OXIDATION BEHAVIOR AND FLEXURAL STRENGTH OF ALUMINUM NITRIDE EXPOSED TO AIR AT ELEVATED-TEMPERATURES

The oxidation behavior of a sintered aluminum nitride containing 3 wt% Y2O3 as a sintering aid was investigated. Samples were exposed to air at elevated temperatures for times up to 100 h. The weights of the sa mples were continuously monitored during exposure at various temperatu res and humidity levels. The effects of oxidation on room-temperature flexural strength were also determined, and correlated to the observed weight changes of the samples. At temperatures less-than-or-equal-to 1200-degrees-C, linear weight gains were observed. However, at tempera tures above 1200-degrees-C, the weight gains became parabolic with res pect to exposure time. The oxidation rates were significantly increase d by water vapor in the air. The oxidation products were found by X-ra y analysis to be a mixture of Al2O3 and 5Al2O3.3Y2O3. The oxide layer formed on the surface was severely cracked because of the thermal expa nsion mismatch between the oxide layer and the substrate. The cracks i nitiated in the oxide layer and propagated into the substrate, resulti ng in severe reduction in the room-temperature flexural strength of th e material. When exposed to ambient air for more than 50 h at temperat ures greater than 1100-degrees-C, the strengths of the samples decreas ed to less than half that of the as-received material.