Research on the damage mechanism of low current 50 keV electron beam on the micro structure, composition and vacuum tribological performance of MoS2-Ti films deposited by unbalanced magnetron sputtering

Abstract

The present study focuses on the influence of 50 keV electron beam (e-beam) irradiation on the micro structure, composition and tribological performance of sputtered MoS2-Ti films. The coated specimens were then irradiated by a fixed-energy e-beam with a flux density of 3.0 × 1010 e/cm2·s until reaching a total fluence of 2.2 × 1014 e/cm2, under a vacuum level of <10−4 Pa. After irradiation, the hardness decreased from 4.2 to 3.6 GPa, and the boundaries between columns became clearer and slightly wider, and the crystallinity of MoS2 decreases, and sub-oxides (TiOx) transforms to TiO2. These results were attributed to the following mechanisms. Firstly, electron bombardment destroyed the MoS2 lattice near the surface; meanwhile, inelastic collision induced thermal energy and its accumulation lead to a reversible phase transformation of Ti3O5 from λ to γ. Then, the phase transformation (i.e. λ to γ of Ti3O5) resulted in both physical and chemical changes, e.g. the initial volume expansion in the λ to γ process and the subsequent shrinkage in the reversed γ to λ process. Thirdly, volume's expansion and shrinkage resulted in microstructure deterioration (e.g. micro cracks), which provided additional (and more effective) inner path for the diffusion of residual oxygen. Finally, diffused oxygen reacts with amorphous sub-oxides (TiOx) to form stable TiO2. In a word, EB irradiation caused softening of sputtered MoS2-Ti films, which lead to a lower friction coefficient and a deterioration of abrasion performance.