Evolution of Phase and Nanostructure in Early Transition Metal Doped Polymer Derived Silicon Carbonitride Ceramics

Abstract

Silicon-based polymer derived ceramics like SiC, SiCN, SiBCN poses excellent resistance towards creep and oxidation are future materials for bond coat applications in high temperature resistant environmental barrier coating systems for the protection of C or SiCbased CMC components. SiCN-based amorphous ceramics developed from the pyrolysis of polysilazane precursor, are believed to be ideal for these applications. Additionally, doping of such precursor with metal oxides provides a special advantage of tailoring the phase assemblage, thermal expansion coefficient, thermal conductivity and oxidation resistance. The current work is based on understanding the evolution of phase and nanostructure with the introduction of early transition metals as molecular source in the preceramic polymer. Further, the oxidation behavior of such metal modified SiCN ceramics were studied and compared with undoped SiCN ceramics. In typical experiments, a commercially available polyvinylsilazane polymer was doped with molecular source of metals (M=Ti /Hf /Zr), crosslinked at 300 oC, and pyrolyzed in N2 atmosphere over a temperature range of 900 oC to 1400 oC to synthesize SiTiCNO, SiHfCNO, and SiZrCNO ceramic hybrids, respectively. Also, a parallel synthesis of undoped SiCN samples using polyvinylsilazane with similar crosslinking and pyrolysis conditions was done for benchmarking the different properties of metal doped SiCN ceramic systems. A detail crosslinking mechanism of pure polyvinylsilazane and metal modified precursors was studied using FTIR spectroscopy. Further, the thermogravimetric study of all crosslinked precursors was performed to estimate ceramic yields and ceramization temperatures in N2 atmosphere. The pure SiCN ceramics predominantly remained single phase amorphous ceramic up to 1400 oC. Two different nanostructured SiTiCNO ceramics were prepared from isopropoxide or n-butoxide sources of Ti-doping in polyvinylsilazane, which appeared predominantly single phase amorphous up to 1100 oC, but anatase-TiO2 precipitated within the SiCN matrix at 1200 oC. Ti-isopropoxide based SiTiCNO remained thermally stable up to 1300 oC, whereas the Ti-butoxide based SiTiCNO system remained stable up to 1400 oC. An exceptionally homogeneous distribution of predominantly TiO2 nanocrystals, in the size range of 2-14 nm was observed throughout the SiCN matrix. Further, the SiHfCNO ceramic hybrids prepared by pyrolysis of Hf-modified polyvinylsilazane precursor appeared predominantly single phase amorphous ceramic up to 900 oC. However, Hf within the SiCN matrix evolved as nanostructured tetragonal phase of HfO2 in the SiCN ceramic matrix at 1000 °C. Interestingly the t-HfO2 nanocrystals were homogeneously distributed with extremely fine crystallite size (2.3 to 5.3 nm up to 1400 °C) throughout SiCN matrix. Similarly, the SiZrCNO ceramic hybrids appeared predominantly single phase amorphous ceramic up to 1100 oC. However, the Zr within SiCN ceramics, above 1100 oC pyrolysis temperatures, nucleated and later precipitates as nanocrystals of t-ZrO2 within 2-9 nm size range, throughout the ceramic microstructure with exceptional homogeneity. The tetragonal phase of ZrO2 and HfO2 remained stable in the SiCN ceramic matrix, even after pyrolysis at 1400 °C. Additionally, coarsening kinetics of ZrO2 in the SiZrCNO system was studied at 1400 oC using the Lifshitz-Slyozov-Wagner model, which exhibited cubic kinetics indicating diffusion controlled growth. Finally, the oxidation behaviour of the SiCN, SiTiCNO, SiHfCNO, and SiZrCNO ceramic nanocomposites was investigated by constant rate heating method using thermogravimetry analyzer in flowing oxygen. SiCN ceramics pyrolyzed at higher temperature showed less mass loss during oxidation. SiTiCNO prepared through Tiisopropoxide modification of polyvinylsilazane improved oxidation properties of SiCN up to 1400 oC. However, SiTiCNO prepared through Ti-n-butoxide improved oxidation properties of SiCN up to 1500 oC and showed less material recession than pure SiCN and SiTiCNO synthesized through polyvinylsilazane and Ti-isopropoxide precursor source. Hf and Zr incorporation in SiCN further enhanced oxidation resistance of the nanohybrid ceramics. Moreover, the effect of Ti, or Hf, or Zr doping on the free nanocarbon phase evolution within the SiCN matrix before and after oxidation was studied through micro- Raman analysis. It was interesting to note that the Cfree existed even after oxidation in all the systems, which shows robustness of the ceramic systems against oxidation. The work exhibits some unique ceramic hybrid materials with exceptional homogeneity of nanocrystals of an oxide phases distributed within an amorphous matrix. The improved oxidation resistance, and their polymorphic stability could provide significant advantages for achieving thermostructurally stable, tough and chemically robust nanocomposite materials for various high temperature uses including bond coat of TBC/EBC systems