A silicon nitride sintered body and its manufacturing method

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PROBLEM TO BE SOLVED : To obtain a uniform silicon nitride-base sintered compact having superior strength and toughness and to provide a method for producing a silicon nitride-base sintered compact by which the mechanical characteristics of the sintered compact are enhanced by an easy method. SOLUTION : A silicon nitride-base sintered compact having a principal crystalline phase of silicon nitride and a non-crystalline phase contg. at least a rare earth element and aluminum in the crystal boundaries of the crystalline phase is heated to a temp. below the thermal shock resistance temp. of the sinteted compact by 50-100 deg.C and is rapidly cooled to room temp. at >=100 deg.C/sec cooling rate to obtain the objective silicon nitride-base sintered compact having >=3.2×10 / deg.C coefft. of thermal expansion in the temp. range from room temp. to 1, 000 deg.C, >=6.5 MPa.m fracture toughness of the surface, difference between the fracture toughness of the surface and that of the central part, >=30 MPa compressive stress imparted to the principal crystalline phase of silicon nitride in the surface and the central part and >=1, 000 MPa strength at a room temp.