Effect of Y2O3 on the oxidation properties of ZrSi2/SiC coating prepared by SAPS on the carbon-carbon composites

In order to improve the oxidation resistance of carbon-carbon (C/C) composites at high temperature, different content of Y₂O₃ modified ZrSi₂/SiC coating for C/C composites were prepared by pack cementation and supersonic atmosphere plasma spraying (SAPS). Microstructure observation and phase identification of the coatings were analyzed by SEM, XRD, DSC/TG and EDS. Experimental results shown that the coating with 10?wt% Y₂O₃ effectively protected C/C composites from oxidation at 1500?°C in air for 301?h with a mass loss of 0.13% and experienced 18 thermal shock times from room temperature (RT) to 1500?°C. First, Y₂O₃ could restrain the phase transition of ZrO₂ to reduce the formation of thermal stresses of the coating; second, the random distribution of ZrO₂ ceramic particles and the formation of ZrSiO₄ enhanced the stability of the SiO₂; third, the formation of Y₂Si₂O₇ and Y₂SiO₅ could relieve the thermal mismatch between ZrSi₂-Y₂O₃ outer layer and the inner layer.     

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.     

Experimental and first-principles simulation study on oxidation behavior at 1700 °C of Lu2O3–SiC-HfB2 ternary coating for SiC coated carbon/carbon composites

The oxidation behavior and microstructure evolution of Lu₂O₃–SiC-HfB₂ ceramic coating specimen at 1700 °C were investigated systematically by experimental study and first-principles simulation. The prepared ternary coating possesses a compact morphology, which effectively defends C/C substrate against oxidation at 1700 °C for 130 h, showing a good antioxidant property. The formed HfSiO₄, Lu₂Si₂O₇, and HfO₂ with high melting points play an active role in developing the thermal stability of the oxidized scale. Besides, Lu and Hf atoms incline to diffuse into SiO₂, which enhances its structural stability. The improved thermal property of the oxidized scale for the Lu₂O₃–SiC-HfB₂/SiC ceramic coating can delay the effective delivery of oxygen inwardly and thus prolong its oxidation protection time. The quick volatilization of SiO₂ at 1700 °C induces that some glass phase evaporates with being not completely stabilized, which causes the formation of holes and the consumption of the inner coating.     

Fabrication of zero-thermal-expansion ZrSiO4/Y2W3O12 sintered body

We focused on the linear negative thermal expansion of Y₂W₃O₁₂ in a wide-temperature range and on the chemical stability of ZrSiO₄ in the fabrication of the composite material ZrSiO₄/Y₂W₃O₁₂ with a zero-thermal-expansion. The compact composed of Y₂W₃O₁₂ and ZrSiO₄ had a thermal shrinkage rate smaller than that of Y₂W₃O₁₂ and higher than that of ZrSiO₄. SEM–EDX observation clarified that the ZrSiO₄/Y₂W₃O₁₂ sintered body fabricated at 1400 °C for 10 h had a microstructure composed of ZrSiO₄ and Y₂W₃O₁₂ grains, and XRD indicated that only ZrSiO₄ and Y₂W₃O₁₂ phases existed in the sintered body. The relative density of the ZrSiO₄/Y₂W₃O₁₂ sintered body reached 92%, which was larger than that of the ZrSiO₄ sintered body because Y₂W₃O₁₂ grains could be sintered at lower temperatures. The average linear thermal expansion coefficients of the ZrSiO₄/Y₂W₃O₁₂ sintered body were −0.4 × 10⁻⁶ and −0.08 × 10⁻⁶ °C⁻¹ in the temperature ranges from 25 to 500 °C and from 25 to 1000 °C, respectively, which showed an almost zero-thermal-expansion.     

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.     

Effect of the Y2O3 amount on the oxidation behavior of ZrB2-SiC-based coatings for carbon/carbon composites

In this paper, the effect of Y₂O₃ addition on the oxidation resistance of ZrB₂-SiC-Y₂O₃ coating for the SiC coated carbon/carbon composites was investigated. Results confirmed the great benefits of adding Y₂O₃ to the oxidation-resistant properties of the original coating from different aspects. The dense structure of surface and cross-section and formation of yttria stabilized zirconia, Y₂Si₂O₇, t-ZrO₂ phases were observed after adding Y₂O₃. Additionally, according to the TEM results, yttrium silicate existed in the form of nanoparticles, while yttria stabilized zirconia existed in the form of agglomeration within the SiO₂ liquid phase. This superior oxidation resistance was attributed to the following reasons: (i) the formed Zr-Si-Y-O glass barrier layer blocked the oxygen diffusion and healed the cracks; (ii) the reduced m-ZrO₂ content weakened the volume expansion of the coating and avoided spallation; (iii) Y₂Si₂O₇ served as a pinning phase which modified the stability of liquid SiO₂ at elevated temperatures.     

Thermal properties of field-assisted-sintered SiCN–Y2O3 composites

Polymer-derived amorphous SiCN has excellent high-temperature stability and properties. To reduce the shrinkage during pyrolysis and to improve the high-temperature oxidation resistance, Y₂O₃ was added as a filler. In this study, polymer-derived SiCN–Y₂O₃ composites were fabricated by mixing a polymeric precursor of SiCN with Y₂O₃ submicron powders in different ratios. The mixtures were cross-linked and pyrolyzed in argon. SiCN–Y₂O₃ composites were processed using field-assisted sintering technology at 1350°C for 5 min under vacuum. Dense SiCN–Y₂O₃ composite pellets were successfully made with relative density higher than 98% and homogeneous microstructure. Due to low temperature and short time of the heat-treatment, the grain growth of Y₂O₃ was substantially inhibited. The Y₂O₃ grain size was ∼1 μm after sintering. The composites’ heat capacity, thermal diffusivity, and thermal expansion coefficients were characterized as a function of temperature. The thermal conductivity of the composites ceramics decreased as the amount of amorphous SiCN increased and the coefficient of thermal expansion (CTE) of the composites increased with Y₂O₃ content. However, the thermal conductivity and CTE did not follow the rule of mixture. This is likely due to the partial oxidation of SiCN and the resultant impurity phases such as Y₂SiO₅, Y₂Si₂O₇, and Y_4.67(SiO₄)₃O.     

Yttrium Bearing Silicon Carbide Matrices for Robust Ceramic Composites

Application of SiC‐based ceramic matrix composites (CMCs) in combustion environments demands the use of an environmental barrier coating (EBC) to prevent volatilization of the protective SiO₂ scale in flowing water vapor. The EBC only provides protection while present on the surface; cracking and spallation of the coating leaves the underlying SiC vulnerable to the oxidation–volatilization processes. A robust matrix material chemically tailored to regrow a yttrium silicate scale in the event of EBC loss has been developed by incorporating yttrium bearing species including YB₂, Y₂O₃, and Y₅Si₃ into the SiC. During oxidation a borosilicate glass helps seal cracks while Y₂O₃ and SiO₂ react to form Y₂Si₂O₇ for environmental protection. Candidate compositions were oxidized for 10 min to 100 h at 1400°C and for 24 h at 1500°C to understand the scale growth. The prospects for effectively applying this approach in CMCs are discussed.     

Autonomous crack healing ability of SiC dispersed Yb2Si2O7 by oxidations in air and water vapor

Yb₂Si₂O₇ is a popular environmental barrier coating; however, it decomposes into Yb₂SiO₅ in high-temperature steam environments. The thermal mismatch between Yb₂Si₂O₇ and Yb₂SiO₅ leads to the cracking and failure of the disilicate coating via oxidation. Dispersing SiC nanofillers into the Yb₂Si₂O₇ matrix is suggested to maintain the Yb₂Si₂O₇ matrix and promote crack self-healing. This study is aimed at clarifying the effect of water vapor on the self-healing ability of such composites. X-ray diffraction analysis and scanning electron microscopy were used to monitor the surface composition and the crack formation, respectively, in 10 vol% SiC-dispersed Yb₂Si₂O₇ composites. Annealing at temperatures higher than 750 °C in air or in a water vapor rich atmosphere led to strength recovery and the self-healing of indentation-induced surface cracks owing to volume expansion during the oxidation of SiC. The self-healing effect was influenced by the oxidation time and temperature. Rapid diffusion of H₂O as an oxidizer into the SiO₂ layer promoted self-healing in a water vapor rich atmosphere. However, accelerated oxidation at temperatures higher than 1150 °C formed bubbles on the surface. Fabricating composites with a small amount of Yb₂SiO₅ will be a solution to these problems.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.     

Oxidation behavior and microstructure evolution of SiC-ZrB2-ZrC coating for C/C composites at 1673 K

To protect carbon/carbon (C/C) composites against oxidation, a SiC-ZrB₂-ZrC coating was prepared by the in-situ reaction between ZrC, B₄C and Si. The thermogravimetric and isothermal oxidation results indicated the as-synthesized coating to show superior oxidation resistance at elevated temperatures, so it could effectively protect C/C composites for more than 221 h at 1673 K in air. The crystalline structure and morphology evolution of the multiphase SiC-ZrB₂-ZrC coating were investigated. With the increase of oxidation time, the SiO₂ oxide layer transformed from amorphous to crystalline. Flower-like and flake-like SiO₂ structures were generated on the glass film during the oxidation process of SiC-ZrB₂-ZrC coating, which might be ascribed to the varying concentration of SiO. The oxide scale presented a two-layered structure ~130 µm thick after oxidation, consisting of a SiO₂-rich glass layer containing ZrO₂/ZrSiO₄ particles and a Si-O-Zr layer. The multiphase SiC-ZrB₂-ZrC ceramic coating exhibited much better oxidation resistance than monophase SiC, ZrB₂ or ZrC ceramic due to the synergistic effect among the different components.     

Microstructures and oxidation behaviors of Al-modified and Al2O3-modified SiC coatings on carbon/carbon composites via pack cementation

Al₂O₃-modified SiC (AOSC) and Al-modified SiC (ASC) coatings were prepared on carbon/carbon (C/C) composites by one-time pack cementation (PC). Their microstructures and anti-oxidation performances were studied. Compared with ASC coating, AOSC coating shows more conspicuous defects (micro-cracks and holes) and lower densification. ASC coating can offer better oxidation resistance and thermal shock resistance to C/C composites than AOSC coating. Al additive can more efficiently improve the sinterability of SiC, which causes the above results. Besides, Al₂O₃ oxidation product is more stable than SiO₂ (l) of oxidized SiC at 1500 °C based on the thermodynamic analysis.     

Reaction mechanisms and properties of in situ porous Al2O3-ZrO2-mullite composites

Customized porous Al₂O₃-ZrO₂-mullite composites were designed and prepared by reaction sintering of zircon (ZrSiO₄) and alumina (Al₂O₃) at sintering temperatures from 1400 to 1600 °C for 3 h. The mechanical properties, microstructural evolution, and reaction mechanisms of the composites were investigated. The reactions between ZrSiO₄ and Al₂O₃ were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS). The results indicate that ZrSiO₄ and Al₂O₃ react and form ZrO₂ and mullite. Sintering temperature has an important effect on the reaction process. At 1400 °C, Al₂O₃ reacts directly with SiO₂ of ZrSiO₄ to form mullite and ZrO₂, which reduces the decomposition temperature of ZrSiO₄ and promotes the decomposition of ZrSiO₄. However, at 1600 °C, ZrSiO₄ first decomposes to form SiO₂ and ZrO₂, and then generates mullite by the diffusion of Si. The migration of Si is the key factor for the formation of mullite. The thermal shock resistance of the composites can be significantly improved by phase transformation toughening of in-situ ZrO₂. Therefore, increasing the proportion of ZrSiO₄ can significantly improve the mechanical properties of sample. The residual ratio of the flexural strength after thermal shock exceeded 90%, when the mass ratio of ZrSiO₄ to Al₂O₃ was 3:1. Besides, the addition of polymethyl methacrylate could improve the porosity of materials and has a direct effect on the thermal conductivity of composites.     

Effect of phase composition on the oxidation resistance of ZrB2-SiC coatings

ZrB₂-SiC-Al₂O₃ (ZSA) and ZrB₂-SiC-Si (ZSS) coatings were prepared on the surface of C/C composites by atmospheric plasma spraying. During oxidation at 1200 ℃ for 10 h, the formation of B₂O₃ in the ZSA coating volatilizes gradually, no glass phase is able to seal cracks and holes, resulting in poor oxidation resistance. On the other hand, the ZSS coating produces a large amount of borosilicate glass phase and still gains 0.8% weight after 10 h. During oxidation at 1500 ℃, a low viscosity SiO₂ glass phase forms on the ZSA coating, due to the presence of Al₂O₃ and this glassy phase has a good ability to seal defects. However, the rapid volatilization of B₂O₃ in the ZSS coating leads to the formation of large holes in the coating, and to progressively increased viscosity of SiO₂ glass phase, resulting in decreased oxidation resistance.     

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.     

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.     

Protective coatings for carbon bonded carbon fibre composites

Carbon bonded carbon fibre composites (CBCF) were modified by direct reaction with molten silicon in order to obtain a silicon carbide layer on the composite surface. Subsequently, the Si-infiltrated CBCF material was coated with a silica-based glass containing yttria and alumina by means of a slurry-dipping technique. On heat treatment the glass yielded a glass-ceramic layer thus giving a multi-layered oxidation and erosion protection system. The microstructural characterisation of the coating was conducted by standard microscopy techniques and by X-ray diffraction. The controlled crystallization of the glass-produced cristobalite, yttrium silicate (Y₂Si₂O₇, keiviite, β-form) and mullite as main crystalline phases. These are excellent ceramic materials for oxidation and erosion protection of SiC-coated carbon-based composites since their coefficients of thermal expansion (CTE) closely match that of SiC. The possibility of healing (closure) of micro cracks by a thermal treatment at 1375 °C, thus exploiting the viscous flow of the residual glass in the glass-ceramic, was explored in order to extend the service life of the protection system.     

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.     

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%.     

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%.