Oxidation protection of carbon/carbon composites with SiC/indialite coating for intermediate temperatures

In order to improve the oxidation resistance of carbon/carbon composites at intermediate temperatures, a novel double-layer SiC/indialite coating was prepared by a simple and low-cost method. The internal SiC transition layer was prepared by pack cementation and the external indialite glass–ceramic coating was produced by in situ crystallization of ternary MgO–Al₂O₃–SiO₂ glass. The microstructures and morphologies of coating were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Oxidation resistance of the as-coated C/C composites was evaluated in ambient air at temperature from 800 °C to 1200 °C. Nearly neglectable mass loss was measured after 100 h isothermal oxidation test, indicating that SiC/indialite coating possesses excellent oxidation protection ability. The as-coated samples have a good thermal shock resistance and no obvious damage was found in the coating even after suffered more than 11 thermal cycles between test temperature and room temperature. The oxidation protection mechanism of this coating was also discussed.     

Mullite-Al2O3-SiC oxidation protective coating for carbon/carbon composites

In order to exploit the unique high temperature mechanical properties of carbon/carbon (C/C) composites, a new type of oxidation protective coating has been produced by a two-step pack cementation technique in an argon atmosphere. XRD analysis showed that the internal coating obtained from the first step was a gradient SiC layer that acts as a buffer layer, and the multi-layer coating formed in the second step was an Al₂O₃-mullite layer. It was found that the as-received coating characterized by excellent thermal shock resistance on the surface of C/C composites during exposure to an oxidizing atmosphere at 1873 K, could effectively protect the C/C composites from oxidation for 45 h. The failure of the coating is due to the formation of bubble holes on the coating surface.     

Microstructure and anti-oxidation properties of multi-composition ceramic coatings for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at elevated temperature, an effective WSi₂-CrSi₂-Si ceramic coating was deposited on the surface of SiC coated C/C composites by a simple and low-cost slurry method. The microstructures of the double-layer coatings were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy analyses. The coating exhibited excellent oxidation resistance and thermal shock resistance. It could protect C/C composites from oxidation in air at 1773 K for 300 h with only 0.1 wt.% mass gain and endure the thermal shock for 30 cycles between 1773 K and room temperature. The excellent anti-oxidation ability of the double-layer WSi₂-CrSi₂-Si/SiC coating is mainly attributed to the dense structure of the coating and the formation of stable vitreous composition including SiO₂ and Cr₂O₃ produced during oxidation.     

SiC coating with high crack resistance property for carbon/carbon composites

A novel SiC coating with a relatively high crack resistance property (crack extension force (G_C): 12.0 J·m^?2) and outstanding thermal shock resistance was achieved merely by pack cementation. Compared with the conventional SiC coating with Al₂O₃ addition (AOSC2), SiC coating with Al–B–C additions (ABSC2) possesses refined and denser microstructure owing to different effects in promoting SiC densification under different additions. Therefore, the improvement in microstructures results in superior mechanical capabilities, antioxidation performance (900 °C), and thermal shock resistance (between 1500 °C and room temperature).     

SiC-Si coating with micro-pores to protect carbon/carbon composites against oxidation

To improve the oxidation resistance of carbon/carbon (C/C) composites at high temperatures, a SiC-Si coating with micro-pores was prepared by slurry and heat-treatment on the surface of C/C composites with SiC-Si inner coating acquired by pack cementation (PC). The microstructure, phase composition, element distribution, and anti-oxidation properties of the dual-layer SiC-Si coating were investigated. The results show that a SiO₂-SiC inlay structure was formed during the oxidation process, due to a large amount of SiO₂ rapidly generated by the oxidation of SiC particles in the porous coating. The coating with this structure could inhibit the cracking of SiO₂ glass and had a good resistance to oxygen diffusion. Moreover, the crack propagation was blocked by the remaining micro-pores of the coating. The coating could protect C/C composites against oxidation for 846 h only with the mass loss of 0.16 % at 1773 K in air.     

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.     

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.     

Catalytic Fabrication of SiC/SiO2 coated graphite and its behaviour in Al2O3–C castable systems

SiC/SiO₂ coated graphite was prepared via a combined sol-gel coating and catalytic conversion route, using graphite flake and tetraethyl orthosilicate as the starting materials, and Fe(NO₃)₃·9H₂O as the catalyst precursor. X-ray diffraction analysis and microstructural examination revealed that a homogeneous coating comprising SiC and cristobalite (SiO₂) and covering the whole surface of graphite was formed. As prepared SiC/SiO₂ coated graphite exhibited better oxidation resistance and water wettability than its uncoated counterpart. Also, oxidation resistance and slag corrosion resistance of a model Al₂O₃–C castable using coated graphite as a carbon source were better than in the case of its counterpart using uncoated graphite.     

Oxidation and thermal shock resistant properties of Al-modified environmental barrier coating on SiCf/SiC composites

SiC_f/SiC ceramic matrix composites (CMCs) are being widely used in the hot-sections of gas-turbines, especially for aerospace applications. These CMCs are subjected to surface recession if exposed to heat-corrosion. In this research, an alternative environmental barrier coating (EBC) is introduced to protect the SiC_f/SiC CMC from high temperature degradation that is, Al film was deposited on the surface of SiC_f/SiC CMC followed by heat-treatment in a vacuum. After that, a dense Al₂O₃ overlay was in-situ synthesized on the surface of CMC, and in this process the microstructure evolution of SiC_f/SiC CMC was analyzed. The oxidation and thermal shock resistance were characterized, showing that the Al-modified SiC_f/SiC CMC has a better oxidation resistance, because the dense Al₂O₃ overlay can hinder oxygen diffusion from environment. What is more, the water-quenching testes show that the Al-modified SiC_f/SiC CMC has a good spallation resistance.     

Thermal stability and oxidation resistance of C/Al2O3 composites fabricated from a sol with high solid content

To improve fracture toughness of monolithic Al₂O₃ ceramics, three-dimensional carbon fiber preform was used as reinforcement, and the C/Al₂O₃ composites without interfacial coating were fabricated through vacuum impregnation-drying-heat treatment route with an Al₂O₃ sol as starting material. Characteristics of the Al₂O₃ sol with high solid content were firstly analyzed. Then thermal stability and oxidation resistance of the C/Al₂O₃ composites were investigated. It is found that the Al₂O₃ sol is an appropriate raw material for the fabrication of C/Al₂O₃ composites. The C/Al₂O₃ composites with a total porosity of 15.5% show a flexural strength of 208.5 MPa and a fracture toughness of 8.1 MPa m^1/2. Strength loss is observed after the composites were annealed at 1400 °C and 1600 °C under inert atmosphere. Oxidation resistance of the C/Al₂O₃ composites is unsatisfactory because of the existence of open pores and microcracks. When Al₂O₃ matrix was modified with SiO₂, the oxidation resistance is remarkably improved due to the viscous flow effect of SiO₂.     

Oxidation resistance of a gradient self-healing coating for carbon/carbon composites

A gradient self-healing coating consisting of three layers, SiC-B₄C/SiC/SiO₂, was examined as a multilayer protection for carbon/carbon composites. The inner layer was made of B₄C and β-SiC, the middle layer was a SiC based layer, and the outer layer was SiO₂ as an airproof layer. Both inner and middle layers were produced to be diphase structure by a pack cementation technique, and the outer airproof layer was prepared by hydrolyzing tetraethylorthosilicate. SEM and EDS investigations showed that the coating had a compositional gradient between B₄C and SiC. The coating showed great self-healing properties from 500 °C to 1500 °C. The weight loss rate of the coated composites was less than 1.3% after 50 h at 1500 °C, and coating represented excellent thermal shock resistance at 1500 °C. The oxidation kinetics of coated carbon/carbon composites showed that the Arrhenius curve consisted of three parts with two broken points at about 700 °C and 1100 °C, and the three parts corresponded to three different self-healing mechanisms in different temperature regions.     

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.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.     

The anti-oxidation mechanism of SiCf/SiC–B4C modified with Al2O3 in wet atmosphere based on machine learning

To study the anti-oxidation mechanism of SiC_f/SiC–B₄C modified with Al₂O₃ in wet atmosphere, the damage evolution of composites after oxidation was explored by unsupervised machine learning technology (k-means). Results display that the mean feature values of cluster-1 (some small cracks in oxidation layer and matrix as well as fiber debonding) in composites modified with Al₂O₃ are larger than that in virgin after oxidation. Meanwhile, as the oxidation time increases, the concentrated area of cluster-2 (fiber breakage, cracks in axial and transverse yarns) in both composites gradually shifts toward the direction of high mean feature values. Because Al₂O₃ can protect the BN from oxidation, the increase of peak frequency and risetime of composites modified with Al₂O₃ is less than that of virgin. Moreover, the proportion of cluster-3 (big transverse matrix cracks) in composites modified with Al₂O₃ is always more than that of virgin. Based on the research results, the corresponding relationship between each damage behavior and each cluster is thoroughly built.     

Preparation of multilayered C–Si–Al2O3 coatings on continuous carbon fibers and C–Si–Al2O3-coated carbon-fiber-reinforced hydroxyapatite composites

Multilayered C–Si–Al coatings with various morphologies were deposited on carbon fibers (CFs) using magnetron sputtering. The thickness of the coatings was increased from 0.5 to 1.5 μm by magnetron sputtering between 90 and 120 min. C–Si–Al coatings of suitable thickness were heat-treated at 600 °C and transformed into C–Si–Al₂O₃ coatings by one-step anodic oxidation (AO). The oxidation time for the one/two-step anodic oxidation and the ratio of oxidation time for the two-step anodic oxidation significantly influenced the morphologies of the C–Si–Al₂O₃(AO) coatings. Al₂O₃ coatings with satisfactory morphologies and structures were prepared by two-step anodic oxidation with a total time of 30 min and a ratio of 1:1 between the initial and secondary oxidation times. The multilayered C–Si–Al₂O₃(AO) coatings were modified to C–Si–Al₂O₃ coatings by secondary heat treatment at 1050 °C. Subsequently, hot-press sintering was used to prepare CFs with multilayered C–Si–Al₂O₃ coating-reinforced hydroxyapatite (CF/C–Si–Al₂O₃/HA) composites. The multilayered C–Si–Al₂O₃-coated CFs demonstrated good resistance to oxidation and thermal shock. This could effectively protect CFs from oxidative damage and maintain its strengthening effect during sintering. The multilayered C, Si, and Al₂O₃ coatings effectively reduced the difference between the coefficient of thermal expansion of the CFs and HA matrixes. The interfacial gaps between the multilayered coatings and HA were reduced. This could enhance the mechanical performance of the composites. The CF/C–Si–Al₂O₃/HA composites exhibited improved mechanical properties with a bending strength of 83.94 ± 12.29 MPa, and fracture toughness of 2.45 ± 0.08 MPa m^1/2. This study can broaden the application of CF/C–Si–Al₂O₃/HA biocomposites as bone-repair materials and help obtain CF-reinforced composites with excellent mechanical properties that are fabricated or serviced at high temperatures.     

Ablation resistance of SiC‐modified ZrC coating prepared by SAPS for SiC‐coated carbon/carbon composites

This study evaluated the ablation resistance of ZrC/SiC coating for carbon/carbon (C/C) composites at different temperatures and heat fluxes, which improved the researches on ultra‐high temperature oxidation of ZrC/SiC system. Results showed that the protection of coating depended on temperature and heat flux. Ablation test for 120 seconds under heat flux of 2.4 MW/m² at 2270°C revealed a good ablation resistance, with the linear ablation rate reduced by 96.4% and the mass gain rate increased by 383.3% compared with those of pure ZrC coating. The good ablation resistance was attributed to the formation of dense oxide scale surface. SiC could improve the compactness of the oxide scale at this temperature by forming SiO₂. A dense scale could not form at 2105°C after ablation for 120 seconds, resulting in a dissatisfactory ablation resistance of the coating. After ablation for 120 seconds at 1738°C, the coating was integrated due to the protection of glassy SiO₂ encapsulated ZrO₂. The coating could not resist the strong shear force from the flame at heat flux of 4.2 MW/m² and was severely damaged after ablation for 60 seconds.     

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.     

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%.     

Aluminum oxide particles/silicon carbide whiskers’ synergistic effect on thermal conductivity of high-density polyethylene composites

Aluminum oxide (Al₂O₃) particles and silicon carbide (SiC) whiskers improved the thermal conductivity of high-density polyethylene (HDPE). To improve the dispersion of inorganic fillers in the matrix, 5 wt% of maleic anhydride-modified polyethylene was added into HDPE as a compatibilizer, and the hybrid matrix was denoted as mHDPE. The thermal conductivity, heat resistance, and tensile properties of resulting HDPE composites were characterized. The results showed that the thermal conductivity reached its maximum value of 0.8876 W/(m K) at 1/4 weight ratio of Al₂O₃/SiC, which was 110.3, 54.8, and 8.8% higher than that of pure HDPE, mHDPE/Al₂O₃, and mHDPE/SiC composites, in the order given, indicating that hybrid fillers have synergistic effect on the thermal conductivity of HDPE composites. Moreover, they also have a synergistic effect on the heat resistance and Young’s modulus. As the SiC content increases, the heat resistance of the composites increases at first and then falls, and the maximum VST is reached at an Al₂O₃/SiC weight ratio of 3/2, which is 5.4 °C higher than that of HDPE. The maximum Young’s modulus of the composites (1160 MPa) is obtained at an Al₂O₃/SiC weight ratio of 1/4, and the yield strength increases gradually as the SiC whiskers’ content increases.     

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.