Thermostructural Evolution of Hf-Doped Polysilazane-Derived Sicn-Hfo2 Ceramics

SiC-based ceramic matrix composites are most preferred materials for modern gas turbine engine components due to their phase stability and creep resistance in ultra-high temperature environments. However, these materials disintegrate easily in moisture containing oxidizing environment at high temperatures, thus requiring the development of a thermal and environmental barrier ceramic protective coating system. The fact that the outer coating system is typically an oxide and the ceramic matrix a carbide, the coating system requires the use of a bond coat. Preceramic polymer derived SiCN materials can be potentially useful for such applications. In the current work such SiCN ceramic is further tailored with Hf incorporation through doping with a molecular precursor in the preceramic polymer. Commercially available polyvinylsilazane precursors was mixed with 20 mol% of an alkoxide of Hf, crosslinked and pyrolyzed in the temperature range of 900 oC to 1400 oC in flowing nitrogen to synthesize the SiHfCNO ceramic composite. The ceramic remained single phase amorphous up to 900 oC. Further annealing caused nucleation followed by crystallization of tetragonal hafnium oxide (t-HfO2) within the SiCN matrix, resulting a unique glass-ceramic nanocomposite of t-HfO2-SiCN. The crystal sizes of t-HfO2 were found to be in the range of 2.3 to 5.1 nm from electron microscopy. Raman spectroscopy was employed to see the effect of Hf-doping on the phase separation and crystallization behavior of sp2-carbon. Further, thermal stability if the ceramics in oxidizing environment of the SiCN-HfO2 ceramic was analyzed by thermogravimetry. Overall results indicated that the incorporation of Hf in the SiCN ceramic system leads to improvement in phase stability and oxidation resistance as compared to the undoped SiCN system