Phase evolution in polymer derived silicon carbonitride ceramic hybrids

Abstract

Polymer-derived ceramics (PDCs) like SiCN, SiBCN poses excellent creep and oxidation resistance. Their unique shaping advantage (due to liquid precursors) and nanostructure make them suitable for coating for coating application, including TBCs and EBCs. SiC based coatings and composites, though perform well at high temperatures, usually disintegrate in atmosphere containing moisture. Polymer derived Si-ceramics are probable materials for applications as bond coats in such high temperature resistant coatings. The purpose of the present work is to explore the evolution of nanostructure in PDC materials with the introduction of transition metal ions. Molecular sources of Zr and/or Hf can affect the nanostructure during the pyrolysis induced conversion of the polymeric phase to ceramic. In the current work, Zr was incorporated into a commercially available polyvinylsilazane and pyrolyzed in an inert atmosphere at different temperatures, ranging from 1000-1400 oC. The pyrolyzed SiZrCN ceramic hybrid appeared as a single phase amorphous ceramic at 1000 oC, and exhibited phase separation of Zr into nanocrystals of t-ZrO2 with higher pyrolysis temperature. Interestingly the nanocrystals exhibited exceptional homogeneity in size (2-6 nm) and distribution in the amorphous SiCN matrix, as confirmed by HRTEM and XRD. The retention of tetragonal phase of ZrO2 in the ceramic matrix, even after pyrolysis at 1400 °C, provides a significant advantage for achieving enhanced toughness of the bond coat. These findings will allow the fabrication of a bond coat material with better toughness, oxidation resistant, hot strength, and tailorable thermal expansion coefficient to match between SiC and other top-coat oxides.