A lubricant additive that prevents sliding metal surfaces from seizing under conditions of extreme pressure. At the high local temperatures associated with metal-to-metal contact, and extreme pressure additive combines chemically with the metal to form a surface film that prevents the welding of opposing asperities, and the consequent scoring that is destructive to sliding surfaces under high loads. Reactive compounds of sulfur, chlorine, or phosphorus are used to form these inorganic films.

Tip excerpted from: The Professional's Guide to Maintenance and Reliability Terminology by Ramesh Gulati, Jerry Kahn and Robert Baldwin