1993 Volume 42 Issue 477 Pages 702-707
The effect of high temperature oxidation on bending strength of Si3N4 containing AlN-Y2O3-HfO2 and of SiC containing B-C additives was studied at room and elevated temperatures up to 1673K.
The surface of all the oxidation specimens was covered by a uniform layer after oxidation. The three-point bending strength of Si3N4 after oxidation was significantly decreased at room temperature but it was nearly equal to that before oxidation at elevated temperatures at which oxidation was performed. On the other hand, the bending strength of SiC after oxidation was not decreased at both room and elevated temperatures. No crack was observed on the oxide layer formed at Si3N4 surface after oxidizing at elevated temperatures and cooling down to room temperature, while many fine cracks were observed on the oxide layer formed at SiC surface.
From the residual stress calculation by FEM after the oxidizing and cooling procedures, it was confirmed that the decrease in bending strength of oxidized Si3N4 at room temperature is caused by the residual stress generated by the difference in thermal expansion between the oxide layer and the base ceramic body. It was also considered that no-decreased in bending strength of SiC at room temperature is due to the relaxation of such residual stress by layer cracking.
