The Impact Behaviour of Low Velocity Charged Microspheres on Planar Target Surfaces

Abstract

Details are given of an experimental and theoretical investigation into the low-velocity (< 50 ms-1) impact behaviour of charged microspheres on planar target surfaces under zero field conditions. The experimental technique is based on a vertically mounted dust-source gun which fires positively charged (≤ 5 x 10-14C) microspheres onto a microzone (~ 200 µm2). Experiments have been performed on a range of microsphere/target combinations, (i.e. Fe/Cu, Ti, Al, W, Mo, stainless steel, Si, glass, p.v.c., mica, ebonite, tufnal, red-fibre and cellulose), and a variety of metal target surface conditions (ambient oxide and highly oxidised): the physical state of the micro-impact zones are monitored using a laser-based ellipsometer. The investigation concentrated on comparing the incidence of "bouncing" events on the various target materials and established the important general distinction between metals and insulators that they respectively promote <1% and >99% of "bouncing" events per incident sample. Highly oxidised metal surfaces show an increased incidence of "bouncing" events that depends inversely on the impact microsphere charge. The mean mechanical behaviour of the "bouncing" events as measured by the coefficient of restitution, exhibits general trends similar to macroscopic experiments, but with a large scatter in the data which is thought to arise from variations in the dynamic hardness and microtopography of the target surface. The electrical behaviour, as measured by the charge modification ratios, indicates that there is negligible charge exchange in a metal/insulator impact event. The “sticking” behaviour on metallic and semi-conducting surfaces is interpreted in terms of thermal heating and results in micro-welding or larger adhesive forces during the impact process, produced by electron-phonon scattering in the oxide contact junction where transient electron tunnelling is taking place. The technological implications of these findings have been exploited in the development of a micron-sized, non-destructive, dynamic hardness measuring device incorporating a computer controlled data acquisition and processing system.