Microstructure Characterization and Mechanical Properties of Polymer‐Derived (HfxTa1−x)C/SiC Ceramic Prepared upon Field‐Assisted Sintering Technique/Spark Plasma Sintering

The high‐temperature microstructural evolution and mechanical properties of two SiC‐based polymer‐derived ceramics with different Hf:Ta molar ratios are investigated using electron microscopy techniques and manipulated by nanoindentation. The as‐pyrolyzed ceramic powder consists of an amorphous Si(Hf x Ta 1− x )C(N,O) structure (where x = 0.2, 0.7) with localized nanocrystalline transition metal carbides (TMCs). Subsequent application of the field‐assisted sintering technique (FAST) for high‐temperature consolidation results in a crystalline (Hf x Ta 1− x )C/SiC ultra‐high temperature ceramic nanocomposite. The microstructure contains powder particle‐sized grains and sinter necks between the former powder particles. The powder particles consist of a β‐SiC matrix and small TMCs. Large TMCs are observed on the internal surfaces of former powder particles. This is due to the pulsed direct current and the resulting Joule heating that facilitates diffusion as well as oxygen impurities. Sinter necks of large β‐SiC grains form during the FAST process. The microstructural regions are assessed using high‐throughput nanoindentation. The hardness for SiC/(Hf 0.7 Ta 0.3 )C is measured on the formed grains and the sinter necks giving mean hardness values of about 27 and 37 GPa, respectively.