The role of tribofilm chemical composition on wear of austenitic stainless steel lubricated with water-glycol containing ionic-liquids as additives

Ionic liquids (ILs) possess good thermal stability, are non-flammable, have high polarity with negligible volatility, etc. In addition, they can potentially be multifunctional lubricant additives simultaneously performing as friction modifiers and anti-wear agents. This makes them ideal additives for polar lubricants, like water-based lubricants. In this work, the friction, wear and tribofilm formation mechanisms of stainless steel 316L tested in water-glycol (WG) formulated with two different ionic liquids (tributylmethylphosphonium dimethylphosphate – PP and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate – BMP) have been investigated and compared with a well-known organic friction modifier (dodecanoic acid – C12). A thorough surface and subsurface study was performed using XPS, SEM/FIB, STEM and EDS to reveal the effect of different types of tribofilms on the frictional and wear behaviour in boundary lubricating conditions. Results showed that the Ni present in the stainless steel provided with electrochemical stability in the lubricant containing BMP resulting in a Ni-rich tribofilm on the surface. The formation of this Ni-rich tribofilm resulted in the best frictional and wear performance. The tribofilms formed with WG-PP and WG-C12 on the stainless steel were oxide-based, leading to higher friction and wear.