Extreme pressure additives react with metal surfaces during operation at high pressures and temperatures, creating a protective layer that that reduces wear between two mating metal surfaces. Anti-wear additives perform in a similar manner, but tend to operate under lower pressures and temperatures.
Extreme pressure additives are used in gear oils, other power-transmitting fluids, load-bearing greases and metalworking fluids. They are usually supplemented with anti-wear additives to make these fluids effective across a wide range of pressure and temperature conditions.
Many extreme pressure and anti-wear additives can be corrosive to certain metals, so they are typically formulated to balance protection with corrosivity in these applications.
Lubricity additives improve boundary lubrication by forming a protective film through deposition, and are generally used in greases, some industrial oils and various break-in lubricants.