1. Solid and Dry Lubrication
Bulk or coating materials that are introduced between two rubbing surfaces for the purpose of reducing friction and wear, without a liquid lubricant, are known as solid lubricants.
A dry lubrication regime is required for applications where a liquid would: a) degrade at high temperatures, b) not function properly, e.g. in space, or c) migrate out of the contact over long periods of time.
Most commonly used solid lubricants in bulk or coating form are graphite, MoS2, WS2, PTFE (Teflon®), and diamond-like carbon. Graphite and MoS2 (shown below) have lamellar structures or hexagonal layer–lattice structures. Within the layers, the atoms are tightly packed and the bonding between the atoms is covalent and strong. These layers are separated by relatively large distances, and held together by weak van der Waals type bonding. Under the action of a relatively small frictional force, displacement of the layers easily occurs which is responsible for solid lubrication (deck of playing cards analogy).
Solid/dry lubricants are used in satellites, orthopedic implants, wind turbines, and many more. S
6.15 Å
Mo (or W)
Weak
Strong
6.7 Å
Mo (or W) disulfide
Graphite (carbon)
Low friction and wear of solid lubricants is a strong function of the operating environment (dry versus humid), contact stress/pressure, sliding speed, temperature, etc. Currently there is no one solid lubricant that can provide low friction and wear in all of these operating conditions, although multi-phase composite solid lubricants are being researched.
Slide created by: Professor Thomas W. Scharf at The University of North Texas
Reference: H.E. Sliney, “Solid Lubricants,” in ASM Handbook: Friction, Lubrication, and Wear Technology, p. 113.