moly disulfide (chemical formula mos) is a silvery black solid that is extracted from the molybdenum mineral molybdenite. It is insoluble in water and dilute acids. Like its cousins tungsten disulfide and boron nitride, moly disulfide is used as a dry lubricant and can be used in applications such as aerospace machinery, two-stroke engines, and gun barrels to reduce friction between the bullet and the gun. It can also be combined with other metals to form composites for specialized purposes. moly disulfide layers can be produced as a powder for direct use, or thin films can be deposited on a substrate using chemical vapor deposition techniques to produce self-lubricating coatings.

Its ability to function as a lubricant is intimately related to its layer structure. Different coordination and stacking sequences lead to three distinct structures of MoS2: 1T, 2H and 3R (Figure 1). The two lower layers of the MoS2 structure are stronger and more resistant to oxidation than the upper layers. This allows it to maintain its lubricant properties in environments where graphite can not.

Molybdenum disulfide exhibits excellent resistance to thermal oxidation at temperatures up to 350oC in an oxidizing environment, but its lubricant properties are reduced to some degree beyond this temperature. It is also very sensitive to corrosion by sulfides, including sulfuric acid, with a corrosion rate similar to that of iron. It is a moderately toxic substance with LD50 values in the range of 30 to 100 g/kg for the sulfide and 15 to 60 g/kg for molybdate trioxide, calcium molybdate, and molybdate dusts. Its toxicity is mainly due to metal ions (Mo2+, Mn2+, and Mg2+) which are present in the soluble forms of the compound.