To obtain a silicon nitride-base sintered compact satisfying both high mechanical strength and fracture toughness at high temp. by forming silicon nitride grains having a specified average major axis size and imparting a specified four-point bending strength at a specified temp. and a specified fracture toughness at room temp.
The sintered compact is obtd. by firing a compsn. based on silicon nitride and contg. an Mg compd., a Zr compd. and a Yb compd. in a prescribed ratio, contains silicon nitride grains having an average major axis size La of 1.20-3.65 represented by L/n [L is major axis size and (n) is the number of silicon nitride grains per unit area] and has ≥650 MPa four-point bending strength in air at 1,100°C and ≥7.5 MPa.m1/2 toughness at room temp. A compsn. consisting of 0.25-0.5 pt.wt. MgO, 0.25-1.0 pt.wt. ZrO2, 4.0-7.0 pts.wt. Yb2O3, and the balance silicon nitride with impurities is prepd. and a compact is formed, temporarily fired in a nitrogen atmosphere at 1,750-1,900°C for 4 hr, subjected to concluding firing by hot isostatic pressing under 75-2,000 kg/cm2 and further heat-treated in a nitrogen atmosphere at 1,550-1,700°C for 10 hr to obtain the sintered compact.
ARAKI ATSUO
SHIMAMORI TORU
Next Patent: CERAMIC SLIDING PARTS