Combining of one atom of molybdenum metal with two atoms of sulfur produces one molecule of molybdenum disulfide (MoS2). In nature, this substance occurs in the important mineral molybdenite. With its black appearance and slippery feel, MoS2 is used as a solid lubricant. It assumes the form of interconnected trigonal prisms, with layers of molecules loosely associated through van der Waal’s forces. Favorable electron distribution may increase the distance between layers, increasing lubricity.
The hexagonal crystal structure of molybdenum disulfide results in extremely low friction, yet with good adherence to metals. MoS2 forms quality burnished or sputtered films on surfaces such as pistons, threads, bearings and valves. It is best suited for low-oxygen applications, such as in space. There, it is used in connection with spacecraft, antenna systems, satellites and solar arrays. In some instances, the disulfide is combined with resins such as acrylics, phenolic, urethane and polyimide to form sprayable coatings.
Molybdenum disulfide can be used even in slightly unfavorable environments, such as moderately elevated temperatures in the presence of oxygen. Although the disulfide does slowly oxidize to the trioxide, the result is still non-abrasive, with an acceptable level of lubrication persisting. In addition, by increasing the molybdenum disulfide particle size, its susceptibility to degradation significantly decreases. Sometimes, as in the case of heavy load-bearing, molybdenum disulfide is the lubricant of choice. Applications may involve more than 250,000 pounds per square inch (17,580 kg per square cm) in some instances.
Molybdenite ore must be obtained and processed to produce commercial grade molybdenum disulfide. Ore is first mined using either the open pit method or the block caving technique, whereby material is dug out from under the ore which then caves in under its own weight. Processing the ore includes its being milled, or ground; flotation follows and involves frothing with air, sometimes under the influence of additives for best results. Interestingly, the useful ore rises whereas the waste rock settles. If necessary, acid leaching is used to remove metallic contaminants.
At this point in processing, the purity may already surpass 92 percent. Patented methods may then be employed to yet further increase purity. The final processed molybdenum sulfide particles frequently run from a few microns down to submicron size. Typical online providers cite molybdenum disulfide purities ranging from 98 to 99 percent. Non-lubricant applications include catalysis in the petroleum industry for desulfurization and as a p-type semiconductor material for the electronics industry.