Ultra-high temperature ceramics (UHTCs) is the group of the most promising materials used in thermal protection system for hypersonic flight vehicles. In this work, ZrB2-SiC and HfB2-SiC based composites were fabricated using Field Assisted Sintering Technology (FAST). The composites containing 25 vol% SiC particles were prepared by in situ reaction of HfSi2/ZrSi2, B4C and carbon black powders, which is a method to densify ZrB2- and HfB2-based composites at low temperatures. Furthermore, Yb2O3, as the rare-earth (RE) based additive with the second lowest ionic radius of RE3+, was used to improve the ablation resistance of ZrB2- and HfB2-SiC composites. The microstructures of the composites were characterized by X-Ray Diffraction and Scanning Electron Microscopy. High temperature (ablation resistance using an oxy-acetylene flame with a temperature up to 2700 °C) properties were investigated. The results showed that homogeneous microstructure and nearly fully dense ZrB2- and HfB2-25vol.%SiC composites with a relative density above 99% were obtained after sintering at the temperature of 1600°C under the pressure of 70 MPa for 10 min. During sintering, all of the initial powders completely transformed to ZrB2/HfB2 and SiC particles, which were homogeneously distributed in the matrix (ZrB2 or HfB2). The Yb2O3 additives were also uniformly distributed at the boundaries of SiC grains. The ablated zone of ZrB2- and HfB2-SiC consisted of three distinct ablation layers. Most importantly, the ablation resistance of ZrB2 and HfB2-based materials was significantly improved by the addition of Yb2O3, and further improved with its increasing amount from 2 to 10 wt.%.

Acknowledgement
This work was supported by the Slovak Research and Development Agency under the contract No. APVV-21-0402 and APVV-SK-CZ-RD-21-0089. The support of VEGA Project No. 2/0116/22, and JRP SAV TUBITAK project No. 546676 is also greatly acknowledged.