Effects of HfC/PyC core-shell structure nanowires on the microstructure and mechanical properties of Hf1-xZrxC coating

To elevate the mechanical and anti-ablation properties of Hf_1-xZr_xC coating on C/C composites, HfC/PyC core-shell structure nanowires (HfCnw/PyC) with different PyC layer thickness were synthesized by two steps of CVD. Influences of HfCnw/PyC on the microstructure and mechanical properties of Hf_1-xZr_xC coating were researched. Toughening mechanism of HfCnw/PyC was also investigated. PyC layer exhibited a lamellar structure and combined well with HfCnw. After incorporating HfCnw/PyC, Hf_1-xZr_xC coating structure converted from columnar crystal to isometric crystal. HfCnw improved H, E, K_c and bonding strength of Hf_1-xZr_xC coating, which is ascribed to the nanowire pullout, debonding, bridging and crack deflection mechanism. With the PyC layer thickness increasing, H and E of the coating reduced, K_c and bonding strength of the coating increased. Because of the moderate bonding strength between HfCnw/PyC and coating matrix, lamellar structure of PyC layer and higher K_c of PyC, toughening effectiveness of the core-shell structures gradually enhanced with the PyC layer thickness increasing.     

Microstructure and ablation behaviour of a strong,dense, and thick interfacial ZrxHf1-xC/SiC multiphase bilayer coating prepared by a new simple one-step method

In order to solve the shortcomings of chemical vapour deposition (CVD), such as CVD-prepared coatings that are weakly bound to the carbon base, Zr_xHf_1-xC/SiC multiphase bilayer ceramic coatings were prepared on substrate surfaces by slurry brushing and the one-step in-situ thermal evaporation reaction method. The coating exhibits multiphase bilayer characteristics due to the self-diffusion of the matrix carbon source and the self-assembly of gaseous Zr and Si with the matrix. The 200-μm-thick Zr_xHf_1-xC solid-solution phase is distributed on the outer coating layer, while the 100-μm-thick SiC phase is distributed in the inner layer such that it contacts the substrate. The coating prepared by brushing with Hf and vapour-deposited with a masterbatch containing 7:3 (w/w) Zr:Si (H-ST) exhibits excellent ablation resistance, attributable to the presence of dense and spallation-free oxide scale and the low oxygen diffusion coefficient of (Zr, Hf)C_yO_z.     

Microstructures and ablation resistance of WSi2/ZrSi2/ZrxHf1-xC/SiC coating based on a pattern strengthening one-step method

To improve the ablation performance of carbon/carbon (C/C) composite materials, a WSi₂–ZrSi₂ composite-reinforced Zr_xHf_1–xC/SiC coating was prepared on the substrate surface via patterning strengthening method. The results show that this coating can protect the substrate from failure for 300 s under an oxyacetylene flame at 2600 °C. Owing to the presence of W, an extremely dense oxide layer was formed on the surface of the coating during initial ablation, which progressively led to the expulsion of the product of oxidation (WO₃) from the inner layer, as well as to holes and cracks on healing the coating surface, thereby significantly improving the ablation performance of the C/C composites. In addition, the excellent ablative performance and mechanism of the coating were analysed using volatility diagrams.     

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.     

A novel Cr-doped Al2O3-SiC-ZrC composite coating for ablative protection of C/C-ZrC-SiC composites

A novel Cr-doped Al₂O₃-SiC-ZrC coating systemis proposed to further improve the ablation resistance of C/C-ZrC-SiC composites. Our approach combines low pressure plasma spray method with slurry impregnation of Zr-Cr-Si-C to achieve a specially tailored oxide-carbide structure. Results show that the as-prepared coating was dense and crack-free, which effectively promoted the ablation resistance of C/C-ZrC-SiC with the mass and linear ablation rates decreased by 66% and 76% respectively, involving a highly dense protective structure of A1_xCr_2-xO₃-SiO₂-ZrO₂ formed during ablation. The compact and continuous multi-oxide scale with “ZrO₂-rosette” skeleton intricated with A1_1.96Cr_0.04O₃ was evidenced in the ablation center.     

Mechanical and ablative properties improvement of HfC-SiC coatings upon introduction of TiC

A novel structural Hf-Ti-Si-C multiphase solid solution coating was designed and manufactured by chemical and solid solution reactions to improve the mechanical and ablation behavior of HfC-SiC coatings. Results show that, with TiC addition, the formed Hf_xTi_1?xC and (Ti_1?xHf_x)₃SiC₂ solid solutions can significantly enhance the micro-mechanical and ablation properties of the coating. The improved hardness and modulus, as well as the reduced ablation rates are mainly attributed to the optimized structure and solid solution reinforcing effect of coating. Moreover, the Ti-additives are conducive to restrain the active oxidation of SiC. Furthermore, HfTiO₂ can reduce the oxides cracking due to the inhibited crystal transformation of HfO₂ and its good self-healing ability, forming a dense and stable oxide scale with superior thermal protection.     

Ablation resistant ZrC coating modified by polymer-derived SiC/TiC nanocomposites for ultra-high temperature application

To improve the ablation resistance of ZrC coating on SiC-coated carbon/carbon composites above 2000 °C, SiC/TiC nanocomposites (SiC/TiC-NCs) powders derived from single-source precursor were incorporated into ZrC coating, denoted as ZrC-SiC/TiC-NCs, via supersonic atmospheric plasma spraying (SAPS). After SAPS, the incorporated SiC/TiC-NCs evolved into TiC/(SiC and Zr_xTi_yC) embedded in amorphous SiC. The ablation resistance of the ZrC-SiC/TiC-NCs coating was evaluated by oxyacetylene flames with a heat flux of 4.18 MW/m². For comparison, the ZrC-SiC-NCs coating without Ti modification was seriously damaged due to rapid gas denudation. The good ablation resistance of ZrC-SiC/TiC-NCs coating is mainly attributed to the distinctive “capsule-like” multi-crystalline microstructure of SiC/TiC-NCs. During ablation, TiO₂ and Zr_xTi_yO₂, due to the oxidation of TiC and Zr_xTi_yC, contributed to the formation of Zr-Ti-Si-O glass with high viscosity and low evaporation pressure, improving the ablation resistance.     

The effect of Gd3+ and Ti4+ co-doping on the thermal radiation performance of (Sm1-xGdx)2(Hf1-xTix)2O7 (0 ≤ x ≤ 0.2) ceramic coatings

In this study, the (Sm_1-xGd_x)₂(Hf_1-xTi_x)₂O₇ (0 ≤ x ≤ 0.2) ceramic coatings were fabricated by atmospheric plasma spraying. The chemical compositions, morphologies and thermo-optical properties of the samples were systemically investigated. It can be found that the infrared emissivity of (Sm_1-xGd_x)₂(Hf_1-xTi_x)₂O₇ ceramic coatings at the wavelength range of 0.76–15 μm increased with the increasing content of Gd³⁺ and Ti⁴⁺. The (Sm_0.8Gd_0.2)₂(Hf_0.8Ti_0.2)₂O₇ ceramic coating exhibited the highest infrared emissivity among the coatings, which was 0.773 and 0.816 at room temperature and 1400 °C, respectively. The mechanism of the increasing infrared emissivity was attributed to the Gd³⁺ and Ti⁴⁺ co-doping can improve the free carrier concentration and the frequency and mode of the lattice vibration. Moreover, the (Sm_0.8Gd_0.2)₂(Hf_0.8Ti_0.2)₂O₇ ceramic coating possessed good thermal resistance, which did not show obvious change in the phase, surface morphology and infrared emissivity after 60 h calcination at 1400 °C.     

Influences of deposition temperature,gas flow rate and ZrC content on the microstructure and anti-ablation performance of CVD-HfC-ZrC coating

To amend the oxidation and ablation resistance of C/C composites, HfC-ZrC biphase coating was synthesized by CVD. Influences of deposition temperature and CH₄ flow rate on the deposition rate, phase constitution and microstructure of the HfC-ZrC coating were investigated. Ablation behavior and ablation mechanism of the HfC-ZrC coating with different ZrC contents were examined. With the deposition temperature rising, the deposition rate and grain size of the HfC-ZrC coating increased. High flow rate of CH₄ was beneficial to improving the deposition rate and reducing the grain size of the HfC-ZrC coating. Moderate ZrC content in the HfC-ZrC coating was conducive to the process of solid solution sintering among HfO₂ and ZrO₂ grains, leading to generating a continuous and compact oxide layer. The coating with HfC/ZrC mole ratio of 1:1 exhibited superior anti-ablation performance, owing to its flat and compact structure and sufficient solid solution sintering among the oxide grains during ablation.     

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.     

A layer-by-layer deposition mechanism for producing a pyrolytic carbon coating on carbon nanotubes

Pyrolytic carbon (PyC) was deposited on carbon nanotubes (CNTs) in order to modify them by introducing defects to their surface. The deposition of PyC was carried out at temperature between 800 and 1000 °C using propane as carbon source with or without a hydrogen carrier gas at low pressure of 20 kPa. The structure of PyC coatings was examined using transmission electron microscopy. The PyC coating could be distinguished from the original CNT walls due to the difference of the structure, with the coating showing a less orderly layer structure. When H₂ was introduced during deposition, PyC coating started to form at 900 °C, and the deposition rate increased rapidly with increasing temperature. Without H₂, PyC coating with a thickness of a few layers could be formed at temperatures between 800 and 900 °C in 10 min. The outmost layer of the PyC coating showed a structure of rough and curved carbon fragment. A layer-by-layer mechanism is proposed for the deposition consisting of alternating fragment formation (nucleation) and lateral growth to layer.     

Structure and optical properties of ZrxHf1-xO2 films deposited by pulsed laser co-ablation of Zr and Hf targets with the assistance of oxygen plasma

Group IVB metal oxides have demonstrated many potential applications in a variety fields owing to their excellent optical, mechanical, electrical, chemical and thermal properties. In this work, ternary oxides Zr_xHf_1-xO₂ films with variable compositions were deposited by pulsed laser co-ablation of a Zr target and a Hf target in an oxygen plasma generated by electron cyclotron resonance microwave discharge of O₂ gas. The oxygen plasma provided an environment containing a high concentration of reactive oxygen species for synthesizing oxides with the Zr and Hf species ablated from the Zr and Hf targets. The structure of the deposited films was characterized and the optical properties were evaluated together with the examination of the effects of post-deposition annealing on the structure and optical properties. The ternary Zr_xHf_1-xO₂ films are much alike with binary ZrO₂ and HfO₂ films in structure and optical properties. They have a monoclinic structure and are highly transparent in a wide spectral region from mid-ultraviolet to mid-infrared. The ultraviolet absorption edge and optical band gap vary slightly with the pulse energies of the laser beams ablating the targets. Annealing in N₂ resulted in the improvement in the film structure.     

Microstructure and mechanical performance evolution of C/C–ZrC composites exposed to oxyacetylene flame

C/C–ZrC composites were prepared by isothermal chemical vapor infiltration (ICVI) combined with reactive melt infiltration (RMI). The ablation behavior of the C/C–ZrC was investigated using an oxyacetylene flame. The effect of ablation time on the microstructure and mechanical property evolution of the composite was studied. The results showed that as the ablation time prolonged, the linear and mass ablation rates of the composite increased firstly and then stabilized. After 15 s ablation, the flexural strength and modulus of the C/C–ZrC were interestingly increased by 141.8% and 40.9%, which reached 138.42 MPa and 6.45 GPa, respectively. During ablation, the preferential oxidation effect of ZrC could mitigate the oxidation of pyrolytic carbon (PyC) and carbon fibers, and the volume change induced by the ZrC →ZrO₂ phase transformation could weaken its bonding with PyC, which was beneficial for releasing the internal residual stresses of the C/C–ZrC and then contributed to the mechanical performance improvement.     

Densification,microstructure, and mechanical properties of V-substituted HfB2-based ceramics

This study firstly developed Hf_1-xV_xB₂ (x = 0, 0.01, 0.02, 0.05) powders, which were derived from borothermal reduction of HfO₂ and V₂O₅ with boron. The results revealed that significantly refined Hf_1-xV_xB₂ powders (0.51 μm) could be obtained by solid solution of VB₂, and x ≥ 0.05 was a premise. However, as the content of V-substitution for Hf increased, Hf_1-xV_xB₂ ceramics sintered by spark plasma sintering at 2000 °C only displayed a slight densification improvement, which was attributed to the grain coarsening effect induced by the solid solution of VB₂. By incorporating 20 vol% SiC, fully dense Hf_1-xV_xB₂-SiC ceramics were successfully fabricated using the same sintering parameters. Compared with HfB₂-SiC ceramics, Hf_0.95V_0.05B₂-20 vol% SiC ceramics exhibited an elevated and comparable value of Vickers hardness (23.64 GPa), but lower fracture toughness (4.09 MPa m^1/2).     

Investigation on bi-layer coating with La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7/YSZ prepared by laser cladding

Thermal barrier coatings are an effective technology for improving the high-temperature performance of hot section components in gas turbine engine. Due to their excellent properties, high-entropy oxides are considered to be promising materials for thermal barrier coatings. Laser cladding is a coating preparation technology and the top coat prepared by laser cladding technology has an important application value for thermal barrier coatings. In this work, to improve the thermal cycling behavior of the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide coating, a bi-layer coating with the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide layer and the YSZ layer was designed and fabricated by laser cladding on the NiCoCrAlY alloy surface. The microstructure, phase and mechanical properties of the coating were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and micro-hardness and nanoindentation tests, respectively. The results show that a bi-layer La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇/YSZ coating was successfully prepared by the laser cladding method, and shows good bonding at the interface between the layers. The high-entropy oxide layer maintains a relatively stable defective fluorite structure and its microstructure exists in the stable cellular and dendrite crystalline state after laser cladding. The high-entropy oxide layer prepared by laser cladding showed an average elastic modulus of 167 GPa and an average hardness of 1022.8HV in nanoindentation tests. Thermal cycling of the coating was carried out at 1050 °C. Failure of the bi-layer coating occurred after 60 thermal cycles at 1050 °C. Thermal stresses between different layers are calculated during thermal cycling. Due to its excellent mechanical properties, the bi-layer coating with the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide and YSZ layers is expected to become an effective high-entropy oxide thermal barrier coating.     

Polymer-metal slurry reactive melt infiltration: A flexible and controllable ceramic modification strategy for irregular C/C components

Herein, we investigate the applicability of the polycarbosilane (PCS)–metal slurry reactive melt infiltration (RMI) process to various metals. The slurry exhibiting the best ceramized ability was used to examine the relationship between the ceramic thickness and reactive time, ceramic thickness and reactive temperature, and infiltration depth and slurry-coating thickness. The results show that the thickness of the ceramic layer increases with reactive time and temperature and the infiltration depth increases with the coating thickness. PCS–Si₉₀Zr₁₀ slurry RMI was selected to modify cylindrical nozzle C/C preforms, and dense C/C–SiC–ZrC composites with a density of ~2.05 g cm⁻³ were obtained. Owing to the good control of the PCS–Si₉₀Zr₁₀ slurry RMI on the interface, matrix, and carbon fiber of the as-received cylindrical composites, the bending strength of the C/C–SiC–ZrC composites was as high as 306.4 MPa, which is considerably higher than that of a C/C preforms (70.4 MPa). Considering the ablation resistance, the mass and linear ablation rates of the C/C–SiC–ZrC composite (~0.29 mg s⁻¹ and ~2.48 × 10⁻³ mm s⁻¹, respectively) were similar to those of the composites prepared using traditional RMI (~0.23 mg s⁻¹ and ~2.29 × 10⁻³ mm s⁻¹). The proposed polymer–metal RMI is more suitable for the modification of C/C preforms with thin-wall structures owing to its advantages including precise control of infiltration dose and flexible operation of slurry coating. Furthermore, it is suitable for the local modification of C/C components.     

Study on La2(Hf1-xTix)2O7 (x ≤ 0.20) pyrochlores for potential thermal/environmental barrier coating applications

Lanthanum hafnate (La₂Hf₂O₇) with a pyrochlore structure has excellent high temperature stability and low thermal conductivity, which is promising for thermal/environmental barrier coatings (T/EBCs) applications. To reduce its thermal expansion coefficient (TEC) so as to better match SiC_f/SiC composites, a smaller tetravalent dopant Ti⁴⁺ has been introduced in the Hf-sites to form La₂(Hf_1-xTi_x)₂O₇ (x ≤ 0.20). The phase composition and microstructure confirms that La₂(Hf_1-xTi_x)₂O₇ solid solutions possess a pure pyrochlore structure. With an increase of x, their TECs are decreasing consistently, whilst their thermal conductivities of La₂(Hf_1-xTi_x)₂O₇ are slightly increasing at high temperature but still much lower than those of meta-stable yttria partially stabilized zirconia, both of which are attributing to an increase of elastic modulus after Ti⁴⁺ doping on Hf-sites. The extremely excellent high temperature stability, relatively low thermal conductivities and low TECs suggest that La₂(Hf_1-xTi_x)₂O₇ is a prospective candidate material for T/EBC applications.     

Effect of Y doping on microstructure and thermophysical properties of yttria stabilized hafnia ceramics

A series of Y₂O₃-doped HfO₂ ceramics (Hf_1-xY_xO_2-0.5×, x?=?0, 0.04, 0.08, 0.12, 0.16 and 0.2) were synthesized by solid-state reaction at 1600?°C. The microstructure, thermophysical properties and phase stability were investigated. Hf_1-xY_xO_2–0.5x ceramics were comprised of monoclinic (M) phase and cubic (C) phase when Y³⁺ ion concentration ranged from 0.04 to 0.16. The thermal conductivity of Hf_1-xY_xO_2–0.5x ceramic decreased as Y³⁺ ion concentration increased and Hf_0.8Y_0.2O_1.9 ceramic revealed the lowest thermal conductivity of ~?1.8?W/m*K at 1200?°C. The average thermal expansion coefficient (TEC) of Hf_1-xY_xO_2–0.5x between 200?°C and 1300?°C increased with the Y³⁺ ion concentration. Hf_0.8Y_0.2O_1.9 yielded the highest TEC of ~?10.4?×?10^?6 K^?1 while keeping good phase stability between room temperature and 1600?°C.     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.