A multilayer MoSi2-SiC-B coating to protect SiC-coated carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a multilayer MoSi₂-SiC-B coating was prepared on the SiC-coated C/C composites by a simple and low-cost slurry method. The phase, microstructure and element distribution of the as-received coating were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The as-received coating could effectively protect C/C composites against oxidation at 850 °C in air for 100 h without mass loss, which exhibits better oxidation protective ability than the multilayer MoSi₂-SiC coating prepared by the same method. At intermediate temperature (850 °C), the excellent oxidation protective ability of the coating is mainly attributed to the formation of the molten B₂O₃ for sealing the microcracks and preventing oxygen from attacking the C/C substrate.     

炭/炭复合材料表面C/SiC/Si-Mo-Cr多层复合防氧化涂层研究(英文)

为提高炭/炭(C/C)复合材料的高温抗氧化性能,采用料浆涂刷法首先在C/C复合材料表面制备了预炭层,然后以Si粉及石墨粉(Si粉与石墨粉的质量配比为:60～80:10～25)为原材料采用包埋法经高温热处理获得C/SiC内涂层,最后在涂有C/SiC内涂层的C/C复合材料表面采用包埋法制备Si-Mo-Cr外涂层。借助扫描电镜、X射线衍射、电子能谱等分析测试手段对涂层试样的微观结构进行了分析,研究了涂层C/C复合材料在1 873 K和1 973 K下的氧化行为。结果表明:由于涂层氧化过程中表面生成了SiO2和Cr2O3复合玻璃层,其在1 873 K温度下表现出优异的防氧化性能,可以有效保护C/C复合材料达135 h。当氧化温度提高至1 973 K并氧化30 h后,该复合涂层氧化过程玻璃层完整性被破坏,涂层失效。     

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.     

炭/炭复合材料用SiC-Glass涂层的高温氧化机理

采用包埋法和预涂-烧结法相结合的组合工艺在炭/炭(C/C)复合材料表面制得SiC-Glass复合涂层, 并借助X射线衍射仪(XRD)和扫描电镜(SEM)对该复合涂层进行了表征, 研究了涂层C/C试样在不同温度下的氧化动力学规律。结果表明: 复合涂层具有双层结构, 包埋SiC内层由β-SiC相和少量游离硅相组成, 外层由MoSi₂颗粒掺杂的硼硅酸盐玻璃构成; 内外层之间结合紧密; 在1300~1600℃的空气气氛中, SiC-Glass涂层表现出良好的抗氧化性能, 其氧化激活能为118.1kJ/mol, 氧化主要受控于氧在Glass层中的体扩散速率; 在1600℃空气气氛中氧化65h后, SiC-Glass涂层C/C试样的氧化失重率仅为1.02%。      

Oxidation Protective C/SiC/Si-SiC Multilayer Coating for Carbon/Carbon Composites Applying at 1873 K

A C/SiC/Si-SiC multilayer coating for protecting carbon/carbon(C/C) composites against oxidation was prepared by slurry and pack cementation.X-ray diffraction(XRD) and energy dispersive spectroscopy(EDS) analysis showed that the inner coating obtained from the slurry and pack cementation was a C/SiC gradient layer acting as bonding layer,and the exterior coating formed in the second pack cementation was a Si-SiC double phase coating.Oxidation tests at 1873 K in air showed that the coating exhibited excellen...     

SiC-Si-ZrSiO_4 Multiphase Oxidation Protective Coating for Carbon/Carbon Composites

In order to improve the anti-oxidation property of carbon/carbon （C/C） composites, a novel SiC-Si-ZrSiO4 multiphase oxidation protective coating was produced on the surface of C/SiC coated carbon/carbon compo ites by a pack cementation technique. The phase composition and microstructure of the as-prepared coatings were characterized by XRD （X-ray diffraction）, SEM （scanning electron microscopy） and EDS （energy dispersive spectroscopy）. Oxidation behavior of the multiphase coated C/C composites was also investigated. It showed that the as-prepared coating characterized by excellent oxidation resistance and thermal shock re- sistance could effectively protect C/C composites from oxidation at 1773 K for 57 h in air and endure the thermal cycle between 1773 K and room temperature for 12 times, whereas the corresponding weight loss is only 1.47%. The excellent oxidation protective ability of the SiC-Si-ZrSiO4 coating could be attributed to the C/SiC gradient inner layer and the multiphase microstructure of the coating.     

Influence of grain size on wear behavior of SiC coating for carbon/carbon composites at elevated temperatures

To improve the wear performance of SiC coating for C/C composites at elevated temperatures, the grain was refined by adding small amounts of titanium, in the raw powders for preparing this coating. The related microstructure and mechanical characteristics were investigated by scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and nano-indention. The results show that the grain size of SiC coating decreased from ∼30 μm to ∼5 μm due to the addition of grain refiner. TiC formed by reacting titanium with graphite, can act as perfect heterogeneous nucleus for the nucleation and growth of β-SiC. The wear resistance and fracture toughness of SiC coating was improved by grain refinement. However, the increasing interfaces increased the friction resistance and resulted in the high friction coefficient of fine-grained coating at room temperature. As the temperature rose, oxides layer formed on the surface of fine-grained coating, which can reduce the adhesive wear and decrease the friction coefficient. The fine-grained coating exhibited relative low friction coefficient of ∼0.41 owing to a compact silica film formed on the worn surface at 600 °C, and the wear was dominated by plastic deformation and shear of silica film. The wear of coarse-grained coating was controlled by the fracture of SiC at high temperature.     

等离子喷涂炭/炭复合材料Cr-Al-Si涂层显微结构及高温抗氧化性能

采用等离子喷涂法在涂覆SiC内涂层的炭/炭复合材料表面制备了Cr-Al-Si外涂层。采用XRD和SEM分析了涂层的物相组成及微观结构, 并测试了复合涂层炭/炭复合材料试样在1500℃静态空气中的抗氧化性能。结果表明: 合金外涂层主要由Al_3.21Si_0.47、 Cr₃Si及Al₂O₃组成, 厚度约为120μm, 无穿透性裂纹; 多孔结构单一β-SiC内涂层的防氧化能力较差, 氧化10h后涂层试样的氧化失重就接近10%, 外加Cr-Al-Si涂层后, 涂层试样的氧化性能显著提高, 氧化61 h后试样的失重仅为5.3%。      

炭/炭复合材料MoSi2/SiC高温抗氧化复合涂层的制备及其结构

采用两段式包埋法工艺可制得涂层结构合理的复合梯度涂层，从里到外涂层结构为：SiC过渡层→SiC致密层→MoSi2／SiC双相层→以MoSi2为主的外层。随着制备工艺中高温阶段保温时间的延长，涂层表面1．2MoSi2为主的薄层越连续。涂层与基体的结合以化学结合为主，并有机械结合，结合强度高。用正硅酸四乙酯对涂层表面进行封闭处理，凝胶形成的SiO2可充填涂层表面裂纹并覆盖在涂层表面。在1500℃高温空气中氧化，未封闭处理的涂层试样表现为氧化失重，封闭处理后的试样氧化增重。     

ZrB2 coating for the oxidation protection of carbon fiber reinforced silicon carbide matrix composites

In order to improve the anti-oxidation performance of carbon fiber reinforced silicon carbide matrix (C/SiC) composites, ZrB₂ coating was prepared on the surface of C/SiC composites by a two-step technique of pack cementation method. The anti-oxidation properties of coated composites were investigated. The results showed that ZrB₂ coating was obtained by the method, which was homogenous and dense. The weight loss percentage of the coated composites was only 1.52 after oxidation in air at 1500 °C for 30 min, which exhibited excellent oxidation resistance.     

Kinetics of chemical vapor infiltration of carbon nanofiber-reinforced carbon/carbon composites

Preforms containing 0, 5, 10, 15 and 20 wt.% carbon nanofibers (CNFs) were fabricated by spreading layers of carbon cloth, and infiltrated by using the technique of isothermal chemical vapor infiltration (ICVI) at the temperature of 1100 °C under the total pressure of 1 kPa and with the flow of the mixture of propane/nitrogen in a ratio of 13:1. The infiltration rates increased with the rising of CNF content, and after 580 h of infiltration, the achievable degree of pore filling was the highest when the CNF content was 5 wt.%, but the composite could not be densified efficiently as the CNF content ranged from 10 to 20 wt.%. An analysis of the results, based on the effective diffusion coefficient and on the in-pore deposition rates, shows that the CNFs, due to their higher aspect ratio, accelerate overgrowth at pore entrances and thus lead to incomplete pore filling.     

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.     

Preparation and microstructural evolution of carbon/carbon composites

Carbon/carbon (C/C) composites with characteristic matrix-crack pattern are key intermediate materials for preparation of carbon/silicon carbide (C/C–SiC) composites. The C/C composites were prepared by pyrolyzing carbon fiber/phenolic resin preform. The change of density, open porosity, mass loss and specially the microstructural evolution of the composites during pyrolysis at 200–900 °C was analyzed, which provided important information for preparation of C/SiC composites by infiltration of silicon. An increasing number of regular spacing cracks were formed above 400 °C. After pyrolysis at 900 °C, the pore volume was 0.17 cm³/g, and the pores in the radius range of 2.44–122.19 μm occupied 81% of the pore volume.     

炭/炭复合材料高温力学行为研究

借助三点弯曲试验和扫描电镜观察,对层压结构C C(2D)、三维整体编织结构C C(3D)在高温1700℃及室温下的弯曲力学行为进行了研究,总结了各自性能及损伤破坏的特点。试验结果表明:3DC C以纤维断裂的形式发生弯曲破坏,其弯曲强度、模量均远大于2DC C;对于2DC C,其弯曲破坏模式为基体的层间开裂,材料性能在很大程度上受到炭基体以及界面状态的控制;C C复合材料在高温下弯曲力学性能大幅提高,强度增加幅度高达45%以上,模量增加幅度达15.3%;高温下界面粘结强度增加,导致3DC C的损伤破坏模式有所变化。     

C/C复合材料磷酸盐防氧化涂层

由涂刷法制备了四种不同配比的新型炭／炭（C／C）复合材料磷酸盐防氧化涂层，通过研究确定了优化涂层方案，涂覆有该涂层的C／C复合材料试样在700℃下空气中氧化100h后，失重率仅为0．952％，热震实验和浸海水恒温氧化实验证明该涂层仍具有良好的抗氧化性能。涂覆有该涂层的C／C复合材料在600℃～800℃时的Arrhenius曲线由两条折线组成，折点为700℃，在600℃～700℃下的氧化表观活化能为139kJ／mol；700oC～800℃下则为93kJ／mol。     

烧蚀过程中C/C复合材料的结构演变

利用SEM和TEM考察了3D C／C复合材料在电弧加热器上烧蚀后的材料表层显微组织结构变化。发现炭纤维和基体炭中的石墨微晶经过烧蚀后都得到了明显的发展。在距烧蚀表面几个纳米的深度范围内形成了高度取向的带状石墨织构，同时，在带状织构中间也形成了许多孔洞和缝隙。在距烧蚀表面几个微米的深度形成了卷曲柱状结构，在这些柱状结构周围有许多缺陷。在纤维和基体炭的界面区域，靠近纤维一侧形成了带状织构，且织构间的缝隙变大。     

Real-time damage evaluation of a SiC coated carbon/carbon composite under cyclic fatigue at high temperature in an oxidizing atmosphere

A SiC coated C/C composite was fatigued at 1300 °C in an oxidizing atmosphere. Damage evolution of the composite was discussed on the basis of modulus reduction and ratchetting strain developments. Real-time electrical resistance and acoustic emission responses were monitored throughout fatigue and the data showed fairly good agreement with composite micro-damage. It is feasible to employ resistance and acoustic emission measurements as real-time damage evolution indicators for fatigue of C/C composites at high temperature in corrosive environments.     

Compressive fracture behavior of 3D needle-punched carbon/carbon composites

Compressive fracture behavior under transverse and longitudinal compressive loading are determined for 3D needle-punched carbon/carbon (C/C) composites with single rough laminar (RL) pyrocarbon matrix or dual matrix of RL pyrocarbon and resin carbon. The results of Weibull statistics analysis indicate that scale parameter σ₀ of transverse and longitudinal compression of the composites with single matrix are 153.41 and 94.26 MPa, and σ₀ of the composites with dual matrix are 205.16 and 105.33 MPa, respectively. The mean compressive strength of both composites is nearly equal to σ₀ under each experimental condition. Failure modes of both composites under transverse and longitudinal compressive loading are shear and extension, respectively. Both composites exhibit quasi-ductile fracture behavior under transverse compression. Many small fragments of fibers and matrix carbon on the fracture surface of the composites are observed for single matrix composites. And the fiber bundle breakage with extensive debonding occurs for dual matrix composites. Under longitudinal loading, the composites with single matrix show quasi-ductile fracture behavior and delamination and splitting of non-woven long carbon fiber cloth layers are observed. The composites with dual matrix exhibit catastrophic failure behavior and crack runs through the composites along compressive loading direction.     

Facile synthesis of boron nitride coating on carbon nanotubes

Boron nitride (BN) coating on the surface of carbon nanotubes (CNTs) was synthesized by the direct reaction of NaBH₄ and NH₄Cl in the temperature range of 500–600 °C. X-ray diffraction, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) confirm the formation of BN coating. It is revealed that the BN coating follows the shape of CNTS without damaging the surface of CNTs, and the elements B and N distribute homogenously along the whole CNTs without chemical bonds between carbon and BN layers. Besides, the oxidation resistance of the CNTs improved a lot after being coated with BN.     

Enhancing both strength and toughness of carbon/carbon composites by heat-treated interface modification

A simple method to increase both strength and toughness of carbon/carbon (C/C) composites is presented. This method is based on the heat treatment of the pre-deposited thin carbon coating, leading to the formation of more orderly pyrolytic carbon (PyC) as a functional interlayer between fiber and matrix that could optimize the interfacial sliding strength in C/C composites. Effects of such a heat-treated PyC layers on the microstructure, tensile strength and fracture behavior of unidirectional C/C composites were investigated. Results showed that although the in-situ fiber strength was deteriorated after the introduction of interfacial layer, tensile strength of the specimen was greatly improved by 38.5% compared with pure C/C composites without any treatment. The interfacial sliding stress sharply decreased, which was interpreted from finite element analysis and verified by Raman spectra. Therefore, the fracture behavior was changed from brittle fracture to multiple-matrix cracking induced non-linear mechanical behavior. Finally, the ultimate strength can be predicted by different models according to the interfacial sliding stress. Our research would provide a meaningful way to improve both strength and toughness of C/C composites.