Friction with Time of Sliding for a Bonded Molybdenum Disulphide Film

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. 

It is unfortunate that with very few exceptions the effect of film hardness on the consolidation behaviour has not been taken into account in reporting coating performance. Even where the authors of a report have obviously been aware of the problems of running in, their test results may show enormous differences between similar films. For example, in a paper by Hopkins and Campbell the authors made the comment All friction coefficients were measured after a brief but complete run- 

The effect of humidity on the friction of a bonded molybdenum disulphide film has been shown to be broadly similar to the effect on a burnished film, as described in Chapter 7. The coefficient of friction increased slowly as the relative humidity increased to about 60%, and then progressively more rapidly as it was further increased to 90%. 

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Figure 11.2 Variation of Friction with Time of Sliding for a Bonded Molybdenum Disulphide Film (Ref. 132)...

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