Role of boron addition on phase composition, microstructural evolution and mechanical properties of nanocrystalline SiBCN monoliths

Nanocrystalline SiBCN monoliths with the same Si/C/N mole ratio and various boron additions ranged from 0 to 3.0 mol were prepared by mechanical alloying plus reactive hot pressing methods. Correction of boron content and microstructural/morphological evolution was investigated in detail by XRD, SEM, TEM and STEM-EDX structure characterization. Except for SiC and BN(C), boron addition contributes to BxC formation. Besides, boron addition promotes the crystallization of SiC, leading to the formation of poor crystallinity of spherical structures in inner SiC. Furthermore, boron addition significantly promotes the grain growth of SiC and BxC and therefore increases the relative volume ratios of BxC/BN(C) and BxC/SiC. Amorphous-like BN(C) changes to belt like structures as boron addition increases. The new formed BxC effectively contributes to the improvement of Vicker's hardness while pull-out of BN(C) belt-like structures benefits the fracture toughness.