Promotive and inhibitive action of polar compounds on the formation of antiwear tribofilm from engine oil

摘要

Various additives are used for engine oil to reduce friction and to prevent wear. One of the most important additives used for engine oil is Zinc dialkyldithiophosphate (ZnDTP) which is known as a multifunctional additive. It is generally known that ZnDTP forms phosphorus tribofilm on metallic surfaces during friction through the processes of thermal degradation, thermo -oxidation, hydrolysis, surface adsorption or a mixture of these mechanisms. The tribofilm formed pad-like protrusions which consists of glassy Zn/Fe phosphate above Zn/Fe sulphide with a thin outer layer of zinc polyphosphate on the metal substrate. Recently, it has been strongly required to reduce the amount of ZnDTP in engine oil for preventing the exhaust gas purification catalyst from poisoning. In addition, since the tribofilm formed from ZnDTP provides relatively high friction resistance in boundary lubrication, this is also a major problem to improve the fuel efficiency. Although molybdenum dithiocarbamate (MoDTC) formulated together with ZnDTP in engine oil gives extremely low friction, seeking alternative ashless FM is still required for the durability aspect. Therefore, it is necessary to discover coexisting additives with ZnDTP which could improve friction-reducing performance as well as retaining the superior antiwear performance of ZnDTP.