Friction of Molybdenum Disulphide

Molybdenum disulphide adheres readily to most substrates. As a result, when sliding takes place between molybdenum disulphide and a solid surface, the phenomena of adhesion and possibly junction growth will take place, and high frictional forces will be generated. This adhesion will be augmented by the action of burnishing (see Chapter 6). However, while adhesive forces between molybdenum disulphide and solid substrates are usually high, the cohesive forces between lamellae of molybdenum disulphide are low. It follows that the coefficient of friction between lamellae will be much lower than that between a lamella and a ductile substrate, and slip will take place preferentially between lamellae. 

The same is not true of all layer-lattice materials . In some the bond energies between layers are very high, for example ionic bonding in the case of mica and n-bonding in the case of graphite. For these it is only when the bond energies are reduced that the shear strength and therefore the coefficient of friction are low. Bond 

The low inter-lamellar attractive forces in molybdenum disulphide consist only of weak Van der Waals forces. In addition the separation distance between the sulphur layers of adjacent lamellae is 3.49A, and is larger than the 3.1 A thickness of an individual lamella. Cleavage or shear of molybdenum disulphide crystals between adjacent lamellae is therefore inherently likely to be easy. 

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. 

With some of the other layer-lattice solid lubricants, the natural electronic structure does not provide the same benefit but a favourable structure can be brought about by intercalation of metallic atoms into their crystal structures, and this is described in more detail in Chapter 14. 

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This exactly follows the relationship found experimentally by several investigators, and provides strong support for the generally-accepted view that the low friction of molybdenum disulphide is due to easy shear between lamellae. 

Figure 5.4 Friction of Molybdenum Disulphide Films Over a Wide Range of Pressures (Ref. 112)...

It can therefore be accepted that whether it is in the natural crystalline form or in a consolidated film, the friction of molybdenum disulphide is adhesive friction, and its low magnitude is due to the easy shear between adjacent lamellae which is made possible by the unusually favourable crystal and electronic structure. However, it must be remembered that the correct orientation of the crystallites is essential for the maintenance of low friction. The shear strength is low only parallel to the basal plane of the lamellae. In other directions the shear strength is high, so that the coefficient of friction will also be high . ... 

It is now generally accepted that the very low sliding friction of molybdenum disulphide is due to the very low shear strength parallel to the basal plane of the crystal lamellae, compared with the high strength or hardness perpendicular to the basal plane. The low shear strength is caused by the wide separation distance... 

In order to reduce the coefficient of friction in bearing applications small amounts of molybdenum disulphide are incorporated although in quantity this material may cause problems through acidic impurities. Blends of polyacetal and FIFE (20-25% P l FE) have a very low coefficient of friction (as low as 0.02)... 

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 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. 

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. 

If this interpretation is correct, then the figure provides strong support for the argument that interfacial friction between molybdenum disulphide films and intracrystalline friction are both determined by the same factors. This is because at the point of intersection between the horizontal and decreasing portions of the curve, where interfacial friction changes to intracrystalline friction, there is no discontinuity, and the same friction value is given for both types of slip. 

Further investigations confirmed that with a variety of contaminants and with several different types of molybdenum disulphide film the presence of contaminant caused a decrease in friction and its removal an increase in friction. The... 

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. 

Figure 6.3 Change in Friction with Running Time for a Rubbed Film of Molybdenum Disulphide (Ref. 132)...

Gansheimer showed that chemical reactions take place between the sulphur of molybdenum disulphide and metallic surfaces during sliding contact, but established no direct correlation with friction, adhesion or wear life. 

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. 

Figure 7.2 Change in Friction of a Rubbed Film of Molybdenum Disulphide with Time of Sliding (Ref. 153)...

Figure 7.4 Variation of Molybdenum Disulphide Friction with Humidity (Ref. 155)...

Vacuum in itself has no harmful effects on the lubrication performance of molybdenum disulphide. The effect of conventional atmospheres containing oxygen and water vapour is to increase the friction and decrease the wear life of molybdenum disulphide lubricants. It follows that in high vacuum, where such contaminants are absent, the friction and wear behaviour are generally improved. 

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. 

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-... 

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 upper limit for the concentration of molybdenum disulphide in such a simple composite is imposed by the low friction and low limiting shear stress of the... 

At one time it was hoped that they might be used for the manufacture of self-lubricating machine components, but compacts of molybdenum disulphide alone had inadequate structural strength. Because the crystallite orientation is random, their coefficients of friction tend to be high, for example Matsunaga et reported... 

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. 

As a result of re-orientation, the film becomes less abrasive, and the coefficient of friction decreases. Because re-orientation begins rapidly with sliding contact, measurements of kinetic friction will often not show the decrease in friction unless friction monitoring is continuous from the commencement of sliding, when a trace such as that shown in Figure 11.2 may be obtained . This is very similar to the trace shown in Figure 6.3 for a rubbed film of molybdenum disulphide powder. 

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