Friction and wear behaviour of sputtered MoSx coatings in dry and humid environments

Friction and wear behaviour of sputtered MoS_x coatings in dry and humid environments

Researcher : Xiaoling Zhang

Keywords : MoS_x coatings; Humidity; Tribology; Sliding; Friction; Wear rate, Fretting

Objective :

Investigate and control the sensitivity of MoS_x coatings to humidity in fretting mode I wear condition.

Research strategy :

MoS_x coatings were deposited by planar magnetron sputtering operated with a pulsed power supply.
The effects of crystallographic orientation, contact stress and sliding speed on the friction and wear behaviour of MoS_x coatings were investigated with fretting wear tests performed at 23 °C in ambient air at a relative humidity (RH) of 10, 50 or 90%.
Wear tracks, debris and the transfer films were analysed with laser profilometer, Normarski optical microscope, TEM, XPS and AES, to understand the fretting wear performaces of MoS_x coatings under different relative humidity conditions.

Results:

1. The coefficient of friction of MoS_x coatings

The coefficient of friction of MoS_x coatings generally increases with increasing relative humidity (Fig. 1). For basal oriented coatings deposited at an Ar pressure of 0.2 and 0.4 Pa, the coefficient of friction is lower in comparison to the random oriented coatings deposited at higher Ar pressure. The lowest coefficient of friction at the three humidity levels tested is obtained with MoS_x coatings deposited at an Ar pressure of 0.4 Pa. For random oriented coatings, the coefficient of friction is not only high (above 0.2), but increases significantly with increasing relative humidity. The coefficient of friction for coatings deposited at an Ar pressure of 2.4 Pa is not shown in Fig. 1 at a relative humidity of 90%, because of the difficulties in identifying a stable friction stage resulting from the very limited wear life of the coatings leading to a premature wear-through.

Fig. 1: The coefficient of friction of MoS_x coatings deposited at different Ar pressures.
Data obtained in fretting tests performed in ambient air of different relative humidity.

The coefficient of friction of MoS_x coatings depends not only on contact stress P, but also on sliding speed V. When the sliding speed is kept constant, the coefficient of friction varies linearly with the inverse contact stress. The higher the contact stress, the lower the coefficient of friction is (Fig. 2). The coefficient of friction also varies linearly with fretting frequency when the contact stress is kept constant. The lower the sliding speed, the lower the coefficient of friction is (Fig. 3). As a first approximation, it can then be assumed that:

µ=S₀/P+b×V (1) With S₀ the shear strength of MoS₂ coatings which corresponds to the slope of each curve in Fig. 2. S₀ appears to be independent of sliding speed when the fretting frequency changes from 10 Hz to 1 Hz for basal oriented coatings. The same was noticed for random oriented coatings (not shown in Fig. 2). The shear strength of MoS₂ coatings depends thus heavily on the crystallographic orientation of the MoS_x coatings. For basal oriented MoSx coatings, S₀ is in the range of 30~50 MPa, while it increases to 90~120 MPa for random oriented MoS_x coatings in our fretting (mode I) tests performed at 50% relative humidity under gross slip conditions. According to literature, S₀ is 24.8 MPa for sputter-deposited MoS₂ in dry air, and 23<S0<33 MPa for burnished MoS₂ in ambient air. The relative higher S₀ value noticed in our tests is considered as resulting from the bi-directional sliding, the higher relative humidity, and the different orientations of the MoS_x coatings.
b is the velocity dependence of the coefficient of friction, which is the slope of the curves in Fig. 3. b appears as independent of contact stress, not sensitive to the crystallographic orientation of MoS_x coatings, and is between 1.0×10^-6~0.2×10^-5 (m s^-1)^-1 in our fretting mode I tests performed under gross slip conditions.

Fig.2: The influence of contact stress on the coefficient of friction
of random and basal oriented MoS_x coatings.

Fig.3: The influence of fretting frequency on the coefficient of friction
of random and basal oriented MoS_x coatings

2. The fretting wear of MoS_x coatings.

The fretting wear volume on MoS_x coatings tested in ambient air of different relative humidity, is given in Fig. 4. The wear volume on basal oriented MoS_x coatings (0.4 Pa Ar pressure) remains at a low level even at increasing relative humidity. Coatings deposited at even lower Ar pressure, 0.2 Pa, show higher and larger scattering in wear volumes at the three relative humidity levels. In contrast, for random oriented MoS_x coatings (0.6 up to 2.4 Pa Ar pressure), the fretting wear volume is not only high in comparison with the basal oriented MoS_x coatings, but it also increases significantly with increasing relative humidity. For coatings deposited at an Ar pressure of 2.4 Pa, the wear resistance is so poor that at 50% and 90% relative humidity wear-through of the coatings occurs within the 10,000 fretting cycles.

Fig.4: Wear volume of MoS_x coatings deposited at different Ar pressures.
Fretting tests were done at different relative humidity for 10,000 cycles (s: 100µm, Fn: 1 N, f: 10Hz)

On plotting dissipated energy against the wear volume in fretting tests done at a constant relative humidity, a linear relation is obtained for each MoS_x coating deposited at different Ar pressures (Fig. 5). The higher the Ar pressures in the deposition processes, the higher the wear rate, expressed in µm³/J, of the coatings is. But the wear rate is higher for coatings deposited at 0.2 Ar pressure than that at 0.4 Ar pressure (not shown in Fig. 5). Such a linear dependence of wear volume on dissipated energy has already been reported for TiN coatings. In that case no dependence on relative humidity was found, but for MoS_x coatings, a dependence of the wear rate in fretting tests on relative humidity is noticed. For basal oriented coatings, the wear rate remains almost unchanged when the relative humidity increases from 10% to 50%, although a slightly higher wear rate is noticed at 90% relative humidity. For random oriented MoSx coatings, the wear rate increases with increasing relative humidity. The higher the Ar pressure during deposition, the lower the humidity resistance of the coatings is.

Fig.5: Wear volume as a function of dissipated energy for MoS_x coatings
deposited at different Ar pressures and tested in fretting mode I

An interesting finding is that the linear relation between wear volume and dissipated energy is valid for different contact stresses and fretting frequencies. Selecting dissipated energy as the governing parameter allows an easy and flexible comparison of fretting wear rate. Loading and fretting cycles affect independently wear rate. Dissipated energy appears thus as a more appropriate tool for comparing wear rate of coatings in general, and MoS_x specifically, tested under different fretting conditions.

3. Analyses on wear track and debris

MoS₂ (Fig. 6) and MoO₃ (Fig. 7) are present in wear debris. Only one form of molybdenum oxide, namely MoO₃, was detected in the wear debris independent of the crystallographic texture of the original MoS_x coatings, and the relative humidity at which the debris were generated. The MoO₃ debris appears to have a lamella structure characterised by a parallel fine sub-microstructure (Fig. 7).

Fig. 6: TEM analysis on wear debris obtained on random oriented MoS_x coating
after 3000 fretting wear cycles in 50% RH (a) dark field image
(b) diffraction pattern k=4.07 mm.nm

Fig. 7: TEM analysis on wear debris obtained on basal oriented MoSx coating
after 10,000 fretting wear cycles in 10% RH (a) dark field image
(b) diffraction pattern k=1.47 mm.nm

The XPS spectra of a random oriented MoS_x coating is shown in Fig. 8. No oxidized sulphur has been detected on the as-deposited surface of a random oriented MoS_x coating. Fretting wear causes significant changes in the surface composition and chemistry of random oriented MoS_x coatings. The Mo 3d spectra recorded in the wear tracks show a great amount of oxidised Mo. The Mo 3d_3/2 peak of Mo in MoO₃ is the highest for the wear track obtained at 90% RH. The S 2p spectra recorded on the wear tracks show the presence of a peak at ~168.6 eV, identified as sulphur in SO₄^2- group, which indicate that sulphur has been oxidised to a great deal in the case of fretting tests performed at high humidity. Basically, oxidised sulphur was not detected on the worn basal oriented surface within the resolution of the equipment used.

Fig. 8: Mo 3d and S 2p XPS spectra of random oriented MoS_x coatings.
Curves 1 are recorded on as-deposited coatings, curves 2 on wear tracks from fretting
tests at 90% RH, and curves 3 on wear tracks from fretting tests at 10% RH

The Auger images in O KLL and S KLL spectra on random oriented MoS_x coatings, show that the rims of the wear track are enriched in O and depleted in S (Fig. 9). This fact hints that MoS_x oxidises during the fretting wear process, and that the oxidised product is pushed to the rims of the wear track. At high humidity, the whole surface of the wear track on a random oriented MoS_x coating, is enriched in O suggesting that an intensive oxidation took place.

Fig. 9: A low humidity wear track on random oriented MoS_x coating.
(b) Auger image in O KLL electrons, and (c) Auger image in S KLL electrons

Three reasons for the difference in resistance to humidity between basal and random oriented MoS_x coatings can be put forward. The edge planes of MoS₂ crystals are more reactive than basal planes. The tribo-chemical reaction on random oriented MoS_x coatings occurs more easily under higher humid conditions than on basal oriented MoS_x coatings, and more MoO₃ particles are generated on the wear tracks and in debris. Secondly, the presence of brittle MoO₃ particles may cause an abrasive wear on these hard to slide edge planes. The formation of MoO₃ will thus affect more badly the tribological performance of the random oriented coatings than in the case of basal oriented ones. And finally, the basal oriented coatings (Fig. 10 a), have a dense and non-columnar structure. In contrast, the random oriented coatings posses a porous columnar structure that becomes more pronounced (Fig. 10 c), at higher Ar pressure in the deposition process. The columnar structure can provide a path for moisture to penetrate easily the coatings.

Fig. 10: Cross-section of MoS_x coatings deposited at the Ar pressure of (a) 0.4, and (c) 1.5 Pa

Publications:

Xiaoling Zhang, R. Vitchev, W. Lauwerens, L. Stals, Jiawen He, J.-P. Celis, Effect of Crystallographic Orientation on the Friction and Wear Behaviour of MoS_x Coatings in Dry and Humid Air, 5th annual meeting, Liege, Nov. 2000.
Xiaoling Zhang, R. Vitchev, W. Lauwerens, L. Stals, Jiawen He, J.-P. Celis, Effect of Crystallographic Orientation on the Friction and Wear Behaviour of MoS_x Coatings in Dry and Humid Air, submitted to Thin Solid Films.
Xiaoling Zhang, R. Vitchev, W. Lauwerens, L. Stals, Jiawen He, J.-P. Celis, The relationship between crystallographic orientation and fretting wear behaviour of MoS_x coatings in humid air, Proceedings of the Second Conference on Materials Science and Technology, Brussels, Oct. 2000, p26-31.
Xiaoling Zhang, W. Lauwerens, L. Stals, Jiawen He, J.-P. Celis, Evaluation of fretting wear rate of sulphur deficient MoS_x coatings based on dissipated energy, in preparation.

Financing source:

This research is a part of the joint scientific and technological project funded by the Flemish government and the P R China government (project BIL 96/35), and the IUAP P4/33 project funded by the Belgian government.

Co-operating institute:

IMO, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium
State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong University, 710049, P R China