Molybdenum disulphide

The base lubricant is usually a petroleum oil while the thickener usually consists of a soap or soap mixture. In addition they may contain small amounts of free alkali, free fatty acid, glycerine, anti-oxidant, extreme-pressure agent, graphite or molybdenum disulphide. 

Materials which reduce the friction of mouldings and other finished products when these are rubbed against adjacent materials which may or may not be of the same composition. The most well-known examples here are graphite and molybdenum disulphide used in quantities of the order of 1-2% in nylons and other thermoplastics used in gear and bearing applications. 

Self-lubricating grades are of particular value in some gear and bearing applications. One commercial nylon compound incorporates 0.20% molybdenum disulphide and 1 % of graphite whilst many other commercial compounds contain only one of these two lubricants. 

In order to reduce the coefficient of friction in bearing applications small amounts of molybdenum disulphide are incorporated although in quantity this material may cause problems through acidic impurities. Blends of polyacetal and FIFE (20-25% P l FE) have a very low coefficient of friction (as low as 0.02)... 

It has been a dream for a tribologiest to create a motion with a super low friction or even no friction between two contact surfaces. In order to reduce friction, great efforts have been made to seek materials that can exhibit lower friction coefficients. It is well known that friction coefficients of high quality lubricants, e.g., polytetrafluoroethylene (PTFE), graphite, molybdenum disulphide (M0S2), etc., are hardly reduced below a limit of 0,01,... 

Danielyan, N. G. et al., Mod. Phys. Lett. B.. 1991, 5(19), 1301 Molybdenum disulphide is capable of forming a thermite type mixture with magnesium, which may explode on grinding. 

Molybdenum is also used as a catalyst in petroleum refining, as a pigment for paints and printer s ink, and as a high-temperature lubricant (molybdenum disulphide-MoS ) for use by spacecraft and high-performance automobiles. 

Molybdenum disulphide is another layered intercalation host, similar to titanium disulphide. This material occurs naturally and formed the basis of the positive electrode for the first high production cylindrical AA-sized cell, manufactured by Moli Energy Ltd in Canada in the 1980s. Cycle life of 100-300 was achieved in practical cells with average discharge voltages of 1.8 V for low rates, giving a theoretical density of approximately 300 Wh/kg. 

This battery was based on a lithium metal anode and a molybdenum disulphide cathode ... 

The electrochemical discharge-charge process of the battery is based on the intercalation/de-intercalation of lithium molybdenum disulphide ... 

Most semiconductor surfaces are reactive and form oxide surfaces in air or water and thus suffer from the same faults as metals in this respect. This is not true of the layer lattice materials which form valence bonds only in the layer plane. Freshly cleaved surfaces of pyrolytic graphite or molybdenum disulphide are thus promising materials, though they are only available in small areas. In principle it might be possible to form LB layers on these materials by epitaxy but very little progress has so far been made in this direction. 

Molybdenum disulphide is capable of forming a thermite type mixture with magnesium, which may explode on grinding. 

Lansdown, A.R., Molybdenum Disulphide Lubrication, Elsevier, Amsterdam, 1999. 

Haycock, D., M. Casrai, and D. S. Urch (1978). Electronic structure of molybdenum disulphide angle resolved x-ray spectroscopy. Japan J. Appl. Phys. 17, 138 0. 

Where does molybdenum disulphide fit into this picture Certainly the use of solid lubricants must be ancient. Loose sand, ice and powdered snow have already been mentioned, but other slippery solids which were available and even locally abundant were graphite, mica, talc and molybdenum disulphide. 

Unfortunately both words were used to describe several different things. "Plumbago" meant graphite, which was easily confused with molybdenum disulphide. "Molybdena" (or molybdaena) also meant graphite, as well as various ores or salts of lead. At certain times and places molybdenite would have been more readily available than graphite, and may well be the subject of some of the early references, but the lack of continuity in the written record makes it impossible to establish when or how the name became more closely associated with molybdenum disulphide. 

The word "molybdenite" which is now clearly identified with natural molybdenum disulphide, and sometimes more generally used for any sample of it... 

The two sections I have put in italics must refer to molybdenum disulphide and not graphite. "Glimmer-Stones" are micas and the various micas all have specific gravities between 2.7 and 3.3, Molybdenite has a specific gravity of 4.6 to 4.75, while that of natural graphites varies between 2.05 and 2.25. Incidentally, the comparison between Molybdaena and mica is very acute, in view of their crystallographic similarity. Similarly the effect of "the strongest open fire" on molybdenite would be to oxidise it to the white or yellow molybdenum trioxide. 

Molybdenum disulphide became readily available in reasonable purity after 1918, but interest in its use was still slow to develop. Johnson has suggested that technical consideration of it mainly followed the establishment of its crystal structure by Pauling and Dickinson in 1923, but if that is so then progress was still very sparse. Koehler used molybdenum disulphide in a composition, patented in 1927, which also included talc, mica and in some cases graphite, but in retrospect that... 

The most important single development in the use of molybdenum disulphide as a lubricant was probably the initiation of studies by the US National Advisory Committee for Aeronautics (NACA) in 1946. Their first report was published in 1948. This work by NACA and its successor the National Aeronautics and Space Administration (NASA) laid the foundations for the great expansion in use during the past forty years. The overall increase in activity in this period was so rapid that by 1952 Climax published a list of 154 different applications. 

Applications in aircraft also increased very quickly. In 1959 Boeing reported from 150 to 200 applications of solid-film lubricants in B-52, KC-135 and Boeing 707 aircraft without any unserviceability reports, and the applications included critical aircraft components. By 1966 over 1000 applications of solid-film lubricants were reported on the North American B-70, and many of these involved molybdenum disulphide. Applications on the General Dynamics F-111 included the heavily-loaded variable geometry wing pivot. 

Van Wyk reported an increase of 100% in sales of molybdenum disulphide as a solid lubricant between 1962 and 1972. However, this rapid expansion led to a number of adverse reports of its performance. BOAC reported accelerated corrosion of Boeing 707 undercarriage bogeys associated with its use, although other reports indicated that corrosion problems on the bogeys disappeared when conventional... 

During the same period use in road vehicles had become widespread. The first reported application was to the leaf-springs of Rolls-Royce cars in 1955, but by 1962 applications were reported by many major car and commercial vehicle manufacturers. Most of these were concerned with such components as ball-joints, shackles, pins, and steering linkages. There was also an increasing use of molybdenum disulphide dispersions in engine oils, but this was generally initiated by the user rather than the vehicle manufacturer. 

In terms of volume, the most important area of application of molybdenum disulphide lubrication is now the automotive field. A major part of this volume consists of molybdenum disulphide greases, and these applications are discussed in more detail in Chapter 13. There is little doubt that their use has made a significant contribution to the extended chassis lubrication intervals in vehicles. 

Utilisation of molybdenum disulphide generally has been increasing steadily, and it seems clear that in many areas its use has achieved technical respectibility after the exaggerated claims and complaints of the 1950 s and early 1960 s. The aviation industry has always been a leading user, but there is now a more widespread acceptance of molybdenum disulphide in various forms. Among the other industries which have accepted its use in a wide variety of applications are metalworking and railways. 

Table 1.1 Some Spacecraft Applications of Molybdenum Disulphide... 

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