represented in Figure 1. Matta et al. [4] showed that superlubricity can be achieved for steel contact in the presence of glycerol solution.
Superlubricity can be defined as a state of motion where the effect of friction is negligible, or the friction vanishes. Matta et al. proposed that the friction reduction mechanism does not follow the classical boundary lubrication model, but instead, the mechanism can be explained by the H-bond network model. Figure 2 is a representation of the H-bond network in glycerol based WBL lubrication confined between ferrous substrates [3]. Later, Joly-Pottuz et al. [5] performed additional tests based on temperature, sliding speed and contact pressure effect using glycerol as a lubricant with steel tribopair. Similarly, they found very low friction under favourable conditions, which follow the same friction reduction mechanism as proposed by Matta et al. Although the outcomes were convincing in terms of friction, the high viscosity of the pure glycerol was a major drawback in the real application. On the other hand, Shi et al. [6] came up with a simple solution to the high viscosity problem. They mixed water with pure glycerol in different percentages and concluded that viscosity can be controlled by mixing glycerol with a certain amount of water. They found that adding 50 weight percentage (wt%) of water in pure glycerol can reduce the viscosity by more than two orders of magnitude. Furthermore, the friction coefficient of pure glycerol in boundary lubrication conditions is at least 30% lower than that of rapeseed oil.
Even glycerol aqueous solution exhibited excellent potential compared to rapeseed oil as a green lubricant when the portion of water content is below 20 wt%. Björling et al. [7] also reported similar friction performance of pure glycerol in line with other findings, even at high pressure rolling/sliding contacts. In addition, they have investigated the friction behaviour of glycerol in boundary, mixed and elastohydrodynamic regimes and in all cases found significant friction improvement.
In addition to glycerol, polyalkylene glycol is another water-soluble and biocompatible compound that shows satisfactory lubricating properties. Recently, the FZG research institute in Germany conducted prototype testing to replicate the transmission gear mechanism in the presence of polyalkylene glycol and conventional PAO-based gear oil [8]. They found that both lubricants performed similar in terms of
film formation capacity under a specific viscosity. Moreover, they also stated that load dependent losses were lower in an FZG gear test when a water containing fluid was used as a lubricating medium instead of conventional oil.
Fraunhofer IVV has also developed lubricants using water as a base medium for the gear drive [9]. They have used carbohydrate-based biopolymers to enhance the viscosity and microbiological stability. Several commercial lubricant manufacturers have also developed their own water-based lubricants over the past couple of years. They have reported desirable performance of WBL from tribological point of view and are continuously developing their technologies to achieve the current state-of-the-art mineral oil-based lubricants [10,11].
Potential as future electric vehicle (EV) fluid Even though many rotating mechanical parts are replaced by electrical components in electric vehicles, the need for lubrication and cooling is still a major concern. Most EV models consist of a single-speed reduction gearbox operated by an electric drive with a high RPM band. Therefore, the need of lubrication is still there to lubricate the gears and bearings. Similarly, there is a higher cooling demand than before to dissipate heat from battery storage and other electrical components to maintain efficiency and longevity. OEMs are constantly looking for a single fluid option that can fulfil the need for coolant and transmission fluid. In this case, the fluid needs to be designed in a balanced way from the tribological, thermal and efficiency points of view. It is quite a challenging task for conventional fluids to meet all the criteria while keeping the environment and sustainability impact in mind.
Formulated water-based lubricant can be an ideal candidate for the single EV fluid concept. Water itself has a high heat capacity which is a desirable requirement for cooling purposes. In most cases, water is mixed with special fluids such as glycols, esters and glycerol which have desirable tribological properties. Therefore, a well-balanced additive chemistry can strengthen the performance of WBLs to use it as a transmission fluid. The iso-viscous nature of WBL results in a lower pressure viscosity response even at high contact pressure, which can be beneficial to reduce power loss and friction. Therefore, WBLs can reduce the losses in the transmission which accounts for around 3% of the overall EV losses [12].
Continued on page 12 LUBE MAGAZINE NO.179 FEBRUARY 2024 11
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