Effects of isothermal annealing on the oxidation behavior, mechanical and thermal properties of AlN ceramics

To evaluate the oxidation behavior of AlN ceramics prepared using a pressureless sintering method, aluminum nitride bulk samples were annealed at temperatures ranging from 900 °C to 1300 °C for 1–5 h under an air atmosphere. The phase composition, microstructure and weight change of the oxidized samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis-mass spectrometry (TGA-MS), respectively. The results showed that certain of the gaseous oxidation products of AlN ceramics were nitrogen oxides (NO_x). The oxidation reaction started at approximately 800 °C to 900 °C. The thermal conductivity and flexural strength of AlN ceramics were enhanced slightly after the heat treatment in the 900–1100 °C temperature range. The entire surface of the AlN block was covered with a thin, compact alumina layer after oxidizing at 1100 °C for 3 h, which may enhance the metal adhesion to the AlN surface in a metallization process. At 1200 °C and higher, the AlN ceramics oxide layer became rough, porous, and cracked, leading to a sharp drop in the flexural strength and thermal conductivity of the AlN ceramics.