Fabrication and Mechanical Properties of Si<sub>3</sub>N<sub>4</sub>/SiC Nanocomposite with Pressureless Sintering and Sinter-Post-HIPing

Abstract

Si3N4/SiC composites containing 0-30 vol% SiC were fabricated by pressureless sintering and sinter-post-HIPing method using fine Si3N4 and nano-sized SiC powders. Specimens reached full density (>96% theoretical density (th.d.)) except the composites made by pressureless sintering which contained a small amount of sintering additives or a large amount of SiC. All composites with 10 mass% sintering additives reached full density (-99% th.d.) by sinter-post-HIPing. The addition of SiC particles greatly influenced the grain morphology of the Si3N4 matrix. The average grain thickness, length and aspect ratio decreased with increasing SiC content because of grain boundary pinning by SiC particles. Also the mechanical properties were markedly dependent upon the fraction of SiC particle reinforcement, as well as matrix grain size and porosity. When the materials were pressureless sintered, the strength of the composite increased with SiC additions below 20 vol%. Further increase in the SiC content resulted in a corresponding decrease in strength. Maximum strength of 1150MPa was attained at 20 vol% SiC addition. The increase in strength by addition of SiC is due to decrease of grain size of Si3N4 matrix grain size, and the decrease by further increase in SiC content is caused by porosity and SiC agglomerate. On the other hand, toughness decreased monotonically with SiC content. The strength of all samples was enhanced by sinter-post-HIPing without any change in fracture toughness. The strength of sintered samples with 10-15 vol% SiC increased by -20% to 1260MPa by post-HIPing. The increase in strength by post-HIPing was interpreted as being due to a decrease flaw size.