Nanosized (Zr,Hf)O2 coating reinforced by AlN whiskers for the ablation protection of SiC coated C/C composites

In this work, AlN whisker-reinforced (Zr, Hf)O₂ coating was prepared on SiC coated carbon/carbon composites by sol-gel and plasma spraying methods. Its cyclic ablation resistance was evaluated using an oxyacetylene torch with a heat flux of 2.38 MW/m². After ablation, the coating showed apparently decreased crack sizes and quantity as compared with the one without AlN whisker addition, pointing out its better crack tolerance. Moreover, the coating had a thinner oxidized region based on the linear ablation rate (−0.042 µm/s), greatly lower than that of pure oxide coating (−0.922 µm/s). After detailed observation and characterization, probable protection mechanism was proposed.     

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.     

Effect of CrSi2 on the ablation resistance of a ZrSi2-Y2O3/SiC coating prepared by SAPS

To improve the ablation resistance of carbon/carbon (C/C) composites, a proportional amount of ZrSi₂-CrSi₂-Y₂O₃ mixed particles were deposited on the surface of SiC-coated C/C composites by supersonic air plasma spraying (SAPS) to form a ZrSi₂-CrSi₂-Y₂O₃/SiC coating. The microstructure and phase compositions of the coating were studied by SEM, EDS, XRD and its anti-ablation performance was tested by oxyacetylene torch. The experimental results showed that the ZrSi₂-CrSi₂-Y₂O₃ outer coating had a dense microstructure without obvious pores and microcracks, and the thickness reached approximately 150 μm. In the process of being eroded and scoured by the oxyacetylene flame, the coating exhibited excellent anti-ablation property, which was attributed to the mosaic microstructure formed by ZrO₂ and a Si-O-Cr liquid film on the coating surface. After experiencing an ablation time of 80 s, the linear ablation rate and the mass ablation rate of the coating were -1.0 ± 0.03 μm s^-1 and -0.16 ± 0.014 mg s^-1, respectively.     

High temperature oxidation resistance of Y2O3 modified ZrB2-SiC coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at high temperature, Y₂O₃ modified ZrB₂-SiC coating was fabricated on C/C composites by atmospheric plasma spraying. The microstructure and chemical composition of the coatings were characterized by SEM, EDS, and XRD. Experiment results showed that the coating with 10 wt% Y₂O₃ presented a relatively compact surface without evident holes and cracks. No peeling off occurred on the interface between the coating and substrate. The ZSY10 coating underwent oxidation at 1450 °C for 10 h with a mass loss of 5.77%, while that of ZS coating was as high as 16.79%. The existence of Y₂O₃ played an important role in inhibiting the phase transition of ZrO₂, thus avoiding the cracks caused by the volume expansion of the coating. Meanwhile, Y₂SiO₅ and ZrSiO₄ had a similar coefficient of thermal expansion (CTE), which could relieve the thermal stress inside the coating. The ceramic phases Y₂SiO₅, Y₂Si₂O₇ and ZrSiO₄ with high thermal stability and low oxygen permeability reduced the volatilization of SiO₂.     

Hf6Ta2O17/Ta2O5 composite ceramic: A new eutectic system

Eutectic oxides are considered as the most promising candidates applied in extreme environment for long periods, and the pace of exploring new eutectic systems has never stopped. In the present work, Hf₆Ta₂O₁₇/Ta₂O₅ eutectic system is firstly prepared by the joint process of solvothermal method and spark plasma sintering (SPS). Eutectic liquid phase produced during the process of SPS fills the closed-pores and promotes the sintering process. Orthorhombic-Hf₆Ta₂O₁₇ and tetragonal-Ta₂O₅ of both eutectic and hypereutectic systems show single crystal characteristics and are combined by coherent interface instead of grain boundaries. The clean and flat interface also shows the characteristics of single crystal diffraction without impurities and amorphous phase. Based on the analysis of microstructure, the sintering mechanism is proposed, and the potential application of Hf₆Ta₂O₁₇/Ta₂O₅ eutectic system is analyzed.     

New anti-ablation candidate for carbon/carbon composites: Preparation,composition and ablation behavior of Y2Hf2O7 coating under an oxyacetylene torch

Y₂Hf₂O₇ possesses low thermal conductivity and high melting point, which make it promising for a new anti-ablation material. For evaluating the thermal stability and the potential applications of Y₂Hf₂O₇ on anti-ablation protection of C/C composites, Y₂Hf₂O₇ ceramic powder was synthesized by solution combustion method and Y₂Hf₂O₇ coating was prepared on the surface of SiC coated C/C composites using SAPS. Results shown that the coating exhibits good ablation resistance under the heat flux of 2.4?MW/m² with the linear and mass ablation rates are 0.16?μm?s^?1 and ?0.028?mg?s^?1, respectively, after ablation for 40?s. With the prolonging of the ablation time, the increasing thermal stress causes the increase of cracks. Moreover, the chemical erosion from SiO₂ and the physical volatilization of low temperature molten products aggravate failure of the Y₂Hf₂O₇ coating.     

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.     

Ablation behavior of a network interlacing ZrC-VC ceramic coating prepared by a pioneering spillover permeation

A novel network interlacing ZrC-VC ceramic coating was prepared by a pioneering spillover permeation. With the increase of Zr content in the blind vias, the content of ZrC in the coating and the density of the coating all decrease. The density of the coating on C/C–ZrC–SiC substrate is obviously higher than that on C/C substrate. The linear ablation rate of the novel ceramic coated C/C–ZrC–SiC composites was ?0.06 μm/s with about 20 and 1.56 times reduction than C/C composites and C/C–ZrC–SiC composites respectively. The improved ablation resistance was attributed to a dense honeycomb ZrO₂ layer filled with liquid vanadium oxide in the ablation center and the improved thermal radiation.     

Ablation behavior of HfC coating with different thickness for carbon/carbon composites at ultra-high temperature

The thickness of the different HfC coatings from 20 μm to 50 μm were prepared on the surface of carbon/carbon (C/C) composites by low pressure chemical vapor deposition (LPCVD). The microstructure and thermal stress of the coatings after ablation were investigated, as well as the effect of thickness and thermal stress on the ablation resistance of the HfC coating was analyzed. After being ablated at a heat flux of 2.4 MW/m² for 60 s, the thermal stress gradually increased at first and then rapidly increased with the increasing thickness of coating. The results indicated that the moderate coating thickness can effectively release the thermal stress generated during the ablation process. The 40 μm-thick HfC coating showed the best ablation resistance with the mass ablation rate and line ablation rate were only 0.13 mg/s and 0.09 μm/s, respectively.     

Effects of HfC nanowire amount on the microstructure and ablation resistance of CVD-HfC coating

To improve the ablation resistance of HfC coating for carbon/carbon (C/C) composites, various fractions of HfC nanowires were incorporated into the HfC coating by chemical vapor deposition (CVD). Effects of HfC nanowire amount on the microstructure and ablation resistance of the CVD-HfC coating were investigated. Results indicated that the HfC nanowire layer became thicker and denser with the deposition time extending. HfC nanowires could inhibit the formation of cracks and interlaminar gaps in the HfC coating. With the increase of HfC nanowire amount, the HfC coating became thicker, while its porosity and roughness firstly decreased and then increased. Ablation tests indicated that the incorporation of HfC nanowires could effectively improve the ablation resistance of the HfC coating, which could be ascribed to the decreasing surface temperature of the coated samples and the effective alleviation of cracking and delamination of the coating during ablation. The HfC coating with HfC nanowires deposited for 1?h exhibited better ablation resistance owing to its compact microstructure, and its mass and linear ablation rates were only 0.41?mg/s and ??1.53?µm/s after ablation for 120?s.     

Mechanical properties and ablation resistance of HfC nanowire modified carbon/carbon composites

Hafnium carbide nanowires (HfCnws) were in-situ grown in carbon/carbon (C/C) composites, and subsquently the preforms were densified by isothermal chemical vapor infiltration to obtain HfCnws modified carbon/carbon (HfCnws-C/C) composites. Morphology and microstructure of HfCnws were examined, and the effect of HfCnws on the mechanical property and ablation resistance of C/C composites were also investigated. Results show that introducing HfCnws refined the grain size of pyrolytic carbon (PyC). The out-of-plane compression, interlaminar shear and flexual strength of HfCnws-C/C composites increased by 120.80%, 45.60% and 94.65%, respectively compared with pure C/C, and the HfCnws-C/C shows good ablation resistance under oxy-acetylene flame ablation.     

Synthesis of Hf6Ta2O17 superstructure via spark plasma sintering for improved oxidation resistance of multi-component ultra-high temperature ceramics

Ultra-high temperature ceramics (UHTCs) have shown aspiration to overcome challenges in the thermal protection system (TPS) by designing new materials referred to as multi-component UHTCs (MC-UHTCs) in the compositional space. MC-UHTCs have shown remarkable improvement in oxidation resistance due to the formation of the Hf₆Ta₂O₁₇ superstructure during plasma exposure. Herein, the Hf₆Ta₂O₁₇ superstructure is synthesized via a solid-state reaction between HfO₂ and Ta₂O₅ powder mixtures during spark plasma sintering (SPS). The compositions chosen are 50 vol% of HfO₂ -50 vol% of Ta₂O₅ (50HO-50TO) and 70 vol% of HfO₂ -30 vol% of Ta₂O₅ (70HO-30TO). The phase quantification via Rietveld analysis showed Hf₆Ta₂O₁₇ as a principal phase with some residual Ta₂O₅ phase in both the samples. The high-temperature thermal stability of the samples was evaluated using high-velocity plasma jet exposure for up to 3 min. 50HO-50TO was able to withstand the intense plasma condition, which is attributed to the higher content of the Hf₆Ta₂O₁₇ phase (～84%) and lower strain in the Ta₂O₅ phase. The augmentation in the Hf₆Ta₂O₁₇ phase to 94.7% (in 50HO-50TO) post plasma exposure has been attributed to the invariant transformation from a liquid state to Hf₆Ta₂O₁₇ at temperatures >2500 °C during testing. The mechanical integrity is elucidated from the insignificant change in the hardness ～13.3 GPa before and 11.2 GPa after plasma exposure of the 50HO-50TO sample. As a result, the Hf₆Ta₂O₁₇ superstructure's thermo-mechanical stability suggests developing novel oxidation-resistant MC-UHTCs in compositional space for reusable space vehicle applications.     

High-temperature erosion resistance of ZrB2-based ceramic coating for lightweight carbon/carbon composites under simulated atmospheric re-entry conditions by high frequency plasma wind tunnel test

A double-layers ZrB₂-based ceramic coatings containing ZrB₂, MoSi₂, SiCw and borosilicate glass were prepared on lightweight and porous ZrB₂-modified carbon–bonded carbon fiber composites (CBCFs/ZrB₂) by a trace oxygen sintered technology. The high-temperature erosion resistance of the coated CBCFs/ZrB₂ was investigated under simulated atmospheric re-entry conditions by a high frequency plasma wind tunnel test with a heat fluxes of 1.3 MW/m and stagnation pressure of 1.2kPa. The results showed that the coated composites exhibited excellent ablation resistance for 800 s and the different positions of coating exhibit different response temperatures and microstructures. After ablation, the micrographs of marginal position show more compact than central position at sample surface. In addition, the side of the samples exhibits crack region, B₂O₃ precipitates region and virgin coating region.     

Ablation resistance of HfB2-SiC coating prepared by in-situ reaction method for SiC coated C/C composites

To improve the ablation resistance of SiC coating, HfB₂-SiC coating was prepared on SiC-coated carbon/carbon (C/C) composites by in-situ reaction method. Owing to the penetration of coating powders, there is no clear boundary between SiC coating and HfB₂-SiC coating. After oxyacetylene ablation for 60 s at heat flux of 2400 kW/m², the mass ablation rate and linear ablation rate of the coated C/C composites were only 0.147 mg/s and 0.267 µm/s, reduced by 21.8% and 60.0%, respectively, compared with SiC coated C/C composites. The good ablation resistance was attributed to the formation of multiple Hf-Si-O glassy layer including SiO₂, HfO₂ and HfSiO₄.     

Integrative improvement on thermophysical properties and ablation resistance of laminated carbon/carbon composites modified by in situ grown HfC nanowires onto carbon fiber cloths

HfC nanowires modified carbon fiber cloth laminated carbon/carbon (HfC_nw-C/C) composites were fabricated by in situ growth of HfC nanowires on carbon cloths via catalytic CVD, followed with lamination of the cloths and densification by pyrolytic carbon (PyC). Morphologies, thermal conductivity, coefficient of thermal expansion (CTE), and ablation resistance of the composites were investigated. Due to the loading of HfC nanowires, the matrix PyC with low texture was obtained; the thermal conductivity of the composites in the Z direction was enhanced from 100℃ to 2500℃; CTE along the X–Y direction also decreased in the range of 2060 ℃ – 2500 ℃, which reaches the maximum of 24 % at 2500℃. Moreover, the 20s-ablation-resistance of HfC_nw-C/C composites exhibits mass and linear ablation rates of 5.3 mg/s and 21.0 μm/s, which are 40 % and 37 % lower than those of pure C/C composites, respectively. Our work shows laminated HfC_nw-C/C composites are a promising candidate for high-temperature applications.     

Bimodal microstructure ZrB2-MoSi2 coating prepared by atmospheric plasma spraying for carbon/carbon composites against long-term ablation

To protect carbon/carbon composites against long-term ablation, a bimodal microstructure ZrB₂-MoSi₂ coating, consisting of an outer ZrB₂-MoSi₂ layer modified by Y₂O₃ and an inner basal ZrB₂-MoSi₂ layer, was prepared by atmospheric plasma spraying. The microstructure, phase composition and ablation resistance of the proposed coating were investigated in detail. Results showed that the bimodal coating maintained integrity in structure except for phase composition. There was no visible interlayer between the inner ZrB₂-MiSi₂ layer and the outer modified one. Mass ablation rate of the bimodal microstructure ZrB₂-MoSi₂ coated C/C composites was −2.02 × 10⁻³ g/s under an oxyacetylene flame ablation at 1873 K for 600 s, which exhibited better ablation resistance than a single ZrB₂-MoSi₂ coating. The excellent ablation resistance was ascribed to the positive effect of Y₂O₃, which not only pined in the glassy phase and alleviated the volatilization of SiO₂ glass phase by reacting with SiO₂ to form high viscosity of Y₂SiO₅, but also stabilized ZrO₂ and promoted its recrystallization and growth.     

Study on properties of Hf6Ta2O17/Ta2O5 system: A potential candidate for environmental barrier coating (EBC)

Ta₂O₅ doped Hf₆Ta₂O₁₇ system (Hf₆Ta₂O₁₇/Ta₂O₅) is considered to have potential application prospect in the field of aero-engine. We herein focus on the thermo-physical, mechanical properties and CMAS corrosion resistance of Hf₆Ta₂O₁₇/Ta₂O₅ to systematically evaluate the possibility for the application of environmental barrier coating (EBC). By changing the content of Ta₂O₅, the gradient adjustment of thermal expansion coefficient can be realized while maintaining low thermal conductivity (1.5–2.2 W/(m·K)). The introduction of Ta₂O₅ significantly reduces the modulus and improves the fracture toughness. Single-phase Hf₆Ta₂O₁₇ shows excellent corrosion resistance against molten calcium-magnesium-alumina-silicate (CMAS). The crystallization of CaTa₂O₆ and HfSiO₄ is the important factor to prevent further corrosion. The introduction of Ta₂O₅ weakens the ability to prevent Si penetration and greatly increases the thickness of the corrosion layer. The results highlight the merit of Hf₆Ta₂O₁₇/Ta₂O₅ system as potential candidate for multi-layer gradient coating on the surface of ceramic matrix composites.     

Preparation and properties of TaSi2-MoSi2-ZrO2-borosilicate glass coating on porous SiCO ceramic composites for thermal protection

A TaSi₂-MoSi₂-ZrO₂-borosilicate glass (TMZG) coating was prepared by a slurry method on a carbon fibre-reinforced porous silicon oxycarbide (SiCO) ceramic composite for thermal protection. The coating was well adhered to the substrate and showed a uniform thickness of approximately 375?µm. After thermal cycling from 1873?K to room temperature six times (total oxidation time of 180?min), the shape and dimension of the TMZG remain almost unchanged with no cracking or peeling of the coating surface. The TMZG-coated sample exhibits good oxidation resistance because of a molten SiO₂ film with ZrSiO₄ particles distributed on the outer layer of the coating. After ablation testing under an oxyacetylene flame at 1927?K for 90?s, the linear ablation rate of the TMZG coated sample are 8.33?×?10^?4 mm/s. The whole coating retains integrity, preventing substrate ablation during the test. The TMZG coating with excellent temperature resistance shows broad applicability in thermal insulation materials.     

Ablation behavior and thermal protection performance of TaSi2 coating for SiC coated carbon/carbon composites

For extending application of TaSi₂ in complex coating system, the ablation behavior and thermal protection performance of TaSi₂ coating is studied to evaluate its potential applications for anti-ablation protection of C/C composites. TaSi₂ coating is prepared by supersonic atmospheric plasma spraying (SAPS) on the surface of SiC coated carbon/carbon (C/C) composites. Phase variation and microstructure are characterized by XRD and SEM, respectively. During the ablation process, the coating is quickly oxidized to SiO₂ and Ta₂O₅ accompanied by a lot of heat consumption. The linear and mass ablation rates are 0.9?µm?s^?1 and ??0.4?mg?s^?1 after ablation for 80?s, respectively Results show that the prepared coating possesses optimal ablation performance under the heat flux of 2.4?MW/m². Moreover, the TaSi₂ coating and SiC inner coating have good chemical and physical compatibility during the ablation process. Therefore, the excellent performance of TaSi₂ coating during the ablation process makes it a candidate for anti-ablation protection for C/C composites.     

Ablation behavior of single and alternate multilayered ZrC-SiC coatings under oxyacetylene torch

Based on the investigation of ablation behavior and thermal stress of the monolayered ZrC-SiC coatings with different SiC amounts, an alternate coating consisting of 4 sublayers with 10 and 70 vol.% SiC was prepared on SiC-coated carbon/carbon (C/C) composites through plasma spraying technique. Ablation tests were carried out under oxyacetylene torch with a heat flux of 2.38 MW/m². The alternate coating could offer 90 s ablation shield for C/C composites, providing superior ablation properties than all monolayered coatings. The improved ablation resistance is mostly induced by the fact that the outmost scale with abundant ZrO₂ particles was able to better endure the mechanical denudation from the torch. Moreover, due to the indirect contact with torch, the innermost sublayers were placed into relatively mild environment, thereby most of Si-based oxides could be retained and further hinder oxygen transport inward during ablation.