Characteristics of tribofilm growth and interfacial friction of DPP and ZDDP anti-wear additives at different temperatures

Abstract

In this study, a Mini Traction Machine (MTM) equipped with the Spacer Layer Interferometry iMaging (SLIM) technique was used to in-situ monitor the formation process of protective (tribo-)films of two anti-wear additives, an ashless dibutyl dithiophosphate (DDP) and a classical zinc dibutyl dithiophosphate (ZDDP). The growing processes of DDP and ZDDP protective films on metallic rubbing surfaces across different lubrication regimes, as well as their influences on the interfacial friction, were investigated by measuring the Stribeck curves repeatedly at different temperatures. In contrast to the ZDDP additive which forms a tribofilm through tribo-chemical reactions activated by thermo-mechanical effects at rubbing interfaces, the DDP additive primarily forms physically adsorbed and thermal films under the studied operating conditions. The DDP protective films exhibit a rapid growth rate, and it is relatively temperature-insensitive, demonstrating superior anti-wear performance. Compared with the fact that ZDDP tribofilms increase the interfacial friction, the adsorption and thermal films formed by the DDP additive show pronounced friction-reduction performance in boundary and mixed lubrication regimes.