Experimental study on ZDDP tribofilm formation in grease lubricated rolling/sliding contacts

The tribofilm formation mechanisms of Zinc dialkyldithiophosphate (ZDDP) anti-wear additives have been extensively studied in oil lubrication. However, they are rarely reported in grease lubrication. In this study, the Mini Traction Machine (MTM) equipped with the Spacer Layer Interferometry iMaging (SLIM) technique is used to monitor the formation of ZDDP tribofilms in situ in Lithium-based grease- and its corresponding base oil- and bled oil- lubricated rolling/sliding contacts at low speeds. The results indicate that, compared to oil lubrication, the thickener deposited films hinder the formation of ZDDP tribofilms through reducing the interfacial friction in grease lubrication under moderate lubrication conditions. Among various operating conditions and ZDDP concentrations, the slide-to-roll ratio (SRR) dominates the formation of the ZDDP tribofilms by removing the grease thickener deposited films. By further extending to severe operating conditions in terms of load, temperature, SRR, and ZDDP concentration, the growing process of tribofilms have been analyzed for low-speed grease lubrication.