Tribology of solid lubricants

Over the last seven decades, extreme operating conditions encountered in many industrial and engineering applications-particularly those within the aerospace industry-have driven the evolution of more advanced commercial lubricants. While many long-standing lubrication techniques utilize liquid or grease-type lubricants, new tribological applications have developed over the last 70 years that have led to the development of lubricants derived from solid materials and coatings with self-lubricating properties. Many tribological applications require two surfaces to slide over one another in relative motion, resulting in friction and wear, such as in cutting and forming operations, gears, bearings, and engine parts. Increasingly, more of these applications are operating in extreme environments (such as high vacuum, microgravity, high/low temperatures, extreme pressure, space radiation, and corrosive gas environments) that are beyond the tolerable and usable domain of liquid and grease-based lubricants. This has propelled the development of dry/solid lubricants that are nonvolatile and can withstand such extreme environmental conditions. In this chapter, a review of the state of solid lubrication and the utilization of solid lubricants as powder transfer films, thin film coatings, colloidal mixtures, and composite matrices are presented for the field of tribology.