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.
Highlights
The production of electric vehicles (EVs) has grown in recent years
The tribological components of EVs will work at a higher speed than those in ICEVs, which makes the lubricant in EVs to function more as a torque transfer rather than as a load-bearing (Van Rensselar, 2019)
It was found that Ionic liquids (ILs) were fully soluble at all concentrations, the friction was stable only at concentrations equal and above 1 wt%, 1 wt% was selected as the concentration for ILs
Summary
The production of electric vehicles (EVs) has grown in recent years. The EVs have higher energy efficiency compared to ICEVs, in which EVs use up to 77% of total electric energy to drive the vehicle, whereas ICEVs only use up to 21% of total fuel energy (Holmberg and Erdemir, 2019). Similar to ICEVs, the components of EVs need lubrication to operate in a tribological environment effectively. The tribological components of EVs will work at a higher speed than those in ICEVs, which makes the lubricant in EVs to function more as a torque transfer rather than as a load-bearing (Van Rensselar, 2019). The higher the speed of the tribological component, the higher the temperature generated in the lubricant. The development of lubricants for EVs nowadays is moving towards low viscosity lubricants, which have better cooling properties and higher temperature stability (Narita and Takekawa, 2019)
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