Microstructure and mechanical properties of AlN-BN based machinable ceramics

AlN based machinable ceramics with h-BN contents in the range of 5 to 30 vol% were prepared by hot-pressing at 1800degreesC for 2 h, using 3 Wt% Y2O3 as sintering aids. The effects of h-BN content on microstructure, mechanical properties, and machinability were investigated. The relative density of sintered compact decreased with increasing h-BN content. The four-point flexural strength also decreased with increasing h-BN content, mainly due to the much lower Young's modulus of BN compared to AlN. In addition, the residual tensile stress, formed by the thermal expansion coefficient difference between AlN and h-BN, might cause the drop of the strength. The h-BN particles had the desired crack deflection increasing effect. However, the fracture toughness decreased with h-BN content. During end-milling process, feed and thrust forces measured for all kinds of specimens decreased with increasing h-BN particles. From the experimental works, excellent machinability of the composites was investigated, and good surface roughness of specimens is observed after machining process. Also, irrespective of h-BN content, relatively good surfaces with roughness less than 0.5 mum (Ra) could be achieved within short lapping time.