Thermal stability of SiCN ceramics studied by spectroscopy and electron microscopy

Processing of two precursor-derived SiCN ceramic monoliths was performed em ploying different liquid polymers, which were obtained by ammonolysis of fu nctionalized chlorosilanes. Preparation of monolithic samples was performed by mixing liquid polysilazane with cross-linked SiCN-powder particles, der ived from the same precursors by heat treatment at 300 degreesC, and subseq uent annealing upon pyrolysis at temperatures exceeding 1000 degreesC to in itiate crystallization. Characterization of the polymerderived ceramics was performed after pyrolysis at 1000 degreesC and, in particular, after annea ling at temperatures ranging from 1400 to 1540 degreesC. Transmission elect ron microscopy was conducted in order to study the devitrification/ thermal stability of the corresponding bulk SiCN glasses. Depending on the functio nalities of the SiCN precursor and the processing conditions, different mic rostructures were obtained. The material prepared from precursor A showed c rystallization of large alpha -Si3N4 grains within the overall homogeneous amorphous bulk material after exposure at 1540 degreesC for 6 h in nitrogen atmosphere. In contrast, the ceramic monoliths derived from SiCN precursor B remained completely amorphous, with no indication of local nucleation or crystallization. It is thought that devitrification of these polymer-deriv ed glasses is promoted by local rearrangements of the glass network within the amorphous bulk. In addition, the role of the excess free carbon, common ly present in polymer-derived SiCN ceramics, on the thermal stability is di scussed. Copyright (C) 2001 John Wiley & Sons, Ltd.