Interaction Between Molybdenum Disulphide and Liquids

Lubricant Load-carrying Capacity (Jaw-Load,lb) Wear Life at 10001b Jaw Load  

Gansheimer observed a difference between the effects of polar and non-polar liquids. Polar liquids caused a much greater and more rapid increase in friction than non-polar liquids, and he suggested that the adverse effects were due to adsorption. Adsorption would be expected to be greater with polar molecules, both on the edge-sites of the molybdenum disulphide and on the metal substrate. 

There may also be more specific effects of certain liquids, such as the softening of the binders in bonded films suggested by Hopkins and Campbell . Any adsorption of a liquid on the surface basal planes, or any intercalation between lamellae would interfere with the ideal charge distributions and inter-lamellar spacing found with uncontaminated molybdenum disulphide. These effects would in fact be similar to the adverse effects of water vapour and other vapours on the friction of burnished films. 

The implication of this work is that the presence of a burnished film on a metal surface should help to improve lubrication by a mineral oil, especially where there might otherwise be some tendency for partial oil starvation. It should be remembered, however, that this film was produced by burnishing molybdenum disulphide powder. Commercial dispersions, bonded films, composites or greases, as well as fully formulated lubricating oils all contain other substances which may significantly affect wetting behaviour. 

While molybdenum disulphide lubrication is adversely affected by liquids, liquid lubrication can be improved by molybdenum disulphide. There is in fact a considerable industry based on the use of molybdenum disulphide in liquids, in dispersions, lubricating oils, greases, anti-seizes and pastes. There is an important contradiction in the fact that in many situations the lubricating action of molybdenum disulphide is either partly or completed destroyed by the presence of liquids, whereas in other situations it can provide useful lubrication benefits in a liquid medium. A great deal of effort has been applied in attempts to establish the mechanism and the conditions by which lubrication by molybdenum disulphide can occur in the presence of a liquid. 

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