Effect of impregnation-pyrolysis process on the properties of pressureless sintered hexagonal boron nitride ceramics

To improve the density and properties of hexagonal boron nitride ceramic fabricated by pressureless sintering process, impregnation treatment was performed using boric acid and urea mixed aqueous solution as h-BN precursor, which is followed by cracking and then secondary pressureless sintering. The influences of concentration of precursor solutions and cycle time on the microstructure and properties of high purity h-BN ceramic were investigated. The results demonstrated that with the increase of concentration of precursor solutions, the bulk density, bending strength, fracture toughness and thermal conductivity increase and then decrease,and all reach the maximum at concentration of 68wt%. When the concentration is too high, h-BN precursor will precipitate from the solution, which is disadvantageous to the impregnation. With the increment of cycle time, the bulk density, bending strength, fracture toughness and thermal conductivity increase monotonously, but with a gradually decreased growth rate. When the cycle time is six, the maximum bulk density of 1.465 g/cm³, bending strength of 84.1 MPa, fracture toughness of 1.52 MPa·m^1/2 and thermal conductivity of 44.36 W·m^-1·K^-1 are obtained, which are 4.7%, 31.6 %, 63.7%, and 31.2% higher than those of the unimpregnated sample, respectively.