Molybdenum disulphide properties

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. 

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... 

Although eighty percent of molybdenum production is used in the metallurgical industries, the fastest-growing sector of use is in chemicals, which has more than doubled in the past thirty years. The most widely-used compound is probably molybdenum disulphide. Apart from its use in lubrication, it is used as an additive to thermoplastics, where it improves the mechanical and thermal properties. It also has a number of potential applications in high density electric batteries, although the extent of commercial use is not clear. 

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. 

It is usual to state " that molybdenum disulphide is a p type semiconductor, while niobium diselenide is a conductor, However Mikhailov has shown that pure molybdenum disulphide is a conductor and that only specimens having a developed film of oxidised material on the surface of the lamellae show semiconductor properties. Correspondingly a composite containing 15% was found to have a specific contact resistance of only 0.4 m.ohm.cm. compared with 0.7 m.ohm.cm ... 

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. 

However, Jamison has intensively studied the relationship between the crystal and electronic structures of layer-lattice solid lubricants and their frictional properties, and has shown that other aspects of its electron distribution give a particularly favourable structure to molybdenum disulphide. In its structure the molybdenum atoms in one layer do not lie directly above or below the molybdenum atoms in an adjacent layer, but are opposite holes in that layer. The sulphur atoms are directly opposite other sulphur atoms, but do not have any unpaired electrons to provide strong bonding. It is this lack of electronic interactions which leads to the high interlamellar spacing, and low interlamellar attraction. 

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... 

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. 

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. 

Molybdenum disulphide is intrinsically an excellent lubricant. No details exist about the way in which it was used in the distant past, but almost certainly it was first used as a free solid. While not deliberately mixed with any other material, it would have been fairly impure. In recent years it has become usual for it to be used in conjunction with other substances to improve some specific property or to overcome some specific problem. From such points of view as ease of application, re-supply, quality control, corrosion prevention or service life, there can be significant advantages in using it in combination with other materials. Nevertheless, there are still many situations for which its use unmixed with other substances can provide a satisfactory, or even optimum, solution. 

For clarity it may be worth mentioning at this point that in all these techniques other substances may be present in small quantities, either by accident or design, so that the description as "molybdenum disulphide alone" is not absolutely accurate. However, the phrase is useful in practice to distinguish these applications from those in which other materials, especially binders, metals or polymers, are present as significant, or even major, components and have a major effect on properties or performance. 

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. 

The initial properties of bonded films as applied vary considerably depending on the type of binder, the ratio of binder to molybdenum disulphide and other components, the nature of the drying or curing process, and the way in which the film was applied, in particular there is a wide variation in hardness from very soft (silicones and alkyds) to very hard (metals and ceramics), and this has important implications for the way in which a film is used and even the purpose for which it is used. 

Like other composite properties, the wear resistance is influenced by the orientation of reinforcing fibres. Sung and Suh found that with biaxially-oriented glass fibre and molybdenum disulphide in a PTFE matrix (Duroid 5813), the wear resistance was greatest when the highest proportion of fibre was normal to the sliding surface. The same effect was found with a graphite fibre/epoxy composite and a Kevlar fibre/epoxy composite. 

It has become the preferred solution for many applications to use molybdenum disulphide and glass fibre together in PTFE, and Table 12.7 shows the properties of two such ternary composites. This development reflects the results of many pioneering research projects such as those of Young et al, Scibbe et al and... 

Table 12.8 Properties of Nylon With or Without Molybdenum Disulphide...

Early attempts to overcome this problem included the addition of hygroscopic compounds, liquid lubricants, metal halides and solid lubricants, including PTFE and molybdenum disulphide ° to the carbon to provide lubrication. However, one of the basic reasons for using carbon for brushes is its good lubricant properties, so that when these are lost and must be supplemented by the addition of solid lubricants, there is no dominant reason for continuing to use carbon. 

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. 

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 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. 

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