Burnished Films

A further implication is that the coefficient of friction of a fully-burnished film is probably determined more by the burnishing pressure then by the subsequent operating pressure, since the films represented by the horizontal line apparently have similar surfaces while the operating pressures vary from 1.5 to lOOOMPa. 

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 -... 

In the simplest case, burnished molybdenum disulphide films are produced by applying a smooth sliding pressure to molybdenum disulphide powder against the hard surface which is to be coated. This can be done by means of a pad of soft material such as fabric or cotton waste under hand pressure, but a variety of mechanical devices has been used in order to produce more consistent films. Figure 6.1 shows an example of a device used to produce burnished films on rings or cylinders . 

In any case, there seems to be little or no advantage in producing burnished films thicker than about 20//m. 

Three separate processes will take place during running-in or burnishing of a soft film. These processes are compression, shear and crystallite re-orientation, and the nature of the burnished film will depend on the relative extents of these processes. 

Brudnyi and Karmadonov described the degree of crystal orientation at the surface of a burnished film in terms of a reflection or texture coefficient C, such that... 

The reflection coefficient of a fully burnished film was found to be the same as that of the MoSj (001) plane and their reflectivities were also the same. They therefore concluded that the surface of the film consisted of an assembly of (001) basal planes. They also inferred that a highly reflective surface occurs when the coefficient of friction is a minimum. 

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. 

The most important aspect of pre-treatment is to provide the optimum surface roughness to give mechanical "keying" of the film to the surface. It is generally accepted that the optimum surface finish is between 0.5 and 2.0//m c.I.a. Most of the published work on surface finish effects has been related to bonded films, and the subject is described in detail in Chapter 11, but similar surface finishes are also desirable for burnished films, especially on hard substrates. However, for sputtered films, which may be only 1//m thick, much smoother substrate surfaces give the maximum life. 

Figure 7.7 Effect of Temperature on Life of a Burnished Film (Ref. 149)...

The thickness of a burnished film can be built up by adding further powder and re-burnishing to a total thickness of at least 15 //m. Such a film may be uniformly consolidated throughout its thickness, so that it will behave as a hard coating, and the wear life will increase with increasing film thickness . 

Test conditions burnished film, conformal contact, ... 

Centers re-examined the published information about the performance of antimony trioxide, and rejected both the sacrificial oxidation theory and the oxide interaction concept. He concluded that antimony trioxide and certain other oxides and sulphides improve performance by providing a soft component which enables molybdenum disulphide more readily to acquire the optimal basal plane orientation for low friction and wear. However, he used compacts, and it is difficult to understand how his conclusion could apply to burnished films or well run-in films which already approach perfect crystallite orientation. His rejection of the sacrificial oxidation theory is based on a report by Gardos and McConnell , which does not include all of... 

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. 

There are basically seven different ways in which molybdenum disulphide can be used alone, namely as a free powder, dispersed in a liquid, as a compact, by in situ formation, as a burnished film, as a transfer film or in a sputtered film. Some... 

Burnished film High quality Laborious, limited Shafts, bushes. 

A more versatile way of applying free powder is in a gas stream. This has the potential advantages of more uniform distribution, penetration into less accessible positions, and re-supply during operation. Several applications of this technique have been described, and some are summarised in Table 9.2. These applications have apparently been restricted to the lubrication of rolling contacts, probably because of the inherent difficulty of applying satisfactory bonded or burnished films to rolling elements, and the problem of re-supply. 

The fundamental aspects of the formation and properties of burnished films have been discussed in detail in Chapters 6 and 7. At this point it is proposed only to describe some practical factors in the preparation and use of burnished films produced directly from powder. 

The optimum conditions for producing burnished films with good endurance and load-carrying capacity and low friction are -... 

Since commercial dispersions and bonded coatings became widely available, the use of powder-burnishing has probably declined, but applications to ball-bearings have been reported " , and a 1981 report described a cold-extrusion machine for steel components in which a burnished film of molybdenum disulphide was applied to the steel billets automatically prior to extrusion . This resulted in a reduction in extrusion loads and improved quality. 

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. 

The influence of the substrate will be discussed later, but in general substrates for sputtered molybdenum disulphide films are very smooth, with surface textures of the order of 0.1 //m. The gross keying effect which has been found important for adhesion of bonded or burnished films is therefore not normally available. It has been... 

The effect of humidity on the friction of a bonded molybdenum disulphide film has been shown to be broadly similar to the effect on a burnished film, as described in Chapter 7. The coefficient of friction increased slowly as the relative humidity increased to about 60%, and then progressively more rapidly as it was further increased to 90%. 

There are indications that polymeric binders may protect films to some extent against oxidation by moist air in storage. A few tests on a fully-burnished soft film with an inorganic binder also showed no deterioration during extended storage in air at 70% relative humidity and 60°C. This gives some confirmation to the concept that in a fully-burnished film the exposed surface consists almost entirely of inert basal planes, and is therefore more resistant to chemical attack. 

As a result Bertrand and Vukasovitch of Climax Molybdenum carried out an intensive study of different cleaning techniques applied to various forms of molybdenum disulphide film. The types of film used were a burnished film from a sub-micron sized powder, a burnished film from a soap-containing molybdenum disulphide powder, an air-cured inorganic-bonded coating, and a film formed from a dispersion in oil. The films were formed on steel pins which were then pressed through an undersized bush. This ensured highly-loaded contact on the film, and consolidation of the film. The burnished powder and the resin-bonded films could be... 

There may also be more specific effects of certain liquids, such as the softening of the binders in bonded films suggested by Hopkins and Campbell . Any adsorption of a liquid on the surface basal planes, or any intercalation between lamellae would interfere with the ideal charge distributions and inter-lamellar spacing found with uncontaminated molybdenum disulphide. These effects would in fact be similar to the adverse effects of water vapour and other vapours on the friction of burnished films. 

One apparently beneficial interaction was described by Braithwaite and Greene . They found that the wettability of steel and cast iron surfaces to a lubricating oil was markedly increased by the presence of a burnished molybdenum disulphide film, in terms of both rate of spread of oil droplets and the area wetted. Both effects are illustrated in Figure 13.1. The improvement in wettability was influenced by the extent of oxidation of the powder used to produce the burnished film. The least oxidised powder was the most effective, but even the most highly oxidised powders had some beneficial effect. 

The implication of this work is that the presence of a burnished film on a metal surface should help to improve lubrication by a mineral oil, especially where there might otherwise be some tendency for partial oil starvation. It should be remembered, however, that this film was produced by burnishing molybdenum disulphide powder. Commercial dispersions, bonded films, composites or greases, as well as fully formulated lubricating oils all contain other substances which may significantly affect wetting behaviour. 

It is fair to say that some of the early reports were ill-founded, and magnified by rumour. There were also categorical reports from some users that molybdenum disulphide, even as free powder or burnished films, positively prevented or cured corrosion problems. These must in most cases have been equally ill-founded, although they provide an interesting parallel with the comments of Cramer in 1764, which were quoted in Chapter 1. 

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