High‐Temperature Creep and Cavitation of Polycrystalline Aluminum Nitride

Abstract

Dense, polycrystalline AlN samples of grain size between 1.8 and 19 μm were fabricated by hot‐pressing. Bar‐shaped samples were subjected to creep in four‐point bending under static loads in nitrogen atmosphere. The outer fiber stress was varied between 20 and 140 MPa and the temperature between 1650 and 1940 K. Steady‐state creep rate, dɛ/dt was proportional to σnd−m where the stress exponent, n, was between 1.27 and 1.43 and grain‐size exponent, m, between ∼ 2.2 and ∼ 2.4. The activation energy for creep ranged between 529 and 625 kJ/mol. Both round (r type) and wedgeshaped (w type) cavities were observed in electron micrographs of the deformed samples. No noticeable change in the dislocation density was observed. Contribution of cavitation to the creep rate was estimated using an unconstrained cavity model. Based on this study it is concluded that the dominant mechanism of creep in polycrystalline AlN is diffusional.