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Evaluation of mechanical properties of a dense SiC/SiCN composite produced via PIP process
Abstract
The material behavior of Polymer Infiltration and Pyrolysis based SiC/SiCN composites is studied and the characteristic thermal and mechanical properties in on- (0/90 °) and off-axis (±45 °) direction are summarized. The tensile properties are determined at room temperature and 1300 °C. Based on the ratio of Young’s modulus and strength between on- and off-axis loading, a new approach for the classification of Weak Matrix Composites (WMC) and Weak Interface Composites (WIC) is proposed, which seems to be reasonable for various CMCs. Even without fibre coating mechanical behavior of SiC/SiCN is similar to that of WIC. In order to explain this, a microstructure model is developed and confirmed by analysis of fracture surface. The effect of temperature on the tensile properties is investigated through analysis of residual thermal stresses. Even though at 1300 °C the strength is slightly lower, the fracture strain increased significantly from RT to 1300 °C.
Increasing the efficiency of jet engines is essential to meet the demanded climate targets. Ceramic matrix composites (CMC) are strong candidates for aircraft applications because they withstand high temperatures, while their density is two-thirds lower than that of conventional nickel-based alloys. This leads to cooling air savings and a lower overall engine weight, resulting in a potential reduction of emissions. To investigate the potential benefits and manufacturing techniques required for the introduction of CMC to the high-pressure turbine of a modern jet engine, the geometry of a nozzle guide vane of an existing turbine was redesigned considering ceramic specific constraints. Then, the liquid silicon infiltration (LSI) process was used to manufacture SiC/SiC nozzle guide vanes. Hi-Nicalon S woven fabric was used together with a CVI-based fiber coating. The outer surface of the vane was ground to meet the requirements for surface roughness, and geometric and positional tolerances. Cylindrical, laser-drilled cooling holes were introduced for trailing edge cooling. In the final step, an environmental barrier coating system (EBC) consisting of yttrium disilicate and yttrium monosilicate layers was applied using PVD processing. Wind tunnel testing under TRL 4 will be performed and vane performance will be evaluated.
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.
SiC/SiCN ceramic matrix composites (CMCs) are promising candidates for components of aero‐engines. To evaluate the properties of these CMCs under realistic conditions, a quasi‐flat panel with effusion cooling holes was investigated in a high pressure combustor rig. A Tyranno SA3 fabric‐based SiC/SiCN composite with high strength and strain to failure was manufactured via polymer infiltration and pyrolysis process. Due to its weak matrix no fiber coating was necessary for damage tolerant behavior. The cooling holes in the panel were introduced via laser drilling. An outer coating of CVD‐based SiC was finally applied for enhanced oxidation resistance. The specimen was tested in the combustor rig and the cooling effectiveness was evaluated. The microstructure of laser machined holes was studied via microscopy and energy‐dispersive X‐ray spectroscopy. The macrostructure was investigated via computing tomography scans before and after the combustor test. Material performances at higher temperatures were estimated via a material performance index. Local microstructure modifications were observed after laser drilling. No crack formation was observed in the CMC panels after rig tests. The measured global cooling effectiveness of 0.76 and the analytical performance evaluation demonstrate the potential benefit of SiC/SiCN materials in combustor applications.
In article number 1903829, Guofeng Xie, Tian Wang, Gang Zhang, and co‐workers summarize the fundamental physical aspects of thermal conductivity in amorphous materials, which have valuable applications in flexible electronics, artificial intelligence chips, thermal protection, and thermoelectrics.
Because of the excellent fracture toughness and oxidation resistance, carbon fiber reinforced silicon carbide (C/C-SiC or C/SiC) exhibits a sound potential in various application areas such as aerospace technology and high-performance braking systems. For the composite’s reliable design, production, examination, quality assurance and verification, however, the statistical distribution of mechanical properties is of crucial interest and has not been investigated in detail yet. In this work, the strength values of C/C-SiC composite, which was developed via Liquid Silicon Infiltration at the Institute of Structures and Design of German Aerospace Center (DLR), were measured under tensile, bending and compression load. The results were analyzed by normal and Weibull distribution statistics and verified by Kolmogorov–Smirnov-test (KS-test) and Anderson–Darling-test (AD-test). Based on the statistical analysis, the 4PB-strength of C/C-SiC composite can be better described by Weibull distribution. In comparison, normal distribution is more suitable for the compression strength. The influence of different numbers of coupons on the mechanical properties has been identified. A scanning electron microscope (SEM) was employed to analyze the fracture surface, which confirmed that the different statistical distribution of strength values was caused by various failure mechanisms.
The holy grail for jet engines is efficiency, and the improved high-temperature capability of CMC systems is giving General Electric a great advantage.
Ceramic matrix composites (CMCs) with low porosity are obtained in one cycle via the well established liquid silicon infiltration (LSI) process, which is characterised by short processing times and fairly low manufacturing costs. Apart from aerospace applications, such as hot structures for re-entry vehicles, more and more applications beyond this classic field of CMCs are of increasing interest, eg brake discs, zero-expansion materials, high-temperature heat exchangers, heat-sink materials, etc. By applying special process parameters the microstructure as well as the physical properties can be tailor-designed to match specific requirements. The thermophysical properties of C/C-SiC, especially specific heat capacity, thermal conductivity as well as total and spectral ernissivity were investigated in order to find out how the selection of fibres and pretreatment of the material affect them. Thermal conductivity of carbon fibres and therefore also of the composite considerably increases with increasing pyrolysis and graphitisation temperature.
Incorporation of porosity into a monolithic material decreases the effective thermal conductivity. Porous ceramics were prepared by different methods to achieve pore volume fractions from 4 to 95%. A toolbox of analytical relations is proposed to describe the effective thermal conductivity as a function of solid phase thermal conductivity, pore thermal conductivity, and pore volume fraction (νp). For νp < 0.65, the Maxwell–Eucken relation for closed porosity and Landauer relation for open porosity give good agreement to experimental data on tin oxide, alumina, and zirconia ceramics. For νp > 0.65, the thermal conductivity of kaolin-based foams and calcium aluminate foams was well described by the Hashin Shtrikman upper bound and Russell’s relation. Finally, numerical simulation on artificially generated microstructures yields accurate predictions of thermal conductivity when fine detail of the spatial distribution of the phases needs to be accounted for, as demonstrated with a bio-aggregate material.
Mechanical testing after neutron irradiation is a critical research tool for evaluating materials for fusion systems, such as silicon carbide fiber silicon carbide matrix (SiC/SiC) composites. However, single-axis tensile testing, which is required to build a fundamental database, requires large specimens. Therefore miniaturization of tensile test specimens has long been pursued as a method to reduce the irradiation volume to fit the capsule size limitation. The objective of this study is to identify specimen size effects on tensile properties of SiC/SiC composites from the viewpoints of the influences of fabric architecture and tensile loading axis, with a final goal to establish a small specimen test technique for tensile testing of the composites. The axial fiber volume fraction plays an important role in achieving good tensile properties. However the size dependent change of the axial fiber volume fraction gives specimen size effect. The composites with much fiber volume content tended to have superior tensile strength, elastic modulus and proportional limit stress. Contrarily, the tensile properties of the composites with the same axial fiber volume fraction were almost independent of the specimen size. This type of size effect is generally common in any types of architecture. The size-relevant fracture mode in off-axis tension: detachment in shorter widths vs. in-plane shear at larger widths, also gives specimen size effect on tensile properties, resulting in strict limitation of miniaturization of the tensile specimen. Finally we proposed a miniature tensile specimen for the composites.
The present article examines the in-plane tensile properties of a two-dimensional (2D) all-oxide ceramic composite. The distinguishing characteristics of the material include fine-scale porosity within the matrix and the absence of a fiber coating. The anisotropy in the elastic-plastic properties has been studied through tension tests in the axial (fiber) direction and at 45° to the fiber axes, both in the presence and the absence of holes or notches. The notch sensitivity in the axial direction is comparable to that of conventional dense-matrix, weak-interface composites, demonstrating the effectiveness of the porous matrix in enabling crack deflection and damage tolerance. Furthermore, the notch sensitivity is rationalized using models that account for the effects of inelastic straining on the local stress distributions around notches and holes, coupled with a scale-dependent failure criterion. In the off-axis orientation, the tensile strength is dictated by a plastic instability, analogous to necking in metals. Following instability, deformation continues within a diffuse localized band, with a length comparable to the specimen width. Similar deformation and fracture characteristics are obtained both with and without holes. The off-axis properties are discussed in terms of the comminution and rearrangement of matrix particles during straining.
The successful replacement of metal alloys by ceramic matrix composites (CMC) in high-temperature engine components will require the development of constituent materials and processes that can provide CMC systems with enhanced thermal capability along with the key thermostructural properties required for long-term component service. This chapter presents information concerning processes and properties for five silicon carbide (SiC) fiber-reinforced SiC matrix composite systems recently developed by NASA that can operate under mechanical loading and oxidizing conditions for hundreds of hours at 1204, 1315, and 1427 C, temperatures well above current metal capability. This advanced capability stems in large part from specific NASA-developed processes that significantly improve the creep-rupture and environmental resistance of the SiC fiber as well as the thermal conductivity, creep resistance, and intrinsic thermal stability of the SiC matrices.
The material behavior of Polymer Infiltration and Pyrolysis based SiC/SiCN composites is studied and the characteristic thermal and mechanical properties in on- (0/90 ◦) and off-axis (±45 ◦) direction are summarized. The tensile properties are determined at room temperature and 1300 ◦C. Based on the ratio of Young’s modulus and strength between on- and off-axis loading, a new approach for the classification of Weak Matrix Composites (WMC) and Weak Interface Composites (WIC) is proposed, which seems to be reasonable for various CMCs. Even without fibre coating mechanical behavior of SiC/SiCN is similar to that of WIC. In order to explain this, a microstructure model is developed and confirmed by analysis of fracture surface. The effect of temperature on the tensile properties is investigated through analysis of residual thermal stresses. Even though at 1300 ◦C the strength is slightly lower, the fracture strain increased significantly from RT to 1300 ◦C.
Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC/SiC CMCs) are promising candidates for components in the hot gas section of jet engines, as they exhibit high temperature resistance and low density compared to their metal alloy counterparts. Three common manufacturing routes are chemical vapor infiltration, reactive melt infiltration (RMI) and polymer infiltration and pyrolysis (PIP). This work describes a new approach of a combined PIP and RMI process. A combination of the processes seems attractive: the remaining porosity after PIP process can be closed by subsequent siliconization, resulting in a dense material. SiC/SiC CMCs were manufactured by PIP process using Hi-Nicalon Type S fibers. Generally, the processing of SiC/SiC, produced solely by PIP route, is rather time-consuming and the composites show a certain residual porosity. In order to obtain a dense matrix and to reduce the processing time, an additional RMI with silicon alloy is carried out after a reduced number of PIP cycles. To protect the fibers during the siliconization, a CVD fiber coating was applied. Microstructure was examined via microCT, SEM and EDS. Bending strength was determined to 433 MPa; strain to failure was 0.60%. The overall processing time was reduced by 55% compared to standard PIP route. The hybrid material contained 70% less unreacted carbon than material produced by LSI process alone.
In order to study the effects of temperature on the material behavior of Liquid Silicon Infiltration (LSI) based continuous carbon fiber reinforced silicon carbide (C/C-SiC), the mechanical properties at room temperature (RT) in in-plane and out-of-plane directions are summarized and the tensile properties of C/C-SiC were then determined at high temperature (HT) 1200 °C and 1400 °C under quasi static and compliance loading. The stress-strain response of both HT tests is similar and almost no permanent strain can be observed compared to the RT, which can be explained through the relaxation of residual thermal stresses and the crack distribution under various states. The different fracture mechanisms are confirmed by the analysis of fracture surface. Furthermore, based on the analysis of hysteresis measurements at RT, a modeling approach for the prediction of material behavior at HT has been developed and a good agreement between test and modeling results can be observed.
Four unidirectional fiber reinforced SiCN ceramic matrix composites were manufactured by means of polymer infiltration and pyrolysis. Two carbon fibers (T800H and Granoc XN90) as well as two silicon carbide fibers (Tyranno ZMI and SA3) without fiber coating were chosen. As matrix precursor, a poly(methylvinyl)silazane was investigated and utilized. The composites with the SA3 and the XN90 fiber had the highest tensile strengths of 478 and 288 MPa, respectively. It is considered that these high modulus fibers with the low modulus SiCN matrix create weak matrix composites. After exposure to air (T = 1200 °C, 10 h), a significant decrease of the mechanical properties was found, caused by the burnout of carbon fibers and the oxidation through open pores stemming from the PIP process and SiCN/SiCN interfaces in case of the SiC fiber based composites.
The paper presents enhanced studies of investigation and modeling of failure properties for wound ceramic matrix composites with varied fiber orientations under tensile loading. Based on mechanical tests and microstructure analysis, the characteristics of a virtual equivalent unidirectional layer (UD-layer) were examined and treated as input for modified Tsai-Wu failure criterion. In order to predict the mechanical properties with more accuracy, particular features of the investigated material have to be taken into consideration: definition of two material modeling groups based on the analysis of microstructure; interaction between failure strength and strain through inelastic deformation; inclusion of inhomogeneities created due to the manufacturing process in the analytical model. Based on the good correlation between the experiments and the modeling results, it can be shown that modeling approaches, considering the above mentioned particular material features, allow a very accurate prediction of the mechanical properties under in-plane tensile loading of CMC laminates.
Full paper: https://authors.elsevier.com/c/1Xef4,UwqXJSJF
Silicon-carbide-fiber-reinforced silicon carbide matrix composites (SiC/SiC) exhibit good thermal shock resistance, a low coefficient of thermal expansion and excellent physical properties as well as chemical stability at elevated temperatures and are therefore regarded as promising candidates for various applications in heavily loaded turbine sections of jet engines. Liquid silicon infiltration was chosen as a technique characterized by short processing times to obtain composites with low porosity in a three-step process: Infiltration of fiber preforms with a phenolic resin, pyrolysis and siliconization. Unfortunately, uncoated Tyranno SA3 fibers were cramped in the matrix, resulting in SiC/SiC with low strength and damage tolerance. In order to protect the fibers and to simultaneously provide a weak fiber matrix bonding, a CVD Si-BN/SiC/pyC fiber coating was chosen. The triple coating leads to a twofold higher bending strength of SiC/SiC as well as to more damage-tolerant fracture behavior compared to composites without fiber coating. Composites with various fiber volume contents are compared with regard to their mechanical properties. The microstructure of the composites was characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and computed tomography (CT), especially with regard to the functionality of the fiber coating.
A damage tolerant weak matrix SiC fiber reinforced composite was developed by utilising a polyvinylsilazane in the polymer infiltration and pyrolysis (PIP) process. The polysilazane was infiltrated via resin transfer moulding in a layup of SA3 fabrics, thermally cured and pyrolyzed. This process was repeated until a residual open porosity of below 5% was reached. During pyrolysis the polyvinylsilazane converts to an amorphous SiCN matrix. In combination with the high modulus Tyranno SA3 SiC fibers a weak matrix composite is created. To protect the composite in oxidative environment at high temperatures, an exterior SiC coating by means of chemical vapour deposition was applied. The polyvinylsilazane was investigated in terms of differential scanning calorimetry and measurement of viscosity to find the ideal temperatures for the polymer infiltration step. Specimens of the precursor were cured and pyrolyzed. The densification during pyrolysis was investigated in terms of He gas pycnometry and X-ray diffraction. The composite was characterized by SEM, µCT and mercury intrusion porosimetry. To determine the suitability of the SiC/SiCN composite for high temperature applications, samples were oxidized and tested by means of 3-point bending.
The thermal conductivities of nanoporous and nanocompositesilicon with incorporated amorphous phases have been computed by molecular dynamics simulations. A systematic investigation of the porosity and the width of the amorphous shell contouring a spherical pore has been made. The impact of amorphous phase nanoinclusions in a crystalline matrix has also been studied with the same amorphous fraction as the porosity of nanoporoussilicon to achieve comparison. The key parameter for all configurations with or without the amorphous phase is proved to be the interface (between the crystalline and amorphous phases or crystalline and void) to volume ratio. We obtain the sub-amorphous thermal conductivity for several configurations by combining pores, amorphous shell, and crystalline phase. These configurations are promising candidates for low cost and not toxic thermoelectric devices based on abundant semiconductors.
Ceramic matrix composites reinforced with long fibers are commonly fabricated by infiltration methods, in which the ceramic matrix is formed from a fluid infiltrating into the fiber structure. Infiltration techniques differ from each other in the types of fluids and the processes for converting the fluid into a ceramic: polymer infiltration and pyrolysis (PIP), chemical vapor infiltration (CVI), reactive melt infiltration (RMI), slurry infiltration, sol-gel infiltration. This chapter discusses the formation of the ceramic matrix microstructure, properties of the interface and the benefits and drawbacks of the composites prepared by the different techniques. Fabrication routes including the stages of preform preparation, interphase deposition, preceramic fluid infiltration and thermal processing are described.
The science and technological development of polymer-derived ceramics (PDCs) are highly interdisciplinary, at the forefront of micro- and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers. This chapter highlights scientific issues related to advanced PDCs research. These issues include general synthesis procedures to produce silicon-based pre-ceramic polymers; special microstructural features of PDCs; and unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines. The chapter also deals with processing strategies to fabricate ceramic components from preceramic polymers, and discussion and presentation of several examples of possible real-life applications that take advantage of the special characteristics of pre-ceramic polymers.
This introductory chapter focuses on the behavior, performance, and experimental results of ceramic matrix composites (CMCs). The strong interest in continuous fiber-reinforced CMCs has arisen primarily because of their ability to retain good tensile strength in the presence of holes and notches. This characteristic is important because CMC components generally need to be attached to other components. This chapter is based on the recognition that mechanism-based models are needed, which allows efficient interpolation between well-conceived experimental matrix composites. The emphasis is on the creation of a framework that allows models to be inserted as they are developed and that can also be validated by carefully chosen experiments. Some of the basic thermomechanical characteristics of composites are first established with emphasis on interfaces and interface properties, as well as residual stresses in this chapter. Then, the fundamental response of unidirectional (1D) materials, subject to tensile loading, is addressed in the chapter, in accordance with several subtopics: mechanisms of nonlinear deformation and failure; constitutive laws that relate macroscopic performance to constituent properties; the use of stress-strain measurements to determine constituent properties in a consistent, straightforward manner; and the simulation of stress-strain curves. The discussion of 1D materials in the chapter is followed by the application of the same concepts to 2D materials, subject to combinations of tensile and shear loading. The objective of this chapter is to address the mechanisms of stress redistribution upon monotonic and cyclic loading as well as the mechanics needed to characterize the notch sensitivity.
SiC fiber-reinforced SiC composites (SiC/SiC), whose preforms had 3D satin weave or 2D non-woven fabric, were fabricated by chemical vapor infiltration (CVI) or polymer impregnation and pyrolysis (PIP). The 3D satin texture made from Hi-Nicalon Type S fiber was successfully woven although the fiber has high elastic modulus. Both CVI and PIP SiC/SiC composites of Type S fiber had higher thermal conductivity than those of Hi-Nicalon fiber. These results can be ascribed to higher thermal conductivity of Type S fiber. The thermal conductivity of the 3D PIP SiC/SiC composites was increased after annealing at 1400°C in vacuum. The bend strength of the 2D CVI SiC/SiC composites of non-coated Hi-Nicalon fiber was higher than that of non-coated Type S fiber, indicating that interfacial modification for Type S fiber is needed to obtain good mechanical properties.
This study presents enhanced studies of inverse approach of the classical laminate theory for prediction of the elastic properties of a wound oxide ceramic matrix composites material (CMC). Based on mechanical tests and microstructure analysis, elastic properties of virtual equivalent unidirectional layers were calculated. To adapt the analytical model to CMCs from different batches which show various fiber volume contents, porosities, and different fiber orientations, a scaling factor Ω was introduced with the help of modified mixing considering these specific properties. A good correlation between experimental and analytically calculated results showed in this study.
Abstract Within the Structures and Materials Division at the National Aeronautics and Space Administration Glenn Research Center (GRC), research is being conducted to develop durable high-temperature materials for the most challenging aerospace applications. Research is advancing material and coating technologies for applications including turbine engine hot section components, rocket engine combustion chamber liners, high-temperature components of advanced space power systems, and atmospheric reentry vehicle surfaces. As part of the volume of papers recognizing 70 years of research at the GRC, this paper summarizes key research contributions that GRC has made to the field of high-temperature aerospace materials.
The goals of the NASA Environmentally Responsible Aviation (ERA) Project are to reduce the NOx emissions, fuel burn, and noise from turbine engines. In order to help meet these goals, commercially-produced ceramic matrix composite (CMC) components and environmental barrier coatings (EBCs) are being evaluated as parts and panels. The components include a CMC combustor liner, a CMC high pressure turbine vane, and a CMC exhaust nozzle as well as advanced EBCs that are tailored to the operating conditions of the CMC combustor and vane. The CMC combustor (w/EBC) could provide 2700oF temperature capability with less component cooling requirements to allow for more efficient combustion and reductions in NOx emissions. The CMC vane (w/EBC) will also have temperature capability up to 2700oF and allow for reduced fuel burn. The CMC mixer nozzle will offer reduced weight and improved mixing efficiency to provide reduced fuel burn. The main objectives are to evaluate the manufacturability of the complex-shaped components and to evaluate their performance under simulated engine operating conditions. Progress in CMC component fabrication, evaluation, and testing is presented in which the goal is to advance from the proof of concept validation (TRL 3) to a system/subsystem or prototype demonstration in a relevant environment (TRL 6).
Non-oxide ceramic fibers are of considerable interest due to the ability to combine the high performance, especially high temperature thermal and creep resistance, with the structural advantages of fibers including their use as reinforcements for metal (MMCs) and ceramic matrix composites (CMCs). In this paper the development of CVD SiC fibers and three generations of polymer derived SiC fibers over the past 50 years are discussed, illustrating the effect of fiber precursor and processing on the microstructure and physical properties of the non-oxide ceramic fibers. Additionally recent advances in research and development related to fibers from SiC and SiCN systems are presented with discussion of the current focus on reducing the costs of the fiber processing, while increasing their thermostructural stability.
Kinking of a plane strain crack out of the interface between two dissimilar isotropic elastic solids is analyzed. The focus is on the initiation of kinking and thus the segment of the crack leaving the interface is imagined to be short compared to the segment in the interface. Accordingly, the analysis provides the stress intensity factors and energy release rate of the kinked crack in terms of the corresponding quantities for the interface crack prior to kinking. Roughly speaking, the energy release rate is enhanced if the crack heads into the more compliant material and is diminished if it kinks into the stiff material. The results suggest a tendency for a crack to be trapped in the interface irrespective of the loading when the compliant material is tough and the stiff material is at least as tough as the interface.
Nanocomposites consisting of precursor-derived Si–C–N ceramics incorporated with carbon nanotubes (CNTs) were successfully prepared by casting of a mixture of CNTs and a liquid precursor polymer followed by cross-linking and thermolysis. The effect of CNTs on the fracture toughness of these nanocomposites was investigated by a thermal loading technique. The results reveal a dependence of the fracture toughness on the type of the CNTs. One type shows a significant increase of the fracture toughness at CNT contents of only 1–2 mass%, whereas the other one exhibits no effect. The microstructural effects of CNTs observed at the fracture surfaces of the nanocomposites by scanning electron microscope (SEM) and transmission electron microscope (TEM) can be correlated with the observed fracture toughness behavior.
A SiC/SiC composite is a candidate material for a demonstration fusion power reactor (DEMO). Identifying the inherent anisotropy of composites is required to predict axial/off-axial mechanical properties for various failure modes. This study evaluated crack propagation behavior by the various modes to provide strength anisotropy maps and we discussed a method to analytically predict this trend. The strength anisotropy maps identified for various fabric orientations clearly indicate that the composites failed in the mixed modes. Specifically, due to the axial anisotropy, five individual mode strengths such as tensile/compressive strengths in the axial/transverse directions, respectively, as well as the in-plane shear strength, are identified to become essential input parameters. With the analytical criterion based on the Tsai–Wu model, the strength anisotropy could satisfactorily be described.
Continuous fibre reinforced ceramic matrix composites (CFCCs) show a scatter in parameters such as first matrix cracking stress, initial elastic modulus, ultimate strength, etc. which characterise their mechanical behaviour. This scatter can be attributed to a large extent to variations in the axial residual stress state from specimen to specimen. Under some circumstances even negative strain accumulation with increasing stress has been observed during tensile loading of CFCCs. Examples of such anomalous stress-strain behaviour are shown, and its occurrence is explained by the presence of an axial residual stress state which differs from that induced by the thermal expansion mismatch between the fibres and the matrix. Microstructural features leading to the establishment of such an abnormal residual stress state are indicated. Combining these observations it appears that variation in the residual stress state explains both the scatter observed in tests exhibiting normal behaviour, and in tests with anomalous stress-strain behaviour. The latter is hence only a manifestation of extreme scatter in the axial residual stresses.
In situ fibre fracture characteristics have been investigated for Si–Ti–C–O fibres after tensile testing up to 1380°C in vacuum and in air. Specimens tested in air at 1100 and 1200°C generally had flat fracture surfaces with less than 20% of fibres exhibiting fracture mirrors: this is attributed to oxygen ingress into the fibre bundles. Fibre strength characteristics normalised to a 10−3 m gauge length indicated that fibres tested in air at elevated temperature have significantly lower strengths and average Weibull parameter, m, compared to the room-temperature, 1200 and 1300°C/vacuum cases, and this is attributed to oxygen damage of the fibre together with oxidation of the fibre/matrix interface. The fibre/matrix interface shear strength, τ, was low for the room-temperature specimens and increased slightly with temperature when tested in vacuum, possibly as a result of a change in the thermal mismatch between fibres and matrix. Values of τ for specimens tested at 1100 and 1200°C in air were an order of magnitude greater than those for room-temperature specimens, indicating a significant degree of oxidation damage at the fibre/matrix interface to have occurred.
The concept of a tensile mastercurve which uniquely represents the mechanical response to time-independent tensile loading is presented. It is shown how the mastercurve can be determined from individual tensile tests with unloading–reloading cycles, and how it provides a rationalisation of both the temperature dependence and the scatter in tensile behaviour. Implications for modelling of the mechanical behaviour are outlined.
A ceramic-matrix composite (CMC) that exhibits damage-tolerant behavior without “weak” interfaces has been demonstrated. The concept relies on the heterogeneous distribution of fiber bundles within a porous matrix having homogeneous, fine porosity. It also depends on the development of residual stress from thermal expansion mismatch. The present demonstration uses Al2O3 fibers with either a silicon nitride or a mullite matrix. The latter is inherently oxidation resistant.
The effects of circular holes and sharp notches on the tensile strength of two Nicalon-reinforced ceramic composites have been investigated. The influence of inelastic straining on the redistribution of stress has been elucidated through measurements of the local strains in the regions of high stress concentration, coupled with finite element simulations of the test geometries, using a nonlinear constitutive law appropriate to ceramic composites. The scale dependence of strength has been inferred from tests performed on specimens of varying size. The utility of two failure models that incorporate both the inelastic straining and the scale dependence has been assessed: one based on the point stress failure criterion and the other on weakest-link fracture statistics. Both approaches provide a reasonably consistent description of the experimental measurements. Some of the implications and limitations associated with the failure models are discussed.
The tensile mechanical properties of ceramic matrix composites (CMC) in directions off the primary axes of the reinforcing fibers are important for the architectural design of CMC components that are subjected to multiaxial stress states. In this study, two-dimensional (2D)-woven melt-infiltrated (MI) SiC/SiC composite panels with balanced fiber content in the 0° and 90° directions were tensile loaded in-plane in the 0° direction and at 45° to this direction. In addition, a 2D triaxially braided MI SiC/SiC composite panel with a higher fiber content in the ±67° bias directions compared with the axial direction was tensile loaded perpendicular to the axial direction tows (i.e., 23° from the bias fibers). Stress–strain behavior, acoustic emission, and optical microscopy were used to quantify stress-dependent matrix cracking and ultimate strength in the panels. It was observed that both off-axis-loaded panels displayed higher composite onset stresses for through-thickness matrix cracking than the 2D-woven 0/90 panels loaded in the primary 0° direction. These improvements for off-axis cracking strength can in part be attributed to higher effective fiber fractions in the loading direction, which in turn reduces internal stresses on weak regions in the architecture, e.g., minicomposite tows oriented normal to the loading direction and/or critical flaws in the matrix for a given composite stress. Both off-axis-oriented panels also showed relatively good ultimate tensile strength when compared with other off-axis-oriented composites in the literature, both on an absolute strength basis as well as when normalized by the average fiber strength within the composites. Initial implications are discussed for constituent and architecture design to improve the directional cracking of SiC/SiC CMC components with MI matrices.
The present article focuses on changes in the mechanical properties of an all-oxide fiber-reinforced composite following long-term exposure (1000 h) at temperatures of 1000–1200°C in air. The composite of interest derives its damage tolerance from a highly porous matrix, precluding the need for an interphase at the fiber–matrix boundary. The key issue involves the stability of the porosity against densification and the associated implications for long-term durability of the composite at elevated temperatures. For this purpose, comparisons are made in the tensile properties and fracture characteristics of a 2D woven fiber composite both along the fiber direction and at 45° to the fiber axes before and after the aging treatments. Additionally, changes in the state of the matrix are probed through measurements of matrix hardness by Vickers indentation and through the determination of the matrix Young's modulus, using the measured composite moduli coupled with classical laminate theory. The study reveals that, despite evidence of some strengthening of the matrix and the fiber–matrix interfaces during aging, the key tensile properties in the 0°/90° orientation, including strength and failure strain, are unchanged. This strengthening is manifested to a more significant extent in the composite properties in the ±45° orientation, wherein the modulus and the tensile strength each exhibit a twofold increase after the 1200°C aging treatment. It also results in a change in the failure mechanism, from one involving predominantly matrix damage and interply delamination to one which is dominated by fiber fracture. Additionally, salient changes in the mechanical response beyond the maximum load suggest the existence of an optimum matrix strength at which the fracture energy in the ±45° orientation attains a maximum. The implications for long-term durability of this class of composite are discussed.
Silicon melt infiltrated, SiC-based ceramic matrix composites (MI-CMCs) have been developed for use in gas turbine engines.
These materials are particularly suited to use in gas turbines due to their low porosity, high thermal conductivity, low thermal
expansion, high toughness and high matrix cracking stress. Several variations of the overall fabrication process for these
materials are possible, but this paper will focus on “prepreg” and “slurry cast” MI-CMCs with particular reference to applications
in power generation gas turbines. These composites have recently been commercialized under the name of HiPerComp™.
Wound highly porous oxide matrix (WHIPOX) ceramic matrix composites consist of oxide fibers (mullite- or alumina-type) which are embedded in mullite- or alumina-rich matrices, respectively. In the ideal case the fiber distribution is homogeneous; in reality, however, fabrication (winding)-induced matrix agglomerations do occur. As knowledge on the homogeneity of the material is crucial for the prediction of the mechanical behavior a technique to describe the mesostructure of WHIPOX quantitatively has been developed by means of optical microscopy (transmitted light). The technique makes use of light conductivity and opacity of fibers and matrix, respectively. Three-dimensional plots of the matrix agglomerations were obtained by tomographic methods using ≈25 individual slices of 1.5 mm thickness for each sample.Samples from different sites of a WHIPOX plate, and samples which have been differently pressed prior to sintering were examined mesostructurally. The study showed that delamination-induced failure of WHIPOX is essentially controlled by localized interlaminate matrix agglomerations. Compression of WHIPOX plates in the pre-sintering moist stage helps to achieve a better homogeneity and thus improved shear strength of WHIPOX components.
As they are potential candidates for fusion reactor structural materials, R&D are being conducted on SiC-based composite materials (CREST-ACE program). To improve the efficiency of the polymer impregnation and pyrolysis (PIP) process for SiC/SiC composite fabrication, a new precursor polymer, poly-vinylsilane (PVS) with SiC filler addition, was adopted as a matrix precursor and process optimization was performed. Consequently, high-density SiC/SiC composite with high mechanical properties was efficiently fabricated. Importantly, near-stoichiometric SiC matrix was developed by blending of poly-carbosilane (PCS) and poly-methylsilane (PMS). Remarkable improvements in tensile properties and fatigue characteristics (at 1573 K) were attained when inorganic powder fillers, BMAS or ZrSiO4, were added to the polymer mixture as the matrix precursor. These results are encouraging to make economically and environmentally attractive fusion reactors utilizing SiC/SiC composites as major structural materials.
Silicon carbide (SiC)-based ceramic composites have been studied for fusion applications for more than a decade. The potential for these materials have been widely discussed and is now understood to be (1) the ability to operate in temperature regimes much higher than for metallic alloys, (2) an inherent low level of long-lived radioisotopes that reduces the radiological burden of the structure, and (3) perceived tolerance against neutron irradiation up to high temperatures. This paper reviews the recent progress in development, characterization, and irradiation effect studies for SiC composites for fusion energy applications. It also makes the case that SiC composites are progressing from the stage of potential viability and proof-of-principle to one where they are ready for system demonstration, i.e., for flow channel inserts in Pb–Li blankets. Finally, remaining general and specific technical issues for SiC composite development for fusion applications are identified.
A continuum damage mechanics constitutive model is developed to describe the mechanical behavior of fiber-reinforced ceramic matrix composites submitted to complex multiaxial loadings. This model relies on the use of a phenomenological internal damage variable defined as the change of the compliance tensor induced by any given loading. Microstructural observations of the damage entities provide qualitative data allowing us to formulate physically founded simplifying hypotheses. The evolution laws of scalar damage variables derived from the components of the compliance tensor are established within a classical thermodynamic framework, using coupled multicriteria expressed in the space of the associated thermodynamic forces. Damage deactivation processes related to the unilateral crack closure effect observed under compressive loadings are introduced through the definition of effective compliance tensor components. This approach is applied to a 2-D woven SiC/SiC composite processed by chemical vapor infiltration (CVI). Major advantages and drawbacks of the proposed approach as observed from various validation tests are discussed.
Ceramic fiber reinforced ceramic matrix composites (CMC) are outstanding ceramics with high fracture toughness. This can be realized if both brittle components of the composite, i.e., fibers and matrix are interacting with each other in an efficient way. Either a weak interface allowing debonding between fiber and matrix controls the fracture processes (WIC–CMC) or the matrix takes this role of a weak and more compliant component (WMC–CMC). An experimental test data base is presented for a WMC-type composite where the materials data are used to establish a model which describes the materials behavior in a macroscopic way. Inelastic deformation and materials damage processes are defined, measured and interpreted on the base of a continuum damage mechanics concept. The elastic and inelastic response is then predictable up to failure as being dependent on the angle between fiber and loading directions of the specimens.
Study on the mechanical behavior of FW12 at room and elevated temperatures
- Tushtev