Design of UHTC for Oxidation Resistance
Abstract
Application of Ultra-High Temperature Ceramics (UHTCs) are currently limited by their poor oxidation resistance. The proposed work seeks to identify UHTC design methodology for improved oxidation resistance focusing on the HfC/TaC system, one of the highest temperature materials systems known. The most thermodynamically stable oxide for HfC/TaC materials is HfO2, an oxide with significant oxygen deficiency and rapid oxygen transport rates, resulting inrapid oxidation of the underlying carbide materials. The proposed work seeks to lower oxidation rates by considering fundamental mechanisms of oxygen diffusion. Diffusion rates can be lowered by decreasing the number of available sites for diffusion or by increasing the energetic barrier for diffusion. Two strategies are proposed to modify the thermally grown hafnium oxide. In the firststrategy, compositions of HfC dilute in TaC will be sought that result in Ta donor doping of the thermally grown HfO2, with the intent to lower the number of oxygen vacancies available for transport. In the second strategy, compositions of HfC with moderate additions of TaC will be sought that result in thermally grown oxides with the Hf6Ta2O17 ordered phase, with the intent to both lower the number of oxygen vacancies available for transport and increase the energeticbarrier for oxygen transport. Both experimental and computational methods will be employed to probe the phase equilibria in the Hf-Ta-C and Hf-Ta-O systems and oxygen transport in the Tadoped HfO2 and ordered Hf6Ta2O17 phases.(Hf,Ta)C materials of varying TaC content will be synthesized by spark plasma sintering.Oxidation experiments will be conducted over a range of relevant times, temperatures, and oxygen partial pressures using resistive or laser heating. Oxidation kinetics will be determined from material consumption rates. The resulting oxide scales will be characterized for phase, composition, and morphology using scanning and transmission electron microscopy, energydispersive spectroscopy, electron energy loss spectroscopy, x-ray photoelectron spectroscopy, xray diffraction analysis, and time-of-flight secondary ion mass spectrometry. Double oxidation exposures in 16O2/18O2 will be used to elucidate oxygen diffusion pathways. 18O2 tracer diffusion studies will be conducted on directly synthesized phase-pure oxides as needed. Rate limitingoxidation mechanisms will be established.Computational efforts include establishing thermodynamic descriptions of the Hf-Ta-C and Hf-Ta-O ternary systems using the CALPHAD method supplemented by the cluster expansion approach and density functional theory. New thermodynamic models based on the Cluster Variation Method will be developed to describe short range order in the oxide phases, thus yieldingmore accurate thermodynamic descriptions for use in the CALPHAD approach. Thermo-Calc software will be used to predict stable phases formed during oxidation of (Hf,Ta) carbides. Kinetic Monte Carlo simulations will be performed to predict oxygen diffusivity in doped and ordered oxides. Migration barriers will be predicted using first-principles calculations with nudged elasticband method. Oxidation simulations will be performed using DICTRA software. Computational results will be compared with experimental results, iteratively improving the thermodynamic and kinetic models.The expected outcome of this work is an improved understanding of design methodologies for oxidation resistant materials applicable for other UHTC material systems via optimization for thermal growth of doped or ordered oxides with reduced oxygen transport rates.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 25, 2019
- Source ID
- N000141912274
Entities
People
- Elizabeth Opila
Organizations
- Office of Naval Research
- United States Navy
- University of Virginia