Transfer of Molybdenum Disulphide

Molybdenum disulphide will transfer readily to suitable clean solid counterfaces from a variety of different sources, including burnished films, bonded films, compacts, composites or single crystals. This ready transfer is of considerable importance in molybdenum disulphide lubrication, for two reasons. 

An obvious disadvantage of easy transfer is that it results in depletion of the molybdenum disulphide in the primary source, and this may be a factor in determining the life of a lubricating film. However, once a smooth transfer film has been formed on a counterface, the transfer process either diminishes considerably or ceases altogether. The problem of depletion of the source is therefore only likely to be serious in systems in which the primary source is a film in continuous or repeated contact with fresh counterface, that is a counterface which is not yet carrying a transfer film. This is most likely to arise in processes such as metalworking, where molybdenum disulphide on a tool or die may be progressively transferred to fresh metal stock. 

The second reason for the importance of transfer is that the lowest coefficients of friction in molybdenum disulphide lubrication are only obtained when a well- 

In some ways transfer of molybdenum disulphide to a metal surface resembles burnishing of loose powder. It follows that descriptions of the mechanism of transfer and of the factors which influence it will be similar in many respects to the description of the burnishing process in Chapter 6. 

As a result there is no clear detailed picture of the way in which transfer takes place, although there is a broad understanding of the nature of the process. It is generally accepted that transfer takes place by the movement of crystallites rather than at the molecular level. Where the source of the molybdenum disulphide is a single crystal, a crystallite in transferring to a counterface must be detached from the source crystal. This can take place by cleavage, fracture or shear, or a combination of one or more of these mechanisms. 

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Lancaster studied the transfer of molybdenum disulphide and graphite to low-carbon steel discs from compacts. The compacts were formed at relatively low pressures, up to 80 MPa, but later work ° indicated that when compacts were formed at higher pressures up to 1500 MPa, there was no obvious difference in the nature of the transfer films produced from them. It can therefore be assumed that the crystallites in the compacts were mainly randomly oriented, although there is some evidence that in unidirectional pressing of a lamellar solid a relatively high degree of orientation occurs in the surface layers. 

Lancaster found that transfer of molybdenum disulphide to very smooth surfaces took place in large aggregated lumps up to 10//m thick on top of a smooth... 

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. 

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. 

Films of molybdenum disulphide can also be formed on solid surfaces by direct transfer from many types of source, including single crystals, composites, and other films. The general subject of transfer is considered in detail in Chapter 8. 

Another chemical aspect which seems to have been clearly established is the generally unsatisfactory performance of molybdenum disulphide on a copper or copper alloy substrate. There is some evidence that sputtered molybdenum disulphide will not adhere properly to copper either in the initial formation of a film or in transfer to a counterface. Reid and Schey also found indications that with their sprayed and burnished films copper became intermingled with the film and transferred to the counterface. 

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... 

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. 

The subject of molybdenum disulphide composites will be described more fully in Chapter 12. Most of them have not been developed specifically to provide transfer lubrication, but it can be assumed that in any situation where molybdenum disulphide is present in a sliding contact, it is capable of producing some transfer to a counterface. 

The PTFE-based retainers typically contain between 15% and 25% of glass fibre reinforcement and 3% to 5% of molybdenum disulphide. The molybdenum disulphide content increases the structural strength, reduces the wear rate, and improves transfer. The transferred lubricant consists mainly of PTFE and molybdenum disulphide. Bearings of this type have been used successfully in spacecraft, but have also been used in many terrestrial industrial applications. 

Molybdenum disulphide is a dark blue-grey or black solid which feels slippery, or greasy, to the touch. Because of its ready transfer to almost any solid surface, and the difficulty in removing it, it is a "dirty" material to handle. It exists in two crystal forms, hexagonal and rhombohedral, and the crystal structure is discussed in detail later. By far the most common form is the hexagonal, and the following data refer to this form. 

The easy transfer to surfaces is probably the reason for the early names "plumbago" and "molybdaena", meaning lead-like, since lead also produces dark marks on paper and fabric. Lead rods were used in ancient times for marking out parchments, and this has led to the expressions "lead pencil" and "black-lead" which have been common until the twentieth century. Both terms now commonly refer to graphite, which resembles molybdenum disulphide in many ways, but the latter was almost certainly used in the same way until the late eighteenth century. The two materials can be easily distinguished by the lower density of graphite. 

The mechanism by which water vapour increases the coefficient of friction has not been established. The effect can arise with well run-in and burnished films in which the exposed surfaces consist for practical purposes entirely of crystallite basal planes, and can typically result in an increase in the coefficient of friction from 0.05 to 0.15. Lancaster pointed out that the higher friction is comparable with that which occurs between a molybdenum disulphide film and a metal substrate during the initial formation of a transferred film. He therefore inferred that the increased friction on exposure to moisture must be due to the replacement of interfaciai sliding by subsurface shear. He postulated that this could only be due to one of the following mechanisms -... 

Because of this ready adherence to a substrate, molybdenum disulphide films can be produced in a wide variety of different ways, including flotation from the surface of a liquid, spraying, brushing or dipping in a volatile dispersant, bonding with adhesive or polymeric compounds, rubbing with powder, transfer, and vacuum sputtering. The nature of the initial film produced depends on the way in which it is applied, and all the important types will be discussed in subsequent chapters. 

The initial thickness of most molybdenum disulphide coatings has an important influence on the performance and life of the coating . The special case of sputtered films is considered in Chapter 10, and there is little information about thickness effects for in situ or transfer films. Many workers have investigated the effect of film thickness on bonded films, but, as was pointed out earlier, much of this work appears confusing, and sometimes contradictory, because of failure to understand and analyse the effects of running-in or burnishing on the consolidation and resulting structure of the films. 

In all these cases, effective lubrication may be obtained by coating only one of the bearing surfaces, and making use of transfer to create a film on the counterface. Two simple precautions need to be taken to make certain of satisfactory operation. The first is to ensure that the primary surface film on the one coated component is not too heavily burnished before assembly, since it is essential for enough molybdenum disulphide to be present to form two viable films. The second precaution is to run in the system under lightly loaded conditions, so that no surface damage or other fault develops before an effective transfer film is formed on the counterface. 

Lubrication by Transfer from a Reservoir is the most important practical application of transfer, as it provides a means for continually supplying molybdenum disulphide to a machine system during operation. 

In practice most composites for use in transfer applications have consisted of three or more components. The most widely-used composites usually contain PTFE as well as molybdenum disulphide, and these must also include a reinforcing material... 

Molybdenum disulphide may be incorporated in a polymer as a friction-reducing additive, to reduce the friction of the composite in sliding contacts, or to provide a reservoir for transfer lubrication. Transfer lubrication has been described in detail in Chapter 8, and the effects of the two different requirements will be discussed briefly in this chapter. 

However, in addition to these layers XPS peak position analysis also suggested evidence of additive interaction on the ferrous surface, from the presence of calcium carbonate (CaCOs) on the surface at 347.leV BE and molybdenum disulphide (M0S2) at 162.2eV. As the samples are etched the CaCOs content is reduced, as is the sulphide layer (M0S2), and phosphates and the substrate material beneath becomes more apparent. When using the Al-Si alloy pin there is evidence of material transfer on top of the iron sulphide/oxide layer. 

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