Role of zinc dialkyl dithiophosphate in carbon black induced abrasive wear

We have explored the contact conditions under which zinc dialkyl dithiophosphate (ZDDP) is corrosive and, in the presence of carbon black, increases wear. It was found that the corrosivity of ZDDP to steel is due to a load dependent mechanochemical reaction and leads to the antagonism between ZDDP and carbon black which is also likely to be load dependent. Depth-profiled Auger spectroscopic analysis of wear scars produced with ZDDP shows that the iron sulfide layer thickness depends on contact load during the wear process. Higher loading produces a thicker iron sulfide layer but, surprisingly, the thickness is relatively insensitive to time. Indenter nanoscratching measurements demonstrate that the highly corroded surface is less scratch resistant than lightly corroded surfaces made under lower load (or of the native steel), explaining increased wear by abrasion of carbon particles (e.g. carbon black or soot) under highly loaded conditions.