Preparation and properties of AlN–BN composite ceramics through combined addition of c-BN and h-BN

“Lamellar” structure of hexagonal boron nitride (h-BN) is prone to the formation of pore defects and the establishment of directional alignment during sintering process, which lead to AlN/BN composite ceramics exhibiting low density and anisotropic flexural strength (σ≡, σ⊥, and σ∥) (Orientation Bias Index |IOP| ≫ 1); these factors greatly limit the application of AlN/BN composite ceramics. In this work, cubic boron nitride (c-BN) was introduced as a reinforcing phase into AlN/BN composite ceramics, which were prepared via hot pressing sintering, and the effects of c-BN addition on microstructure, mechanical properties, and anisotropy were systematically investigated. The results show that during hot pressing sintering, c-BN phase changes into h-BN with “onion” structure, which effectively fills pore defects and reduces the degree of alignment of h-BN. |IOP| of sample 0C25H (170) continues to decrease with the increase in c-BN amount up to 2.7%. and anisotropy is significantly reduced. With the increase in c-BN amount, σ≡ and σ⊥ first increase and then decrease, while σ∥ only increases. σ≡ and σ⊥ reach the highest values (σ≡ = 362 MPa and σ⊥ = 340 MPa) at a c-BN amount of 15% (sample 15C10H); these values are 20.5% and 15.6% higher than those of sample 0C25H (no c-BN added), respectively, and σ∥ reaches the highest value (σ∥ = 239 MPa) for sample 25C0H, which is 136.6% higher than that of sample 0C25H. Sample 25C0H exhibits the highest σ∥ value (239 MPa), which is 136.6% higher than that of sample 0C25H. The onion-like h-BN formed by phase transformation of c-BN not only provides significant toughening to AlN/BN composite ceramics but can also effectively reduce anisotropy in their mechanical properties.