Pressure-Induced Structural Phase Transformation and Yield Strength of AlN

Aluminum nitride, a significant ceramic material used in electronic technological applications, was investigated by in situ synchrotron radiation X-ray diffraction in a diamond anvil cell at ambient temperature. The starting sample of AlN powder with a 1–2 μm average grain size was compressed to 36.9 GPa under the hydrostatic condition and 36.2 GPa under the nonhydrostatic condition, respectively. The phase transformation from hexagonal wurtzite to cubic rock salt (B4-to-B1) for the hydrostatic condition 20.5 and 20.4 GPa for the nonhydrostatic condition. We found that the phase transition was irreversible and that pure cubic phase AlN was obtained after pressure relief to reestablish ambient pressure. The experimental results reveal that the bulk modulus of hexagonal AlN is B0 = 287.1 GPa at a fixed B0′ = 4 under hydrostatic compression. For the recompressed pure cubic sample in the diamond anvil, the equation of state (EOS) of cubic AlN is B0 = 357.0 GPa when fit from ambient pressure to 39.5 GPa. The high pressure leads to the changes of atomic positions, bond distances, and bond angles, which were obtained from analysis using the refined GSAS package software, and explains the phase transition mechanism of hexagonal to cubic structure. In addition, the hexagonal AlN starts to exhibit a plastic deformation at approximately 8.5 GPa under nonhydrostatic compression, which is very close that of TiN by comparison.