The effect of steel hardness on the performance of antiwear additives

Wear tests were performed in five oil formulations using AISI-52100 balls softened down to 35 Re. Two configurations were used: four tempered balls, and three tempered balls with a hard spindle ball. The antiwear additives gave low wear constants at all hardnesses except for the softer balls in the four-tempered-balls configuration. Indeed, in many instances moderate softening of the steel actually reduced the rate of wear. The better additives gave wear protection by the formation of films that embedded into the softer metal surfaces forming a distinctively contorted mechanical mixture of film and metal. The mixture tended to flow in a thick sheet off the sides and trailing edge of scars on the softest steel. When wearing four softened balls the mixed layer was further deformed by the formation of meshed grooves on the scar and ring ball. The glassy films formed in oils containing ZDDP on steel of intermediate hardness developed into very large pads several microns thick and up to 300 mu m long that embedded into the steel. Film material formed in base oil, and the base oil with acidless tallow did not embed in the softened steel. Wear resistance in these oils also improved with moderate degrees of softening, though steel softened to below 450 VHN wore rapidly due to scuffing. All the forms of surface damage seen on the softened steel when using the better additives were judged to be undesirable for engineering purposes, even though the nominal wear rates might be acceptable.