Search Results (15)

HfC, TaC, and HfTaC composite coatings were successfully fabricated on SiC-coated carbon/carbon (C/C) composites using the double glow plasma alloying (DGPA) technique. The microstructure, mechanical properties, and tribological behaviors of the coatings were systematically investigated. The HfTaC coating exhibited a dense and uniform structure with good interfacial integrity and a compositionally graded transition layer, effectively relieving thermal stress. The hardness of HfTaC and HfC coatings (approximately 12 GPa) was higher than that of the TaC coating. Moreover, the higher K value (1.02) and H/E ratio (H/E = 0.09, H³/E² = 0.085 GPa) indicate that the HfTaC coating exhibits good load-bearing capacity and toughness. Under both 5 N and 15 N loads in the reciprocating friction, the HfTaC coating maintained the lowest and most stable friction coefficient (~0.18). Under the 15 N load, it exhibited the smallest specific wear rate. Observation of the wear scars revealed that the HfC and TaC coatings suffered from pore formation and flake-like spallation, while the HfTaC coating retained structural integrity with only minor cracks. In high-temperature ball-on-disc friction tests up to 500 °C, the wear mechanism of the HfTaC coating gradually transitioned from mild abrasive wear to severe oxidative and adhesive wear, yet the HfTaC coating still provided effective protection. These findings demonstrate that the DGPA-fabricated HfTaC coating is a promising candidate for enhancing the wear resistance and service durability of C/C composites. Full article

Using low-cost transition-metal chlorides and furfuryl alcohol as raw materials, the (TiZrHfNbTa)C precursor was prepared, and a three-dimensional braided carbon fiber preform (C/C) coated with pyrolytic carbon (PyC) was used as the reinforcing material. A C/C-(TiZrHfNbTa)C composite was successfully fabricated through the precursor impregnation pyrolysis (PIP) process. Under extreme oxyacetylene ablation conditions (2311 °C/60 s), this composite material demonstrated outstanding ablation resistance, with a mass ablation rate as low as 0.67 mg/s and a linear ablation rate of only 20 μm/s. This excellent performance can be attributed to the dense (HfZr)₆(TaNb)₂O₁₇ oxide layer formed during ablation. This oxide layer not only has an excellent anti-erosion capability but also effectively acts as an oxygen diffusion barrier, thereby significantly suppressing further ablation and oxidation within the matrix. This study provides an innovative strategy for the development of low-cost ultra-high-temperature ceramic precursors and opens up a feasible path for the efficient preparation of C/C-(TiZrHfNbTa)C composites. Full article

The reinforcement of cast Cantor’s-type high-entropy alloys by MC carbides and their effect on the hot oxidation behavior were investigated. Three equimolar CoNiFeMnCr alloys without or with carbon and with either hafnium or tantalum were elaborated. Their as-cast microstructures were specified. Oxidation tests were carried out in air at 1100 °C. Flexural creep tests were performed at 1100 °C at 10 MPa. The carbide-free CoNiFeMnCr alloy was single-phased. The version with Hf and C added and the one with Ta and C added contained interdendritic eutectic script HfC and TaC carbides, respectively. After oxidation for 50 h at 1100 °C, all alloys were covered by a (Cr,Mn)₂O₃ scale with various proportions of Cr and Mn. HfO₂ or CrTaO₄ also formed. Oxidation resulted in a deep depletion in Cr and in Mn in the subsurface. Oxidation is much faster for the three alloys by comparison with chromia-forming alloys. Their bad oxidation behavior is obviously due to Mn and protection by coating is to be considered. The creep deformation of the carbide-free CoNiFeMnCr alloy was very fast. The creep resistance of the two versions reinforced by either HfC or TaC deformed much slower. The addition of these MC carbides led to a deformation rate divided by five to ten times. Now, creep behavior comparisons with commercial alloys are to be conducted. They will be performed soon. Full article

The large-scale application of stainless steel (SS) bipolar plates (BPs) in proton exchange membrane fuel cells (PEMFCs) is mainly limited by insufficient corrosion resistance and electrical conductivity. In this work, Ta-Nb-N coatings were prepared on 316L SS substrates by unbalanced magnetron sputtering to improve corrosion resistance and conductivity. The Ta-Nb-N coatings had a dense structure without obvious defects. In simulated PEMFC cathode environments consisting of 0.5 M H₂SO₄ + 2 ppm HF at 70 ± 0.5 °C, which is harsher than the U.S. Department of Energy specification, the corrosion current density of Ta-Nb-N-coated BPs was reduced to 2.2 × 10⁻² $\mathsf{\mu }$A$·$cm⁻². Ta-Nb-N-coated samples showed better electrical conductivity than 316L SS, which had an excellent interfacial contact resistance of 9.2 mΩ$·$cm². In addition, the Ta-Nb-N-coated samples had a water contact angle of 100.7°, showing good hydrophobicity for water management. These results indicate that Ta-Nb-N coatings could be a promising material for BPs. Full article

Ni-based composite coatings reinforced by high-entropy intermetallic compounds (HEICs) were prepared by detonation spraying (DS) on low alloy steel substrates. To this end, first (Ti-Nb)(V-Cr-Ni-Fe) and Al₃(TiZrNbCrHfTa) HEIC powders were fabricated by arc melting followed by ball milling. The as-milled HEIC powders were then employed as reinforcement particles to prepare Ni-7wt.% HEIC composite coatings. The average particle size of the (Ti-Nb)(V-Cr-Ni-Fe) and Al₃(TiZrNbCrHfTa) HEIC powders were 18 and 35 µm, respectively, while the average particle size of the Ni powder was 56 µm. (Ti-Nb)(V-Cr-Ni-Fe) exhibited a single hexagonal C14 Laves phase in spite of Ti and Nb segregations. The XRD pattern of Al₃(TiZrNbCrHfTa) indicated the presence of a tetragonal D0₂₂-type structure along with some minor CrTi and Cr₅Al₈ phases. The sprayed Ni-7wt.% FeNiCrV-TiNb and Ni-7wt.% Al₃(TiZrNbCrHfTa) composite coatings retained crystal structures of the powder mixtures, suggesting proper thermal stability for both powders. The coatings exhibited a dense microstructure consisting of a lamellar microstructure with low porosity and sound bonding with the substrate. The microhardness of Ni-7wt.% FeNiCrV-TiNb (450 HV) was higher than that of Al₃(TiZrNbCrHfTa) (338 HV), and it exhibited lower fluctuation than that of Ni-7wt.% Al₃(TiZrNbCrHfTa). DS is an effective method to fabricate metal matrix composites reinforced by HEICs with a low level of porosity. Full article

In order to improve the high-temperature oxidation resistance of Ti60 alloy, a Ti-Hf-Mo-Ta-Nb-B composite coating was prepared on Ti60 alloy with Ti, Hf, Mo, Ta and Nb powder and B powder as raw materials using laser cladding. The microstructure and oxidation behavior of the coating before and after oxidation at 1100 °C × 120 h in static air were studied with XRD, SEM, EDS and isothermal oxidation techniques. The results show that the coating was mainly composed of six phases, (Ti_0.2Hf_0.2Mo_0.2Ta_0.2 Nb_0.2)B₂, TiB, HfB₂, Mo_4.00 B_3.40, TiHf and Hf_1.86Mo_0.14. The high-temperature oxidation of the coating and Ti60 alloy followed parabolic law, and the oxidation weight gain rate of the coating after 110 °C × 120 h was only 1/4.8 of that of the Ti60 alloy. The improvement of the high-temperature oxidation resistance of the coating may benefit from high-temperature oxidation resistance (Ti_0.2Hf_0.2 Mo_0.2Ta_0.2Nb_0.2)B₂, HfB₂ and TiB boride ceramic phases. Full article

Ultra-high Temperature Oxidation-Resistant Alloys (UTORAs) have received a lot of attention due to the increased research demand for deep space exploration around the world. However, UTORAs have the disadvantages of easy oxidation and chalking. So, in this study, a UTORAs is prepared by hot-press sintering on VZrHfNbTa (HEA: High Entropy Alloys can generally be defined as more than five elements by the equal atomic ratio or close to the equal atomic ratio alloying, the mixing entropy is higher than the melting entropy of the alloy, generally forming a high entropy solid solution phase of a class of alloys.) a substrate coated with hafnium. The bonding mechanism, resistance to high-temperature oxidation, and hardness of the sample tests are carried out. The results show that zirconium in the matrix will diffuse into the hafnium coating during the high-temperature sintering process and form the HfZr alloy transition layer, the coating thickness of the composite is about 120 μm, and the diffusion distance of zirconium in the hafnium coating is about 60 μm, this transition layer chemically combines the hafnium coating and the HEA substrate into a monolithic alloy composite. The results of high-temperature oxidation experiments show that the oxidation degree of the hafnium-coated VZrHfNbTa composite material is significantly lower than that of the VZrHfNbTa HEA after oxidation in air at 1600 °C for 5 h. The weight gain of the coated sample after oxidation is 56.56 mg/cm², which is only 57.7% compared to the weight gain of the uncoated sample (98.09 mg/cm² for uncoated), and the surface of the uncoated HEA shows obvious dents, oxidation, and pulverization occurred on the surface and interior of the sample. In contrast, the coated composite alloy sample mainly undergoes surface oxidation sintering to form a dense HfO₂ protective layer, and the internal oxidation of the hafnium-coated VZrHfNbTa composite alloy is significantly lower than that of the uncoated VZrHfNbTa HEA. Full article

To improve the infrared emissivity and the ablation resistance of HfB₂/SiC/TaSi₂ coatings for serving in heat flux of 4400 kW/m², HfB₂/SiC/TaSi₂ coatings with different contents of high-emissivity Gd₂O₃ were prepared using atmospheric plasma spraying. The highest emissivity in 3–5 μm can reach up to 0.92 at 1273 K when the Gd₂O₃ content is at 10 vol.%. The increase in the emissivity is attributed to the additional electronic transitions induced by oxygen vacancies, which are generated by substituting Hf⁴⁺ with Gd³⁺. Due to the high emissivity, the surface temperature of the coating modified with 10 vol.% Gd₂O₃ was decreased by ~100 K. Meanwhile, the mass and the liner ablation rate are confirmed to be 4.28 × 10⁻⁷ kg/s and 2.15 × 10⁻⁷ m/s, respectively, which are decreased by 80% and 31% compared to the undoped HfB₂/SiC/TaSi₂ coating. During ablation, HfB₂/SiC/TaSi₂/Gd₂O₃ coating was oxidized to HfO₂, Gd₂Ta₂O₇, HfSiO₄, and GdTaO₄. A stable Hf–Ta–Gd–Si–O multiphase glass was formed on the surface of the coating, which could restrain oxygen penetration. However, the excessive amount of Gd₂O₃ is detrimental to the ablation performance due to its consumption of the SiO₂ glass layer. These findings indicate that the addition of an appropriate amount of Gd₂O₃ could improve the anti-ablation performance of the modified coating. Full article

Five kinds of multi-component entropy-stabilized oxide ceramics were prepared by a solid-state reaction method for thermal barrier coatings, namely La_0.125Y_0.125Yb_0.125Gd_0.125Zr_0.5O_1.75 (LaYYbGdZr), Y_0.125Yb_0.125Gd_0.125Ta_0.125Zr_0.5O_1.875 (YYbGdTaZr), La_0.1Y_0.1Yb_0.1Gd_0.1Ta_0.1Zr_0.5O_1.85 (LaYYbGdTaZr), Y_0.125Yb_0.125Gd_0.125Ta_0.125Hf_0.25Zr_0.25O_1.875 (YYbGdTaHfZr), and La_0.1Y_0.1Yb_0.1Gd_0.1Ta_0.1Hf_0.25Zr_0.25O_1.85 (LaYYbGdTaHfZr). Many properties of the materials were studied, such as their microscopic morphology, crystal structure, thermophysical properties, and ablation resistance. The results show that the oxide ceramics synthesized in this paper have a uniform single-phase defect fluorite structure, and can still maintain this structure after high-temperature treatment at 1500 °C. The YYbGdTaHfZr coatings had the lowest thermal conductivity (0.61~0.89 W·m^–1·K^–1), which was much lower than that of YSZ. The ceramic blocks also exhibited excellent thermal expansion properties. The thermal expansion coefficient of LaYYbGdTaZr could reach 11.09 × 10⁻⁶ K⁻¹ (1400 °C), which was slightly higher than that of 8YSZ (11.0 × 10⁻⁶ K⁻¹). The antioxidant ablation results proved that the YYbGdTaHfZr coating showed the best heat-insulating property. All the results showed that the YYbGdTaHfZr coating is a promising thermal barrier coating. Full article

Thermal barrier coatings (TBCs) have been seriously threatened by calcium-magnesium-alumina-silicate (CMAS) corrosion. The search for novel ceramic coatings for TBCs with excellent resistance to CMAS corrosion is ongoing. Herein, CMAS corrosion resistance behavior and the mechanism of a promising Hf₆Ta₂O₁₇ ceramic coating for TBCs are investigated. The results show that temperature is the most important factor affecting the CMAS behavior and mechanism. At 1250 °C, the corrosion products are composed of dense reaction products (HfSiO₄, Ca_XHf_6−xTa₂O_17−x) and CMAS self-crystallization products. At 1300 and 1400 °C, the corrosion products are mainly dense CaTa₂O₆ and HfO₂, which prevent further CMAS infiltration. Full article

HfC and HfTaC₂ coatings with gradient composition manufactured by double-cathode glow discharge plasma surface metallurgy technology were designed to improve the wear resistance of TA15 titanium alloy. The deposition mechanism of plasma and diffusion mechanism of atoms were investigated, and the growth process of coatings was revealed. The mechanical properties comprising microhardness and elastic modulus were investigated via first-principles calculations and experimental verification. The results reveal that the wear resistance of HfC and HfTaC₂ coatings with abrasive wear mechanism is always better than that of the substrate with abrasive wear, adhesive wear and oxidation wear mechanism. The volume wear rates of the coatings are reduced by 90%–97% compared with the substrate, and that of HfTaC₂ coatings are reduced by 29.9%–45.5% compared with HfC coatings. Furthermore, V-shaped cross section profiles of wear scars formed on HfC coatings, and U-shaped on HfTaC₂ coatings, which is attributed to the addition of tantalum which causes HfC to form a sufficient solid solution, a 0.187–0.030 Å elongation of Ta-C bond length and 0.039–0.051 Å shortening of Hf-C bond length led to the unit cell shrinkage and the Bragg lattice changes from face-centered cubic to face-centered square lattice; accordingly, hardness and wear behavior were further improved. Full article

In this research, we report on a comparative study of the corro-erosive properties of TaC and HfC individual coatings and (Ta₇₀-Hf₃₀-C) and (Ta₃₀-Hf₇₀-C) coatings. These were subjected to different impact angles of abrasive particles, corresponding with angles of 30°, 60° and 90°. It was determined that at 90°, a higher structural damage of the coatings obtained was presented. In addition, for all the systems, it was possible to conclude that the coating formed by (Ta₃₀-Hf₇₀-C) presented a roughness of approximately 7.11 nm, which influenced corrosive properties such as the corrosion potential and corrosion rate. Finally, it was possible to conclude that the (Ta₃₀-Hf₇₀-C) coating presented a higher resistance against corro-erosive environments. This was attributed to structural and mainly superficial factors, making this coating the best option to be implemented as a protective coating against highly aggressive environments in industry. Full article

The TaC, HfC, and Hf-Ta-C coatings are successfully prepared by non-reactively DC magnetron sputtering. The effects of working pressure and deposition temperature on the structure and mechanical properties of Ta-Hf-C coating are analyzed. The scratch performance of the Ta-Hf-C coating deposited on 304 stainless steel and tungsten substrates are investigated. Results show the hardness and elastic modulus of Ta-Hf-C solid solution coating both increase to 37.8 ± 1.1 GPa and 435.8 ± 13.8 GPa due to the solid solution strengthening effect. Plastic deformation resistance H³/E² of Ta-Hf-C coating can reach 0.285, which is more than twice that of binary coating. Furthermore, the scratch performance and failure mechanism show that Ta-Hf-C coating has a weaker plastic deformation resistance on soft substrate and low friction characteristic (0.01) on hard substrate, which implies that Ta-Hf-C coating is a good protective coating that can be applied to cutting tools. Full article

Ta, Hf, TaC_x, HfC_x, and Ta_xHf_1-xC_y coatings were deposited by reactive pulsed Direct Current (DC) magnetron sputtering of Ta or Hf pure metallic targets in Ar plus CH₄ gas mixtures. The properties have been investigated as a function of the carbon content, which is tuned via the CH₄ flow rate. The discharge was characterized by means of Optical Emission Spectroscopy and, in our conditions, both Ta-C and Hf-C systems seem to be weakly reactive. The structure of the as-deposited pure tantalum film is metastable tetragonal β-Ta. The fcc-MeC_x carbide phases (Me = Ta or Hf) are {111} textured at low carbon concentrations and then lose their preferred orientation for higher carbon concentrations. Transmission Electron Microscopy (TEM) analysis has highlighted the presence of an amorphous phase at higher carbon concentrations. When the carbon content increases, the coating’s morphology is first compact-columnar and becomes glassy because of the nano-sized grains and then returns to an open columnar morphology for the higher carbon concentrations. The hardness and Young’s modulus of TaC_x coatings reach 36 and 405 GPa, respectively. For HfC_x coatings, these values are 29 and 318 GPa. The MeC_x coating residual stresses increase with the addition of carbon (from one-hundredth of 1 MPa to 1.5 GPa approximately). Nevertheless, the columnar morphology at a high carbon content allows the residual stresses to decrease. Concerning Ta_xHf_1-xC_y coatings, the structure and the microstructure analyses have revealed the creation of a nanostructured coating, with the formation of an fcc superlattice. The hardness is relatively constant independently of the chemical composition (22 GPa). The residual stress was strongly reduced compared to that of binary carbides coatings, due to the rotation of substrates. Full article

The object of this work was the deposition of a Ta-Hf-C thin film with a gold interlayer on stainless steel, via the physical vapor deposition (PVD) technique, in order to evaluate the properties of different systems subjected to micro-abrasive wear phenomena generated by alumina particles in Ringer's solution. The surface characterization was performed using a scanning electron microscope (SEM) and atomic force microscope (AFM). The crystallographic phases exhibited for each coating were obtained by X-ray diffraction (XRD). As a consequence of modifying the composition of Ta-Hf there was evidence of an improvement in the micro-abrasive wear resistance and, for each system, the wear constants that confirm the enhancement of the surface were calculated. Likewise, these surfaces can be bioactive, generating an alternative to improve the biological fixation of the implants, therefore, the coatings of TaC-HfC/Au contribute in the development of the new generation of orthopedic implants. Full article