A Vacuum (10−9Torr) Friction Apparatus for Determining Friction and Endurance Life of MoSxFilms

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Abstract
The first part of this paper describes an ultrahigh vacuum friction apparatus (tribometer). The tribometer can be used in a ball-on-disk configuration and is specifically designed to measure the friction and endurance life of solid lubricating films such as MoSx in vacuum at a pressure of 10−7 Pa, 10−9 torr. The sliding mode is typically unidirectional at a constant rotating speed. The second part of this paper presents some representative friction and endurance life data for magnetron sputtered MoSx films, 110 nm thick, deposited on sputter-cleaned 440C stainless-steel disk substrates, which were slid against a 6-mm-diameter 440C stainless-steel bearing ball. All experiments were conducted with loads of 0.49 to 3.6 N, average Hertzian contact pressure, 0.33 to 0.69 GPa, at a constant rotating speed of 120 rpm, sliding velocity ranging from 31 to 107 mm/s due to the range of wear track radii involved in the experiments, in a vacuum of 7×10−7 Pa, 5× 10−9 torr, and at room temperature. The results indicate that there are similarities in friction behavior of MoSx films over their life cycles regardless of load applied. The coefficient of friction μ decreases as load W increases according to μ = kW−1/3 , which is in agreement with the Hertzian contact model. The endurance life E of MoSx films decreases as the load W increases according to E = KW−1.4 for the load range. The load- (contact pressure-) dependent endurance life allows a reduction in the time needed for wear experiments and an acceleration of endurance life testing of MoSx films. For the magnetron-sputtered MoSx films deposited on 440C stainless-steel disks, the specific wear rate normalized to the load and the number of revolutions was 3 × 10−8 mm3/N -revolution; the specific wear rate normalized to the load and the total sliding distance was 8 × 10−7 mm3/N·m; and the nondimensional wear coefficient was approximately 5 × 10−6. The values are almost independent of load in the range 0.49 to 3.6 N, average Hertzian contact pressures of 0.33 to 0.69 GPa.

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