MECHANICAL-PROPERTIES OF SI3N4 CERAMICS PREPARED FROM CARBON-COATED POWDERS

The purpose of the present work is to toughen the weak grain boundary phase in Si3N4 ceramics by forming a nano-composite grain boundary phase of SiC particles. The SiC particles are formed by a reaction between carbon and silicon oxide impurity phase on Si3N4 powder surfaces. The carbon is coated on Si3N4 powder surfaces by thermal decomposition of CH4 gas. The sintered bodies are obtained by hot-pressing under 30 MPa applied pressure at 1800-degrees-C for 60 min in N2 atmosphere. Results of mechanical property measurements show that the bending strength increases to 1250 MPa for the 0.29 wt% carbon coated Si3N4 from 990 MPa for the non-coated one, and the fracture toughness shows a similar tendency, i.e., from 5.2 MN/m3/2 to 6.3 MN/m3/2. The crystalline phases of the hot-pressed bodies are determined by X-ray diffraction, and the existence of beta-SiC is confirmed. The fractured surfaces are observed by SEM, and the fracture mode is observed to change from intergranular to transgranular fracture by applying the carbon coating. The generated SiC particles toughen the grain boundary phase, and improve mechanical properties by eventually producing a nano-composite phase of the grain boundary phase and nano-size SiC particles.