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.     

Effect of processing parameters on microstructure and ablation behavior of SiC/ZrB2 coating for graphite

In this research, a SiC/ZrB₂ coating was produced on graphite by reactive melt infiltration and plasma spraying method. The coating characterization was performed using XRD analysis, electron microscopy equipped with energy dispersive spectrometer (EDS), and supersonic flame ablation test at 2073 K. The results indicated that the dense C/SiC coating with good ablation resistance can be obtained at 1873 K. The coating thickness decreased with increasing infiltration temperature. The results of ablation test showed that by increasing the infiltration temperature and holding time, weight loss and mass ablation rate decreased from 22.63% to 9.83% and 3.63 × 10⁻³ g cm⁻² s⁻¹ to 1.34 × 10⁻³ g cm⁻² s⁻¹, respectively. The results showed that by using the ZrB₂ as outer coating the ablation resistance improved remarkably. The weight loss and mass ablation rates for the SiC/ZrB₂ coating were 12.79% and 1.857 × 10⁻³ g cm⁻² s⁻¹, respectively.     

Structural design and ablation performance of ZrB2/MoSi2 laminated coating for SiC coated carbon/carbon composites

A protective coating alternated with ZrB₂ and MoSi₂ laminated layers was designed and prepared on carbon/carbon (C/C) composites with SiC inner layer by supersonic atmosphere plasma spraying. After ablated at a heat flux of 2.4 MW/m² for 30s, ZrB₂/MoSi₂ laminated coating was in good condition with a linear growth rate and mass gain rate of 1.67 μm/s and 0.44 mg/s, respectively. From the central region to the border region, the calculated residual thermal stress of ZrB₂/MoSi₂ laminated coating decreased at first and then increased rapidly, illustrating the size change of the generated laminated cracks. The alternate design of ZrB₂ layers for erosion and MoSi₂ layers for oxidation resulted in the laminated stress distribution and improved ablation resistance.     

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.     

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.     

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.     

WSi2 modified HfB2-SiC coating: Microstructure and ablation resistance

To enhance the ablation resistance of carbon/carbon composites, WSi₂ modified HfB₂-SiC coating was prepared by slurry dipping combined with vapor silicon infiltration. The effect of WSi₂ contents on the ablation performance of the coating was investigated. The results showed that the coating with 20 wt% WSi₂ had the best ablation resistance under oxyacetylene torch. During ablation, a dense Hf-Si-W-O oxide layer was covered on the coating surface, inhibiting the diffusion of oxygen. Additionally, WSi₂ with high emissivity and the formed W with good thermal conductivity played a role in reducing the ablation temperature. These effects contributed to the improvement of ablation performance.     

Boosting bonding strength of hydroxyapatite coating for carbon/carbon composites via applying tree-planting interface structure

Hydroxyapatite (HA) coated carbon/carbon composites (CC) is a potential material for orthopedic application because of the combination of good biocompatibility and mechanical properties. In this work, we synthesize a tree-planting interface which is composed of holes formed by micro-oxidized CC substrates and carbon nanotubes (CNTs) to achieve a high bonding strength of HA coating. The holes include annular gaps between carbon fiber and pyrolytic carbon, as well as irregular holes formed by oxidized pyrolytic carbon. The CNTs can grow inside the holes and extend into the HA coating. As a result, the bonding strength of HA coating with tree-planting interface achieves 11.14 ± 0.78 MPa. It increases by 181.3% comparing with the HA coating on CC without interface (3.96 ± 0.30 MPa). The in-vitro bioactivity evaluated by the response of mesenchymal stem cells (MSCs) shows promotions of cell proliferation and cell activity with increasing culture time. After applied with tree-planting interface, the HA coating with strong bonding and good bioactivity may be applied in orthopedic field in the future.     

Effect of CVI SiC content on ablation and mechanism of C/C-SiC-ZrC-Cu composites

C/C-SiC-ZrC-Cu composites were fabricated by chemical vapor infiltration, precursor infiltration-pyrolysis and vacuum-pressure infiltration methods. During Cu infiltration, the Cu_6·69Si and Cu₃Si new phases are generated through reaction between SiC and molten Cu. The formed Cu_6·69Si, Cu₃Si, ZrC and SiC phases can improve the wettability and interface combination between Cu and the doped carbon matrix. The ablation tests demonstrate that the CVI SiC content significantly affects the structure of protective oxide layer, and induces inverse effects in ablation center at 2500 ^°C and 3000 ^°C. The relatively high CVI SiC content enhances the ablation resistance of composites at 2500 ^°C, but increases the linear ablation rate at 3000 ^°C due to the excessive evaporation and mechanical denudation. During ablation, the formed Si-Zr-C-O layer underneath ablation center and the Si-Cu-C-O layer on transition or marginal areas can prevent carbon matrix from serious oxidation. After ablation for 20 s, the C/C-SiC-ZrC-Cu composites with high CVI SiC content display the best anti-ablation property at 2500 ^°C, and the ablation rates are 3.5 ± 0.1 μm/s and 3.4 ± 0.1 mg/s.     

Dynamic oxidation resistance and residual mechanical strength of ZrB2-CrSi2-SiC-Si/SiC coated C/C composites

To improve oxidation resistance of carbon/carbon (C/C) composites, a multiphase double-layer ZrB₂-CrSi₂-SiC-Si/SiC coating was prepared on the surface of C/C composites by pack cementation. Thermogravimetry analysis showed that the as-prepared coating could provide effective oxidative protection for C/C composites from room temperature to 1490 °C. After thermal cycling between 1500 °C and room temperature, the fracture behaviors of the as-prepared specimens changed and their residual flexural strengths decreased as thermal cycles increased. The specimen after 20 thermal cycles presented pseudo-plastic fracture characteristics and relatively high residual flexural strength (83.1%), while the specimen after 30 thermal cycles failed catastrophically without fiber pullout due to the severe oxidation damage of C/C substrate especially the brittleness of the reinforcement fibers.     

SiC nanowire-toughened MoSi2-WSi2-SiC-Si multiphase coating for improved oxidation resistance of C/C composites

To improve the oxidation resistance of carbon/carbon (C/C) composites at high temperature, a SiC nanowire-toughened MoSi₂-WSi₂-SiC-Si multiphase coating was prepared by chemical vapor deposition (CVD) and pack cementation. The microstructure, mechanical properties and oxidation resistance of the coating were investigated. After the introduction of SiC nanowires, the elastic modulus, hardness, and fracture toughness of the MoSi₂-WSi₂-SiC-Si coating were increased by 25.48%, 4.09% and 45.03%, respectively. The weight loss of the coated sample with SiC nanowires was deceased from 4.83–2.08% after thermal shock between 1773 K and room temperature for 30 cycles and the weight loss is only 3.24% after isothermal oxidation at 1773 K in air for 82 h. The good oxidation resistance of the coating is mainly attributed to that SiC nanowires can effectively inhibit the propagation of cracks in the coating by the toughening mechanisms including bridging and pull-out.     

Effect of TaSi2 addition on long-term ablation behavior of HfB2-SiC coating

In order to improve the thermal protective ability of HfB₂-SiC coating, TaSi₂ was introduced into the coating and its function on long-term ablation resistance was investigated by plasma ablation tests under two heat fluxes. The variations in ablation behavior and structural evolution of the coatings under low and high heat fluxes were elaborated. The results indicated that TaSi₂ significantly improved the ablation resistance of HfB₂-SiC coating due to the alleviation of volume expansion when the ablation temperature was above the melting point of Ta₂O₅ (∼1872 ℃). However, the results are reversed when the temperature was below the fusing point. The reasons for the variation were explained by the effect of temperature on the active oxidation of SiC and the oxidation behavior of TaSi₂. The improvement of the ablation resistance also can be attributed to the avoidance of the SiC-depleted region and the formation of Hf-Ta-Si-B-O compound glassy layer with low oxygen permeability and high viscosity.     

Strength degradation of SiC fibers with a porous ZrB2-SiC coating: Role of the coating porous structure

ZrB₂-SiC coatings with varied porous structures were deposited on SiC fiber tows using the sol-gel method and cured at 1400 ℃ in vacuum. Tensile strength of the coated SiC fibers were much lower than that of the uncoated fibers. The bimodal distribution in the Weibull plot of the coated SiC fibers demonstrated that the fracture of the coated fiber can be attributed to two types of defects: the porous structure of the coating and the fiber defects. Detailed morphology and microstructure characterization of the coating and fiber combined with strength calculation were carried out to investigate the individual contribution of the fiber defects and the porous coating layer respectively. The results revealed that apart from the fiber damage during the coating process the porous structure of the fiber coating has a non-negligible effect on the fiber strength, presumably due to a relatively strong bonding between the fiber and coating.     

Effect of SiCnws on flexural strength of SiCf/HfC-SiC composites after impact and ablation

SiC nanowires (SiC_nws) modified SiC_f/HfC-SiC composites were prepared by precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) methods. The microstructure, flexural strengths, impact and impact-ablation tests of the composites with and without SiC_nws were investigated. The results showed that after introducing SiC_nws, not only the retention rate of HfC ceramic produced by PIP was increased obviously, but also the fracture displacement of the modified composites was reduced due to the enhancement effect of SiC_nws at interface between SiC fiber and matrix. After impact and impact-ablation, the strength retention of SiC_nws modified composites was 91.6 % and 69.1 % respectively, higher than that of the composites without SiC_nws (85.2 % and 54.8 %). As the impact resistance of the modified composites was improved by the pull-out and bridging of SiC_nws, the ablation resistance of the impacted composites was enhanced as well.     

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.     

Exploring Hf-Ta-O precipitation upon ablation of Hf-Ta-Si-C coating on C/C composites

To improve the ablation resistance of C/C composites, a Hf-Ta-Si-C coating was fabricated by chemical vapor co-deposition. The ablation resistances of Hf-Si-C and Hf-Ta-Si-C coatings were characterized comparatively. After introducing Ta, the ablation resistance, smoothness and integrity of the ablated coating improved. Hf-Si-O and Hf-Ta-Si-O glass wires were observed. To investigate the reaction processes of the two types of glass wires, TEM observation was carried out and coupled to molecular dynamics simulations. These illustrated that Hf and Ta atoms tend gather together at high temperature. To investigate the effect of Hf-Ta-O precipitation on the ablation behavior, supplementary experiments were carried out on bulk ceramics with the same target composition, confirming that precipitation reaction could accelerated the formation of a smooth and intact oxide layer, playing a positive role in protecting C/C composites.     

Microstructure,thermophysical properties,and ablation resistance of C/HfC-ZrC-SiC composites

Carbon/carbon composites modified by HfC-ZrC-SiC were fabricated by reactive melt infiltration with the aim of improving their ablation resistance for application in aerothermal environments. Their microstructure, thermophysical and ablation properties were investigated. Results show that the thermal diffusivity decreases with increasing temperature for all composites. The thermal conductivity of the C/HfC-ZrC-SiC composites decreasing with increasing HfC molar fraction is related to decreased grain size and increased porosity, which impede phonon interaction and increase the phonon scattering. High HfC content effectively improves the oxidation and ablation resistance of the composites. C/HfC-ZrC-SiC composites containing 8.8?mol.% HfC exhibited the best ablation resistance owing to a compact and continuous HfO₂-ZrO₂ mixed layer that formed on the ablated surface.     

Microstructure and ablation resistance of ZrCxNy-modified ZrC-SiC composite coating for carbon/carbon composites

Nitrogen-modified ZrC-SiC coatings were prepared by thermal evaporation, in situ reaction, and nitriding process, and the microstructure and ablation property of the coatings were studied. The results showed that nitrogen atoms could replace the carbon atoms and fill the vacancies of ZrC. In addition, the interface of the ZrC phase was optimized. The nitrogen atom solid solution was limited on the coating surface, and the interior of the coating was composed of high-melting point ZrC and SiC ceramics. The ablation test showed a reduction in the ablation rate of the coating after nitriding due to the formation of a dense ZrO₂ layer.     

Influence of HfC nanowires on the growth behavior,microstructure and ablation resistance of CVD-HfC coating

HfC nanowire-toughened HfC ablation resistant coating was prepared on carbon/carbon composites by two steps of chemical vapor deposition. Effects of HfC nanowires on the growth behavior, microstructure and ablation resistance of the HfC coating were researched. Due to the incorporation of HfC nanowires, the deposition rate of the HfC coating was improved, the HfC coating was composed of particle-stacked crystals. After incorporating HfC nanowires, the bonding strength and fracture toughness of the HfC coating increased. HfC nanowires could restrain the crack propagation of HfC coating during ablation, contributing to improving the ablation resistance of HfC coating. After ablation for 60?s, the mass ablation rate of the HfC-coated C/C sample reduced from 0.44 to 0.26?mg/s because of the incorporation of HfC nanowires.     

Effect of SiC content on electrical,thermal and ablative properties of pressureless sintered ZrB2-based ultrahigh temperature ceramic composites

The effects of SiC content (10–40 vol.%) on electrical, thermal and ablation properties of pressureless sintered ZrB₂-SiC composites showing interfacial segregation of W-rich phases have been studied. The electrical resistivity was measured by four-probe method, whereas thermal diffusivity and coefficient of thermal expansion (CTE) were determined using laser-flash method and thermo-mechanical analyzer, respectively. Whereas thermal conductivities calculated from experimentally obtained thermal diffusivity values are found to be the highest for the ZrB₂-20 SiC composite, both electrical conductivity and CTE decrease with increasing SiC content. The specimens were subjected to thermal shock by soaking at 800–1200 °C, followed by water-quenching. Further, some specimens were exposed to oxyacetylene flame (2200 °C) for 10 min. The damage was estimated from changes in mass, Young’s modulus, and hardness. The highest thermal shock and ablation resistance have been observed for the ZrB₂-20 SiC composite, as thermal properties and formation of protective oxide scale play key role.