HIGH-TEMPERATURE CRACK ANNEALING EFFECTS ON FRACTURE-TOUGHNESS OF ALUMINA AND ALUMINA-SIC COMPOSITE

Fracture toughness and bend strength of an alumina ceramic and a compo site (Al2O3-17 vol.-%SiC) have been investigated in ail between 800 an d 1400 degrees C, both before and after high temperature annealing. In unannealed material the toughness in the monolithic alumina was found to decrease with increasing test temperature, while the opposite effe ct was observed in the reinforced alumina where K-1c was highest at 14 00 degrees C. High temperature annealing increased room temperature to ughness considerably, although after cooling to ambient and testing at the annealing temperature, the toughness was found to be similar to t hat of unannealed specimens. Bend strength dependence on annealing tem perature was found to differ for uncracked and precracked alumina samp les. In uncracked material the strength levels between 1000 and 1400 d egrees C were similar, whereas in precracked and annealed specimens a progressive increase in strength with higher annealing temperatures wa s observed. Election microscopy observations confirmed that the result s could be explained by crack healing which had occurred after prolang ed annealing above 1200 degrees C.