Molybdenum disulphide reduction

Figure 7.8 Reduction in Friction of an In Situ Molybdenum Disulphide with Irradiation (Ref. 164 - 166)...

On the basis of their results, they put forward an "oxide interaction concept , according to which the friction and wear of bonded films, composites and simple transfer films of molybdenum disulphide are improved by the presence of low-melting oxides which either combine easily or form desirable eutectics with molybdenum oxides. Such a concept would provide valuable guidance to the development of better solid lubricants, but, as Buckley pointed out, the authors had in fact produced no evidence for the "oxide interaction" concept. The lower friction can in fact be explained by a reduction in the rate of oxidation, since the films exhibiting lower friction had all been run at some stage in air. 

An alternative explanation is suggested by some later work of Fleischauer and Bauer °°. They found that the best performance of transfer films of molybdenum disulphide was obtained when molybdic oxide was present in the lowest layers of the film adjacent to the steel surface. Oxidation to molybdic oxide is increased in the presence of moisture, so that reduction of moisture content due to frictional heating may reduce the amount of molybdic oxide present, and thus have a direct adverse effect on transfer film life as well as reducing the rate of film formation. Fleischauer and Bauer also found indications that transfer film life was improved if a slight excess of sulphur was present at the interface between the film and the substrate. The presence of molybdic oxide or excess sulphur are undesirable in the bulk of the lubricant and especially on the sliding surface and they suggested that for optimum... 

The behaviour of these oil-soluble compounds is of great practical importance, because of the potential value of effective oil-soluble additives, especially for friction reduction. Whether they act by in situ formation of molybdenum disulphide is of more academic interest, since an understanding of their mechanism is important mainly in indicating the best lines of future development. 

Since commercial dispersions and bonded coatings became widely available, the use of powder-burnishing has probably declined, but applications to ball-bearings have been reported " , and a 1981 report described a cold-extrusion machine for steel components in which a burnished film of molybdenum disulphide was applied to the steel billets automatically prior to extrusion . This resulted in a reduction in extrusion loads and improved quality. 

The main advantage of polyimides is that they retain their structural strength to temperatures approaching 350 C, but they also have good outgassing characteristics in vacuum. For such applications the thermal stability of a PTFE filler is a limitation, and 5% of molybdenum disulphide may be added to a polyimide for friction reduction. The wear resistance of this material was inferior to some of the filled nylons, acetals and polycarbonates, and it would probably be chosen mainly for its high temperature stability. 

This would also explain the major effects of the grafting, which were reduced friction and easier plastic deformation. The marked reduction in friction is shown in Figure 12.4. In retrospect it seems probable that the grafted molybdenum disulphide would have been particularly useful in polymeric composites because of improved wettability, reduced friction and easier plastic deformation, but it is not clear that they were ever used in this way. 

Rolek et al tried to distinguish between viscosity, temperature and composition in their influence on the effects of molybdenum disulphide dispersions in oils. They used a series of white oils, mineral oils, and the same mineral oils with some polar additives removed. The results were not entirely clear, but they supported Tsuya s findings (see below) on the effect of viscosity. However, they also seemed to indicate a more specific effect of temperature and the presence of polar additives. It seemed that there was a specific inhibiting effect of polar additives in suppressing any friction reduction by the molybdenum disulphide. In addition they identified a temperature effect distinct from its effect on viscosity, and suggested that this might be related to a transition temperature, possibly associated with desorption of polar compounds. 

Under boundary or mixed lubrication conditions there is usually a reduction in friction when molybdenum disulphide is dispersed in an oil but it can have no useful effect under fully hydrodynamic lubrication conditions, because there is no contact between the bearing surfaces. 

Thus any advantage of molybdenum disulphide in an oil in reducing friction will be limited to the boundary and mixed lubrication regions, where the reduction in friction may be considerable. One theoretical analysis suggested that the influence of the dispersed powder depends only on particle shape, size and concentration , or in other words that the dispersed powder is simply forming a physical barrier between the interacting surfaces. It would follow that the same effect could be produced by other dispersed solids, and this was confirmed by studies with zinc sulphide, zinc pyrophosphate and calcium hydroxide. 

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. 

They also reported a general reduction in sludge, varnish, wear and oil thickening due to oxidation, with no adverse effect in any of the test engines. The improvement in deposits was also confirmed by Muller and Bartz , who reported improved piston cleanliness. Further investigation suggested that this was due to an improvement in oxidation stability, but the way in which molybdenum disulphide can improve oxidation stability is not clear. Other beneficial mechanisms which have been suggested are improved dispersion of solid contaminants by the dispersant used to stabilise the molybdenum disulphide dispersion, and prevention of surface deposits by the formation of a molybdenum disulphide film. 

Overall, similar improvements in wear in engines have also been found Shadow et al used radioactive tracers to study the wear in piston engines. They found a consistent decrease in piston wear in engines when a dispersion of molybdenum disulphide in the engine oil was used. Figure 13.3 shows the effect on wear rate when the molybdenum disulphide was added, and the reduction in wear rate of the bearings was over 40%. 

There have also been several laboratory studies which showed significant reductions in wear in a variety of tests at molybdenum disulphide concentrations between 0.5% and 10%, but unfortunately most of these studies used a paraffinic white oil, so that no useful practical inferences can be obtained from them. White oils are not typical of mineral lubricating oils in their friction and wear behaviour, as they have inferior anti-wear properties and are known to be untypical in their response to additives. 

When reduced wear is associated with the addition of molybdenum disulphide to a lubricating oil, the type of wear is always likely to be adhesive. The effects of molybdenum disulphide in such a case would be a direct reduction in the degree of adhesion by the interpolation of a molybdenum disulphide film, as well as a reduction in the overall stress levels due to lowering of friction. 

Niederhauser et al ° used the alternative approach of coating the molybdenum disulphide in a sputtered coating with PTFE to protect it from attack by moisture. They found that there was some reduction in corrosion of a steel substrate, although a greater reduction was obtained when the coating was sputtered onto a rhodium interlayer. Zhai et al also reported some improvement in the stability of a composite plating when cerium was incorporated to coat the molybdenum disulphide and reduce moisture attack, but they did not directly assess corrosion resistance. 

The degree of improvement in a few years is clearly indicated by the marked reduction in the number of publications about corrosion after 1976. With the elimination of graphite and of excessive ball-milling, and the incorporation of corrosion inhibitors, it should now be possible to use molybdenum disulphide in bonded films, composites, greases or anti-seizes with confidence that no corrosion problem will arise, although commonsence precautions should always be taken ... 

The main reaction is the electrochemical oxidation of lithium metal and the reduction of molybdenum disulphide via the intercalation reaction ... 

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