Tungsten disulphide

Sliney indicated that the comparison between tungsten disulphide and molybdenum disulphide is more complicated. He showed that both compounds oxidise at temperatures as low as 300°C, and that below 340°C tungsten disulphide oxidises more rapidly than molybdenum disulphide. The oxidation rates in that temperature range are very low, and with loosely-compacted powders of 1jt/m average particle size the time required to oxidise 50% of the materials at 300 C was approximately one to two weeks. Above 340°C molybdenum disulphide oxidises more rapidly than tungsten disulphide, and it is at these higher temperatures that the relative oxidation resistance is more important. The relationships are shown in Figure 14.4. 

Figure 14.4 Oxidation Rates of Molybdenum Disulphide and Tungsten Disulphide (Ref.533)...

The changes in friction and wear behaviour with change in temperature in air are presumably determined by oxidation. Figure 14.5 shows that at low temperatures both molybdenum disulphide and tungsten disulphide have similar coefficients of friction of about 0.07 to 0.08, but a marked increase in friction occurs at 400°C with molybdenum disulphide and 600"C with tungsten disulphide. The corresponding temperatures for increasing friction reported by Tsuya were 300 C and 450°C. 

The effective temperatures in the sliding contact will of course, be significantly affected by frictional heating and therefore by sliding speed and load, but the important conclusion is that the increase in friction with rising temperature occurs at a temperature at least 100°C higher with tungsten disulphide than with molybdenum disulphide. 

The higher possible operating temperature with tungsten disulphide, due to its greater oxidation resistance, and the resulting maintenance of low friction to higher temperature, have been the main reasons for its use in practical engineering... 

Kirner reported the successful use of plasma spraying for niobium diselenide, niobium ditelluride, and a mixture of tungsten disulphide and silver, but the performances in high vacuum and high temperature were inferior to those obtained with molybdenum disulphide. There has been a great deal of Russian work on the... 

The temperature limits are more curious. There seems to be no evidence from other published literature for the much higher temperature performance in vacuum of the niobium diselenide and tungsten diselenide than tungsten disulphide, and the very inferior performance of molybdenum disulphide. Similarly, there is no obvious reason... 

The composite of 52.9% tungsten disulphide in a cobalt and silver matrix, described as AF-SL 14, was one of a series of composites developed by the USAF, and was tested satisfactorily by several investigators " . 

There is very little information available about corrosion risks with the other lubricating dichalcogenides. Spalvins mentioned that the friction and endurance of a sputtered tungsten disulphide film deteriorated considerably when tested in the atmosphere instead of in vacuum, and the deterioration was associated with formation of sulphuric acid and corrosion of the substrate. These results are not very different from those obtained with molybdenum disulphide in the same period, and there seems to be no reason to expect the problem to be any greater or any less with the other dichalcogenides. 

Davis, B.T. and Presland, A.E. B., Tribolytic Deposition of Tungsten Disulphide, Nature Physical Science, 230, 119, (1971). 

The usual industrial catalyst consists of molybdenum (or occasionally tungsten) disulphide supported on y-alumina and promoted by cobalt (or more rarely nickel). An account was given in the earlier Report and more recent reviews of hds catalysts and reactions have been published. Delmon has published an excellent critique of work on the structure of Co-Mo catalysts. Technical processes have been reviewed, ... 

Another material of this kind is tungsten disulphide, originally developed by NASA for aerospace applications, now also applicable to specialty industries [58]. Ti3SiC2 is a thermodynamically stable, nano-layered, ternary carbide and part of a family of over 50 ternary carbides and nitrides, the MAX phases [62]. These phases are a new class of solids possessing unique combinations of properties they are readily machinable, relatively soft for ceramics, but elastically stiff, and electrically and thermally conductive. They combine the good properties of both metals and ceramics that could lead to this technology contributing to future lubricant developments. 

A thermogravimetric method has been used to investigate the kinetics of reaction of tungsten metal with pure sulphur vapour at temperatures between 295 and 550 °C. The rate of sulphurization of the metal was rapid and increased systematically with temperature to a rate maximum at 460—475 °C thereafter the rates decreased. The initial sulphurization rate increased, in a complex manner, with increasing sulphur pressure for temperatures below the rate maximum above this temperature the rate increased directly with the partial pressure of Sg. Tungsten disulphide was the only reaction product detected under all experimental conditions. 

Prasad, S. V. and Zaenski, J. S., Tribology of tungsten disulphide (WS2) characterization of wear-induced itanslsT films. Journal of Materials Science Letters, 12, 1993, 1413-1415. 

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