High temperature mechanical properties of Si3N4–MoSi2 and Si3N4–SiC composites with network structures of second phases

Si3N4–MoSi2 composites have been employed as ceramic glow plugs for an application to functional parts. The ceramic glow plugs were developed and mass-produced by DENSO Co. in Japan. The ceramic glow plugs are used to help start engines by heating them installed in the combustion chamber of the diesel engine. They can be heated at a higher heating rate, that is, the higher heat resistance than a metal glow plug with the result that the diesel engine can be started faster. To the purpose of improving the high temperature mechanical properties of Si3N4–MoSi2 composites, the relationship between the composition and high temperature strength of the Si3N4–MoSi2 composites were investigated at 1400°C. The 1400°C strength could be improved by controlling Y2O3 contents. Si3N4–SiC composites with a three dimensional network structure of SiC particles has been investigated. The composites consist of a plurality of Si3N4 grains surrounded by fine SiC particles which are discontinuously dispersed. It was found that the creep resistance and oxidation resistance as well as electrical and thermal properties of the composite can be considerably improved. Furthermore, the simultaneous improvement of both superplastic deformation and creep resistance by forming a network structure of Si3N4–SiC phase in the Si3N4–SiC composites was also examined at elevated temperatures. It was found that the superplastic deformation and creep resistance of the composites with the network structure of continuous Si3N4–SiC phase could been improved in one sintered body.