Research On Preparation And Properties Of AlN-BN Composite Ceramics

AIN and AIN-BN composite ceramics were fabricated by hot pressing (HP) and by hot isostatic pressing (HIP) without sintering aid, using AIN and BN powders as raw materials in this paper.The effects of HP temperature and BN contents on the microstructures and properties of composites were investigated to establish the specific model, which was dramatically beneficial to predict thermal conductivity and volume resistivity of AIN-BN composite ceramics fabricated by hot isostatic pressing.As a result, the preparations of AIN and AIN-BN composite ceramics treated by HP showed that AIN-BN composite ceramics were made up of pure AIN and BN. Besides, the relative density, thermal conductivity, volume resistivity and hardness were increased with the increase of sintering temperature when the content of BN phase kept constant. Apart from that, grains tended to grow gradually and to form a relatively concentrated AIN-BN composite ceramics, meanwhile, the volume resistivity, quality wear rate and average coefficient of friction also showed a upward trend, while the content of BN went up constantly. In contrast, the thermal conductivity and hardness dropped slightly. However, under the condition of nAlN:nBN=50:50, BN was significantly characterized by cross stacking tendency, grain coarsening and porous, as a result, density of ceramic had a downward trend. More specifically, AIN-BN composites treated by HP, with the ratio of nAlN:nBN= 75:25, were prepared at 1700℃ and a dwell time of 1h HP, and its relative density, thermal conductivity and volume resistivity were 95.58%,58.13W/(m.K) and 7.32× 1014Q.cm respectively.AIN-BN composite ceramics prepared by HIP indicated that the relative density, thermal conductivity, volume resistivity and hardness of these ceramics increased with the increase of sintering temperature and pressure under the same content of BN, conversely, the average coefficient friction and quality wear rate went down. Furthermore, the effect of HIP sintering temperature on compactness was more effective than the pressure for AIN-BN composite ceramics. Hot isostatic pressing could effectively break BN "cards" structure due to its extremely high pressure, which leaded to little influence on densification through the addition of BN. while the hardness and thermal conductivity decreased with the increase of content of BN, in contrast, the volume resistivity increased under this condition. To be specific, AIN-BN composites prepared by HIP, with the ratio of nAlN:nBN= 75:25, were prepared at 1600℃,90MPa and a dwell time of 3h, and it’s relative density, thermal conductivity and volume resistivity were 98.03%,77.29W/(m.K) and 1.35×1015Ω.cm respectively.According to the two-phase composite model of composite ceramics,the thermal conductivity and volume resistivity is exactly conformed with the formula: λcom=λ2(1-F)(1-φ)(λ2-λ1)/λ2(λ1+F(1-φ)(λ2-λ1)).This result can accurately predict the thennal conductivity and volume resistivity of AlN-BN composite ceramic which was prepared by HIP.