Solid solution synthesis of tantalum carbide‐hafnium carbide by spark plasma sintering

Solid solutions of Tantalum carbide (TaC) and Hafnium carbide (HfC) were synthesized by spark plasma sintering. Five different compositions (pure HfC, HfC‐20 vol% TaC, HfC‐ 50 vol% TaC, HfC‐ 80 vol% TaC, and pure TaC) were sintered at 1850°C, 60 MPa pressure and a holding time of 10 min without any sintering aids. Near‐full density was achieved for all samples, especially in the HfC‐contained samples. The porosity in pure TaC samples was caused by the oxygen contamination (Ta₂O₅) on the starting powder surface. The addition of HfC increased the overall densification by transferring the oxygen contamination from TaC surface and forming ultrafine HfO₂ and Hf‐O‐C grains. With the increasing HfC concentration, the overall grain size was reduced by 50% from HfC‐ 80 vol% TaC to HfC‐20 vol% TaC sample. The solid solution formation required extra energy, which restricted the grain growth. The lattice parameters for the solid solution samples were obtained using X‐ray diffraction which had an excellent match with the theoretical values computed using Vegard's Law. The mechanical properties of the solid solution samples outperformed the pure TaC and HfC carbides samples due to the increased densification and smaller grain size.     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Influence of hafnium carbide on vacuum plasma spray processed tantalum carbide microstructures

Five specimens of (TaC)_1?x(HfC)_x, where x is 0.0, 0.3, 3.0, 16.5, and 19.8 at.%, were fabricated by vacuum plasma spraying. As HfC content increased, the grain size was reduced and the volume fraction of TaC, Ta₂C and Ta₄C₃ changed, with the TaC phase being more dominate. Reduced grain sizes also lead to an increase in Knoop hardness values. The reduction of grain size with increasing HfC content has been explained by the system being driven further into a compositionally lower melting temperature phase field. This increase in liquid fraction caused greater under-cooling and the formation of more nucleation sites that lead to a finer grain size. The changing volume fraction of (TaC)_1?x(HfC)_x and sub-stoichiometric tantalum carbide phases has been contributed to the loss of carbon intrinsic to vacuum plasma spray processing.     

Mechanical properties of reactively processed W/Ta2C-based composites

The mechanical properties of reactively processed W/Ta₂C cermet composites were studied. Dense W/Ta₂C cermets with a nominal composition of 40.4 vol% W(Ta)ss, 48.9 vol% Ta₂C and 10.6 vol% Ta₂WO₈ were fabricated using a pressureless reactive processing method. Four-point bend strength, fracture toughness, elastic modulus, and microhardness were measured at room temperature. The average flexure strength was 584 MPa which is lower than for pure W; however the strength of pure Ta₂C is unknown. The fracture toughness was 8.3 MPa m^1/2 which fits a rule of mixtures between literature values for the fracture toughness of the W and Ta₂C phases. The elastic modulus was 476 GPa, and the microhardness was 13.4 GPa. Both Young's modulus and hardness were higher than values reported for other W-based cermets.     

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.     

Preparation of silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction and liquid phase sintering

Dense silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction method and liquid phase sintering was reported. Ammonium peroxydisulfate ((NH₄)₂S₂O₈) and ammonium carbonate ((NH₄)₂CO₃) were used as acid and base solutions to treat the silicon carbide powder, respectively. Influence of silicon carbide powder with chemical treatment on the preparation of silicon carbide suspension was studied. It was indicated that 50 vol% and 52 vol% silicon carbide suspensions with viscosities of 0.71 Pa s and 0.80 Pa s could be prepared using acid and base treated powders. Influence of silicon carbide powder with chemical treatment on the coagulation process and properties of green bodies and sintered ceramics were studied. It was indicated that silicon carbide green bodies with compressive strength of 1.13 MPa could be prepared using base treated powder. Dense silicon carbide ceramics with relative density above 99.3% and flexural strength of 697 ± 30 MPa had been prepared by DCC via dispersant reaction and liquid phase sintering using Al₂O₃ and Y₂O₃ as additives at 1950 °C for 2 h.     

Low-temperature synthesis and oxidation resistance of random combination of Hf,Nb, and Ta carbides microcuboids

Based on the dissolution-precipitation mechanism, this article successfully synthesized binary and ternary transition metal carbide microcuboids with random combinations of Hf, Nb, and Ta by annealing monocarbides/cobalt powders. Accelerated mass transport rate through the flow of molten alloys (Co-Hf-Nb-Ta) instead of slow solid diffusion made the low-temperature pressureless sintering technique (1500°C) a reality. Furthermore, the equilibrium morphology was driven by the gradient Gibbs potential of carbides induced by the different local curvature of powders and anisotropic interfacial energy. (Hf_0.5Ta_0.5)C possessed the optimal oxidation resistance among all mentioned carbides, even competed with (Hf_1/3Nb_1/3Ta_1/3)C. During the isothermal oxidation at 800∼1200°C, the doping of Nb and Ta in carbides assisted the monoclinic-orthorhombic HfO₂ transition at ambient pressure, besides, TaC can also restrain the orthorhombic-monoclinic transition of Nb₂O₅. Moreover, oxidation kinetics parameters concluded that the addition of HfC and TaC contributed to the decreasing reaction order and the increasing activation energy, respectively.     

Microstructure refinement and mechanical properties improvement of HfB2–SiC composites with the incorporation of HfC

HfB₂ and HfB₂–10 vol% HfC fine powders were synthesized by carbo/boronthermal reduction of HfO₂, which showed high sinterability. Using the as-synthesized powders and commercially available SiC as starting powders, nearly full dense HfB₂–20 vol% SiC (HS) and HfB₂–8 vol% HfC–20 vol% SiC (HHS) ceramics were obtained by hot pressing at 2000 °C/30 MPa. With the incorporation of HfC, the grain size of HHS was much finer than HS. As well, the fracture toughness and bending strength of HHS (5.09 MPa m^1/2, 863 MPa) increased significantly compared with HS (3.95 MPa m^1/2, 654 MPa). Therefore, it could be concluded that the incorporation of HfC refined the microstructure and improved the mechanical properties of HfB₂–SiC ceramics.     

Oxidation/ablation behaviors of hafnium carbide-silicon carbonitride systems at 1500 and 2500 C

The oxide scales of hafnium carbide (HfC) typically exhibit a porous structure after oxidation/ablation due to the release of gas oxidation products, which allows oxygen penetration to promote the rapid oxidation of the HfC matrices. Here, we report that the oxidation/ablation resistance of HfC was enhanced by the incorporation of amorphous silicon carbonitride (SiCN). HfC-SiCN ceramics with 10 vol % SiCN showed a significant improvement in the oxidation/ablation resistance compared with pure HfC. The HfC-10 vol % SiCN ceramic has a higher density with good mechanical properties. After being oxidized at 1500 °C for 2 h, a dense and homogeneous HfO₂-HfSiO₄ layer with low oxygen permeability is formed. The ablation resistance of the HfC-10 vol % SiCN ceramic is improved due to the formation of the triple-layer structure oxide with good thermal stability and mechanical scouring resistance. After ablation under an oxyacetylene flame for 60 s, the mass and linear ablation rates of HfC-10 vol % SiCN ceramic are −0.019 mg cm⁻² s⁻¹ and -0.156 μm s⁻¹, respectively.     

Synthesis of ultra-fine hafnium carbide powders combining the methods of liquid precursor conversion and plasma activated sintering

Ultra-fine hafnium carbide (HfC) powders were synthesized using a novel method combining liquid precursor conversion and plasma activated sintering (PAS). Solution-based processing was used to achieve a fine-scale mixing of the reactants, and further treatment by PAS allowed fast formation of HfC. We investigated the effect of the type of acid used during the liquid precursor conversion on the synthesized powders, where mixtures were prepared using salicylic acid, citric acid, or a combination of these. The results show that pure HfC powders (with an average particle sizes of 350 nm) were obtained at a relatively low temperature (1550 °C) using a HfOCl₂·8H₂O precursor with the mixed acids. The oxygen content of the synthesized powders was only 0.97 wt%. The type of acid had a significant effect on the synthesis product. When using only citric acid, the temperature required to produce pure hafnium carbide increased to 1700 °C. In the case of a salicylic acid precursor, pure HfC was not obtained, even at a synthesis temperature of 1700 °C.     

UHTC–carbon fibre composites: Preparation,oxyacetylene torch testing and characterisation

Current generation carbon–carbon (C–C) and carbon–silicon carbide (C–SiC) materials are limited to service temperatures below 1800 °C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre–UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB₂, ZrB₂–20 vol% SiC, ZrB₂–20 vol% SiC–10 vol% LaB₆, HfB₂ and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 °C and the results are compared with that of a C–C benchmark composite.     

Spark plasma sintering of TaC–HfC UHTC via disilicides sintering aids

Ta_0.8Hf_0.2C ceramic has the highest melting point among the known materials (4000 °C). Spark plasma sintering is a new route for consolidation of materials, specially ultra high temperature ceramics (UHTCs), which are difficult to be sintered at temperatures lower than 2000 °C.The purpose of this study is to consolidate Ta_0.8Hf_0.2C by spark plasma sintering at low temperature using MoSi₂ and TaSi₂ as sintering aid. In this regard, effect of different amounts of sintering aids and carbides ratio on densification behavior and mechanical properties of Ta_1?xHf_xC were investigated.Fully consolidation of Ta_0.8Hf_0.2C was achieved in presence of 12 vol.% sintering aid after sintering at 1650 °C for 5 min under 30 MPa. The first stage of sintering was due to plastic deformation of sintering aids particles and consequent rearrangement. The second stage was occurred via Ta_1?xHf_xC solid solution and liquid phase formation.     

Spark plasma sintering and characterization of ZrC-TiB2 composites

ZrC-based composites were consolidated from ZrC and TiB₂ powders by the Spark Plasma Sintering (SPS) technique at 1685 °C and 1700 °C for 300 s under 40-50-60 MPa. Densification, crystalline phases, microstructure, mechanical properties and oxidation behavior of the composites were investigated. The sintered bodies were composed of a (Zr,Ti)C solid solution and a ZrB phase. The densification behaviors of the composites were improved by increasing the TiB₂ content and applied pressure. The highest value of hardness, 21.64 GPa, was attained with the addition of 30 vol% TiB₂. Oxidation tests were performed at 900 °C for 2 h and the formation of ZrO₂, TiO₂ and B₂O₃ phases were identified by using XRD.     

Nanosized Hf6Ta2O17 particles reinforced HfC ceramic coating for high temperature applications

There is an urgent need for the thermal protection of carbon/carbon composites to increase their service span in aerospace field. As members of ultra-high temperature ceramics, HfC and ZrC are thought to be alternative materials, but their poor oxidation behavior and fracture toughness at temperature over 2000 ℃ prevent them from taking full advantages. Herein, we synthesized nanosized Hf₆Ta₂O₁₇ powders and introduced them into HfC ceramic to tackle the above problems. Plasma-sprayed HfC coatings with Hf₆Ta₂O₁₇ varied from 0 to 15 mol.% (0, 1.25, 2.5, 5, 15) were exposed to an oxyacetylene torch with a heat flux of 2.38 MW/m². The one with 2.5 mol.% Hf₆Ta₂O₁₇ showed the best ablation resistance. Smaller doping was unable to effectively hinder oxygen diffusion given the inadequate compactness of the formed scale, conversely substantial humps and ruptures formed on the samples with higher amounts, acting as straightforward paths for oxygen.     

Low-temperature synthesis of tantalum carbide by facile one-pot reaction

Tantalum carbide (TaC) was synthesized by polycondensation and carbothermal reduction reactions from an inorganic hybrid. Tantalum pentachloride (TaCl₅) and phenolic resin were used as the sources of tantalum and carbon, respectively. FTIR of as-synthesized dried complexes revealed formation of Ta-O. Pyrolysis of the complexes at 800 °C/1 h under argon resulted in tantalum oxide which after heat treatment at 1000–1200 °C transformed to tantalum carbide. The mean crystallite size of the precursor-derived TaC ceramics was less than 40 nm and Ta and C elements were homogeneously distributed in the ceramic samples. Mechanism for formation of TaC ceramic was analyzed.     

Oxidation of ZrB2–SiC ultra-high temperature composites over a wide range of SiC content

The oxidation performance of ZrB₂–SiC ultra-high temperature ceramics with SiC content ranging from 20 to 80 vol% has been evaluated at 1773 K for 50 h and at 2073 K for 20 min. Oxidation reaction pathways were interpreted using volatility diagrams of the ZrB₂–SiC system. At 1773 K for 50 h, all ZrB₂–SiC composites from 20 to 80 vol% SiC formed a protective SiO₂ surface coating. Samples with ≤50 vol% SiC developed a distinguishable SiC-depleted layer at 1773 K and 2073 K. High temperature torch testing for 20 min at approximately 2073 K revealed that samples with ≥65 vol% SiC exhibit a depression under the torch flame. Samples rich in ZrB₂ were dominated by a ZrO₂ layer after a similar exposure. The overall weight density of ultra-high temperature ceramics can be reduced with improved oxidation performance at 1773 K by adding at least 65 vol% SiC.     

High temperature oxidation of Zr- and Hf-carbides: Influence of matrix and sintering additive

Ultra-high temperature ceramics having melting points above 3500 K and high thermal conductivities are envisaged as future receivers of concentrating solar power plants. The high pressure and solar temperature reactor (Réacteur Hautes Pression et Température Solaire, REHPTS) implemented at the focus of the Odeillo 5 kW solar furnace was used to investigate the oxidation of three refractory carbides containing different sintering additives (HfC/MoSi₂, ZrC/MoSi₂, ZrC/TaSi₂) that could be considered as promising candidates. The concentration of the additive, TaSi₂ or MoSi₂, was 20 vol%. Each kind of sample was oxidized in air for 20 min at 1800, 2000 and 2200 K. Experiments were filmed using a video camera and the gaseous phases were analyzed in situ by mass spectrometry. Various post-test characterizations have shown that the nature of the carbide and additive strongly affects the composition of the oxide layer and therefore the high-temperature behaviour.     

Temperature and frequency dependence of dielectric loss of Ba(Mg1/3Ta2/3)O3 microwave ceramics

Ba(Mg_1/3Ta_2/3)O₃ ceramic possessing extremely high Q × f value of more than 300 THz at microwave frequency was developed in our previous study. It is of great interest to understand the mechanism of microwave absorption in such a practical material. In the present study we report on the temperature dependence of the dielectric loss in the Ba(Mg_1/3Ta_2/3)O₃. The mechanism of the microwave absorption is discussed using two phonons difference process. The samples were prepared by conventional solid state reaction and sintered at 1893 K in oxygen atmosphere. Dielectric properties in the microwave range were measured by Hakki & Colemann and resonant cavity methods in the temperature range of 20–300 K. Whispering gallery mode technique was used for the measurement of the dielectric properties at the millimeter wave frequency. Dielectric loss of the Ba(Mg_1/3Ta_2/3)O₃ at the microwave frequency increases with temperature between 200 and 300 K in general agreement with the theory of intrinsic dielectric loss derived from the two phonon difference process. However below 200 K, the dielectric loss has shown a distinctive behavior with a loss peak at 40 K. It was inferred that the loss peak of the Ba(Mg_1/3Ta_2/3)O₃ was caused by the local orientation polarization having dispersion at the microwave frequency.     

Synthesis of a Fine (Ta0.8,Hf0.2)C Powder from Carbide or Oxide Powder Mixtures

Tantalum hafnium carbide ((Ta_0.8,Hf_0.2)C) powders were successfully synthesized using a modified spark plasma sintering (SPS) apparatus with TaC/HfC or Ta₂O₅/HfO₂/C starting materials. The (Ta_0.8,Hf_0.2)C obtained from the carbides had a finer particle size of 220 nm, whereas those obtained from the oxides had less contamination during the milling process (0.35 wt%) than the other case. Particle coarsening of the solid‐solution phase was effectively suppressed by using a modified SPS apparatus because of the fast heating/cooling rate. High‐energy ball milling promoted a solid‐solution reaction for the formation of (Ta_0.8,Hf_0.2)C by refining the size and inducing the homogeneous mixing of the starting materials. By the combination of the fast heating and high‐energy ball milling, fine tantalum hafnium carbide powders with low contamination were successfully synthesized.     

Oxidation and self-healing behaviour of spark plasma sintered Ta2AlC

Self-healing and oxidation of spark plasma sintered Ta₂AlC was investigated using a newly developed wedge loaded compact specimen to determine strength recovery in a single specimen. Previous work had predicted dominant Al oxidation leading to dense and strong reaction products to result in favourable healing properties. However, crack-gap filling and strength recovery of Ta₂AlC were not achieved by oxidation at 600 °C. Oxidation below 900 °C in synthetic and atmospheric air resulted in porous Ta-oxides, with no Al₂O₃ formation. DTA up to 1200 °C revealed a two-step reaction process with the final products Ta₂O₅ and TaAlO₄. The study shows that the kinetics may overrule the self-healing MAX-phase design criteria based on thermodynamics.