EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The group of ultra-high temperature ceramics (UHTCs) are promising materials for extreme environmental conditions such as high temperatures (≥ 1300 °C). Among them, ZrB2 experienced extensive research due to its high melting point, low density and thermal shock resistance. However, ZrB2 shows rather low oxidation resistance, which is paramount for most applications in extreme environments. By adding silicon containing secondary phases to ZrB2 the oxidation behavior can be improved.

In the last few years research focused on using silicon-based polymer-derived ceramics (PDCs) as Si additive (e.g. SiCO, SiCN) to further improve high temperature properties of ZrB2. PDCs can be chemically modified by using metal alkoxides for introducing other elements (such as B, Zr, Hf), which leads to polymer-derived ceramic nanocomposites (PDC-NCs) with an improved oxidation resistance. However, there is scarcity of information on the influence of PDC-NCs on the oxidation behavior of ZrB2-based ceramics, which is the focus of this study. Therefore, ZrB2 powder was coated with polymer-derived SiCN, SiZrCN or SiZrBCN, respectively, and hot-pressed, producing ceramic monoliths with skeletal densities ≥ 4,85 g/cm³. The oxidation behavior and kinetics were investigated using thermogravimetric analysis at 1300 °C in synthetic air with dwell times of 50 h and 100 h. A detailed study of materials and oxide microstructure was carried out using optical microscopy, electron probe micro analysis and X-ray diffraction. The experimental findings were compared to thermodynamic equilibrium calculations, which lead to a better understanding of the oxidation mechanism. From the calculations the formation of gaseous species in particular CO, B2O3 and SiO within the oxide scale is derived, which explains the oxidation behavior and the developed microstructure. Overall, the results show an improved oxidation resistance for all three investigated materials in comparison to literature data of ZrB2-SiC.