Surface functionalization of metal and metal oxide nanoparticles for dispersion and tribological applications – A review

Friction has caused a substantial amount of property damage in many industries worldwide, and the continuous progress of industrial production requires the development of high-performance lubricants. The performance of lubricants largely depends on the additives in their formulation. Notably, metal and metal oxide nanoparticles (NPs) have been widely used to improve lubricants and have shown great potential. However, their efficacies as lubricant additives are limited due to their high surface energy, as well tendency to aggregate, agglomerate, and coagulate in lubricating media. Owing to these concerns, researchers have focused on tunable surface functionalization of metal and metal oxide NPs. Surface functionalization has been demonstrated as an effective way to improve the performance of such NPs as reinforcing phases and lubricant additives in composites. Therefore, this article provides a systematic review of the current research status on the surface functionalization of metal and metal oxide NPs, which has not been comprehensively reviewed yet. To begin with, we outline the existing synthesis methods of metal and metal oxide NPs and discuss the existing surface functionalization strategies. Following, we review the dispersion evaluation methods and tribological studies of metal and metal oxide NPs. Subsequently, we also assess the impact of surface functionalization strategies on the dispersion and tribological performance of metal and metal oxide NPs, as well as their underlying mechanisms. Finally, we discuss the existing challenges and suggest potential future research directions. We anticipate that this article could serve as an excellent guide for the chemical functionalization of metal and metal oxide NPs, which could be beneficial for various applications such as tribology, thus opening up a pathway to improve the physical and chemical properties of metal and metal oxide NPs.