This work was performed in an effort to fundamentally understand how nanoscale contacts between materials evolve due to coupled stresses, specifically those arising from mechanical and electrical stimuli. To investigate the stress response of contact between these materials, atomic force microscopy (AFM) experiments was performed with the AFM tip-substrate interface subjected to normal mechanical loading and electric current, individually and in combination. Experiments to study the effect of coupled external stresses on metal/metal and metal/graphite interfaces are largely based upon AFM. We chose this experimental tool because the AFM tip mimics the contact of a single nanoscale asperity which provides a means of studying the very complex process by which a contact area evolves with stress. AFM is also advantageous for this study because 1) a predefined force can be applied to the tip-substrate interface enabling measurement of forces down to the pN level; 2) multiple characteristics of an interface can be measured simultaneously under various external stimuli at the nanoscale; and 3) measurements can be made in a non-vacuum environment unlike other nanoscopic equipment. After creating a nanoscale contact by bringing a metal-coated AFM probe or all-metal probe into contact with a sample, coated with either a metal or a graphitic layer, mechanical and/or electrical stresses are applied to the tip-substrate interface. Then the resulting adhesion force and electrical contact resistance are measured. These measurements are used not only to understand how stresses couple to affect the size of nanoscale material interfaces but also to explore limits of continuum contact area expressions for predicting contact evolution and to identify atomic-scale metrics for characterizing nanoscale contacts.

Research supported by the US AFOSR (Grant # FA9550-15-1-0256).