The effect of relative humidity on wear of a diamond-like carbon coating

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1 Surface and Coatings Technology 167 (003) 1 5 The effect of relative humidity on wear of a diamond-like carbon coating a, b c Jiaren Jiang *, Sam Zhang, R.D. Arnell a Integrated Manufacturing Technologies Institute, National Research Council Canada, 800 Collip Circle, London, Ont., Canada N6G 4X8 b School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore c Centre for Advanced Materials and Surface Engineering, University of Salford, Salford M5 4WT, UK Abstract Sliding wear behaviour of a diamond-like carbon (DLC) coating deposited using the combined closed field unbalanced magnetron sputter ion plating and plasma assisted chemical vapour deposition technique has been investigated against tungsten carbide balls in air with various relative humidity (RH) levels as well as in water. A smooth transfer layer was always formed on the ball surface and wear of the ball was negligible. The wear rate of the coating decreased significantly with increase in RH, with sliding in water showing the lowest wear rate. When sliding in dry air or in a vacuum, the coating failed very quickly, producing a wear rate of approximately two orders that for sliding in humid air. Under the investigated conditions, sliding speed (0.5 and 0.05 m s ) had no significant effect on the wear behaviour of the DLC coating in the various environments. The role of humid air in reducing wear rate of the DLC coating has been attributed to the formation of low energy surfaces by the adsorption of water at the dangling carbon bonds on the edge-faces of the sp clusters in the DLC coating. 00 Elsevier Science B.V. All rights reserved. Keywords: Wear; Tribological behaviour; Diamond-like carbon; Relative humidity; Environmental effect 1. Introduction The high hardness, high chemical stability and low friction and wear make diamond-like carbon (DLC) coatings good candidates for a wide range of tribological applications. However, studies have shown that their tribological behaviour can be very sensitive to environmental factors, especially to relative humidity (RH), and is dependent on the deposition method and deposition conditions w1 11x. Under most of the conditions, the hydrogenated DLC coatings showed low values of friction coefficients in dry environment and in vacuum w,3,5 8x and very low friction coefficients have been observed in ultrahigh vacuum w5,7,8x. Friction coefficients increase with increase in RH w,3,6 11x. Oxygen has been found to increase the friction coefficients of such DLC coatings w3x. On the other hand, Voevodin et al. w1,13x showed an opposite effect of environment on friction of a-c and a-c:h coatings deposited using the pulsed laser deposition technique. The friction coefficient after a certain number of cycles of sliding against steel or sapphire balls decreased with increase in RH *Corresponding author. Tel.: q ; fax: q address: jiaren.jiang@nrc.ca (J. Jiang). (;80% RH). Friction coefficients were higher in vacuum (10 Pa) than in dry nitrogen and in humid air. Data on the wear performance of DLC coatings in different environmental conditions is relatively less available compared with that for their frictional behaviour. There seems to be no direct relationship between frictional and wear behaviour w,5,6,9,14,15x. Various theories have been proposed to explain the tribological behaviour of DLC coatings w16 5x and they are subject to controversy. The role of environments on friction and wear of amorphous carbon coatings is still not well understood. Considering that air is the most commonly encountered environment in DLC applications and RH is one of the main factors affecting tribological properties of DLC coatings, in this study, the tribological behaviour of an unhydrogenated composite DLC coating has been investigated in air with various RH using a ball-on-disk sliding wear rig.. Experimental method DLC coatings were deposited on an M4 tool steel substrate using the combined closed field unbalanced magnetron sputtering and plasma assisted chemical vapour deposition technique w6,7x. The coating was /03/$- see front matter 00 Elsevier Science B.V. All rights reserved. doi: /s (0)00905-

2 J. Jiang et al. / Surface and Coatings Technology 167 (003) 1 5 Fig. 1. Typical variation of wear volume as a function of sliding distance at various RH levels and at sliding speeds of 0.05 and 0.5 m s. composed of multilayers of, from the substrate outwards, TiyTiNytransitional Ti(N,C)yTiCyDLC. The thickness of the DLC coating layer, measured using a ball crater method, was approximately 1. mm. The hardness and elastic modulus of the DLC coating, measured using a nanoindenter, were 7 and 180 GPa, respectively. Wear tests were carried out on a continuous ball-ondisk sliding wear rig with a controllable environment chamber. The RH of the environment was controlled by mixing different proportions (flow rates) of dry compressed air and air saturated with water vapour before introducing the gas into the chamber. The actual RH in the testing chamber enclosing the specimen was measured using a hygrometer. The balls were made of sintered tungsten carbide and had a diameter of 6.35 mm. Two sliding speeds were used: 0.05 and 0.5 m s. The applied normal load was 10 N. During sliding, wear volumes of the disk specimens were measured in situ after a certain sliding distance using a profilometre attached to a PC. The average cross-section area of a wear track was calculated from at least 1 measurements. The reproducibility of this technique was within 1.6%. After each wear measurement, an optical photograph of the wear scar formed on the ball was taken. Then, the specimen chamber was closed and purged for more than 0 min and the specimens were put back into contact again to restart the sliding. Detailed examination of scars on the balls in SEM showed that a significant amount of material was transferred from the coating to the ball surface and little wear from the ball had actually occurred; this was confirmed by profilometre scanning over the surface of the ball wear scar. 3. Results 3.1. Wear as a function of relative humidity Fig. 1 shows some wear curves for sliding in different RH at sliding speeds of 0.05 and 0.5 m s. The variation of wear volume as a function of sliding distance was roughly linear. Specific wear rate decreased with increase in RH. Fig. summarises the results for the effect of RH on specific wear rate of the DLC coating at sliding speeds of 0.05 and 0.5 m s. The normal load was 10 N except that a load of 0 N was applied when sliding in water because the wear measurement in this case was very difficult at a load of 10 N. The results obtained from sliding in water are plotted in Fig. at a RH of 100%. The results for the two sliding speeds essentially followed the same trend except at the low RH end where the wear rate at 0.5 m s seemed to be slightly higher than at 0.05 m s. Under dry conditions (RH;0%), the coating failed very rapidly. The wear life of the coating was approximately m, giving a specific wear rate of approximately.0= m N m which is almost two orders that for sliding in wet air. There was a relatively wide spread in the values of wear rate at low RH, indicating some unstable situations during the sliding. However, the reasons are not clear. With increase in RH from dry condition to approximately 0%, the wear rate decreased rapidly. Between RH of 0 and 60%, the variation in wear rate with increase in RH became insignificant. Above approximately 60% RH, wear rate decreased rapidly again with further increase in RH. The wear rate for sliding in water was significantly lower than in air; but it seemed to fall on the same curve as the results for sliding in high humidity air.

3 J. Jiang et al. / Surface and Coatings Technology 167 (003) Fig.. The effect of RH in air on wear rate of the DLC coating at sliding speeds of 0.05 and 0.5 m s a RH of 100%.. Data for sliding in water is plotted at When the sliding wear test was carried out in a y6 vacuum of 8=10 Torr at a speed of 0.5 m s, the coating failed within a sliding distance of less than 50 m. Large and loose wear debris particles were formed. 3.. Examination of wear surfaces According to SEM observations, the major noticeable difference in the topography of wear surfaces formed at different RH was that wear surfaces were much smoother in high RH than in relatively dry conditions (Fig. 3). The photographs were taken on wear surfaces of the top DLC coating layer which was not won through at this stage. Wear surfaces produced by sliding in water (Fig. 3c) were very similar to those formed at high RH as shown in Fig. 3b. Wear scars formed on the ball specimens observed under optical microscope were very similar under the various conditions. Fig. 4 shows the variation of scars on the ball specimen as a function of sliding distance for sliding in RH of 16%. Material transfer from the coating to the ball surface was apparent, while wear of the ball was negligible. With the progress of sliding, wear debris particles were accumulated and compacted surrounding the ball wear scars. 4. Discussion Rabinowicz w8x analysed the formation of wear debris particles during sliding wear due to plastic deformation and obtained a relationship for the particle size, D, as a function of material properties and surface energy, g, g Dsk (1) p where k is a material-related constant and p is the yield strength of the wear material. Eq. (1) indicates that reducing the surface energy of the rubbing material will lead to the formation of wear debris particles with reduced sizes which are prone to agglomerate and get compacted onto the wear surfaces to form wear protective layers w9x. A high RH may also assist in the agglomeration and compaction of the wear debris particles to the wear surfaces by the capillary effect, further reducing wear. For the unhydrogenated DLC coating as investigated in the present study, its structure is primarily constituted of an amorphous or nanocrystallised framework of sp - coordinated carbon atoms with a very small fraction of 3 sp -bondings, as has been shown by infra-red spectros- copy w30x. In the sp configuration, three of the four valence electrons of a carbon atom are assigned to the trigonally directed sp hybrid orbitals to form strong covalent bonds with nearest neighbouring atoms. The fourth electron, also known as the p electron, lies in an orbital normal to the sp hybrids. This bonding configuration would ideally lead to the formation of the perfect graphite lattice in which layers of atoms in hexagonal network are stacked in an ABAB sequence with weak Van der Wall s force between the adjacent layers. In the DLC coatings, clusters of sp bonds with localised ordering are formed. On the material surface of such clusters is composed of cleavage-faces (with high degrees of disorder) and edge-faces. The cleavage-face of the sp structure, which is graphite-like, is a lowenergy surface that is essentially formed by the basal planes. The dangling covalent bonds on the edge-face of the sp -bonded clusters have high energy levels and are highly active. However, they can be readily passivated by the adsorption of water vapour or other gases from the environment to give a low energy surface.

4 J. Jiang et al. / Surface and Coatings Technology 167 (003) 1 5 amorphous carbon structures into more ordered graphitic structures which are subject to dusting under vacuum and dry conditions.

(1)), which are difficult to be incorporated into the compact transfer layers on the rubbing surfaces but rather are easily removed from the rubbing interface to cause wear loss.

4 4 J. Jiang et al. / Surface and Coatings Technology 167 (003) 1 5 amorphous carbon structures into more ordered graphitic structures which are subject to dusting under vacuum and dry conditions. The high surface energy dictates the formation of large wear debris particles (Eq. (1)), which are difficult to be incorporated into the compact transfer layers on the rubbing surfaces but rather are easily removed from the rubbing interface to cause wear loss. As a result, a very high rate of wear of the DLC coating was observed when sliding in vacuum and in very dry conditions. On the other hand, as RH increases, the freshly formed dangling bonds on the wear surface can be passivated rapidly by the adsorption of water vapour to keep low energy surfaces, leading to the formation of finer wear debris particles than those formed under dry conditions (Eq. (1)). Therefore, wear rate decreases with increase in RH (Fig. ). Fig. 3. SEM observations on the wear surfaces formed during sliding at different RH: (a) RH 3.1%; (b) RH 8% and (c) water. When sliding is carried out in vacuum or in a dry environment, the newly created wear surface areas with the dangling carbon bonds due to wear cannot be passivated rapidly after each cycle of sliding. Strong adhesion can occur to cause high frictional forces (friction coefficient of ). This is different from situations for hydrogenated DLC coatings where the dangling bonds are passivated by hydrogen atoms within the coating structure to give a low energy surface and low friction coefficients. The high frictional heat generated during sliding in vacuum or in dry environment can lead to a fast transformation of the metastable Fig. 4. The evolution of wear scars on the ball specimen during sliding in air (RH 16%, 10 N, 0.5 m s ). (a) Sliding distance 390 m and (b) 1845 m.

5 J. Jiang et al. / Surface and Coatings Technology 167 (003) From the SEM observations, abrasive wear might have occurred when sliding in low RH air due to the entrapment of large hard wear debris particles within the rubbing interface before being removed, leaving rough grooves on the wear track (Fig. 3a). However, at higher RH levels, smooth wear track surfaces were formed due to the formation of fine wear debris particles and wear protective transfer layers (Fig. 3b). These are consistent with the predictions of Eq. (1) and support the argument that low surface energy due to water adsorption in humid air reduces wear rate of the unhydrogenated DLC coating. 5. Summary The wear behaviour of a multi-layered unhydrogenated DLC coating has been investigated on a ball-ondisk wear rig in air with various RH levels as well as in water. Tungsten carbide balls were used as the countering rubbing surface. While the wear of the balls was negligible, the wear rate of the DLC coating decreased significantly with increase in RH, especially below 0% RH. In dry air and in vacuum, the wear rate of the coating was two orders of magnitude of that for sliding in humid air. The decreasing wear rate with increase in RH has been attributed to the formation of low energy surfaces by the adsorption of water at the dangling carbon bonds on the edge-faces of the sp clusters in the DLC coating. References w1x C. Donnet, Condensed Matter News 4 (1995) wx D.S. Kim, T.E. Fischer, B. Gallois, Surf. Coat. Technol. 49 (1991) w3x R. Memming, H.J. Tolle, P.E. Wierenga, Thin Solid Films 143 (1986) w4x A. Erdemir, M. Switala, T. Wei, P. Wilbur, Surf. Coat. Technol. 50 (1991) w5x C. Donnet, M. Belin, J.C. Auge, J.M. Martin, A. Grill, V. Patel, Surf. Coat. Technol. 68y69 (1994) w6x T. Le Huu, H. Zaidi, D. Paulmier, P. Voumard, Thin Sold Films (1996) w7x K. Enke, H. Dimigen, H. Hubsch, Appl. Phys. Lett. 36 (1980) w8x K. Enke, Thin Solid Films 80 (1981) w9x A.K. Gangopadhyay, W.C. Vassel, M.A. Tamor, P.A. Willerment, J. Tribol. ASME Trans. 116 (1994) w10x C. Donnet, T. Le Mongne, L. Ponsonnet, et al., Tribol. Lett. 4 (1998) w11x E.-S. Yoon, H. Kong, K.-R. Lee, Wear 17 (1998) w1x A.A. Voevodin, M.S. Donley, J.S. Zabinski, J.E. Bultman, Surf. Coat. Technol (1995) w13x A.A. Voevodin, A.W. Phelps, J.S. Zabinski, M.S. Donley, Diamond Relat. Mater. 5 (1996) w14x M. Hilden, J. Lee, G. Ouano, N. Nayak, A. Wu, IEEE Trans. Magn. 6 (1990) w15x Yilmaz Ozmen, Akihiro Tanaka, Touru Sumiya, Surf. Coat. Technol (000) w16x K. Miyoshi, Surf. Coat. Technol. 43y44 (1990) w17x J.-P. Hirvonen, R. Lappalainen, J. Koskinen, A. Anttila, T.R. Jervis, M. Trakula, J. Mater. Res. 5 (1990) w18x J. Robertson, Adv. Phys. 35 (1986) w19x Y. Liu, A. Erdemir, E.I. Meletis, Surf. Coat. Technol. 81 (1996) w0x Y. Liu, A. Erdemir, E.I. Meletis, Surf. Coat. Technol (1997) w1x Y. Liu, E.I. Meletis, J. Mater. Sci. 3 (1997) wx Y. Kokaku, M. Kitoh, J. Vac. Sci. Technol. A 7 (1989) w3x S.V. Pepper, J. Vac. Sci. Technol. 0 (198) w4x M.N. Gardos, B.L. Soriano, J. Mater. Res. 5 (1990) w5x S. Jahanmir, Wear 133 (1989) w6x D.P. Monaghan, D.G. Teer, P.A. Logan, I. Efeoglu, R.D. Arnell, Surf. Coat. Technol. 60 (1993) w7x D.P. Monaghan, D.G. Teer, K.C. Laing, I. Efeoglu, R.D. Arnell, Surf. Coat. Technol. 59 (1993) 1 5. w8x E. Rabinowicz, Wear 7 (1964) 9. w9x J. Jiang, F.G. Stott, M.M. Stack, Tribol. Int. 31 (1998) w30x J. Jiang, R.D. Arnell, Development and application of diamondlike carbon coatings, Progress Report No. 10, DTI-EPSRC Link Project, UK, July, 1998.

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