Antiwear Film Formation by ZnDTP, Detergent, and Dispersant Components of Passenger Car Motor Oils

Lubricant suppliers are under increasing pressure to provide high quality lubricants with superior oxidation and wear performance. In recent years, governmental regulations have required exhaust gas catalysts to reduce emissions of hydrocarbon and nitrogen oxides, key components of smog. Exhaust gas catalysts are sensitive to the types and levels of additives present in engine oil. The next generation of motor oils may have significantly lower sulfur, phosphorus, and sulfated ash contents to ensure exhaust gas catalyst performance throughout the life of the engine. However, phosphorus is a key component in commercial engine oils, used for both antiwear and oxidation inhibition. Engine testing of new low-sulfur, low-phosphorus antiwear components is expensive and time consuming so bench testing of potential candidates is highly desirable as a first step evaluation. Electrical contact resistance (ECR) has been shown to be a convenient method to assess antiwear film formation in a ball-on-flat bench wear test. Correlation of the bench test to fired engines was demonstrated for lubricants varying only in the type of detergent. Previous papers have examined film formation by one and two component formulations of zinc dialkyidithiophosphate (ZnDTP) and detergents. In this study, the ECR technique is systematically extended to formulations including ZnDTP, detergent, and dispersant. Both type and level of components are considered and the implications for engine performance are discussed.