Effect of water presence on tribological performance and tribofilm hierarchical transition of graphite cast iron lubricated with 5W-30 formulated engine oil at 40 ℃

This study was designed to enhance the understanding of the extent to which water entered engine oil in terms of friction and wear performance at 40 ℃. To this end, a commercial 5W-30 engine oil was adopted in order to mix with different levels of water. Results indicated that the kinetic viscosity of engine oil gradually decreased while its contact angle increased with increasing water content of engine oil. When the water content was below 1.0 vol%, no severe tribological response occurred. However, when the water content in engine oil was more than 2.0 vol%, severe wear was observed. Specifically, for 5.0 vol% water in engine oil, pits were evident on the rubbing surface due to corrosion. Compared to tribofilm formed by engine oils, water content affected the interfacial reaction between engine oil and ferrous surface to form a composite tribolayer composed of iron oxide
and sulfides on a cast iron substrate. Furthermore, the study discussed the interactions of oil specimens containing different water contents with ferrous surface and proposed a possible mechanism by which Zinc dialkyl dithiophosphates (ZDDP) generated a tribofilm in the presence of water.