Microstructural Evolution Of Surface Layers During Electrolytic Plasma Processing

Abstract

Electrolytic Plasma Processing is an emerging technology for surface modification. The EPP process is based on electrolysis of an aqueous electrolyte by application of an electrical potential between the workpiece and counter-electrode, and the production of plasma (micro-arc discharges on the workpiece surface). The plasma micro-arcs provide a heat source for surface modification via localized surface melting and rapid cooling (cleaning process) and, if desirable, enhance ion deposition on a given substrate (coating). Three substrates (low carbon steel, pure Al and pure Ti) were "cleaned" by EPP and their near surface layer microstructure was studied. It was found that the uppermost layer for all three substrate materials was developing a "hill and valley" surface morphology, with individual characteristics influence by material's properties such as melting point, undercooling and surface energy. The affected layer extends up to 2μm for all three substrates and the top layer was found to exhibit ultrafine grains. The EPP modified surface layer developed compressive residual stresses. The magnitude of the stress is correlated to the melting point which controls the annealing and grain growth kinetics, for each material. Three coating materials, Zn, Ni and Mo, with a wide range of melting temperature were deposited by EPP on steel substrates. The topography exhibited by each coating was found to be influenced by its surface energy. Low surface energy coating material will present large nodules, high deposition rate and increased porosity. Medium surface energy coating materials were found to develop large nodules, lower deposition rates and low porosity. High surface energy coatings tend to present nodule coalescence and may exhibit crack formation, depending on the difference in TM between the substrate and the coating material. The microstructure at the coating/substrate interface was studied. Two controlling parameters were found regarding substrate/coating interface evolution. These are difference in melting point between substrate and coating material and the phase diagram characteristics. The extent of the interface was related to the difference in TM between the coating and the substrate material. A low TM coating material (Zn) interface will form intermetallics as predicted by the phase diagram. A coating with comparable TM with the substrate (Ni) results in a liquid phase with both elements soluble and depending on the phase diagram characteristics, significant mixing can occur at the interface. The formation of the high melting temperature coating (Mo) was found to be dominated by its large TM compared to the substrate. The continuous presence of a liquid phase of the substrate material was resulting in the extension of the interface far into the coating. Depending on the phase diagram, intermetallics may form at the interface. The present findings show that a judicious selection of coating materials can be made by considering the coupling substrate - coating TM and their binary phase diagram.