Oxidation resistance of tantalum carbide-hafnium carbide solid solutions under the extreme conditions of a plasma jet

The oxidation behaviors of tantalum carbide (TaC)- hafnium carbide (HfC) solid solutions with five different compositions, pure HfC, HfC-20 vol% TaC (T20H80), HfC- 50 vol% TaC (T50H50), HfC- 80 vol% TaC (T80H20), and pure TaC have been investigated by exposing to a plasma torch which has a temperature of approximately 2800 °C with a gas flow speed greater than 300 m/s for 60 s, 180 s, and 300 s, respectively. The solid solution samples showed significantly improved oxidation resistance compared to the pure carbide samples, and the T50H50 samples exhibited the best oxidation resistance of all samples. The thickness of the oxide scales in T50H50 was reduced more than 90% compared to the pure TaC samples, and more than 85% compared to the pure HfC samples after 300 s oxidation tests. A new Ta₂Hf₆O₁₇ phase was found to be responsible for the improved oxidation performance exhibited by solid solutions. The oxide scale constitutes of a scaffold-like structure consisting of HfO₂ and Ta₂Hf₆O₁₇ filled with Ta₂O₅ which was beneficial to the oxidation resistance by limiting the availability of oxygen.     

High-temperature compressive creep behavior and mechanism of Hf6Ta2O17 ceramic as a candidate for thermal barrier coatings

A novel Hf₆Ta₂O₁₇ ceramics is prepared by a solid-state reaction method. High-temperature creep behavior of Hf₆Ta₂O₁₇ and 8YSZ ceramics are investigated by compressive creep test combined with a digital image correlation (DIC) method. It is found that the creep mechanism of Hf₆Ta₂O₁₇ ceramics is controlled by grain boundary sliding associated with dislocation movement (stress exponent ∼2-3, and activation energy of 600–620 kJ/mol). Grain boundary sliding accommodated to the interface reaction is the main creep mechanism of 8YSZ ceramics (stress exponent ∼2, and activation energy of 425∼465 kJ/mol). Hf₆Ta₂O₁₇ ceramics have higher creep resistance than 8YSZ ceramics under the same conditions.     

Synthesis of Hf6Ta2O17 superstructure via spark plasma sintering for improved oxidation resistance of multi-component ultra-high temperature ceramics

Ultra-high temperature ceramics (UHTCs) have shown aspiration to overcome challenges in the thermal protection system (TPS) by designing new materials referred to as multi-component UHTCs (MC-UHTCs) in the compositional space. MC-UHTCs have shown remarkable improvement in oxidation resistance due to the formation of the Hf₆Ta₂O₁₇ superstructure during plasma exposure. Herein, the Hf₆Ta₂O₁₇ superstructure is synthesized via a solid-state reaction between HfO₂ and Ta₂O₅ powder mixtures during spark plasma sintering (SPS). The compositions chosen are 50 vol% of HfO₂ -50 vol% of Ta₂O₅ (50HO-50TO) and 70 vol% of HfO₂ -30 vol% of Ta₂O₅ (70HO-30TO). The phase quantification via Rietveld analysis showed Hf₆Ta₂O₁₇ as a principal phase with some residual Ta₂O₅ phase in both the samples. The high-temperature thermal stability of the samples was evaluated using high-velocity plasma jet exposure for up to 3 min. 50HO-50TO was able to withstand the intense plasma condition, which is attributed to the higher content of the Hf₆Ta₂O₁₇ phase (～84%) and lower strain in the Ta₂O₅ phase. The augmentation in the Hf₆Ta₂O₁₇ phase to 94.7% (in 50HO-50TO) post plasma exposure has been attributed to the invariant transformation from a liquid state to Hf₆Ta₂O₁₇ at temperatures >2500 °C during testing. The mechanical integrity is elucidated from the insignificant change in the hardness ～13.3 GPa before and 11.2 GPa after plasma exposure of the 50HO-50TO sample. As a result, the Hf₆Ta₂O₁₇ superstructure's thermo-mechanical stability suggests developing novel oxidation-resistant MC-UHTCs in compositional space for reusable space vehicle applications.     

Synthesis,sintering, and grain growth kinetics of Hf6Ta2O17

A systematic study of the solid-state synthesis, pressureless sintering, and grain growth kinetics of Hf₆Ta₂O₁₇ is presented. The ideal conditions for solids-state synthesis of Hf₆Ta₂O₁₇ powder with minimal particle necking was 1250 °C for 2 h in air. The resultant powder has an average particle size of 210 ± 70 nm. The combined synthesis and ball-milling procedure produces highly sinterable Hf₆Ta₂O₁₇ powder, achieving > 97 % of theoretical density after pressureless sintering at 1600 °C for 2 h in air. The grain growth mechanism was sensitive to processing conditions, appearing to be primarily driven by surface diffusion below 1600 °C and grain boundary diffusion above 1650 °C. The respective activation energies for grain growth were found to be Q_S = 659 ± 79 kJ mol⁻¹ and Q_GB = 478 ± 63 kJ mol⁻¹.     

Harmonized toughening and strengthening in pressurelessly reactive-sintered Ta0.8Hf0.2C-SiC composite

Ta_0.8Hf_0.2C-27?vol%SiC (99.0% in relative density) composite was toughened and strengthened via pressurelessly in-situ reactive sintering process. HfC and β-SiC particles were formed after reaction of HfSi₂ and carbon black at 1650?°C. Ta_0.8Hf_0.2C was obtained from solid solutioning of HfC and commercial TaC. The β-α phase transformation of SiC proceeded below 2200?°C. High aspect ratio, platelet-like α-SiC grains formed and interconnected as interlocking structures. Toughness and flexural strength values of 5.4?±?1.2?MPa?m^1/2 and 443?±?22?MPa were measured respectively. The toughening mechanisms by highly directional growth of discontinuous α-SiC grains were crack branching, bridging and deflection behaviors.     

Study on properties of Hf6Ta2O17/Ta2O5 system: A potential candidate for environmental barrier coating (EBC)

Ta₂O₅ doped Hf₆Ta₂O₁₇ system (Hf₆Ta₂O₁₇/Ta₂O₅) is considered to have potential application prospect in the field of aero-engine. We herein focus on the thermo-physical, mechanical properties and CMAS corrosion resistance of Hf₆Ta₂O₁₇/Ta₂O₅ to systematically evaluate the possibility for the application of environmental barrier coating (EBC). By changing the content of Ta₂O₅, the gradient adjustment of thermal expansion coefficient can be realized while maintaining low thermal conductivity (1.5–2.2 W/(m·K)). The introduction of Ta₂O₅ significantly reduces the modulus and improves the fracture toughness. Single-phase Hf₆Ta₂O₁₇ shows excellent corrosion resistance against molten calcium-magnesium-alumina-silicate (CMAS). The crystallization of CaTa₂O₆ and HfSiO₄ is the important factor to prevent further corrosion. The introduction of Ta₂O₅ weakens the ability to prevent Si penetration and greatly increases the thickness of the corrosion layer. The results highlight the merit of Hf₆Ta₂O₁₇/Ta₂O₅ system as potential candidate for multi-layer gradient coating on the surface of ceramic matrix composites.     

Toughness mechanism and plastic insensitivity of submicron second phase Ta in a novel Ta–Hf6Ta2O17 composite ceramic

The introduction of metal-second phase can improve the fracture toughness of metal-ceramic composite (MCC) material, but usually degrades the strength and hardness. The pace of exploring the process and materials to both improve the toughness and hardness has never stopped. In this study, a novel Ta–Hf₆Ta₂O₁₇ composite ceramic is successfully prepared by spark plasma sintering. The effects of Ta content on microstructure and mechanical properties of the as-sintered ceramic are investigated. The fracture toughness of Ta–Hf₆Ta₂O₁₇ composite ceramic first increases and then decreased slightly with the increase in Ta content, reaching the maximum value of 4.21 ± 0.09 MPa m^1/2 at 20 vol% Ta. The improvement of the fracture toughness does not affect the hardness, whose value is stable between 16.74 GPa and 18.43 GPa. Based on the results of Selsing’s model, Raman spectra and TEM, it is confirmed that the toughness mechanism of Ta–Hf₆Ta₂O₁₇ composite ceramics originates from good inherent interface strength and crack deflection caused by the second phase. The maintenance of hardness comes from the plastic insensitivity of submicron Ta caused by the interfacial tensile stress, which provides a potential mechanism for the design of metal-ceramic composite with excellent strength and toughness.     

Ablation-resistant Ta0.78Hf0.22C solid solution ceramic modified C/C composites for oxidizing environments over 2200 °C

Ta_0.78Hf_0.22C solid solution ceramic was synthesized and introduced into carbon/carbon (C/C) composites by polymer infiltration and pyrolysis (PIP). Effect of the introduction of Ta_0.78Hf_0.22C on the microstructure, ablation resistance, and flexural performance of C/C composites were investigated. Results showed that the flexural strength and modulus of the composites were increased by 108 % and 117 %, respectively, after adding Ta_0.78Hf_0.22C solid solution into C/C composites. In addition, the introduction of Ta_0.78Hf_0.22C improved the ablation resistance of C/C composites under oxyacetylene ablation environment, over 2200 °C. The linear and mass ablation rates were decreased 73 % and 70 %, respectively. The oxide with low melting point, Ta₂O₅, exhibited good sealing and oxygen-barrier capacity, and the formation of new solid solution oxide particles, Hf₆Ta₂O₁₇, could pin cracks during ablation, both of them were contributed to better ablation resistance of modified C/C composites.     

Hardness and toughness improvement of SiC-based ceramics with the addition of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2

The influences of different contents ranging 0–15 wt% of high-entropy boride (HEB) (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ on the mechanical properties of SiC-based ceramics using Al₂O₃-Y₂O₃ sintering additives sintered by spark plasma sintering process were investigated in this study. The results showed that the introduction of 5 and 10 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ could facilitate the densification and the grain growth of SiC-based ceramics via the mechanism of liquid phase sintering. However, the grain growth of SiC-based ceramics was inhibited by the grain boundary pinning effect with the addition of 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂. The SiC-based ceramics with 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ showed the enhanced hardness (21.9±0.7 GPa) and high toughness (4.88±0.88 MPa·m^1/2) as compared with high-entropy phase-free SiC-based ceramics, which exhibited a hardness of 16.6 GPa and toughness of 3.10 MPa·m^1/2. The enhancement in mechanical properties was attributed to the addition of higher hardness of HEB phase, crack deflection toughening mechanism, and presence of residual stress due to the mismatch of coefficient of thermal expansion.     

Ablation behavior and mechanisms of 3D-Cf/Ta0.8Hf0.2C-SiC composite at temperatures up to 2500 °C

In this work, ablation behavior and mechanisms of 3D-C_f/Ta_0.8Hf_0.2C-SiC composite were studied via air plasma test at temperatures up to 2500 °C. At temperatures below 2000 °C, a continuous oxides layer composed of o-Ta₂O_5(H) – t-Ta₂O₅ – Hf₆Ta₂O_17(H) – SiO₂ is formed on the ablation surface, which turns to o-Ta₂O_5(H) – SiO₂ above 2200 °C. During ablation, o-Ta₂O_5(H) precipitates from the glassy SiO₂ and grows up following Oswald ripening. Although the volatilization of SiO₂ aggravates with ablation temperature increase and ablation time extension, o-Ta₂O_5(H) – SiO₂ melt can still serve as effective self-healing similar to SiO₂ glass. Accordingly, the multiphase oxides layer formed on the ablation surface provides a stable protective effect for the internal composite under all the tested ablation conditions. As a consequence, the 3D-C_f/Ta_0.8Hf_0.2C-SiC composite presents outstanding ablation-resistant performance even at 2500 °C for 300 s, with a linear recession rate of ～ 5.7 µm/s and a mass recession rate of 2.91 mg/s.     

Ablation behavior of (TiZrHfNbTa)C high-entropy ceramics with the addition of SiC secondary under an oxyacetylene flame

The ablation behavior of high-entropy ceramics (HECs) was investigated in this study using an oxyacetylene flame at 2000 °C. Spark plasma sintering was used to construct a dense HEC (TiZrHfNbTa)C with a 20 vol% of SiC addition (HEC-20SiC). The densification of HEC-20SiC can be improved to a certain extent by adding SiC particles, increasing the hardness of HEC-20SiC to up to 24.6 GPa, and the crack deflection observed through the addition of SiC particles were considered to be the strengthening and toughening mechanisms. After ablation, Hf₆Ta₂O₁₇, Ti_5.1Ta_4.9O₂₀, Nb₂Zr₆O₁₇, TaZr_2.75O₈, and SiO₂ can be detected on an ablated surface and HEC-20SiC possesses the minimum mass ablation rate (?1.9 mg s^?1) and line ablation rate (2.1 μm s^?1) among the comparative ceramics. On the one hand, the SiC phase forms gaseous CO, CO₂, and SiO as well as viscous SiO₂ during ablation and some part of the heat can be dissipated by the evaporation of gaseous CO, CO₂, and SiO; further, pore defects can be healed by viscous SiO₂, thus inhibiting the diffusion of reactive oxygen species. On the other hand, the HEC phase with a lattice-distortion caused by single-phase solid-solution can effectively inhibit the invasion of reactive oxygen species and the outward migration of metal atoms. The invasion rate of reactive oxygen is considered to be the main step during HEC-20SiC ablation, and it is believed that higher principal component HECs can improve ablation performance even further.     

Valence state of europium and samarium in Ln2Hf2O7 (Ln = Eu,Sm) based oxygen ion conductors

Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) pyrochlores have been prepared via mechanical activation of oxide mixtures, followed by heat treatment for 4h at 1450 and 1600 °C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf_1-xEu_xO₆ octahedra in pyrochlore Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 °C for 24h in air reduces the total conductivity of the Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) by a factor of 2.5–6, due to the decrease in the concentrations of oxygen vacancies and Ln²⁺ ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) at ~800 °C can be understood in terms of the changes in the oxygen sublattice of the pyrochlore structure as a result of the oxidation of divalent europium and partial filling of oxygen vacancies at this temperature.     

Ablation properties and mechanism of a novel La2Hf2O7 coating

Based on excellent thermal stability and high melting point, La₂Hf₂O₇ coating was prepared on SiC coated C/C composites by SAPS. When exposed to a heat flux of 3.2 MW/m² for 20 s, the maximum surface temperature reaches 2240 °C, and the linear and mass ablation rates are 0.0030 mm/s and 0.0001 g/s, respectively. The chemical composition of the ablated La₂Hf₂O₇ coating remained unchanged, and the coarsening of La₂Hf₂O₇ grains caused a denser surface morphology with some pinholes and microcracks, which was attributed to thermal accumulation and scouring of heat flux. Owing to the low thermal conductivity, most of the heat was concentrated on the superficial region, promoting the outer La₂Hf₂O₇ coating to evolve into a dense block structure with some defects, and the inner La₂Hf₂O₇ coating maintained the initial structure. Because of the dense structure and low oxygen permeability of LaHf₂O₇ coating, the inner SiC coating was well protected without oxidation. Moreover, the nano-indentation results show that the hardness of La₂Hf₂O₇ coating increased from 6.39 to 15.67 Gpa, which was ascribed to the intense solid-state sintering of La₂Hf₂O₇ coating.     

Effects of Ta2O5 content on mechanical properties and high-temperature performance of Zr6Ta2O17 thermal barrier coatings

Zr₆Ta₂O₁₇/YSZ double ceramic top coat thermal barrier coatings (TBCs) with different Ta₂O₅ content are prepared by atmospheric plasma spraying (APS). The effects of Ta₂O₅ content on mechanical properties and high-temperature performance of Zr₆Ta₂O₁₇ TBCs are investigated. With the decrease in Ta₂O₅ content, the hardness and fracture toughness of the Zr₆Ta₂O₁₇ coating increases from 12.833 to 15.117 GPa and from 3.3592 to 3.7753 MPa m^1/2, respectively. The Zr₆Ta₂O₁₇ coating specimens with different Ta₂O₅ content do not delaminate from the substrate except the edge peeling after 180 h of oxidation or 2000 thermal cycles at 1150°C. The Zr-12Ta specimen (the molar ratio of ZrO₂/Ta₂O₅ of 0.88:0.12) presents the more excellent mechanical properties and high-temperature performance. The reduction in Ta₂O₅ content will improve the mechanical properties, high-temperature oxidation, and thermal cycle performance of Zr₆Ta₂O₁₇/YSZ double ceramic top coat TBCs.     

Enhanced photocatalytic activity of nonstoichiometric crystalline TaO2F and Ta2O5 with carbon coating

Pure and carbon-coated tantalum-based oxides photocatalysts were synthesized via the mesocrystalline precursor transformation method by annealing pure and polydopamine-coated (NH₄)₂Ta₂O₃F₆ mesocrystals in Ar. The oxygen-poor atmosphere thermal annealing process assisted the formation of nonstoichiometric TaO₂F mesocrystals with more F and Ta₂O₅ nanorods with oxygen vacancies and the associated lower valence state Ta ions (Ta⁴⁺). Furthermore, the carbon coating, decomposed from coated polydopamine, helped to control their particle size within 100 nm by isolating the connection of (NH₄)₂Ta₂O₃F₆ subunits. Hence, as-synthesized products, particularly carbon-coated Ta₂O₅ nanosheets, owning large surface area (67.6 m² g^?1), fine particle size (<100 nm), excellent electronic conductivity, decreased bandgap energy, enhanced and extended absorption in the visible range, exhibited preferable photocatalytic activity in the photodegradation of methylene blue, reaching a 76.54 % and 41.71 % removal under ultraviolet and visible light illumination, suggesting a promising candidate for wide-range responsive photocatalytic applications.     

Transformation of metallic polymer precursor into nanosized HfTaC2 ceramics

Polymeric precursor for HfTaC₂ ceramic was prepared by coordination reaction between metal alkoxides and acetylacetone, subsequent hydrolysis, and blend with oligomeric novolac carbon sources. The phase analysis and microstructure study were conducted using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). It showed that after pyrolyzed at 1450 °C for 90 min, HfTaC₂ was obtained with particle size ~50 nm and uniform elemental distribution. Phase transition from precursor to ceramic was studied by in situ XRD measurements at temperatures from 800 to 1200 °C. In the early stage, oxide solid solution Hf₆Ta₂O₁₇ was firstly detected at ~950 °C, followed by Ta₂O₅ and TaO. As temperature was raised, signals for TaO and Hf₆Ta₂O₁₇ gradually weakened and disappeared, while those for other phases strongly strengthened. Furthermore, formation of Hf_xTa_yC_z solid solution was monitored and confirmed by peak migration during 1300 °C isothermal treatment. When the sample was pyrolyzed at 1450 °C, solid solution Hf_xTa_yC_z was detected at different holding time. Phase structure at 90 min was the closest to standard HfTaC₂ with particle size D_v (90) ~ 200 nm.     

Low-temperature reactive hot-pressing of Ta0.2Hf0.8C–SiC ceramics at 1700°C

Temperature above 2000°C and additional pressure is generally required to achieve the full densification of Ta_xHf_1−xC-based ceramics. This work proposed a novel method to fabricate dense Ta_0.2Hf_0.8C ceramics at relatively low temperature. Using a small amount of Si as a sintering aid, Ta_0.2Hf_0.8C was densified at 1700°C by reactive hot-pressing (RHP), with SiC formed in situ. Microstructure evolution mechanisms of the ceramics during RHP were investigated. The effect of silicon content on the densification and mechanical properties of the ceramics was revealed. It is indicated that the apparent porosity of the Ta_0.2Hf_0.8C–SiC ceramics was as low as 0.5%, whereas bending strength and fracture toughness of the ceramics were as high as ∼637 MPa and 6.7 MPa m^1/2, respectively, when the silicon content was 8 wt.%. This work provides a new idea for the low-temperature densification of Ta_xHf_1−xC and other ultrahigh temperature ceramics with high performance.     

Ablation behavior of high-entropy carbides ceramics (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C upon exposition to an oxyacetylene torch at 2000 °C

The ablation performance of a high-entropy ceramic carbide, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C, was performed by oxyacetylene ablation flame, simulating the extreme service environment at 2000 ºC. Phase and microstructure characterization at multi-length scales was carried out. During ablation, a compositionally and microstructurally complex oxidation layer formed on the ablation surface, which consisted of a combination of (Zr_xHf_1?x)₆(Nb_yTa_1?y)₂O₁₇, Ti(Nb_xTa_1?x)₂O₇, and Ti_x(Zr_aHf_bNb_cTa_1?a-b-c)_1?xO₂. Based on the microstructure information, the ablation mechanisms were proposed considering the oxidation thermodynamics and kinetics. Comparable rates of O inward diffusion and Ti outward diffusion are suggested, and a particular innermost dense layer composed of isolated (Zr_xHf_1?x)₆(Nb_yTa_1?y)₂O₁₇ grains embedded in a continuous Ti(Nb_xTa_1?x)₂O₇ matrix is considered to be beneficial for a better ablation resistance.     

Hf6Ta2O17/Ta2O5 composite ceramic: A new eutectic system

Eutectic oxides are considered as the most promising candidates applied in extreme environment for long periods, and the pace of exploring new eutectic systems has never stopped. In the present work, Hf₆Ta₂O₁₇/Ta₂O₅ eutectic system is firstly prepared by the joint process of solvothermal method and spark plasma sintering (SPS). Eutectic liquid phase produced during the process of SPS fills the closed-pores and promotes the sintering process. Orthorhombic-Hf₆Ta₂O₁₇ and tetragonal-Ta₂O₅ of both eutectic and hypereutectic systems show single crystal characteristics and are combined by coherent interface instead of grain boundaries. The clean and flat interface also shows the characteristics of single crystal diffraction without impurities and amorphous phase. Based on the analysis of microstructure, the sintering mechanism is proposed, and the potential application of Hf₆Ta₂O₁₇/Ta₂O₅ eutectic system is analyzed.     

Thermal shock performance and failure behavior of Zr6Ta2O17-8YSZ double-ceramic-layer thermal barrier coatings prepared by atmospheric plasma spraying

Zr₆Ta₂O₁₇ has higher fracture toughness, better phase stability, thermal insulation performance and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (8 YSZ, 7–8 wt%) at temperatures above 1200 °C. However, the thermal expansion coefficients between Zr₆Ta₂O₁₇ coating and bond coating do not match well. A double-ceramic-layer design is applied to alleviate the thermal stress mismatch. The Zr₆Ta₂O₁₇/8 YSZ double-ceramic-layer thermal barrier coatings (TBCs) are prepared by atmospheric plasma spraying (APS). During the thermal shock test, Zr₆Ta₂O₁₇/8 YSZ double-ceramic-layer TBCs exhibit a better thermal shock resistance than 8 YSZ and Zr₆Ta₂O₁₇ single-layer TBCs. The thermal shock performance and failure mechanism of TBCs in the thermal shock test are investigated and discussed in detail.