Triboelectrochemical friction control of W- and Ag-doped DLC coatings in water–glycol with ionic liquids as lubricant additives

In the last years, diamond like carbon (DLC) coatings doped with both carbide forming and non-carbide forming metallic elements have attracted great interest as novel self-lubricating coatings. Due to the inherent properties of DLC, the doping process can provide adsorption sites for lubricant additives depending on the chemical and electrochemical state of the surface. Ionic liquids (ILs) are potential lubricant additives with good thermal stability, non-flammability, high polarity, and negligible volatility. These characteristics make them also ideal for polar fluids, like water-based lubricants. In this work, three different DLC coatings (DLC, W- and Ag-doped DLC) were deposited on stainless steel substrates and their friction in dry and lubricated conditions in water-based lubricants was studied. Three ILs, tributylmethylphosphonium dimethylphosphate (PP), 1,3-dimethylimidazolium dimethylphosphate (IM) and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP) were used as additives and compared with a well-known organic friction modifier (dodecanoic acid). The results showed better mechanical integrity, toughness and adhesion of the doped coatings compared to the undoped DLC. The Ag-doped DLC coating had the best mechanical properties of all the coatings. W formed tungsten carbide precipitates in the DLC coating. Two different additive-adsorption mechanisms controlled friction: a triboelectrochemical activation mechanism for Ag-DLC, and an electron-transfer mechanism for W-DLC resulting in the largest reduction in friction.