Effect of Zr and Ti on the Phase Evolution of Silicon Oxycarbide Ceramics

Abstract

Si based polymer-derived ceramics (PDCs) have been gaining attention due to their exceptional functional and ultrahigh temperature properties. With excellent creep resistance even at temperatures far beyond 1000°C and resistance in oxidative as well as corrosive environment, PDCs have emerged as potential materials in the fabrication of thermo-structural composites. Among the many PDCs, including SiC, SiCN, SiOC, and SiCBN, SiOC can be fabricated from commercially available polysiloxanes or polysilsesquioxanes. However, silicon oxycarbide based PDCs exhibit relatively limited oxidation resistance and high temperature performance compared to their nitride/carbonitride counterparts. In addition, these ceramics should be compatible with carbide based substrates and oxide based top coats for thermal protection systems. Preliminary work has indicated that incorporation of transition metals into the Si-O-C system exhibits interesting crystallization, oxidation, and thermal stability. This work demonstrates the behaviour of Zr and Ti, introduced as a molecular source, in the phase evolution, crystallization, and oxidation resistance of the polymer-derived SiMOC (where M = Zr, Ti) ceramics. A commercially available preceramic polymer was doped with 5−20mol% of the dopant metal ion (through alkoxide source), and cross-linked. Inert atmosphere pyrolysis of thus produced powders yielded black coloured SiMOC ceramics. X-ray diffraction (XRD) analysis was employed to observe the phase evolution. It was revealed that Ti ions crystallized into carbide phase at temperatures as low as 1200°C, while the Zr ions phase separated into t-ZrO2 whose crystallite size was calculated from the X-ray diffractograms using Scherrer formula. In both of the systems the phase separation of SiO2 was observed at temperatures beyond 1300°C. The crystallites of the formed SiC, TiC and ZrO2 were further analyzed by transmission electron microscopy (TEM). Since carbide phase was formed in the Ti-doped ceramic, thermogravimetric (TG) tests were performed to study the mass loss of the ceramic in both inert and oxidative atmospheres. The chemical nature of SiTiOC ceramic was investigated by Fourier-transform infrared (FTIR) spectroscopy. To support a hypothesis on the placement of the dopant atoms in the SiOC structure, ab initio calculations based on simple bond-counting method were also performed.