne elektrode) in premik napetosti v anodno smer. Pri nastanku anodne poškodbe na napetostni referenčni elektrodi se to opazi kot premik napetosti v katodno smer. Torej je sočasen tokovni in napetostni odziv v anodni smeri pogoj za detekcijo korozijskega dogodka na delovni elektrodi. Če je merilni sistem sestavljen iz ohmskega predočajevalca in visokoimpedančnega predočajevalca, lahko omogočimo hkratno spremljanje tudi potenciala, ki ga merimo med referenčno elektrodo in delovno elektrodo. Ob uporabi 18-bit A/D pretvornika lahko dosežemo resolucijo za tok nekaj nA, za meritve napetosti pa ^V. Tovrstnemu eksperimentu, ki je za tribokorozijsko preskušanje zelo redko, pravimo meritve elektrokemijskega šuma. Pri potenciodinamskih meritvah različno hitro spreminjamo potencial. S to metodo lahko opazujemo vpliv trenja na različne reakcije, ki potekajo med drgnjenjem. Ta vpliv se lahko opazi le, če je razmerje med obrabljeno in neobrabljeno površino dovolj veliko. 6. Primeri tribokorozijskega preskušanja Tribokorozijske lastnosti izbranega materiala v korozivni raztopini preskušamo s klasičnim tribološkim spremljanjem obrabe, povezanim s hkratnim elektrokemijskim eksperimentom. Povezovanje mehanskih in elektrokemijskih metod omogoča hkratno spremljanje koeficientov in sile trenja pri definiranih korozijskih razmerah. Na slikah 7-9 so prikazani tribokorozijski eksperimenti na ploščici iz nerjavnega jekla 316L v raztopini 0,5 M H2SO4. Predstavljeni eksperimenti so narejeni pri potencialu odprtega kroga (angl. Open circuit potential) brez zunanje narinjene napetosti. 0.6 ùj 0.4 - 0.2 - 0.0 - -0.2 - 0.1 0 200 400 600 800 1000 1200 1400 1600 t / s -1-1-1-1-1-1— 500 1000 1500 2000 2500 3000 3500 t / s Slika 7: Spremljanje korozijskega potenciala nerjavnega jekla 316 L pri različnih silah obremenitve med tribološko obrabo Nas sliki 7 lahko spremljamo odvisnost uporabljene sile pri drgnjenju. Večja, ko je sila, večja je sprememba v zmanjšanju korozijskega potenciala elektrode iz nerjavnega jekla tipa 316 L. Vidimo lahko, da so potenciali pri silah 5 in 10 N podobni, iz česar lahko sklepamo, da sila 1 N ni zadosti velika, da bi povsem odstanila pasivno plast na obrabni poti. Zato je korozijski potencial pri sili 1 N nekoliko bolj pozitiven med časom obrabe. Po končani obrabi se potencial povrne nazačetno vrednost v razičnih časih, to je čas, ki ga kovina potrebuje, da se ponovno pasivira. ÙÙ 1000 2000 3000 t / s 4000 5000 Slika 8: Spremljanje korozijskega potenciala nerjavnega jekla 316 L pri različnih poteh med tribološko obrabo Nas sliki 8 lahko spremljamo vpliv časa obrabe v raztopini 0,5 M H2SO4 pri konstanti sili obremenitve. Korozijski potencial elektrode se ob začetku tribološke obrabe zmanjša in ostaja konstantne vrednosti pri vseh neodvisno od časa obrabe. Po koncu obrabe se potencial začne povečevati. Čas repasivacije je odvisen od več dejavnikov in je različen za tri različna preskušanja. 0 0.2 - 0.1 - 0.0 - > ùù -0.1 - -0.2 - -0.3 - 1000 2000 t / s 3000 4000 Slika 9: Spremljanje korozijskega potenciala nerjavnega jekla 316 L pri različnih hitrostih tribološke obrabe Nas sliki 9 je predstavljena odvisnost korozijskega potenciala od hitrosti obrabe pri konstantni sili in poti obrabe za sistem nerjavno jeklo 316 L v 0,5 m raztopini H2SO4. Večja, ko je hitrost, večja je sprememba v zmanjšanju korozijskega potenciala. Prav tako hitrost obrabe vpliva na repasivacijo elektrode. Opazimo lahko, da se po koncu obrabe potencial začne pomikati k bolj pozitivnim vrednostim. Ta čas je najdaljši pri največji hitrosti obrabe. Predvidimo lahko, da je opažena lastnost posledica dejstva, da so se korozijske razmete tik ob elektrodi bistveno spremenile, saj je hitro drgnjenje povzročilo večje spremembe v električni dvoplasti in celotnem okoliškem korozijskem sistemu. Do vzpostavitve ravnovesnega stanja in repasivacije je potrebno več časa, kar lahko opazimo s počasnim večanjem korozijskega potenciala. 7. Sklep Kovine so v agresivnem okolju izpostavljene korozijskem propadanju. V mnogih primerih lahko korozijsko stanje kovinskega materiala poslabša hkratna obraba, kar povzroča še izrazitejše propadanje kovinskih površin. Področje uporabe kovin je zelo široko, zato je poznavanje tribokorozijskih procesov in njihove kontrole velikega pomena. Ker je propadanje kovine elektrokemijski proces, lahko njegove lastnosti spremljamo z uporabo različnih elektrokemijskih tehnik. Za spremljanje tribokorozijskih lastnosti pa potrebujemo posebno preskuševališče, ki je sestavljeno iz tribometra ter posebej predelane korozijske celice, ki vsebuje tako delovno, referenčno ter števno elektrodo. Korozijo kovinskega materiala med obrabo lahko spremljamo na več različnih načinov. V prispevku so predstavljeni različni primeri tribokorozijskega preskušanja nerjavnega jekla 316 L v 0,5 M raztopini žveplove(VI) kisline pri potencialu odprtega kroga v odvisnosti od velikosti sile obrabe, časa oziroma poti obrabe ter hitrosti obrabe. 8. Literatura: [I] A. Legat, M. Bajt Leban, Ž. Bajt, Electrochim. acta. 49 (2004) 2741-2751. [2 ] M. Bajt Leban, Ž. Bajt, A. Legat, Electrochim. acta. 49(2004) 2795-2801. [3] A. Legat, Electrochim. Acta 52 (2007) 7590-7598. [4] F. P. Bowden, D. 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