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Molybdenum disulfide is used as a dry lubricant in, e.g. greases, dispersions, friction materials and bonded coatings.
Molybdenum-sulfur complexes may be used in suspension but more commonly dissolved in lubricating oils at concentrations of a few percent.
Molybdenum disulfide, MoS2, the most common natural form of molybdenum, is extracted from the ore and then purified for direct use in lubrication. Since molybdenum disulfide is of geothermal origin, it has the durability to withstand heat and pressure. This is particularly so if small amounts of sulfur are available to react with iron and provide a sulfide layer which is compatible with MoS2 in maintaining the lubricating film.
A number of unique properties distinguish molybdenum disulfide from other solid lubricants:
A low coefficient of friction (0.03-0.06) which, unlike graphite, is inherent and not a result of absorbed films or gases;
A strong affinity for metallic surfaces;
Film forming structure;
A yield strength as high as 3450 MPa (500 x 103 psi);
Stability in the presence of most solvents;
Effective lubricating properties from cryogenic temperatures to about 350oC in air (1200oC in inert or vacuum conditions).
Molybdenum disulfide will perform as a lubricant in vacuo where graphite fails.
A combination of molybdate and water soluble sulfides can provide both lubrication and corrosion inhibition in cutting fluids and metal forming materials. Oil soluble molybdenum-sulfur compounds, such as thiophosphates and thiocarbamates, provide engine protection against wear, oxidation and corrosion. Several commercial manufactures supply these additives to the lubrication industry.
Some molybdenum disulfide formulations are given in Table 1.