Review of the lubrication of metallic surfaces by zinc dialkyl-dithiophosphates

Implementation of tighter environmental restrictions on combustion engine emissions has resulted in the need to create new, more environment friendly engine oil additives. By far the most studied and effective class of antiwear additive are the zinc dialkyl(aryl)-dithiophopshates (ZDDPs). To date, there is no single, effective replacement additive for ZDDP. In order to create an effective replacement, it is first necessary to understand how and why an additive works as well as it does, on all length scales from macro to nano. This comprehensive review of the literature over the last 50 years provides insight into the overall picture of ZDDP by focussing on their chemical characterization, film formation mechanisms, properties and structure with the intent of exposing gaps that still remain in understanding the conundrum of ZDDP.