34. Velocity Dependence of Nanoscale Friction, Adhesion and Wear

The advent of micro/nanostructures and the subsequent miniaturization of moving components for various nanotechnology applications, such as micro/nanoelectromechanical systems (MEMS/NEMS), have ascribed paramount importance to tribology and mechanics on the nanoscale. Most of these micro/nanodevices and components operate at very high sliding velocities (of the order of tens of mm/s to few m/s). Atomic force microscopy (AFM) studies into potential materials, coatings and lubricants for these devices have been rendered inadequate due to the inherent limitations on the highest sliding velocities achievable with commercial AFMs (<250  μm/s). The development of a new AFM-based technique has allowed nanotribological investigations to be performed over a wide range of velocities (up to 10 mm/s). Research conducted on various materials, coatings and lubricants reveals a strong velocity dependence of friction, adhesion and wear on the nanoscale. Based on the experimental evidence, theoretical formulations have been realized for nanoscale friction behavior in order to design a comprehensive analytical model that explains the velocity dependence. The model takes into consideration the contributions of adhesion at the tip–sample interface, high impact velocity related deformations at the contacting asperities, and atomic-scale stick-slip. Dominant friction mechanisms are identified and critical operating parameters for their transitions are defined. Wear studies are conducted at high sliding velocities to elucidate the primary failure mechanisms. A novel AFM-based nanowear mapping technique to map wear on the nanoscale is developed, and the interdependence of normal load and sliding velocity on sample surface wear is studied. This technique helps identify and classify wear mechanisms and determine the critical parameters responsible for their transitions. The interdependence of mechanical and tribological properties for various materials is explored and tribologically ideal materials with low adhesion and friction, suitable for nanotechnology applications, are identified.