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.     

A mullite/SiC oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites against oxidation, a mullite coating was prepared on SiC precoated C/C composites by a hydrothermal electrophoretic deposition process. The phase composition, microstructure and oxidation resistance of the prepared mullite/SiC coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to achieve crack-free mullite coatings. The mullite/SiC coating displays excellent oxidation resistance and can protect C/C composites from oxidation at 1773 K for 322 h with a weight loss rate of only 4.89 × 10^?4 g/cm² h. The failure of the multi-layer coatings is considered to be caused by the volatilization of silicate glass layer, the formation of microholes and microcracks on the coating surface and the formation of penetrative holes between the SiC bonding layer and the C/C matrix at 1773 K. The corresponding high temperature oxidation activation energy of the coated C/C composites at 1573–1773 K is calculated to be 111.11 kJ/mol.     

MoSi2-based oxidation protective coatings for SiC-coated carbon/carbon composites prepared by supersonic plasma spraying

To protect carbon/carbon (C/C) composites against oxidation, MoSi₂-based oxidation protective coatings for SiC-coated carbon/carbon composites were prepared on them by supersonic plasma spraying. The MoSi₂-based coatings primarily consist of MoSi₂, Mo₅Si₃ and glassy SiO₂. Only a few pinholes and some microcracks are observed on the surface and no through-thickness cracks penetrate the cross-section. Weight loss of the MoSi₂-based coated specimens is only 1.14% after 400 h oxidation in air at 1773 K and the coated C/C composites remain intact after 11 thermal cycles between 1773 K and room temperature. The outstanding anti-oxidation ability is mainly attributable to the formation of SiO₂-based layer on the surface of MoSi₂-based coatings.     

A SiC–Si–ZrB2 multiphase oxidation protective ceramic coating for SiC-coated carbon/carbon composites

In order to improve the oxidation protective ability of SiC-coated carbon/carbon (C/C) composites, a SiC–Si–ZrB₂ multiphase ceramic coating was prepared on the surface of SiC-coated C/C composite by the process of pack cementation. The microstructures of the coating were characterized using X-ray diffraction and scanning electron microscopy. The coating was found to be composed of SiC, Si and ZrB₂. The oxidation resistance of the coated specimens was investigated at 1773 K. The results show that the SiC–Si–ZrB₂ can protect C/C against oxidation at 1773 K for more than 386 h. The excellent oxidation protective performance is attributed to the integrity and stability of SiO₂ glass improved by the formation of ZrSiO₄ phase during oxidation. The coated specimens were given thermal shocks between 1773 K and room temperature for 20 times. After thermal shocks, the residual flexural strength of the coated C/C composites was decreased by 16.3%.     

Design of an inlaid interface structure to improve the oxidation protective ability of SiC–MoSi2–ZrB2 coating for C/C composites

To improve the oxidation protective ability of SiC–MoSi₂–ZrB₂ coating for carbon/carbon (C/C) composites, pre-oxidation treatment and pack cementation were applied to construct a buffer interface layer between C/C substrate and SiC–MoSi₂–ZrB₂ coating. The tensile strength increased from 2.29 to 3.35 MPa after pre-oxidation treatment, and the mass loss was only 1.91% after oxidation at 1500 °C for 30 h. Compared with the coated C/C composites without pre-oxidation treatment, after 18 thermal cycles from 1500 °C and room temperature, the mass loss was decreased by 30.6%. The improvements of oxidation resistance and mechanical property are primarily attributed to the formation of inlaid interface between the C/C substrate and SiC–MoSi₂–ZrB₂ coating.     

The oxidation behavior of MoSi2-CrSi2-Si/SiC coating for C/C composites in H2O-O2-Ar atmosphere: Experiment and first-principle investigation

The MoSi₂-CrSi₂-Si/SiC multi-component coating was prepared by two-step pack cementation on carbon/carbon composites. To investigate the effect of water vapor on the anti-oxidation of the coated samples, two kinds of atmosphere (50%H₂O-50%O₂, 50%O₂-50%Ar) were designed for comparison with a total pressure of 1 atm at 1773 K. The results showed that, after being tested for 10 h, the weight loss of the coated samples in O₂+Ar and H₂O+O₂ were 0.243% and 0.436% respectively. The reasons for different weight losses can be attributed to the water vapor, which could degrade the protective ability of the glass layer formed by SiO₂ and Cr₂O₃ and thereby accelerate the oxidation of MoSi₂ and CrSi₂. Based on the Mulliken analysis calculated by the first principle, the corresponding water vapor corrosion resistance of the prepared coating was in the following order: SiC>MoSi₂>CrSi₂, which was consistent with the experimental phenomenon.     

High-temperature erosion resistance and aerodynamic oxidation mechanism of multi-layer MoSi2–CrSi2–Si/SiC coated carbon/carbon composites in a wind tunnel at 1873 K

The high-temperature erosion resistance of multi-layer MoSi₂–CrSi₂–Si/SiC coated carbon/carbon (C/C) composites was investigated in a wind tunnel. To study the aerodynamic oxidation mechanism and analyze the failure of the coated C/C composites, the shear force and bending moment distribution of the tested specimens in a wind tunnel were calculated. Flexural strengths and thermogravimetric analysis of the coated specimens were measured. These results show that the multi-layer MoSi₂–CrSi₂–Si/SiC antioxidation coating can protect the C/C composites from high-temperature erosion in a wind tunnel at 1873 K for more than 86 h. Due to the high viscosity of SiO₂, the multi-layer coating lacked effective oxidation resistance from 900 to 1500 K, resulting in extensive mechanical damage and the fracture of the tested specimens.     

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.     

Effect of co-deposited SiC nanowires and carbon nanotubes on oxidation resistance for SiC-coated C/C composites

To protect carbon/carbon composites (C/Cs) against oxidation, SiC coating toughened by SiC nanowires (SiCNWs) and carbon nanotubes (CNTs) hybrid nano-reinforcements was prepared on C/Cs by a two-step technique involving electrophoretic co-deposition and reactive melt infiltration. Co-deposited SiCNWs and CNTs with different shapes including straight-line, fusiform, curved and bamboo dispersed uniformly on the surface of C/Cs forming three-dimensional networks, which efficiently refined the SiC grains and meanwhile suppressed the cracking deflection of the coating during the fabrication process. The presence of SiCNWs and CNTs contributed to the formation of continuous glass layer during oxidation, while toughed the coating by introducing toughing methods such as bridging effect, crack deflection and nanowire pull out. Results showed that after oxidation for 45 h at 1773 K, the weight loss percentage of SiC coated specimen was 1.35%, while the weight gain percentage of the SiCNWs/CNTs reinforced SiC coating was 0.03052% due to the formation of continuous glass layer. After being exposed for 100 h, the weight loss percentage of the SiCNWs/CNTs reinforced SiC coating was 1.08%, which is relatively low.     

Fabrication of a SiC/Si/MoSi2 multi-coating on graphite materials by a two-step technique

A SiC/Si/MoSi₂ multi-coating for graphite materials was prepared by a two-step technique. SiC whisker reinforcement coating was produced by pyrolysis of hydrogen silicone oil (H-PSO) at 1600 °C, and then the dense coating was formed by embedding with the powder mixture of Si, graphite and MoSi₂ at 1600 °C in argon atmosphere. The microstructure, thickness, phase and oxidation resistance of the coating were investigated. Research results showed that, the phase of multi-coating was composed of SiC, Si and MoSi₂. The thickness of the coating was about 300 μm. In addition, the coating combined with matrix well, and surface was continuous and dense. The oxidation pretreatment experiment was carried out in the static air at 1400 °C for 4 h before thermal failure tests and the specimens had 0.045% weight gain. Subsequent thermal failure tests showed that, the SiC/Si/MoSi₂ multi-coating had excellent anti-oxidation property, which could protect graphite materials from oxidation at 1000 °C in air for 12 h and the corresponding weight loss was below 1 wt%. Based on the surface morphology changes, oxidation pretreatment experiment and thermal failure tests enhanced densification of multi-coating and the coating had a certain self-healing ability.     

HfC nanowires to improve the toughness and oxidation resistance of Si-Mo-Cr/SiC coating for C/C composites

To improve the oxidation resistance of carbon/carbon (C/C) composites, a dense HfC nanowire-toughened Si-Mo-Cr/SiC multilayer coating was prepared by chemical vapor deposition (CVD) and pack cementation. The microstructure, thermal shock and isothermal oxidation resistance of the coating were investigated. HfC nanowires could improve the toughness of the coating and suppress the coating cracking. After incorporating HfC nanowires in the coating, both of the thermal shock and isothermal oxidation resistance of the coating were obviously improved. The multilayer coating with HfC nanowires could effectively protect C/C composites at 1773 K for 270 h, whose weight loss is only 0.19%. The good oxidation resistance is mainly attributed to the formation of a compound glass layer containing SiO₂ and Cr₂O₃.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

Microstructure and oxidation resistance of C-AlPO4–mullite coating prepared by hydrothermal electrophoretic deposition for SiC-C/C composites

Oxidation resistant C-AlPO₄–mullite coating for SiC pre-coated carbon/carbon composites (SiC-C/C) was prepared by a novel hydrothermal electrophoretic deposition process. The phase composition, surface and cross-section microstructure of the as-prepared multi-layer coatings were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of deposition voltage on phase composition, microstructure and oxidation resistance of the as-prepared coatings was particularly investigated. Results show that the outer layer coating mainly composed of C-AlPO₄ and mullite phase can be achieved after the hydrothermal electrophoretic deposition. The thickness, density and anti-oxidation property of the C-AlPO₄–mullite coating was improved with the increase of deposition voltage from 160 V to 200 V. The multi-layer coating prepared at a voltage of 200 V exhibit excellent anti-oxidation property, which can effectively protect C/C composites from oxidation in air at 1773 K for 324 h with a weight loss of 1.01%. The failure of the multi-layer coatings is due to the generation of cross-holes in the coating, which cannot be self-cured by the metaphosphate and silicate glass layer after long time oxidation at 1773 K.     

Physical,thermal and ablative performance of CVI densified urethane-mimetic SiC preforms containing in situ grown Si3N4 whiskers

A two-step process has been developed for silicon carbide (SiC) coated polyurethane mimetic SiC preform containing silicon nitride (Si₃N₄) whiskers. SiC/Si₃N₄ preforms were prepared by pyrolysis/siliconization treatment at 1600 °C, of powder compacts containing rigid polyurethane, novolac and Si, forming a porous body with in situ grown Si₃N₄ whiskers. The properties were controlled by varying Si/C mole ratios such as 1–2.5. After densification using a chemical vapour infiltration, the resulting SiC/Si₃N₄/SiC composites showed excellent oxidation resistance, thermal conductivity of 4.32–6.62 Wm⁻¹ K⁻¹, ablation rate of 2.38 × 10⁻³ − 3.24 × 10⁻³ g cm⁻² s and a flexural strength 43.12–55.33 MPa for a final density of 1.39–1.62 gcm⁻³. The presence of a Si₃N₄ phase reduced the thermal expansion mismatch resulting in relatively small cracks and well-bonded layers even after ablation testing. This innovative two-step processing can provide opportunities for expanded design for using SiC/Si₃N₄/SiC composites being lightweight, inexpensive, homogeneous and isotropic for various high temperature applications.     

Enhanced bonding strength and thermal cycling performance of MoSi2–CrSi2–SiC–Si coating for carbon/carbon composites by surface modification via blasting treatment

Before the preparation of MoSi₂–CrSi₂–SiC–Si coating, blasting treatment of carbon/carbon (C/C) composites, as a surface modification method, was conducted under oxyacetylene torch. MoSi₂–CrSi₂–SiC–Si coating was prepared on the treated C/C composites by pack cementation, where an interlock interface was formed between the coating and the C/C substrate. After blasting treatment, the thermal expansion coefficient mismatch between the coating and C/C substrate was alleviated efficiently, and the bonding strength of the coating was increased by 45.6% and reached 26.2 MPa. To simulate the real working condition, thermal cycling test was conducted under oxyacetylene torch from 1600 °C to room temperature to construct an environment of combustion gas erosion. Due to the improvement of bonding strength and the alleviation of thermal expansion coefficient mismatch between the coating and the C/C substrate, thermal cycling performance of MoSi₂–CrSi₂–SiC–Si coating was enhanced. After 25 thermal cycles, the mass loss of the coated C/C composites without blasting treatment was up to 2.4%, and the C/C substrate was partially exposed. In contrast, the mass loss of the coated C/C composites with blasting treatment was only 1.1%.     

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.     

Mullite whisker toughened mullite coating to enhance the thermal shock resistance of SiC pre-coated carbon/carbon composites

In order to improve the thermal shock resistance of the coated carbon/carbon (C/C) composites, a mullite whisker toughened mullite coating was fabricated on the surface of SiC pre-coated C/C composites (SiC-C/C) by molten-salt method with a later hot dipping process. The phase compositions, surface and cross-section microstructures, high temperature thermal shock resistance of the as-prepared multi-layer coatings were investigated. Results show that the introduction of mullite whiskers can effectively improve the density of the mullite outer coating and decrease the cracking of the coating during the thermal shock cycle process. After 100 times thermal shock cycles between 1773 K and room temperature, only 1.87 × 10⁻³ g cm⁻² weight loss has been detected, indicating the achievement of the excellent thermal shock resistance.     

In situ pressureless sintering of SiC/MoSi2 composites

SiC-reinforced MoSi₂ composites have been successfully prepared by in situ pressureless sintering from elemental powders of Mo, Si and C. Meanwhile, the evolutions of the samples’ microstructure and phase at different temperatures were investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS). It can be seen that at the temperature of 1100 °C, the main phases were Mo and Si, accompanying with a small amount of rich molybdenum products Mo₅Si₃ and Mo₃Si. Then the main phases changed to MoSi₂ and SiC when the sintering temperature reached 1300 °C. Finally we obtained MoSi₂/SiC composites with well-dispersed SiC particles after sintering at the temperature of 1550 °C for 120 min. The evolution of porosity in these composites fits the porosity reduction model well developed by Pines and Bruck, which revealed the particle agglomeration in the composites. The flexural strength and fracture toughness of 10% SiC/MoSi₂ composites were up to 274.5 MPa and 5.5 MPa m^1/2, increased by approximately 40.8% and 30.6% compared with those of monolithic MoSi₂, respectively.     

Oxidation of ZrB2–SiC ultra-high temperature composites over a wide range of SiC content

The oxidation performance of ZrB₂–SiC ultra-high temperature ceramics with SiC content ranging from 20 to 80 vol% has been evaluated at 1773 K for 50 h and at 2073 K for 20 min. Oxidation reaction pathways were interpreted using volatility diagrams of the ZrB₂–SiC system. At 1773 K for 50 h, all ZrB₂–SiC composites from 20 to 80 vol% SiC formed a protective SiO₂ surface coating. Samples with ≤50 vol% SiC developed a distinguishable SiC-depleted layer at 1773 K and 2073 K. High temperature torch testing for 20 min at approximately 2073 K revealed that samples with ≥65 vol% SiC exhibit a depression under the torch flame. Samples rich in ZrB₂ were dominated by a ZrO₂ layer after a similar exposure. The overall weight density of ultra-high temperature ceramics can be reduced with improved oxidation performance at 1773 K by adding at least 65 vol% SiC.     

Effect of temperature gradient on the erosion behavior of SiC coating for carbon/carbon composites in a combustion environment

SiC coating with a thickness of 50–70 µm was prepared on the surface of C/C composites by in-situ reaction method. The SiC coated C/C composites were then tested in a wind tunnel where a temperature gradient from 200 to 1600 °C could be obtained to investigate their erosion behavior. The results of wind tunnel test indicated that the service life of C/C composites was prolonged from 0.5 to 44 h after applying the SiC coating. After the wind tunnel test, three typical oxidation morphologies, including glassy SiO₂ layer, porous SiO₂ layer and clusters of honeycomb-like SiO₂ grains, were found on the SiC coated C/C composites. With the decrease of oxidation temperature, the amount of glassy SiO₂ declined and the thermal stress increased, which induced the cracking followed by the degradation of the SiC coating.