Molybdenum (IV) Sulfide
aka molybdenum disulfide, "Moly", "Moly-D", etc.
CAS No. 1317-33-5
NFPA Rating: Health 1, Flammability 1, Reactivity 0
Appearance: Black crystalline powder
Odor: Slight sulphurous or petrolium odor
Solubility: Insoluble in water and dilute acids
Specific Gravity: 5.060
Melting Point: 1185C (2165F)
Molybdenum disulfide occurs in a crystalline lamella structure, similar to graphite. Individual layers are comprised of a molybdenum plane between sulfide planes in a hexagonal space lattice; these layers are bonded by van der Waals interaction between the sulfide surfaces. This strucure allows for the layers to slide easily over each other, making the substance useful as a solid lubricant (see below). Also, since it displays only the sulfide anions it bonds well to metallic surfaces, increasing its value as a lubricant.
While MoS2 melts at 1185C, it begins to oxidize in air at about 315C. Its main oxidation product is MoO3.
Safety and Health
Reactivity: Not generally significant. Incompatible with strong acids and oxidizers; reacts violently with hydrogen peroxide.
Fire Hazard: Slightly flammable; releases toxic fumes when burned.
Health Effects: May cause eye and skin irritation. Follow basic laboratory safety procedures.
MoS2 is used extensively as a solid lubricant; its performance is superior to graphite, particularly in high-temperature, high-pressure, and high-vacuum applications (although high-temperature use in air may raise oxidation issues). Since the slipping between layers is not dependent on a water monolayer, friction is actually decreased under these circumstances.
One common everyday applications of MoS2 is in the lubrication of automotive CV joints. It is also sometimes included as an additive to oil to improve performance; in this role it also provides an additional margin of safety against dry running, as it adheres well to metal surfaces. (It should be noted that MoS2 as an additive is incompatible with many detergents, and combination with such may actually increase wear.)
Because of oxidation, MoS2 is not suitable for use in air at temperatures above 350-400C, depending on the system and environmental factors; however, it may be safely used at higher temperatures in nonoxidizing environments. (For comparison, graphite degrades in air above 450C.) While the oxidation product MoO3 is not itself abrasive, it is hydroscopic and will absorb water to contaminate the system and destroy performance.
MoS2 is also used as an industrial catalyst for hydrodesulfurization (using hydrogen to remove sulfur from fuels). Since the catalytic sites are on the edges of the molecular sheets, the active surface is relatively small. New research shows that efficiency may be increased by special preparation into microparticles.