Thermal stability of SiCN ceramics studied by spectroscopy and electron microscopy

Processing of two precursor‐derived SiCN ceramic monoliths was performed employing different liquid polymers, which were obtained by ammonolysis of functionalized chlorosilanes. Preparation of monolithic samples was performed by mixing liquid polysilazane with cross‐linked SiCN‐powder particles, derived from the same precursors by heat treatment at 300 °C, and subsequent annealing upon pyrolysis at temperatures exceeding 1000 °C to initiate crystallization. Characterization of the polymer‐derived ceramics was performed after pyrolysis at 1000 °C and, in particular, after annealing at temperatures ranging from 1400 to 1540 °C. Transmission electron microscopy was conducted in order to study the devitrification/thermal stability of the corresponding bulk SiCN glasses. Depending on the functionalities of the SiCN precursor and the processing conditions, different microstructures were obtained. The material prepared from precursor A showed crystallization of large α‐Si 3 N 4 grains within the overall homogeneous amorphous bulk material after exposure at 1540 °C 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‐derived glasses is promoted by local rearrangements of the glass network within the amorphous bulk. In addition, the role of the excess free carbon, commonly present in polymer‐derived SiCN ceramics,­on the thermal stability is discussed.