The thermal stability and lubricity of zinc thiomolybdenate at temperatures up to 500°C

ZnMoO2S2·H2O and ZnMoS4 were synthesized in laboratory. Their thermal stability was investigated by means of thermogravimetric analysis (TGA), and their lubricating characteristics at temperatures up to 500°C were examined by sliding a Sialon ceramic ball against a W18Cr4V steel disc on a high temperature ball-on-disc tribometer. The morphologies and the elemental distributions in the worn surfaces of the ceramic ball were observed and determined by scanning electron microscopy (SEM) equipped with energy dispersion spectrometry (EDS). The chemical states of some typical elements in the worn surfaces of the ceramic ball were explored with X-ray photoelectron spectroscopy (XPS). As the results, both ZnMoO2S2·H2O and ZnMoS4 showed good lubricity for Sialon ceramic/W18Cr4V steel pair from room temperature to 400°C. The XPS analytical results indicate that both ZnMoO2S2·H2O and ZnMoS4 experienced tribochemical reactions during the friction process at 300°C, thereby generating S and Mo of higher valence. SEM observations of the worn surfaces of the Sialon ceramic ball show that a surface protective film composed of the lubricants and their decomposed products was formed thereon, which contributes to keep the sliding surfaces off contact and abate the adhesion and scuffing on the lubricated surfaces, therefore to decrease the friction coefficients.