Fundamental Mechanisms Underlying the Effectiveness of Nanoparticle Additives to Lubricants: 25 Examples Linking Nano- to Macroscale Friction

Abstract

Studies of the fundamental origins of friction have undergone rapid acceleration in recent years by providing valuable information on the nanoscale mechanisms responsible for friction at the macroscopic level. Significant efforts have been directed at developing composite nanofluids and nanoparticle additives to unlock new tribological properties unattainable by traditional lubricants. The studies are now further evolving by developing methods to achieve active control over nano- and/or mesoscale friction through the application of magnetic and electric fields external to the contact. These methods constitute an area of rapidly growing interest, and they also illuminate how the performance of conventional lubricants could be enhanced through the synergistic addition of nanoparticles (NPs). This mini review highlights 25 publications that collectively reveal significant progress, as well as important outstanding challenges, to the fundamental understanding of how the addition of NPs impacts lubricant performance. The first two topics focus on how Quartz Crystal Microbalance (QCM) nanotribological response to solid contacts can be linked to macroscale friction coefficients in the boundary lubrication regime and how QCM response upon immersion into a liquid is linked to macroscale lubricity in the mixed and hydrodynamic regimes. The third and fourth topics highlight the pivotal role of nanoparticle charge and surface treatments, while also indicating that the rolling of nanoparticles is ineffective and/or detrimental. The fifth topic focuses on applications that demonstrate the tuning of friction by varying nanoparticle electric charge and/or an external electric potential. The highlighted literature was selected to demonstrate a range of experimental and theoretical research, to provide direct connections between the nanoscale and macroscale tribological attributes, and to emphasize environmentally friendly lubricating materials such as water-based nanofluids.