Effect of interfacial voltage on tribological characteristics of polysilicon micro-contacts tested in ambient air under dynamic impact conditions

Abstract

A polycrystalline silicon microtribometer was used to examine the evolution of tribological properties under dynamic mechanical impact and hot switching conditions. Changes in surface properties were tracked via periodic static adhesion tests. Surfaces experienced two distinct periods in the course of the dynamic lifetime—a run-in phase with little measurable surface modification and a degradation phase with significant surface modification and corresponding increases in the measured adhesion force. No statistically significant differences were found in either the run-in period length or the degradation rate between hot switching conditions and purely mechanical impacting. However, when combining these two parameters, it was discovered that longer run-in periods correspond to slower degradation rates for the mechanical switching but correspond to faster degradation rates for the hot switching. This can be attributed to the action of the surface hydrocarbons physisorbed from the ambient air. These hydrocarbons have high surface mobility because they are bound by weak van der Waals forces. Under hot switching conditions, mobility is enhanced by increasing surface temperature caused by electrical current flow through nano-asperities. Mobile hydrocarbons diffuse across the surface (driven by concentration gradients) to damaged areas resulting in a healing effect that is dominant in the hot switching tests. Results are interpreted in light of the principle tribological degradation and healing mechanisms and the experimental sources of these mechanisms.