Synergistic effects of metals co-sputtered with MoS 2

Abstract

Sputtered MoS 2 films providing satisfactory lubricating properties can be achieved by proper control of processing parameters. The sputtering process is ideally suited to applications which require very thin films of MoS 2 as it yields coatings with excellent substrate adhesion and which have endurances per unit thickness in excess of those of conventional dry film coatings. Sputtered interfacial coatings of other materials can be deposited onto certain substrates to enhance lubricating qualities and adherence. In the work presented here, the evaluation method was a sliding friction test run at a normal load of 63.4 kgf and a speed representing 800 load applications per minute. With this set of test parameters sputtered MoS 2 films produced under widely reported process parameters manifest irregular coefficients of friction and variability of endurance. The films also exhibit time-related changes in their coefficients of friction which stabilize from 6 to 12 h after sputtering. The observed irregularities are not peculiar to any particular method of sputtering and are most probably caused by residual stresses or inhomogeneous crystal growth as the film increases in thickness. The observed frictional instabilities and irregularities can be eliminated or greatly reduced by the codeposition of certain metals with MoS 2. These metal additives appear to increase the size of the acicular crystallites by approximately 35%, and in appropriate concentrations they provide synergistic effects such as a lowering of the coefficient of friction and an increase in the endurance per unit thickness. The ability to vary the ratio of metals to MoS 2 enables films to be produced which are controllably harder than those obtained with MoS 2 alone. Up to about 10% the metal additives do not adversely affect the lubricating qualities. The codeposited films have stable coefficients of friction with increased endurance per unit thickness and are clearly superior to standard sputtered MoS 2 films.