Molybdenum Disulphide Alone

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. 

Molybdenum disulphide alone can be used as the reservoir material, either in the form of single crystal or as a compact. It is difficult to define the structural strength of single-crystal molybdenum disulphide. Because of its anisotropic nature, the ultimate stress in shear, tension or compression varies critically with the direction of the applied stress in relation to the crystal orientation, as discussed in Chapter 4, but some indication is given by the hardness values on the crystal faces and edges of 1.5 and 8 Mohs respectively. 

Table 9.1 Processes Using Molybdenum Disulphide Alone...

For clarity it may be worth mentioning at this point that in all these techniques other substances may be present in small quantities, either by accident or design, so that the description as "molybdenum disulphide alone" is not absolutely accurate. However, the phrase is useful in practice to distinguish these applications from those in which other materials, especially binders, metals or polymers, are present as significant, or even major, components and have a major effect on properties or performance. 

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 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. 

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... 

The more interesting results obtained were for the use of both additives together, all of which showed a further increase in load-carrying capacity, so that any interaction in these tests was beneficial. The greatest improvement was approximately 39 kg increase in Initial Seizure Load compared with the solution of ZDDP alone. Curiously, 1% of molybdenum disulphide gave only about 18 kg improvement over the ZDDP solution. There was virtually no increase in weld load compared with the ZDDP, and this again suggests that the concentrations of molybdenum disulphide were too low to be very effective. Thorp explained these results on the basis that molybdenum disulphide cannot compete with the base oil for adsorption on the steel surfaces, but can adsorb on top of an adsorbed ZDDP film, but there is no real proof of this explanation. 

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