UHTC (Ultra High Temperature Ceramics) based on metal borides such as ZrB₂ and HfB₂ are materials that are difficult to sinter. This makes the use of additives facilitating sintering reasonable. These additives, apart from improving the sinterability of these materials, should not adversely affect the special properties of these materials such as high thermal conductivity, high melting point, mechanical strength or oxidation resistance. Such materials include SiC, B₄C and WC as well as MoSi₂ and CrSi₂ silicides.  On the basis of the available literature [1,2] and conducted studies, it was found that both SiC and MoSi₂ contribute to the chemical resistance of composites.
In this study, we investigated HP pressure assisted sintering and SPS pressure and electric field assisted sintering of composites composed of 40% vol. ZrB₂- 40% vol. HfB₂- 20% vol. MX. The above mentioned materials were used as MX additives. Sintering was carried out at a temperature adjusted to the starting composition, which ranged from 1400 to 2000 °C. The effects of the additives used on the densification degree of the obtained composites, their microstructure, flexural strength, hardness and oxidation resistance were investigated. It was found that a promising additive is chromium silicide, which was used for the first time in this type of research.

1) W.G. Fahrenholtz, G. E. Hilmas, I.G. Talmy, J.A. Zaykoski. J. Am. Ceram. Soc., 90 (2007) 1347–1364.
2) W.G. Fahrenholtz. J. Am. Ceram. Soc. 90 (2007) 43-148, 2007

This work was carried out under the (AGH-University of Science and Technology, Krakow, Poland) Excellence Initiative - Research University project number: 501.D4.II.696.7996