UDK 621.793:539.92:539.375.6	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 49(3)429(2015)
INTERACTION OF Cr2N AND Cr2N/Ag THIN FILMS WITH CuZn-BRASS COUNTERPART DURING BALL-ON-DISC TESTING
INTERAKCIJA Cr2N IN Cr2N/Ag TANKIH PLASTI V PARU S CuZn-MEDENINO MED PREIZKUSOM KROGLA NA DISK
Pavel Bilek, Peter Jurci, Petra Dulova, Maria Hudakova, Jana Ptacinova,
Matej Pa{ak
Institute of Materials Science, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava,
Pauli'nska 16, 917 24 Trnava, Slovak Republic pavel-bilek@email.cz
Prejem rokopisa - received: 2014-07-28; sprejem za objavo - accepted for publication: 2014-09-04
doi:10.17222/mit.2014.121
Cr2N- and Cr2N/Ag-nanocomposite thin films were deposited on a substrate made of Cr-V ledeburitic tool steel Vanadis 6 by reactive magnetron sputtering, at a deposition temperature of 500 °C, using pure Cr and Ag targets, in a composite, low-pressure N2/Ar atmosphere. The additions of silver to the Cr2N/Ag coatings were w = (3, 7, 11 and 15) %. Tribological testing using a ball-on-disc apparatus was realized at ambient temperature and, for the Cr2N with the additions of mass fractions of Ag 7 % and 11 %, at the elevated temperatures of (300, 400 and 500) °C, respectively. Balls made of binary CuZn-brass (55 % Cu, 45 % Zn) were used as the counterparts. The wear tracks after the ball-on-disc testing and the worn balls were analyzed with scanning electron microscopy (SEM) and a microanalysis (EDX), and the wear rates were calculated. The adhesive wear was derived from a quantitative-point metallographic analysis. The obtained results infer that a considerable material transfer from the counterpart onto the surface of the coatings takes place during dry sliding. The material transfer (and the adhesive wear of the counterpart) is mainly due to the low shear strength of the brass used. Two main trends were observed. The first one shows that the adhesive material transfer decreases with the increasing silver content when tested at ambient temperature. The second trend indicates that the use of a higher testing temperature leads to a higher adhesive wear of the counterpart.
Keywords: Cr2N/Ag-nanocomposite PVD coatings, ball-on-disc, adhesion, friction coefficient, wear rate
Cr2N- in Cr2N/Ag-nanokompozitne tanke plasti so bile nanesene na podlago iz Cr-V ledeburitnega orodnega jekla Vanadis 6 z reaktivnim nanašanjem z magnetronom pri temperaturi nanašanja 500 °C z uporabo tarč iz čistega Cr in Ag v kompozit v nizkotlačni atmosferi N2/Ar. Dodatki srebra v nanose Cr2N/Ag so bili w = (3, 7, 11 in 15) %. Tribološki preizkusi z uporabo naprave krogla na disk so bili izvršeni pri sobni temperaturi in pri Cr2N tudi z masnim deležem dodatka 7 % in 11 % Ag pri povišanih temperaturah (300, 400 in 500) °C. Krogle, izdelane iz binarne CuZn-medenine (55 % Cu, 45 % Zn), so bile izbrane kot par. Sledovi obrabe po preizkusu krogla na disk in obrabljene krogle so bili analizirani z vrstično elektronsko mikroskopijo (SEM) in mikroanalizo (EDX), izračunane pa so bile tudi hitrosti obrabe. Adhezivna obraba je bila prikazana s kvantitativno točkasto metalografsko analizo. Dobljeni rezultati kažejo, da se med suhim drsenjem pojavi občuten prenos materiala iz krogle na površino nanosa. Prenos materiala (in adhezijska obraba krogle) je nastal večinoma zaradi majhne strižne trdnosti medenine. Opaženi sta bili dve glavni usmeritvi. Prva je bila, da se pri preizkušanju pri sobni temperaturi adhezivni prenos materiala zmanjšuje z naraščajočo vsebnostjo srebra. Druga pa, da uporaba višje temperature pri preizkusu povzroči večjo adhezivno obrabo krogle.
Ključne besede: Cr2N/Ag-nanokompozitni PVD-nanosi, krogla na disk, adhezija, koeficient trenja, hitrost obrabe
1 INTRODUCTION
or more nanocrystalline phases in a functional matrix to provide improved mechanical and tribological properties
Hard ceramic coatings like CrN and TiN have been	and/or corrosion resistance. Some coatings are designed
used for the last three decades due to their high hardness	to be adaptive,that is, their properties follow the changes
and chemical stability, high oxidation resistance and low	in the operating conditions. An example of an adaptive
wear rate1,2. They have gained great scientific interest	coating is a hard wear-resistance matrix with an incorpo-
and industrial popularity due to these properties in	ration of soft metals like Cu, Ag or Au. This method can
copper machining3, alumina die casting and forming, and	improve the lubricating in specific tribological condi-wood processing4. However, the friction coefficient of tions9. Chromium nitrides combined with noble metals
most transition-metal nitride coatings is fairly high	are relatively easy to co-deposit by reactive magnetron
(0.6-0.8) and the tribological effectiveness, especially at	sputtering and they form nanocomposite structures due
elevated temperatures, is insufficient5,6. Therefore, a lot	to a lack of miscibility between the matrix and the lubri-
of effort has been made in recent years to decrease the	cant10,11.
friction coefficient at room as well as elevated tempera-	Silver is most commonly used as an addition to the
tures. Multi-functional coatings combining soft lubri-	TM-nitride thin films. It exhibits a stable chemical beha-
cating phases within a hard wear-resistance matrix offer	viour over a wide temperature range as well as in a
good properties7,8. These coatings generally include one	variety of aggressive environments. Ag is capable to mi-
grate to a free surface to form Ag particles providing lubrication above 300 °C and, thereby, distinctively reducing the friction coefficient12.
Our recent investigations of the magnetron-sputtered CraN-films with w = (3, 7, 11 and 15) % of silver amounts, deposited on the Cr-V ledeburitic steel Vanadis 6, can be summarized as follows1317: the incorporation of silver in the CraN matrix led to an improvement in the tribological properties at elevated temperatures, especially at 400 °C and 500 °C. Nevertheless, the addition of 15 % of Ag made the film too soft and sensitive to the wear, which resulted in a worsening of the tribological performance. On the other hand, the films with w = (7 and 11) % of Ag additions seem to be very promising.
In this paper, the tribological performance against a CuZn-brass counterpart of the nanocomposite coatings consisting of a hard CraN matrix co-deposited with different Ag additions is investigated. The films were deposited onto the Cr-V ledeburitic steel Vanadis 6 using the magnetron-sputter technique.
2 EXPERIMENTAL WORK
The substrate material was the PM ledeburitic tool steel Vanadis 6 with mass fractions 2.1 % C, 1.0 % Si, 0.4 % Mn, 6.8 % Cr, 1.5 % Mo, 5.4 % V and Fe as the balance element.
The samples used for the investigation and the conditions for depositing CraN- and CraN/Ag- coatings were reported elsewhere11. In the case of the CraN coatings, during the deposition, the power was a.Q kW per cathode (both Cr). For the production of the Ag-containing coatings, the power of the Cr cathode was kept at 5.8 kW, while the power of the Ag cathode was varied (0.10, 0.21, 0.34 and 0.45) kW in order to prepare the films with different Ag concentrations (3, 7, 11 and 15) %.
The tribological properties of the coatings were measured using a CSM ball-on-disc tribometer at room temperature and, for the coatings with 7 % and 11 % of Ag, also at the elevated temperatures up to 500 °C. The balls of 6 mm in diameter, made from CuZn-brass (55 % Cu, 45 % Zn), were used for the tests. No external lubricant was added during the measurements. The
normal load used for the investigation was 1 N and the total sliding distance for each measurement was 100 m. The volume losses V of the worn balls were calculated on the basis of the sketch shown on Figure 1 using the following formula:
V = ((w -h)- (3p2+h2))/6	(1)
where R is the ball radius, h and p are the height and the radius of the worn spherical segment of the ball. Relating the volume loss to the normal load and sliding distance, the wear rates W were calculated.
After the testing, the wear tracks and the worn balls were examined with a scanning electron microscope (SEM) JEOL JSM-7600F and an energy dispersive X-ray analysis (EDX).
The adhesive wear was derived from the quantitative-point metallographic analysis carried out on the SEM micrographs of the tracks after the ball-on-disc testing.
3 RESULTS AND DISCUSSIONS
Figure 2 shows a detail of the track after the ballon-disc test of the CraN/HAg coating tested at room temperature against the CuZn-brass counterpart. The surface of the coating did not show any indications of damage. On the other hand, thanks to the corresponding EDX maps, a considerable material transfer from the counterpart to the surface of the coating was detected; it was mainly due to the low shear strength of the brass used.
Figure 1: Sketch of a worn ball for the volume-loss calculation Slika 1: Skica prikaza obrabe krogle za računanje volumenske izgube
Figure 2: Transferred material of the CuZn-brass counterpart to the surface of the CraN/11Ag coating after the ball-on-disc test at room temperature: a) overview, b) EDX of chromium, c) EDX of silver, d) EDX of copper, e) EDX of zinc
Slika 2: Prenesen material CuZn-medenine iz krogle na površino Cr2N/11Ag-nanosa po preizkusu krogla na disk; preizkušeno pri sobni temperaturi: a) videz, b) EDX-kroma, c) EDX-srebra, d) EDX-bakra, e) EDX cinka
Figure 3: Worn CuZn-brass ball after the ball-on-disc test of the Cr2N/11Ag coating at room temperature
Slika 3: Obrabljena krogla iz CuZn-medenine po preizkusu krogla na disku na CriN/llAg-nanosu, preizkušano pri sobni temperaturi
Figure 3 depicts the worn CuZn-brass ball after the ball-on-disc test of the CriN/llAg coating at room temperature. The parallel grooves oriented along the sliding direction are well visible and the diameter of the worn spherical segment is easily measurable. The surfaces of the worn balls were investigated with an EDX analysis to confirm the presence of silver. However, no signal of silver was obtained and one could assume that the silver content on the surface was too low to be detected with the EDX analysis.
The mean value of the friction coefficient examined at room temperature decreased with the increasing silver content in the CriN matrix until 7 % (Figure 4). However, the influence of a higher silver content was not as positive for the friction coefficient as for the coating with 7 % Ag. The wear rates of the balls decreased by about 50 % for the coatings with the silver addition in comparison with the pure CriN, and in the case of the coating with 15 % of Ag the decrease was about 75 % (Figure 4). These results were expected since silver can act as a solid lubricant, facilitating the sliding of the balls12. In our previous works11,17 silver particles were well visible on the surface of the coatings after the deposition. On the
Figure 5: Wear rates W and friction coefficient ^ of Cr2N + 7Ag* and Cr2N + 11Ag coatings tested at room and elevated temperatures, *pre-vious result16
Slika 5: Hitrost obrabe W in koeficient trenja ß pri Cr2N + 7Ag* in Cr2N + 11Ag-nanosih, preizkušanih pri sobni in povišanih temperaturah, *predhodni rezultati16
other hand, the experiments carried out at the elevated temperature showed the opposite tendency. An increase in the testing temperature led to a slight increase in the mean value of the friction coefficient (Figure 5). As reported in the previous works16,17, a decrease in the friction coefficient was observed with the increasing testing temperature, but there an alumina counterpart was used. In the case of the CuZn-brass counterpart the mechanism of the wear was different and a softening of the CuZn-brass alloy also took place at high temperatures.
Nevertheless, the wear rates of the CuZn-brass balls during the tribological testing of the coatings with mass fractions 7 % and 11 % of Ag at elevated temperatures were lower in comparison with the measurement at room temperature (Figure 5). The minimum values for both coatings were found at the temperature of 300 °C.
Figure 6 depicts the dependence of the friction coefficient on the sliding distance for the CriN + 11Ag coating tested at ambient and elevated temperatures. All the measurements show a considerable instability. The friction coefficient is oscillating around the mean value in the range of 0.2. This can be explained with the cre-
Figure 4: Wear rates W and friction coefficient ß of Cr2N and Ag coatings tested at room temperature Slika 4: Hitrost obrabe W in koeficient trenja ß pri Cr2N- in Ag-prevlekah, preizkušanih pri sobni temperaturi
CriN/ CriN/
Figure 6: Dependence of friction coefficient ß on sliding distance L of the CriN + 11Ag coating tested at different temperatures against the CuZn-brass counterpart
Slika 6: Odvisnost koeficienta trenja ß od dolžine drsenja L pri CriN + 11Ag-nanosu, preizkušanem pri različnih temperaturah, v paru z CuZn-medeninasto kroglo
Figure 7: Area A of adhesion interactions between CuZn-brass and uncoated steel Vanadis 6, Cr2N and Cr2N/Ag coatings during ball-on-disc testing, at room temperature
Slika 7: Podro~je A adhezijske interakcije med CuZn-medenino in neprekritim jeklom Vanadis 6 ter Cr2N- in Cr2N/Ag-nanosi med preizkusom krogla na disk pri sobni temperaturi
ation of adhesion joins between the surface of the coating and the ball during the sliding and their subsequent release, accompanied with the oscillating of the friction coefficient.
A quantitative-point metallographic analysis was used to describe the adhesion interaction during the ball-on-disc testing. Figure 7 shows the results of the testing at room temperature for the uncoated steel Vana-dis 6, the pure Cr2N and Cr2N/Ag coatings. The highest adhesion wear was found for the uncoated substrate, where the area of adhesion interaction was A = (81 ± 5) %. For the coated substrate it is clearly evident that the adhesion wear decreases with the increasing silver content; the minimum A = (55 ± 8) % was found for the Cr2N coating with the highest silver addition of 15 %. One can assume that the incorporation of silver into the Cr2N matrix improves the wear of the CuZn-brass ball front view and also the area of adhesion interaction at room temperature.
On the other hand, the material transfer is more remarkable in the conditions of higher testing temperatures
(Figure 8). Both coatings, Cr2N with mass fractions of Ag w = (7 and 11) %, showed the same tendency, although the coating with 11 % of Ag exhibited a slower increase in the adhesion wear with the increasing testing temperature than the coating with 7 % of Ag. Most likely, the higher adhesion interaction at higher temperatures is caused by the softening of the CuZn-brass material, when the atoms of the ball material easily create the adhesion joins.
4 CONCLUSIONS
The friction and wear characteristics of the Cr2N and Cr2N/Ag coatings prepared with the magnetron-sputter-deposition method were investigated at room temperature and elevated temperatures during a ball-on-disc testing against a CuZn-brass counterpart. The results can be summarized as follows:
•	During the tribological testing, a considerable material transfer from the CuZn-brass counterpart to the surfaces of all the tested coatings was observed.
•	For the Cr2N/Ag coatings, a lower friction coefficient and also lower wear rates of the balls were found during the testing at room temperature in comparison with the pure Cr2N. This phenomenon is attributed to the silver incorporated into the Cr2N matrix.
•	Higher testing temperatures led to a slight increase in the friction coefficient. On the other hand, the wear rates of the balls were further decreasing, with the minimum values at 300 °C.
•	During the testing at room temperature, the highest material transfer was found for the uncoated steel Vanadis 6. The adhesion wear was lower when the Cr2N coating was tested. The decrease is more remarkable with the increasing silver content incorporated into the Cr2N matrix.
•	Higher testing temperatures led to increased adhesion interaction of both Cr2N coatings, with mass fractions of Ag 7 % and 11 %.
Figure 8: Area A of adhesion interactions between CuZn-brass and Cr2N coatings with mass fractions of Ag 7 % and 11 % during the ball-on-disc testing at different temperatures
Slika 8: Podro~je A adhezijske interakcije med CuZn-medenino in Cr2N-nanosi z masnim deležem Ag 7 % in 11 % med preizkusi krogla na disk pri razli~nih temperaturah
Acknowledgements
This publication is the result of implementing the project: CE for development and application of advanced diagnostic methods in processing of metallic and non-metallic materials, ITMS: 26220120048, supported by the Research & Development Operational Programme funded by the ERDF. This research was supported by grant project VEGA 1/1035/12.
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