DEGRADATION OF SIC PARTICLES IN ALUMINUM-BASED COMPOSITES

The technological process used for the production and processing of me tal 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 contac t. During the contact a chemical interaction takes place in the interf acial zone as a consequence of the free-energy difference existing bet ween ceramic compound and metal. Aluminium and aluminium alloys reinfo rced 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 de gradation of the reinforcement produced by molten matrix is the major problem for some production technologies, This problem has only been s olved partially by using a coating or changing the chemical nature of the matrix. In particular, the technological problem of interfacial re action in the SiC-Al system can be solved by adding elemental silicon to the matrix to achieve the eutectic composition. However, this exped ient gives rise to a consequent significant lowering of the melting po int. The problem can be overcome and the production process improved w ithout changing the characteristics of the material by the control of processing parameters. The interfacial reaction also produces elementa ry silicon and this has been found as aluminium-silicon eutectic segre gated at the aluminium grain boundary. An accurate description of the kinetic process can be obtained by determining the silicon content pre sent in the matrix by original derivations obtained by means of calori metric analysis.