Synthesis, microstructure and properties of SiCN ceramics prepared from tailored polymers

Different liquid polymers in the system SiCN with tailored structures were prepared by ammonolysis from functionalized chlorosilanes. Crosslinking to an unmeltable polymer with initiators at low temperatures and subsequent ceramization were studied applying Si-29 solid-state nuclear magnetic resonance (NMR) spectroscopy in combination with Fourier transformed infrared (FTIR) spectroscopy and thermoanalytical techniques. Microstructure development, in particular, the devitrification of the corresponding bulk polymer-derived SiCN glasses was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Preparation of monolithic samples was performed by mixing liquid polysilazane with SiCN-powder particles, derived from the same precursors by heat treatment at 300 degrees C, and subsequent annealing at temperatures exceeding 1000 degrees C to initiate crystallization. Depending on the functionalities of the SiCN-precursor and the processing conditions, different microstructures were obtained. The material prepared from the HVNG precursor revealed a homogeneous amorphous micro structure with only a small fraction of crystallized spherical inclusions after exposure at 1540 degrees C: for 6 h in nitrogen atmosphere. In contrast, investigating ceramic monoliths derived from another SiCN precursor, a different crystallization sequence was observed. The material derived from the HPS precursor showed crystallization of large alpha-Si3N4 grains within the bulk. As will be discussed in detail, devitrification of these polymer-derived glasses is promoted by local rearrangements and possible phase separations within the amorphous bulk. Moreover, local decomposition and residual porosity can affect the crystallization behavior, which strongly differs depending on the polymer employed. In addition to the crystallization phenomena observed, different oxidation response was monitored for the two SiCN ceramics discussed here. Moreover, fracture strength and hardness data were recorded, which, however, did not substantially differ between the polymer-derived ceramics investigated. (C) 1999 Elsevier Science S.A. All rights reserved.