The combined effects of ZDDP, surface texture and hardness on the running-in of ferrous metals

Abstract

The effects of surface characteristics including roughness, lay direction and hardness of rubbing pairs on the antiwear performance of secondary short chain ZDDPs under a boundary lubrication condition are studied experimentally. The antiwear performance of the ZDDPs is evaluated by the duration of running-in periods recorded in wear tests of specially prepared specimens. A running-in period is defined as the time interval from the beginning of a wear test to the time at which the contact resistance between the rubbing pair approaches infinity. The shorter running-in period yields the better antiwear performance. The wear tests were conducted with a reciprocating sliding contact made by flat-on-flat specimens which were made from grey cast iron, quenched medium carbon steel and bearing steel. Two lay directions of the surface texture, namely, parallel and perpendicular to the sliding direction were ground, which made four possible combinations in a rubbing action. The CLA roughness of the specimens ranged from 0.35 μm to 0.04 μm. It is found that to obtain a shorter running-in period and to enhance the antiwear performance of ZDDPs, the following rules should be obeyed. If the hardness numbers of the two rubbing members are near the same, the combination of their lay directions should be both in parallel but perpendicular to the sliding direction and, the roughness values should be smaller than 0.09 μm. If the hardness number of one member is much greater than that of the other, the soft member should be the smaller one and the surface of the hard member should be as smooth as possible. Under any circumstance, the smaller members should be chamfered.