Diverse mechanisms of friction induced self-organisation into a low-friction material - An overview of WS2 tribofilm formation

A clever way to accomplish low friction in sliding contacts is to use materials that combine high hardness with low shear strength. Such seemingly paradoxical combination of high resistance and low resistance to plastic deformation can only be realized by combining a hard substrate material with a thin easy-shear coating. Some prominent such coating candidates are sulfides and selenides of mainly molybdenum and tungsten. This paper focuses on tungsten disulfide, WS2, and explores the many routes to formation and regeneration of such low-friction tribofilms. The initial surfaces involve various types of coatings, materials and fluids, including W and S in different states, but none of which include crystalline WS2. All formation routes result in remarkably similar, pure crystalline WS2 tribofilms.