Evolution of Microstructure, Oxidation Kinetics, and Thermal Shock Behavior of SiBCNZr Ceramics

Zr-doped amorphous SiBCNZr ceramics were prepared via the polymer-derived ceramics (PDCs) route to address the reliability issues in harsh environments. The structural evolution and failure mechanisms of SiBCNZr ceramics under high-temperature oxidation and thermal shock cycles were systematically investigated. The results show that the introduction of Zr atoms was integrated into the amorphous network through covalent bonds such as Zr─B and Zr─Si. The oxidation kinetics of the ceramic at 1400°C followed the parabolic law. A dense borosilicate glass layer was formed on the ceramics’ surface, which effectively blocked oxygen diffusion during the early stage of oxidation. As oxidation time increased, ZrO₂ nanocrystals and cristobalite precipitated within the oxide layer, accompanied by the escape of CO and N₂ gases that created pores on the surface. The continued growth of ZrO₂ and cristobalite eventually made the oxide layer brittle, leading to cracks and failure caused by volume contraction during the β→α phase transition. The residual strength of the samples followed a non-monotonic trend as the number of cycles increased. The strength first dropped sharply due to thermal stress, followed by a slight recovery as the oxide layer spread and blunted microcracks, and then decreased again because of internal oxidation and matrix damage.