Molybdenum and its applications
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 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.
| Mo Content (%) | Product Type | Uses |
|---|---|---|
| 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).
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).
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)
Fig 3: Molybdenum sulfur compounds
used as soluble lubricating oil additives
Fig 4: Application of a soluble
Mo S lubricating oil additive
