- Email: [email protected]
Different tribological behavior of MoS2 coatings under fretting and pin-on-disk conditions
Surface and Coatings Technology 163 – 164 (2003) 422–428
Different tribological behavior of MoS2 coatings under fretting and pinon-disk conditions Xiaodong Zhua,b, W. Lauwerensc,*, P. Cosemansc, M. Van Stappenc, J.P. Celisd, L.M. Stalsb, Jiawen Hea a
State Key Laboratory of Mechanical Behavior of Metals, Xi’an Jiaotong University, Xi’an 710049, PR China b Institute for Materials Research, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium c WTCM, Wetenschapspark 3, B-3590 Diepenbeek, Belgium d Department MTM, Katholieke Universiteit Leuven, Kasteelplein Arenberg 44, B-3001 Leuven, Belgium
Abstract The performance of tribological coatings is often evaluated by fretting or pin-on-disk measurements. However, only a few papers dealt with the comparison of the two methods on the same coating. In this paper, a comparison of the tribological behavior of sputtered MoS2 coatings in fretting and pin-on-disk tests under different conditions is studied. The relationship between the mechanical properties of the coatings and their wear performance in both type of sliding tests is discussed. MoS2 coatings were deposited by magnetron sputtering from a MoS2 target. The coating thickness and adhesion were evaluated by respectively the ball crater and scratch test. The hardness as well as the toughness were obtained by a Vickers microhardness test. After the tribological tests the surface morphology of the wear scar in the coating was observed by scanning electron microscope. Also transfer films on the counterbody were investigated. It appears that the fretting and pin-on-disk tests can give considerably different friction and wear results. Coatings that show excellent performance in fretting tests have shown a low endurance in pinon-disk tests. Sliding speed appears to be a crucial factor. Also the material of the counterbody, steel or corundum, influences the results in both tests in a different way. The different behavior of a coating in both tests is related to the toughness of the coating and to the way that the debris is involved in the friction and wear mechanisms. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: MoS2 coatings; Wear mechanisms; Friction; Pin-on-disk test; Fretting test
1. Introduction Coatings of sputter-deposited molybdenum disulfide have been used in high vacuum and aerospace applications for lubrication purposes where low friction is desirable w1x. Interest is now growing in applying this solid lubricant in ambient atmosphere or high humidity environments w2,3x. The reported results of the tribological properties of MoS2 coatings are sometimes quite different. The reasons could be that the deposition apparatus andyor techniques used bring about different coating properties. However, besides the properties of the coatings, the testing methods can also affect the performance of the coatings. *Corresponding author. Tel.: q32-11-260856; fax: q32-11260859. E-mail address: [email protected] (W. Lauwerens).
According to practical applications, most machine parts work at continuous or oscillatory sliding conditions. Thus, the method of evaluation of the coatings should be close to their working conditions. In literature, most of the researchers evaluated the tribological properties of MoS2 coatings based on pin-on-disk sliding test, which is the most common way for sliding wear tests in the laboratory w4–6x. Other methods, including reciprocating sliding and mode I fretting tests were less referred w7,8x. Furthermore, only a few papers concerned the application of two different methods on the same coating w9x. Understanding the test conditions, especially the uni- and bi-directional sliding modes, on the tribological performance of the coatings could be important for selecting the proper evaluation methods. Results of hard coatings showed significant difference in wear rate between these two types of sliding wear tests w10x. It is
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 6 3 8 - 2
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
423
Fig. 1. Schematic of the fretting wear testing equipment.
also of interest to examine if these phenomena are found in softer lubricant coatings like MoS2. In the present study, MoS2 coatings were deposited by magnetron sputtering on high-speed steel substrate. The tribological behavior of sputtered MoS2 coatings in both fretting and pin-on-disk tests under different conditions is studied. In addition, a MoS2 yTi composite coating with approximately 20 at.% Ti homogeneously incorporated, known as MoST娃 w11x, deposited by Teer Coatings Ltd (UK) on an identical substrate was used for comparison. The wear mechanism of MoS2 coatings and its relationship with the mechanical properties of the coatings in both test methods are also discussed. 2. Experimental The MoS2 coatings were deposited by planar magnetron sputtering in a Balzers BAI640R equipment (WTCM, Belgium). The MoS2 target had a purity of 99%. Hardened and polished (Ras0.05 mm) ASP23 high-speed steel disks (64 HRC) were used as the substrates. Before the deposition of the MoS2 coatings, a thin titanium interlayer approximately 100 nm thick was applied to improve the adhesion. The temperature of the substrate during the deposition of MoS2 was below 80 8C. The substrate bias voltage was y50 V and the deposition time was 120 min, with a background argon pressure of 0.4 Pa. The MoS2 yTi coating was deposited by a closed field unbalanced magnetron sputter ion plating process starting with a 100 nm Ti interlayer to enhance the adhesion (Teer Coatings Ltd, UK) w11x. X-ray diffraction (XRD) measurement was
done with a Siemens D5000 diffractometer using Cobalt Ka radiation (ls0.17889 nm) for the structural analysis of the MoS2 coatings. The wear morphology of the coatings and the steel balls were investigated with a Philips XL 30 scanning electron microscope (SEM) and an optical microscope. The thickness of the MoS2 and MoS2 yTi coating was measured by a ball crater test and the adhesion of the coatings was measured by the scratch test with a standard diamond stylus with maximum load up to 80 N. Optical examination of the scratch was used to determine the failure of the coatings. A series of indentations by a microhardness tester using a Vickers indentor at loads ranging from 5 to 500 g were also made to examine the toughness and the hardness of the coatings. The loads at which fractures of the coatings occurred were recorded for toughness comparison. Tribological properties of the coatings were investigated by fretting (mode I) under gross slip conditions and by pin-on-disk tests at 23 8C in ambient air at a relative humidity of 50%. Corundum and 100Cr6 steel balls of a diameter of 10 mm and roughness of Ras0.2 mm were used as the counterbody. In the fretting test, the ball was loaded on top of the coating by springs at a normal load of 1.0 and 10.0 N. Fig. 1 shows the setup of the fretting wear test equipment. In pin-on-disk tests, the loads of 5 and 10 N were applied on the ball by dead weight. The wear track diameters ranged from 20 to 40 mm. The testing parameters are shown in Table 1. In fretting displacements of 100 and 500 mm were used. At 500 mm displacement the test is rather a reciprocating test since the observed contact radii typi-
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
424
cally ranged between 80 and 150 mm. However, this condition was selected to investigate the tribological behavior under conditions between those of fretting and pin-on-disk tests. After the wear tests, the wear volume was measured by a WYKO-NT3300 profilometer with white light interferometry. The wear volumes of the wear pits in fretting tests were measured directly and that of the wear track in pin-on-disk tests were calculated by averaging the cross-section of eight sectors. The wear rate was calculated by dividing the wear volume by normal load and sliding distance. For fretting this gives an averaged value since the wear rate is not uniform throughout the wearing area. 3. Experimental results Fig. 2. XRD pattern of the MoS2 coating.
3.1. Structure and mechanical properties of the coatings Fig. 2 shows the XRD pattern of the MoS2 coating. No detectable typical MoS2 peaks can be seen and this indicates that the coating is amorphous or near amorphous. Also the structure of the MoS2 yTi coating is amorphous as appears from Ref. w9x. The thickness of the MoS2 and MoS2 yTi coatings amount to be 1.5 and 1.7 mm respectively. The scratch test results show that both the MoS2 and MoS2 yTi coatings are well adherent to the substrate and that no debonding occurred at the maximum load of 80 N. Since the thickness of the MoS2 coating is only approximately 1.5 mm, the hardness is difficult to obtain from the microhardness tests because of the substrate effect. However, a thick MoS2 coating of 5 mm deposited at the same parameters gives a hardness of 3.6 GPa (load 5 g). This value is lower than some published data of 4.7 GPa for MoS2 coatings w9x. For the hardness of the MoS2 yTi coating, we refer to Ref. w9x where a value of 15 GPa is reported, obtained by a depth-sensing hardness test at 4 mN. Fig. 3 reveals that the toughness of the MoS2 coating is very low in comparison to the MoS2 yTi coating because fracture and delamination occur at 50 g load, yet the indentation of the MoS2 yTi at 500 g load is
perfect. The mechanical properties of the MoS2 coating, i.e. the low toughness and hardness, are probably related to the amorphous structure. Although the MoS2 yTi coating also shows an amorphous structure, it is well known that the addition of titanium increases the hardness and probably also improves the toughness of the coating. 3.2. Friction and wear behavior Fig. 4 shows the coefficient of friction of the MoS2 and MoS2 yTi coating as a function of test cycles in fretting tests when small load and displacement were used. In the test with a corundum counterbody and after an obvious running-in stage, the coefficient of friction of the MoS2 coating became stable and stayed at approximately 0.1. The running-in stage was slightly longer and the friction coefficient was less stable at a level of approximately 0.2 when a steel ball was used as the counterbody. The coefficient of friction of the MoS2 yTi coating is higher than that of the MoS2 coating when a corundum ball is used, but it is comparable in case of a steel ball. Opposite to the MoS2 coating, the friction of MoS2 yTi with a steel ball is lower than with a corundum ball.
Table 1 Variables of fretting and pin-on-disk tests Fretting
MoS2
MoS2yTi a b
Pin-on-disk
Load (N)
Displacement (mm)
Frequency (Hz)
Speed (mmys)
Load (N)
Speed (mmys)
Frequency (Hz)
1 1 10 10
100a 500b 100a 500a
10 2 10 2
2 2 2 2
5 10 10 10
200a 100b 20b 2b
1.5 0.8 0.2 0.03
1
100a
10
2
5
787a
6.3
Counterbody are steel and corundum balls. Counterbody is steel ball.
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
425
Fig. 5. Coefficient of friction of MoS2 at different loads and displacements in fretting test.
Fig. 3. Comparison of toughness of MoS2 and MoS2yTi by indentation (a) MoS2 (b) MoS2yTi.
The coefficient of friction of the MoS2 coating tested with a steel ball when higher load and large displacement were used is shown in Fig. 5. Wear-through occurred after approximately 600 cycles in the case
Fig. 4. Coefficient of friction of MoS2 and MoS2yTi in fretting test.
when high load (10 N) and large displacement (500 mm) are applied. No wear-through could be observed at the same testing conditions with a corundum ball. Fig. 6 shows the comparison of friction in the pinon-disk test with steel and corundum balls. The MoS2 coating was removed after only 40–80 revolutions against the steel counterbody, yet the wear with a corundum ball is much lower and the coefficient of friction remains at 0.2 after 10 000 revolutions. The coefficient of friction of the MoS2 yTi coating is lower than MoS2 either against a corundum or steel ball. The trend that with a steel ball the friction is lower in case of MoS2 yTi corresponds to the results in fretting tests. The wear rates of MoS2 and MoS2 yTi coatings in pin-on-disk and fretting tests (1 N, 100 mm) are shown in Fig. 7a. The results show the trend that the wear rate in the fretting test is higher than that in the pin-on-disk test, except the MoS2 coating tested in pin-on-disk conditions against the steel counterbody (wear-through). From Fig. 7b it can be concluded that the sliding
Fig. 6. Coefficient of friction of MoS2 and MoS2yTi in pin-on-disk test.
426
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
Fig. 8. Coefficient of friction of MoS2 at different sliding speeds in pin-on-disk test.
at different displacements. The debris on the counterbody in case of the small displacement (Fig. 10a) is accumulated around the contact area. In case of the large displacement (Fig. 10b), it is scattered in a larger area along the direction of movement. Transfer films
Fig. 7. Wear rate of (a) MoS2yTi and MoS2 in pin-on-disk and fretting test (1 N, 100 mm) and (b) MoS2 in fretting at different test conditions.
displacement has a stronger effect than the load on the wear rate in fretting. The effect of sliding speeds on the friction of MoS2 against a steel ball is shown in Fig. 8. It is noticed that at lower sliding speed the friction is also lower. At the speed of 2 mmys, which is similar to the condition in fretting, the friction is lower than 0.1, even better than that in fretting conditions. No wear-through can be observed at 2 mmys sliding speed after 200 cycles. 3.3. Analysis of wear morphology Fig. 9 shows the micrograph of MoS2 wear pits obtained after testing at 1 N, 100 mm, 30 000 cycles in fretting against a corundum (Fig. 9a) and a steel ball (Fig. 9b). The morphology of the wear of the coating against a corundum ball after fretting and pin-on-disk tests is smooth and featureless, and no wear is detectable on the ball. A thin and uniform transfer film on the corundum ball is observed at any test condition. Fig. 10 shows the contacting area on the steel balls after testing
Fig. 9. Optical micrograph of the wear pit of the MoS2 coating obtained in fretting tests (a) with a corundum ball (b) with a steel ball.
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
427
film on the steel counterbody in Fig. 10b also shows a small area where the transfer film is missing, resulting in a local steel–MoS2 contact. This can point to an unstable, not very well adherent transfer film, which is also in agreement with the unstable friction coefficient in case of a steel ball. With the steel counterbody the MoS2 coating showed a large difference in tribological performance when tested under fretting or pin-on-disk conditions. Clear differences between the two methods are the wearing area exposed to the open air, the sliding direction and the sliding speed, leading to different wear mechanisms. In a fretting test with small displacement (100 mm), most of the contact area is covered by the counterbody, but the covered area in pin-on-disk is negligible. Wahl and Singer presented a model describing lubricant transfer processes between the coating, transfer film, wear
Fig. 10. SEM micrograph of contact area of steel counterbody (a) 10 N, 100 mm, 10 000 cycles; (b) 10 N, 500 mm, 1000 cycles.
can be seen on the steel counterbody in both cases, but it is partially missing in case of Fig. 10b. Fig. 11a shows the wear track of the MoS2 coating against a steel ball in the pin-on-disk test obtained after 45 cycles. No abrasive wear can be seen, however, the coating in the center of the track is torn off and some cavities are left. The contact area on the steel counterbody is smooth without wear and no transfer film was formed, instead, the debris piled up in front of the contact zone (Fig. 11b). The detail of the debris reveals that it consists of large pieces (Fig. 11c). 4. Discussion The experimental results indicate clearly that the test conditions have strong effects on the performance of the MoS2 coating. The use of a corundum counterbody results in better tribological performance than a steel ball in both the fretting and the pin-on-disk tests. This could be due to the high hardness and chemical inertness of the corundum ball or to the observed transfer films on corundum. Indeed the presence of a stable transfer film, covering the whole contact zone of the counterbody, is essential for good tribological behavior w12x. Singer et al. also reported that for DC triode sputtering deposited MoS2 coatings the transfer film on WC–Co and sapphire balls were uniform, but the one on a steel counterbody was thick and patchy, and the coating had abrasion grooves w13x. The morphology of the transfer
Fig. 11. SEM micrograph of (a) pin-on-disk wear track of MoS2 obtained after 45 revolutions tested at 5 N, 200 mmys against steel ball; (b) the steel counterbody; (c) detail of debris on steel ball.
428
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
patches and ejected debris w14x. According to this model, the wear is controlled by the loss flux of the depletion material (i.e. the coating) and the depletion material trapped in the contact (detached material) can be beneficial. If the generation rate of detached coating material is high, which is expected in bi-directional fretting, the wear rate can still be low in case the detached material remains in the contact zone. Considering the low hardness and toughness, the MoS2 coating is easily removed under the shear stress, meaning that the production rate of detached material is high. However, since a large amount of the wearing area is covered in the fretting test, the detached material is confined within the contact zone. Instead of being ejected as debris, the trapped coating material converts to a transfer layer under the cyclic movements. This is confirmed by the comparison of the wear rates in Fig. 7b where a large displacement, i.e. large exposed area, has the largest effect on the wear rate. Therefore, the wear rate in fretting is controlled by the removal of detached material, which is a slow process in fretting. The pin-on-disk is an extreme case where the contact zone changes continuously and most of the detached material is ejected. The influence of the sliding speed on the tribological performance is explained by the formation of the transfer film. High sliding speed causes higher tangential stress that removes easily the stuck debris on the steel ball and makes it difficult to form the transfer film. Therefore, the steel–MoS2 contact remains until the coating is completely removed. The reason why the MoS2 yTi coating shows better performance in a pin-on-disk test than MoS2 can be explained by its mechanical properties. In comparison to the MoS2 coating, the MoS2 yTi coating is tougher and harder, and the production of detached coating material is slower. Hence, the wear is controlled by the formation of the detached material. This can be advantageous in pin-on-disk conditions when ejection of debris is easy. The result that the MoS2 yTi and MoS2 coatings with a corundum ball showed lower wear rate in pin-on-disk conditions than in fretting is corresponding to the result of hard TiN coatings where the wear rate of bi-directional sliding is about twice that of unidirectional sliding w10x. This implies that the production of detached coating material in fretting is more efficient than in pin-on-disk conditions. However, the reasons why the two coatings show different behavior when steel or corundum balls are used, is not clear. It is probably related to a difference in mechanical properties andyor a possible tribo-chemical role of Ti in the wear mechanism.
5. Conclusions The selection of the wear test method and contact conditions are important for the evaluation of a coating. MoS2 shows a better performance in fretting with a corundum counterbody and a comparable performance with a steel counterbody. The MoS2 yTi composite coating is much better in pin-on-disk tests. The amorphous MoS2 coating has poor mechanical properties and this results in a low performance when a steel counterbody is used in pin-on-disk tests. The low strength of the MoS2 coating can be an advantage when a transfer film is easily formed under fretting conditions. The area exposed to the ambient air that determines the rate of ejection of detached coating material is responsible for the marked difference in performance of the MoS2 coating in fretting and pin-on-disk tests. Acknowledgments The authors would like to thank the Flemish and China government for the support (project BIL99y11). The authors are also grateful to Teer Coatings Ltd for the deposition of the MoS2 yTi coating. References w1x M.R. Hilton, P.D. Fleischauer, Surf. Coat. Technol. 54y55 (1992) 435–441. w2x P. Niederhauser, H.E. Hinermann, M. Maillat, Thin Solid films 109 (1983) 209–218. w3x V. Fox, J. Hampshire, D. Teer, Surf. Coat. Technol. 112 (1999) 118–122. w4x J. Rechberger, P. Brunner, Surf. Coat. Technol. 62 (1993) 393–398. w5x M.C. Simmonds, A. Savan, H. Van Swygenhoven, E. Pfluger, ¨ S. Mikhailov, Surf. Coat. Technol. 108–109 (1998) 340–344. w6x R. Gilmore, M.A. Baker, P.N. Gibson, et al., Surf. Coat. Technol. 108–109 (1998) 345–351. w7x K.J. Wahl, I.L. Singer, Tribol. Lett. 1 (1995) 59–66. w8x X. Zhang, R. Vitchev, W. Lauwerens, L. Stals, J. He, J.-P. Celis, Thin Solid Films 396 (1y2) (2001) 69–77. w9x N.M. Renevier, V.C. Fox, D.G. Teer, J. Hampshire, Surf. Coat. Technol. 127 (2001) 24–37. w10x P.Q. Wu, D. Drees, L. Stals, J.P. Celis, Surf. Coat. Technol. 113 (1999) 251–258. w11x N.M. Renevier, N. Lobiondo, V.C. Fox, D.G. Teer, J. Hampshire, Surf. Coat. Technol. 127 (2001) 24–37. w12x S. Debaud, S. Mischler, D. Landolt, in: D. Dowson, et al. (Eds.), Lubrication at the Frontier, Elsevier Science, 1999, pp. 405–412. w13x I.L. Singer, S. Fayeulle, P.D. Ehni, Wear 195 (1996) 7–20. w14x K.J. Wahl, I.L. Singer, Role of the third body in life enhancement of MoS2, in: D. Dowson, et al. (Eds.), The Third Body Concept, Elsevier Science, 1996, pp. 407–413.
Different tribological behavior of MoS2 coatings under fretting and pinon-disk conditions Xiaodong Zhua,b, W. Lauwerensc,*, P. Cosemansc, M. Van Stappenc, J.P. Celisd, L.M. Stalsb, Jiawen Hea a
State Key Laboratory of Mechanical Behavior of Metals, Xi’an Jiaotong University, Xi’an 710049, PR China b Institute for Materials Research, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium c WTCM, Wetenschapspark 3, B-3590 Diepenbeek, Belgium d Department MTM, Katholieke Universiteit Leuven, Kasteelplein Arenberg 44, B-3001 Leuven, Belgium
Abstract The performance of tribological coatings is often evaluated by fretting or pin-on-disk measurements. However, only a few papers dealt with the comparison of the two methods on the same coating. In this paper, a comparison of the tribological behavior of sputtered MoS2 coatings in fretting and pin-on-disk tests under different conditions is studied. The relationship between the mechanical properties of the coatings and their wear performance in both type of sliding tests is discussed. MoS2 coatings were deposited by magnetron sputtering from a MoS2 target. The coating thickness and adhesion were evaluated by respectively the ball crater and scratch test. The hardness as well as the toughness were obtained by a Vickers microhardness test. After the tribological tests the surface morphology of the wear scar in the coating was observed by scanning electron microscope. Also transfer films on the counterbody were investigated. It appears that the fretting and pin-on-disk tests can give considerably different friction and wear results. Coatings that show excellent performance in fretting tests have shown a low endurance in pinon-disk tests. Sliding speed appears to be a crucial factor. Also the material of the counterbody, steel or corundum, influences the results in both tests in a different way. The different behavior of a coating in both tests is related to the toughness of the coating and to the way that the debris is involved in the friction and wear mechanisms. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: MoS2 coatings; Wear mechanisms; Friction; Pin-on-disk test; Fretting test
1. Introduction Coatings of sputter-deposited molybdenum disulfide have been used in high vacuum and aerospace applications for lubrication purposes where low friction is desirable w1x. Interest is now growing in applying this solid lubricant in ambient atmosphere or high humidity environments w2,3x. The reported results of the tribological properties of MoS2 coatings are sometimes quite different. The reasons could be that the deposition apparatus andyor techniques used bring about different coating properties. However, besides the properties of the coatings, the testing methods can also affect the performance of the coatings. *Corresponding author. Tel.: q32-11-260856; fax: q32-11260859. E-mail address: [email protected] (W. Lauwerens).
According to practical applications, most machine parts work at continuous or oscillatory sliding conditions. Thus, the method of evaluation of the coatings should be close to their working conditions. In literature, most of the researchers evaluated the tribological properties of MoS2 coatings based on pin-on-disk sliding test, which is the most common way for sliding wear tests in the laboratory w4–6x. Other methods, including reciprocating sliding and mode I fretting tests were less referred w7,8x. Furthermore, only a few papers concerned the application of two different methods on the same coating w9x. Understanding the test conditions, especially the uni- and bi-directional sliding modes, on the tribological performance of the coatings could be important for selecting the proper evaluation methods. Results of hard coatings showed significant difference in wear rate between these two types of sliding wear tests w10x. It is
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 6 3 8 - 2
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
423
Fig. 1. Schematic of the fretting wear testing equipment.
also of interest to examine if these phenomena are found in softer lubricant coatings like MoS2. In the present study, MoS2 coatings were deposited by magnetron sputtering on high-speed steel substrate. The tribological behavior of sputtered MoS2 coatings in both fretting and pin-on-disk tests under different conditions is studied. In addition, a MoS2 yTi composite coating with approximately 20 at.% Ti homogeneously incorporated, known as MoST娃 w11x, deposited by Teer Coatings Ltd (UK) on an identical substrate was used for comparison. The wear mechanism of MoS2 coatings and its relationship with the mechanical properties of the coatings in both test methods are also discussed. 2. Experimental The MoS2 coatings were deposited by planar magnetron sputtering in a Balzers BAI640R equipment (WTCM, Belgium). The MoS2 target had a purity of 99%. Hardened and polished (Ras0.05 mm) ASP23 high-speed steel disks (64 HRC) were used as the substrates. Before the deposition of the MoS2 coatings, a thin titanium interlayer approximately 100 nm thick was applied to improve the adhesion. The temperature of the substrate during the deposition of MoS2 was below 80 8C. The substrate bias voltage was y50 V and the deposition time was 120 min, with a background argon pressure of 0.4 Pa. The MoS2 yTi coating was deposited by a closed field unbalanced magnetron sputter ion plating process starting with a 100 nm Ti interlayer to enhance the adhesion (Teer Coatings Ltd, UK) w11x. X-ray diffraction (XRD) measurement was
done with a Siemens D5000 diffractometer using Cobalt Ka radiation (ls0.17889 nm) for the structural analysis of the MoS2 coatings. The wear morphology of the coatings and the steel balls were investigated with a Philips XL 30 scanning electron microscope (SEM) and an optical microscope. The thickness of the MoS2 and MoS2 yTi coating was measured by a ball crater test and the adhesion of the coatings was measured by the scratch test with a standard diamond stylus with maximum load up to 80 N. Optical examination of the scratch was used to determine the failure of the coatings. A series of indentations by a microhardness tester using a Vickers indentor at loads ranging from 5 to 500 g were also made to examine the toughness and the hardness of the coatings. The loads at which fractures of the coatings occurred were recorded for toughness comparison. Tribological properties of the coatings were investigated by fretting (mode I) under gross slip conditions and by pin-on-disk tests at 23 8C in ambient air at a relative humidity of 50%. Corundum and 100Cr6 steel balls of a diameter of 10 mm and roughness of Ras0.2 mm were used as the counterbody. In the fretting test, the ball was loaded on top of the coating by springs at a normal load of 1.0 and 10.0 N. Fig. 1 shows the setup of the fretting wear test equipment. In pin-on-disk tests, the loads of 5 and 10 N were applied on the ball by dead weight. The wear track diameters ranged from 20 to 40 mm. The testing parameters are shown in Table 1. In fretting displacements of 100 and 500 mm were used. At 500 mm displacement the test is rather a reciprocating test since the observed contact radii typi-
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
424
cally ranged between 80 and 150 mm. However, this condition was selected to investigate the tribological behavior under conditions between those of fretting and pin-on-disk tests. After the wear tests, the wear volume was measured by a WYKO-NT3300 profilometer with white light interferometry. The wear volumes of the wear pits in fretting tests were measured directly and that of the wear track in pin-on-disk tests were calculated by averaging the cross-section of eight sectors. The wear rate was calculated by dividing the wear volume by normal load and sliding distance. For fretting this gives an averaged value since the wear rate is not uniform throughout the wearing area. 3. Experimental results Fig. 2. XRD pattern of the MoS2 coating.
3.1. Structure and mechanical properties of the coatings Fig. 2 shows the XRD pattern of the MoS2 coating. No detectable typical MoS2 peaks can be seen and this indicates that the coating is amorphous or near amorphous. Also the structure of the MoS2 yTi coating is amorphous as appears from Ref. w9x. The thickness of the MoS2 and MoS2 yTi coatings amount to be 1.5 and 1.7 mm respectively. The scratch test results show that both the MoS2 and MoS2 yTi coatings are well adherent to the substrate and that no debonding occurred at the maximum load of 80 N. Since the thickness of the MoS2 coating is only approximately 1.5 mm, the hardness is difficult to obtain from the microhardness tests because of the substrate effect. However, a thick MoS2 coating of 5 mm deposited at the same parameters gives a hardness of 3.6 GPa (load 5 g). This value is lower than some published data of 4.7 GPa for MoS2 coatings w9x. For the hardness of the MoS2 yTi coating, we refer to Ref. w9x where a value of 15 GPa is reported, obtained by a depth-sensing hardness test at 4 mN. Fig. 3 reveals that the toughness of the MoS2 coating is very low in comparison to the MoS2 yTi coating because fracture and delamination occur at 50 g load, yet the indentation of the MoS2 yTi at 500 g load is
perfect. The mechanical properties of the MoS2 coating, i.e. the low toughness and hardness, are probably related to the amorphous structure. Although the MoS2 yTi coating also shows an amorphous structure, it is well known that the addition of titanium increases the hardness and probably also improves the toughness of the coating. 3.2. Friction and wear behavior Fig. 4 shows the coefficient of friction of the MoS2 and MoS2 yTi coating as a function of test cycles in fretting tests when small load and displacement were used. In the test with a corundum counterbody and after an obvious running-in stage, the coefficient of friction of the MoS2 coating became stable and stayed at approximately 0.1. The running-in stage was slightly longer and the friction coefficient was less stable at a level of approximately 0.2 when a steel ball was used as the counterbody. The coefficient of friction of the MoS2 yTi coating is higher than that of the MoS2 coating when a corundum ball is used, but it is comparable in case of a steel ball. Opposite to the MoS2 coating, the friction of MoS2 yTi with a steel ball is lower than with a corundum ball.
Table 1 Variables of fretting and pin-on-disk tests Fretting
MoS2
MoS2yTi a b
Pin-on-disk
Load (N)
Displacement (mm)
Frequency (Hz)
Speed (mmys)
Load (N)
Speed (mmys)
Frequency (Hz)
1 1 10 10
100a 500b 100a 500a
10 2 10 2
2 2 2 2
5 10 10 10
200a 100b 20b 2b
1.5 0.8 0.2 0.03
1
100a
10
2
5
787a
6.3
Counterbody are steel and corundum balls. Counterbody is steel ball.
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
425
Fig. 5. Coefficient of friction of MoS2 at different loads and displacements in fretting test.
Fig. 3. Comparison of toughness of MoS2 and MoS2yTi by indentation (a) MoS2 (b) MoS2yTi.
The coefficient of friction of the MoS2 coating tested with a steel ball when higher load and large displacement were used is shown in Fig. 5. Wear-through occurred after approximately 600 cycles in the case
Fig. 4. Coefficient of friction of MoS2 and MoS2yTi in fretting test.
when high load (10 N) and large displacement (500 mm) are applied. No wear-through could be observed at the same testing conditions with a corundum ball. Fig. 6 shows the comparison of friction in the pinon-disk test with steel and corundum balls. The MoS2 coating was removed after only 40–80 revolutions against the steel counterbody, yet the wear with a corundum ball is much lower and the coefficient of friction remains at 0.2 after 10 000 revolutions. The coefficient of friction of the MoS2 yTi coating is lower than MoS2 either against a corundum or steel ball. The trend that with a steel ball the friction is lower in case of MoS2 yTi corresponds to the results in fretting tests. The wear rates of MoS2 and MoS2 yTi coatings in pin-on-disk and fretting tests (1 N, 100 mm) are shown in Fig. 7a. The results show the trend that the wear rate in the fretting test is higher than that in the pin-on-disk test, except the MoS2 coating tested in pin-on-disk conditions against the steel counterbody (wear-through). From Fig. 7b it can be concluded that the sliding
Fig. 6. Coefficient of friction of MoS2 and MoS2yTi in pin-on-disk test.
426
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
Fig. 8. Coefficient of friction of MoS2 at different sliding speeds in pin-on-disk test.
at different displacements. The debris on the counterbody in case of the small displacement (Fig. 10a) is accumulated around the contact area. In case of the large displacement (Fig. 10b), it is scattered in a larger area along the direction of movement. Transfer films
Fig. 7. Wear rate of (a) MoS2yTi and MoS2 in pin-on-disk and fretting test (1 N, 100 mm) and (b) MoS2 in fretting at different test conditions.
displacement has a stronger effect than the load on the wear rate in fretting. The effect of sliding speeds on the friction of MoS2 against a steel ball is shown in Fig. 8. It is noticed that at lower sliding speed the friction is also lower. At the speed of 2 mmys, which is similar to the condition in fretting, the friction is lower than 0.1, even better than that in fretting conditions. No wear-through can be observed at 2 mmys sliding speed after 200 cycles. 3.3. Analysis of wear morphology Fig. 9 shows the micrograph of MoS2 wear pits obtained after testing at 1 N, 100 mm, 30 000 cycles in fretting against a corundum (Fig. 9a) and a steel ball (Fig. 9b). The morphology of the wear of the coating against a corundum ball after fretting and pin-on-disk tests is smooth and featureless, and no wear is detectable on the ball. A thin and uniform transfer film on the corundum ball is observed at any test condition. Fig. 10 shows the contacting area on the steel balls after testing
Fig. 9. Optical micrograph of the wear pit of the MoS2 coating obtained in fretting tests (a) with a corundum ball (b) with a steel ball.
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
427
film on the steel counterbody in Fig. 10b also shows a small area where the transfer film is missing, resulting in a local steel–MoS2 contact. This can point to an unstable, not very well adherent transfer film, which is also in agreement with the unstable friction coefficient in case of a steel ball. With the steel counterbody the MoS2 coating showed a large difference in tribological performance when tested under fretting or pin-on-disk conditions. Clear differences between the two methods are the wearing area exposed to the open air, the sliding direction and the sliding speed, leading to different wear mechanisms. In a fretting test with small displacement (100 mm), most of the contact area is covered by the counterbody, but the covered area in pin-on-disk is negligible. Wahl and Singer presented a model describing lubricant transfer processes between the coating, transfer film, wear
Fig. 10. SEM micrograph of contact area of steel counterbody (a) 10 N, 100 mm, 10 000 cycles; (b) 10 N, 500 mm, 1000 cycles.
can be seen on the steel counterbody in both cases, but it is partially missing in case of Fig. 10b. Fig. 11a shows the wear track of the MoS2 coating against a steel ball in the pin-on-disk test obtained after 45 cycles. No abrasive wear can be seen, however, the coating in the center of the track is torn off and some cavities are left. The contact area on the steel counterbody is smooth without wear and no transfer film was formed, instead, the debris piled up in front of the contact zone (Fig. 11b). The detail of the debris reveals that it consists of large pieces (Fig. 11c). 4. Discussion The experimental results indicate clearly that the test conditions have strong effects on the performance of the MoS2 coating. The use of a corundum counterbody results in better tribological performance than a steel ball in both the fretting and the pin-on-disk tests. This could be due to the high hardness and chemical inertness of the corundum ball or to the observed transfer films on corundum. Indeed the presence of a stable transfer film, covering the whole contact zone of the counterbody, is essential for good tribological behavior w12x. Singer et al. also reported that for DC triode sputtering deposited MoS2 coatings the transfer film on WC–Co and sapphire balls were uniform, but the one on a steel counterbody was thick and patchy, and the coating had abrasion grooves w13x. The morphology of the transfer
Fig. 11. SEM micrograph of (a) pin-on-disk wear track of MoS2 obtained after 45 revolutions tested at 5 N, 200 mmys against steel ball; (b) the steel counterbody; (c) detail of debris on steel ball.
428
X. Zhu et al. / Surface and Coatings Technology 163 – 164 (2003) 422–428
patches and ejected debris w14x. According to this model, the wear is controlled by the loss flux of the depletion material (i.e. the coating) and the depletion material trapped in the contact (detached material) can be beneficial. If the generation rate of detached coating material is high, which is expected in bi-directional fretting, the wear rate can still be low in case the detached material remains in the contact zone. Considering the low hardness and toughness, the MoS2 coating is easily removed under the shear stress, meaning that the production rate of detached material is high. However, since a large amount of the wearing area is covered in the fretting test, the detached material is confined within the contact zone. Instead of being ejected as debris, the trapped coating material converts to a transfer layer under the cyclic movements. This is confirmed by the comparison of the wear rates in Fig. 7b where a large displacement, i.e. large exposed area, has the largest effect on the wear rate. Therefore, the wear rate in fretting is controlled by the removal of detached material, which is a slow process in fretting. The pin-on-disk is an extreme case where the contact zone changes continuously and most of the detached material is ejected. The influence of the sliding speed on the tribological performance is explained by the formation of the transfer film. High sliding speed causes higher tangential stress that removes easily the stuck debris on the steel ball and makes it difficult to form the transfer film. Therefore, the steel–MoS2 contact remains until the coating is completely removed. The reason why the MoS2 yTi coating shows better performance in a pin-on-disk test than MoS2 can be explained by its mechanical properties. In comparison to the MoS2 coating, the MoS2 yTi coating is tougher and harder, and the production of detached coating material is slower. Hence, the wear is controlled by the formation of the detached material. This can be advantageous in pin-on-disk conditions when ejection of debris is easy. The result that the MoS2 yTi and MoS2 coatings with a corundum ball showed lower wear rate in pin-on-disk conditions than in fretting is corresponding to the result of hard TiN coatings where the wear rate of bi-directional sliding is about twice that of unidirectional sliding w10x. This implies that the production of detached coating material in fretting is more efficient than in pin-on-disk conditions. However, the reasons why the two coatings show different behavior when steel or corundum balls are used, is not clear. It is probably related to a difference in mechanical properties andyor a possible tribo-chemical role of Ti in the wear mechanism.
5. Conclusions The selection of the wear test method and contact conditions are important for the evaluation of a coating. MoS2 shows a better performance in fretting with a corundum counterbody and a comparable performance with a steel counterbody. The MoS2 yTi composite coating is much better in pin-on-disk tests. The amorphous MoS2 coating has poor mechanical properties and this results in a low performance when a steel counterbody is used in pin-on-disk tests. The low strength of the MoS2 coating can be an advantage when a transfer film is easily formed under fretting conditions. The area exposed to the ambient air that determines the rate of ejection of detached coating material is responsible for the marked difference in performance of the MoS2 coating in fretting and pin-on-disk tests. Acknowledgments The authors would like to thank the Flemish and China government for the support (project BIL99y11). The authors are also grateful to Teer Coatings Ltd for the deposition of the MoS2 yTi coating. References w1x M.R. Hilton, P.D. Fleischauer, Surf. Coat. Technol. 54y55 (1992) 435–441. w2x P. Niederhauser, H.E. Hinermann, M. Maillat, Thin Solid films 109 (1983) 209–218. w3x V. Fox, J. Hampshire, D. Teer, Surf. Coat. Technol. 112 (1999) 118–122. w4x J. Rechberger, P. Brunner, Surf. Coat. Technol. 62 (1993) 393–398. w5x M.C. Simmonds, A. Savan, H. Van Swygenhoven, E. Pfluger, ¨ S. Mikhailov, Surf. Coat. Technol. 108–109 (1998) 340–344. w6x R. Gilmore, M.A. Baker, P.N. Gibson, et al., Surf. Coat. Technol. 108–109 (1998) 345–351. w7x K.J. Wahl, I.L. Singer, Tribol. Lett. 1 (1995) 59–66. w8x X. Zhang, R. Vitchev, W. Lauwerens, L. Stals, J. He, J.-P. Celis, Thin Solid Films 396 (1y2) (2001) 69–77. w9x N.M. Renevier, V.C. Fox, D.G. Teer, J. Hampshire, Surf. Coat. Technol. 127 (2001) 24–37. w10x P.Q. Wu, D. Drees, L. Stals, J.P. Celis, Surf. Coat. Technol. 113 (1999) 251–258. w11x N.M. Renevier, N. Lobiondo, V.C. Fox, D.G. Teer, J. Hampshire, Surf. Coat. Technol. 127 (2001) 24–37. w12x S. Debaud, S. Mischler, D. Landolt, in: D. Dowson, et al. (Eds.), Lubrication at the Frontier, Elsevier Science, 1999, pp. 405–412. w13x I.L. Singer, S. Fayeulle, P.D. Ehni, Wear 195 (1996) 7–20. w14x K.J. Wahl, I.L. Singer, Role of the third body in life enhancement of MoS2, in: D. Dowson, et al. (Eds.), The Third Body Concept, Elsevier Science, 1996, pp. 407–413.