Insights into the microstructure transformation and ultralow tribological behavior of WS ₂ coatings

Tungsten disulfide (WS₂)-based coatings have excellent thermal stability and self-lubricating properties, and are considered to be better lubricating material at elevated temperatures. This paper reports that WS₂ coating can achieve ultra-low friction behavior after sliding for a period of time when the ambient temperature rises to 400 °C, that is, the friction coefficient drops sharply from 0.08 to about 0.025 (a decrease of 68.7%), which is a very interesting phenomenon. There is no doubt that WS₂ coating will undergo more significant oxidation at the medium temperature. It is generally believed that oxidation is detrimental to the tribological properties of disulfide coatings because oxides formed at intermediate temperatures act as abrasive phases to increase friction and wear, which cannot explain the special ultra-low friction behavior of WS₂ coatings at 400 °C. Therefore, the microscopic structural evolution of transfer film during the friction process was investigated by Raman and high-resolution transmission electron microscopy. It is found that the formation of WO₃ nanocrystalline (~ 15 nm) promotes the structural ordering of WS₂ around WO₃ nanocrystalline. Then, incommensurate contact interfaces are spontaneously formed between WS₂ crystals with (002) plane preference and adjacent WO3 nanoparticles, thus achieving ultra-low friction state. This ultra-low friction mechanism of WS₂ coating provides guidance for the design of superlubricating coating for the elevated-temperature atmosphere.