The extravagance of the lithium paradigm in lubrication: mechanochemistry reveals weaknesses and alternative strategies

Lithium salts are by far the most common type of thickener in lubricating greases. The battery industry is placing increasing demands on lithium supply and pricing, however, and a probable toxin classification of precursor material looms. It is thus important to explore a transition to a lithium-independent lubricant industry. In addition to providing the consistency properties of grease, a primary function of the thickener is to act as a passive lubricant supply system, releasing oil into the contact and making functional additives available to the surface as required. In this study, the performance of lithium complex grease was stress-tested by subjecting it to highly loaded rolling-sliding contact conditions that are now commonplace in modern machines. Tribological evaluation and Time-of-Flight Secondary Ion Mass Spectrometry analysis of tribofilms together reveal serious limitations in the properties of lithium thickeners; instead, a lithium-free grease matrix system, based on polypropylene, is shown to display better lubricating performance, rendering it a viable and more effective alternative.