Active coatings for SiC particles to reduce the degradation by liquid aluminium during processing of aluminium matrix composites: study of interfacial reactions

The application of a surface coating on SiC particles is studied as an alternative means of solving problems of reactivity between SiC reinforcements and molten aluminium and problems of low wetting which limit the application of casting routes for fabrication of Al–SiC_p composites. The selected active barrier was a ceramic composed of SiO₂, which was generated by controlled oxidation of the SiC particles. The coating behaves as an active barrier, preventing a direct reaction between molten aluminium and SiC to form Al₄C₃ as the main degradation product. At the same time, the SiO₂ provokes other interfacial reactions, which are responsible for an improvement in wetting behaviour.
Composites were prepared by mixing and compacting SiC particles with Al powders followed by melting in a vacuum furnace, and varying the residence time. Transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and field emission TEM were employed as the main characterization techniques to study the interfacial reactions occurring between the barrier and the molten aluminium. These studies showed that the SiO₂ coating behaves as an active barrier which reacts with the molten Al to form a glassy phase Al–Si–O. This compound underwent partial crystallization during the composite manufacture to form mullite. The formation of an outer crystalline layer, composed mainly of Al₂O₃, was also detected. Participation of other secondary interface reactions inside the active barrier was also identified by HREM techniques.