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

A Semi-deterministic Wear Model Considering the Effect of Zinc Dialkyl Dithiophosphate Tribofilm

Tribochemistry plays a very important role in the behaviour of systems in tribologically loaded contacts under boundary lubrication conditions. Previous works have mainly reported contact mechanics simulations for capturing the boundary lubrication regime, but the real mechanism in which tribofilms reduce wear is still unclear. In this paper, the wear prediction capabilities of a recently published mechanochemical simulation approach (Ghanbarzadeh et al. in Tribol Int, 2014) are tested. The wear model, which involves a time- and spatially dependent coefficient of wear, was tested for two additive concentrations and three temperatures at different times, and the predictions are validated against experimental results. The experiments were conducted using a mini-traction machine in a sliding/rolling condition, and the spacer layer interferometry method was used to measure the tribofilm thickness. Wear measurements have been taken using a white-light interferometry. Good agreement is seen between simulation and experiment in terms of tribofilm thickness and wear depth predictions

Study of the Interfacial Mechanism of ZDDP Tribofilm in Humid Environment and its Effect on Tribochemical Wear; Part II: Numerical

Relative humidity and the effect on tribochemistry and wear of boundary lubricated systems are examined experimentally in part one of this study. In part II of this study the tribofilm thickness and wear results obtained experimentally are used to develop a semi-deterministic approach to implement the effect of humidity in wear prediction of boundary lubrication for the first time. Two approaches were used for this purpose; firstly, a modification factor was found to be suitable for Archard's wear equation to be able to account for the effect of relative humidity. This factor is found to be good for engineering designers to be able to predict the lifetime of machine parts. Secondly, the effect of humidity on the tribofilm growth on the surfaces was captured in the model and its effect on wear was tested based on a recent model developed by the authors. It is shown that, as expected, if the tribofilm growth is captured effectively, wear can be predicted. The prediction results were validated with the experimental results showing good agreement. Calibration of the tribochemical model at different levels of relative humidity suggest that the maximum tribofilm formation factor (hmax) is varying linearly with the humidity percentage. This led to the further development of the tribochemical model of the authors to adapt to the humid environments

An experimental and analytical study of the effect of water and its tribochemistry on the tribocorrosive wear of boundary lubricated systems with ZDDP-containing oil

Water has long been recognised as a contaminant in lubricants. It can affect wear performance, especially in bearing systems, in complex ways. Water can also induce corrosion, which in turn can enhance wear. The individual parts of any tribocorrosion system are related to a complex mix of system parameters such as lubricant and additives, relative humidity and temperature. The effect of different water concentrations and different temperatures has been studied experimentally in this work. A modification to Archard[U+05F3]s wear coefficient was applied with respect to the experimental measurements. The new wear model considering the effect of water was implemented into the previously-reported numerical model to develop a new semi-deterministic numerical wear model adapted to the tribo-corrosion system in this work