A Si–SiC Oxidation‐resistant Coating for Carbon/Carbon Composites by Hot‐pressing Reaction Technique

A Si–SiC coating was prepared by hot‐pressing reactive sintering (HPRS) technique for protecting carbon/carbon (C/C) composites against oxidation. The Si–SiC coating has a dense and crack‐free structure with a thickness of 70–90 μm. The Si–SiC coating by HPRS has a higher SiC content and lower Si content than the coating by pressure‐less reactive sintering (PRS). It also exhibits better oxidation‐protective ability than that prepared by PRS. With hot‐pressing, the flexural strength of the Si–SiC coated C/C composites decreases from 121 MPa to 99 MPa, and the interface bonding strength increases from 6 MPa to 10 MPa.     

Fabrication of short carbon fibre reinforced SiC multilayer composites by tape casting

Silicon carbide multilayer composites containing short carbon fibres (C_sf/SiC) were prepared by tape casting and pressureless sintering. C fibres were dispersed in solvents and then mixed with SiC slurry to make green C_sf/SiC tape. Triton X-100 was found to be the best one for Toho Tenax HTC124 fibres (with water soluble coating) among BYK-163, BYK-410, BYK-2150, BYK-9076, BYK-9077 and Triton X-100 dispersants. C_sf/SiC multilayer composites containing 5 vol.% fibre (mean fibre length of 3, 4.5, and 6 mm) were obtained. Addition of short C fibres seems to worsen the densification process in the C_sf/SiC multilayer composites, whereas anisotropy shrinkage in C_sf/SiC was also observed. Open pores size was increased slightly after the addition of C fibre but it decreased with the mean fibre length. Mechanical properties were affected by high residual porosity. The addition of short C fibre has not changed the crack deflection at weak interfaces. C_sf/SiC multilayer composites containing longer fibres (4.5 and 6 mm) presented higher elastic modulus, bending strength and Vickers hardness as compared to shorter fibres (3 mm). Improved sintering performance and fibre content are necessary to improve mechanical properties.     

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.     

SiC coating toughened by SiC nanowires to protect C/C composites against oxidation

A dense SiC coating toughened by SiC nanowires was prepared on carbon/carbon (C/C) composites using a two-step technique of chemical vapor deposition (CVD) to protect them against oxidation. The morphologies and crystalline structures of the coatings were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. SiC nanowires played a role in decreasing the size of the cracks and improving the thermal shock resistance of the coating. The result of thermal shock between 1773 K and room temperature for 21 times indicates that, compared with the SiC coating without SiC nanowires, the average size of the cracks in the SiC coating toughened with SiC nanowires reduced from 5 ± 0.5 to 3 ± 0.5 μm. The weight loss of the SiC coated C/C composites decreased from 9.32 to 4.45% by the introduction of SiC nanowires.     

A Si–SiC oxidation protective coating for carbon/carbon composites prepared by a two-step pack cementation

To prevent carbon/carbon (C/C) composites from oxidation, a Si–SiC coating has been prepared by a two-step pack cementation technique. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis show that the coating obtained by the first step pack cementation is a porous β-SiC structure, and a dense structure consisting α-SiC, β-SiC and Si is obtained after heat-treatment by the second step pack cementation. By energy dispersive spectroscopy (EDS) analysis, a gradient C–SiC transition layer can be formed at the C/C-coating interface. The as-received coating has excellent oxidation protection ability and can protect C/C composites from oxidation for 166 h at 1773 K in air. The weigh loss of the coated C/C is due to the formation of bubble holes on the coating surface and through-coating cracks in the coating.     

A multilayer coating of dense SiC alternated with porous Si-Mo for the oxidation protection of carbon/carbon silicon carbide composites

In order to improve the oxidation behavior of carbon/carbon silicide carbide composites prepared by liquid silicon infiltration of carbon/carbon porous preforms, a multilayer coating of dense SiC alternated with porous Si-Mo was prepared by chemical vapor deposition combined with slurry painting. Oxidation test showed that weight loss of the coated sample was only 0.25% after 150 h oxidation in air at 1673 K. And the coated sample gained weight in the course of 46 cycles of thermal shock test between 1673 K and 373 K. The coating remained intact during the two kinds of tests and no obvious failure was found. The excellent oxidation protective ability and thermal shock resistance of the SiC/Si-Mo coating can be attributed to the alternated structure.     

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.     

Oxidation and erosion resistance of MoSi2-CrSi2-Si/SiC coated C/C composites in static and aerodynamic oxidation environment

A MoSi₂-CrSi₂-Si multi-composition coating was prepared on the surface of SiC coated carbon/carbon (C/C) composites by pack cementation in argon. The crystalline structure of the multi-composition coating was measured by X-ray diffraction. The morphology and element distribution were analyzed by scanning electron microscopy and energy dispersive spectroscopy, respectively. The isothermal oxidation and erosion resistance of the multi-layer coating were investigated in electrical furnace and high temperature wind tunnel, respectively. These results indicated that the weight loss of the coated C/C composites was only 0.8% after oxidation in air at 1873 K for 500 h, and the sample was fractured after erosion at 1873 K for 32 h in wind tunnel. The weight loss of the coated C/C composites was considered due to the excessive depletion of the outer coating and the appearance of defects in the coating. The mismatch of thermal expansion coefficient between the multi-layer coating and C/C composites caused excessive local stress and resulted in the fracture of the coated C/C composites.     

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.     

ZrB2–SiC gradient oxidation protective coating for carbon/carbon composites

To improve the oxidation protective ability of carbon/carbon composites, ZrB₂–SiC gradient coating was prepared on the surface of C/C composites by an in-situ reaction method. The ZrB₂–SiC gradient coating consisted of an inner ZrB₂–SiC layer and an outer ZrB₂–SiC–Si coating. The phase composition and microstructures of the multiphase coating were characterized by XRD, EDS and SEM. Results showed that the inner coating is mainly composed of ZrB₂ and SiC, while the outer multiphase coating is composed of ZrB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The oxidation behavior of the coated C/C composites at 1773 K in air was investigated. Results show that the gradient ZrB₂–SiC oxidation protective coating could protect C/C from oxidation for 207 h with only (4.56±1.2)×10⁻³ g/cm² weight loss, owing to the compound silicate glass layer with the existence of thermally stable phase ZrSiO₄.     

Thermal diffusivity of 3D C/SiC composites from room temperature to 1400 °C

A 3D C/SiC composite and a bulk CVD SiC material were prepared. The effects of the CVD SiC coating and the heat treatment on the longitudinal and transverse thermal diffusivity of the C/SiC composites were investigated. The thermal diffusivity of the C/SiC composites could be well fitted by a multinomial function from room temperature to 1400 °C which includes a power term, an exponential term and a constant term. The exponential term affected the thermal diffusivity and led to its increase above 1200 °C with activation energy of 77 kcal/mol. The microstructure change in the composites was the reason that the thermal diffusivity was increased above 1200 °C. The longitudinal thermal diffusivity of the composite was twice or more than the transverse one and increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it. The heat treatment could increase the thermal diffusivity and make the exponential term disappeared in the functions. The functional curve before the treatment intersected that after the treatment at the treatment temperature.     

Synthesis of a silicon carbide coating on carbon fibers by deposition of a layer of pyrolytic carbon and reacting it with silicon monoxide

A method for preparing a SiC coating on carbon fibers is presented. The SiC coating was generated from the reaction of silicon monoxide (SiO) with a pyrolytic carbon (PyC) layer deposited on the fibers. The influence of holding time on the microstructure of the SiC layer was discussed. The oxidation behaviors of the uncoated and SiC coated carbon fibers were compared. The formation mechanism of the SiC coating was evaluated. With increased reaction time, the SiC coating becomes thicker and its surface becomes rough. The oxidation resistance of the carbon fiber was improved by the SiC coating. The initial oxidation temperature of the SiC coated carbon fiber is about 200 °C higher than that of the uncoated carbon fiber. The growth of the SiC coating is mainly attributed to the indirect reactions of SiO with PyC in the SiO/SiC/C system, in which silicon is considered a critical intermediate product.     

Oxidation behavior of C–SiC composites with a Si–W coating from room temperature to 1500°C

Oxidation tests of carbon fiber reinforced silicon carbide composites with a Si–W coating were conducted in dry air from room temperature to 1500°C for 5 h. A continuous series of empirical functions relating weight change to temperature after 5 h oxidation was found to fit the test results quite well over the whole temperature range. This approach was used to interpret the different oxidation mechanisms. There were two cracking temperatures of the matrix and the coating for the C–SiC composite. Oxidation behavior of the C–SiC composite was nearly the same as that of the coated C–C composite above the coating cracking temperature, but weight loss of the C–SiC composite was half an order lower than that of the coated C–C composite below the cracking temperature. As an inhibitor, the SiC matrix increased the oxidation resistance of C–SiC composites by decreasing active sites available for oxidation. As an interfacial layer, pyrolytic carbon decreased the activation energy below 700°C. From 800°C to 1030°C, uniform oxidation took place for the C–SiC composite, but non-uniform oxidation took place for the coated C–C composite in the same temperature range. The Knudsen diffusion coefficient could be used to explain the relationship between weight loss and temperature below the coating cracking temperature and the matrix cracking temperature.     

Oxidation behavior of SiC/glaze-precursor coating on carbon/carbon composites

In order to improve the oxidation behavior of carbon/carbon composites in a wide range of temperature, a new SiC/glaze-precursor coating was developed.The SiC layer was produced by slurry and sintering, while the glaze precursor layer was prepared by slurry and drying. The microstructures and phase compositions of the coating were analyzed by SEM and XRD, respectively. The oxidation resistance of the coated composites was investigated using both isothermal and temperature-programmed thermogravimetric analysis in the temperature range from room temperature to 1600 °C. The results showed that the oxidation behavior of the coating was mainly controlled by the diffusion of oxygen during the test.The coating showed excellent oxidation resistance and self-healing ability in a wide range of temperature.     

Fabrication and characterization of random chopped fiber reinforced reaction bonded silicon carbide composite

This paper deals with the microstructure and mechanical properties of reaction bonded silicon carbide reinforced with random chopped carbon fibers of 3 mm length. The composites were fabricated by dispersing chopped carbon fibers into bimodal SiC/C suspension, forming green body through slip casting, and then reaction sintering at 1700 °C. The effect of the chopped fiber fraction on microstructure and mechanical properties was evaluated. A significant increase of fracture toughness was obtained as the carbon fiber fraction approaches 30 vol.%. The chopped fibers had reacted with liquid silicon during reaction sintering, so little fiber pullout was observed. Crack deflection and bridging is the predominant mechanism for the composite toughening.     

Oxidation analysis of 2D C/ZrC–SiC composites with different coating structures in CH4 combustion gas environment

2D C/ZrC–SiC composites were fabricated by chemical vapor infiltration combined with polymer slurry infiltration and pyrolysis. Liquid highly branched polycarbosilane was used as the pre-ceramic precursor. In order to improve the oxidation resistance, three kinds of coating structures were prepared on C/ZrC–SiC composites: pure zirconium carbide coating, SiC–ZrC coating, and ZrB₂–SiC coating. Structural evolutions of the as-produced composites after oxidation in CH₄ combustion gas atmosphere at about 1800 °C were investigated and compared. Based on a model of the oxidation process, the mixture ZrB₂–CVD SiC showed the best oxidation resistance.     

Combustion joinining of carbon/carbon composites by a reactive mixture of titanium and mechanically activated nickel/aluminum powders

Combustion joining of carbon/carbon (C/C) composites using a mixture of titanium and mechanically activated Ni/Al powders as a reactive medium is reported. A minimum preheating of the sample stack to 630 K is required to initiate the joining process. A robust crack- and pore-free joint layer (∼75−100 μm in thickness), which is composed of NiAl_x and TiC_y(O_z) phases, is produced. Tensile-strength testing of the joined C/C composites shows that the fracture does not occur along the joint layer.     

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.     

Oxidation microstructure studies of reinforced carbon/carbon

Laboratory oxidation studies of reinforced carbon/carbon (RCC) are discussed with particular emphasis on the resulting microstructures. This study involves laboratory furnace (500-1500 °C) and arc-jet exposures (1538 °C) on various forms of RCC. RCC without oxidation protection oxidized at 800 and 1100 °C exhibits pointed and reduced diameter fibers, due to preferential attack along the fiber edges. The 800 °C sample showed uniform attack, suggesting reaction control of the oxidation process; whereas the 1100 °C sample showed attack at the edges, suggesting diffusion control of the oxidation process. RCC with a SiC conversion coating exhibits limited attack of the carbon substrate at 500, 700 and 1500 °C. However samples oxidized at 900, 1100, and 1300 °C show small oxidation cavities at the SiC/carbon interface below through-thickness cracks in the SiC coating. These cavities at the outer edges suggest diffusion control. The cavities have rough edges with denuded fibers and can be easily distinguished from cavities created in processing. Arc-jet tests at 1538 °C show limited oxidation attack when the SiC coating and glass sealants are intact. When the SiC/sealant protection system is damaged, attack is extensive and proceeds through cracks, creating denuded fibers in and along the cracks. Even at 1538 °C, where diffusion control dominates, attack is non-uniform with fiber edges oxidizing preferentially.     

Effect of heat treatment on the mechanical properties of PIP–SiC/SiC composites fabricated with a consolidation process

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been considered as candidates for heat resistant and nuclear materials. Three-dimensional (3D) SiC/SiC composites were fabricated by the polymer impregnation and pyrolysis (PIP) method with a consolidation process, mechanical properties of the composites were found to be significantly improved by the consolidation process. The SiC/SiC composites were then heat treated at 1400 °C, 1600 °C and 1800 °C in an inert atmosphere for 1 h, respectively. The effect of heat treatment temperature on the mechanical properties of the composites was investigated, the mechanical properties of the SiC/SiC composites were improved after heat treatment at 1400 °C, and conversely decreased with increased heat treatment temperature. Furthermore, the effect of heat treatment duration on the properties of the SiC/SiC composites was studied, the composites exhibited excellent thermal stability after heat treatment at 1400 °C within 3 h.