Film Life and Mechanism of Failure

There are probably four important modes of failure of molybdenum disulphide films, namely loss of adhesion or film break-up), wear, oxidation, and possibly fatigue. 

One form of film break-up has been mentioned previously, in which the surface of a relatively thick film becomes consolidated over a soft unconsolidated layer. This can lead to shear in the soft layer, especially with high non-conformal loading, and the consolidated layer will break away. Loss of adhesion and film break-up can also occur, especially with a bonded film, if the surface pre-treatment has been badly performed, so that the coating simply fails to adhere and breaks away. These cases may be considered as premature failures caused by poor film preparation. 

Where a film has been well prepared and burnished to a high degree of consolidation and reflectivity, and is run against a similar film on the counterface, sliding will take place almost entirely between the smooth surfaces. Under those circumstances the wear rate will be extremely low, and may for a period even be nil, so that the mean film thickness remains unchanged for the remainder of the film 

When lubrication failure finally occurs, the surface film contains large quantities of molybdic oxide and sulphate, and very little molybdenum disulphide. Presumably the disordered nature of the products causes a volume increase, which leads to the formation of the blisters if the nature of the contact permits it. Kinner considered that blister formation was unlikely to be a cause of failure in conformal contacts because the presence of uniform loading over the surface prevents any vertical development. The same oxidation process takes place in conformal sliding, except to the extent that conformal contact inhibits oxygen access, and even if blisters are physically prevented from forming, there must be a slow increase in friction and film break-up. 

In the whole life cycle, a beneficial effect of humidity has been demonstrated, in that a relative humidity of 7% at the bearing surfaces (20% in the environment) gave an increase of over 100% in the duration of the smooth running period compared with a completely dry system. This may be due to improved running-in in a slightly humid atmosphere with perhaps a thicker fully-ordered surface film. 

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