Protection of Carbon/Carbon Composites against Oxidation Zaščita kompozitov tipa ogljik/ogljik pred oksidacijo Borchardt G.,* Institut fur Allgemaine Metallurgie, Technische Universitat Clausthal, Deutschland R. Turk, S. Javorič, V. Nardin, Oddelek za metalurgijo in materiale, FNT, Univerza v Ljubljani, Slovenia A mayor problem of carbon/carbon composites is use in oxidative environments. Protective coating in this study consists of SiC outer and B4C inner coating and offer efficient protective of composites against oxidation. Because of cracks and erosion of microstructure as a result of oxidation oxidized samples have lovver mechanical properties. Key vvords: carbon/carbon composites, oxidation, protective coatings, mechanical properties of oxidized composites. Največji problem kompozitov tipa ogljik/ogljik je njihova uporaba v oksidativnih atmosferah. Ugotavljali smo učinkovitost večplastne zaščite grafita pred oksidacijo, sestavljene iz zunanje SiC in notranje B4C plasti. Rezultati kažejo, da je takšna zaščita učinkovita v temperaturnem intervalu 600-1000°C. Zaradi razpok in erozije strukture grafita, ki so posledica oksidacije, so mehanske lastnosti oksidiranih kompozitov slabše. Ključne besede: kompoziti tipa ogljik/ogljik, oksidacija, zaščitne plasti, mehanske lastnosti oksidiranih kompozitov. 1. Introduction Carbon fibers and carbon/carbon (C/C) composites are at-traetive materials because of strength-to-weight properties supe-rior to those of any other materials. Potential uses range from those in aircraft, autoindustrv to medical and šport applications (boat making, fishing rod...). C/C composites consist of carbon fibers sel in a graphite mata. Mechanical properties of C/C composites depend 011 fibers. which exhibit high Young's modulus E (E = 250-500 GPa) com-pared to the graphite matrix (Ev = 30 GPa).1 Strong bonding betvveen the matrix and the fibers results in high shear strength while weak bonding inereases the toughness. so that crack propagation in the matrix can bc arrested at the fiber surface. High tensile strength is a consequence of very strong covalent bonds betueen carbon atoms and high anizotropic crys-talline fibers.; 2. Protection of C/C Composites against Oxidation The most obvious advantage of using C/C composites in aerospace application is their high relative strength compared vvith low vveight. A tnavor problem is using sueh materials in oxidizing environments (hot flovving gases). Carbon rapidly reacts vvith oxygen at temperatures as lovv as 500 C. forming gaseous products (CO, Prof dr OL NTHER BORCHARDT. Inst. tur Allgemcinc Metallurgie. Tehn. t niversitat Maustal, Deutsehland CO,). Gasifications leads to a rapid degradation of the composite. This oxidation process results in the erosion of the strueture and in the degradation of the mechanical properties vvhieh the material originally posessed. Many protective coatings are being considered to prevent contact betvveen oxygen and carbon. Most of these coatings rclv on oxide films formed during oxidation as oxygen diffusion bar-riers. 2.1. Protective Coatings The most important 19.34 U.S. patent ' in vvork on protection against oxidation for C/C composites deseribes a coating system for graphite materials composed of a SiC and vitreous overlav coatings. Work 011 oxidation protection for C/C composites started in 1970's. The coating system vvas very similar to that in the 1934 patent and vvas composed of a SiC conversion laycr and silicate glaze overlav. Any coating material used to protect the composite from o.\-idation niust prevent the invard diffusion of oxygen, and has lovv volatility to prevent erosion. Coating issues associated vvith oxi-dation protection are coating erosion, spallation and oxygen per-meation of the intact coating system. Erosion resistance requires the use of outer coatings that have lovv vapor pressures. The high thermal expansion coeffi-cient is a strong negative factor because the large differences in thermal expansion behavior often results intension-induced coating cracks. In the presence of an oxide film 011 the surface, the oxygen has to diffuse through it to reach the substrate/oxide surface. Oxide film acts as a diffusion barrier. The oxidation of a substrate involves five steps, shown schematically in Figure l:4 Figure I: The oxidation of a protected graphite Slika I: Oksidacija zaščitenega grafita 1. gas phase diffusion aeross the boundary layer: 2. oxygen diffusion through cracks: if the oxide film is cracked, (oxygen diffuse through it to the substrate/oxide film interface); 3. condensed phase diffusion (if the oxide film is nonporous); 4. reaction at the interface substrate/oxide; 5. counter diffusion of gas products back to atmosphere. Oxides have usualy higher thermal expansion coefficients than corresponding carbides. For this reason it is advantageous to start with a nonoxide outer coating that converts to an appro-priate oxide upon exposure. Such deposition reduce diferences betvveen thermal expansion bchaviour of coating and substrate. Studies have shovvn the utility of borate glasses for protect-ing C/C composites front oxidation. Current coating system for proteeting C/C composites are composed of an outer coating of SiC and an inner B4C coating. Due to oxidation SiC is convert-ed to SiO, vvhich acts as primary oxygen barrier.'' Oxidation of the inner B4C coating through cracks in the SiC outer coating produces a borate glass B,0,. Above the melting point of B,0< it flovvs to fill cracks present in the inner and outer coatings. 3. Mechanical Properties of (Kidized C/C Composites Oxidation of the graphite leads to pitting, degradation and porosity of C/C composites. Gasification appeared to lead pro-gressively to the formation of pores in the matrix. follovved by propagation of longitudinal channels along the fiber axes. Cracks, vvhich appears under loading, leads to a loss in strength of the composite. One of the most oxidized and loaded fibers rupture and this leads to crack initiation and propagation of transversional cracks because of shear stresses.7 Strength of oxidized composites is reduced because of cracks and microstructure erosion. Bending strains for oxidized C/C composites are much lovv-er than that for unoxidized ones. 4. Experimental Procedure Graphite EK 986 used in this study vvere supplied by Ringsdorf, Germany. The average density vvas 1.85 g/cm and open porosity 8%. Graphite samples vvere cut in approximatelv cube form, side 0.7 +- 0.01 mm. Multilayer coating vvas composed of an outer coating of SiC and of an inner B ,C coating. SiC and B4C coatings various thicknesses vvere deposited bv Physical Vapor Deposition (PVD) process vvith High Frequency (HF) Sputtering. Samples vvere sputtered in vacuum at 2-3 x 10 mbar. fre-quency 10 MHz and voltage lkV. 80 min of sputtering vvas suf-ficient to deposit a B4C film of 0.30 um and 75 min to deposit a SiC film of 0.70 pm on the surface of the samples. Some graphite samples vvere only B4C coated. Measurements of the oxidation kinetics of ali graphite samples vvere carried out in a vertically open-ended furnace in stag-nant air, betvveen 600 and 1200°C. Mass changes of the specimens vvere measured bv Thermo Gravimetry Analyses (TGA) using an automatic "SARTO-TIOUS M 25 D-V" thermobalance (sensitivitv +-0.001 mg) vvhich vvas connected vvith computer. Ali kinetics data vvere collected and approximated vvith Computer operated data on line acquisition system. Samples vvere cooled dovvn in air and vvere observed using Scanning Electron Microscope (SEM). Jeal JSM - 35. C/C composites materials BB 7655 used in this studv vvere made from SCHUNK. Germany. Samples vvere preprotected vvith Si, vvhich vvas deposed into the graphite matrix ("Kapillarziliziert").8 Samples vvith average dimensions 20 x 20 x 1.7 mm. the average density 1.64 g/cm' and the average porosity 8.2% vvere ox-idized betvveen 400° and 8()0°C. Oxidized samples vvere cooled dovvn in air and tensioned bv GLEEBLE 1500 (DUFFERS SCTENT1FIC) at room temperature in air at a rate of 0.1 mm/sec. 5. Results and Discussion 5.1 Oxidation hehaviour of uncoated graphite Oxidation of the graphite appeared to occur at specific active sites, leading to pitting, degradation and porosity at the surface. Figure 2 shovvs the relative vveight loss for oxidized samples per unit of calculated geometric area (dm/A,,) as a funetion of time at 600°C and 1()()0°C. Due to the presence of porosity, the effective surface area over vvhich reaction can occur is possiblv 10 to 100 times higher than the geometric area. This is consistent vvith the high poros-ity of the C/C composites. Oxidation process of graphite results in the erosion of the strueture (Figure 3). Because of higher erosion the oxidation rate for uncoated graphite samples (Figure 2.b-A) at 1000°C is higher than rate at 600°C (Figure 2.a-A). The lovv temperature rate - limiting step is probably a surface reaction - desorption of oxidation products (CO. CO:) from the carbon netvvork. At higher temperatures the release of the oxidation products becomes easier and leaves defects in the carbon netvvork. The rate is then probably controlled by oxygen diffusion into pores. ( tirne (sec) Figure 2: Relative vveight loss per unit of calculated geometric area (dm/A„) as a function of tirne at 600°(a) and 1000°C(b). A ... uncoated graphit C.... graphit, eoated with SiC and B,C proteetive eoating Slika 2: Relativna i/guba mase na enoto izračunane geometrične površine (dm/A„) kol funkcija časa pri 600°(a) and l()00°C(b). A ... nezaščiten grafit B ... grafit, zaščiten s SiC in B,C zaščitno plastjo With continued oxidation the porosity and active surface area in-creased resulting in the increase in the oxidation rate. An important question is vvhether oxidation of C/C compos-ites proceeds more readilv along the carbon fiber axes or in the less vvell crvstallized matrix of the composite. Because of the higher incidence of reactive edge sitcs, amor-phous carbons tend to be more susceptile to gasification than crvstalline graphite. Oxidation of the graphite occured simultaneously at specific active sites (vacancies. pores). Burn off of oxidation products leads progrcsivelv to formation of pores in the matrix (Figure 4.a). followed by the propagation of longitudinal channels along the fiber axes (Figure 4.b). Facile oxygen diffusion along sueh channels allovved a rapid excavation of material and growth of larger pores betvveen the fiber bundles. Gasification also occured rapidly at the exposed ends of the fibers. leading to diffusion of oxygen along the fiber axes. 5.2 Oxidation behariour of eoated graphite In the temperature range 600 C-10()0°C samples covered vvith SiC and B4C eoating (Figure 2.a,b-C) shovved better oxi-dation resistance and reduetion in the oxidation rate compared vvith uneovered graphite samples (Figure 2.a,b-A). Figure 3: Surface of uncoated graphite after oxidation at 60(1 C (a) and 1000'C (h) Slika 3: Površina nezaščitenega grafita po oksidaciji pri 600 C' (a) in 1000°C (b) Figure 4: Oxidation of the graphite occured simultaneously at specific active siles. appeared to lead progressivelv to the formation of pores in llie matrix (a), follovved by the propagation of longitudinal channels along the fiber axes (b) and grovvth of larger pores betvveen the fiber bundles (c). Slika 4: Oksidacija grafita prične simultano na specifičnih aktivnih mestih, kar vodi do tvorbe por v matrici (a), podolžnih kanalov vzdolž osi vlaken (b) in rasti večjih por med vlakni (c) -35 a) T= 600°C 0 20000 40000 60000 80000 100C00 tirne (sec) graphite a graphite - a B4C > a SiC 4 1 graphite - 41 B4C > 4 'SiC T=450"C Figure 5: Surface of coated graphit after oxidation at 600' C (a) and 1000' C (b) Slika 5: Površina zaščitenega grafita po oksidaciji pri 000 C (a) in 1000 C (In Figure 7: After removal from the furnace we noticed that B:C' plus SiC coating resignate from graphite matrix and cracked. Slika 7: Po oksidaciji smo ugotov ili. daje zaščitna plast B,C plus SiC odstopila od grafitne matrice in razpokala. Oxide films formed during oxidation prevent direct contact between oxygen and undcrlving graphite and reduee numbers of specific active sites and presence of porositv over vvhich oxida-tion can oeeur (Figure 5). Examination of the B4C plus SiC coated specimen on removal from the furnace suggested that oxidation vvas initiated at the eorners and edges of the sample. Bonding of the coating at these sites vvas probablv poorer than on the faees and that local mismatehes caused mierocraek-ing in these regions. SiC prevent direct oxygen attack on the carbon matri.v Cooling composite from relativ elv high deposition temperatures leads to cracking in the coating as a result of thermal evpansion mismatehes (Figure 6.a). Upon exposure to o.xygen SiC becomes an oxide SiC), vv h i c h is an excelent oxygen barrier so oxidation bv diffusion through Sit) laver is not the limiting factor. Porous structure of SiC and eraeks in the coating degrades the oxidation resistanee of graphite. Oxygen diffuse through cracks and causes oxidation of tinderlv ing carbon. (Figure 6.bi. Thal is the reason vv liv SiC coating offered onlv limited protection of oxidation. The tise of B4C gave good protection at temperatures up to 1000°C. B,C forms borate glass B:(), on oxidation vvhich has surface energies less than 100 mJ m 2 for carbon vvetting and v is-cosities of 10' to K)2 dPa s (in the 600 to 1 100 C range). Figure 6: Cooling composite leads tu cracking in the coating as a result of thermal e.vpansion mismatehes (a). (Kvgcn diffuse through cracks and causes oxidation of undcrlving carbon (hi. B,(), flovvs to fill thermal expansion mismaleh cracks in the outher SiC coating and in the matrix (c). Slika 6: Pri ohlajanju kompozita se v zaščitnih plasteh pojavijo razpoke zaradi razlik v temperaturni razteznosti zaščitnih plasti grafita (al. Kisik difundira sko/i razpoke in povzroča oksidacijo spodaj ležečega ogljika (b). B,0,. ki se tvori med oksidacijo, zapolni razpoke v SiC plasti in v grafitni matrici (c) a SiC = 4.7 X 10-«/ K a B4C = 5.29 - 6.25 X 10"6/ K " graphite = 7.86x 10^/K Above the melting point of B:0, (450 C) it flows to lili thermal expansion mismatch cracks in the outhcr SiC eoating and in the matrix (Figure 6.e) providing a diffusion barrier in the composite. BO; is segregated in ciusters at the aetive sites on the graphite surface where oxidation normallv occured and blocks this aetive sites. The use of borate eoating is limited bv the volatililv ofthe bo-ratc. Volatization cd' the eoating leaves the underlving material exposed. Rapid oxidation and a 250% volume inerease at the con-version of B4C to B:0, are cssential features of these coatings. After removal from the furnace vve noticcd that B ,C plus SiC" eoating resignate from graphite matrix and crack (Figure 7). Hvdrolvsis of B .O; produces orthoboric acid and a 125% volume inerease: B O, + H.O = 2B 0; + HO = B.O; + H,BO, Heating relcases vvater and produccs a mixture of boric ox-ide and mctaboric acid. Under drv conditions the complete con-version H,BO, back to B.O, completed al about 450 . 5 J Mcchanical properties ofo.\idi:cd C/C eompozites Because of cracks and erosion of microstructure of C/C composites oxidized samples have lovver mcchanical properties. 3500, 2800