Self‐Generating High‐Temperature Oxidation‐Resistant Glass‐Ceramic Coatings for C–C Composites Using UHTCs

Carbon–carbon (C–C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high‐temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high‐temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600°C. Oxidation protection for C–C composites can be achieved by duplicating the self‐generating oxide chemistry of bulk UHTCs formed by a “composite effect” upon oxidation of ZrB₂–SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE‐TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600°C. Pure SiC‐based fillers are ineffective at protecting C–C from oxidation, whereas ZrB₂–SiC filled C–C composites retain up to 90% initial mass. B₂O₃ in SiO₂ scale reduces initial viscosity of self‐generating coating, allowing oxide layer to spread across C–C surface, forming a protective oxide layer. Formation of a ZrO₂–SiO₂ glass‐ceramic coating on C–C composite is believed to be responsible for enhanced oxidation protection. The glass‐ceramic coating compares to bulk monolithic ZrB₂–SiC ceramic oxide scale formed during DNE‐TGA where a comparable glass‐ceramic chemistry and surface layer forms, limiting oxygen diffusion.     

Effect of silicon/graphite ratio and temperature on oxidation protective properties of SiC/ZrB2–SiC coatings prepared by pack cementation

To investigate the silicon/graphite ratio and temperature on preparation and properties of ZrB₂–SiC coatings, ZrB₂, silicon, and graphite powders were used as pack powders to prepare ZrB₂–SiC coatings on SiC coated graphite samples at different temperatures by pack cementation method. The composition, microstructure, thermal shock, and oxidation resistance of these coatings were characterized and assessed. High silicon/graphite ratio (in this case, 2) did not guarantee higher coating density, instead could be harmful to coating formation and led to the lump of pack powders, especially at temperatures of 2100 and 2200 °C. But residual silicon in the coating is beneficial for high density and oxidation protection ability. The SiC/ZrB₂–SiC (ZS50-2) coating prepared at 2000 °C showed excellent oxidation protective ability, owing to the residual silicon in the coating and dense coating structure. The weight loss of ZS50-2 after 15 thermal shocks between 1500 °C and room temperature, and oxidation for 19 h at 1500 °C are 6.5% and 2.9%, respectively.     

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.     

Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.     

Ultra‐High‐Temperature Ceramic HfB2‐SiC Coating for Oxidation Protection of SiC‐Coated Carbon/Carbon Composites

To protect the carbon/carbon (C/C) composites from oxidation, an outer ultra‐high‐temperature ceramics (UHTCs) HfB₂‐SiC coating was prepared on SiC‐coated C/C composites by in situ reaction method. The outer HfB₂‐SiC coating consists of HfB₂ and SiC, which are synchronously obtained. During the heat treatment process, the formed fluid silicon melt is responsible for the preparation of the outer HfB₂‐SiC coating. The HfB₂‐SiC/SiC coating could protect the C/C from oxidation for 265 h with only 0.41 × 10^?2 g/cm² weight loss at 1773 K in air. During the oxidation process, SiO₂ glass and HfO₂ are generated. SiO₂ glass has a self‐sealing ability, which can cover the defects in the coating, thus blocking the penetration of oxygen and providing an effective protection for the C/C substrate. In addition, SiO₂ glass can react with the formed HfO₂, thus forming the HfSiO₄ phase. Owing to the “pinning effect” of HfSiO₄ phase, crack deflecting and crack termination are occurred, which will prevent the spread of cracks and effectively improve the oxidation resistance of the coating.     

High temperature anti‐oxidation behavior of in situ and ex situ nanostructured C/SiC/ZrB2‐SiC gradient coatings: Thermodynamical evolution,microstructural characterization,and residual stress analysis

Nanostructured C/SiC/ZrB₂–SiC oxidation protective gradient coating was prepared by a two‐step reactive melt infiltration method. In order to reduce production cost, ZrB₂ phase was synthesized by the in situ reactive that included low‐cost ZrO₂ and B₂O₃ powders as raw materials. High‐temperature oxidation behavior of coatings was evaluated by isothermal oxidation test at 1773 K in air for 10 hours. Thermodynamical behavior of the coatings at various temperatures during oxidation test and coating process was predicted by HSC Chemistry 6.0 software. Compressive residual stresses of 36.9 MPa and 41 MPa were calculated for in situ and ex situ coatings by Williamson‐Hall method. After 10 hours of isothermal oxidation at 1773K, in situ and ex situ coatings showed 12.84% and 15.69% of weight losses with oxidation rates of 1.87 × 10^?2 g cm^?3 h^?1 and 0.91 × 10^?2 g cm^?3 h^?1, respectively. These results indicated that the oxidation protection ability of the coating produced by the in situ method was very close to ex situ coating.     

Intermediate Temperature Internal Oxidation of a SiC/SiCN Composite with a Polymer‐Derived Matrix

SiC‐based composites exhibit oxidative embrittlement at intermediate temperatures. Although the mechanisms of internal oxidation in composites with initially hermetic matrices have been studied extensively, comparable studies on composites with semipermeable matrices, such as those produced by polymer infiltration and pyrolysis, have not been reported. The present article focuses on the latter class of composites, specifically a SiC_f /SiCN_m with a dual BN/Si₃N₄ fiber coating. It describes detailed SEM and TEM analyses of the microstructure before and after oxidation in dry air or water vapor at 800°C. The results show that internal oxidation is more aggressive in water vapor and occurs appreciably even in the absence of an applied stress. The sequence of oxidation of the constituent phases appears to be consistent with the underlying thermodynamic hierarchy for the respective oxidation reactions. Notably, contrary to existing models based on preferential oxidation of BN coatings, oxidation occurs first on the SiC fiber surfaces and the Si₃N₄ overcoat; crystalline BN remains even after significant fiber and matrix oxidation has occurred. The results are discussed in terms of rate‐controlling kinetic processes, the effect of oxidant type, and applied stress.     

High‐temperature mechanical behavior of ZrB2‐based composites with micrometer‐ and nano‐sized SiC particles

Using micrometer‐ and nano‐sized SiC particles as reinforcement phase, two ZrB₂‐SiC composites with high strength up to 1600°C were prepared using high‐energy ball milling, followed by hot pressing. The composite microstructure comprised finer equiaxed ZrB₂ and SiC grains and intergranular amorphous phase. The temperature dependency of flexure strength related to the initial particle size of SiC. In the case of micrometer‐sized SiC, the high‐temperature strength was improved up to 1500°C compared to room‐temperature strength, but the strength degraded at 1600°C, with strength values of 600‐770 MPa. In the case of nano‐sized SiC, the enhanced high‐temperature strength was observed up to 1600°C, with strength values of 680‐840 MPa.     

Mechanical Characterization of Annealed ZrB2–SiC Composites

Composites consisting of 70 vol% ZrB₂ and 30 vol% α‐SiC particles were hot pressed to near full density and subsequently annealed at temperatures ranging from 1000°C to 2000°C. Strength, elastic modulus, and hardness were measured for as‐processed and annealed composites. Raman spectroscopy was employed to measure the thermal residual stresses within the silicon carbide (SiC) phase of the composites. Elastic modulus and hardness were unaffected by annealing conditions. Strength was not affected by annealing at 1400°C or above; however, strength increased for samples annealed below 1400°C. Annealing under uniaxial pressure was found to be more effective than annealing without applied pressure. The average strength of materials annealed at 1400°C or above was ~700 MPa, whereas that of materials annealed at 1000°C, under a 100 MPa applied pressure, averaged ~910 MPa. Raman stress measurements revealed that the distribution of stresses in the composites was altered for samples annealed below 1400°C resulting in increased strength.     

Microwave Absorption of SiC/HfCxN1−x/C Ceramic Nanocomposites with HfCxN1−x‐Carbon Core–Shell Particles

The dielectric properties of high‐temperature stable single‐source precursor‐derived SiC/HfC_xN_1?x/C ceramic nanocomposites are determined by microwave absorption in the X‐band (8.2–12.4 GHz) at room temperature. The samples synthesized at 1700°C, denoted as SiC/5HfC_xN_1?x/C‐1700°C and SiC/15HfC_xN_1?x/C‐1700°C ceramics, comprising ≈ 1.3 and ≈ 4.2 vol% HfC_xN_1?x, respectively, show enhanced microwave absorption capability superior to hafnium‐free SiC/C‐1700°C. The minimum reflection loss of SiC/5HfC_xN_1?x/C‐1700°C and SiC/15HfC_xN_1?x/C‐1700°C are ?47 and ?32 dB, and the effective absorption bandwidth amount to 3.1 and 3.6 GHz, respectively. Segregated carbon, including graphitic carbon homogeneously dispersed in the SiC matrix and less ordered carbon deposited as a thin film on HfC_xN_1?x nanoparticles, accounts for the unique dielectric behavior of the SiC/HfC_xN_1?x/C ceramics. Due to their large reflection loss and their high chemical and temperature stability, SiC/5HfC_xN_1?x/C‐1700°C and SiC/15HfC_xN_1?x/C‐1700°C ceramics are promising candidate materials for electromagnetic interference applications in harsh environment.     

Effect of precoated carbon layer on microstructure and anti‐erosion properties of SiC coating for 2D‐C/C composites

To improve the erosion resistant of carbon‐carbon composites, an SiC coating was synthesized on carbon‐carbon composites by the in situ reaction method. They are firstly coated with carbon layer by slurry, and then SiC coatings are obtained by chemical vapor reaction. The effects of precoated carbon layer on the microstructure and anti‐erosion properties of SiC‐coated C‐C composites were studied and characterized. The thickness of the SiC coating increased with the increase in the precoated carbon layer thickness. The different thickness of carbon layer affects hardness of the SiC coatings, resulting in diverse erosion resistance of the coatings. The SiC coating prepared with moderate thickness of precoated carbon layer exhibits the best erosion resistance, and show better resistance at an impact angle of 30° than 90°. The eroded surface revealed that coating cracking and brittle fracture, fiber‐matrix debonding, fiber breakage, and material removal, and the additional microcutting and microploughing at oblique impact angle are the major erosion mechanism of SiC coating for C/C composites.     

Improved oxidation resistance of expanded graphite through nano SiC coating

Expanded graphite with nano SiC and amorphous SiC_xO_y coating was successfully prepared through pyrolysing silane coupling agent (SCA), where the grafting of SCA dominated the final products. The results show that mainly amorphous SiC_xO_y coating covers expanded graphite at 1000 °C, regardless of the SCA concentration. In comparison, nano SiC coating can be synthesized at 1200 °C depending on the good dispersion of SCA (with a SCA concentration of 50 vol%). The formed SiC coating contributes to much higher peak oxidation temperature (812.1 °C) than 678.0 °C of the pure expanded graphite. Meanwhile, the oxidation activation energies of expanded graphite are remarkably improved from 149.15 kJ/mol to 176.16 kJ/mol (based on Kissinger method), attributing to the derived nano SiC and SiC_xO_y coating.     

Synthesis of Ti3SiC2 coatings onto SiC monoliths from molten salts

Coatings with composition close to Ti₃SiC₂ were obtained on SiC substrates from Ti and Si powders with the molten NaCl method. In this work, the growth of coatings by reaction in the salt between monolithic SiC substrates and titanium powder is obtained between 1000 and 1200 °C. At 1000 °C, a coating of 8 µm thickness is formed in 10 h whereas a thin coating of 0.5 µm has been grown in 2 h. A lack in silicon was first found in the coatings prepared at 1100 and 1200 °C. For these temperatures, the addition of silicon powder in the melt had a favorable effect on the final composition, which is found very close to the composition of Ti₃SiC₂. The reaction mechanism implies the formation of TiC_x layers in direct contact with the SiC substrate and the presence of more or less important quantities of Ti₃SiC₂ and Ti₅Si₃C_x in the upper layers.     

Bonding strength and thermal shock resistance of a novel bilayer (c-AlPO4-SiCw-mullite)/SiC coated carbon fiber reinforced CMCs

Mullite coating, SiC whiskers toughened mullite coating (SiC_w-mullite), and cristobalite aluminum phosphate (c-AlPO₄) particle modified SiC_w-mullite coating (c-AlPO₄-SiC_w-mullite) were prepared on SiC coated C/SiC composites using a novel sol-gel method combined with air spraying. Results show that molten SiO₂ formed by the oxidation of SiC whiskers and molten c-AlPO₄ improved the bonding strength between mullite outer coating and SiC–C/SiC composites due to their high-temperature bonding properties. The bonding strength between mullite, SiC_w-mullite, c-AlPO₄-SiC_w-mullite outer coatings and SiC–C/SiC composites were 2.41, 4.31, and 7.38 MPa, respectively. After 48 thermal cycles between 1773 K and room temperature, the weight loss of mullite/SiC coating coated C/SiC composites was up to 11.61%, while the weight losses of SiC_w-mullite/SiC and c-AlPO₄-SiC_w-mullite/SiC coatings coated C/SiC composites were reduced to 7.40% and 5.12%, respectively. The addition of appropriate SiC whiskers can considerably improve the thermal shock resistance of mullite coating owing to their excellent mechanical properties at high temperature. In addition, c-AlPO₄ particles can further improve the thermal shock resistance of SiC_w-mullite coating due to their high-temperature bonding and sealing properties. No obvious micro-pores and cracks were observed on the surface of c-AlPO₄-SiC_w-mullite coating after 48 thermal cycles due to timely healing effect by formation of secondary mullite.     

A novel approach for preparing a SiC coating on a C/C-SiC composite by slurry painting and chemical vapor reaction

In order to improve the oxidation resistance of C/C-SiC composites, a SiC coating was prepared on a C/C-SiC composite by slurry painting combined with a chemical vapor reaction process. The oxidation resistance and microstructural evolution of the coated samples were investigated. The results show that the as-prepared SiC coating contained a large amount of residual silicon, and the presence of these Si promoted the formation of a complete SiO₂ glass layer in the initial stage of oxidation. However, the evaporation of the residual Si also accelerated the failure of the SiC coating, which caused the weight loss of the sample to be about 2.2% after oxidation in static air at 1500 °C for 300 h. Attributed to a large number of SiC ceramics in the C/C-SiC composite, the oxidation weight loss rate of the coating sample after coating failure was reduced.     

Oxidation of SiC/BN/SiC ceramic matrix composites in dry and wet oxygen at intermediate temperatures

The oxidation behavior of SiC/BN/SiC ceramic matrix composites (CMCs) was evaluated from 400° to 800 °C in 100% O₂ and 50% H₂O/50% O₂ gas mixtures. Thermogravimetric analysis (TGA) was utilized to measure weight change during controlled environment exposures at elevated temperatures for 1 and 50 hours. Oxidized CMCs and their oxides were studied post-exposure with scanning electron microscopy and energy dispersive spectroscopy. The oxidation onset and composition transition temperatures were evaluated. Key observations include oxide composition, oxide wetting, oxygen solubility in Hi-Nicalon SiC fibers and BN fiber coating oxidation and volatility behavior as a function of temperature. Degradation in wet environments at 600 °C was most extensive due to the formation of a non-wetting, non-protective surface oxide, allowing oxidant access to the BN fiber coatings followed by oxidation and volatilization. Implications of the CMC oxidation behavior are discussed for CMCs in service.     

Oxidation protection and mechanism of the HfB2-SiC-Si/SiC coatings modified by in-situ strengthening of SiC whiskers for C/C composites

To improve the oxidation resistance and alleviate the thermal stress of the HfB₂-SiC-Si/SiC coatings for C/C composites, in-situ formed SiC whiskers (SiC_w) were introduced into the HfB₂-SiC-Si/SiC coatings via chemical vapor deposition (CVD). Effects of SiC_w on isothermal oxidation and thermal shock resistance for the HfB₂-SiC-Si/SiC coatings were investigated. Results showed that the SiC_w-HfB₂-SiC-Si/SiC coatings exhibited excellent oxidation resistance for C/C composites with only 0.88% weight loss after oxidation for 468?h at 1500?°C, which was markedly superior to 4.86% weight loss for coatings without SiC_w. Meanwhile, after 50 times thermal cycling, the weight loss of the SiC_w-HfB₂-SiC-Si/SiC coated samples was 4.48%, which showed an obvious decrease compared with that of the HfB₂-SiC-Si/SiC coated samples. The SiC_w-HfB₂-SiC-Si/SiC coatings exhibited excellent adhesion to the C/C substrate and had no penetrating cracks after oxidation. The improved performance of the SiC_w-HfB₂-SiC-Si/SiC coatings could be ascribed to the SiC_w, which effectively relieved CTE mismatch and remarkably suppressed the cracks through toughening mechanisms including whiskers pull-out and bridging strengthening. The above results were confirmed by thermal analysis based on the finite element method, which demonstrated that SiC_w could effectively alleviate thermal stress generated by temperature variation. Furthermore, the SiC_w-HfB₂-SiC-Si/SiC coating can provide a promising fail-safe mechanism during the high temperature oxidation by the formation of HfSiO₄ and SiO₂, which can deflect cracks and heal imperfections.     

Strength of hot pressed ZrB2–SiC composite after exposure to high temperatures (1000–1700 °C)

Residual strength (room temperature strength after exposure in air at high temperatures) of hot pressed ZrB₂–SiC composites was evaluated as function of SiC contents (10–30 vol%) as well as exposure temperatures for 5 h (1000–1700 °C). Multilayer oxide scale structures were found after exposures. The composition and thickness of these multilayered oxide scale structure was dependent on exposure temperature and SiC contents in composites. After exposure to 1000 °C for 5 h, the residual strength of ZrB₂–SiC composites improved by nearly 60% compared to the as-hot pressed composites with 20 and 30 vol% SiC. On the other hand, the residual strength of these composites remained unchanged after 1500 °C for 5 h. A drastic degradation in residual strength was observed in composites with 20 and 30 vol% SiC after exposure to 1700 °C for 5 h in ZrB₂–SiC. An attempt was made to correlate the microstructural changes and oxide scales with residual strength with respect to variation in SiC content and temperature of expsoure.     

Evaluation of poly(methylsilane‐carbosilane) synthesized from methyl‐dichlorosilane and chloromethyldichloromethylsilane as a precursor for SiC

A new poly(methylsilane‐carbosilane) (PMSCS) for silicon carbide precursor was synthesized by Wurtz‐type copolycondensation of methyldichlorosilane (MeHSiCl₂) with chloromethyldichloromethylsilane (ClCH₂MeSiCl₂) and terminated with vinylmagnesium chloride (ViMgCl). The use of insufficient sodium made the reaction more economic and safe. By changing the ratios of two monomers and the end‐block agent, the properties of the obtained PMSCS and the C/Si ratio of its derived ceramic could be tuned. Upon pyrolysis at 1000 °C under argon, silicon carbide with nearly stoichiometric C/Si ratio and low oxygen content was obtained in 64% of ceramic yield. PMSCS showed high potential as an economical SiC ceramic precursor for the fabrication of SiC matrix, coating, and adhesives. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46610.     

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.