Microstructure and mechanical properties of ZTA/BN machinable ceramics fabricated by reactive hot pressing

Zirconia-toughened alumina (ZTA)/boron nitride (BN) machinable ceramics have been successfully fabricated by reactive hot pressing. BN was in-situ introduced into the composite by the reaction of aluminum nitride (AlN) and boric acid (H3BO3). The microstructure of ZTA/BN composite was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The results indicated that BN grains were uniformly and compactly distributed among Al2O3 and ZrO2 grains. The relative density, flexural strength, fracture toughness, and Vickers hardness increased with increasing temperature. The flexural strength and fracture toughness of the ZTA/BN composite with BN content of 12.5 vol% sintered at 1800 °C achieved high values of 731 MPa and 7.48 MPa m1/2, respectively. These values were much higher than those of the composite with the same composition prepared by conventional powder sintering method, and even higher than those of the monolithic ZTA ceramics. The enhancements of the fracture strength and fracture toughness were primarily ascribed to the reactive hot pressing method which introduced BN in-situ and prevented the aggregation of BN and reduced the flaw sizes. The ZTA/BN composite exhibited excellent machinability and could be drilled using conventional cemented carbide drill bits.