Effect of oxygen pressure from 10−9 to 10−6 Torr on the friction of sputtered MoSx films

Abstract

The effect of oxygen pressure in the range 10 −9 to 10 −6 Torr on the friction of sputtered MoS x films was determined in a pin-on-disk tribometer at a hertzian contact pressure of 0.44 GPa. The coefficient of friction was 0.04–0.05 at oxygen pressures of 1 × 10 −8 Torr or less. When 2 × 10 −8 Torr or more of oxygen was present during sliding the coefficient of friction decreased gradually to 0.01–0.02, independent of the pressure. However, static exposure to oxygen at any pressure did not affect the subsequent friction in vacuum. The wear scars with high friction were much larger than those with low friction. Oxygen may have reduced the friction force by causing a reduction in the shear strength of the film or a reduction in the area of contact. It is argued that the results are not consistent with a reduction in film shear strength. Rather, it is proposed that oxygen reduces the friction by influencing the transfer of material to the pin in such a way as to reduce the area of contact. With this hypothesis, a shear strength of 4.8 ± 0.6 MPa, independent of oxygen pressure, was deduced for a representative film.