Effect of additive combinations on tribofilm formation in fully formulated 0W‐8 oil

Abstract

Reducing engine oil viscosity is an effective way to meet automotive fuel efficiency requirements. Clarifying the effects of multiple coexisting additives in low-viscosity oil is important for the development of low-viscosity engine oil. In this study, six kinds of 0W‐8 engine oil, including those without a Ca-derived detergent, a dispersant or molybdenum dialkyldithiocarbamate (MoDTC), were selected to study the effects of these additives on the tribofilm formation and the friction characteristics of fully formulated low-viscosity oil. A custom-made rolling and sliding tribometer was used to evaluate the tribological properties and form the tribofilms. The elemental distribution of the tribofilms was characterized by electron probe microanalysis (EPMA), and the chemical compositions of the films were determined using X-ray photoelectron spectroscopy (XPS). The Ca-derived detergent hindered both the ZnDTP-derived and MoDTC-derived tribofilm formation in the fully formulated low-viscosity oil due to adsorption competition. The effect of the ashless dispersants on the ZnDTP-derived tribofilm formation in the fully formulated 0W‐8 oil showed opposite results in the presence or absence of MoDTC. The addition of MoDTC had different effects on the ZnDTP-derived tribofilm formation under different additive combinations.