When two surfaces are brought into contact the surface oxide films prevent intermetallic welding. The films may be ruptured by sliding the surfaces over each other, or by applying sufficient pressure to cause plastic flow at the interface. The ease with which surfaces weld on sliding depends upon their rigidity and the nature and thickness of the surface films.Extreme pressure additives react with the surface oxide to form a compound which is more fusible than the metal. If the local temperature should rise sufficiently during sliding, this compound will melt and smear over the surface, and act as a temporary lubricant.