Use of Molybdenum Disulphide

The most important single development in the use of molybdenum disulphide as a lubricant was probably the initiation of studies by the US National Advisory Committee for Aeronautics (NACA) in 1946. Their first report was published in 1948. This work by NACA and its successor the National Aeronautics and Space Administration (NASA) laid the foundations for the great expansion in use during the past forty years. The overall increase in activity in this period was so rapid that by 1952 Climax published a list of 154 different applications. 

During the same period use in road vehicles had become widespread. The first reported application was to the leaf-springs of Rolls-Royce cars in 1955, but by 1962 applications were reported by many major car and commercial vehicle manufacturers. Most of these were concerned with such components as ball-joints, shackles, pins, and steering linkages. There was also an increasing use of molybdenum disulphide dispersions in engine oils, but this was generally initiated by the user rather than the vehicle manufacturer. 

Several of the thermoplastics benefit from lubrication with small quantities of liquids, including oils and water, and their friction and wear resistance may then be superior to those of conventionally lubricated bearing materials. The use of molybdenum disulphide fillers probably conveys little or no advantage in these... 

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. 

Several publications have described the use of molybdenum disulphide pastes for the lubrication of screw threads. This is an application in which the paste assists assembly and subsequently acts as an anti-seize. The critical factors in screw or bolt assembly are the friction between male and female thread and between bolt-head and workpiece, and their effect on the relationship between assembly torque and bolt tension (clamping force). The stiffness and integrity of a bolted assembly are determined by the bolt tensions, but since it is impracticable to measure bolt tension directly, the required bolt tension is controlled by the torque applied to the bolt or nut. However, the relationship between assembly torque and bolt tension is determined by head friction and thread friction. If the friction is reduced, the required bolt tension will be obtained at a lower assembly torque. 

One other area in which the use of molybdenum disulphide may be undesirable for pre-assembly or anti-seize use is when nickel alloys are used in critical applications, especially at high temperatures. Such alloys are extensively used in aviation gas turbine engines, and there have been cases in which serious failures of gas turbines have been attributed to the effect of molybdenum disulphide-based lubricants on high-temperature components Provided these two hazards are kept in mind, molybdenum disulphide dispersions and pastes are of great value as antiseize compounds and as assembly aids. 

Table 16.5 lists a number of engineering components with the most appropriate form of molybdenum disulphide to use for lubricating them. Table 16.6 summarises some of the important factors to consider in designing for the use of molybdenum disulphide films. 

Table 16.6 Important Factors in Designing for the Use of Molybdenum Disulphide Films...

In the past fifteen years the situation with regard to the technology of molybdenum disulphide lubrication has stabilised in many respects, and a measure of consensus has been reached about some of the mechanisms involved. The use of molybdenum disulphide has become routine in some industries, and there are many well-established and reputable commercial products available. Except in the high-technology field of physical deposition techniques, especially sputtering, the output of new research publications has fallen from perhaps two hundred a year in the nineteen-seventies to fewer than ten a year in the nineteen-nineties. 

Van Wyk reported an increase of 100% in sales of molybdenum disulphide as a solid lubricant between 1962 and 1972. However, this rapid expansion led to a number of adverse reports of its performance. BOAC reported accelerated corrosion of Boeing 707 undercarriage bogeys associated with its use, although other reports indicated that corrosion problems on the bogeys disappeared when conventional... 

In terms of volume, the most important area of application of molybdenum disulphide lubrication is now the automotive field. A major part of this volume consists of molybdenum disulphide greases, and these applications are discussed in more detail in Chapter 13. There is little doubt that their use has made a significant contribution to the extended chassis lubrication intervals in vehicles. 

Utilisation of molybdenum disulphide generally has been increasing steadily, and it seems clear that in many areas its use has achieved technical respectibility after the exaggerated claims and complaints of the 1950 s and early 1960 s. The aviation industry has always been a leading user, but there is now a more widespread acceptance of molybdenum disulphide in various forms. Among the other industries which have accepted its use in a wide variety of applications are metalworking and railways. 

A vast amount of information has been published on the testing of molybdenum disulphide materials for space use. It seems probable that most if not all American satellites and spacecraft have contained some application of molybdenum disulphide, and a number of space applications are listed in Table 1.1. A notable early example was its use on the extendible legs of the Apollo Lunar Module in 1969. Application of molybdenum disulphide in more conventional bearing systems is described in Chapters 9 to 13, but the wide variety of lubricant uses is shown in Table 1.2 by a list of applications not described in more detail elsewhere in the book. 

Molybdenum trioxide, or "molybdic oxide", melts at 795 C and boils at 1155°C at normal atmospheric pressure. It has a fairly high hardness and in many early publications was described as a harmful abrasive product from the oxidation of molybdenum disulphide in service. In fact the trioxide is not highly abrasive, being possibly less abrasive in some circumstances than molybdenum disulphide itself , and it has been recommended for use as a lubricant at elevated temperatures to 700°C. 

These organo-molybdenum compounds are in commercial use, for example as friction-modifiers. Their mechanism of action is considered in Chapter 9, in connection with the in situ production of molybdenum disulphide films. 

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 oxidation behaviour of molybdenum disulphide is of considerable practical importance. The presence of oxidation products causes an increase in friction, and the life of a burnished or bonded film in air may be largely determined by oxidation. The maximum temperature for satisfactory use in air or any other oxidising medium is therefore also controlled by oxidation. 

While studying the influence of molybdenum disulphide on the wettability behaviour of steel, Braithwaite and Greene found that the pH of a powder fell from 6.07 to 3.55 in one hour and to 2.60 in two hours when heated at 350°C in air. This powder was more typical of a fine lubricant grade, having a BET specific surface of 3 m /g, but the temperature used was very much higher and it is difficult to compare their results with those of Ducas because of the different technique used to assess the acidity produced. 

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 general subject of film formation is considered in the next chapter, but at this point it will be useful to mention a few aspects of film behaviour in order to clarify the nature of friction between lubricated components. In the first place, if two surfaces slide against one another with only free molybdenum disulphide powder present as a lubricant, then initially the coefficient of friction is quite high. It is only when a smooth adherent film has formed on at least one of the surfaces that lower friction occurs. In the second place, if a smooth adherent film of molybdenum disulphide is present on only one of the surfaces, then the lowest possible friction will still not be obtained. It is only when a useful film is also present on the second surface, either formed in advance or formed by transfer from the film on the first surface, that the lowest values of friction will be found. 

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. 

It is obvious from simple geometrical considerations that in any process for applying molybdenum disulphide to a solid substrate, the first contact is likely to be at the peaks of the asperities on the substrate. However, Johnston and Moore were the first to study the burnishing process in detail, using a cylinder covered with fabric to apply molybdenum disulphide powder to a flat copper substrate. They found that in their tests the first hundred traverses of the burnishing device filled the low spots on the substrate so as to produce a smooth surface. Subsequent traverses built up further layers of molybdenum disulphide onto the film, and the film thickness appeared to increase indefinitely without any significant subsequent change in the texture of the surface. 

Much thicker films can be produced by the use of dispersions of molybdenum disulphide powder in volatile liquids. The dispersion can be applied to a solid surface by dipping, brushing or spraying, and the liquid is then allowed to evaporate, either at room temperature or with additional heating. Although the dry films are much... 

Fleischauer has carried out a detailed analysis of the electronic structure of molybdenum disulphide. This analysis showed that all the accessible orbital electrons for both molybdenum and sulphur are used in intralaver bonding, leaving only high-energy antibonding orbitals available for bonding between layers or for basal surface adhesion to substrates. There are no accessible orbital electrons on either... 

A very large variety of different chemicals have been used in conjunction with molybdenum disulphide to improve different aspects of performance of bonded films or composites, and these will be more suitably discussed in Chapters 10 and 11. A few have been used more generally to improve the life of molybdenum disulphide films, and it will be more appropriate to describe them here. 

He used spectrochemical techniques to study the composition of burnished molybdenum disulphide films from preparation to eventual failure. He found that the film surface at failure contained little molybdenum disulphide but contained molybdic oxide, sulphur, sulphate and iron compounds. In the presence of antimony trioxide, however, there was preferential oxidation of the antimony trioxide to the tetroxide Sb204. On heating a mixture of molybdenum disulphide and antimony trioxide in air at 500 C and 600 C, he found by X-ray diffraction that the products were mixtures of molybdenum disulphide and antimony tetroxide. Neither molybdic oxide M0O3 nor antimony trioxide was present. When molybdenum disulphide alone was heated under the same conditions, it was almost completely converted to molybdic oxide. 

There are basically three ways in which transfer of molybdenum disulphide can be deliberately used for lubrication. These are the pre-coating of a bearing surface with a molybdenum disulphide film, transfer from one bearing surface to an uncoated counterface, and continuing replenishment from a reservoir during machine operation. 

Lancaster has suggested that because transfer is an inefficient process, the concentration of solid lubricant in a reservoir composite must be at least 25%. Successful results have been claimed for composites with much lower concentrations, but comparisons are difficult because different workers have used different criteria for successful operation. Certainly most of the successful composites which consist only of molybdenum disulphide in a strong matrix have contained at least 20% of the solid lubricant. 

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