Insights into the wear track evolution with sliding cycles of carbon-alloyed transition metal dichalcogenide coatings

Abstract

This study is aimed towards a better understanding of the tribological performance of the optimized Mo-Se-C dry lubricant coatings. The coatings are deposited with a lower carbon content (< 30 at. %), as compared to the literature. The detailed analysis of the wear track and the tribolayer evolution as a function of the C content and the number of sliding cycles is discussed, using Raman spectroscopy, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) analyses. The coatings are highly compact, homogeneous, with (002) basal planes parallel to the surface as observed by X-ray diffraction (XRD) studies. In unidirectional tests, under ambient conditions, the friction coefficient and the specific wear rate decrease with the number of sliding cycles. After 25,000 cycles, the wear tracks are not fully covered with MoSe2 tribolayer, instead displayed zones of the tribolayer as well as the as-deposited coating. After 100,000 cycles, the wear track is covered by a thick MoSe2 tribolayer. Scanning-TEM (STEM) chemical composition mapping also confirms that MoSe2 tribolayer is dominating during sliding in ambient air. Reciprocating sliding tests are also performed in ambient air, dry N2, and at 200 °C. The transition metal dichalcogenide (TMD) phase is always governing the low friction, with coatings not showing the chameleon behavior claimed in the literature for C-containing TMD coatings.