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.     

Consolidation and characterization of highly dense single-phase Ta–Hf–C solid solution ceramics

Herein, Ta–Hf–C solid solution ceramics were consolidated from nano-scale Ta–Hf–C solid solution powders for the first time. Four different compositions (4TaC–1HfC, 1TaC–1HfC, 1TaC–3HfC, and 1TaC–4HfC) were prepared by hot-pressed sintering at 2100°C, 70 MPa pressure and a holding time of 30 minutes. The densification, formation of single-phase solid solution and mechanical properties of the samples were systemically investigated. Relative density >95% was achieved for all four compositions with some improvement when TaC content was increased. And the formation of single-phase Ta–Hf–C solid solution was strongly demonstrated by phase analysis and crystal measurement using XRD and TEM. A significant improvement of hardness up to ~30 GPa was achieved, which was much higher than that of pure TaC (18.9 GPa) and HfC (22.1 GPa), due to the high densification and solid solution strengthening mechanism.     

Controlling phase separation of TaxHf1−xC solid solution nanopowders during carbothermal reduction synthesis

Synthesis of single‐phase tantalum hafnium carbide (Ta_xHf_1?xC, 0<x<1) solid solution nanopowders via carbothermal reduction (CTR) reaction is complicated due to the difference in reactivity of parent oxides with carbon and presence of a miscibility gap in TaC‐HfC phase diagram below ~887°C. These can lead to phase separation, ie, formation of two distinct carbides instead of a single‐phase solid solution. In this study, nanocrystalline Ta_xHf_1?xC powders were synthesized via CTR of finely mixed amorphous tantalum‐hafnium oxide(s) and carbon obtained from a low‐cost aqueous solution processing of tantalum pentachloride, hafnium tetrachloride, and sucrose. Particular emphasis was given to investigate the influences of starting compositions and processing conditions on phase separation during the formation of carbide phase(s). It was found that due to the immiscibility of Ta‐Hf oxides and relatively fast CTR reaction, individual nano‐HfC and TaC phases form quickly (within minutes at 1600°C), then go through interdiffusion forming carbide solid solution phase. Moreover, the presence of excess carbon in the CTR product slows down the interdiffusion of Ta and Hf dramatically and delays the solid solution formation, whereas DC electrical field (applied through the use of a spark plasma sintering system) accelerates interdiffusion significantly but leads to more grain growth.     

Low‐Temperature Sintering of HfC/SiC Nanocomposites Using HfSi2‐C Additives

HfC/SiC nanocomposites were fabricated via the reactive spark plasma sintering (R‐SPS) of a nano‐HfC powder and HfSi₂‐C sintering additives. The densification temperature decreased to 1750°C as the additive content increased. XRD analysis indicated the formation of pure HfC–(19.3–33.8 vol%) SiC within the sintered composites without residual silicide or oxide phases or secondary nonoxide phases. Ultrafine and homogeneously distributed HfC (470 nm) and SiC (300 nm) grains were obtained in the dense composites using nano‐HfC powder through the high‐energy ball‐milling of the raw powders and R‐SPS. Grain growth was further suppressed by the low‐temperature sintering using R‐SPS. No amorphous phase was identified at the grain boundary. The maximum Vickers hardness, Young's modulus, and fracture toughness values of the HfC/SiC nanocomposites were 22 GPa, 292 GPa, and 2.44 MPa·m^1/2, respectively.     

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.     

Sintering and densification mechanisms of tantalum carbide ceramics

Dense tantalum carbide (TaC) ceramics were prepared using TaC nanopowder via spark plasma sintering (SPS). The effects of the sintering temperature and applied pressure on the densification and grain growth behaviour of TaC ceramics were investigated. The results showed that high temperature and pressure promoted sintering densification, while their increase caused an increase in the grain size of TaC ceramics. A highly dense TaC ceramic (∼97.19%) with a fine grain size of 2.67 μm was obtained by sintering at 1800 °C for 10 min under 80 MPa. The Vickers hardness, Young's modulus and fracture toughness were 15.60 GPa, 512.66 GPa and 3.59 MPa·m^1/2, respectively. The densification kinetics were investigated using a creep deformation model. Diffusion and grain boundary sliding were proven to be the dominant densification mechanisms based on the stress and grain size exponents combined with the microstructural characteristics. The apparent activation energy of the mechanism controlling densification was 252.94 kJ/mol.     

Effect of residual excess carbon on the densification of ultra-fine HfC powder

The residual carbon content of ultra-fine hafnium carbide (HfC) powder was controlled by the optimization of the synthesis process, and the effect of residual carbon on the densification of HfC powder was analyzed. The amount of residual carbon in the HfC powder could be reduced by the de-agglomeration of HfO₂ powder before the carbo-thermal reduction (CTR) process. The average particle size of HfO₂ powder decreased from 230 to 130 nm after the de-agglomeration treatment. Ultra-fine (d₅₀: 110 nm) and highly pure (metal basis purity: >99.9 % except for Zr) HfC powder was obtained after the CTR at 1600 °C for 1 h using the C/Hf mixing ratio of 3.3. In contrast, the C/Hf ratio increased to 3.6 without the de-agglomeration treatment, indicating that a large amount of excess carbon was required for the complete reduction of the agglomerated HfO₂ particles. HfC ceramics with high relative density (>98 %) were obtained after spark plasma sintering at 2000 °C under 80 MPa pressure when using the HfC powder with low excess carbon content. In contrast, the densification did not complete at a higher temperature (2300 °C) and pressure (100 MPa) when the HfC powder contained a large amount of residual carbon. The results clearly indicated that residual carbon suppressed the densification of HfC powder in case the carbide powder had low oxygen content, and the residual carbon content could be controlled by the optimization of the synthesis process. The average grain size and Vickers hardness of the sintered specimen were 6.7(±0.7) μm and 19.6 GPa, respectively.     

Synthesis and properties of (Hf1-xTax)C solid solution carbides

Thermodynamically stable (Hf_1–xTa_x)C (x?=?0.1–0.3) compositions were selected by First Principle Calculation and synthesized in nanopowders via high-energy ball milling and carbothermal reduction of commercial oxides at 1450?°C. The formation of a solid solution during powder synthesis was investigated. The solid solution carbide powders were sintered at 1900?°C by spark plasma sintering without a sintering aid. As a result, the (Hf_1–xTa_x)C solid solution carbides exhibited high densities, excellent hardness and fracture toughness (ρ: 98.7–100.0%, HVN: 19.69–19.98?GPa, K_IC: 5.09–5.15?MPa?m^1/2) compared with previously reported HfC and HfC–TaC solid solution carbides.     

Reaction Sintering of HfC/W Cermets with High Strength and Toughness

Hafnium carbide/tungsten (HfC/W) cermets were prepared by an in situ reaction sintering process, using hafnium oxide (HfO₂) and tungsten carbide (WC) as the raw materials. The reaction path, densification behavior, microstructure development, and mechanical properties of the cermets were comprehensively investigated. It was found that WC decomposed to tungsten semicarbide (W₂C) and tungsten (W) in sequence, and meanwhile HfC was formed by carbothermal reduction between HfO₂ and as‐released carbon from the dissociation of WC. The solid solution formation between HfC and W during sintering was also studied. The obtained cermets (>98% TD) have a Vickers' hardness of 8.16 GPa, a fracture toughness of 14.45 MPa m^1/2, and a high flexural strength of 1211 MPa.     

Microstructure and mechanical properties of TaC ceramics with 1–7.5 mol% Si as sintering aid

Metallic Si as sintering aid was effective in densifying tantalum carbide ceramic (TaC) by spark plasma sintering (SPS) at 1700°C. Full density was reached at 5.0 mol% Si addition (equivalent to 1.088% Si in weight) and above. Enhanced densification of TaC ceramic with Si was associated with decrease in oxygen content from ~0.24 wt% in TaC powder to ~0.03 wt% in consolidated specimen. Rest of the oxygen species was collected at multigrain conjunctions to form SiO₂‐based liquid at high temperatures. Upon cooling, Ta, Si, O, and C dissolving in the liquid precipitated minor phases of TaSi_x and SiC of low concentrations. Microstructure of TaC ceramics was refined by the Si addition, with average grain size decreasing from 11±8 μm at 1.0 mol% Si to 3±2 μm at 7.5 mol% Si addition. Ta solute in SiC and Si solute in TaC were evidenced. TaC ceramic containing 7.5 mol% Si had a relatively good flexural strength and fracture toughness of 646±51 MPa and 5.0 MPa·m^1/2, respectively.     

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.     

Bond characteristics,mechanical properties,and high-temperature thermal conductivity of (Hf1−xTax)C composites

Bond characteristics, mechanical properties, and high-temperature thermal conductivity of ultrahigh-temperature ceramics (UHTCs), hafnium carbide (HfC), tantalum carbide (TaC), and their solid solution composites, were investigated using first-principles calculations. Mulliken analyses revealed that Ta formed stronger covalent bonds with C than did Hf. Bond overlap analyses indicated that the Hf–C bond possessed mixed covalent and ionic bond characteristics, compared with the more covalent character of the Ta–C bond. Consequently, the overall elastic properties were enhanced with increasing number of Ta–C bonds in the composites. The overall metallicity of the composites also increased with increasing TaC content; thus, the mechanical properties did not improve monotonically. Our results indicate that adding a small amount of TaC to HfC or vice versa to produce a composite would create a new UHTC with greatly improved elastic and mechanical properties as well as high-temperature thermal conductivity.     

Synthesis of nonstoichiometric high entropy Nb-added carbides

A novel class of nonstoichiometric high-entropy carbide (HEC_x) materials, namely, Nb/TiC/TaC, Nb/TiC/TaC/VC and Nb/TiC/TaC/VC/WC, were produced by mechanically milled and spark plasma sintering (SPS) from Nb and carbides. XRD, SEM-EDS and S/TEM-EDS were used to characterize the phase constitution, microstructure and compositional distribution of samples, respectively. HEC_x exhibits a single-phase rock-salt crystal structure with a relatively uniform elemental distribution. Among the three different HEC_x materials, Nb/TiC/TaC/VC/WC with an average grain size of 2.15 μm sintered at 1600 °C shows an enhanced fracture toughness of 5.1 ± 0.1 MPa m^1/2 compared with transition metal carbides. The mechanically mixed and low sintering temperature lead to the formation of finer grains. The higher fracture toughness can be attributed to atomic relaxation resulting from carbon vacancies and solid solution strengthening.     

Synthesis,processing and properties of TaC–TaB2–C ceramics

Multi-phase ceramics in the TaC–TaB₂–C system were prepared from TaC and B₄C mixtures by reactive pressureless sintering at 1700–1900 °C. The pressureless densification was promoted by the use of nano-TaC and by the presence of active carbon in the reaction products. The presence of TaB₂ inhibited grain growth of TaC and increased the hardness compared to pure TaC. If a coarse TaC powder was used, the compositions did not densify. In contrast, pure nano-TaC was pressureless sintered at 1800 °C by the addition of 2 wt.% carbon introduced as carbon black or graphite. The introduction of carbon black resulted in fully dense TaC ceramics at temperatures as low as 1500 °C. The grain size of nominally pure TaC ceramics was a strong function of carbon stoichiometry. Enhanced grain size in sub-stoichiometric TaC, compared to stoichiometric TaC, was observed. Additional work is necessary to optimize processing parameters and evaluate the properties of ceramics in the TaC–TaB₂–C system.     

Electrochemical deposition of tantalum carbide coatings in molten LiCl‐KCl‐K2CO3

A method for the preparation of tantalum carbide (TaC) coatings on tantalum by electrochemical reduction in carbonate ions in molten LiCl‐KCl was developed. Carbide coatings were obtained on the tantalum substrate at 900°C with a bias voltage of ?1.8 V versus the graphite counter electrode. The phase composition, morphology and strength of the carbide coating were characterized by XRD, SEM, and XPS analyses, as well as scratch testing. Kinetic mechanism for the formation of TaC coatings and evolution of chemical bonds between the carbide layer and substrate were schematically discussed. The coatings consist of a single phase of TaC with a thickness of approximately 5 μm. Ta₂O₅ and tantalate derivatives in molten salt restrict TaC formation. Electro‐deoxidation of Ta substrate can favorably eliminate tantalum‐involved compounds to produce TaC. TaC coatings improve the surface strength of Ta substrate obviously. The formation of a metal‐carbon solid solution in molten salt determines the existence of excess carbon on Ta substrate. Chemical bonds on the TaC coating were investigated in comparison with those at the interface of the metal‐oxygen‐carbon and carbon film.     

Synthesis,densification, and microstructure of TaC‐TaB2‐SiC ceramics

The usual way to prepare TaC‐TaB₂ ceramics by adding B₄C to TaC leads to formation of residual C, which degrades samples’ mechanical properties. To eliminate the residual C, we suggest incorporating Si together with B₄C into TaC ceramics, resulting in new ultrahigh‐temperature ceramics (TaC‐TaB₂‐SiC). Dense ceramics (>99%) with SiC volume fraction ranging from 15.86% to 41.04% were fabricated by reactive spark plasma sintering at 1900°C for 5 minutes. The formation of SiO₂‐based transient liquid phase was evidenced by the “film” in intermediate products, which can promote densification. The fine‐grained microstructure in final products was found to be associated with the in situ formed SiC, which impeded TaC and TaB₂ grains from coarsening by the pinning effect. Besides, ultrafine TaB₂ grains (~100 nm) produced during the reaction and then rearranged in liquid also contributed to grain refinement. Compared to TaC‐TaB₂(‐C) ceramics prepared from TaC and B₄C, the acquired composites exhibit better mechanical properties, due to their fine‐grained microstructures and the elimination of residual C.     

Influence of solid loading on densification behavior,micromorphology and dielectric properties of Ba0.67Sr0.33TiO3 ceramics fabricated by a novel DCC-HVCI method

Ba_0.67Sr_0.33TiO₃ (BST) ceramics with highly improved dielectric performance were fabricated by a novel direct coagulation casting via high valence counter ions (DCC-HVCI) method. The influence of solid loading on densification behavior, micromorphology, and dielectric performance of the samples was investigated. With the increase of solid loading from 40 to 50 vol%, the maximum densification rate of BST ceramics increased from 0.090 to 0.122 s⁻¹, and the densification temperature decreased from 1424 to 1343 °C, which indicated that high solid loading could promote the densification behavior of samples during sintering. BST ceramics fabricated by the DCC-HVCI method showed uniform grain size and microstructure, which was beneficial for the dielectric properties of BST ceramics. Samples obtained from 45 vol% suspensions possessed the lowest dielectric permittivity (ε_r ≈ 2801), and the dielectric loss (tanδ≈0.0262) was about 1/10 of that of dry-pressed samples (tanδ≈0.301), which could be attributed to the composition homogenization.     

Facile one‐step high‐temperature spray pyrolysis route toward metal carbide nanopowders

Fine ultrahigh‐temperature ceramic (UHTC) powders have found very important applications in many fields. In this work, a facile high‐temperature spray pyrolysis (HTSP) approach is implemented for the synthesis of HfC and TaC UHTC nanopowders starting from organic solvent (e.g., ethanol or 1‐pentanol) solutions of metal precursors (HfCl₄ or TaCl₅). It is proposed that, during HTSP, the precursor solution droplets would continuously undergo rapid drying, thermolysis (i.e., removal of low molecular weight species such as H₂, H₂O, and CO), and finally in situ carbothermal reduction (CTR) process to give rise to metal carbide nanopowders. The as‐obtained materials are shown by SEM as uniform and separated nanoparticles (~90 nm), whereas TEM reveals the carbide (e.g., HfC) nanoparticles are actually even smaller (~10‐20 nm) and embedded in amorphous carbon from excess solvent decomposition. It is found that among different processing parameters, the organic solvent used and the metal precursor concentration could largely influence the formation of metal carbide. In addition, lower HTSP temperatures (≤~1500°C for HfC) only lead to oxide‐carbon mixtures while higher temperatures (≥~1650°C) promote carbide formation. The HTSP method developed in this work is simple, low‐cost and efficient, and could potentially be optimized further for future large‐scale manufacturing of ultrafine UHTC nanopowders.     

Mechanical properties and rapid consolidation of binderless nanostructured tantalum carbide

The rapid sintering of nanostructured TaC hard material was investigated with a focus on the manufacturing potential of high-frequency induction heated sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostructured materials. A dense pure TaC hard material with a relative density of up to 96% was produced with simultaneous application of 80 MPa pressure and induced current within 3 min. The finer the initial TaC powder size, the higher the density and the better mechanical properties. The fracture toughness and hardness values obtained from 10 h milled powder were 5.1 ± 0.3 MPa m^1/2 and 22 GPa, respectively, under 80 MPa pressure and 80% output of total power capacity (15 kW).     

Synthesis of transition metal carbides and high-entropy carbide TiZrNbHfTaC5 in self-shielding DC arc discharge plasma

The paper shows the feasibility of synthesizing micro- and nano-sized particles of binary metal carbides (Me–C) and high-entropy carbide (HEC) TiZrNbHfTaC₅ by vacuum-free electric arc method. The method is based on the effect of self-shielding of the reaction volume from atmospheric oxygen by carbon monoxide CO, which is generated during arcing in air. The synthesis results in a solid solution with a NaCl-type carbide with a cubic lattice, which simultaneously contains atoms of titanium, zirconium, niobium, hafnium, tantalum, and carbon. The lattice parameter of the HEC TiZrNbHfTaC₅ phase is ～4.532 Å that is in line with the known data on this compound. The synthesis product contains micro-sized particle agglomerates of transition metal carbides. The synthesis products also contain nano-sized particles with a shell-core structure, in which the core can consist of metal carbide (TiC, ZrC, NbC, HfC, TaC) or HEC TiZrNbHfTaC₅, and the shell is a graphite phase.