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 ratiosare investigated using electron microscopy techniques and manipulated bynanoindentation. The as-pyrolyzed ceramic powder consists of an amorphous (HfxTa1-x)C(N,O) structure (where x=0.2, 0.7) with localized nanocrystallinetransition metal carbides (TMCs). Subsequent application of the field-assisted sintering technique (FAST) for high-temperature consolidation results in acrystalline (HfxTa1-x)C/SiC ultra-high temperature ceramic nanocomposite. Themicrostructure contains powder particle-sized grains and sinter necks betweenthe former powder particles. The powder particles consist of a beta-SiC matrix andsmall TMCs. Large TMCs are observed on the internal surfaces of former powderparticles. This is due to the pulsed direct current and the resulting Joule heatingthat facilitates diffusion as well as oxygen impurities. Sinter necks of large beta-SiCgrains form during the FAST process. The microstructural regions are assessedusing high-throughput nanoindentation. The hardness for SiC/(Hf0.7Ta0.3)C ismeasured on the formed grains and the sinter necks giving mean hardnessvalues of about 27 and 37 GPa, respectively