Ionic Liquid Additives in Water-Based Lubricants for Bearing Steel – Effect of Electrical Conductivity and pH on Surface Chemistry, Friction and Wear

Abstract

Water-based lubricants have the potential to become the largest environmentally friendly lubricants in applications such as electric vehicles and the newly emerging green technologies of the future due to their inherent low viscosity and cooling properties. In order to be environmentally acceptable (EAL), both base lubricants and additives should comply with biodegradability, non-toxicity, and non-bioaccumulation requirements. Additives for water-based lubricants should ideally be polar and soluble in water and, at the same time, should not increase the electrical conductivity to critical levels for corrosion. However, most additives used in synthetic or mineral oils are non-polar. Ionic liquids have recently gained attention as lubricant additives due to their high polarity, making them highly surface-active (i.e. high tendency to adsorb on metal surfaces). However, they are seen as highly corrosive for many metal alloys. In this work, a water-glycol lubricant containing two different ionic liquids has been investigated as a potential green lubricant for a bearing steel AISI 52100 with accurate control on electrical conductivity and pH. The selected ionic liquids were tributylmethylphosphonium dimethylphosphate (PP) and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP). The tribological behaviour of the ionic liquids was compared with a well-known organic friction modifier, dodecanoic acid (C12). The ionic liquids showed lower friction and wear rate than the water-based lubricant alone. However, they showed higher friction than the lubricant formulated with C12, in which PP gave lower friction than BMP due to low pH. A detailed subsurface analysis of the wear track using scanning-transmission electron microscopy (STEM) showed that a thick oxide tribofilm was built on the wear track for both lubricants formulated with ionic liquids due to high electrical conductivity. This tribofilm gave beneficial effect on wear. Although PP and BMP gave thicker tribofilms than C12, it was not durable, resulting in cracking and detachment.

Effect of the Polar Head Type on the Surface Adsorption and Tribofilm Formation of Organic Friction Modifiers in Water-Based Lubricants

“…The degree of the material loss is defined as the ratio between the area loss and the groove area, and its determination has been published somewhere else. 29,33 The friction evolution plots show a coefficient of friction around 0.3 for the base WG lubricant and almost the same value for the base lubricant (WG) with methyl laurate and monolaurin. The lubricant containing C12 shows a very stable and low coefficient of friction around 0.12 with a very short running in period.…”

“…This function is typically achieved by other groups of chemicals than the ones of FMs through the formation of a protective tribofilm that ultimately controls friction and wear at the tribosurface. This mechanism is activated by in situ mechanochemical reactions between species from the lubricant and the metallic elements of the tribosurface. , Not surprisingly, the ability of OFMs to form tribofilms has attracted little attention, , and most studies focus on their adsorption and frictional performance. ,− However, in former works performed in our group, the ability of OFMs to form protective tribofilms in WBLs has been observed. ,,,, If FMs can lead to both friction and wear reduction, they can eventually have a double function (multifunction) in the lubricant, leading to simpler and greener formulations.…”

Facile Synthesis of Lignin-Castor Oil-Based Oleogels as Green Lubricating Greases with Excellent Lubricating and Antioxidation Properties

“…As a natural phenolic compound found in plant biomass, lignin is the second most abundant biopolymer material on earth. Lignin incorporation into polymers has been widely explored by researchers due to its antimicrobial properties, UV shielding, antioxidant activity, thermal stabilization, lubricity, and other properties. − As one of the three main components of lignocellulose, lignin is the only aromatic biopolymer. Depending on the applied isolation method, lignin can be categorized into different types, such as acid lignin, alkali lignin, and organosolv lignin. − The way lignin is extracted affects its final chemical properties as the bond cleavage and formation occur at varying degrees during the process.…”

Effect of Steel Hardness and Composition on the Boundary Lubricating Behavior of Low-Viscosity PAO Formulated with Dodecanoic Acid and Ionic Liquid Additives

“…Subsequently, the adjacent IL in the lubricant is attracted to the first adsorbed layer in the same manner, forming a multilayer structure at the surface. − For the second mechanism, localized high temperature and high pressure are generated at the contact area, decomposing the ILs. Consequently, the decomposition products of ILs reacts with the nascent worn surface to form a protective tribofilm. − Most research on ILs as lubricant additives focuses on the boundary lubricating condition and tribofilm formation. − Only a few study the adsorption mechanisms of ILs; ,, therefore, the lubricating mechanism of ILs is still far from being fully understood.…”