Correlation between microstructure and creep behavior in liquid-phase-sintered α-silicon carbide

The influence of increasing the sintering time from 1 to 7 h on the microstructure evolution and the mechanical properties at high temperature was studied in a-silicon carbide (alpha-SiC) sintered in argon atmosphere with Y2O3-Al2O3 (10% weight) as liquid phase (LPS-alpha-SiC). The density decreased from 98.8% to 94.9% of the theoretical value, the grain size increased from 0.64 to 1.61 mu m, and some of the grains became elongated. The compression tests were performed in argon atmosphere, between 1450 degrees C and 1625 degrees C and stresses between 25 and 450 MPa, with the strain rate being between 4.2 x 10(-8) and 1.5 x 10(-6) s(-1). The stress exponent and the activation energy Q were determined, finding values of it between 2.4 +/- 0.1 and 4.5 +/- 0.2 and Q = 680 +/- 35 kJ/mol for samples sintered for 1 h, and it between 1.2 +/- 0.1 and 2.4 +/- 0.1 and Q = 710 +/- 90 kJ/mol for samples sintered for 7 h. The correlation between these results and the microstructure indicates that grain-boundary sliding and the glide and climb of dislocations, both accommodated by bulk diffusion, may be two independent deformation mechanisms operating. At the temperatures of the tests, the existence of solid-state reactions between SiC and the sintering additives is responsible of the microstructural changes observed. These effects are not a consequence of the process of deformation, but rather they are because of the thermal treatment of the material during the creep.