Molybdenum Disulphide Friction with Humidity

Another theory of the reason for increased friction in the presence of moisture was proposed by Gao et al . They found that in a humid environment molybdenum disulphide films were more readily thinned by sliding contact, due to increased ease of interlamellar slip. They suggested that adsorption of water softened the films, and that resulting increased deformation by plowing in sliding contact led to a poorly oriented film and thus to increased friction. However, they considered that this was a short-term reversible effect which was not in conflict with theories of chemical breakdown. Gao et al also poiinted out the possibility that an increase In friction is caused by capillary pressure effects of moisture at asperity contacts. 

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Figure 7.4 Variation of Molybdenum Disulphide Friction with Humidity (Ref. 155)...

The actual coefficient of friction of a molybdenum disulphide film will depend on the integrity of the film, contact pressure, temperature, humidity, film thickness and presence of contaminants. For a pure, smooth, dense, properly-oriented film at high contact pressure in a clean, dry atmosphere in unidirectional sliding, coefficients of friction as low as 0.02 have been reported. With impurities, poor orientation, humidity and low pressure, the coefficient of friction may be as high as 0.3. 

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. 

In general the effect of moisture on molybdenum disulphide is to increase both friction and wear rate. Figure 7.1 shows the effect of relative humidity on the coefficient of static friction for annular washers coated with an unbonded molybdenum disulphide film. The actual values of the coefficient of friction are high, because the films were unburnished, but the increase from 0.12 to 0.49 is significant. 

Subject to this complication, the friction properties of the group are generally similar to those of molybdenum disulphide. The actual coefficients of friction vary with load, speed, temperature and humidity, but some reported figures are shown in Table 14.6. The chemical properties are also similar to those of molybdenum disulphide. They are resistant to attack by water, alkalis and most acids, but are attacked by aqua regia and hot concentrated hydrochloric, nitric or sulphuric acids. The most significant differences are in their electrical conductivity and their oxidation resistance. 

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