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.     

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.     

涂层工艺对C/C复合材料结构和弯曲性能的影响

采用热处理和包埋工艺制备了Ｃ／Ｃ复合材料的ＭｏＳｉ２／ＳｉＣ抗氧化涂层,对组织结构、界面、弯曲断口进行了显微观察,分析了氧化保护涂层及其工艺对其机械性能的影响,结果表明,该工艺在Ｃ／Ｃ复合材料表面生成涂层的同时,使基材内部的界面也被硅化;并且发现,热解炭基体比炭纤维更易与Ｓｉ反应生成ＳｉＣ。Ｃ／Ｃ复合材料经涂层工艺处理后,弯曲强度降低;热处理过程中发生的材料氧化是弯曲强度下降的主要原因     

碳纤维增强碳复合材料表面多层复合涂层中SiC和TiC内层的比较(英文)

采用包埋法制备了碳纤维增强碳(carbon fiber reinforced carb on composites,C/C)复合材料表面多层涂层,包括SiC,TiC内层,SiC,TiC中间层以及SiC+TiC复合外层。利用场发射扫描电镜和X射线衍射对其表面和断面的结构进行研究。结果显示:和TiC内层相比较,SiC内层较厚而且致密,具多孔结构且和C/C复合材料结合紧密;TiC内层较薄且和C/C复合材料结合松散。制备的SiC+TiC复合外层由SiC,TiC和Ti3SiC2组成。     

Strength degradation of SiC fibers with a porous ZrB2-SiC coating: Role of the coating porous structure

ZrB₂-SiC coatings with varied porous structures were deposited on SiC fiber tows using the sol-gel method and cured at 1400 ℃ in vacuum. Tensile strength of the coated SiC fibers were much lower than that of the uncoated fibers. The bimodal distribution in the Weibull plot of the coated SiC fibers demonstrated that the fracture of the coated fiber can be attributed to two types of defects: the porous structure of the coating and the fiber defects. Detailed morphology and microstructure characterization of the coating and fiber combined with strength calculation were carried out to investigate the individual contribution of the fiber defects and the porous coating layer respectively. The results revealed that apart from the fiber damage during the coating process the porous structure of the fiber coating has a non-negligible effect on the fiber strength, presumably due to a relatively strong bonding between the fiber and coating.     

Erosion Behavior of C/SiC Composites in Atomic Oxygen

A T300 carbon fiber and a SiC‐coated C/SiC composite made from the same fiber were studied in atomic oxygen environment. The carbon fiber shows significant degradation while the erosion rate of SiC‐coating of C/SiC is about 50 times lower. Evidence shows that Si is preferentially etched from the SiC surface. And XPS information showed that amorphous carbon and diamond‐like carbonare periodically generated on the tested composite surface. Statistical analysis shows that the C/SiC specimens have no significant change in flexural properties after 1‐year fluence AO treatment.     

空间望远镜用C/SiC镜面研制综述

综述了空间望远镜的主镜用高强度、高表面精度、低热膨胀系数的低温（约4K）用镜面的制备和检测过程.日本将Φ710mm的高强度反应烧结SiC材料已用于红外望远镜镜面.在短切炭纤维增强C/C复合材料毛坯的基础上进行液相硅渗透（LSI）而制备的C/SiC复合材料在光学镜面方面具有更大的优势.通过提高C/C复合材料毛坯中沥青基炭纤维体积分数及控制硅化速度,可有效地提高LSI-C/SiC复合材料的机械性能和表面光学精度;通过不同规格的炭纤维的混杂化,可使C/SiC复合材料热膨胀系数的各向异性降低至小于4％的差异.SiC、Si-SiC浆料涂层处理可有效地提高表面精度至2 nm rms的极高要求.     

碳/碳复合材料表面SiC/c-AlPO_4复合涂层及其抗氧化机制

采用二次包埋法和水热电泳沉积法相结合的工艺在碳纤维增强碳复合材料表面制得SiC/方石英型磷酸铝(cristobalite aluminum phosphate,c-AlPO4)复合涂层。借助X射线衍射仪和扫描电镜对复合涂层的晶相组成和显微结构进行了表征。研究了复合涂层的高温氧化性能,讨论了复合涂层氧化、失效的机理。结果表明:复合涂层具有双层结构,包埋的SiC内层主要由α-SiC,β-SiC和少量的游离硅组成,外层由c-AlPO4颗粒构成,内外层结合紧密。复合涂层在1300～1500℃范围内具有良好的抗氧化性能,其氧化激活能为117.2kJ/mol,氧化过程主要受氧在c-AlPO4层中的体扩散速率所控制;氧化气体逸出留下的孔洞是复合涂层防氧化失效的主要原因。     

化学气相渗C／SiC材料结构与性能的研究

采用碳布层叠然后用化学气相渗方法制备了Ｃ／ＳｉＣ复合材料，这种材料纤维与基体间的界面是决策材料力学行为的重要因素，带有热解碳作为界面层的Ｃ／ＳｉＣ材料，在断裂进表现出大范围的脱粘，纤维与周围的基体不同发生断裂，有大量的纤维拨出，断口类似毛刷，无界央层材料表现为脆性平面断口，裂纹直接通过纤维和基体向前扩展，没有发生脱粘。     

Lifetime-Applied Stress Response in Air of a SiC-Based Nicalon-Fiber-Reinforced Composite with a Carbon Interfacial Layer: Effects of Temperature (300° to 1150°C)

The lifetimes in air as a function of applied flexure stress and temperature (300–1150°C) are described for a Si–O–C based (Nicalon) fiber plain-weave cloth reinforced SiC-matrix composite (∼7% closed porosity) with an ∼0.3 µm thick carbon interfacial layer. The measured lifetimes of both samples with and without an external SiC seal coating were similar and decreased with applied flexural stress (for stresses greater than ∼90 MPa) and with temperature. At temperatures of ≥600°C, the external CVD SiC coating had negligible effect on the lifetimes; however, at 425°C, a detectable improvement in the lifetime was observed with an external SiC coating. When the applied stress was decreased below an apparent "threshold stress" (e.g., ∼90 MPa) for tests conducted at temperatures ≤950°C, no failures were observed for times of ≥1000 h. Electron microscopy observations show that the interfacial carbon layer is progressively removed during tests at 425° and 600°C. In these cases, failure is associated with fiber failure and pullout. At 950° and 1150°C, the carbon interface layer is eliminated and replaced by a thick silica layer due to the oxidation of the Nicalon fiber and the SiC matrix. This results in embrittling the composite.     

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.     

Wear behavior of SiC nanowire-reinforced SiC coating for C/C composites at elevated temperatures

To improve the wear resistance of SiC coating on carbon/carbon (C/C) composites, SiC nanowires (SiCNWs) were introduced into the SiC wear resistant coating. The dense SiC nanowire-reinforced SiC coating (SiCNW-SiC coating) was prepared on C/C composites using a two-step method consisting of chemical vapor deposition and pack cementation. The incorporation of SiCNWs improved the fracture toughness of SiC coating, which is an advantage in wear resistance. Wear behavior of the as-prepared coatings was investigated at elevated temperatures. The results show that the wear resistance of SiCNW-SiC coating was improved significantly by introducing SiC nanowires. It is worth noting that the wear rate of SiCNW-SiC coating was an order of magnitude lower than that of the SiC coating without SiCNWs at 800 °C. The wear mechanisms of SiCNW-SiC coating at 800 °C were abrasive wear and delamination. Pullout and breakage of SiC grains resulted in failure of SiC coating without SiCNWs at 800 °C.     

Oxidation protection of C/C composites with a multilayer coating of SiC and Si + SiC + SiC nanowires

An oxidation protective Si–SiC coating with randomly oriented SiC nanowires was prepared on the SiC-coated carbon/carbon (C/C) composites by a two-step technique. First, a porous network of SiC nanowires was produced using chemical vapor deposition. This material was subjected to pack cementation to infiltrate the porous layer with a mixture of Si and SiC. The nanowires in the coating could efficiently suppress the cracking of the coating by various toughening mechanisms including nanowire pullout, nanowire bridging, microcrack deflection and good interaction between nanowire/matrix interface. The results of thermogravimetric analysis and thermal shock showed that the coating had excellent oxidation protection for C/C composites between room temperature and 1500 °C. These results were confirmed by two additional oxidation experiments conducted at temperature of 900 and 1400 °C, which demonstrated that the coating could efficiently protect C/C composites from oxidation at 900 °C for more than 313 h or at 1400 °C for more than 112 h.     

Carbon fiber reinforced ceramic matrix composites with an oxidation resistant boron nitride interface coating

Toughening a ceramic in a ceramic matrix composite (CMC) depends on an ability of the composite to tolerate an accumulation of matrix cracks. When the reinforcement phase is carbon fiber, these cracks leave the fiber susceptible to destructive oxidation by ingress of air during high temperature exposure. Generally, a graphitic carbon interface coating is applied to carbon fibers because it provides for a weak bond between fiber and matrix that is required to promote toughening. This investigation seeks to utilize a BN coating instead of a C coating in order to promote oxidation resistance. Like graphitic carbon, BN is soft and easily cleavable. Preliminary observations that C/BN/SiC CMC's using Toray T300 carbon fibers were highly brittle and of low strength lead to a requirement of heat treating the fibers prior to the CVD of BN for toughened composites to be fabricated. It is likely heat treating removed reactive functionalities from the fiber surface to yield a weakly adhered and compliant interface.     

Thermo-mechanical characterization of carbon fiber reinforced silicon carbide composites having boron nitride as an interphase material

The carbon fiber reinforced silicon carbide composites were prepared by an isothermal chemical vapour infiltration process. In order to achieve the required density, the carbon fiber preforms in the form of rectangular panels were infiltrated by silicon carbide (SiC) matrix. Prior to the matrix infiltration, a thin coating of boron nitride, as an interphase, was applied on the fiber preform. The test samples were subjected to seal coating of silicon carbide by chemical vapour deposition process. The effect of protective SiC seal coating was examined by testing (3-point bend test) the uncoated and the seal coated samples at different temperatures. Higher value of the flexural strength was observed for the seal coated samples as compared to the uncoated samples, when got tested at high temperature (up to 1400?°C). The detailed analysis of the fractured surfaces of the tested samples was carried out.     

Fatigue of three advanced SiC/SiC ceramic matrix composites at 1200°C in air and in steam

High‐temperature mechanical properties and tension‐tension fatigue behavior of three advanced SiC/SiC composites are discussed. The effects of steam on high‐temperature fatigue performance of the ceramic‐matrix composites are evaluated. The three composites consist of a SiC matrix reinforced with laminated, woven SiC (Hi‐Nicalon?) fibers. Composite 1 was processed by chemical vapor infiltration (CVI) of SiC into the Hi‐Nicalon? fiber preforms coated with boron nitride (BN) fiber coating. Composite 2 had an oxidation inhibited matrix consisting of alternating layers of silicon carbide and boron carbide and was also processed by CVI. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Composite 3 had a melt‐infiltrated (MI) matrix consolidated by combining CVI‐SiC with SiC particulate slurry and molten silicon infiltration. Fiber preforms had a CVI BN fiber coating applied. Tensile stress‐strain behavior of the three composites was investigated and the tensile properties measured at 1200°C. Tension‐tension fatigue behavior was studied for fatigue stresses ranging from 80 to 160 MPa in air and from 60 to 140 MPa in steam. Fatigue run‐out was defined as 2 × 10⁵ cycles. Presence of steam significantly degraded the fatigue performance of the CVI SiC/SiC composite 1 and of the MI SiC/SiC composite 3, but had little influence on the fatigue performance of the SiC/SiC composite 2 with the oxidation inhibited matrix. The retained tensile properties of all specimens that achieved fatigue run‐out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Strengthening/toughening of 2D C/SiC composites by coating

SiC and SiC_w/SiC coatings were prepared on two-dimensional carbon fiber reinforced silicon carbide ceramic matrix composites (2D C/SiC), and strengthening/toughening of the composite by the coatings was investigated. After coating, the density of the C/SiC composites was increased effectively and the mechanical properties were improved significantly. Compared with SiC coating, SiC_w/SiC coating showed the more significant effect on strength/toughness of the composites. Coatings had two effects: surface strengthening and matrix strengthening. The latter was the dominant effect. The surface strengthening can increase the crack initiation stress, while the matrix strengthening can enhance the crack propagation resistance. The former effect increased the strength and the latter effect increased the toughness.     

Catalytic Fabrication of SiC/SiO2 coated graphite and its behaviour in Al2O3–C castable systems

SiC/SiO₂ coated graphite was prepared via a combined sol-gel coating and catalytic conversion route, using graphite flake and tetraethyl orthosilicate as the starting materials, and Fe(NO₃)₃·9H₂O as the catalyst precursor. X-ray diffraction analysis and microstructural examination revealed that a homogeneous coating comprising SiC and cristobalite (SiO₂) and covering the whole surface of graphite was formed. As prepared SiC/SiO₂ coated graphite exhibited better oxidation resistance and water wettability than its uncoated counterpart. Also, oxidation resistance and slag corrosion resistance of a model Al₂O₃–C castable using coated graphite as a carbon source were better than in the case of its counterpart using uncoated graphite.     

Processing of Cf/SiC composites by hot pressing using polymer binders followed by polymer impregnation and pyrolysis

Continuous carbon fiber (C_f) reinforced silicon carbide (SiC) matrix composite (C_f/SiC) was processed through hot pressing (HP) using polycarbosilane (PCS) in matrix and polysilazane in interphase regions as polymer binders. HP experiments were conducted at 4 MPa, 1200 °C and 1 h; followed by PCS polymer impregnation and pyrolysis (PIP) at 1200 °C under vacuum. The BN/SiC-Si₃N₄ interphase formed on the C_f cloth during BN dispersed polysilazane polymer coating and pyrolysis. The influence of PCS quantity during HP experiments on C_f/SiC composites was studied. Results suggest that sintering of SiC matrix in C_f/SiC composite improves by increasing PCS content during HP; however, high PCS content increases the liquidity of SiC-PCS mixture to flow out of the composite structure. The C_f/SiC composites with relative density ranging from 79 to 83% and flexural strength from 67 to 138 MPa was achieved.     

SiC改性C/C复合材料的制备及其烧蚀性能

采用超声波震荡法将SiC微粉添加到二维针刺碳毡预制体中,利用热梯度化学气相浸渗工艺沉积热解碳制备了SiC改性碳纤维增强碳基(carbon fiber reinforced catbon,C/C)复合材料.借助x射线衍射与扫描电子显微镜检测和观察材料的微观结构,利用氧-乙炔烧蚀实验测试材料的抗烧蚀性能.结果表明:SiC微...