Effect of additive composition on mechanical properties of pressureless sintered silicon carbide ceramics sintered with alumina, aluminum nitride and yttria

Silicon carbide (SiC) ceramics were pressureless sintered with 3 vol% Al 2 O 3 -Y 2 O 3 -AlN additives with the AlN/(Al 2 O 3 +AlN) molar ratios of 0-0.75 at 1850-2000 °C for 1 hr and the effects of additive composition (i.e., changes in the AlN/(Al 2 O 3 +AlN) molar ratio while maintaining constant Y 2 O 3 content) on the mechanical properties of the pressureless-sintered SiC ceramics were investigated. Self-reinforced microstructures consisting of relatively large platelet SiC grains and relatively small equiaxed grains have been obtained in all specimens when sintered at 1900 °C for 1 h in an argon atmosphere. The achievement of self-reinforced microstructures under such mild conditions (holding for 1 hr at 1900 °C) is caused by the beneficial effects of additive composition and the acceleration of the β→α phase transformation of SiC by seeding, i.e., the addition of 1 vol% α-SiC into β-SiC. The typical flexural strength and fracture toughness of the pressureless-sintered SiC ceramics with an AlN/(Al 2 O 3 +AlN) mole ratio of 0.5 were 433 MPa and 6.6 MPa·m 1/2 at room temperature, respectively.