Molybdenum Disulfide (MoS₂) Coatings Lubrication Review

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Molybdenum disulfide is the inorganic compound with the formula MoS₂.

This black crystalline sulfide of molybdenum occurs as the mineral molybdenite. It is the principal ore from which molybdenum metal is extracted. MoS2 is relatively unreactive, being unaffected by dilute acids and oxygen. In its appearance and feel, molybdenum disulfide is similar to graphite. Indeed, like graphite, it is widely used as a solid lubricant because of its low friction properties and robustness.

Molybdenum disulfide (MoS₂ ). Like graphite, MoS₂ has a low friction coefficient, but, unlike  graphite, it does not rely on adsorbed vapors or moisture. In fact, adsorbed vapors may actually result in a slight, but insignificant, increase in friction. MoS₂ also has greater load-carrying capacity and its manufacturing quality is better controlled. Thermal stability in non oxidizing environments is acceptable to 1100C (2012 °F), but in air it may be reduced to a range of 350 to 400 °C (662 to 752 °F).

There are currently no clear lubrication alternatives to molybdenum disulfide or the very similar tungsten disulfide that can resist temperatures higher than 350 °C in oxidizing environments. Research has been conducted on compacted oxide layer glazes, which form during metallic surface sliding wear at several hundred degrees Celsius. However, because these oxide layers are physically-unstable, their use has currently not proven practical.

Applications:

MoS₂ with particle sizes in the range of 1–100 µm is a common dry lubricant. Few alternatives exist that can confer the high lubricity and stability up to 350 °C in oxidizing environments. Sliding friction tests of MoS2 using a pin on disc tester at low loads (0.1–2 N) give friction coefficient values of <0.1.

Molybdenum disulfide is often a component of blends and composites where low friction is sought. A variety of oils and greases are used, because they retain their lubricity even in cases of almost complete oil loss, thus finding a use in critical applications such as aircraft engines. When added to plastics, MoS₂ forms a composite with improved strength as well as reduced friction. Polymers that have been filled with MoS₂ include nylon (with the trade name Nylatron), Teflon, and Vespel. Self-lubricating composite coatings for high-temperature applications have been developed consisting of molybdenum disulfide and titanium nitride by chemical vapor deposition.

MoS₂ is often used in two-stroke engines; e.g., motorcycle engines. It is also used in CV and universal joints. MoS₂-coatings allow bullets easier passage through the rifle barrel causing less barrel fouling allowing the barrel to retain ballistic accuracy much longer. This resistance to barrel fouling comes at a cost of lower muzzle velocity with the same load due to a decreased chamber pressure. MoS2 is applied to bearings in ultra-high vacuum applications up to 10−9 torr (at −226 to 399 °C). The lubricant is applied by burnishing and the excess is wiped from the bearing surface.

MoS₂ is also used in ski wax to prevent static buildup in dry snow conditions and to add glide when sliding in dirty snow.

It is often used in two-stroke engines; e.g., motorcycle engines. MoS₂ is also used in CV and universal joints. During the Vietnam War, the molybdenum disulfide product "Dri-Slide" was used to lubricate weapons, although it was supplied from private sources, not the military. MoS₂ -coatings allow bullets easier passage through the rifle barrel with less deformation and better ballistic accuracy.

Typical Molybdenum Disulfide Coating Characteristics
Load Capacity (ASTM 2625 B)	250,000 psi
Wear life (ASTM 2625 A)	250 min.
Salt Spray Resistance (ASTM B117)	500 hrs.
Coefficient of Friction (ASTM D1894)	.19 static .16 kinetic
Abrasion resistance (mil STD 141A)	10,000 cycles/mil
Thickness	.001" - .003"
Use Temperature	400°F max