Effect of zirconium on precursor chemistry, phase stability, and oxidation of polyvinylsilazane-derived SiCN ceramics

The present work focuses on the structural changes of polyvinylsilazane precursor upon modification by a molecular precursor of zirconium and processing of high temperature stable nanostructured SiCN and SiZrCNO ceramics. The bonding characteristics of the polymerized as well as pyrolyzed samples of pure and Zr-modified polyvinylsilazane precursor and their polymer to ceramic conversion processes have been analyzed. Zr doping shifts the ceramization process to an earlier temperature as compared to the undoped preceramic polymer. The structural evolution of metastable SiCN and SiZrCNO systems was studied by using X-ray diffraction, high-resolution transmission electron microscopy, and diffraction intensity profiles. The pure SiCN system remained as a single-phase amorphous ceramic up to 1400 degrees C. While the Zr-doped SiCN ceramics also appeared as monophasic amorphous ceramic at 1000 degrees C, nanocrystals of t-ZrO2 were found to precipitate throughout the ceramic microstructure with exceptional homogeneity for samples pyrolyzed at higher temperatures. The high temperature stability of t-ZrO2 in the amorphous SiCN matrix has also been demonstrated. Constant rate heating oxidation studies indicated remarkable improvement in mass retention of the Zr-doped SiCN system as compared to the undoped ceramic for temperatures as high as 1500 degrees C. The retention of tetragonal phase of ZrO2 in the ceramic matrix, even after pyrolysis at 1400 degrees C and improved oxidation resistance provide significant advantages for achieving tough and thermally stable SiCN-ZrO2 ceramic nanocomposites for bond coat applications.