Influence of chemical and physical contaminants on the antiwear performance of model automotive engine oil

The antiwear performance of simulated used-engine-oil that contained a chemical contaminant (degraded zinc dialkyldithiophosphate (ZnDTP)) was studied with and without physical contamination (carbon black) using a four-ball tribometer. By reacting with cumene hydroperoxide, sec-C6-ZnDTP was degraded and produced many compounds containing both phosphorous and sulphur. The simulated used-oils were found to promote wear. This wear was considered to be due to corrosive wear by the excess reaction of surfaces with the sulphur contained in the degraded compounds. Carbon black was used to model carbon soot, which is another key substance of degraded engine oils, especially in diesel engines, to study the synergism between chemical contamination (ZnDTP degradation) and physical contamination (carbon soot contamination). Carbon black increased wear irrespective of the level of ZnDTP degradation, and the acceleration was much greater in the degraded oils. The wear acceleration by carbon black was observed even when the antiwear film from ZnDTP was already present on the surface. It was suggested that the wear acceleration by carbon black was due to abrasion.