Molybdenum-Sulfur Compounds in Lubrication

Molybdenum disulfide is used as a dry lubricant in, e.g. greases, dispersions, friction materials and bonded coatings.

Molybdenum-sulfur complexes may be used in suspension but more commonly dissolved in lubricating oils at concentrations of a few percent.

Molybdenum disulfide, MoS2, the most common natural form of molybdenum, is extracted from the ore and then purified for direct use in lubrication. Since molybdenum disulfide is of geothermal origin, it has the durability to withstand heat and pressure. This is particularly so if small amounts of sulfur are available to react with iron and provide a sulfide layer which is compatible with MoS2 in maintaining the lubricating film.

A number of unique properties distinguish molybdenum disulfide from other solid lubricants:

  • A low coefficient of friction (0.03-0.06) which, unlike graphite, is inherent and not a result of absorbed films or gases;
  • A strong affinity for metallic surfaces;
  • Film forming structure;
  • A yield strength as high as 3450 MPa (500 x 103 psi);
  • Stability in the presence of most solvents;
  • Effective lubricating properties from cryogenic temperatures to about 350oC in air (1200oC in inert or vacuum conditions).
  • Molybdenum disulfide will perform as a lubricant in vacuo where graphite fails.

A combination of molybdate and water soluble sulfides can provide both lubrication and corrosion inhibition in cutting fluids and metal forming materials. Oil soluble molybdenum-sulfur compounds, such as thiophosphates and thiocarbamates, provide engine protection against wear, oxidation and corrosion. Several commercial manufactures supply these additives to the lubrication industry.

Some molybdenum disulfide formulations are given in Table 1.

Molybdenum disulfide formulations
Mo Content (%)Product TypeUses
1 - 20 Greases - for manufacturing, mining and transportation Ball and roller bearings, splines, chassis, conveyors
20 - 60 Pastes - mineral or synthetic base Assembly of machinery, splines, gears, universal joints, metal forming
0.5 - 5 Industrial and Motor Oils or Synthetic fluids All automotive and industrial gears, reducers, cams, etc
1 - 20 Water Suspensions Metalworking and process lubrication, threads, slices, packaging, die casting
Up to 85 Bonded Coatings - air or heat cured, organic, inorganic Threads, tools, switches, locks, valves, slide, process lubrication, metalworking
1 - 40 Metal working compounds, Soaps, Powders, etc Extrusion, cold forming, wire drawing, deep drawing
10 - 100 Pure or Mixed Powders Punch line, stamping, forming, relays, switches, packing
Composites
1 - 10 Friction Products, Sintered Cu brakes, Semi-metallic and Non-asbestos pads Aircraft, automotive and rail brake pads & linings
1 - 30 Plastic, Rubber & Metal Composites Gears, slides, bearings, thrust washers, O-rings

Table 1: Molybdenum disulfide formulations

The ability of molybdenum disulfide to function as a lubricant is intimately related to its layer structure (Fig. 1).

layer structure of molybdenum disulfide

Fig 1: A side view of the layer structure of molybdenum disulfide. The lighter spheres represent molybdenum atoms and the darker spheres sulfur. The outer layers of sulfur atoms bind to each metal surface. Contact between the metal surfaces is prevented.

A layer of molybdenum atoms is sandwiched between two layers of sulfur atoms. When molybdenum disulfide is dispersed between two metal surfaces a layer binds to each metal surface through the sulfur atoms. Then the asperities (surface irregularities on the metals) are prevented from coming into contact. Sliding contact is between the outer layers of sulfur atoms which are only weakly interacting. The surfaces are therefore able to slide easily relative to one another (Fig. 2).

Compounds typically used as oil soluble additives are shown (Fig. 3) which are used dissolved in lubricating oils (Fig. 4).

Lubricating action of MoS2

Fig 2: Lubricating action of MoS2 . Molybdenum disulfide is dispersed between two metal surfaces; the asperities (surface irregularities on the metals) are prevented from coming into contact. (Reproduced with permission of Acheson Industries: http://www.achesonindustries.com)

Molybdenum sulfur compounds

Fig 3: Molybdenum sulfur compounds
used as soluble lubricating oil additives

Application of a soluble

Fig 4: Application of a soluble
Mo S lubricating oil additive