Effect of water on ZDDP anti-wear performance and related tribochemistry in lubricated steel/steel pure sliding contacts

Zinc Dialkyldithiophosphate (ZDDP) is an effective anti-oxidant and anti-wear additive in lubricants. Only a few studies have addressed its anti-wear performance when water is present in the lubricant. The mechanism by which water affects the tribofilm formation processes is not fully understood. The objective of this study is to investigate the effect of water on the tribological performance of ZDDP-containing lubricants and related tribochemical processes when lubricating steel/steel contacts under pure sliding in boundary lubrication/extreme pressure conditions.The experimental work presented here is mainly separated into three phases. In the pre-test phase, the impact of water on the bulk properties of the lubricant (viscosity, Total Acid Number (TAN), FTIR information) has been investigated. In the tribological test phase, performance of these lubricants was evaluated in a ball-on-disc tribometer under boundary lubrication/extreme pressure conditions whilst controlling the relative humidity. In the post-test phase, the chemical nature of the tribofilms formed in tribological tests was addressed using Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy.It has been concluded that water is detrimental to the performance of the ZDDP additive. Physical and chemical analysis of the lubricant as well as the lubricated surfaces in the pre-test and post-test phases has indicated that water mainly impacts the tribofilm-forming ability of the lubricant rather than the physical properties of the lubricant itself. The chain length of phosphate and thickness of reaction layer on the wear scar, measured by XPS, was reduced with increased water concentration/relative humidity. In this paper, the mechanism by which water affects the tribological performance is discussed.

► Viscosity and TAN did not change significantly due to water.
► In the tribological tests, water increases wear.
► Water could possibly affect the formation of ZDDP tribofilm.
► XPS showed sulphate formed on the wear scar when more water was involved.
► Shorter chain length phosphate and thinner reaction layer formed with more water.