Degradation of SiC particles in aluminium-based composites

Abstract

The technological process used for the production and processing of metal matrix composites (MMCs) can require contact over extended periods of time between matrix and the ceramic reinforcement or at least the permanence at high temperature and pressure of the two parts in contact. During the contact a chemical interaction takes place in the interfacial zone as a consequence of the free-energy difference existing between ceramic compound and metal. Aluminium and aluminium alloys reinforced with silicon carbide are widely utilized materials. The chemical interaction between matrix and reinforcement is not very fast but the reaction equally occurs, and a harmful layer of interfacial compound (Al₄C₃) is developed after a sufficiently long time. At present, the degradation of the reinforcement produced by molten matrix is the major problem for some production technologies. This problem has only been solved partially by using a coating or changing the chemical nature of the matrix. In particular, the technological problem of interfacial reaction in the SiC-Al system can be solved by adding elemental silicon to the matrix to achieve the eutectic composition. However, this expedient gives rise to a consequent significant lowering of the melting point. The problem can be overcome and the production process improved without changing the characteristics of the material by the control of processing parameters. The interfacial reaction also produces elementary silicon and this has been found as aluminium-silicon eutectic segregated at the aluminium grain boundary. An accurate description of the kinetic process can be obtained by determining the silicon content present in the matrix by original derivations obtained by means of calorimetric analysis.