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The compatibility between additives and base oils is a critical aspect of lubricant formulation design. Previous research has shown that high-polarity base oils impede the formation of zinc dialkyl dithiophospate (ZDDP) tribofilms, which is generally attributed to competitive surface adsorption. However, experimental evidence supporting this hypothesis remains limited, necessitating further insights into the film growth process. In this work, primary and secondary ZDDPs were blended with non-polar PAO, as well as polar dioctyl sebacate (DOS) and castor oils (CAS). Both conventional ball-on-disc tests and in lubro AFM experiments that achieved an in-situ observation on tribofilm evolutions were performed. ZDDP addition led to anti-wear improvement that varied by its molecular structure, while CAS showed obviously lower friction. Secondary ZDDP exhibited fast tribofilm growth rate, as evidenced by both the abrupt increase in friction coefficient and the direct observations from AFM experiments. The volumetric growth rate of the tribofilms followed the order PAO > DOS > CAS, inversely correlating with base oil polarity. An initial adsorption antagonism between ZDDP and polar molecules on the steel surface was confirmed in following tests. In addition, the rupture of pre-formed glassy tribofilms was observed in highly polar CAS, possibly due to the solubilization of the topmost polyphosphate.
Jiang et al. (Sat,) studied this question.