Molybdenum shear structures

Fig. 14. Schematic representation of the way spillover oxygen inhibits the formation of shear structures leading to molybdenum suboxidcs during ally tic oxidation (1,27,28).

A number of substances such as graphite, talc, and molybdenum disulfide have sheetlike crystal structures, and it might be supposed that the shear strength along such layers would be small and hence the coefficient of friction. It is true... 

Molybdenum disulhde (M0S2), graphite, hexagonal boron nitride, and boric acid are examples of lamella materials commonly applied as solid lubricants. The self-lubricating nature of the materials results from the lamella crystalline structure that can shear easily to provide low friction. Some of these materials used to be added to oils and greases in powder forms to enhance their lubricity. Attention has been shifted in recent years to the production and use of nanosize particles of M0S2, WS2, and graphite to be dispersed in liquid lubricants, which yields substantial decreases in friction and wear. 

By increasing the value given to n, the Mo03 structure is ultimately obtained in all these series. It is evident that, at least in theory, topochemical reduction of MoOs should be made possible by such a shear mechanism initially appearing along very widely separated dislocation lines this may be the clue to the structure of the discolored molybdenum trioxide crystals which are the main product when Mo03 is heated strongly in vacuo (12). 

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 . ... 

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. 

In the case of the equivalent niobium compounds, the same electronic effects are not present. He postulated that in pure stoichiometric niobium disulphide this results in poor lubrication. When good lubrication behaviour is observed, it is probably caused by additional niobium atoms intercalated between the lamellae, which contribute non-bonding electrons. On the basis of this theory, non-bonded atoms intercalated between the lamellae can increase the inter-lamellar spacing, whereas bonded intercalated atoms increase the resistance to inter-lamellar shear, and therefore the friction. However, an alternative interpretation is that certain intercalated atoms alter the interaction between the niobium atoms, allowing rearrangement to the 2H structure of molybdenum disulphide, and it is the favourable structure which provides good lubrication performance. 

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

Hexagonal BN powder exhibits the same characteristics of solid lubricants as seen in graphite and molybdenum disulfide. These include crystalline structure, low shear strength, adherence of the solid lubricant film, low abrasivity, and thermochemical stability [3]. In many instances, (h)BN exceeds the performance levels of these conventional solid lubricant characteristics, particularly adherence and thermochemical stability. Eigure 19.5 [39] is a typical scanning electron micrograph (SEM) of a commercial BN powder. 

The electrochemical behavior of nonstoichiometric molybdenum oxides M0O3 5 suggests that both charmel/site size and electronic conductivity are the predominant factors influencing the extent of reversible lithium incorporation by metal oxides with framework structures shear-related to those of Re03 and M0O3. Distortions and nonequivalence of the sites available for LU accommodation should affect the thermodynamics of the reduction processes. The nonstoichiometric... 

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