Oleophilic molybdenum disulphide

An alternative method of improving wettability was first described by Groszek , and has been described in Section 7.6. This is the production of oleophilic molybdenum disulphide by ball milling in oil. The product is readily wetted by organic liquids, and can in fact be used as a grease thickener. Groszek described dispersions in several different organic liquids and it is clear that the improved wettability leads to the formation of much more stable dispersions. 

Apart from the frictional effects, two other performance characteristics need to be considered, namely load-carrying capacity and wear. Most of the available information on these subjects has been obtained from the practical use of commercial materials and will be discussed later, but Groszek and Witheridge found that molybdenum disulphide ground conventionally in air gave a small increase in Mean Hertz Load and Weld Load in a Four-Ball Test as 5% dispersions in a mineral oil. However, oleophilic molybdenum disulphide, ground in n-heptane, gave a 170% increase in Mean Hertz Load and a 260% increase in Weld Load. 

This is of course consistent with the work of Groszek and others on oleophilic molybdenum disulphide. They showed that molybdenum disulphide which had been ground in a non-polar liquid (n-heptane) was more readily wetted by nonpolar liquids than similar powders which had been ground in air and were therefore, by implication, more highly oxidised. 

The situation is quite different when moiybdenum disulphide powder is used in a liquid. As has been shown, friction reduction and film formation only arise when the geometry permits particles of the powder to be trapped between bearing surfaces, and probably sheared. Such break-up of particles within a non-polar liquid is directly comparable with the procedure used by Goszek for the production of oleophilic molybdenum disulphide, so that the resulting fractured particles will presumably also be oleophilic. 

Oleophilic molybdenum disulphide (see Chapters 7 and 9) has been reported as... 

Groszek and Witheridge showed that such oleophilic powders are highly effective in increasing load-carrying capacity. They provided some evidence that the effectiveness is due to the fact that oleophilic particles preferentially attach to bearing surfaces in a basal plane orientation. In that orientation the molybdenum disulphide would provide less opportunity for capillary penetration, and would be less susceptible to chemical reactions. 

The relative inefficiency of film formation in a liquid may be due to either one or both of two different factors. The first is that since the molybdenum disulphide originally present in a dispersion will normally have been processed or ground in air the proportion of oleophilic particles present will be small until the dispersion has been subjected to considerable further comminution in the oil. This possiblity is supported... 

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