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.     

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.     

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.     

A Functionally Graded Carbide in the Ta–C System

Hard materials used in such abrasive wear applications as cutting tools and wear inserts in drilling tools require high hardness to resist wear, high fracture toughness to withstand mechanical and thermal shock, and high chemical and thermal stability. Such a combination of properties is difficult to achieve in single‐phase materials. Functional grading is an approach that overcomes this limitation by designing and processing a graded microstructure that provides high hardness and chemical resistance at the surface with a tough interior or bulk. While functional grading is a widely used practice in the cemented carbides industry, it has not been demonstrated with “pure” carbides. This article reports the feasibility of designing and processing a graded carbide in the Ta–C binary system. It is shown that a simple carburization treatment of the high‐toughness carbide, ζ‐Ta₄C_3?x, can lead to the formation of the hard carbide phase, γ‐TaC_y, on the surface. The thickness, microstructure (grain size), and composition (C/Ta atomic ratio, y) of the γ‐TaC_y layer can be optimized to obtain both high hardness and high strength for the graded material.     

Effect of TiC addition on the microstructure and properties of Ni3Ta–TaC reinforced Ni-based wear-resistant coating

The present study focuses on the surface wear-resistant strengthening technology of the tunnel boring machine disc cutter ring. Ni₃Ta–TaC reinforced Ni-based wear-resistant coatings were synthesized in-situ on the surface of 5Cr5MoSiV steel by laser cladding with pure Ni spherical powder, pure Ta spherical powder and Ni coated graphite. To inhibit coating cracking, TiC powder was added to promote the in-situ formation of TaC. The effect of adding TiC on the microstructure and properties of Ni-based wear-resistant coating has been investigated by experiments and first-principles calculation. The results show that Ni₃Ta and TaC particles are synthesized in-situ in the coating, and small TaC particles are aggregated around TiC. The orientation relationship of TaC (100)//TiC (100) is confirmed by EBSD and TEM. The properties of interface of TaC (100) and TiC (100) are calculated by the first-principles, showing that the C–Ti and Ta–C interface has high adhesion and good stability. The wear resistance of the two coatings is 4 times higher than that of the substrate. The addition of TiC can effectively inhibit the formation of the lath-shaped Ni₃Ta intermetallic compound and cracks, resulting in excellent wear resistance and toughness.     

ζ‐Ta4C3−x: A High Fracture Toughness Carbide with Rising‐Crack‐Growth‐Resistance (R‐Curve) Behavior

The microstructures and mechanical properties of tantalum carbides containing predominantly the ζ‐Ta₄C_3?x phase are compared with the properties of the monocarbide (γ‐TaC) and the hemicarbide (α‐Ta₂C) and two‐phase composites. It is shown that a Ta and γ‐TaC powder mixture corresponding to a C/Ta at. ratio of 0.66 can be hot‐pressed (1800°C, 2 h) to obtain ~95 wt% of ζ‐Ta₄C_3?x with a density of 98% of theoretical. This material has an attractive combination of high fracture toughness (13.8 ± 0.2 MPa√m) and fracture strength (759 ± 24 MPa) with modest hardness (5.6 ± 0.5 GPa). The fracture toughness and strength measured for this material were the highest among all the materials with C/Ta ratio ranging from 0.5 (hemicarbide) to 1.0 (monocarbide). It is also shown that a material containing 86 wt% ζ‐Ta₄C_3?x can be consolidated by pressureless sintering of a hydrogenated Ta and γ‐TaC powder mixture without significant drop in density (97% of theoretical) or mechanical properties (13.4 ± 0.2 MPa√m, 700 ± 20 MPa, 6.0 ± 0.4 GPa). Materials containing high weight fraction of the ζ‐Ta₄C_3?x phase exhibited rising crack‐growth‐resistance (R‐curve) behavior. Optical and scanning electron microscope observations suggested crack‐face bridging was the dominant toughening mechanism. The crack‐bridging ligaments were lamellae of the basal planes of the ζ‐Ta₄C_3?x phase produced by their easy cleavage. The thickness of the lamellae ranged from 40 to 2000 nm, significantly less than the grain size.     

Oxidation behavior of high-entropy carbide (Hf0.2Ta0.2Zr0.2Ti0.2Nb0.2)C at 1400–1600 °C

Oxidation behavior of high-entropy carbide (Hf_0.2Ta_0.2Zr_0.2Ti_0.2Nb_0.2)C (HTZTNC) was investigated over temperature range of 1400–1600 °C. Results showed improved oxidation resistance of high-entropy carbide compared with individual carbide ceramics. In oxide layer, Ta₂O₅ and Nb₂O₅ were found to be dominant phases at 1400 °C, whereas ZrTiO₄ and HfTiO₄ were main phases obtained at 1500 and 1600 °C. Moreover, these complex dense oxide layer structures on the surface of HTZTNC at high temperature led to excellent oxidation resistance. The observation of Ti-depleted layer at 1500 and 1600 °C after 20 min of oxidation indicated that oxidation mechanism involved outward diffusion of titanium oxide, which was further confirmed by reoxidation experiments. In sum, these findings are promising for future development of high-entropy ultrahigh temperature ceramics with good oxidation resistance.     

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.     

Processing of Dense ζ‐Ta4C3−x by Reaction Sintering of Ta and TaC Powder Mixture

Tantalum carbides are commonly processed by hot pressing, canned hot‐isostatic‐pressing, or spark plasma sintering because of their high melting temperatures and low diffusivities. This paper reports processing of dense ζ‐Ta₄C_3?x by reaction sintering of a Ta and TaC powder mixture (C/Ta atomic ratio = 0.66). ζ‐Ta₄C_3?x is of interest due to its rhombohedral (trigonal) crystal structure that may be characterized as a polytype with both face‐centered‐cubic and hexagonal‐close‐packed Ta stacking sequences interrupted by stacking faults and missing carbon layers. This structure leads to easy cleaving on the basal planes and high fracture toughness. A key step in processing is the hydrogenation of the Ta powder to produce β‐TaH_x, a hard and brittle phase that enables efficient comminution during milling and production of small, equiaxed Ta particles that can be packed to high green density with the TaC powder. Studies of phase evolution by quantitative X‐ray diffraction during sintering revealed several intermediate reactions: (1) decomposition of β‐TaH_x to Ta; (2) diffusion of C from γ‐TaC to Ta leading to the formation of α‐Ta₂C_y' with the kinetics described by the Avrami equation with an exponent, n = 0.5, and an activation energy of 219 kJ/mole; (3) equilibration of α‐Ta₂C_y' and γ‐TaC_0.78 phases; and (4) formation of ζ‐Ta₄C_2.56 from the equilibrated α‐Ta₂C and γ‐TaC_0.78 phases with the kinetics characterized by a higher Avrami exponent (n ≈ 3) and higher activation energy (1007 kJ/mole). The sintered material contained ~0.86 weight fraction ζ‐Ta₄C_2.56 and ~0.14 weight fraction γ‐TaC_0.78 phases. The microstructure showed evidence of nucleation and growth of the ζ‐Ta₄C_2.56 phase in both the α‐Ta₂C and γ‐TaC_0.78 parent phases with distinct difference in the morphology due to the different number of variants of the habit plane.     

Variations in Tantalum Carbide Microstructures with Changing Carbon Content

A wide variety of microstructures have been obtained by vacuum plasma spraying (VPS) 39Ta:61C atomic percent feedstock powders. During processing, the powder feed was fed through a high energy VPS plasma plume, where altering nozzle angle changed the overall retained carbon concentration in the deposited material. The samples were subsequently sintered and hot isostatic pressed to homogenize and consolidate the microstructure. The microstructures consisted of grains that were either equiaxed or acicular. In the samples with less carbon loss, the equiaxed grains were either the TaC phase or a TaC matrix that encased fine laths of Ta₄C₃. In the sample with the most carbon loss, acicular grains were found containing layered and parallel TaC, Ta₂C, and Ta₄C₃ laths along the major‐axis of the grains. The phases of the compounds have been determined by using complimentary X‐ray diffraction and electron diffraction techniques. Focused ion beam serial sectioning and transmission electron microscopy tilt series tomography were performed to generate three‐dimensional reconstructions of the microstructure morphologies. This article addresses how tantalum carbide microstructures are controlled by the overall concentration and phase fraction content in each of these samples.     

Oxidation behavior of graphene nanoplatelet reinforced tantalum carbide composites in high temperature plasma flow

Graphene nanoplatelets (GNP) reinforced tantalum carbide (TaC) composites are exposed to a high temperature plasma flow in order to evaluate the effects of GNP on the oxidation behavior of TaC at conditions approaching those of hypersonic flight environments. The addition of GNP is found to suppress the formation of the oxide layer by up to 60%. The high thermal conductivity of GNPs dissipates heat throughout the sample thereby reducing thermal gradients and reducing the intensity of heating at the surface exposed to plasma. In addition, GNPs enhance oxidation resistance by providing toughening which suppresses crack formation and bursting that accelerates oxidation. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) reveal that GNPs have the ability to survive the intense high temperature of the plasma. GNPs are believed to seal oxide grain boundaries and hinder the further influx of oxygen. GNPs also provide nano sized carbon needed to induce the localized reduction of Ta₂O₅ to TaC. Micro computed X-ray tomography (MicroCT) validates that the above mechanisms protect the underlying unoxidized material from the structural damage caused by thermal shocks and high shear forces, by reducing thermal gradients and providing toughness.     

High-temperature oxidation behavior of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics in air

High-entropy metal carbides have recently been arousing considerable interest. Nevertheless, their high-temperature oxidation behavior is rarely studied. Herein the high-temperature oxidation behavior of (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy metal carbide (HEC-1) was investigated at 1573-1773 K in air for 120 minutes. The results showed that HEC-1 had good oxidation resistance and its oxidation obeyed a parabolic law at 1573-1673 K, while HEC-1 was completely oxidized after isothermal oxidation at 1773 K for 60 minutes and thereby its oxidation followed a parabolic-linear law at 1773 K. An interesting triple-layered structure was observed within the formed oxide layer at 1673 K, which was attributed to the inward diffusion of O₂ and the outward diffusion of Ti element and CO or CO₂ gaseous products.     

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.     

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.     

Synthesis of ultra-fine tantalum carbide powders by a combinational method of sol–gel and spark plasma sintering

Tantalum carbide (TaC) nanopowders were synthesized by a novel method combining the sol–gel and spark plasma sintering (SPS) processes using tantalum pentachloride (TaCl₅) and phenolic resin as the sources of tantalum (Ta) and carbon (C), respectively. Gels of Ta-containing chelate with good uniformity and high stability were prepared by solution-based processing. The products with the structure of carbon-coated tantalum pentoxide (Ta₂O₅) were obtained after pyrolysis at 800?°C. Further heat treatment by SPS resulted in the fast formation of TaC at a relatively low temperature. The effects of the C/Ta molar ratio in the raw materials and the heat treatment temperature on the prepared powders were investigated. With increase in the C/Ta molar ratio from 3.75 to 4.25, the synthesis temperature, oxygen content and average crystallite size of the TaC powders decreased. Furthermore, the oxygen content of the powders prepared at the C/Ta molar ratio of 4.25 could be reduce by increasing the heat treatment temperature from 1400° to 1600°C, which unfortunately also induced a mean crystallite size increase from 30 to 100?nm. The TaC powders obtained at a comparatively low C/Ta molar ratios of 4.25 at 1500?°C had an average particle size of about 50?nm and a low oxygen content of about 0.43?wt%.     

Characteristics,wear and corrosion properties of borided pure titanium by pack boriding near α → β phase transition temperature

The boride layer characteristics, wear and corrosion properties of borided commercially pure titanium by pack boriding near the α → β phase transition temperature were investigated using X-ray diffraction, scanning electron microscopy, electron probe microanalysis, dry reciprocating friction tests, and electrochemical experiments in this work. The pack boriding was carried out at the temperatures of 860 °C, 880 °C, 900 °C, and 920 °C for 5, 10, 15 and 20 h. The results indicated that, in both α and β phase temperatures, the boride layer is composed of the outer TiB₂ layer and the inner TiB layer. The α→β phase transition temperature of commercially pure Ti in this work is in the range of 882–900 °C, and the growth of TiB layer can be enhanced at this temperature range. Commercially pure Ti borided at 920 °C for 20 h has the best wear resistance and corrosion resistance. Finally, wear and corrosion mechanisms were also discussed.     

Comprehensive properties and thermal stability of Ta2O5-doped PIN-PHT ceramics

Herein, high-performance 0.11 Pb(In_1/2Nb_1/2)O₃-0.89 Pb(Hf_0.47Ti_0.53)O₃-0.8Ta₂O₅ (PIN-PHT-0.8Ta) ceramics are successfully synthesized. In addition, performance improvement is comprehensively analyzed from viewpoints of microstructure, phase structure and electrical properties. Experimental results reveal that the addition of Ta₂O₅ changes phase structure of PIN-PHT ceramics from ferroelectric tetragonal phase to rhombohedral phase. This leads to the appearance of morphotropic phase boundaries (MPBs). At the same time, the addition of Ta₂O₅ reduces grain size and enhances grain uniformity. Also, Ta₂O₅ doping improves internal and external contribution of piezoelectric response, which greatly improves dielectric, piezoelectric and ferroelectric properties of PIN-PHT. Key performance parameters include d₃₃, k_p, T_C, ε_r and tanδ, which are found to be 630 pC/N, 0.73, 322.6 °C, 1917 and 1.55%, respectively. In particular, thermal stability of PIN-PHT-0.8Ta ceramics is found to be higher than PZT-based ceramics, as well as d₃₃ value and performance retention rate of PIN-PHT-0.8Ta are found to be 560 pC/N and 89% at 300 °C, respectively, which are far superior to commercial PZT-5 and PZT-8 ceramics. These properties indicate potential of PIN-PHT-0.8Ta ceramics in high-temperature applications.     

Effect of pulse electromagnetic coupling treatment on thermal conductivity of WC-8Co cemented carbide

In this study, investigations were conducted focusing on the WC-8Co cemented carbide. A dry turning test of TC4 titanium alloy with WC-8Co cemented carbide tool treated by pulsed electromagnetic coupling treatment (PEMCT) was conducted. Tool wear was observed and analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS). The thermal conductivity of WC-8Co cemented carbide before and after the PEMCT was measured using Hotdisk thermal conductivity analyzer. The finite element software, DEFORM, was used to simulate the cutting process, and the stable cutting temperature range was obtained. The high-temperature oxidation test was conducted in a muffle furnace to study the effect of the PEMCT on the oxidation resistance of WC-8Co cemented carbide. This study obtained the effect of the PEMCT on the thermal conductivity of tungsten cobalt cemented carbide. The results show that the PEMCT can reduce the adhesive, diffusion, and oxidation wear of the tools, thus, improving the wear resistance and service life of the tools. The PEMCT improves the thermal conductivity and diffusivity of WC-8Co cemented carbide. Moreover, the oxidation resistance of WC-8Co cemented carbide in high-temperature conditions can be improved.     

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.     

Enhancing cutting performance of Ti(C,N)-based cermet tools on nodular cast iron by incorporating high-entropy carbide

In this study, we investigated the cutting performance and wear mechanisms of Ti(C,N)-based cutting tools containing varying weight percentages (0%, 5%, 10%, and 15%) of high-entropy carbide (HEC) (V_0.2Nb_0.2 Mo_0.2Ta_0.2W_0.2)C phase, when used for turning nodular cast iron. According to the turning test results, the cermet cutting tools containing 0 wt%, 5 wt%, 10 wt%, and 15 wt% HEC phases demonstrated effective cutting lives of 402, 720, 632, and 465 s, respectively. The tool with 5 wt% HEC phase showed the best cutting performance. When cutting nodular cast iron with cermet cutting tools, the main wear mechanisms observed were diffusion, oxidation, adhesion, and abrasion on the flank surface, along with diffusion, oxidation, and abrasion on the rake surface. The results of this study indicated that (V_0.2Nb_0.2Mo_0.2Ta_0.2W_0.2)C could be adopted as an effective reinforced phase in the cermet cutting tools.