Abstract
Ionic liquids have been widely discussed as potential lubricants, however, their properties make them also very good potential candidates as lubricant additives (e.g., friction modifiers and anti-wear). In this work, the tribological study of two ionic liquids (tributylmethylphosphonium dimethylphosphate (PP), and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP)) as lubricant additives has been performed on stainless steel (AISI 316L) exposed to polar (water-glycol) and non-polar (polyalphaolefin) based lubricants under boundary lubricating conditions. The performance of these ionic liquids as lubricant additives has been compared to a classical organic friction modifier (dodecanoic acid (C12)). The water-glycol lubricant formulated with the two ionic liquids showed friction values higher than the same base lubricant formulated with dodecanoic acid, however, opposite results were observed for polyalphaolefin (PAO). A detailed surface chemical analysis using X-ray photoelectron spectroscopy (XPS) revealed differences in the passive/tribofilm thickness and chemical composition of the stainless steel surface tested in all lubricants. In the case of the polar lubricant additivated with ionic liquids, the tribochemical reaction accompanied by a tribocorrosion process led to the formation of an unstable passive/tribofilm resulting in high friction and wear. However, in the absence of tribocorrosion process (polyalphaolefin base lubricant), the tribochemical reaction led to the formation of a stable passive/tribofilm resulting in low friction and wear. A detailed surface and subsurface investigation of the microstructure using scanning electron microscopy equipped with a focused ion beam (SEM-FIB) showed that high wear rates resulted in thicker recrystallization region under the wear track surface. Among all lubricant additives tested in this work, BMP in non-polar lubricant media showed the best tribological performance.
Highlights
In this modern era, energy efficiency has become a significant issue in automotive and industrial sectors worldwide
Sliding tests were performed on stainless steel under the boundary lubricating regime conditions to study the influence of ionic liquids as boundary lubricant additives in polar and non-polar base lubricants
The reference lubricant additive used in this work was a classical organic friction modifier (OFM), dodecanoic acid (C12)
Summary
Energy efficiency has become a significant issue in automotive and industrial sectors worldwide. When the mechanical conditions in a tribological system become severe (very high pressure in the boundary lubricating regime), the friction modifier on the moving surface tends to desorb In this extreme boundary lubricating condition, anti-wear additives are required to preserve the integrity of the surfaces in contact. They offer other benefits as well, such as low melting points (liquid at room temperature), chemical and thermal stability (hightemperature operation), non-flammability, high ionic conductivity, high heat capacity, high thermal conductivity (facilitating the removal of excessive heat), and high polarity (highly surface active and adsorbing) [10,11,12] Their chemical and physical properties can be tuned by changing the anion and cation composition to obtain task-specific ILs structures. The tribological performance of the different additives is discussed based on a detailed XPS analysis inside and outside the wear tracks, and a microstructural analysis of the wear track subsurface cross-section
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