Preparation and characterization of silica-doped aluminum nitride-aluminum nitride polytypoid composites

Fine-grained AlN-AlN polytypoid composites doped with SiO2 were prepared in situ by pressureless sintering of AlN-Al2O3 mixtures (3.7 and 10.5 mol% Al2O3) at 1870 °C using Y2O3 as sintering aid. SiO2-doping (1 wt.%) was used to stabilize the AlN polytypoids and reduce the average grain size of the polytypoids. The composites consisted of elongated SiO2 containing AlN polytypoids grains (30 and 61 vol.%) embedded in equi-axed AlN matrix. Vickers hardness and bending strength increased with polytypoid composite formation. The Young's modulus showed a maximum at 3.7 mol% Al2O3 (30 vol.% polytypoids). The fracture toughness increased up to 4.0 ± 0.3 MPa m1/2 for the composites. The toughening was caused by crack deflection due to residual strain both in the polytypoids and the AlN matrix.