Oxidation barriers on SiC particles for use in aluminium matrix composites manufactured by casting route:: Mechanisms of interfacial protection

This paper is centred on a study of the interface reaction mechanisms which participate in the fabrication of an aluminium-SiC composite by a casting route, when reinforcements (particles, in this case) have been previously coated by oxidation with a SiO2 layer. The studies, which were carried out using transmission electron microscopy and differential scanning calorimetry, made it possible to propose a model of action of the SiO2 barrier in relation to the coating thickness and the reaction time. The first reaction that occurred in this SiC-SiO2-molten Al system was the formation of an Al-Si-O glassy phase which progressively consumed the SiO2 barrier, reducing the matrix-particle interface energy and favouring wetting of the SiC surfaces. When the oxidation coating was completely consumed, the SiC was preferentially dissolved by the glassy phase, inside which the formation of amorphous carbon was detected. These studies also show that carbon enrichment of the reaction layer activated the precipitation of metallic impurities (such as Fe or Cu) in the reaction. Longer reaction times (8 h) could also favour crystallization of the glassy phase to form mullite and the formation of microcrystalline alumina at the reaction interface. (C) 2002 Kluwer Academic Publishers.