Creep behaviour of Si3N4 ceramics sintered with RE2O3

The objective of this work was to evaluate the creep behaviour Of Si3N4 based ceramics obtained by uniaxial hot-pressing. As sintering additive, an yttrium-rare earth oxide solid solution, designed RE2O3, that shows similar characteristics to pure Y2O3, was used. Samples were sintered using high-purity alpha-Si3N4 powder, with additive mixtures based on RE2O3/Al2O3 or RE2O3/AIN, at 5 and 20 vol.%, respectively. The sintered samples were characterized by X-ray diffractometry, scanning electron microscopy and density. Specimens of 3x3x6 mm(3) were submitted to creep tests, under compressive stresses between 100 and 350 MPa at temperatures ranging from 1250 to 1375 degrees C in air. Samples with RE2O3/Al2O3 showed beta-Si3N4 as crystalline phase, with grains of high aspect ratio, and a relative density around 99% of the theoretical density. The Si3N4/RE2O3/AIN samples presented alpha-Si3N4 solid solution, designed alpha-SiAlON, with a more equiaxed microstructure and slightly lower relative density (96-98%). The results of creep tests indicated that these cera mics containing alpha-SiAlON are the more creep resistant, with steady-state creep rates around 10(-4) h(-1), with stress exponents (n) in the range 0.67-2.53, indicating grain boundary sliding as the main creep mechanism.