This paper identifies gaps in the present state of knowledge and describes emerging research directions for ultra-high temperature ceramics. Borides, carbides, and nitrides of early transition metals such as Zr, Hf, Nb, and Ta have the highest melting points of any known compounds, making them suitable for use in extreme environments. Studies of synthesis, processing, densification, thermal properties, mechanical behavior, and oxidation of ultra-high temperature ceramics have generated a substantial base of knowledge, but left unanswered questions. Emerging research directions include testing/characterization in extreme environments, composites, computational studies, and new materials.

Experimental data on the oxidation kinetics of SiC‐containing diborides of Zr and Hf in the temperature regime of 1473–2273 K are interpreted using a mechanistic model. The model encompasses counter‐current gas diffusion in the internal SiC depleted zone, oxygen permeation through borosilicate glass channels in the oxide scale, and boundary layer evaporation at the surface. The model uses available viscosity, thermodynamic and kinetic data for boria, silica, and borosilicate glasses, and a logarithmic mean approximation for compositional variations. The internal depletion region of SiC is modeled with CO/CO2 counter diffusion as the oxygen transport mechanism. Data reported for pure SiC in air/oxygen, for ZrB2 containing varying volume fractions of SiC, and for SiC–HfB2 ultra‐high temperature ceramics (UHTCs) by different investigations were compared with quantitative predictions of the model. The model is found to provide good correspondence with laboratory‐furnace‐based experimental data for weight gain, scale thicknesses, and depletion layer thicknesses. Experimental data obtained from arc‐jet tests at high enthalpies are found to fall well outside the model predictions, whereas lower enthalpy data were closer to model predictions, suggesting a transition in mechanism in the arc‐jet environment.

MAX-phase Zr2SC ceramics were synthesized by hot pressing mixtures containing ZrH2, carbon or ZrC, and ZrS2, WS2, or FeS2 as sulfur sources. The Zr2SC synthesized with FeS2 had a typical MAX-phase laminated structure, good thermal shock resistance, thermal stability to 2100 degrees C, and were readily machineable. The flexural strength was about 250 MPa up to 800 degrees C. The thermal expansion coefficient was 8.8 x 10(-6)/degrees C in the 25-2000 degrees C-temperature range, thermal conductivity was 38 W/m-K at 100 degrees C and about 30 W/m-K at 1100 degrees C, and specific heat at temperatures from 100 to 1100 degrees C ranged from 0.4 to 0.5 kJ/kg degrees C. Load-deflection curves exhibited plastic deformation from RT to 2066 degrees C. The sample deflection at fracture demonstrated significant dependence on temperature with a minimum at 1510 degrees C. In arc heater testing at 2150 degrees C for 23 s, the material developed an adherent, protective scale. Published by Elsevier B.V.

The broader context of this discussion, based on a workshop where materials technologists and computational scientists engaged in a dialogue, is an awareness that modeling and simulation techniques and computational capabilities may have matured sufficiently to provide heretofore unavailable insights into the complex microstructural evolution of materials in extreme environments. As an example, this article examines the study of ultrahigh-temperature oxidation-resistant ceramics, through the combination of atomistic simulation and selected experiments. We describe a strategy to investigate oxygen transport through a multi-oxide scale-the protective layer of ultrahigh-temperature ceramic composites ZrB2-SiC and HfB2-SiC-by combining first-principles and atomistic modeling and simulation with selected experiments.

Details of the high-pressure combustion-driven powder compaction (CDC) process are presented with particular reference to the consolidation of refractory metal and alloy powders. Rapid load cycles (ms) at pressures up to 2,069 MPa (150 tsi) result in high-green-density net- or near-net-shape parts that exhibit minimal shrinkage during sintering and fine-grain equiaxed microstructures. CDC tungsten and rhenium-base alloys retain strength levels >172 MPa (25,000 psi) at 1,927 degrees C. As an example of the utility of CDC, the performance of tungsten and molybdenum in X-ray targets is assessed. The properties of CDC refractory metals and alloys offer potential for new commercial applications in the energy, electronics, defense, and biomedical sectors.

A mechanistic model that interprets the oxidation behavior of the diborides of Zr, Hf and Ti in the temperature range of 1000– 1800 C was formulated. Available thermodynamic data and literature data for vapor pressures and diffusivities were used to evaluate the model. Good correspondence was obtained between theory and experiments for weight gain, recession and scale thickness as functions of temperature and oxygen partial pressure. At temperatures below about 1400 C, the rate-limiting step is the diffusion of dissolved oxygen through a film of liquid boria in capillaries at the base of the oxidation product. At higher temperatures, the boria is lost by evaporation, and the oxidation rate is limited by Knudsen diffusion of molecular oxygen through the capillaries between nearly columnar blocks of the oxide, MO2. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A wide range of experimental data on the oxidation of ZrB2 and HfB2 as a function of temperature (800 degrees-2500 degrees C) is interpreted using a mechanistic model that relaxes two significant assumptions made in prior work. First, inclusion of the effect of volume change associated with monoclinic to tetragonal phase change of the MeO2 phases is found to rationalize the observations by several investigators of abrupt changes in weight gain, recession, and oxygen consumed, as the temperature is raised through the transformation temperatures for ZrO2 and HfO2. Second, the inclusion of oxygen permeability in ZrO2 is found to rationalize the enhancement in oxidation behavior at very high temperatures (>1800 degrees C) of ZrB2, while the effect of oxygen permeability in HfO2 is negligible. Based on these considerations, the significant advantage of HfB2 over ZrB2 is credited to the higher transformation temperature and lower oxygen permeability of HfO2 compared with ZrO2.

A wide range of experimental data on the oxidation of ZrB2 and HfB2 as a function of temperature (800 degrees-2500 degrees C) is interpreted using a mechanistic model that relaxes two significant assumptions made in prior work. First, inclusion of the effect of volume change associated with monoclinic to tetragonal phase change of the MeO2 phases is found to rationalize the observations by several investigators of abrupt changes in weight gain, recession, and oxygen consumed, as the temperature is raised through the transformation temperatures for ZrO2 and HfO2. Second, the inclusion of oxygen permeability in ZrO2 is found to rationalize the enhancement in oxidation behavior at very high temperatures (>1800 degrees C) of ZrB2, while the effect of oxygen permeability in HfO2 is negligible. Based on these considerations, the significant advantage of HfB2 over ZrB2 is credited to the higher transformation temperature and lower oxygen permeability of HfO2 compared with ZrO2.

A mechanistic model that interprets the transition in oxidation behavior of zirconium diboride as the temperature is varied from 600°C to 2500°C is presented. Available thermodynamic data and literature data for vapor pressures, oxygen permeability in boria, and viscosity of boria were used to evaluate the model. Three regimes and the temperatures of transition between them were identified. In the intermediate temperature regime, viz., 1000°C to 1800°C, good correspondence was obtained between theory and experiments for weight gain, recession, and scale thickness as functions of temperature and oxygen partial pressure. In this regime, the rate-limiting step is the diffusion of dissolved oxygen through a film of liquid boria in capillaries at the base of the oxidation product. At lower temperatures, an external boria scale forms, but it was not found to contribute significantly to oxidation resistance. Comparison with literature data on recession is very good, but weight gain is predicted to be higher than experimentally observed unless flow of viscous boria is included. At higher temperatures, the boria is lost by evaporation, and the oxidation rate is limited by diffusion of molecular oxygen through the capillaries between nearly columnar blocks of the oxide MO2.; this regime is soon followed by a rapid acceleration of recession due to vaporization of the oxide MO2 itself.

The effect of Si3N4, Ta5Si3, and TaSi2 additions on the oxidation behavior of ZrB2 was characterized at 1200°–1500°C and compared with both ZrB2 and ZrB2/SiC. Significantly improved oxidation resistance of all Si‐containing compositions relative to ZrB2 was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si3N4‐modified ceramics increased with increasing Si3N4 content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide‐containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB2, with the effect increasing with temperature. The most oxidation‐resistant ceramics contained 15 vol% Ta5Si3, 30 vol% TaSi2, 35 vol% Si3N4, or 20 vol% Si3N4 with 10 mol% CrB2. These materials exceeded the oxidation resistance of the ZrB2/SiC ceramics below 1300°–1400°C. However, the ZrB2/SiC ceramics showed slightly superior oxidation resistance at 1500°C.