Ultra-high Temperature Applications Research Articles

Radio frequency‐assisted zirconium carbide matrix deposition for continuous fiber‐reinforced ultra high temperature ceramic matrix composites

AbstractZirconium carbide (ZrC) is considered to be a potential candidate for ultra high temperature applications due to its high melting point, good chemical inertness, and ablation resistance, but the monolithic form suffers from low fracture toughness and hence poor thermal shock resistance. Reinforcing it using continuous carbon fibers (Cf) to create an ultra high temperature ceramic matrix composite is an obvious solution, however densifying ZrC requires the use of very high temperatures combined with significant pressure, such as obtained by using hot pressing or spark plasma sintering, which risks damaging fibers. In the present work, radio frequency‐assisted chemical vapor infiltration (RF‐CVI) has been investigated with a view to forming Cf/ZrC composites. These initial experiments revealed the ability to deposit pure, nano‐grained, and near stoichiometric ZrC with deposition occurring preferentially from the center of the sample due to the nature of the inverse temperature profile developed. The deposited ZrC grains were in the range of 4–9 nm in size and had a lattice parameter of 0.4750 nm. The work also showed that the use of RF‐CVI enabled the minimization of early pore sealing, a common problem for conventional CVI.

A simple recipe for designing multicomponent ultra-high temperature ceramic classes by using structure maps coupled with machine learning

Successful synthesis of novel high entropy ceramic (HEC) for ultra-high temperature application classes, namely, borides, carbides, and nitrides, has been experiencing a bottleneck in having a suitable design and successful synthesis strategy. Producing high-entropy ultra-high-temperature ceramics from their oxides offers a major processing benefit, while employing a design approach using machine learning enhances the efficiency of the formation of single-phase HECs. In this regard, we propose a generalized strategy to generate a semi-synthetic database for each of these classes using literature data and atomic environment mapping-based structure plots, which can further be used to build machine learning models. The imbalance of the dataset was addressed using adaptive synthetic sampling and the edited nearest neighbors technique. The trained models are able to accurately predict over 90% of the single-phase chemistry for each of the classes. Furthermore, a few compositions representing these classes were successfully synthesized from the corresponding oxide mixture to validate the effectiveness of the proposed strategy.

Effect of Microstructure on Oxidation and Micro-mechanical Behavior of Arc Consolidated Mo-Ti-Si-(B) Alloys

AbstractThe present study deals with the development and characterization of Mo-35Ti-10Si and Mo-35Ti-10Si-2B (wt.%) alloy for ultra-high temperature applications beyond the temperature limit of existing super alloys. The microstructural characterization using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), electron back scattered diffraction (EBSD), x-ray diffraction (XRD) revealed that the Mo-35Ti-10Si-2B alloy was consisted of three phases, namely, (Mo, Ti)ss, (Mo, Ti)5SiB2 and (Ti, Mo)5Si3; whereas, Mo-35Ti-10Si alloy was found to be consisting of (Mo, Ti)ss, and (Mo,Ti)3Si phases. Since quantification of boron is difficult by EDS, Particle Induced Gamma-ray Emission (PIGE), a nuclear reaction analysis technique was used for chemical composition analysis of boron. The oxidation behavior of the Mo-35Ti-10Si-2B alloy in the temperature regime of 825-1250 °C was studied in detail and compared with boron-free Mo-35Ti-10Si alloy. Mo-35Ti-10Si-2B alloy exhibited superior oxidation behavior at intermediate temperatures of 825 °C, and excellent oxidation resistance at higher temperatures between 1000 and 1250 °C due to the formation of the protective borosilica and double oxide layers (TiO2 and duplex borosilica-TiO2), respectively. High-temperature oxidation mechanisms were discussed using detailed microstructural cross section analysis of the oxidized alloy samples. The micro-mechanical behavior of constitutive phases of the Mo-35Ti-10Si-2B alloy were studied by microhardness, nano-indentation and micropillar compression testing. The micropillar compression of (Mo, Ti)ss phase showed fairly ductile behavior with the evidence of activation of dislocation in the form of slip lines revealed through the post-deformation fractography. Deformation studies of (Mo, Ti)5SiB2 and (Ti, Mo)5Si3 phases were also carried out which showed large strain bursts indicating possibility of activation of dislocation activities even at room temperatures imparting low level of ductility.

Effect of Microstructure on Oxidation and Micro-mechanical Behavior of Arc Consolidated Mo-Ti-Si-(B) Alloys

AbstractThe present study deals with the development and characterization of Mo-35Ti-10Si and Mo-35Ti-10Si-2B (wt.%) alloy for ultra-high temperature applications beyond the temperature limit of existing super alloys. The microstructural characterization using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), electron back scattered diffraction (EBSD), x-ray diffraction (XRD) revealed that the Mo-35Ti-10Si-2B alloy was consisted of three phases, namely, (Mo, Ti)ss, (Mo, Ti)5SiB2 and (Ti, Mo)5Si3; whereas, Mo-35Ti-10Si alloy was found to be consisting of (Mo, Ti)ss, and (Mo,Ti)3Si phases. Since quantification of boron is difficult by EDS, Particle Induced Gamma-ray Emission (PIGE), a nuclear reaction analysis technique was used for chemical composition analysis of boron. The oxidation behavior of the Mo-35Ti-10Si-2B alloy in the temperature regime of 825-1250 °C was studied in detail and compared with boron-free Mo-35Ti-10Si alloy. Mo-35Ti-10Si-2B alloy exhibited superior oxidation behavior at intermediate temperatures of 825 °C, and excellent oxidation resistance at higher temperatures between 1000 and 1250 °C due to the formation of the protective borosilica and double oxide layers (TiO2 and duplex borosilica-TiO2), respectively. High-temperature oxidation mechanisms were discussed using detailed microstructural cross section analysis of the oxidized alloy samples. The micro-mechanical behavior of constitutive phases of the Mo-35Ti-10Si-2B alloy were studied by microhardness, nano-indentation and micropillar compression testing. The micropillar compression of (Mo, Ti)ss phase showed fairly ductile behavior with the evidence of activation of dislocation in the form of slip lines revealed through the post-deformation fractography. Deformation studies of (Mo, Ti)5SiB2 and (Ti, Mo)5Si3 phases were also carried out which showed large strain bursts indicating possibility of activation of dislocation activities even at room temperatures imparting low level of ductility.

Realization of ultra-high temperature stability and excellent electrical properties in LTO-based ceramics via A/B-site co-doing

La2Ti2O7 (LTO) ceramics, ideal for ultra-high temperature applications with a Curie temperature near 1460 °C, were previously limited by a low piezoelectric coefficient (d33) of 1.5 pC/N. In this work, the incorporation of Bi3+ and V5+ dopants have been employed to augment the piezoelectric properties of LTO. An enhanced d33 ∼ 4.0 pC/N is achieved in La1.91Bi0.09Ti1.97V0.03O7 (LBTV9) ceramics, while maintaining an ultra-high Curie temperature of 1437 °C. This improvement is attributed to local heterogeneity in the lattice structure induced by the doping. LBTV9 ceramics also exhibit excellent high-temperature stability and insulation properties, maintaining a d33 at 4.0 pC/N after annealing at 1000 °C for 48 h and stabilizing at 4.2 pC/N across temperatures from 200 °C to 1600 °C, with a DC resistivity of 1.7 × 106 Ω·cm at 1000 °C. These enhancements make LBTV9 ceramics suitable for extreme condition applications, thus unlocking new possibilities for their use in ultra-high temperature piezoelectric devices.

W-doped concentration improved the hardness, elastic modulus, elastic anisotropy and thermodynamic properties of ZrC ultrahigh temperature ceramics

Although ZrC is a promising ultrahigh temperature ceramics due to the high melting point, high strength, high hardness and low density, the improvement of mechanical properties and thermal stability is very significance for it's ultrahigh temperature application. To improve the mechanical and thermodynamic properties of ZrC ultrahigh temperature ceramics, we apply the first-principles calculations to study the influence of W-doped concentration on the structural stability, hardness, elastic modulus, elastic anisotropy and thermodynamic properties of the cubic ZrC ceramics. The calculated results show that three W-doped ZrC are thermodynamic stability. In particular, the calculated impurity formation energy of the W-doped ZrC increases with increasing W concentration. Importantly, the calculated elastic modulus of the W-doped ZrC increases with increasing W concentration. Although the hardness of W-doped ZrC is bigger than the parent ZrC, the hardness of W-3.125 at.% is higher than the W-doped ZrC with the high concentration of W. Naturally, the high hardness and elastic modulus of the W-doped ZrC is that the metal W enhances the localized hybridization between Zr and C, which is demonstrated by the change of ZrC bond. In addition, the parent ZrC and the W-doped ZrC show excellent elastic anisotropy. Furthermore, the calculated Debye temperature of W-3.125 at.% ZrC is higher than the parent ZrC and the other W-doped ZrC. Therefore, we predict that the W-3.125 at.% ZrC shows excellent mechanical properties and better thermodynamic properties in comparison to the other W-doped ZrC ceramics.

Multicomponent (Hf-Zr-Ta)B2 coatings for carbon/carbon composites and structural optimization enabling superior ablation resistance

Multicomponent (Hf-Zr-Ta)B2 potentially provides improved ablation resistance compared with silicon-based ceramics. Here we deposited (Hf0.5–x/2Zr0.5–x/2Tax)B2 (x = 0, 0.1, and 0.2) coatings onto C/C composites, and investigated their ablation behaviors under an oxyacetylene torch with a heat flux of 2.4 MW m–2. It was observed that the x = 0.1 oxide scale bulged but was denser, and the x = 0.2 oxide scale was blown away due to the formation of excessive liquid. Based on these findings, we further developed a duplex (Hf-Zr-Ta)B2 coating that showed a linear recession rate close to zero (0.11 μm s–1) after two 120-s ablation cycles. It is identified that the resulting oxide scale is mainly composed of (Hf, Zr)6Ta2O17 and (Hf, Zr, Ta)O2 by performing aberration-corrected (scanning) transmission electron microscopy. The protective mechanism is related to the peritectic transformation of orthorhombic-(Hf, Zr)6Ta2O17 to tetragonal-(Hf, Zr, Ta)O2 plus Ta-dominated liquid. This study contributes to the development of Ta-containing multicomponent UHTC bulk and coatings for ultra-high temperature applications.

Intrinsic factors responsible for brittle versus ductile nature of refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) are of interest for ultrahigh-temperature applications. To overcome their drawbacks — low-temperature brittleness and poor creep strength at high temperatures — improved fundamental understanding is needed. Using experiments, theory, and modeling, we investigated prototypical body-centered cubic (BCC) RHEAs, TiZrHfNbTa and VNbMoTaW. The former is compressible to 77 K, whereas the latter is not below 298 K. Hexagonal close-packed (HCP) elements in TiZrHfNbTa lower its dislocation core energy, increase lattice distortion, and lower its shear modulus relative to VNbMoTaW whose elements are all BCC. Screw dislocations dominate TiZrHfNbTa plasticity, but equal numbers of edges and screws exist in VNbTaMoW. Dislocation cores are compact in VNbTaMoW and extended in TiZrHfNbTa, and different macroscopic slip planes are activated in the two RHEAs, which we attribute to the concentration of HCP elements. Our findings demonstrate how ductility and strength can be controlled through the ratio of HCP to BCC elements in RHEAs.

Synthesis of porous ZrC particles derived from MOF-801

Porous zirconium carbide (ZrC) nanoparticles were synthesized by carbothermal reduction using MOF-801, a zirconium-based metal-organic framework (Zr-MOF), and sucrose, which serves as an environment-friendly carbon source. The sucrose was adsorbed into the pores of MOF-801 for adjusting the carbon content. The results showed that the final ZrC particles retained the morphology of the initial MOF-801 particles with a round shape and a uniform size of 261 ± 37 nm. Importantly, the material was able to maintain its high surface area up to 1500 °C. The specific surface area of ZrC nanoparticles synthesized using a 1.5:1 Sucrose: MOF-801 molar ratio gradually decreased from 186 m2/g to 13 m2/g as the reaction temperature increased from 1500 °C to 2000 °C, revealing that the porosity of the porous ZrC particles can be controlled due to partial sintering inside the particles at different temperatures. Porous ZrC particles show promise for ultra-high temperature applications such as thermal insulation and as porous substrates.

ScB2C2: The first high damage tolerant ultra-high temperature ceramic with hydrolysis resistance

The first high damage resistant ultra-high temperature ceramic ScB2C2 with excellent hydrolysis resistance was successfully prepared through boron carbon thermal reduction and hot pressing sintering using Sc2O3, B4C and graphite as raw materials. The Vickers hardness, flexural strength, fracture toughness, Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio of as-prepared ScB2C2 ceramic are 6.0 ± 0.2 GPa, 246 ± 17 MPa, 4.11 ± 0.18 MPa·m1/2, 240 GPa, 101 GPa, 127 GPa and 0.18, respectively. In the a-b plane (B2C2 sheet) and along the c-axis direction, ScB2C2 exhibits extremely strong anisotropy of chemical bonds and elastic properties. The high residual strength, damage tolerance factor Dt and fracture toughness to strength ratio KIc/σf indicates that ScB2C2 has excellent damage tolerance. In addition, ScB2C2 displayed remarkable thermal shock resistance and hydrolysis resistance. These unique features endow ScB2C2 with significant potential for future ultra-high temperature applications.

Enhanced fracture properties of continuous Cf/ZrB2-based composites with polydopamine-derived carbon boundary layer

Continuous carbon fiber-toughened ZrB2-based composites (Cf/ZrB2-based composites) are potential thermo-structural materials suitable for ultra-high temperature applications. To improve the fracture properties, polydopamine-derived carbon (PDC) was deposited on carbon fibers and used as a boundary layer of Cf/ZrB2-based composite (Cf/PDC/ZrB2). Results indicated that PDC was applied uniformly to fibers, with no instances of cross-sticking observed among adjacent fibers. The matrix could effectively infiltrate into the intra-bundle region, resulting in the formation of a highly dense Cf/PDC/ZrB2 composite. PDC can effectively modulate the interface of Cf/ZrB2-based composite. Using polydopamine-derived carbon (PDC) as a boundary layer, the average interfacial shear strength of the composite was decreased from 369 MPa to 133 MPa. The fracture toughness of Cf/PDC/ZrB2 composite was 8.4 MPa·m1/2, representing a 19 % enhancement in comparison with the composite without a PDC layer. The work of fracture of Cf/PDC/ZrB2 composite was 8.5 kJ·m−2, significantly surpassing that of the composite without a PDC layer (1.7 kJ·m−2). The outstanding fracture properties can be attributed to the beneficial interfacial bonding established by PDC layer between fiber and matrix, which promotes the activation of toughening mechanisms including crack branching/deflection and fiber bridging/pull-out.

Ir–Zr coating through a pack cementation method for ultra-high temperature applications

An Ir–Zr intermetallic coating was prepared onto Re/Ir coated graphite through a pack cementation method. The microstructure, nanoindentation and ablation behaviors of the coating system were investigated. The Ir–Zr coating was about 20 μm thick and consisted of 4 intermetallic phases which showed a quasi-gradient transition in composition. Nanoindentation measurements indicated that most of the intermetallic phases had higher hardness but lower Young's modulus compared to pure iridium. By ablation test in a high-frequency plasma wind tunnel, the heat flux limit of the Ir–Zr coating was measured to be about 6.1 MW/m2, and the endurance time was about 100 s under this condition. The coating failed at ∼2240 °C during the ablation test when the iridium melted, but the unfused parts remained complete and in good adhesion with the iridium beneath. The average normal emissivity between 2 and 5 μm of the sample was measured to be ∼0.82 at around 2092 °C before failure, which was higher than pure iridium and bulk ZrO2.

Combustion synthesis of ultra-high-temperature solid solutions (ZrxNb1-x)B2. Part 2: Fine-tuning the mechanical properties and thermal conductivity of Zr–Nb–B diboride solid solutions for ultra-high temperature applications

In Part 1 of this study, we investigated the macrokinetic parameters and phase formation mechanism in the combustion front of Zr–Nb–B mixtures. In the second part, we produced consolidated samples of solid solutions NbB2-(0.100%)ZrB2 by hot pressing the combustion-derived powders at 1900 °C. By measuring the crystallographic parameters, mechanical and thermophysical properties of the produced bulk samples, we obtained valuable insights into the behavior of the solid solutions. Our results pave the way for the application-specific fine-tuning of the mechanical properties and thermal conductivity of diboride solid solutions in the Zr–Nb–B system for ultra-high temperature applications. Our findings have significant implications for the development of advanced materials with outstanding performance in extreme environments.

A comparative study on the stability of six Ir/Ir3X (X = Ti, V, Zr, Nb, Hf, Ta) interfaces by first-principle and AIMD calculations

Interfacial stability is critical for dual-phase Ir/Ir3X materials used in ultra-high temperature applications. Despite this, this property remains poorly recognized and understood. In the present work, the interfacial energies and the generalized stacking fault energies (GSFE) curves are calculated to investigate the thermodynamic and kinetic stability of the six Ir/Ir3X interfaces by DFT and AIMD methods. It is found that Ir/Ir3Ti possesses the lowest interfacial energy at 0 K and 2000 K, as well as the highest unstable stacking fault energy, indicating it is the most stable interface. The comparison shows that the considered interfaces containing IVB elements (Ti, Hf and Zr) are more stable than those containing VB elements (V, Nb and Ta). Furthermore, the analysis reveals that interfacial energy differences in Ir/Ir3X may contribute to interfacial lattice misfit and charge redistribution at the interface. We aim to provide essential theoretical guidance for the design of novel high-temperature dual-phase Ir-based alloys.

YB2C2: The first damage tolerant ceramic with melting point over 2500 oC

The ultra-high temperature thermal stability of YB2C2 between 2300 oC and 2550 oC in argon was systematically investigated by induction heating method. The phase composition and microstructure of the samples after heat treatment at different temperatures were characterized with X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. The results show that YB2C2 has excellent ultra-high temperature thermal stability, and it can maintain the structure and phase stability after heat treatment at 2500 oC for 20 min. It would melt rather than decompose with the rising of the temperature. The melting point of YB2C2 is between 2500 oC and 2550 oC, and it has great potential for future ultra-high temperature applications.

Powder Fabrication and Laser Powder Bed Fusion of a MoSiBTiC-La2O3 Alloy

In the present work, an approach of freeze-dry pulsated orifice ejection method (FD-POEM) was utilized to fabricate monodispersed MoSiBTiC-La2O3 composite powders for laser powder bed fusion (L-PBF). The FD-POEM powders were spherically shaped, possessing a narrow size range and uniform element distribution. As revealed by the single-track and single-layer experiments, the porous FD-POEM particles were sufficiently fused under laser irradiation, leading to the generation of continuous laser tracks and low surface roughness layers, which proved a feasible L-PBF processability of MoSiBTiC-La2O3 powders. Careful microstructural observations confirmed that the microstructure of the molten pools was primarily composed of Mo solid solution dendrites reinforced with La2O3 nanoparticles. Consequently, the single MoSiBTiC-La2O3 track had a high Martens hardness of 3955 HM. The result of this work reveals that the combination of FD-POEM and L-PBF has a great potential of developing advanced heat-resistant Mo-based alloys with tailored structures for ultrahigh-temperature applications.

Microstructure and oxidation behavior of Co–Cr–Ta ternary alloys

The improvement of oxidation resistance of alloys is one of the key points to develop components for ultra-high temperature applications. The formation of a CrTaO4 oxide layer has attracted attention because of its thermal stability and good protection against oxidation above 1000 °C where more commonly used protective oxide layers such as Cr2O3 show their limits. This study investigates the microstructure evolution and oxidation resistance of the Co-Cr-Ta ternary alloys in comparison to the Calphad predictions. The Co-Cr-Ta samples tested here are composed of an FCC Co-rich phase and a C15 laves phase, and an increase of Cr content was linked to the formation and growth of an additional σ phase. Thermogravimetric analysis, together with the isothermal oxidation tests in air at 1200 °C proved that alloys exhibited good oxidation resistance formed a continuous CrTaO4 layer. Also, optimization of Co-Cr-Ta composition with improved oxidation resistance was attempted using data-driven strategy and the results demonstrated the potential of such approach for the future of alloy design.

Microstructural characteristics and mechanical properties of Si-B alloys for functional and structural ultra-high temperature applications

Silicon boron alloys have been recognized as important materials for e.g. a direct usage in ultra-high temperature latent heat thermal energy storage systems or as a batch materials for processing boron enhanced silicide-based composites. In this work, we put new experimentally driven insights on a structure of selected Si-B binary alloys. For this reason, four Si-xB alloys (x = 2.5; 8, 13.5; 86 (at%)) were fabricated from pure elements by using a crucible-less electric arc melting technique. Nominal compositions of the alloys were selected in accordance to Si-B phase diagram as hypoeutectic, eutectic, hypereutectic and corresponding to the stoichiometric composition of SiB6 intermetallic phase, respectively. The fabricated Si-xB alloys were subjected to a detailed structural characterization. Additionally, a hardness, elastic moduli and fracture toughness of each phase constituent were examined through a micro-indentation technique. A co-existence of various silicon borides, namely SiB4, SiB6 and SiBn, has been experimentally documented. Obtained results allows receiving a description of each alloy in terms of microstructure, crystal structure and mechanical properties of involved phases. It was found that mechanical properties (both microhardness and Young modulus) increases with raising boron content in recognized Si-B phases. The fracture toughness of silicon borides assessed through indentation experiments was found to be in the rage of 2.13–3.29 MPa·m1/2.

Mechanical properties of pristine and nanocrystalline graphene up to ultra-high temperatures

Despite being the basic building block of most carbon materials used in high and ultra-high temperature applications, the mechanical properties of graphene at such temperatures remain totally unknown. Here we compute these properties using tensile molecular dynamics simulations at temperatures ranging from room temperature to 5000 K (i. e. about the melting temperature) for pristine and nanocrystalline graphene. Mechanical simulations at such temperatures are made possible via the use of the screened environment dependent reactive empirical bond order potential [Perriot et al. Phys. Rev. B 88, 064101 (2013)]. We show that even at the highest temperature, both graphene and nanocrystalline graphene retain high stiffness, strength and fracture strain. Analyses of tensile curves and trajectories (including an analysis of in-plane non-affine atomic displacements) show that both pristine and nanocrystalline graphene show brittle rupture, whatever the temperature, even though some early sign of ductility can be detected for the nanocrystalline graphene at 5000 K. This rules out the possibility of a ductile fracture in graphene at any temperature below the melting point. Finally, the simulations also highlight the importance of bond rotation (Stone-Wales) based defects in the fracture initiation mechanism for pristine graphene. Especially at intermediate temperature (3000 K) crack nucleation in pristine graphene is shown to take place exactly at the moment and place of the first Stone-Wales defect formation.

Grain boundary segregation induced strong UHTCs at elevated temperatures: A universal mechanism from conventional UHTCs to high entropy UHTCs

Ultra-high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties, including ultra-high melting point, excellent chemical stability, and good oxidation resistance, which make them promising candidates for aerospace and nuclear applications. However, the degradation of high-temperature strength is one of the main limitations for their ultra-high temperature applications. Thus, searching for mechanisms that can help to develop high-performance UHTCs with good high-temperature mechanical properties is urgently needed. To achieve this goal, grain boundary segregation of a series of carbides, including conventional, medium entropy, and high entropy transition metal carbides, i.e., Zr0.95W0.05C, TiZrHfC3, ZrHfNbTaC4, TiZrHfNbTaC5, were studied by atomistic simulations with a fitted Deep Potential (DP), and the effects of segregation on grain boundary strength were emphasized. For all the studied carbides, grain boundary segregations are realized, which are dominated by the atomic size effect. In addition, tensile simulations indicate that grain boundaries (GBs) will usually be strengthened due to segregation. Our simulation results reveal that grain boundary segregation may be a universal mechanism in enhancing the high-temperature strength of both conventional UHTCs and medium/high entropy UHTCs, since GBs play a key role in controlling the fracture of UHTCs at elevated temperatures.