Molybdenum Disulfide (MoS2)
Molybdenum was not discovered until the latter part of the 18th century, and does not occur in the metallic form in nature. Despite this, its predominant mineral - molybdenite - was used in ancient times (a 14th-century Japanese sword has been found to contain molybdenum) but would have been indistinguishable from other similar materials such as lead, galena and graphite. Collectively, these substances were known by the Greek word “molybdos”, which means lead-like.
Molybdenum was first positively identified in 1778 by a Swedish scientist, Carl Wilhelm Scheele. Shortly thereafter, in 1782, Peter Jacob Hjelm reduced molybdenite oxide with carbon to obtain a dark metallic powder which he named “molybdenum”.
Molybdenum remained a laboratory curiosity throughout most of the 19th century until the technology for the extraction of commercial quantities became practical. In 1891, the French company Schneider and Co. first used molybdenum as an alloying element in the production of armor plates. It was quickly noted that molybdenum was an effective replacement for tungsten in numerous steel alloying applications because its density is only slightly more than half that of tungsten.
MoS2 occurs naturally in the form of thin solid veins within granite. It is mined and highly refined to achieve purity suitable for lubricants. MoS2 has a hexagonal crystalline structure as shown in Figure 1. The intrinsic property of easy shear occurs at the interface between the sulfur molecules. The interaction between layers provides an effect similar to what a person may experience if trying to maneuver across a floor completely covered with brand new playing cards. Each playing card slides easily with respect to each individual layer. As the total surface resistance is reduced or redistributed among many other interacting surfaces, the net effect is reduced total surface friction and resistance.
Because there is no vapor present between lattice plates, MoS2 is effective in high-vacuum conditions, where graphite will not work. The particle size and film thickness are important parameters that should be matched to the surface roughness of the lubricated component. Particle size selection is much larger for rough cut surfaces, such as hobbed open gears, than for highly finished surfaces, such as those found on bearings. Improperly matched particle sizes may result in excessive wear by abrasion caused by impurities in the MoS2.
The temperature limitation of MoS2 at 400°C (752°F) is imposed by oxidation. MoS2 oxidizes slowly in atmospheres up to 600°F. In a dry, oxygen-free atmosphere it can function as a lubricant up to 1300°F. The oxidation products of MoS2 are molybdenum trioxide (MoO3) and sulfur dioxide. MoS3 is hydroscopic and causes many of the friction problems in standard atmosphere. MoO3 is a preferred form of the metal used as an additive for various other metals, which is its primary use.
The issue of where molybdenum disulfide should be used, versus graphite or tungsten disulfide, is generally best addressed by a lubrication engineer. For most commercial applications, these are relatively simple judgments. In aerospace applications where unique environments and exotic materials are employed, these questions often take substantial research to provide the best answers.3
The low friction coefficients of MoS2 often exceed that of graphite.