Multi-layer CVD-SiC/MoSi2–CrSi2–Si/B-modified SiC oxidation protective coating for carbon/carbon composites

To further improve the oxidation resistance of coating for carbon/carbon (C/C) composites, a multi-layer CVD-SiC/MoSi₂–CrSi₂–Si/B-modified SiC coating was prepared on the surface of C/C composites by pack cementation and chemical vapour deposition method, respectively. The microstructures, oxidation and thermal shock resistance of the coating were studied. The influence of B content in pack powder on the microstructure and oxidation resistance of B-modified SiC coating was also investigated. The results show that the B-modified SiC coating prepared with 10 wt.% B exhibited the best oxidation protection ability for C/C composites at 1173 K. The multi-layer coatings could protect the C/C composites at 1173 K for 30 h and 1873 K for 200 h, and endure 30 thermal cycles between 1873 K and room temperatures. The oxidation resistance and thermal shock resistance is mainly attributed to their dense structure and self-sealing property.     

高温氧化及热震对SiC/ZrB2-SiC/SiC涂层炭/炭复合材料力学行为的影响

为提高炭/炭(C/C)复合材料的高温抗氧化性能,同时分析涂层制备及高温氧化对涂层材料力学行为的影响,在C/C复合材料表面采用反应熔渗、料浆涂刷结合化学气相沉积工艺制备了SiC/ZrB2-SiC/SiC三层高温抗氧化涂层。利用SEM和XRD分析复合涂层的微观结构和相组成,考察涂层复合材料1500℃高温抗氧化和1500℃-室温的抗热震性能,研究高温氧化及热震对涂层C/C复合材料力学行为的影响。结果表明,复合涂层试样1500℃静态空气环境下具有优异的抗氧化及抗热震性能:1500℃氧化20 h后试样保持增重,1500℃至室温热震50次后增重为0.69%。因涂层制备过程中粉料的渗入反应,复合材料弯曲强度增长了7.08%。在经历1500℃氧化20 h和1500℃至室温50次热震后,涂层复合材料弯曲强度有所下降,且因材料界面结合力的减弱使得纤维拔出特征明显,材料塑性断裂特征增强。     

Interfacial control of uni-directional SiCf/SiC composites based on electrophoretic deposition and their mechanical properties

Interfacial control of uni-directional SiC_f/SiC composites were performed by EPD, and their mechanical properties at room temperature were evaluated. The effect of the thickness of carbon interphase on SiC fibers by EPD on mechanical properties of uni-directional SiC_f/SiC composites was also investigated. The average thickness of carbon coating on SiC fibers increased from 42 nm to 164 nm with an increase in the concentration of colloidal graphite suspension for EPD. Dense SiC_f/SiC composites were achieved and their fiber volume fraction was 47–51%. The SiC_f/SiC composites had a bending strength of 210–240 MPa. As the thickness of carbon coating was below 100 nm, the SiC_f/SiC composites (SC01 and SC02) fractured in almost brittle manner. In contrast, the SiC_f/SiC composites (SC03) showed a pseudo-ductile fracture behavior with a large number of fiber pullout as the thickness of carbon coating was above 100 nm. The fracture energy of SC03 was 3–4 times as high as those of SC01 and SC02 and the value was about 1.7 kJ/m². In consideration of the results of mechanical properties, the thickness of carbon coating on SiC fibers should be at least 100 nm to obtain high-performance SiC_f/SiC composites. The fabrication process based on EPD method is expected to be an effective way to control the interfaces of SiC_f/SiC composites and to obtain high-performance SiC_f/SiC composites.     

A carbon nanotube–enhanced SiC coating for the oxidation protection of C/C composite materials

A carbon nanotube–enhanced SiC (CNT–SiC) coating was deposited on C/C composites to improve the oxidation resistance of C/C. The CNT–SiC coating was prepared by direct growth of CNTs on C/C surface at 700 °C followed by deposition of SiC using chemical vapor deposition at 1150 °C for 1 h. SiC was deposited on the CNTs as well as the interface between CNTs and C/C, making CNTs strongly rooted on C/C surface. The final CNT–SiC coating consisted of two layers: the CNT–SiC layer and SiC layer. In comparison to the SiC coating, the CNT–SiC coating showed fewer cracks and a better oxidation resistance because the CNTs reduce the stress in the coating caused by the mismatch of the coefficient of thermal expansion between C/C and SiC.     

C/SiC/Si-Mo-Cr复合涂层碳/碳复合材料力学性能研究

采用包埋法和涂刷法在碳/碳复合材料表面制备了一种新型的C/SiC/Si-Mo-Cr复合高温抗氧化涂层. 借助XRD和SEM等测试手段对所制备复合涂层的微观结构进行了表征, 采用三点弯曲试验研究了涂层处理及热震试验对碳/碳复合材料力学性能的影响规律. 结果表明: 制备的多相涂层结构致密, 涂层后碳/碳复合材料弯曲强度有所增大, 断裂特征由假塑性向脆性转变. 涂层试样经1500℃至室温20次热震后, 涂层试样的弯曲强度降低, 塑性增强.     

Improvement of SiCf/SiC density by slurry infiltration and tape stacking

SiC fiber-reinforced SiC–matrix ceramic composites (SiC_f/SiC) were fabricated by vacuum infiltration of a SiC slurry into Tyranno™-SA grade-3 fabrics coated with a 200 nm-thick pyrolytic carbon (PyC) layer followed by hot pressing using a transient eutectic-phase. The density of the composite was improved using a special infiltration apparatus with a pressure gradient and alternating tape insertion between fabrics. Their overall properties were compared with those of monolithic SiC and composite containing chopped fibers. Although the density of the composites decreased with increasing fiber fraction, SiC_f/SiC containing 50 vol.% fibers had a density of 3.13 g/cm³, which is the highest reported thus far. The composites containing continuous fibers had a maximum flexural strength of 607 MPa and a step increase in the stress–displacement behavior during the three-point bending test due to fiber reinforcement, which was not observed in the monolith.     

Study on Mechanical Behavior of CVD-SiC Coated C/SiC Composites under Simulated Space Environments

SiC coating was prepared on the surface of C/SiC composites by chemical vapor deposition (CVD) method, and then mechanical behavior of CVD-SiC coated C/SiC composites under cold and thermal cycling had been investigated. Specimens were thermally cycled between the temperatures of ?100 °C and 100 °C for up to 200 cycles, respectively. The coating was characterized by XRD, SEM and EDS. The results showed that there were no significant changes in the flexural property. CVD-SiC coated C/SiC composites had good mechanical stability in above simulated space environments. While great changes occurred on both elements and structure of the coating, from homogeneous single-phase of SiC into the inner layer of SiC and the outer of C, which caused the change of the bending strength.     

Interlaminar shear strength of SiC matrix composites reinforced by continuous fibers at 900 °C in air

To reveal the shear properties of SiC matrix composites, interlaminar shear strength (ILSS) of three kinds of silicon carbide matrix composites was investigated by compression of the double notched shear specimen (DNS) at 900 °C in air. The investigated composites included a woven plain carbon fiber reinforced silicon carbide composite (2D-C/SiC), a two-and-a-half-dimensional carbon fiber-reinforced silicon carbide composite (2.5D-C/SiC) and a woven plain silicon carbon fiber reinforced silicon carbide composite (2D-SiC/SiC). A scanning electron microscope was employed to observe the microstructure and fracture morphologies. It can be found that the fiber type and reinforcement architecture have significant impacts on the ILSS of the SiC matrix composites. Great anisotropy of ILSS can be found for 2.5D-C/SiC because of the different fracture resistance of the warp fibers. Larger ILSS can be obtained when the specimens was loaded along the weft direction. In addition, the SiC fibers could enhance the ILSS, compared with carbon fibers. The improvement is attributed to the higher oxidation resistance of SiC fibers and the similar thermal expansion coefficients between the matrix and the fibers.     

Evaluating the effect of oxygen content in BN interfacial coatings on the stability of SiC/BN/SiC composites

Boron nitride was studied as a fiber–matrix interface coating for Nicalon™/SiC composites. The effect of initial O-impurity content within the as-processed BN coatings on the long-term interface stability was investigated at elevated temperatures in flowing oxygen. Two types of Nicalon™/SiC composites were used for this study; one composite had a BN coating with <2% oxygen (low-O BN) and another composite had BN with an oxygen concentration >11% (high-O BN) in the as-processed state. The high-O BN is actually most representative of BN coatings available commercially. The BN coatings in both the high-O and low-O BN containing composites were structurally similar. The samples used here were thinned to <200 μm before oxidation and the final preparation for electron microscopy examination of the interface region was done after the reactions were completed. Thin samples were used to simulate maximum corrosion effects that would occur at the surface of an actual part during service. Ech sample was exposed to flowing oxygen at temperatures as high as 950°C for times up to 400 h. After each oxidation experiment, the BN coatings were examined by TEM to quantify the extent of any reaction which occurred at either the fiber/BN and BN/SiC matrix interfaces. At 950°C for 100 h, there were no interface microstructural changes observed in the low-O BN but there was extensive silica formation at the fiber/BN interfaces in the high-O BN. After 400 h at 950°C, large voids formed at the fiber/BN interface in the high-O BN sample only. Oxygen present within the initial BN coating contributed significantly to the degradation of the interfacial properties of the composite. Several techniques, including transmission electron microscopy (TEM), Auger electron spectroscopy (AES), energy-dispersive spectrometry (EDS), and electron energy-loss spectroscopy (EELS) were used to characterize changes in structure and chemistry of the fiber–matrix interface region and to elucidate and quantify composite degradation mechanisms.     

Mechanism of synthesizing dense Si–SiC matrix C/C composites

The following technique is known to synthesize C/C (carbon fiber-reinforced carbon) composites. The organic matter in the preformed yarn (plastic straw covered yarn including bundles of long carbon fibers, carbon powder, and organic binder) is pyrolyzed at 500 °C and concurrently hot-pressed. Then, the carbon ingredient is graphitized in an atmosphere of nitrogen at 2000 °C. The authors used the above mentioned C/C composites as a starting material and developed a dense Si–SiC matrix C/C composites in which most long carbon fibers remain without reacting with Si which is infiltrated in argon at 1600 °C and 100 Pa. As a result, production of 1 × 2 m large size plates free from warps and cracks was attained in NGK Insulators, Ltd. This mechanism consists of three steps. First, a trunk-shaped Si–SiC matrix is synthesized between yarn and yarn. Then a trunk-shaped Si–SiC matrix extends a yarn by force. Only differential gap is made in a yarn surface. Finally, branch-shaped Si–SiC matrix is synthesized so that a trunk-shaped Si–SiC matrix leads to the yarn inside.     

Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating

A new kind of oxidation protection coating of Si-MoSi₂ was developed for three dimensional carbon fiber reinforced silicon carbide composites which could be serviced upto 1550 °C. The overall oxidation behavior could be divided into three stages: (i) 500 °C < T < 800 °C, the oxidation mechanism was considered to be controlled by the chemical reaction between carbon and oxygen; (ii) 800 °C < T < 1100 °C, the oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (iii) T > 1100 °C, the oxidation of SiC became significant and was controlled by oxygen diffusion through the SiC layer. Microstructural analysis revealed that the oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, the media layer is Si + MoSi₂ layer, and the inside layer is SiC layer. The coated C/SiC composites exhibited excellent oxidation resistance and thermal shock resistance. After the composites annealed at 1550 °C for 50 h in air and 1550 °C 100 °C thermal shock for 50 times, the flexural strength was maintained by 85% and 80% respectively. The relationship between oxidation weight change and flexural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failure starting point for oxidation protection coating.     

SiC Nanowires Toughed HfC Ablative Coating for C/C Composites

In order to improve ablation resistance of carbon/carbon(C/C) composites,SiC nanowires were prepared on C/C composites surface in prior through chemical vapor reaction before HfC coating.SiC nanowires grew randomly and had good combination with HfC coating.SiC nanowires toughed HfC coating had lower linear and mass ablation rates than original HfC coating.The surface was much flatter and exhibited smaller cracks in center region.The ablation mechanism of HfC coating has been changed by SiC nanowires.Thicker HfO2 fused layer was formed on the surface of the toughed HfC coating,which could provide efficient protection for C/C composites.Therefore,SiC nanowires toughed HfC coating behaved in better ablation resistance.     

High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.     

Oxidation behavior of 3D Hi–Nicalon/SiC composite

Oxidation behavior of a three dimensional (3D) Hi–Nicalon/SiC composite with CVD SiC coating was investigated in the simulated air using a thermogravimetric analysis (TGA) device. Below 1100 °C, the oxidation kinetics was controlled by gas diffusion through the defects in the SiC matrix and coating and resulted in the consumption of PyC interphase. The residual flexural strength did have not a remarkable fluctuation and the relationship between the residual strength to temperature and weight change to temperature of the 3D Hi–Nicalon/PyC/SiC composite indicated the same regularity. Above 1200 °C, the oxidation kinetics was controlled by oxygen diffusion through the SiO₂ scale formed on the SiC coating and matrix. And the residual flexural strength of the composites was governed by the strength degradation of the Hi–Nicalon fiber. After oxidation, the fracture displacement in flexural tests increased with the weight loss increasing and the fracture mode showed a non-brittle pattern.     

W-Mo-Si/SiC Oxidation Protective Coating for Carbon/Carbon Composites

A W-Mo-Si/SiC double-layer oxidation protective coating for carbon/carbon （C/C） composites was prepared by a two-step pack cementation technique. XRD （X-ray diffraction） and SEM （scanning electron microscopy）results show that the coating obtained by the first step pack cementation was a thin inner buffer layer of SiC with some cracks and pores, and a new phase of （WxMo1-x）Si2 appeared after the second step pack cementation. Oxidation test shows that, after oxidation in air at 1773 K for 175 h and thermal cycling between 1773 K and room temperature for 18 times, the weight loss of the W-Mo-Si/SiC coated C/C composites was only 2.06%. The oxidation protective failure of the W-Mo-Si/SiC coating was attributed to the formation of some penetrable cracks in the coating.     

Microstructure and oxidation resistance of SiC coated carbon-carbon composites via pressureless reaction sintering

The effects of processing parameters on the microstructure and oxidation resistance of silicon carbide (SiC) coated carbon-carbon (C-C) composites were investigated. C-C composites were made from plain woven carbon cloths and phenolic derived carbon matrices in the laboratory. Pressureless reaction sintering has been used to apply SiC coating to C-C composites using epoxy resin and silicon powder as the precursor. Results showed that the oxidation resistance of C-C composites was enhanced by coating with SiC. The pressureless reaction sintering process exhibits good processability. -SiC was formed after heat treatment at 1800 °C and the -SiC formed after heat treatment at 2200 °C. The SiC coated C-C composites exhibit good oxidation resistance at 1000 °C for 100 h under the test conditions.     

Effects of polymer derived SiC interphase on the properties of C/ZrC composites

Polymer derived silicon carbide (SiC) interphase was introduced by precursor infiltration and pyrolysis (PIP) to prevent carbon fiber erosion and to improve the fiber–matrix interface bonding of C/ZrC composites prepared by PIP. Introducing SiC interphase increased the density of the composites. The SiC interphase not only protected carbon fibers effectively from erosion by carbo-thermal reduction, but also enhanced the mechanical properties of C/ZrC composites by strengthening the interface bond. The flexural strength and fracture toughness of C/ZrC composites with SiC interphase prepared by two PIP cycles were 319 MPa and 18.8 MPa m^1/2 respectively. The ablation properties of C/ZrC composites were with rising content of SiC interphase but then decreased when excessive. The mass loss rate and the linear recession rate of the C/ZrC composites with SiC interphase prepared by one PIP cycle were 0.0079 g/s and 0.0084 mm/s, respectively.     

Oxidation behaviour of three-dimensional woven C/SiC composites

Abstract

The oxidation behaviour of a three-dimensional woven C/SiC composite protected with an SiC seal coating and with an SiC coating combined with an SiO₂–B₂O₃ glassy coating have been respectively investigated through an experimental approach based on mass and flexural strength changes. Three main temperature domains exist for C/SiC composites protected with an SiC seal coating. At low temperatures (<700°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics. At an intermediate temperatures (between 700 and 1100°C), the oxidation kinetics are controlled by gas phase diffusion through a network of microcracks in the SiC matrix and coating. At high temperatures (>1100°C), the oxidation kinetics are controlled by oxygen diffusion through the SiO₂ scale formed on the SiC coating. Composites of C/SiC with an SiC/(SiO₂–B₂O₃) coating exhibit better oxidation resistance. The filling of the pores and the microcracks and the flow of the glassy coating at higher temperatures result in a global decrease of mass loss in the composites. By researching the relationship between the residual flexural strength and the mass variation in different temperature ranges, it is shown that the change in the residual flexural strength is dominated by the degradation of carbon phase.     

Al/SiC composite coatings of steels by thermal spraying

Thermal spraying has been used to coat carbon steels (F112) and austenitic stainless steels (AISI 304) with aluminium matrix composites. Mixtures of aluminium powder and SiC particles were used as spraying material. A sol-gel silica coating was laid on SiC particles to reduce the porosity of the composite coatings and to inhibit the formation of aluminium carbide. The sol-gel silica coating acts as an active barrier enhancing the wettability of the reinforcement by molten aluminium. Coatings with a reinforcement volume fraction up to 30 vol.% were obtained with porosities of about 1.0 vol.%. The incorporation of sol-gel silica coated SiC particles reduces the coefficient of thermal expansion of the composite coating and enhances its adhesion to the substrates more than when uncoated SiC particles were used.     

Ablation behavior and mechanism of SiC/Zr–Si–C multilayer coating for PIP-C/SiC composites under oxyacetylene torch flame

To improve the ablation resistance of PIP-C/SiC composites, SiC/Zr–Si–C multilayer coating was prepared by chemical vapor deposition (CVD) using methyltrichlorosilane (MTS) and hydrogen as the precursors and molten salt reaction using KCl–NaCl, sponge Zr and K₂ZrF₆, then the ablation capability of the coated composites was tested under oxyacetylene torch flame. The linear and mass ablation rates were much lower than those of uncoated samples. The linear and mass ablation rates of the three coating coated samples reached 0.0452 mm/s and 0.031 g/s, decreased by 27.3% and 27.1%, respectively. Moreover, the linear and mass ablation rates of the five coating coated samples reached 0.0255 mm/s and 0.0274 g/s, decreased by 59.0% and 35.5%. The gases released during ablation could take away a lot of heat, which was also helpful to the protection of the composites.