Initial nucleation of amorphous Si-B-C-N ceramics derived from polymer-precursors

Nucleation behavior of amorphous Si-B-C-N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy (TEM) combined with spatially-resolved electron energy-loss spectroscopy (EELS) analysis. The ceramics were pyrolyzed at 1000 degrees C followed by further annealing in N-2, and SiC nano-crystallites start to emerge at 1200 degrees C and dominate at 1500 degrees C. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400 degrees C. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCx and Si3N4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si3N4-like clusters at 1200-1400 degrees C. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si3N4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCx layers that developed between SiC and Si3N4-like clusters as well as from the excessive oxygen to form the stable SiO2. (C) 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.