A Surface Effect Associated with the Use of Oils Containing Zinc Dialkyl Dithiophosphate

It is shown that the operation of engines on crankcase oils containing appreciable amounts of zinc dialkyl dithiophosphate can increase the small-scale roughness of the lifter-foot surfaces. In addition, the small-scale roughness can be reduced by subsequent operation of the same engine parts on oils containing no zinc dialkyl dithiophosphate. This change in roughness coincides with a change in reflectivity that can be detected visually. A similar effect of the additive has been demonstrated in a bench test machine. The increase in roughness which accompanies the operation on the additive-treated oil occurs rapidly and is maintained to high contact loads although some smoothing may occur at high loads. The evidence suggests that the change in roughness results from a reaction of additive decomposition products with the metal surface. The mechanism responsible, however, is not completely understood.