Microstructure and ablation performance of HfC/PyC core-shell structure nanowire-reinforced Hf1-xZrxC coating

To improve the ablation performance of C/C composites, HfC/PyC core-shell structure nanowire (HfCnw/PyC)-reinforced Hf_1-xZr_xC coating was prepared via three-step chemical vapor deposition (CVD) method. Effects of HfCnw/PyC and PyC layer thickness on the microstructure, residual stress and ablation performance of Hf_1-xZr_xC coating were studied. HfCnw/PyC-reinforced coatings exhibited equiaxial crystal structure. After incorporating HfCnw/PyC, ablation property of Hf_1-xZr_xC coating was enhanced because of the skeleton role of HfO₂ nanowires. PyC possessed low coefficient of thermal expansion (CTE) and high heat conductivity, but poor ablation performance. Hence, with the increase in thickness of PyC layer, ablation property of the coating first increased and then decreased. HfCnw/PyC-reinforced Hf_1-xZr_xC coating with PyC layer thickness of about 50 nm exhibited the best ablation property.     

Multicomponent (Hf0.25Zr0.25Ti0.25Cr0.25)B2 ceramic modified SiC–Si composite coatings: In-situ synthesis and high-temperature oxidation behavior

High-entropy ceramics, a novel type of multicomponent materials with broad application prospects, have stirred up world-wide interests for over a decade. In the current work, in-situ high-entropy (Hf_0.25Zr_0.25Ti_0.25Cr_0.25)B₂ ceramic modified SiC–Si (HETMB₂-SiC-Si) coating was deposited on carbon/carbon (C/C) composites via gaseous reactive infiltration of Si assisted slurry painting (GRSI-SP) method, to improve the oxidation protective ability of C/C composites at 1973 K. The formation and oxidation mechanisms of the coating was explored by first-principles simulation, experiment and thermodynamic analyses. The coating prepared at 2373 K shows dense mosaic structure filled with HETMB₂-rich Si-based multiphase. This coating adheres well with the C/C substrate, which is ascribed to the formed zigzagged SiC–Si transition layer. This coating protected C/Cs from oxidation for more than 205 h at 1973 K. The enhanced oxidation protective ability is mostly ascribed to the subsequently generated compact and stable Hf-Zr-Ti-Cr-Si-O composite oxidation scale. This research will start up novel research ares of developing high-entropy materials modified coatings with improved protective ability under extreme environments.     

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.     

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.     

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).     

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.     

Deposition and ablation resistance of HfC-based coatings prepared on SiC-coated C/C composites by supersonic atmospheric plasma spraying

In order to improve the ablation properties of C/C composites, HfC-based coatings with different mass ratios of SiC were deposited on the surface of SiC-coated carbon/carbon composites by supersonic atmospheric plasma spraying. The morphologies and microstructures of the HfC-based coatings were characterised. The ablation resistance test was carried out by oxyacetylene torch. The results show that the as-prepared coatings are multiphase coatings consisting of HfC, HfO₂, SiC and SiO₂. The structure of different coatings is dense. After ablation for 60?s, the ablation centre region of coating is smooth without obvious microcrack and pinhole, and no interlaminar crack can be observed at the cross-section. An Hf–Si–O compound oxide layer is generated on the surface of coating, which is beneficial for protecting the C/C composites from being ablated. Meanwhile, the further generated HfSiO₄ can play a pinning effect, which can prevent crack extension.     

Structure,mechanical properties and thermal stability of Ti1-xSixN coatings

Ti_1-xSi_xN coating is a promising candidate for wear resistant applications due to their super-hardness and high thermal stability. Here, we explored the structure, mechanical properties and thermal stability of Ti_1-xSi_xN (x?=?0, 0.13, 0.17 and 0.22) coatings deposited by cathodic arc evaporation. Monolithically grown Si-containing Ti_1-xSi_xN coatings, which are Si-solution in TiN for x?=?0.13 and 0.17, reveal a high hardness of 39.4?±?0.67 and 40.6?±?0.72?GPa, respectively. Then Ti_1-xSi_xN transforms into a nanocomposite structure consisting of cubic Ti(Si)N nanocrystallite enveloped by the amorphous SiN_x tissue phase for x?=?0.22, which exhibits a high hardness of 40.0?±?0.6?GPa. However, increasing of Si content leads to a significant increase in compressive stress from ?0.63?GPa for x?=?0 to ?3.78?GPa for x?=?0.13 to ?4.54?GPa for x?=?0.17 to ?5.51?GPa for x?=?0.22. The hardness of Ti_1-xSi_xN coatings can be maintained up to ~ 1000?°C due to the suppressed grain growth, and then decreases for further elevated annealing temperature, whereas the TiN coating exhibits a continuous drop in hardness towards its intrinsic value of ~ 21.3?GPa.     

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.     

Multiphase composite Hf0.8Ti0·2B2–SiC–Si coating providing oxidation and ablation protection for graphite under different high temperature oxygen-containing environments

A novel multiphase composite coating composed of Hf_0.8Ti_0·2B₂ solid solution, SiC and Si was prepared by a joint procedure of slurry method and silicon reactive infiltration (SRI). The oxidation and ablation experiments were conducted to investigate oxidation and ablation resistance of the Hf_0.8Ti_0·2B₂–SiC–Si coated graphite samples, respectively. The results revealed that the coated sample was oxidized at 1823 K for 108 h with a mass gain of 1.49%, which was ascribed to the high viscosity oxide layer improved by HfSiO₄ and TiO₂ in conjunction with dense structure of the coating, thereby presenting excellent high temperature stability. Furthermore, after 90 s ablation at 3273 K under a heat flux of 5.62 MW/m², the composite coating was not peeled off, which had mass ablation rate (MAR) and linear ablation rate (LAR) of 3.1 mg/s and 1.5 μm/s, respectively. The refractory oxide layer comprising oxides of Hf and Ti on the surface acted as an oxygen barrier, which can weaken the mechanical erosion force of oxyacetylene flame, finally protecting the inner coating and graphite matrix from further consumption.     

TiB2–ZrB2–SiC composite ceramic coating with the formation of solid-phase (TixZr1-x)B2 deposited by atmospheric plasma spraying as a barrier to molten cryolite-based salt

TiB₂, ZrB₂, and SiC powders with particles measuring several micrometers were first agglomerated and then deposited onto the substrate of a porous carbon block as a barrier to molten cryolite-based salt. Scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were conducted to investigate the fine microstructure of the obtained ceramic coating and to elucidate the evolution of agglomerated powder from sprayed particles in plasma jet to splats on a substrate. Results indicated that a highly dense ceramic coating consisting of a solid solution (Ti_xZr_1-x)B₂, residual TiB₂, and ZrB₂ was obtained. The compactness of the coating and the formation of a solid solution phase was mainly caused by a SiC-rich liquid phase, as determined from a boride and silicon carbide pseudodiagram. After being submerged in molten cryolite-based salt for 8 h, the ceramic coating was firmly bonded to a carbon block. No molten slat permeated the ceramic coating.     

Si/SiC coated Cf/SiC composites via tape casting and reaction bonding: The effect of carbon content

In this paper, a novel surface modification method for C_f/SiC composites is proposed. Si/SiC coating on C_f/SiC composites is prepared by tape casting and reaction bonding method. The effects of carbon content on the rheological property of the slurries along with the microstructure of the sintered coatings are investigated. The best result has been obtained by infiltrating liquid silicon into a porous green tape with a carbon density of 0.84 g/cm³. In addition, the effect of sintering parameters on the phase composition of the coatings is studied. Dense Si/SiC coating with high density as well as strong bonding onto the substrate is obtained. This Si/SiC coating exhibits an excellent mechanical property with HV hardness of 16.29±0.53 GPa and fracture toughness of 3.01±0.32 MPa m^1/2. Fine surface with roughness (RMS) as low as 2.164 nm is achieved after precision grinding and polishing. This study inspires a novel and effective surface modification method for C_f/SiC composites.     

B-site acceptor doped AgNbO3 lead-free antiferroelectric ceramics: The role of dopant on microstructure and breakdown strength

B-site aliovalent modification of AgNbO₃ with a nominal composition of Ag(Nb_1-xM_x)O_3-x/2 (x = 0.01, M = Ti, Zr and Hf) was prepared. The effects of dopants on microstructure, dielectric, ferroelectric and conduction properties were investigated. The results indicate that the introduction of acceptor dopant does not lead to grain coarsening. Zr⁴⁺ and Hf⁴⁺ doping are beneficial to stabilize the antiferroelectric phase of AgNbO₃. Among all the samples, Ti⁴⁺ doped AgNbO₃ has the minimum resistivity while Hf⁴⁺ doped AgNbO₃ has the maximum resistivity, therefore, Hf⁴⁺ doped AgNbO₃ has high BDS. The XPS results indicate that the conduction behaviour is associated with the concentration of oxygen vacancies. This work hints that acceptor dopant is also effective on the microstructure control and chemical modification of AgNbO₃-based ceramics.     

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.     

ZrB2–SiC gradient oxidation protective coating for carbon/carbon composites

To improve the oxidation protective ability of carbon/carbon composites, ZrB₂–SiC gradient coating was prepared on the surface of C/C composites by an in-situ reaction method. The ZrB₂–SiC gradient coating consisted of an inner ZrB₂–SiC layer and an outer ZrB₂–SiC–Si coating. The phase composition and microstructures of the multiphase coating were characterized by XRD, EDS and SEM. Results showed that the inner coating is mainly composed of ZrB₂ and SiC, while the outer multiphase coating is composed of ZrB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The oxidation behavior of the coated C/C composites at 1773 K in air was investigated. Results show that the gradient ZrB₂–SiC oxidation protective coating could protect C/C from oxidation for 207 h with only (4.56±1.2)×10⁻³ g/cm² weight loss, owing to the compound silicate glass layer with the existence of thermally stable phase ZrSiO₄.     

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.     

Synthesis of new lead-containing MAX phases of Zr3PbC2 and Hf3PbC2

In this work, two new 312 MAX phases of Zr₃PbC₂ and Hf₃PbC₂ were successfully synthesized by spark plasma sintering. It is the first discovery of lead-containing 312 MAX phases, which together with M₂PbC (M = Ti, Zr, Hf) form the lead-containing MAX phase family. Considering the extremely low electrical conductivity of Hf₂PbC, these two new compounds are of great research value. Based on the Rietveld refinement results, their lattice parameters and atomic positions were well determined, as a = 3.3771(5) Å, c = 20.0070(9) Å for Zr₃PbC₂ and a = 3.3357(1) Å, c = 19.7659(8) Å for Hf₃PbC₂, where M atoms are located at (0, 0, 0) and (1/3, 2/3, 0.1258(6)[Zr]; 0.1255(2)[Hf]), Pb atoms are located at (0, 0, 1/4), and C atoms are located at (1/3, 2/3, 0.0663(2)[Zr]; 0.0641(3)[Hf]), respectively. Additionally, the typical laminar microstructure of Zr₃PbC₂ and Hf₃PbC₂ grains was observed.     

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.     

TaB2–SiC–Si multiphase oxidation protective coating for SiC-coated carbon/carbon composites

To improve the oxidation resistance of the carbon/carbon (C/C) composites, a TaB₂–SiC–Si multiphase oxidation protective ceramic coating was prepared on the surface of SiC coated C/C composites by pack cementation. Results showed that the outer multiphase coating was mainly composed of TaB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The coating could protect C/C from oxidation for 300 h with only 0.26 × 10^?2 g²/cm² mass loss at 1773 K in air. The formed silicate glass layer containing SiO₂ and tantalum oxides can not only seal the defects in the coating, but also reduce oxygen diffusion rates, thus improving the oxidation resistance.