Effect of atmosphere and sintering time on the microstructure and mechanical properties at high temperatures of $\alpha$-SiC sintered with liquid phase Y$_2$O$_3$ and Al$_2$O$_3$

Abstract

The influence that the atmosphere (N_2 or Ar) and sintering time have on microstructure evolution in liquid-phase-sintered alpha-sic and on its mechanical properties at high temperature was investigated. The microstructure of the samples sintered in N2 was equiaxed with a grain size of 0.70 {\mu}m and a density of 98% of the theoretical value regardless of the sintering time. In contrast, samples sintered in Ar had an elongated-grain microstructure with a density decreasing from 99% to 95% and a grain size increasing from 0.64 to 1.61 {\mu}m as the sintering time increased from 1 to 7 hours. The mechanical behaviour at 1450 {\deg}C showed the samples sintered in nitrogen to be brittle and fail at very low strains, with a fracture stress increasing from 400 to 800 MPa as the sintering time increased. In contrast, the samples sintered in Ar were quasi-ductile with increasing strain to failure as the sintering time increased, and a fracture stress strongly linked to the form and size of the grains. These differences in the mechanical properties of the two materials are discussed in the text. During mechanical tests, a loss of intergranular phase takes place in a region, between 50 and 150 {\mu}m thick, close to the surface of the samples--the effect being more important in the samples sintered in Ar