In many cases, grease and oil cannot be used in extreme special environments such as vacuum, high or low temperature. Solid lubricants can show different lubrication characteristics according to the special application conditions and environment of bearings. To select the most suitable solid lubricants, it is necessary to understand the characteristics of different solid lubricants. Typical solid lubricants for bearings are shown in Table 1.
Table 1 typical solid lubricants for bearings The crystal structure, thermal stability, friction coefficient, load capacity, dust generation and gas release performance of solid lubricants such as soft metals, layered solid lubricating materials and high polymer materials are given in Table 1. These data are the basis for selecting the most suitable solid lubricants. (1) Soft metal Silver (Ag) and lead (PB) can be used as lubricants for very high vacuum bearings and high temperature bearings because of their good self-healing ability and low steam pressure. In many cases, the surface of rolling element (steel ball) used in bearings is covered with a thin film (usually about 0.0 to several microns) by ion plating or spray plating. However, due to the fact that AG is easy to oxidize, it must be carefully applied in air. Other soft metal lubricants include gold (AU), cadmium (CD), indium (in), etc. (2) Layered solid lubricating materialsMolybdenum disulfide (MoS2) with layered lattice structure is a well-known solid lubricant, which can be coated on cages and bearing rings by spray plating and thermosetting methods. Molybdenum disulfide is mainly used in vacuum bearings, while tungsten disulfide (WS2) and graphite (c) are sometimes used as cage materials (composite materials), but are usually used as lubricants for high temperature bearings. (3) High polymer materials High polymer materials, such as polytetrafluoroethylene (PTFE) and polyimide (PI), can be used to coat rolling elements and cages, and also used as cage materials. Because of their excellent chemical resistance, good adhesion and relatively unaffected by the environment, these materials are commonly used in bearings in dust environment and bearings moving repeatedly in atmosphere and vacuum environment.