Crack-healing and oxidation behavior of silicon nitride ceramics

Si3N4/SiC composite ceramic specimens, made to JIS standard, were sintered and subjected to three-point bending. Semi-elliptical surface cracks of 50–400 μm in diameter were made on the tensile side of each specimen. Crack-healing behavior as a function of environment, temperature, time, and crack size, and oxidation behavior as a function of temperature and time were studied. The main conclusions are as follows: (1) Cracks healed completely in air, but did not heal in Ar gas, N2 gas nor in a vacuum. (2) This Si3N4 ceramic has the ability to heal a crack at temperature from 900 to 1400 °C completely. (3) The maximum surface crack size that can be healed completely was 200 μm in diameter. (4) The activation energies for crack-healing and oxidation were 150 and 131KJ/mol, respectively.