OXIDATION BEHAVIOR AND EFFECTS OF OXIDATION ON THE STRENGTH OF SIC-WHISKER REINFORCED ALUMINA

The oxidation behaviour and effect of oxidation on the strength of a SiC-whisker-reinforced-alumina composite material (Al2O3-SiCw) were investigated. The oxidation mechanism of the composite material was determined by thermogravimetric analysis (TGA) and compositional analysis. Changes in the fracture strength and surface morphology were also determined and related to the oxidation mechanism. Weight changes of samples exposed to flowing Ar with various levels of oxygen partial pressure, P-O2 at 1400 degrees C were monitored continuously with a microbalance. Changes in strength were measured after exposure to flowing Ar with different P-O2 at 1400 degrees C for various periods of time. The P-O2-range employed in this experiment was from 5 x 10(-7) to 1 x 10(-3) MPa. In contrast to the oxidation behaviour of monolithic SiC materials, weight gains were detected for the whole P-O2-range investigated. However, despite the weight gains in the low P-O2-region (P-O2 < 1 x 10(-5) MPa), significant reductions in strength were observed which were apparently due to the loss of SiC whiskers from the sample surface via the formation of volatile SiO. This SiO gas reacted with the Al2O3 matrix to form a non-protective aluminosilicate glass on the surface, resulting in a linear weight gain with time. In the high P-O2-region, typical parabolic weight gains were observed as a result of the formation of an aluminosilicate glass on the surface by a reaction between SiO2, formed by the oxidation of SiC whiskers, and the matrix alumina. The observed increases in strength of the specimens with exposure are believed to be due to blunting of existing surface flaws with a product oxide.