Tribochemistry and the development of AW and EP oil additives — a review

The antiwear (AW) and extreme pressure (EP) properties of chlorine, sulphur, phosphorus‐containing organic compounds and zinc dialkyldithiophosphate (ZDDP) oil additives are affected greatly by their reactive ability to the metal surface, as well as by the chemical composition, chemical state, physical and mechanical properties of the protective films formed. Over four decades, research has taken place on boundary lubrication, and a much better understanding of the AW and EP action mechanism of one additive by itself in base oil has been obtained, and much more knowledge on the relationship between wear and decomposition of additives, adsorption and reaction of the additive or its decomposed products with metal, has been gained. A series of analytical methods and a great number of modern surface analytical tools have been set up and used for research in this area. The problems and some suggestions for the future study on boundary lubrication of oil additive are proposed.