Reaction Synthesis of Aluminum Nitride-Boron Nitride Composites Based on the Nitridation of Aluminum Boride

摘要

Aluminum nitride-boron nitride (AlN-BN) composites were prepared based on the nitridation of aluminum boride (AlB_2). AlN powder was added to change the BN volume fraction in the obtained composites. Thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffractometry, and the nitridation ratio were used to investigate the nitridation process of AlB_2. At ～1000℃, a sharp exothermic peak occurred in the DTA curve, corresponding to the rapid nitridation of aluminum in AlB_2. On the other hand, the nitridation of the transient phase, Al_(1.67)B_(22), was very slow when the temperature was <1400℃. However, the nitridation speed obviously accelerated at temperatures >1600℃. The pressure of the nitrogen atmosphere was also an important factor; high nitrogen pressure remarkably promoted nitridation. Treatment at 2000℃ was disadvantageous for nitridation, because of the rapid formation of a dense surface layer that inhibited nitrogen diffusion into the specimen interior. Three specimens, with 5 wt% Y_2O_3 additive and different BN contents, were prepared by pressureless reactive sintering, according to the determined sintering schedule. Electron microscopy (scanning and transmission) observations revealed that the in-situ-formed BN flakes were homogeneously and isotropically distributed in the AlN matrix. A schematic mechanism for microstructural formation was developed, based on the results of nitridation and the microstructural features of the obtained composites. The obtained composites, with a low BN content, exhibited a high bending strength, comparable to that of reported hot-pressed AlN-BN composites.