Under heavy polymer-to-metal friction duties, modification of the contacting surfaces is of great importance. The first reason for this modification is transfer of the metal to the polymer surface and of the polymer on to the metal surface. The intensity of this process depends mainly on the relative hardness of the contacting materials, and on the natures of the gas phase and the liquid medium. Frictional copper plating of reinforced thermosetting plastics may occur with glycerin as the lubricant. In the absence of mutual transfer, passing from friction in the air to friction in a vacuum of about 10 −5 torr does not affect the friction duty. Therefore, when there is no transfer, the oxide layers on the metal surface do not play the determinative role they do in metal-to-metal friction. The second cause of modification of the metal and polymer surfaces are the thermal processes which, in the case of thermosetting resins and high-melting polymers, may be accompanied by thermal or thermo-oxidative degradation of the polymers with the formation of highly reactive low-molecular compounds. In the case of thermoplastics a surface film of the melt may form. This sets up conditions for fluid instead of dry friction, makes the friction coefficient vs. load dependence extreme in character, and renders the friction force independent of the load at high values of the latter. Some powdered thermoplastics display high anti-wear action under heavy steel-to-steel friction duties, this being related not only to their ability to withstand being squeezed out of the friction zones at high specific loads, but also to tribochemical processes.