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.
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Ultra-high temperature ceramics Fahrenholtz, William G; Wuchina, Eric J; Lee, William E ...
2014., 2014, 2014-10-10, 2014-09-29
eBook
"This book will capture historic aspects and recent progress on the research and development of ultra-high temperature ceramics. This will be the first comprehensive book focused on this class of ...materials in more than 20 years. The book will review historic studies and recent progress in the field. The intent is to provide a broad overview and critical analysis rather than focus on the latest scientific results. The content will include synthesis, powder processing, densification, property measurement, and characterization of boride and carbide ceramics. Emphasis will be on materials for hypersonic aerospace applications such as wing leading edges and propulsion components for vehicles traveling faster than Mach 5, but will also include materials used in the extreme environments associated with high speed cutting tools and nuclear power generation"-- Provided by publisher. "This book provides a snapshot of the current state-of-the-art in the processing, densification, properties, and performance of boride and carbide ceramics. The book contains contributions from leading experts who have active research in ultra-high temperature ceramics"-- Provided by publisher.
High-entropy carbide powders were produced by a two-step synthesis process consisting of carbothermal reduction followed by solid solution formation. Nominally pure (Hf,Zr,Ti,Ta,Nb)C in a ...single-phase rock salt structure had an average particle size of about 550 nm and an oxygen content of 0.2 wt%. The fine particle size was due to the use of high-energy ball milling prior to carbothermal reduction combined with the relatively low synthesis temperature of 1600 °C. Oxygen content was minimized by completion of the carbothermal reduction reactions under vacuum. This is the first report of synthesizing a high-entropy carbide powder using individual transition metal oxides and carbon as precursors.
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Dense (Hf, Zr, Ti, Ta, Nb)C high‐entropy ceramics were produced by hot pressing (HP) of carbide powders synthesized by carbothermal reduction (CTR). The relative density increased from 95% to 99.3% ...as the HP temperature increased from 1750°C to 1900°C. Nominally phase pure ceramics with the rock salt structure had grain sizes ranging from 0.6 µm to 1.2 µm. The mixed carbide powders were synthesized by high‐energy ball milling (HEBM) followed by CTR at 1600°C, which resulted in an average particle size of ~100 nm and an oxygen content of 0.8 wt%. Low sintering temperature, high relative densities, and fine grain sizes were achieved through the use of synthesized powders. These are the first reported results for low‐temperature densification and fine microstructure of high‐entropy carbide ceramics.
A thermodynamic model was developed to explain the formation of a SiC‐depleted layer during ZrB2–SiC oxidation in air at 1500°C. The proposed model suggests that a structure consisting of (1) a ...silica‐rich layer, (2) a Zr‐rich oxidized layer, and (3) a SiC‐depleted zirconium diboride layer is thermodynamically stable. The SiC‐depleted layer developed due to active oxidation of SiC. The oxygen partial pressure in the SiC‐depleted layer was calculated to lie between 4.0 × 10−14 and 1.8 × 10−11 Pa. Even though SiC underwent active oxidation, the overall process was consistent with passive oxidation and the formation of a protective surface layer.
The mechanical properties of single‐phase (Hf,Zr,Ti,Ta,Nb)C high‐entropy carbide (HEC) ceramics were investigated. Ceramics with relative density >99% and an average grain size of 0.9 ± 0.3 µm were ...produced by a two‐step process that involved carbothermal reduction at 1600°C and hot pressing at 1900°C. At room temperature, Vickers hardness was 25.0 ± 1.0 GPa at a load of 4.9 N, Young's modulus was 450 GPa, chevron notch fracture toughness was 3.5 ± 0.3 MPa·m1/2, and four‐point flexural strength was 421 ± 27 MPa. With increasing temperature, flexural strength stayed above ~400 MPa up to 1800°C, then decreased nearly linearly to 318 ± 21 MPa at 2000°C and to 93 ± 10 MPa at 2300°C. No significant changes in relative density or average grain size were noted after testing at elevated temperatures. The degradation of flexural strength above 1800°C was attributed to a decrease in dislocation density that was accompanied by an increase in dislocation motion. These are the first reported flexural strengths of HEC ceramics at elevated temperatures.
High‐entropy diboride powders were produced by a two‐step synthesis process consisting of boro/carbothermal reduction followed by solid solution formation. Nominally phase‐pure (Hf,Zr,Ti,Ta,Nb)B2 in ...a single‐phase hexagonal structure had an average particle size of just over 400 nm and contained 0.3 wt% carbon and 0.3 wt% oxygen. The fine particle size was due to the use of high‐energy ball milling prior to boro/carbothermal reduction, which led to a relatively low synthesis temperature of 1650°C. Oxygen and carbon contents were minimized by completion of the boro/carbothermal reduction reactions under vacuum. This is the first report of synthesis of a nominally phase pure high‐entropy diboride powder from oxides using a two‐step process.
Ceramics based on group IV-V transition metal borides and carbides possess melting points above 3000 °C, are ablation resistant and are, therefore, candidates for the design of components of next ...generation space vehicles, rocket nozzle inserts, and nose cones or leading edges for hypersonic aerospace vehicles. As such, they will have to bear high thermo-mechanical loads, which makes strength at high temperature of great importance. While testing of these materials above 2000 °C is necessary to prove their capabilities at anticipated operating temperatures, literature reports are quite limited. Reported strength values for zirconium diboride (ZrB
) ceramics can exceed 1 GPa at room temperature, but these values rapidly decrease, with all previously reported strengths being less than 340 MPa at 1500 °C or above. Here, we show how the strength of ZrB
ceramics can be increased to more than 800 MPa at temperatures in the range of 1500-2100 °C. These exceptional strengths are due to a core-shell microstructure, which leads to in-situ toughening and sub-grain refinement at elevated temperatures. Our findings promise to open a new avenue to designing materials that are super-strong at ultra-high temperatures.
Herein, we critically evaluate computational and experimental studies in the emerging field of high-entropy ultra-high-temperature ceramics. High-entropy ultra-high-temperature ceramics are ...candidates for use in extreme environments that include temperatures over 2,000°C, heat fluxes of hundreds of watts per square centimeter, or irradiation from neutrons with energies of several megaelectron volts. Computational studies have been used to predict the ability to synthesize stable high-entropy materials as well as the resulting properties but face challenges such as the number and complexity of unique bonding environments that are possible for these compositionally complex compounds. Experimental studies have synthesized and densified a large number of different high-entropy borides and carbides, but no systematic studies of composition-structure-property relationships have been completed. Overall, this emerging field presents a number of exciting research challenges and numerous opportunities for future studies.
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