Some Organo-Molybdenum Compounds Studied for Lubricant Performance

The various types of compound and their mechanisms of action have been reviewed by Mitchell . He concluded that the question of the extent to which the molybdenum additives decompose to produce molybdenum disulphide, if at all, was not yet resolved and certainly depended on the type of compound and the operating conditions. There was little doubt that molybdenum dithiocarbamates form molybdenum disulphide, probably by decomposition at hot spots caused by asperity interactions. It was less certain that molybdenum dithiophosphates form molybdenum disulphide, although mixtures of zinc dialkyidithiophosphate and molybdenum complexes had been shown to do so . 

Similar materials were used by Skeldon et al to produce a self-lubricating surface film on aluminium. They first produced a porous alumina film on the surface by anodizing, and then re-anodized in an electrolyte containing 0.01 M ammonium tetrathiomolybdate. They then found that the pores of the primary anodized film contained mainly amorphous molybdenum trisulphide, and this was converted by vacuum annealing to hexagonal molybdenum disulphide. The film gave a marked improvement in wear resistance. 

Jin and Zhou found that molybdenum disulphide was formed by a different approach which did not depend on the use of a soluble molybdenum compound. They were studying the performance of a mixed ether/ester oil containing 3% of a zinc dialkyidithiophosphate at 450°C, in contact with a molybdenum alloy. They found that an in situ film of graphite and molybdenum disulphide was formed, and the 

The effect of operating conditions in the formation of these in situ films is likely to be complex. In 1972 Forbes showed that even under fairly mild conditions with an anti-wear additive such as tricresyl phosphate surface films are formed which contain both phosphorus and organic fragments. Similarly Cann showed that a cross-linked thick surface film is formed from zinc dialkyidithiophosphate under mild rubbing conditions. It seems probable that at least some of the soluble molybdenum compounds would also experience partial breakdown under mild rubbing conditions, and that these would encourage the formation of protective surface films. 

Under more severe rubbing conditions, further breakdown occurs, and the products are likely to be mainly inorganic, such as phosphates and sulphides. Finally, where contacts are severe enough to remove the protective oxide and leave exposed fresh unoxidised metal, Morecroft found that organic molecules, even saturated hydrocarbons, will break down to give fragments such as carbon, hydrogen and carbon monoxide. 

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Table 9.7 Some Organo-Molybdenum Compounds Studied for Lubricant Performance...

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