Ultra-high Temperature Ceramics Research Articles

Analysis of Test Specimen Temperature Gradients Incurred in Resistive Heating System Oxidation Studies of Ultra-High Temperature Ceramics

AbstractThe need for advanced materials that can meet application requirements at ultra-high temperatures in oxidizing environments is an area of active research. One challenge facing the high temperature materials community is the ability to conduct controlled ultra-high temperature oxidation tests with minimal to no contamination or reaction with the chamber. A unique resistive heating system (RHS) capable of achieving ultra-high temperatures (> 1700 °C) to enable such experimentation is described. A concern of such a system is the potential presence of thermal gradients in directions not reflective of actual material applications, e.g., the hottest region being in the center of the sample. Experimental results from the oxidation of ZrB2 specimens at nominal temperatures of 1500°, 1700° and 1800 °C in low pO2 (0.1–1% O2 in Ar) environments are presented. Specimen thermal gradients generated during oxidation 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 ultra-high temperature ceramics.

Electromagnetic wave-absorbing of polymer derived rod-like ZrB2 ultrahigh-temperature ceramic composites

Ultra-high temperature ceramics (UHTCs) are candidates that can be used in extremely high temperature environments relative to the Si-based ceramic due to their excellent high-temperature performance. However, there are few researches on the electromagnetic wave absorption of the UHTCs themselves. In this work, rod-like ZrB2 and its composites were prepared by polymer derived method at a lower temperature. The phase composition and electromagnetic wave absorption properties of the ceramics at different pyrolyzed temperatures were analyzed. The rod-like ZrB2 pyrolyzed at 1500 °C shows a RLmin of -54.77dB at a thickness of 4.7mm. The rod-like ZrB2/SiC has an absorption bandwidth of 5.68GHz (RL <-10 dB, 7.6-13.28GHz) at a thickness of 4.25mm, exceeding the X-band. An absorption bandwidth of 2.24GHz for RL <-20 dB (99% absorbed) was obtained by the fabricated composites. The strong absorption of rod-like ZrB2/SiC composites is due to the strong conductive loss and multiple scattering of electromagnetic wave by the rod-like network structure. The formation of core-shell ZrB2/SiC increases the polarization and enhance the electromagnetic wave absorbing properties of the ceramics. The results of this study show that the polymer derived UHTCs have the strong ability to absorb electromagnetic wave.

Multiscale structural understanding of plasma spraying anti-ablation coating: an example of Ta-Hf-W-C ultrahigh temperature ceramics

The coating formation and structural design of ultra-high temperature ceramics (UHTCs) for aerospace vehicle anti-ablation coatings through plasma spraying remain uncertain despite extensive engineering applications. In this study, (Ta,Hf,W)C/HfC micro-composite, (Ta,Hf,W)C single-phase, and (Ta,Hf,C)/W nanocomposite coatings were successfully prepared using induction plasma spheroidization (IPS), vacuum plasma spraying (VPS), and spark plasma sintering (SPS). Density functional theory (DFT) was employed to capture the electronic structure of (Ta,Hf,W)Cy (y = 1, 0.875, 0.75, 0.5), confirming the influence of carbon vacancies and ceramic metallicity on the plastic forming behavior of sprayed coatings. Additionally, the contribution of HfC to structural stability and WC to the metallicity of carbon-based rock salt-structured UHTCs was elucidated. The pivotal role of carbon vacancies in inducing metastable phase formation and metal phase precipitation for (Ta,Hf,W)C was identified. Ablation behavior variations resulted from differences in multi-scale coating structures, where transverse cracks exhibited more detrimental effects than vertical cracks. Notably, a scale effect of oxidation related to the uniformity of metal elements in the multicomponent ceramic coating was uncovered. It was determined that components capable of forming high melting point oxides should not be uniformly dissolved in the matrix.

Mechanical properties of hafnium tantalum carbides via field assisted sintering technology for hypersonic vehicles

Ultra-high temperature ceramics (UHTCs) are refractory transition-metal carbides, nitrides, and borides with the highest melting temperatures known materials, making them prime candidates for applications in aerospace and hypersonic vehicles. Of the UHTCs, tantalum carbide (TaC) and hafnium carbide (HfC) feature the highest melting temperatures. We investigated the binderless consolidation of HfC/TaC powder blends using Field Assisted Sintering Technology (FAST). Powders consisting of 90/10, 50/50, and 10/90 vol% HfC:TaC were sintered to high densities (>94 %). Bulk and nanomechanical, chemical, and microstructural characterization revealed substantially greater strength, hardness, and stiffness for ternary alloys. Mechanical properties correlated with physiochemical analysis indicated trace oxygen phases, solid-solution strengthening, and nonstoichiometric carbon were the key mechanisms driving the peak property enhancement of the 50 vol% solid-solution sample, despite lower densities. This study provides insight into optimizing the compositional design of HfC-TaC alloys by balancing influences from solid solution strengthening and the thermodynamic effects of oxygen/carbon stoichiometry.

Zr/Hf-containing metallocene-based ceramic precursors

Polymer-derived-ceramic (PDC) route is an excellent strategy to tailor components at the molecular level, while existing synthetic methods face various problems such as complex preparation, multi-step reaction, and high conversion temperature, etc. Here, we propose an approach to prepare Zr-, Hf-, or Zr/Hf-containing metallocene-based ceramic precursors, which are constructed by the alkylation reaction between cyclopentadienyl ligand and methylene. Through the homogeneous and one-pot reaction occurs, we collect the target products. The reaction condition is mild, and the conversion temperature (1400 °C) is relatively low, and with a high ceramic yield (57.50–66.34%). The obtained metal-containing polymers are further converted to the orthorhombic phase (M7O8.79F9.71, M = Zr, Hf) after pyrolysis at 800 °C. Annealing at an intermediate temperature of 1400 °C in an argon atmosphere results in cubic crystalline ZrC or HfC nanoparticles embedded in a ceramic matrix. The free carbons are in-situ formed after high-temperature treatment, and higher disorder of carbons is observed with increasing temperature. This work provides a good strategy for the potential precursor of ultrahigh-temperature ceramics.

Spark plasma sintering and mechanical properties of tantalum carbonitrides Ta2CxNy

Dense tantalum-based carbonitride materials featuring >95 % relative density, TaC, Ta2C1.5N0.5, Ta2CN, Ta2C0.5N1.5 and TaN, were prepared by Spark Plasma Sintering (SPS) at 1873 K with a dwell time of 10 min and a pressure of 50 MPa. Despite the presence of an oxide phase (i.e. 5 vol%) and some W inclusions (i.e. 1 vol%), the mechanical properties of such ultra-high temperature ceramics (UHTC) show promising values. Indeed, the Young's moduli measured by nano-indentation were approx. 600–700 GPa, which is higher than literature values reported for similar UHTC. The hardness values increased from 16.4 ± 0.8 GPa for TaC to 27.9 ± 1.3 GPa for TaN, with an approximately linear trend for the carbonitride samples while the ratio of plastic deformation work over the total indentation work followed the opposite trend.

A universal strategy towards the fabrication of ultra-high temperature ceramic matrix composites with outstanding mechanical properties and ablation resistance

Materials with both excellent mechanical properties and good ablation resistance are urgently needed for the applications of thermal protection system in extreme high temperature oxidizing environments. However, owing to the lack of efficient fabrication processes, the existing materials suffer from either insufficient mechanical properties or poor ablation resistance. Herein, we report a novel solid–liquid combination fabrication strategy that successfully achieves the efficient incorporation of ultra-high temperature ceramics into 3D continuous carbon fiber performs as well as rapid densification. The relative density of the green body approaches the cubic close packing of spherical particles. Using Cf/ZrB2–SiC as an example, the as-prepared composite exhibits outstanding mechanical properties with a flexural strength surpasses 600 MPa and an unprecedented work of fracture of 11,851 ± 838 J/m2, which is two orders of magnitude higher than that of the currently reported monolithic ultra-high temperature ceramics. Moreover, the high ZrB2 content endows the composites with excellent ablation resistance and is thus capable of maintaining long-term non-ablative conditions at 2500 °C. The strategy possesses remarkable universality and high design flexibility, providing a time-saving and cost-effective universal strategy for the on-demand design and fabrication of high-performance ceramic, carbon, metal, and polymer matrix composites.

Pressureless sintered high-entropy transition metal diboride ceramics doped with yttrium tetraboride

High entropy diborides (HEBs) are new candidates of ultra-high temperature ceramics. However, it is difficult to obtain dense HEB ceramics via pressureless sintering. In this study, yttrium tetraboride (YB4) was introduced to consolidate (Ti0.25Zr0.25Hf0.25Ta0.25)B2 (4TMB2) ceramics and its effects on the microstructures and mechanical properties were studied. YB4 addition facilitates the densification and grain elongation of 4TMB2 ceramics by forming eutectic liquid phases and eliminating oxide impurities. Relative density of 4TMB2 ceramics containing 5 vol% YB4 reaches to 94.4 % after pressureless sintered at 2000 °C for 3 h. The hardness and toughness determined by indention method are 26.9±1.5 GPa and 5.73±1.65 MPa∙m1/2. This ceramics also exhibit excellent four-point bending creep resistance with the strain rate of 1.48×10−7 s−1 at 1900 °C under 20 MPa. The observed creep behavior is attributed to grain boundary sliding, plastic deformation, grain refinement and enhanced atomic diffusion of Y element, all driven by high-temperature stresses.

Irradiation resistance of thermo-optical properties of zirconium diboride by 3 MeV electrons

Due to good thermal conductivity and thermal shock resistance, ultra-high temperature ceramics such as zirconium diboride (ZrB2) have been investigated as promising materials to be used in reusable thermal protection systems TPSs are vital to the heat balance of a spacecraft during atmospheric reentry and subsequent operation in space. Hence, the thermal and optical properties are especially critical for such applications. Meanwhile, radiation exposure in space can pose risks of degrading such material properties, especially over a prolonged mission duration. The interaction of electron radiation-which can be found in the outer Van Allen belt, with ZrB2 has not been studied previously and was chosen as the main scope of this study. An electron source of 3 MeV with different radiation exposure time was used. The response of thermo-optical properties of ZrB2 to increasing electron radiation fluences was investigated. ZrB2 samples were made through spark plasma sintering into sintered pellets and then exposed to 3 MeV electron irradiation. These ZrB2 samples were characterized by their microstructure, thermal conductivity, coefficient of thermal expansion (CTE), emittance, absorptivity, and surface roughness before and after irradiation. It was found that ZrB2’s thermo-optical properties showed high radiation resistance at these fluences, and no apparent microstructural change was observed after irradiation. However, the irradiated samples had, on average, a 29% lower surface roughness than the unirradiated samples, possibly originating from electron sputtering.

Fabrication and Ablation Properties of SiC Nanowires-Network Modified Carbon/Carbon–Ultrahigh Temperature Ceramics Composites

In order to address the anti-ablation of carbon/carbon (C/C) composites, SiC nanowires (SiCnws) network-modified C/C-ZrB2-ZrC-SiC composites were prepared through the one-step precursor conversion method. With an optimized B-Si-Zr sol-precursor, uniformly dispersed SiCnws and ceramic particles forming an interlocking structure were synchronous in situ grown in the C/C matrix. During ablation, the partially oxidized SiCnws networks with molten SiO2 surface can stabilize the oxide protecting layer and heal the microcracks efficiently in combination with the refractory ZrO2 particles, thus significantly improving the anti-ablation properties of the composites. This study lays the foundation for the high temperature and long-term anti-oxidation and anti-ablation application of C/C composites in the aerospace industry.

Viable route to the manufacture of short carbon fiber-rich UHTC complex shapes with enhanced toughness

ZrB2 - based pasted, containing 5, 20, 45, 60 vol% short carbon fibers (Cf), were prepared in order to obtain complex shapes via direct ink writing, which were then sintered by hot pressing. The rheological behavior of the pastes indicated that at low Cf concentrations, the fibers act as a lubricant decreasing the paste viscosity. Above 20 vol% Cf, they act also as a binder creating an interconnected network that dominates over the lubrication effect, resulting in a viscosity increase. Complex shapes in the form of a hollow cylinder and a winglet with a functionally graded composition were then extruded and hot pressed, using a versatile powder bed method that enabled full densification with no need to use ad-hoc machined graphite inserts. Bars obtained from the direct ink writing process resulted in high alignment of the short fiber and displayed fracture toughness increase of 54% over that of the unreinforced matrix, while demonstrating a 42% increase with respect to conventionally hot pressed dry mixtures of ZrB2 and short fibers.This work shows the great potential of manufacturing complex shapes in ultra-high temperature ceramics matrices reinforced with short fibers for a plethora of design possibilities, including functional grading with control over layer composition, thickness and even alignment of short fiber.

Phase-Tunable Molybdenum Boride Ceramics as an Emerging Sensitive and Reliable SERS Platform in Harsh Environments.

Traditional surface-enhanced Raman scattering (SERS) sensors rely heavily on the use of plasmonic noble metals, which have limitations due to their high cost and lack of physical and chemical stability. Hence, it is imperative to explore new materials as SERS platforms that can withstand high temperatures and harsh conditions. In this study, the SERS effect of molybdenum boride ceramic powders is presented with an enhancement factor of 5 orders, which is comparable to conventional noble metal substrates. The molybdenum boride powders synthesized through liquid-phase precursor and carbothermal reduction have β-MoB, MoB2 , and Mo2 B5 phases. Among these phases, β-MoB demonstrates the most significant SERS activity, with a detection limit for rhodamine 6G (R6G) molecules of 10-9 m. The impressive SERS enhancement can be attributed to strong molecule interactions and prominent charge interactions between R6G and the various phases of molybdenum boride, as supported by theoretical calculations. Additionally, Raman measurements show that the SERS activity remains intact after exposure to high temperature, strong acids, and alkalis. This research introduces a novel molybdenum boride all-ceramic SERS platform capable of functioning in harsh conditions, thereby showing the promising of boride ultrahigh-temperature ceramics for detection applications in extreme environments.

Composition dependence of oxidation resistance in high entropy ultra-high temperature ceramics

High entropy ultra-high temperature ceramics (HE-UHTCs) have garnered intense research interest due to the potential for optimized oxidation and mechanical properties for extreme environment applications. HE-UHTCs are expected to oxidize according to the thermodynamic favorability of their respective oxidation reactions, which varies according to the periodic grouping. Based on this, the oxidation resistance of equimolar (metals-basis) group IV + V (HfZrTiTaNb), group IV + V + VI (HfZrTiTaMo) and group IV + VI (HfZrTiMoW) carbides and borides were evaluated at 1700°C in 1 mol% O2 for 5 min and compared. Group IV elements oxidized preferentially in all three compositions. Group V element-containing carbides exhibited the lowest oxidation resistance, attributed to the formation of intergranular liquid oxides. (HfZrTiMoW)C exhibited the best resistance among the carbides. The diborides exhibited similar material consumption, reinforcing the hypothesis that the oxidation behavior under these conditions is controlled by the presence of boria. These findings provide direction for HE-UHTC composition design for oxidation resistance.

Modulation of particle residence time in powder size-dependent ZrC coating on carbon-carbon composite with vacuum plasma spray

Recently, carbon-carbon (C/C) composites have been widely employed in various application fields due to their exceptional properties, but their oxidation at relatively low temperature around 500 °C poses a challenge to be used in high-temperature conditions. To address the drawback, the coating of ultra-high temperature ceramic (UHTC) layer on the C/C composites has been suggested as one of promising solutions. However, the quality of UHTC coating layers obtained through vacuum plasma spraying (VPS) is heavily influenced by the size of the introducing raw powders to coat the layers. Therefore, we investigated the size effects of ZrC powders in the range of (10–50) μm, one of UHTCs, to optimize the coating quality. By modulating process conditions, the coating quality with thick ZrC layer having lower porosity was obtained when using the powder around 34 μm. From the results, we found the gas residence time related to the particle velocity in the plasma flame is one of key parameters to be modulated by VPS operational parameters, which could improve the quality of the coating layer even for large powder cases.

Spark plasma sintered TiB2-SiC UHTC derived from B4C and stoichiometric intermetallic phases from Ti-Si system

Previously conducted studies determined that the reactive sintering of B4C and stoichiometric additions of TiSi2 or Ti5Si3 intermetallics created a dense TiB2-SiC double-phase system that could be classified as ultra-high temperature ceramics (UHTC). In this work, a spark plasma sintering (SPS) technique was used to improve the sintering process and thus further reduce the obtaining temperature and improve the final microstructure and mechanical properties of the proposed, double-phase TiB2-SiC UHTC systems. To evaluate the effects of the experiment, a comprehensive study of phase composition, microstructure and mechanical properties was added. As a result, minimal sintering temperatures were further reduced from 1420 - 1650 ºC to a range of 1200–1400 ºC. Among the prepared composites those sintered at 1400 ºC achieved the best properties. The prepared composites maintained high hardness (up to 24.85 GPa), high tensile strength (up to 309 GPa), relatively high fracture toughness (up to 4.80 MPa∙m1/2) and relatively low density, all of which characterized TiB2-SiC UHTC systems.

Research Progress of High-entropy Carbide Ultra-high Temperature Ceramics

Research Progress of High-entropy Carbide Ultra-high Temperature Ceramics

Microstructure, mechanical properties and ultra-high temperature thermal stability of GdB6 ceramics fabricated by reactive spark plasma sintering

In this study, a highly dense GdB6 bulk ceramic material was fabricated using one-step in-situ reaction spark plasma sintering technology. The sample with a relative density of 96.7 % and a purity of 96.9 wt% was successfully obtained by sintering at 1800 °C for 30 min under a uniaxial pressure of 50 MPa. The Vickers hardness, elastic modulus and fracture toughness of the as-obtained GdB6 bulk ceramics were 18.9 GPa, 209 GPa and 1.6 MPa·m1/2, respectively. The thermal conductivity of GdB6 at room temperature was 28.68 W·m−1·K−1, 70 % of which was mainly attributed to phonon vibration, while the remaining 30 % to the electrical thermal conductivity. In addition, the as-obtained GdB6 ceramics showed no phase transformation after heat treatment at a relatively high temperature of 2100 °C for 20 min in Ar atmosphere. The highly covalent octahedral boron framework played a critical role in maintaining the phase stability of the GdB6 crystal lattice. This suggested that GdB6 ceramics could be used as potential ultra-high temperature ceramics for aerospace applications and high-power vacuum electronic devices operated at high temperatures.

The ablation behaviors of ZrB2-based UHTC coating with La2O3-modified SiC bond coat

Ultra-high temperature ceramic (UHTC) has been developed to protect C/C composites exposed to high-temperature oxidizing environment. In this study, the effect of additive for the ablation behavior of ZrB2-based coating was investigated. The SiC–La2O3 bond coat was firstly prepared on the C/C composites by the method of pack cementation (PC). ZrB2–SiC–Si (ZSS) and ZrB2–SiC–Al2O3 (ZSA) coatings with dense microstructure and excellent interfacial bonding then were prepared on the surface of C/C composites coated with SiC–La2O3 layer by the method of atmospheric plasma spraying (APS). The ablation experiments of all coated samples were conducted under oxyacetylene torch with a heat flux of 4.18 MW/m2. The microstructure, phase compositions, interfacial bonding, ablation and oxidation behaviors of coating were investigated. After ablation test, the ZrO2 sketon with prorous structure was formed in the central region of ZSS coating. However, denser structure still can be observed at ZSA coating due to the formation of liquid phase. The SiC–La2O3–C/C interface are still without cracks. Furthermore, the diffusion of La element into ZSS and ZSA coatings from the SiC–La2O3 layer was beneficial for the improvement of ablation resistance. The result indicated that SiC–La2O3/ZSA could effectively provide protection for C/C composites.

Ultra-high temperature ceramics for extreme environments

Ultra-high temperature ceramics for extreme environments

High-entropy diboride ceramics with graphite addition

Graphite is a widely used oxygen impurity-removing additive and a property-tuning second phase for diboride and carbide-based ultrahigh temperature ceramics (UHTCs). In recent years, high-entropy diboride (HEB) ceramics have attracted much attention for the development of better UHTCs. However, oxide impurity generally inhibits their densification and harms their properties. Here, (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 ceramics added with different contents of graphite (HEB-xC, x = 0, 1, 2.5, 5, 10, 20 vol%) were prepared by spark plasma sintering. It was found that the added graphite acted as an oxygen-removing agent and the oxide impurity phase t-(Zr, Hf)O2 was converted to the (Hf, Ta)C-based solid solution carbide. With the increase in graphite content, the relative density increased from 89.4% for HEB-0 C to 100% for HEB-10 C. Based on the evaluated properties of the prepared specimens, including Young’s modulus, hardness, toughness, and thermal conductivity, the added graphite with an optimized content is important for a given started powder and processing to achieve good and balanced material properties.