Shear properties of C/C-SiC sandwich structures

Carbon fiber reinforced carbon-silicon carbide (C/C-SiC) sandwich structures have been developed using the Liquid Silicon Infiltration process and the in situ joining method. They offer high mass-specific stiffness, low thermal expansion, and high environmental stability. Potential application areas are highly precise satellite structures, like optical benches. In this study, sandwich samples were manufactured using prepregs based on 2D carbon fibre fabrics and a phenolic resin precursor. Carbon fibre reinforced polymer preforms for folded and grid-cores, as well as for the skin panels were manufactured using autoclave technique. In the second step, the sandwich components were pyrolyzed, leading to C/C preforms. For the build-up of the sandwich samples, two skin panels were joined to a core structure and subsequently, the resulting C/C sandwich preform was siliconized. C/C-SiC sandwich samples were tested under shear load. Shear strength, modulus, and fracture strain were determined and compared to the results obtained by analytical calculation. The shear properties were dependent on the fiber orientation in the core structure as well as on the core type and orientation. The sandwich shear stiffness obtained in the tests was close to the expected theoretical values, calculated on the basis of the material properties and the core geometry.     

Experimental evaluation and theoretical prediction of elastic properties and failure of C/C-SiC composite

The paper presents experimental characterization and theoretical predictions of elastic and failure properties of continuous carbon fiber reinforced silicon carbide (C/C-SiC) composite fabricated by Liquid Silicon Infiltration (LSI). Its mechanical properties were determined under uniaxial tensile, compression, and pure shear loads in two sets of principal coordinate systems, 0°–90° and ±45°, respectively. The properties measured in the 0°–90° coordinate system were employed as the input data to predict their counterparts in the ±45° coordinate system. Through coordinate transformations of stress and strain tensors, the elastic constants and stress-strain behaviors were predicted and found to be in good agreement with the experimental results. In the same way, three different failure criteria, maximum stress, Tsai-Wu, and maximum strain, have been selected for the evaluation of the failure of C/C-SiC as a type of genuinely orthotropic material. Based on the comparisons with experimental results, supported by necessary practical justifications, the Tsai-Wu criterion was found to offer a reasonable prediction of the strengths, which can be assisted by the maximum stress criterion to obtain an indicative prediction of the respective failure modes.     

Thermoset IM-LSI-based C/C-SiC: Influence of flow direction and weld lines on microstructure and mechanical properties

The production of ceramic matrix composites (CMC) based on C/C-SiC is still very cost-intensive and therefore only economical for a few applications. The fabrication of the preforms involves many costs that need to be reduced. In this work, the shaping of the CFRP-preforms is realized by thermoset injection molding, which enables large-scale production. The polymeric matrix used is a multi-component matrix consisting of novolak resin, curing agent and lubricant. Six millimeter chopped carbon fiber with a proportion of 50 wt.% were used as a reinforcement. These ingredients are processed by an industrial equipment for compounding and injection molding in order to manufacture a CFRP demonstrator representing a brake disc. Test specimens are cut out of the demonstrator in different directions in order to investigate influences of flow direction and weld lines on microstructural and mechanical properties. Afterward, the CFRP samples were converted to C/C-SiC composites by the liquid silicon infiltration process. The article addresses the flow behavior of the compound during the injection molding and the building of the weld lines in the demonstrator. In addition, results of the directional dependence of the microstructural and mechanical properties within the fabricated disc in the different production steps are presented.     

Effects of Preform Shear on Tensile Properties of a Woven C/SiC Composite

This article addresses effects of weave defects in an angle‐interlock C‐fiber preform on the tensile properties of the resulting fully processed C‐fiber/SiC‐matrix composite. For this purpose, a preform was intentionally sheared in a controlled manner after weaving. The resulting distortions were quantified by analyzing high‐resolution images of the preform surface after the first step of matrix processing, while the tows were still clearly visible. Comparisons are made of tensile test results on specimens cut from this composite panel and from a pristine panel in select loading orientations. Strain maps obtained by digital image correlation are used to identify local strain variations that are attributable to weave defects. The results are discussed in terms of: (i) the shear‐normal coupling that arises in loading orientations of present interest, and (ii) the geometric effects of tow misalignment on tow continuity along the specimen gauge length. The composite is found to perform in a robust manner, in the sense that the tensile properties are not sensitive to the presence of the defects.     

Electrical properties of C/C and C/C-SiC ceramic fibre composites

The electrical properties of carbon/carbon (C/C) and carbon/carbon-silicon carbide (C/C-SiC) ceramic composites were measured. The results show that the capacitance decreases rapidly with an increase in frequency and it becomes constant above a frequency of 500 kHz, whereas the dissipation factor increases with increasing frequency. C/C-SiC composites give higher value than C/C composites due to the presence of microcracks.     

Fabrication of C/C-SiC composites using high-char-yield resin

Carbon fiber reinforced ceramic matrix composites (C/C-SiC composites) were fabricated using a type of high-char-yield phenolic resin with the char yield of 81.17 wt.%. Firstly, the fabric prepreg was prepared by spreading the phenolic resin solution onto the two dimensional carbon fiber plain weave fabric and dried consequently. Afterward, the resin was cured and then the carbon fiber reinforced polymer (CFRP) was pyrolyzed to get amorphous carbon. Finally, C/C-SiC composites were obtained through liquid silicon infiltration (LSI) process. SEM micrographs showed that the Si/SiC area was homogeneously dispersed in the matrix, and during the siliconization process, a layer of SiC was formed along the surface of carbon fibers or carbon matrix. The fiber volume of CFRP was about 40 vol.%, which was much lower than other studies. XRD result indicated that only β-SiC type was formed. The result of X-ray computed tomography clearly showed the structure changes before and after the melt infiltration process. Mechanical property test showed that the composites had fracture strength of 186 ± 23 MPa, and a flexural modulus of 106 ± 8 GPa.     

Fabrication of 2D C/C-SiC composites using PIP based hybrid process and investigation of mechanical properties degradation under cyclic heating

2D C/C-SiC composites were fabricated using PIP process by repeated impregnations of porous C/C composite preforms with polycarbosilane followed by pyrolysis. Effect of cyclic heating on flexural and shear strength of these composites was studied by exposing the test specimens to oxyacetylene flame for 20 s and cooling by a blast of air. The cyclic heating tests were repeated up to five times. Average flexural and shear strength of the as fabricated composites were about 330 MPa and 14.5 MPa respectively. After five heating and cooling cycles, average flexural and shear strength were reduced to 120 MPa and 5.5 MPa respectively. SEM, XRD, EDAX and XPS studies were also carried out to investigate the causes of strength reduction. Oxidation started preferentially at carbon matrix through the cut ends of the weft fibers. Oxidative damage due to repeatedly heating cooling was found to be much smaller in through-thickness direction due to passive oxidation of SiC matrix while severe damage was observed parallel to the fabric layers.     

Damage analysis of 2.5D C/C-SiC composites subjected to fatigue loadings

Damage analyses of a ceramic matrix composite during fatigue and quasi-static loads were performed by acoustic emission (A.E.) monitoring. The material studied was a 2.5D C/C-SiC composite produced by chemical vapor infiltration followed by liquid silicon infiltration. The analysis done during the first 200 cycles of a fatigue test showed that the number of A.E. hits is a good parameter for the quantification of damage. Furthermore, the A.E. hit energy was associated with the type of damage. In this sense, the damage developed during the fatigue loading was related to matrix crack initiation, propagation and re-opening, as well as fiber-matrix friction. Quasi-static tests on post-fatigue samples showed that the previous fatigue loadings increased the material`s damage threshold and hindered the development of new damage. Particular attention was given to the sample after 2,000,000 cycles as this sample showed distinct A.E. signals that could be related to fiber debonding.     

Mechanical hysteresis and damage evolution in C/SiC composites under fatigue loading at room and elevated temperatures

In this paper, the mechanical hysteresis and damage evolution in C/SiC ceramic matrix composites (CMCs) under cyclic tension-tension fatigue loading at room and elevated temperatures in air and in inert atmosphere and different loading frequencies are investigated. The fatigue hysteresis loops models considering multiple matrix cracking modes are developed to establish the relationships between fatigue hysteresis loops, fatigue hysteresis dissipated energy, and fiber/matrix interface shear stress. The evolution of fatigue hysteresis dissipated energy and interface shear stress vs applied cycles is analyzed. It was found that the interface shear stress degradation rate increases with fatigue peak stress, and loading frequency from 40 to 375 Hz.     

Evaluation of dynamic modulus measurement for C/C-SiC composites at different temperatures

The determination of elastic properties at application temperature is fundamental for the design of fibre reinforced ceramic composite components. An attractive method to characterize the flexural modulus at room and high temperature under specific atmosphere is the nondestructive Resonant Frequency Damping Analysis (RFDA). The objective of this paper was to evaluate and validate the modulus measurement via RFDA for orthotropic C/C-SiC composites at the application temperature. At room temperature flexural moduli of C/C-SiC with 0/90° reinforcement were measured under quasi-static 4-point bending loads and compared with dynamic moduli measured via RFDA longitudinally to fibre direction. The dynamic modulus of C/C-SiC was then measured via RFDA up to 1250°C under flowing inert gas and showed an increase with temperature which fitted with literature values. The measured fundamental frequencies were finally compared to those resulting from numerical modal analyses. Dynamic and quasi-static flexural moduli are comparable and the numerical analyses proved that bending modes are correctly modeled by means of dynamic modulus measured via RFDA. The nondestructive RFDA as well as the numerical modeling approach are suitable for evaluation of C/C-SiC and may be transferred to other fibre reinforced ceramic composite materials.     

Theoretical prediction on structure evolution and optimal properties of silicon modified hexagonal boron nitride as interphase in SiCf/SiC composite

To predict the effects of Si doping on hexagonal boron nitride (h-BN) and to achieve a balance between mechanical and oxidation properties for the interphase modification in SiC_f/SiC composites, we herein calculate and analyze the crystal structures and mechanical properties of (BN)₆₄Si_x (x = 4, 8, 16, 32) models by means of density functional theory (DFT) calculations and ab initio molecular dynamics (aiMD) simulations. The possible trends of crack deflection and self-healing ability are discussed. The modeling shows an obvious transition of (BN)₆₄Si_x from the layered crystal structure and anisotropic mechanical property to amorphous structure and isotropic mechanical property as the Si doping content up to 36.1 wt%. Regarding to the application of interphase in SiC_f/SiC composites, (BN)₆₄Si₁₆ model structure possess the highest debonding potential according to Cook and Gordon’s criteria and illustrates the higher self-healing capacity at elevated temperature.     

Properties of C/C-SiC composites produced via transfer moulding and inner siliconization

Short carbon fiber reinforced polymers (CFRP) are successfully prepared by transfer moulding technology. For this purpose, compounds on the basis of novolac/urotropin with different 6 mm chopped carbon fibers and silicon powder contents are produced utilizing a laboratory tempered sigma-blade kneader. These compounds are then shaped into 46 × 8 × 3 mm³ CFRP specimens using a transfer moulding machine. Depending on the material composition, the conversion to C/C-SiC composites is performed through liquid silicon infiltration (LSI) process or inner siliconization. First, the short fiber content is varied between 30 and 50 wt% and its influence on the process and properties of the composites is studied. Second, an investigation of the inner siliconization through the co-mixing of silicon powder (1-23 wt% in CFRPs) during the compound production as well as a comparison with the external silicon infiltration process are presented and discussed. According to the results, the best mechanical properties are achieved at a fiber content of 40 wt% in the case of the external silicon infiltration and at silicon content below 14 wt% for composites produced by the inner siliconization process.     

2维C/SiC复合材料的拉伸损伤演变过程和微观结构特征

通过单向拉伸和分段式加载-卸载实验,研究了二维编织C/SiC复合材料的宏观力学特性和损伤的变化过程.用扫描电镜对样品进行微观结构分析,并监测了载荷作用下复合材料的声发射行为.结果表明:在拉伸应力低于50MPa时,复合材料的应力-应变为线弹性;随着应力的增加,材料模量减小,非弹性应变变大,复合材料的应力-应变行为表现为非线性直至断裂.复合材料的平均断裂强度和断裂应变分别为23426MPa和0.6%.拉伸破坏损伤表现为:基体开裂,横向纤维束开裂,界面层脱粘,纤维断裂,层间剥离和纤维束断裂.损伤累积后最终导致复合材料交叉编织节点处纤维束逐层断裂和拔出,形成斜口断裂和平口断裂.     

Application of radio frequency glow discharge plasma for enhancing adhesion bonds in polymer/polymer joints

It was found in this study, that air radio frequency glow discharge (rfGD) plasma is increasing surface roughness and is enhancing wettability of poly(propylene) (PP), poly(ethylene) (PE), and poly(carbonate) (PC). As an addition, the increased surface microhardness of PP and PE was found. Results indicate that rfGD air plasma treatment is very effective tool for improvement of adhesive properties of studied polymers as tested on single lap and double lap joints. The value of maximum load rise from 107 to 1926 N (approximately 18‐fold increase) for 20‐min‐treated PE and from 314 to 834 N (approximately three‐fold increase) for PP. Artificial accelerated ageing rapidly decreased the quality of adhesive joint. The value of maximum load decreased from 1926 to 221 N (approximately nine‐fold decrease) in the case of 20‐min‐treated PE. This result indicates that the ageing of the adhesive joint of plasma treated polymers can be a limitation factor in the possible application of the final component because of the extremely high loss of its initially‐enhanced mechanical properties. However, the strength of the thus prepared joints was higher in comparison to the plasma nontreated virgin samples. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1827–1833, 2006     

C/C-SiC梯度基复合材料氧化行为研究

研究比较了Ｃ／Ｃ－ＳｉＣ梯度基复合材料和Ｃ／Ｃ复合材料的氧化行为．实验结果表明：ＳｉＣ通过占据表面活性点提高了共沉积基体的氧化起始温度;由于减少了碳与氧的接触面积,阻挡氧化凹坑的扩展,降低了材料的氧化质量损失速率．利用ＳＥＭ观察了梯度基复合材料微观氧化过程     

Joining oxide ceramic matrix composites by ionotropic gelation

The production of complex-shaped all-oxide ceramic matrix composites (Ox-CMC) is somewhat restricted by their current processing methods, as well as by the lack of applicable joining techniques. Thus, we present a new method for joining Ox-CMCs based on the gelation of slurries with the polysaccharide polymer alginate. For this investigation, Nextel 610/alumina-zirconia composites were produced using alginate as binder and aluminum acetate as gelling agent. The joining capabilities of this technique were investigated with microstructural analyses and single-lap compression shear tests. For that, a slurry-containing alginate was used to join two composite plates at different stages of the processing: gel state, dried green body and after sintering. Joining composites plates in their gel or green state was successful as the joints showed shear strength values similar to the interlaminar shear strength of the composites plates. The quality of the joints was attributed to the interactions between the alginate chains of the composite plates and the joint. We also show that even the joining of already sintered Ox-CMCs is feasible. However, densification cracks and lower shear strength are observed for such cases.     

Surface engineering of SiCf/SiC composites by selective thermal removal

Surface engineering based on the Selective Thermal Removal (STR) of SiC fibers from SiC_f/SiC composites was used to obtain a brush‐like surface in view of joining processes. As observed through 3D confocal microscopy, the thermal treatment led to a selective removal of the surface fibers so that the specific surface increased. Wetting tests were performed using a Ag–Cu–Ti brazing alloy. The STR led to a negligible increase in the contact angles, which ranged from 16° to 21° for as‐received composites and increased to 28° for composites after STR. Microstructural observations showed that the brazing alloy perfectly adhered to the brush‐like surfaces were giving a mechanical interlocking expected to enhance the strength of the joint.     

C/C多孔体对C/C-SiC复合材料微观结构和弯曲性能的影响

以4种纤维含量相同(32%,体积分数,下同),用化学气相渗透(chemical vapor infiltration,CVI)法制备了4种密度的碳纤维增强碳(carbon fiber reinforced carbon,C/C)多孔体,基体炭含量约20%～50%.利用液相渗硅法(liquid silicon infiltration,LSI)制备了C/C-SiC复合材料,研究了C/C多孔体对所制备的C/C-SiC复合材料微观结构和弯曲性能的影响.结果表明:不同密度的C/C多孔体反应渗硅后,复合材料的物相组成均为SiC,C及单质Si;随着C/C多孔体中基体炭含量的增加,C/C-SiC复合材料中SiC含量逐渐减少而热解炭含量逐渐增加.C/C-SiC复合材料弯曲强度随着材料中残留热解炭含量增加而逐渐增加,热解炭含量为约42%的C/C多孔体所制备的C/C-SiC复合材料的弯曲强度最大,达到320 MPa.     

Fatigue of a SiC/SiC ceramic composite with an ytterbium-disilicate environmental barrier coating at elevated temperature*

Tension-tension fatigue performance of a SiC/SiC composite with an ytterbium-disilicate environmental barrier coating (EBC) was investigated at 1200°C in air and steam. The composite is reinforced with Hi-Nicalon™ SiC fibers and has a melt-infiltrated matrix processed by chemical vapor infiltration of SiC with subsequent infiltration with SiC particulate slurry and molten silicon. The EBC consists of a Si bond coat and an Yb₂Si₂O₇ top coat applied via air plasma spraying. Tensile properties were evaluated at 1200°C. Tension-tension fatigue was examined for maximum stresses of 110-140 MPa. To assess the efficacy of EBC, experimental results obtained for the coated composite are compared to those for a control uncoated composite. Surface grit-blasting inherent in the EBC application process degrades tensile strength of the composite. However, the EBC effectively protects the composite from oxidation embrittlement during cyclic loading in air or steam. Fatigue runout set to 200 000 cycles (55.6 hours at a frequency of 1.0 Hz) was achieved at 110 MPa in air and steam. Retained properties of pre-fatigued specimens were characterized. Composite microstructure, along with damage and failure mechanisms were investigated. Damage and failure of the composite are attributed to the growth of cracks originating from numerous processing defects in the composite interior.     

Damage evolution in SiC/SiC unidirectional composites by X-ray tomography

Melt infiltrated SiC/SiC ceramic matrix composite unidirectional (UD) composite specimens were imaged under load using X-ray microtomography techniques in order to visualize the evolution of damage accumulation and to quantify damage mechanisms within the composite such as matrix cracking and fiber breaking. The data obtained from these in situ tensile tests were used in comparison with current models and literature results. Three-dimensional (3D) tomography images were used to measure the location and spacing of matrix cracking that occurred at increasing stress increments during testing within two UD composite specimens. The number of broken fibers and the location of each fiber break gap that occurred within the volume of both specimens were also quantified. The 3D locations of fiber breaks were correlated with the location of each matrix crack within the volume of the specimen and it was found that at the stress scanned directly before failure, most of the fiber breaks occur within 100 microns of a matrix crack.