Role of sintering and clay particle additions on coating formation during PEO processing of AM50 magnesium alloy

Abstract

In this study sintering of electrochemical conversion products on the surface of AM50 magnesium alloy by the discharges was identified as an essential step in the coating formation in PEO processing. Clay particles were selected as suitable additives because of their relative low melting point and possible reactions with the substrate-electrolyte conversion products. The comparison of coatings formed in electrolytes with and without particles clearly indicates a complete change in the microstructure. In the standard electrolytes the coatings are crystalline while they are amorphous if particles are present though the energy input (process parameters) was more or less the same. Only the presence and incorporation of the particles into the coating has obviously induced a low temperature reactive liquid phase formation process, reaching a coating composition with glass forming ability which is likely due to the fast cooling by the electrolyte. In contrast, the coating formation in standard electrolytes has to be considered more like a solid phase sintering. However the results clearly indicate that the sintering of reaction/conversion products by the high energy discharges is a very important step in the coating formation and that the sintering by the discharges in PEO processing can be influenced by the use of additives similar to traditional sintering processes.