Preparation and properties of sintering additive-free AlN–BN composite ceramics by hot-pressing sintering

With high activity h-BN powders as raw materials and without the using of any sintering additives, AlN–BN composite ceramics with 0–30 vol% were prepared by hot-pressing sintering in N₂ at 1850 °C. Effect of h-BN content on the microstructure and properties of the composite ceramics was investigated. The results show that fully dense composite ceramics with h-BN content up to 20 vol% can be obtained and all composites show a fine-grain-microstructure with clean and clear boundaries. No “card room structure” of h-BN was formed in the samples. With the increasing h-BN content, the composites show a decreased bending strength, fracture toughness, Vickers hardness, thermal conductivity and dielectric constant. The application of high activity raw materials produces a significant grain-refinement strengthening effect and makes the composites exhibit a much higher strength and toughness compared with the results reported by other researchers. Due to the absence of sintering additive, the composite ceramics also show a quite low dielectric loss at both low and high frequency. An optimal comprehensive properties are obtained for the AlN-20 vol% BN composite: bending strength of 410 ± 21 MPa, fracture toughness of 4.21 ± 0.19 MPa·m^1/2, Vickers hardness of 7.7 ± 0.2 GPa, thermal conductivity of 46.6 W·m⁻¹·K⁻¹, dielectric constant of 7.4 (1 MHz) and 7.48 (18 GHz), dielectric loss of 6.7 × 10⁻⁴ (1 MHz) and 6.18 × 10⁻³ (18 GHz).