Effect of annealing on microstructure and properties of Si_3N_4-AlN composite ceramics

Aiming at developing novel microwave-transparent ceramics with low dielectric loss, high thermal conductivity and high strength, Si₃N₄-AlN (30%, mass fraction) composite ceramics with La₂O₃ as sintering additive were prepared by hot-pressing at 1 800 °C and subsequently annealed at 1 450 °C and 1 850 °C for 2 h and 4 h, respectively. The materials were characterized by XRD and SEM. The effect of annealing process on the phase composition, sintering performance, microstructure, bending strength, dielectric loss and thermal conductivity of the materials was investigated. The results showed that both annealing at 1 850 °C and 1 450 °C promoted the phase transformation of α-Si₃N₄ to β-Si₃N₄. After annealing at 1 850 °C, grain growth to a certain extent occurred in the materials. Especially, the elongated β-Si₃N₄ grains showed a slight increase in diameter from 0.2 μm to 0.6 μm approximately and a decrease in aspect ratio. As a result, as the annealing time increased to 4 h, the bending strength declined from 456 MPa to 390 MPa, whereas the dielectric loss decreased to 2.15×10^?3 and the thermal conductivity increased to 16.3 W/(m·K) gradually. When annealed at 1 450 °C, increasing the annealing time to 4 h significantly promoted the crystallization of glassy phase to La₂Si₆N₈O₃ phase in the materials, which led to the increase in bending strength to 619 MPa and thermal conductivity to 15.9 W/(m·K), respectively, and simultaneously the decrease in dielectric loss to 1.53×10^?3.