Tribological Behavior and Mechanism of Surface-Modified Mullite as Polyurea Grease Additive

Abstract The excellent tribological performance of nanoparticles as additives in lubricating greases has attracted widespread attention. In this study, a silane coupling agent (KH550) was used to chemically modify nanoscale mullite (composed of Al2O3 and SiO2), and the modified nano-KH-mullite was dispersed as an additive in polyurea grease at different ratios (0.01, 0.05, 0.07, 0.1, and 0.3 wt%). The friction and wear performance of the compound polyurea greases were evaluated using a four-ball friction tester. Professional instruments will be utilized to analyze the micro-morphology, 3D profile, elemental content, and valence state of the worn surface, aiming to investigate the lubrication and antiwear mechanism of the nano-KH-mullite additive. The results indicate that nano-mullite effectively improves the antiwear and antifriction performance of the polyurea grease. When the additional content is 0.05 wt%, the friction coefficient (COF) and the wear scar diameter (WSD) are reduced by approximately 19.97% and 11.1%, respectively, and the roughness of the worn surface is also reduced. The excellent tribological performance of the composite grease is attributed not only to the good dispersibility of KH-mullite but also to its ability to promote the formation of a composite lubrication film on the friction pair surface. The component KH-Al2O3 in the KH-mullite can fill and repair scratches and grooves on the worn surface, while the other component KH-SiO2 can bear the load and convert the friction mode to rolling friction.