CORROSION AND STRENGTH OF SIC-WHISKER-REINFORCED ALUMINA EXPOSED AT HIGH-TEMPERATURES TO H2-H2O ATMOSPHERES

The flexural strengths and corrosion behavior of a SiC-whisker-reinforced alumina composite material (Al2O3-SiC(w)) were studied as functions of time, temperature, and P(H2O) in H-2-H2O environments. Experimental conditions included temperatures of 1300-degrees and 1400-degrees-C, exposure times up to 30 h, and water vapor levels ranging from very dry (P(H2O) = 4.6 x 10(-7) MPa) to relatively wet (P(H2O)) 2.2 x 10(-3) MPa). The study showed that the weight, strength, composition, and microstructure were strongly dependent on the P(H2O) level in the environment. When the P(H2O) in the H-2 atmosphere was low, active oxidation of the SiC whiskers in the composite occurred, causing severe reductions in the weight and strength of the samples. At 1400-degrees-C, the most severe degradation was observed following exposure to an atmosphere with P(H2O) = 1.3 x 10(-5) MPa. Exposure for 10 h to such an environment reduced the room-temperature flexural strength of the material to less than 50% of that of the unexposed samples. At higher water vapor levels, the reductions in weight and strength became less severe because of the formation of aluminosilicate glass and mullite at the sample surface. Finally, when the composite was exposed at 1400-degrees-C to H-2 atmospheres containing water vapor pressures of 5 x 10(-4) MPa and above, weight gains were observed; and the strength of the material was significantly higher than that of the unexposed material. The increase in strength was attributed to crack healing or blunting as a result of the formation of aluminosilicate glass and mullite on the sample surface. Similar dependencies of strength and weight on the level of oxidant (H2O) in the atmosphere were noted following the exposures at 1300-degrees-C; however, the effects were smaller in magnitude.