Effect of Water on the Interfacial Mechanisms of the Tribofilms Formed by Zinc Dialkyl Dithiophosphate: Experimental and Analytical Study

Understanding the true interfacial mechanisms of the growth of the tribofilms generated by Zinc Dialkyl Dithiophosphate (ZDDP) is important because it is the most widely used anti-wear additive and there is legislative pressure to find efficient environmentally-friendly replacements. The main focus of this study is to investigate the effect of water on the interfacial mechanisms involved in the formation of the ZDDP tribofilms and correlate it to the chemical properties of the glassy polyphosphates. The effect of different parameters such as temperature, humidity, mixed-water in oil, load and rubbing time on the tribofilm formation and its durability has been studied experimentally and analytically using a Mini Traction Machine (MTM) with the Spacer Layer Interferometry Method (SLIM) attachment. The role of additive depletion on the pre-formed tribofilm thickness under mechanical stress has also been studied. Results show that physical parameters such as temperature, humidity and pressure significantly influence the tribofilm. X-ray Photoelectron Spectroscopy (XPS) analysis was carried out to assess the evolution of the chemical structure of the tribofilm during the test. The chemical analysis suggests that there are different chemical properties across the thickness of the tribofilm and these determine the durability characteristics. A humidity control system was designed and integrated with the Mini Traction Machine (MTM) and Spacer Layer Interferometry Method (SLIM) for the first time to evaluate the effect of relative humidity and the tribochemical changes on the tribological performance and tribofilm characteristics of boundary lubricated systems. One of the key aspects in this study is the use of continuous humidity control system, which can provide steady humid environment during the tribological tests. In the present study, the tribofilm thickness and wear results obtained experimentally were used to develop a semi-deterministic approach to implement the effect of humidity and mixed-water in wear prediction of boundary lubrication.