Chemical And Physical Properties Of Tribofilms Formed By The Interaction Of Ashless Dithiophosphate Anti-Wear Additives

Physiochemical properties of tribofilms generated from octadecylthioperoxydiphosphate (SSC18) and octadecylphosphorofluoridothioate (PFC18) were compared with zinc dialkyl dithiophosphates (ZDDP) in this study. PFC18 and SSC18 were chemically synthesised and mixed in mineral base oil to prepare test oil formulations. Additionally, PFC18 and SSC18 were mixed at different concentrations and evaluated for their compatibility and tribological performance using high frequency reciprocating rig (HFRR) tribometer. All lubricant formulations with ashless additives exhibited superior anti-wear characteristics compared to ZDDP. Comprehensive surface analysis of tribofilms was conducted using scanning electron microscopy, nano-indentation and X-ray absorption near edge spectroscopy. Formulations containing only SSC18 and PFC18 exhibited a uniform and continuous tribofilm formation, while formulations with a combination of SSC18 and PFC18 had a stable pad-like structure. X-ray absorption near edge structure (XANES) results indicate ashless PFC18 and SSC18 by themselves and in combination form iron phosphate enriched tribofilms with a mixture of iron sulfides and/or sulfates.