Burnished Films from Powder

In general use the word burnish means to produce a shiny or glossy surface on a material by rubbing. This may be achieved by a variety of techniques from the use of a soft pad to the use of a hard burnishing tool, depending on the nature and material of the surface. 

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. 

Micrographs obtained by Takahashi and Kashiwaya from bulk solid. 

There are believed to be three different mechanisms by which the first layer of crystallites becomes attached, although none of the three has been proved beyond doubt to take place, nor their relative importance. 

The first of these mechanisms is a simple infilling of low spots on the surface. There is no doubt that such an infilling takes place, as shown by the work of Johnston and Moore and Bartz and Muller mentioned previously. Further support for this process is provided by the fact that optimum film formation is strongly influenced by the surface texture of the substrate. It is difficult to accept that this geometrical process alone can account for any effective attachment of crystals, since loose molybdenum disulphide powder applied to a machined surface shows little tendency to form an attached film unless some pressure is applied to it. Compression of powder into a low spot would almost certainly be needed to interlock particles with each other and with the slopes of the cavity, and this interlocking could be expected to be retained when the applied pressure is removed. 

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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... 

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. 

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. 

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. 

There had obviously been a serious communication gap with respect to the consolidation of bonded films. The phenomenon of burnishing of powder had been described almost twenty years earlier, and the effects of running-in for bonded films had been known to scientists for several years. In spite of this, important users had not been aware of the necessity, either from trade articles, or the product manufacturer s literature, or, I was ashamed to realise, through my own first two ESRO reports. 

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