Burnishing of Soft Films

Loose molybdenum disulphide powder has only a limited tendency to adhere to solid surfaces. Very fine powders will attach loosely to metal surfaces, but the coarser grades commonly used for lubrication will not. However, there are several techniques which will produce soft adherent coatings. 

The flotation process used by Matsunaga and Tsuya involves floating fine molybdenum disulphide powder on the surface of a liquid, and lifting it off onto the surface of a flat metal plate. After the liquid is removed by draining and drying, a weakly-adherent thin uniform film of the powder is left on the metal surface. The film appears to consist of randomly-oriented crystals, and has been extensively used for research purposes, but not for use in practical machinery. 

Much thicker films can be produced by the use of dispersions of molybdenum disulphide powder in volatile liquids. The dispersion can be applied to a solid surface by dipping, brushing or spraying, and the liquid is then allowed to evaporate, either at room temperature or with additional heating. Although the dry films are much 

Three separate processes will take place during running-in or burnishing of a soft film. These processes are compression, shear and crystallite re-orientation, and the nature of the burnished film will depend on the relative extents of these processes. 

The compressive yield stress is also low, so that pressure applied normal to the surface results in compression and densification of the film, with a resulting increase in the limiting shear stress and the compressive yield stress. Burnishing without excessive loss of film material is therefore best achieved if compression takes place simultaneously with sliding, in other words if the counterface is curved or slightiy inclined to the film surface. 

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