Friction and Wear Properties of Bonded Films

The initial properties of bonded films as applied vary considerably depending on the type of binder, the ratio of binder to molybdenum disulphide and other components, the nature of the drying or curing process, and the way in which the film was applied, in particular there is a wide variation in hardness from very soft (silicones and alkyds) to very hard (metals and ceramics), and this has important implications for the way in which a film is used and even the purpose for which it is used. 

In a bonded coating as applied, the molybdenum disulphide crystallites are randomly oriented . The static coefficient of friction is therefore quite high, typically between 0.1 and 0.3, and the films can be quite abrasive. The actual degree of abrasivity depends on the hardness of the coating, the particle size of the coating, and the way in which the coated surface is loaded against a counterface. 

Better control and more repeatable consolidation can be achieved by a separate burnishing process before the coated surface is put into normal operation. Burnishing is carried out in much the same way as in the burnishing of loose powder. It is in fact easier because a uniform layer of molybdenum disulphide is already in position, thus avoiding the problems of controlling loose powder. 

Films can be burnished with almost any inert smooth soft material such as leather or a low-modulus polymer, but a simple method is with a hand-held soft cloth, or a similar cloth mounted on a harder backing device. As with spraying, better control and repeatability are achieved with some form of mechanical system in which the contact pressure, speed and duration can be accurately determined and maintained. These requirements are similar to those of a wear test machine, and in fact a simple wear test rig can often be used as a burnishing device. The contact pressure should initially be low, and should be increased progressively to a pressure approaching that which will be applied to the film in service. 

In practice there is a continuous range of coating hardnesses varying from very soft to very hard, and the hardness is determined not only by the binder but by the concentrations of molybdenum disulphide and other solid components. The definition of hardness is also relative to the service contact loads. A coating of intermediate hardness may behave like a hard film under low contact pressures or like a soft film under high contact pressures. As a result, it is very difficult to compare the quoted load-carrying capacity and wear lives of different films tested under different conditions by different research groups. 

---