Abstract
Ultra-high temperature ceramics (UHTCs), most notably transition metal carbides and borides, exhibit melting temperatures exceeding 3000°C, making them appropriate candidates to withstand the extreme temperatures (~2000°C) expected to occur at the leading edges of hypersonic vehicles. However, their propensity to react rapidly with oxygen limits their sustained application. Oxidation resistant materials require formation of dense, adherent and solid oxides that provide an effective barrier, slowing the diffusion of oxygen and/or metal atoms to the reaction interface under ultra-high temperature conditions. The work completed was part of a collaborative effort to identify new UHTCs with the potential for improved properties based on entropic stabilization of the base material. The goals of this work were to determine the parameters influencing the oxidation behavior of group IV, V and VI transition metal HE-UHTCs at ultra-high temperatures. Candidate compositions for oxidation studies were identified based on initial thermodynamic assessments to be HfZrTiTaNb, HfZrTiTaMo and HfZrTiMoW carbides and diborides. The bulk of the analytical and experimental studies were performed on HfZrTiTaNb carbide and diboride.
Thermodynamic calculations using FactSage software and databases were performed for oxidation reactions of the constituents in equimolar, five-component carbide and diboride ceramics in the group IV, V and VI elemental palette. Periodic trends in oxide stability and phase were observed. The relative stability of the oxide phases formed from the constituent carbides and from binary carbide solutions were used to investigate preferential oxidation. This analysis was extended to ternary and higher order alloys and carbides. The thermodynamic models thus developed were compared to the experimentally determined oxidation behavior of the five-component HfZrTiTaNb carbide and diboride. Experimental results verified that the thermodynamically favorable group IV oxides dominated in the observed oxide scale. It was concluded that, given any high entropy material, even a slight relative favorability for a given oxide formation reaction will result in preferential oxidation, reducing the configurational entropy in the underlying substrate material.
Specimen thermal gradients generated during oxidation in a unique resistive heating system (RHS) capable of achieving ultra-high temperatures (>1700°C) were evaluated using finite element analysis models. Thermal gradients on the order of the uncertainty in temperature measurements were calculated, confirming the RHS suitability for conducting ultra-high temperature oxidation exposures on HE-UHTCs. The oxidation kinetics of the high entropy group IV+V (HfZrTiTaNb)C and (HfZrTiTaNb)B<sub>2</sub> materials were extensively evaluated at 1500°C-1800°C using the RHS in one atmosphere 0.1%-1% oxygen/argon gas mixtures for times up to 15 minutes. Possible mechanisms based on the resulting time, temperature and oxygen partial pressure dependence underscored the complex oxidation behavior of these materials. The carbides formed porous and intergranular oxides. Oxidation resistance was improved upon external scale formation. The diborides formed dense external scales and exhibited better oxidation resistance compared to the carbides. This improvement was attributed to the formation of liquid boria. Both compositions showed an unexpected reduction in material consumption at 1800°C for all times tested, compared to the lower temperatures tested. An in-depth analysis of the composition and morphology of the oxide scale and sub-surface regions for specimens tested at 1800°C revealed that the formation of denser group IV-rich (Hf, Zr, Ti) oxides mitigated the formation of the otherwise detrimental liquid-forming group V (Ta, Nb) oxides, leading to the improved oxidation resistance.
Group IV+V+VI (HfZrTiTaMo) and group IV+VI (HfZrTiMoW) carbides and borides, were exposed at 1700°C in 1%O<sub>2</sub> for five minutes, conditions at which group IV+V HfZrTiTaNb compositions exhibited the poorest oxidation resistance. Oxidation behavior of the high entropy compositions were also compared to the baseline ZrC and ZrB<sub>2</sub> ceramics. All compositions exhibited preferential oxidation of the group IV elements. Group V (Ta) element-containing carbides were found to exhibit the lowest oxidation resistance, which was attributed to the tendency to form intergranular and liquid oxides, while (HfZrTiTaMo)C performed comparably with ZrC. All the diboride compositions exhibited similar material consumption, reinforcing the hypothesis that the oxidation behavior under these conditions is at least partly attributable to liquid boria acting as an oxidant diffusion barrier.
The findings in this study demonstrate that the oxidation behavior of HE-UHTCs can be understood primarily with thermodynamic principles, with minimal application of kinetic considerations. Further, they show that the oxidation behavior of HE carbides are compositionally driven, with the most promising composition being (HfZrTiMoW)C. Therefore, the thermodynamic models developed in this work can be used to tailor the design of a carbide HE-UHTC composition for oxidation resistance. Under the conditions explored in this study, the boride HE-UHTCs exhibited better oxidation resistance. Further studies are recommended at temperatures above the boiling point of boria (>1860°C) to probe the oxidation behavior of boride HE-UHTCs without the influence of boria acting as a diffusion barrier.
