The synthesis of composites and solid solutions of α-SiCAlN by field-activated combustion

Abstract

One approach to improve the relatively low fracture toughness of SiC is to add reinforcement phases in the form of fibers, whiskers, or particles. But the residual defects in composites sintered from mixtures of such reinforcement and matrix phases causes a reduction in the strength of these composites. Another way to improve the fracture toughness of SiC is to form SiC-AlN composites in situ. The crystal structures of SiC and AlN are very similar and a complete solid solution of SiC-AlN can be formed. However, the phase diagram of the SiC-AlN system shows the existence of a miscibility gap. A solid solution of SiC-AlN will decompose into two isostructural 2H phases by annealing at temperature below about 2,100 C. SiC-AlN composites with two separate phases in a modulated structure have shown much improved fracture toughness. The possibility of synthesizing composites or solid solutions of SiC and AlN by self-propagating high-temperature synthesis (SHS) is an attractive energy-saving alternative. However, while the calculated adiabatic combustion temperatures (T{sub a}) of several possible synthesis routes are high, no self-propagating combustion reaction can be initiated. In previous studies it was demonstrated that the application of an electrical field activates SHS processes, and thus the formationmore » of SiC-AlN composites as a solid solution may be possible under the influence of an electric field.« less