Insights into the microstructure transformation and ultralow tribological behavior of WS₂ coatings

Tungsten disulfide (WS₂)-based coatings have excellent thermal stability and self-lubricating properties and are considered better lubricating materials at elevated temperatures. This work reports that a WS₂ coating can achieve ultralow friction behavior after sliding for a period of time when the ambient temperature increases to 400 °C; that is, the friction coefficient decreases sharply from 0.08 to approximately 0.025 (a decrease of 68.7%), which is a very interesting phenomenon. There is no doubt that the WS₂ coating will undergo more significant oxidation at a 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 ultralow friction behavior of WS₂ coatings at 400 °C. Therefore, the microscopic structural evolution of the transfer film during the friction process was investigated via Raman and high-resolution transmission electron microscopy. The formation of WO₃ nanocrystals (~15 nm) promotes the structural ordering of WS₂ around WO₃ nanocrystals. Then, incommensurate contact interfaces are spontaneously formed between WS₂ crystals with a (002) plane preference and adjacent WO₃ nanoparticles, thus achieving an ultralow friction state. This ultralow friction mechanism of the WS₂ coating provides guidance for the design of superlubricating coatings for elevated-temperature environments.