Some Reported Coefficients of Friction for Transition Metal Dichalcogenides

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. 

The effective temperatures in the sliding contact will of course, be significantly affected by frictional heating and therefore by sliding speed and load, but the important conclusion is that the increase in friction with rising temperature occurs at a temperature at least 100°C higher with tungsten disulphide than with molybdenum disulphide. 

Both Tsuya and Ducas found that in moist air the friction of tungsten diselenide rose more than that of molybdenum disulphide. More surprisingly Ducas found that the friction of other dichalcogenides fell in moist air. His results are listed in Table 14.8. 

The higher possible operating temperature with tungsten disulphide, due to its greater oxidation resistance, and the resulting maintenance of low friction to higher temperature, have been the main reasons for its use in practical engineering 

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Table 14.6 Some Reported Coefficients of Friction for Transition Metal Dichalcogenides...

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