Sintering and HIPping of silicon nitride-silicon carbide composite materials

Abstract

Si 3N 4 composite materials containing up to 60 vol.% of dispersed β-SiC particles were sintered with Y 2O 3 and Al 2O 3 at 1850°C and 0·1 MPa N 2. Fractional density decreased from 0·97 to 0·91 when the SiC content increased from 0 to 60 vol.%. Simultaneously a retardation of grain growth and reduced pore size was found. Subsequent HIPping at 2000°C with N 2-pressure of 100 MPa resulted in almost complete elimination of presintered closed pores and a final density of 0·99 up to 20 vol.% of SiC. During HIPping Si 3N 4 is formed by reaction of SiC or C with the SiO 2 intergranular glass in the presence of high N 2-pressure. While fracture toughness shows no significant influence of SiC content, a reduction of critical defect size with increasing SiC content results in a distinct increase of fracture strength particularly after HIPping.