Failure criteria in thin film lubrication with EP additives

The scuffing behaviour of sliding couples made from 12 NiCr 13 steel, lubricated with SAE 90 and SAE 20 W30 oils with and without additives, was studied as a function of relative sliding speed and bulk oil temperature. It was found that the scuffing load decreases almost inversely with sliding speed. Calculations which take into account the decrease in hardness at increasing temperature show a fairly good constancy of total contact temperature (bulk + flash temperature) at scuffing (i.e. values ranging from 550 ° to 650 °C for all oils and test conditions). At low speeds (up to 1 m s^?1) a well-defined increase in scuffing load was found when EP additives were used; at higher speeds this effect was found to have vanished completely. Variations in nominal contact pressure in the range 1 to 3 had no appreciable influence on the scuffing load, indicating that, in the present case, scuffing was associated with a transition from the boundary lubrication regime to the severe wear regime.