Fabrication of machinable AlN-BN composites with high thermal conductivity by pressureless sintering turbostatic BN-coated AlN nanocomposite powders

To impart machinability to hard and brittle AIN ceramics without losing their high thermal conductivity, a homogeneous dispersion of fine BN particles in an AIN matrix was investigated. A homogeneous dispersion of BN particles was obtained by pressureless sintering of turbostatic BN-coated AIN nanocomposite powder (AIN-BN nanocomposite powder), which was prepared by reducing and heating AIN particles containing a mixture of boric acid, urea, and carbon. Though AIN is slightly oxidized by boric acid during the reduction, the addition of carbon reduced the oxygen content of the AIN-BN composite powder by carbothermal reduction of the oxidized AIN particles. As a result, the thermal conductivity of the sintered material increased with decreasing oxygen content of the nanocomposite powder. AIN-BN nanocomposites containing more than 20 vol% BN showed high strength, machinability, and relatively high thermal conductivity in comparison with the conventional microcomposites.