Toughening effect and mechanisms of AlN whiskers on a low-temperature sintered AlNw/AlN ceramics

Conventional aluminum nitride (AlN) ceramics exhibited insufficient mechanical properties expected from their potential applications in the presence of dynamic loads and intensive stresses caused by thermal shock. In this study, the mechanical properties of AlN ceramics were enhanced by toughening them using AlN whiskers (AlN_w) via gel casting followed by low-temperature sintering at 1650°C. The addition of AlN_w simultaneously increased the bending strength, toughness and thermal conductivity of the AlN ceramics. The maximum values of the bending strength, toughness, and thermal conductivity of 9.0 wt% AlN_w/AlN were 303.92 MPa, 3.92 MPa·m^1/2 and 186.53 W·m⁻¹·K⁻¹, respectively, which were higher than those of the AlN ceramics by 39.93%, 61.31% and, 15.61%, respectively. The AlN_w in AlN_w/AlN ceramics tightly bonded with the ceramic matrix, leading to two characteristic toughening mechanisms in the ceramics: crack pinning and deflection at irregular grain boundaries caused by doped whiskers, as well as bridging and stress relaxation caused by whiskers incorporated with the grains. Moreover, the one-dimensional morphology of AlN_w can provide a channel for quick photon transport, thereby enhancing the thermal conductivity of AlN_w/AlN.