From molecular liquids to ionic: advancing tribology for extreme conditions

Our ability to lubricate machine components effectively sets the boundaries for the technologies we can reliably deploy. This is patently clear in fields like wind power, wave and tidal power, and space. These applications face challenges due to low-speed oscillating motions where conventional lubrication methods struggle. By means of ionic material design, we probe these limitations with the objective of enabling progress in key machine technologies towards sustainable development. Our research shows that ionic liquids (ILs), when used as grease additives, can delay lubricant ejection from fretting contacts and provide remarkable lubricity. Nuclear magnetic resonance reveals that the ability of the IL to enact this effect depends on whether it is sequestered with the other grease components (thickener – base oil blend) or is readily available and mobile. Wide angle x-ray scattering shows that when these ionic liquids are subjected to pressures in the GPa range they have similar structural compliance and liquid-to-solid transitions when compared to a conventional PAO synthetic oil, and that these can be modified by changing the structure of the constituent ions. These findings illustrate that the effect of lubricant retention within the contact is linked to the ability of the IL to reach the surfaces and strongly adsorb, and that distinct behaviors under pressure may be achieved by tuning the architecture of the ionic species. We then explore the impact of these effects at the component level in oscillating bearings, using a custom-built frameless bearing test rig. Furthermore, these ILs are shown to have remarkable performance in vacuum environments, reducing wear by multiple orders of magnitude when compared to heritage space lubricants, and offering a pathway towards PFAS-free vacuum lubricants. The unique versatility and potential of these non-halogenated ILs is further highlighted when we then show that it is possible to use carbon capture and biomass products to synthesize ionic lubricants. Overall, it is shown that ionic materials can be leveraged to expand the limits set by our current technology in lubrication practice.