Crystallization and microstructural evolution process from the mechanically alloyed amorphous SiBCN powder to the hot-pressed nano SiC/BN(C) ceramic

The mechanically alloyed amorphous SiBCN powders were hot pressed at 1500, 1600, 1700, 1800, and 1900 A degrees C under a pressure of 80 MPa in the nitrogen atmosphere for 30 min. The crystallization, the microstructural evolution, and the properties of the prepared ceramics were carefully studied by XRD, TEM, HRTEM, and property testing. Results show that the crystallization of beta-SiC, turbostratic BN(C), and alpha-SiC in the amorphous matrix starts at about 1500, 1600, and 1700 A degrees C, respectively. When the powder is hot pressed at the temperatures higher than 1700 A degrees C, the prepared ceramics always consist of nano beta-SiC, alpha-SiC, turbostratic BN(C), and amorphous body. With the increase of the sintering temperature, the ceramic crystallinity becomes higher, the grains get larger, and the amorphous content becomes lower. At the temperatures lower than 1800 A degrees C, the bulk density, the relative density, the flexural strength, the Young's modulus, and the fracture toughness of the prepared ceramics show persistent but insignificant increase. However, when the ceramic is sintered at 1,900 A degrees C, these properties are rapidly improving to 2.6 g/cm(3), 91.8 %, 331.0 MPa, 139.4 GPa, and 2.8 MPa m(1/2).