Shear strength measurements of parallel MoSx thin films

The effect of Hertzian contact pressure on the friction coefficient of parallel MoSx, films was investigated by sliding sapphire and AISI 52100 steel against parallel MoS,(x)-coated AISI 440C steel under a wide range of contact loads. Mean initial Hertzian contact pressures in the range 0.493-1.258 GPa were created between balls and parallel MoS,(x)-coated 440C disks by varying the applied load and the elastic moduli of the balls. The friction coefficient decreased with the increasing load. Using the Hertzian contact model, we estimated that the shear strength of parallel MoS, films was approximately 40 MPa. This value is higher than that obtained with the usual lamellar MoS2, films. An explanation in terms of an increasing reactivity of basal surfaces in parallel films is given. The effect of gaseous environment on friction measurement was also studied by varying the atmosphere conditions. For this purpose, friction tests were performed in open air and in dry Argon (Ar). The friction coefficient was shown to increase in the presence of water vapour. The shear strength increased with humidity from 40 MPa up to about 80 MPa. An explanation, taking into account the deformation microstructures that have been developed during sliding, is proposed. (C) 1997 Elsevier Science S.A.