Properties of Molybdenum Disulphide Films

The frictional properties of molybdenum disulphide films have been discussed in the previous two chapters, and it is not necessary to repeat the same information here. However, before proceeding to discuss the influence of various factors on the magnitude of the friction, it may be worth emphasizing the fact that friction varies with the gaseous environment, humidity, temperature, load, purity and the state of orientation and consolidation of a film. No-one has ever attempted the huge task of carrying out a parametric study of all these factors together, and most of the published work has failed to define one or more of the influential conditions. As a result it is very difficult to establish absolute values of the coefficient of friction in any particular situation. 

It also follows that it is impossible in most cases to compare the results published by different investigators with any reliability. Where a report shows a variation in the friction with change in one test condition, the relative values can usually be accepted with a great deal of confidence, but the absolute values will often not agree from one investigation to another. In order to predict the value of the friction in a situation, it is necessary to analyse a variety of reports and infer those conditions which are not completely defined. An attempt is made to do this later in this book, and Table 16.7 lists a range of typical coefficients of friction under different conditions. 

In summary, the friction is at its lowest for fully ordered surface films in dry air or vacuum at high load and highest for randomly-oriented films in the presence of water vapour or certain other vapours at low load. 

Except for the anomalous low friction reported for an ultra-pure sputtered film (see Section 10.5) and under radiation (see Section 7.4), typical minimum coefficients of friction in dry air or vacuum are 0.02 to 0.03. For fully disordered films in dry air the static coefficient of friction can be between 0.12 and 0.15, but because reorientation begins rapidly when movement starts any figures for the kinetic friction of disordered films must be suspect. 

The effects of moisture and other vapours will be discussed in detail in the next section, but the friction can be more than trebled in the presence of moisture. 

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