Effects of Vapours and Other Contaminants

The work of Jamison has been referred to earlier , in which he showed how the electronic structure of the hexagonal molybdenum disulphide crystal is uniquely favourable for producing low sliding friction. Any contamination by vapours or reagents Is therefore inherently likely to affect this favourable structure adversely rather than beneficially, and to produce an increase in friction rather than a decrease. This has in fact been shown experimentally to be the case, and a wide range of publications over many years reported increases in sliding friction parallel to the basal planes of the crystallites, resulting from the presence of moisture or other vapours 

After a shut-down in high vacuum of 10 Torr or better it was sometimes found that there is a brief increase in the friction. This disappears after a short period of operation . In other cases there may be a decrease in friction during a shutdown. The effect is known as the stop time effect . Matsunaga s first investigation confirmed that the presence of contaminants was involved in this phenomenon, and showed that the friction increase on re-starting could be described by an equation based on a simple model of contaminant diffusion within the lubricant film. 

Further investigations confirmed that with a variety of contaminants and with several different types of molybdenum disulphide film the presence of contaminant caused a decrease in friction and its removal an increase in friction. The 

The mechanism by which water vapour increases the coefficient of friction has not been established. The effect can arise with well run-in and burnished films in which the exposed surfaces consist for practical purposes entirely of crystallite basal planes, and can typically result in an increase in the coefficient of friction from 0.05 to 0.15. Lancaster pointed out that the higher friction is comparable with that which occurs between a molybdenum disulphide film and a metal substrate during the initial formation of a transferred film. He therefore inferred that the increased friction on exposure to moisture must be due to the replacement of interfaciai sliding by subsurface shear. He postulated that this could only be due to one of the following mechanisms - 

He went on to point out that both mechanisms are inconsistent with other evidence, such as (1) that the increase in friction on vapour admission is virtually instantaneous, and occurs with compacts in the same way as with films on metals, and (2) that vapours adsorb more readily on crystallite edges than on basal planes. 

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