Effects of weak boundary phases (WBP) on the microstructure and mechanical properties of pressureless sintered Al₂O₃/h-BN machinable composites

Al₂O₃/h-BN machinable composites were fabricated by pressureless sintering at 1750 °C for 2 h in nitrogen atmosphere. The relative density of the sintered composites decreased, while the porosity increased with increasing h-BN content. By adding 20 vol.% h-BN to the composites, the porosity increased up to 16.7%. The effects of weak boundary phases (WBP), including h-BN and pores, on the microstructure, mechanical properties and machinability of the composites were investigated. The flexural strength, fracture toughness, Young's modulus and hardness of the composites decreased with increasing WBP content. When WBP content increased up to 18.8 vol.%, the composites can be machined easily by cemented carbide drills. Furthermore, the machining mechanisms of the composites were investigated using Hertzian contact tests.