Properties of Molybdenum Disulphide

Overall the electrical properties of molybdenum disulphide are obviously both interesting and complex. The influences of anisotropy, heat, light, contaminants and intercalation have already been shown to be associated with a range of properties from semiconductivity to superconductivity, as well as power generation. 

The presence of water has considerable influence on the lubrication properties of molybdenum disulphide, and their interaction has been studied in some detail. Nevertheless there is still conflict and confusion about this aspect of behaviour, as about many others. 

A number of investigations have been made into the influence of contact load on the frictional properties of molybdenum disulphide. Puchkov and Pashkov used a technique which they claimed to differentiate between shear stress and surface friction. They studied the effect of varying compressive stress on the resistance to... 

The frictional properties of molybdenum disulphide films have been discussed in the previous two chapters, and it is not necessary to repeat the same information here. However, before proceeding to discuss the influence of various factors on the magnitude of the friction, it may be worth emphasizing the fact that friction varies with the gaseous environment, humidity, temperature, load, purity and the state of orientation and consolidation of a film. No-one has ever attempted the huge task of carrying out a parametric study of all these factors together, and most of the published work has failed to define one or more of the influential conditions. As a result it is very difficult to establish absolute values of the coefficient of friction in any particular situation. 

A further complicating fact is that the presence of solid particles in general is now recognised as having a detrimental effect on the life of rolling-element bearings . Thus the improvements reported by Popinceanu et al, Scott and Blackwell, and Kuhnell and Stecki differ from the usual effect of solid particles, and must represent some specific property of molybdenum disulphide. 

Table 16.2 Comparative Properties of Molybdenum Disulphide, Graphite and PTFE...

Potoczek M, Przybylski K, Rekas M, Defect structure and electrical properties of molybdenum disulphide , J Phys Chem Solids, 2006 67 2528-2535. 

Because of the abundance of naturally-occurring molybdenite, there is little real incentive for the synthesis of molybdenum disulphide, but it has been synthesised in small quantities. In most earlier work syntheses have been carried out only for research purposes, either to investigate the synthesis reactions themseives or to compare the properties of natural and synthetic material. Larger quantities seem to have been synthesized only when a country with insufficient natural sources wanted to ensure a reliable indigenous supply. 

The physics and chemistry of molybdenum disulphide intercalation compounds have been reviewed by Woollam and Somoano . Perhaps the most interesting of these properties is superconductivity below 6.9°K, ° obtained with either organic bases or alkali metals. Some of the intercalation compounds show high alkali ion diffusivity, and this has led to them being considered for use in electrodes for high energy-density batteries . 

The application of molybdenum disulphide and other dichalcogenides has become important in electrical brushes, especially in spacecraft, and its electrical properties are of considerable interest. It is therefore surprising to find that there is no clear agreement about its electrical conductivity. 

Apart from its low-friction properties, the other attribute of molybdenum disulphide which is important in lubrication is its very high load-carrying capacity. Having said that, it is then impossible to give a specific value for the load-carrying capacity, because it depends entirely on the form and conditions in which it is used. 

Other industrial applications still exist, but the use of dispersions is cleaner and more convenient. Burnishing of molybdenum disulphide films applied by means of dispersions can be carried out in exactly the same way as for free powder, and the resulting burnished coatings have similar properties, but there are no detailed reports about them other than those of Matsunaga described in Chapter 6. Films from dispersions will also be burnished in use by the effects of sliding under contact load, and their eventual form and behaviour are likely to be similar in all respects to those produced from loose powder. Similar burnished films are likely to be the end-product of many of the softer bonded coatings, and these will be discussed further in Chapter 11. 

Until the late nineteen-eighties it was generally accepted that the desirable sputtered films of molybdenum disulphide had the type of structure shown in Figure 10.5. The plate-like or rod-like crystals are oriented with their basal planes perpendicular to the substrate surface, and are superimposed on an amorphous, or perhaps partly micro-crystalline sub-layer. Such films are often referred to as Type I or Type A films, and their production and properties are now well understood. 

More complex ceramic binders have been used, but in general the detailed composition of the binder has not been described. One very complex one consisting basically of molybdenum disulphide in silica, had the composition in Table 11.4. Molybdenum disulphide has also been incorporated in fused-fluoride lubricant coatings to improve their properties at temperatures below 500°C. In tests over 450°C in air the molybdenum disulphide was effective for one test, but was then no longer available because of oxidation. However, such coatings would presumably have useful lives at high temperature in vacuum or inert atmosphere. 

When molybdenum disulphide is added to a grease, the effect on the flow properties depends critically on the concentration of molybdenum disulphide. Where the quantity added is high enough to cause a major change in the flow properties, the result is a product which no longer behaves as a grease, but behaves like a paste. Such materials are generally used as anti-seize compounds rather than lubricants, and they will be described in Section 13.4. 

The indications are therefore that the incorporation of molybdenum disulphide in greases will usually give an improvement in the load-carrying performance with little or no adverse effect on other grease properties at concentrations up to 10%. 

There is a general tendency for wear to be reduced in molybdenum disulphide greases, but the improvement is not as great nor as consistent as for load-carrying capacity. Friction is also generally lower. The effects on physical and chemical properties of greases are more complicated, and addition of molybdenum disulphide has been shown to affect the stability of the gel structure, the oxidation resistance and the corrosion resistance. 

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. 

Ermakov, A.T., et al, Lubricating Properties of Diffusion Type Coatings of Molybdenum Disulphide, Russian Engineering J., 14, 36, (1974). 

Several different processes have been used, the simplest being by the reaction of hydrogen sulphide with molybdenum pentachloride, or the reaction of sulphur vapour with molybdic oxide or molybdenum metal. The last of these processes has been called the SHS process (Self-Propagating High-Temperature Synthesis) and Russian workers have reported that the product is less contaminated with impurities and has almost identical lubricating properties to natural molybdenum disulphide. The crystal structure is considered in more detail later, but it seems probable that the initial product of syntheses has a disordered... 

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