A Combined QCM and AFM Study Exploring the Nanoscale Lubrication Mechanism of Silica Nanoparticles in Aqueous Suspension

Addition of nanoparticles to liquid lubricants often leads to a reduction in both friction and wear rates for a wide range of solid-liquid-nanoparticle combinations. While the lubricating properties of nanoparticles are well documented, the detailed physical mechanisms remain to be fully explored. In a step toward such an understanding, the nano-tribological properties of gold surfaces immersed in aqueous suspensions of negatively charged SiO2 nanoparticles were examined by means of Quartz Crystal Microbalance (QCM) and Atomic Force Microscopy methods. The SiO2 nanoparticles were found to reduce the resistance to shear motion at the QCM's solid-liquid interface. The effect was observed to be concentration dependent, with ca. 1.5 wt% yielding the maximum reduction in shear. An electrokinetic mechanism is proposed whereby the loosely bound nanoparticles roll and/or slide on the surface, while upper layers of nanoparticles slip over the surface layer because of the repulsive electrostatic forces between the individual particles. The nanoparticles were observed to remove the electrode material from the gold surface and slightly increase the overall roughness with the major change happening within the first hour of the exposure. This study inherently provides insight into a complex interface of solid, liquid and nanoparticles at a nanometer scale.