Microstructure and mechanical properties of AlN–hBN based machinable ceramics prepared by pressureless sintering

AlN based machinable ceramics containing hBN were prepared by economical pressureless sintering at 1800 °C for 2 h in a nitrogen gas atmosphere. The effects of hBN content on microstructure, mechanical properties and machinability were investigated. The relative density of sintered compacts decreased with increasing hBN content. The four-point flexural strength of 340 MPa for monolithic AlN also decreased to about 120 MPa by the addition of 20 vol% hBN, mainly due to increased porosity. With increasing hBN content, AlN–hBN based ceramic composites revealed increased crack deflection. The Vickers indentation crack paths in specimens were sinusoidal indicating crack bridging and pull-outs of composite grains during crack propagation. During end-milling and microdrilling processes for AlN ceramic composites with more than 10 vol% hBN, measured feed and thrust forces decreased with increasing hBN content, demonstrating a good machinability of the composites.