Structure Characterization Of Two Boron Nitride Fibers And Preparation By PAS And Properties Of BN_f/Si₃N₄Composite Ceramics

As ceramic fibers, boron nitride fiber (BNf) combines a variety of excellent performance of nitride ceramic materials and fiber materials. Such as elevated temperature mechanical strength, excellent dielectric properties, high temperature and chemical resistance and other characteristics are all the excellent performance. α-Si3N4ceramics have excellent high-temperature mechanical strength and good dielectric properties. Fiber reinforced ceramic matrix composite with both ceramic materials and fibers’ excellent characteristics has wider application space. The composite became the best choice of special ceramic material for high temperature. This paper focuses on adding BN fibers as reinforcing phase into the Si3N4ceramic matrix. Firstly, we characterize the BNf-1and BNf-2fibers’ composition, phase and structure to contrast their differences. The BNf/Si3N4composite ceramics were prepared by plasma activated sintering. The phase composition, structure, dielectric properties and mechanical properties of composite ceramics were studied systematically.Three elements B, N, and O are the mainly content of fibers and impurity elements are very low characterized by XRF, ICP and XPS. The main phase of the two BN fibers is hexagonal boron nitride (h-BN) substantially analyzed by XRD and FT-IR. The surface of BNf-1fiber is homogeneous smooth. There is almost no obvious defect and the diameter is5-9μm. The amorphous (a-BN) and turbine-like (t-BN) regions show that the fibers’ crystallization is not entirely. The microstructure of BNf-2fiber is looser than BNf-1fiber and has more defects.The BNf/Si3N4multiphase ceramics were prepared by plasma activated sintering with amount of0～40vol.%. The optimum sintering temperature is1400℃, and the optimum sintering aid is3wt.%Al2O3+4wt.%MgO. After sintering we obtained samples with main phase a-Si3N4, and the relative density is higher than95.0%. The relative density of BNf-1fiber composite ceramics is higher than BNf-2fiber composite ceramics. The dielectric constant and dielectric loss of Si3N4ceramic is7.2and1.5×10-2. As the increasing of the two BN fibers, the dielectric constant and dielectric loss decreased obviously. When the composite ceramics’ amount of BNf-1is40vol.%. The dielectric constant and dielectric loss is5.8and8×10-3. With the two BN fiber content increasing, the composite ceramics’ Young’s modulus, hardness and bending strength are showing a downward trend, in which BNf-1fiber composite ceramics’ modulus higher than BNf-2fiber composite ceramics’. The hardness and bending strength of BNf-1fiber composite ceramics were lower than BNf-2fiber composite ceramics’. The fracture toughness of composite ceramics has been significantly improved with the addition of fibers obviously. The fracture toughness of Si3N4ceramic matrix is3.59MPa·m1/2. The highest value of fracture toughness is6.41MPa·m1/2with10vol.%addition of BNf1fiber, increased by about79%, and the BNf-1fiber composite ceramics’ fracture toughness is generally higher than BNf-2fiber composite ceramics’ fracture toughness.