Ultra-Low-Friction Films on Modified Surfaces

Thin films of molybdenum disulphide produced by the technique of sputtering have been assessed in terms of their friction and wear properties under high vacuum. Improvements in the tribological properties of such films longer endurances, lower friction have been realised through the use of hard, ceramic substrate materials by modifying the surface texture of the substrate to which the film is applied by laser irradiation of the deposited film and by modification of the film through ion-beam mixing. Additionally, a round- robin exercise has been undertaken in which the properties of MoS2 films produced by different sources were characterised. As part of this round-robin exercise, comparative data were obtained on the tribological properties of MoS2 films produced by three different sputtering techniques, these processes being those developed at three independent laboratories operating either in the UK or USA. These data indicate that 1 micron-thick films, when tested under purely sliding motion at high contact stresses, have durabilities which differ by an order of magnitude and friction coefficients which range from below 0.01 to about 0.03. Thicker films 6 microns have essentially similar endurances and yield slightly higher friction coefficients. It is further shown, in an extension of the round-robin exercise, that storage of sputtered MoS2 films under dessicated conditions for up to 12 weeks does not, in general, adversely affect their tribological properties. It is shown that film friction and durability are profoundly affected by the substrate material to which the film is applied. Thus MoS2 films deposited on hot-pressed silicon nitride have appreciably longer endurances and lower friction than similar films applied to metallic substrates. Models are presented which explain the observed variations in friction and durability in terms of the mechanical properties of the substrate and film-to-substrate adhesion respectively.