Formation mechanism of wear-resistant composite film by Span 80-decorated halloysite nanotubes

Phyllosilicate clay mineral, despite being a high-performance and low-cost lubricant additive, faces the problems of possible biotoxicity and inconsistent mechanisms. Halloysite nanotube (HNT) clays have attracted much research attention because of their good biocompatibility and potential film-forming properties. The tribological properties of Span 80-decorated HNTs dispersed in a base oil at various concentrations were investigated using a pin-on-disk tribometer. The results show that at low concentrations (0.1–0.6 wt%), the modified HNTs, which have higher friction coefficients, exhibited excellent anti-wear performance, especially at 0.6 wt% concentration, at which the wear was readily reduced by 46.5%. The outstanding wear resistance is attributed to the formation of a composite tribofilm on the surface. The tribofilm comprises an outer layer of soft amorphous carbon film and an inner hard transition layer containing Al2O3, SiOx, and iron oxide. In contrast, at high HNT concentrations (0.8–1.0 wt%), the friction coefficient decreased by 13.5% attributed to the conversion of adhesive wear to abrasive wear by unfilmed halloysites. Further experiments demonstrate that the key factor to the formation of the composite biofilm is the crushing of halloysite, which is also affected by the load. These findings provide new insights into the tribological mechanism of phyllosilicates as potential eco-friendly additives.