Microstructural evolution of Si(HfₓTa₁₋ₓ)(C)N polymer-derived ceramics upon high-temperature anneal

Ultra-high temperature ceramic nanocomposites (UHTC-NC) within the Si(HfxTa1−x)(C)N system were synthesized via the polymer-derived ceramics (PDC) synthesis route. The microstructure evolution of the materials was investigated upon pyrolysis and subsequent heat treatment. The crystallization behavior and phase composition were studied utilizing X-ray diffraction, scanning- and transmission electron microscopy. Single-source-precursors were converted into amorphous single-phase ceramics, with the exception of surface crystallization effects, at 1000 °C in NH3. Annealing in N2 at 1600 °C resulted in fully crystalline UHTCs. The powder samples revealed microstructures consisting of two characteristic regions, bulk and surface; displaying intrinsic microstructure and phase composition differences. Instead of the expected nitrides, transition metal carbides (TMC) were detected upon high-temperature anneal. The residual carbon available in the system triggered a decomposition reaction, resulting in the formation of TMCs plus gaseous nitrogen and SiC. Experimental data underline that N-containing PDCs are prone to phase separation accompanied by thermal decomposition and diffusion-controlled coarsening.