Insights into the origins of friction in hard materials via two-axis nanoindentation

Abstract Friction is commonly thought to be controlled by processes in small asperities between two sliding surfaces. These asperities can dramatically increase the local contact stresses to levels at which local yielding can occur, even in hard materials. In this study, a unique two-axis nanoindentation system was used to examine the mechanics of deformation of micron-scale asperity contacts by sliding a nominally spherical diamond indenter over a flat plate of fused silica under a combination of normal and transverse loading. The nanoindentation system is composed of two independently controlled nanoindentation actuators oriented transverse to each other in such a way that the forces and displacements of each can be independently controlled and monitored. Tests were conducted by holding the normal force on the spherical indenter constant and ramping the lateral force at a constant rate. Results are presented and discussed in terms of prevailing ideas on the mechanisms that control friction. Graphical Abstract