A study of the optimization mechanism of solid lubricant concentration in self-lubricating composite

Four NiMoS2 self-lubricating composites with varied MoS2 concentrations were hot-pressed. Their friction and wear behaviors were tested using a ball-on-disk wear machine at both room temperature and 250°C. Morphologies of the worn composite surfaces were observed under SEM. It was shown that the self-lubricating properties of the composites change in accordance with the formation situation of the surface lubricating film. With increasing MoS2 concentration, the integrity of the formed lubricating film increases, and both the friction coefficient and wear rate decrease almost linearly. Above 60% MoS2, however, severe spalling of the lubricating film occurs, and the self-lubricating property drops remarkably. From the viewpoint of thin solid film lubrication, corresponding frictional models were proposed for theoretically analyzing the optimum MoS2 concentration. It was indicated that the optimum solid lubricant concentration may be determined as such a level that a homogeneous and continuous lubricating film is just formed on the whole frictional surface, while the mechanical property of the composite has not declined enough to cause a remarkable decrease in the film's self-lubricity.