Low Friction Mechanism Survey of Tungsten Disulfide by Using XRD, XPS, and XAFS

Abstract

Solid lubricants are used to lubricate the mechanical equipment of various spacecraft and launch vehicles. Tungsten disulfide (WS2) is one of the solid lubricants that can be used in a variety of applications at elevated temperatures in outer space. In this study, WS2 coatings on a metal disc were applied via the shot peening method using WS2 powder (diameter of 2.0 μm). Friction experiments were carried out to investigate the friction characteristics of WS2 using a pin-on-disc-type rotary tribometer at high temperature in a vacuum. The friction coefficients of the WS2 shot peening coating were extremely low at room temperature and increased with the test temperature. Nevertheless, WS2 coatings demonstrated superior frictional properties at elevated temperatures and thus they are applicable at 400 °C in vacuum. All WS2 specimens showed a preferred orientation of the (0002) basal plane parallel to the disc surface. Atomic arrangements of the second nearest neighbor atoms were distorted from the original sites during the wear process. The room temperature friction tests of the shot peened WS2 specimen, annealed at 400 °C, demonstrated that the friction coefficients of the specimen with a higher degree of planar alignment on the (0002) plane were larger than those of the specimens with planner imperfections. Lattice defects such as stacking faults and dislocations assisted in the sliding motions of the adjacent sulfur atoms on the (0002) plane by extremely low shear stress.