SiC Fibers Research Articles

Novel multifibrillar carbon and oxidation-stable carbon/ceramic hybrid fibers consisting of thousands of individual nanofibers with high tensile strength

In this study, multifibrillar carbon and carbon/ceramic (C/SiCON) fibers consisting of thousands of single nanofibers are continuously manufactured. The process starts with electrospinning of polyacrylonitrile (PAN) and PAN/oligosilazane precursors resulting in poorly aligned polymer fibers. Subsequent stretching leads to parallel aligned multifibrillar fibers, which are continuously stabilized and pyrolyzed to C or C/SiCON hybrid fibers. The multifibrillar carbon fibers show a high tensile strength of 911 MPa and Young’s modulus of 154 GPa, whereas the multifibrillar C/SiCON fibers initially have only tensile strengths of 407 MPa and Young’s modulus of 77 GPa, due to sticking of the nanofibers during the stabilization in air. Additional curing with electron beam radiation, results in a remarkable increase in tensile strength of 707 MPa and Young's modulus of 98 GPa. The good mechanical properties are highlighted by the low linear density of the multifibrillar C/SiCON fibers (~ 1 tex) compared to conventional C and SiC fiber bundles (~ 200 tex). In combination with the large surface area of the fibers better mechanical properties of respective composites with a reduced fiber content can be achieved. In addition, the developed approach offers high potential to produce advanced endless multifibrillar carbon and C/SiCON nanofibers in an industrial scale.

Heterogeneous components removal mechanism and grinding force model from energy aspect in ultrasonic grinding continuous fiber reinforced metal matrix composites

Continuous fiber reinforced metal matrix composites (CFMMCs) offer higher specific strength, specific modulus, and operating temperature than matrix metals due to the unique enforcement mechanism of the one-to-one scale arrangement of matrix and reinforced phases. Due to the heterogeneous characteristics between the plastic matrix and brittle fibers, the removal mechanism of CFMMCs during processing is exceptionally complex. Ultrasonic vibration-assisted grinding (UVAG) shows great advantages in machining difficult-to-cut materials (i.e., ceramics and composites) by changing the motion trajectory between grains and workpieces, effectively reducing grinding force and improving machining quality. However, little is known about the removal mechanism of UVAG for CFMMCs composed of the ductile (i.e., metal matrix) and brittle (i.e., SiC fiber) phases with highly anisotropic structure characteristics. This raises the question of how CFMMCs with heterogeneous components perform under abrasive processing and how to predict their processing forces. Hence, UVAG and conventional grinding (CG) experiments with single CBN grain were carried out on SiC fiber reinforced TC17 matrix composites (SiCf/TC17) in this work. A grinding force model considering both phases and materials structure from energy aspect was proposed. A theoretical model for suppressing SiC fiber damage has been proposed, which is expected to guide low-damage processing of brittle materials. According to the results, the removal models of CFMMCs are revealed including: i) macro fracture of SiC fiber, ii) neat fracture of SiC fiber, and iii) TC17 matrix massive adhesion on the SiC fiber. Besides, no cracks crossing fibers are observed on the subsurface of SiC fiber under both UVAG and CG due to the good support of the TC17 matrix on SiC fibers. The grinding force predicted model error decreases as ap increases. When ap is 50 μm, the errors between predicted and experimental values are 7.8 % and 9.1 % for normal forces (Fn) and tangential forces (Ft), respectively. Ultrasound suppresses the severe wear behavior of grains, thereby improving the tool life. This paper aims to comprehensively reveal the characteristics of abrasive processing of CFMMCs from various aspects (surface morphology, subsurface features, grinding force prediction, and tool wear), which will promote the industrial application of CFMMCs.

An efficient multi-scale method for failure mechanism analysis of SiCf/Ti composites with experimental validation

This study focuses on developing and validating a high-precision concurrent multi-scale finite element model considering the progressive damage model (PDM) and the cohesive zone model (CZM) for predicting the damage evolution of SiCf/Ti composites. At the macro-scale, a constitutive model describing the anisotropic damage criterion of SiCf/Ti composites was implemented by means of a user-defined subroutine (VUMAT). Meanwhile, based on the submodelling, a representative volume element (RVE) model was developed to systematically analyze the failure modes of the SiC fibers, the matrix, and the interface, respectively. Specifically, the crack initiation and propagation in the stress concentration zones were further discussed. Simultaneously, experimental methods were employed to verify the feasibility of the current model. During the uniaxial compression, the brittle fracture of the SiC fibers and ductile fracture of the matrix are the predominant failure modes of SiCf/Ti composites, with fiber fracture being the dominant factor. The crack initiates at the fibers, propagates rapidly to the fiber-matrix interface, and then extends into the matrix. Moreover, the fracture locations are suscepticle to stress concentrations, leading to the crack initiation and propagation. The predicted results show that the local damage has a significant effect on the failure mechanism of SiCf/Ti composites and this multi-scale model provides a scientific reference for the design and optimization of composites.

Microstructural evolution and phase transformation of cansas 3303 SiC fibers during thermal shock at 1200 °C

A self-built automatic platform was built to evaluate the thermal shock performances of Cansas-III SiC fibers at 1200 °C in air up to 500 cycles, the corresponding oxidation and damage mechanisms were then revealed. The results showed that oxide scale was formed during thermal shock with a thickness of about 1 μm, the core was not oxidized via its protection. The fiber roughness increased before 100 cycles due to the decomposition of the amorphous SiCxOy phase, and then decreased as the formation of oxide scale. Within Cansas-III SiC fibers, structural defects of free carbon increased as the increasing ID/IG values, in addition, the TO peak shifted to the left, which both demonstrated that the relative movements occurred among SiC grains and residual stresses were then emerged. To clarify the axis and radial cracking of the oxide scale, a modified formulation considering nonzero shear strain components was then proposed to calculate the related residual stresses. The peeling and cracking of the oxide scale was ascribed to the residual stresses of GPa level among SiC grains and the thermal expansion mismatch between the oxide scale and the core.

Feature analysis and recognition of fiber breakage AE signals after propagation

Acoustic emission (AE) is a non-destructive testing technique, and establishing correlations between AE signals and material damage modes is one of its primary challenges. However, it is difficult to identify damage modes in ceramic matrix composites (CMCs) due to AE signal attenuation occurring after propagation and complex damage modes. In this study, AE signals generated by the breakage of C and SiC fibers were monitored at different distances and angles on the C/SiC plate. The attenuation of energy and the frequency spectra were analyzed. The Mel-frequency cepstral coefficient (MFCC) method was used to analyze the waveform data of AE signals and extract MFCC features. To identify the damage caused by C and SiC fiber breakage, AE parameter features and MFCC features were selected as inputs, and a fully connected neural network was constructed to train a supervised pattern recognition model. The results show that the MFCC feature has higher recognition accuracy than the traditional feature when AE is used for damage identification.

Formation mechanism of TiC twin during densification of SiCf/Ti2AlNb composites

SiCf/Ti2AlNb composites were fabricated using the magnetron sputtering precursor wires method in conjunction with the hot isostatic pressing (HIP) technique. The morphology, elemental distribution, and structural characteristics of the interfacial reaction layer (RL) were examined through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and wavelength dispersive spectroscopy (WDS). The results showed that the interface between SiC fibers and Ti2AlNb reacted to form TiC and α2. Due to the interdiffusion of elements at the interface, a small amount of Al atoms existed in the TiC layer. These Al atoms reduced the stacking fault energy (SFE) of TiC, which is beneficial for the formation and stability of Σ3{111} twin boundaries.

Development of 1400°C(2552°F) class Ceramic Matrix Composite Turbine Shroud and Demonstration Test with JAXA F7 Aircraft Engine

Abstract The Authors developed 1400°C(2552°F) class CMC material system which consist of SiC fibers and SiC matrix and ytterbium silicate base matrix, aiming for higher temperature capability. Then they designed and manufactured high pressure turbine shrouds for aircraft engines using that 1400°C class material system, and they conducted strength tests and thermal cycle tests for turbine shroud components. After that they conducted engine tests for the CMC turbine shrouds demonstration in the actual engine environment jointly with the Japan Aerospace Exploration Agency (JAXA) in 2021. The engine test was conducted for over 75 hours including over 35 hour hot time. After the test teardown inspection was conducted. No spallation of EBC, no recession and no wear on CMC turbine shrouds were found. As the result of the microstructure observation for cut faces of CMC turbine shrouds, no oxidation in SiC fibers, no chemical reaction in matrix, and no microcrack in matrix were found, but, some oxidation in fiber interface coating and microcrack in EBC were found. Bending strength tests with specimens cut out from CMC turbine shrouds were conducted in order to survey the degradation of material. As the result of bending test, the strength of the specimens cut out from engine tested shrouds were equivalent to the strength of the specimens cut out from unused shrouds. The CMC turbine shrouds after engine test were determined to be serviceable, therefore the developed 1400°C class CMC shrouds was proven to be sound in an actual engine environment.

Li2O addition as a means of achieving low-temperature densification of SiC ceramic

In order to achieve low-temperature densification of SiC ceramic, β-SiC nanopowder was doped with Li2O together with Y2O3–Al2O3–MgO as sintering aids, and then spark plasma sintered at temperatures ranging from 1450 °C to 1600 °C for 30 min under 30 MPa pressure in argon. The densities, microstructures and mechanical properties of SiC ceramics were systematically investigated, in comparison to the counterparts sintered with Y2O3–Al2O3–MgO additives. Since Li2O has a low melting point, Li2O addition effectively decreased the formation temperature of liquid secondary phase, enabling SiC ceramics to obtain higher density and lower porosity after low-temperature sintering (T ≤ 1550 °C). Besides, Li2O addition was found to decrease the viscosity of liquid phase and enhance grain growth of SiC through dissolution-reprecipitation process. When sintered at 1550 °C, Li2O addition led to a remarkable increase of density from 3.03 g/cm3 to 3.21 g/cm3 for SiC ceramic. Meanwhile, the SiC ceramic with Li2O addition exhibited a high Vickers hardness of 22.75 ± 0.35 GPa and a good fracture toughness of 3.83 ± 0.27 MPa m1/2. The research results are expected to benefit the low-cost fabrication of SiC ceramics based on pressure-assisted sintering and alleviate the thermal damage of SiC fiber in hot-pressed SiCf/SiC composites.

Microstructure and mechanical properties of SiCf/SiC–AlN composites prepared by low temperature reactive melt infiltration

In this work, SiCf/SiC–AlN composites were fabricated by low temperature reactive melt infiltration of Si–Al alloy into porous SiCf/C–Si3N4 preforms prepared through slurry brushing. The effect of the preforms pore structure on the melt infiltration was studied, and the erosion behaviors of Si–Al alloy melt on the fibers and the interphases were investigated. The results indicate that the preforms with smaller pore size can hinder the alloy melt infiltration, making it difficult for the infiltration process to proceed adequately. In contrast, when the preforms with larger pore size are utilized, the alloy melt can completely infiltrate into the preforms and form dense SiC–AlN matrix in the intra-fiber bundle by in-situ reaction. Moreover, the bending strength of the composites derived from larger pore size preforms could reach 160.2 MPa, which is about 1.5 times higher than that of the composites derived from smaller pore size preforms. In addition, the damage of the SiC interphase caused by the preforms preparation aggravates the erosion of the internal BN interphase and SiC fibers by Al melt. The eroded interphases and fibers could adversely affect the mechanical properties of both composites. This study lays the foundation for the preparation of the composites with excellent properties.

Processing and properties of 3D woven SiC/SiC composites with hybrid matrix

3D woven SiC/SiC composites with Sylramic™-iBN SiC fibers have been fabricated by first partially infiltrating 3D woven fiber preforms with SiC by chemical vapor infiltration (CVI) and then by polymer infiltration and pyrolysis (PIP) of SiC-yielding polymer. The influence of fiber architecture on damage evolution and failure mechanisms during tensile loading, in-plane tensile properties, transverse thermal conductivity, and interlaminar tensile strength of the composites has been investigated. Wherever possible, the data are compared with those of 2D woven melt infiltrated (MI), CVI, and (CVI+PIP) hybrid SiC/SiC composites. The composites exhibit 300-hr creep durability at 1482 °C in air at stresses up to 138 MPa and show potential for elevated-temperature applications. Further improvements in composite properties are possible by optimizing composite constituents and fiber architecture.

Improvement of carbon fiber oxidation resistance by thin ceramic coating using silica particles

Carbon fiber-reinforced plastics have become indispensable materials in energy saving and new energy technologies, such as automobiles, wind power generators, and hydrogen tanks for fuel cells. However, the lack of oxidation resistance limits the high-temperature applications of carbon fibers. In this study, we developed a coating technology for carbon fibers to improve their oxidative resistance. Carbon fibers with silicon carbide coatings were obtained by the adsorption of silica colloids on carbon fibers using electro adsorption, followed by heating. The resulting coating has small thickness and strong chemical connections. The thermogravimetric analysis confirmed that the oxidation start temperature of the coated carbon fibers was higher than that of carbon fibers. Moreover, the mechanical properties of the coated carbon fibers were maintained. Therefore, the proposed coating technology can broaden the application of carbon fibers as the alternative to some SiC fiber applications. Further, it is applicable for recycling carbon fibers to increase their value in terms of thermal oxidation stability.

Multi-pulse femtosecond laser ablation of SiC/SiC composites: Morphological evolution, oxidation mechanisms and component characterization

SiC fiber reinforced SiC matrix (SiosC/SiC) composites processed by femtosecond laser play a pivotal role in thermal protection systems. This paper reveals the ablation characteristics of multi-pulse femtosecond laser ablation of SiC/SiC composites, including morphological evolution, oxidation mechanisms and component characterization by heat accumulation simulation and advanced experimental analysis. The results show that the ablation depth induced by the heat accumulation effect is an exponential function of laser parameters. Morphological analysis and component characterization indicate that two types of ablation regions are formed, normal ablation region is an oxidized zone induced by passive oxidation mechanism, and central ablation region is a non-oxidized zone induced by active oxidation mechanism. The increase of single pulse energy and decrease of scanning speed enhance the driving force of chemical reactions, thus dramatically affecting the gas hole and graphite properties in the normal ablation region. Furthermore, the propagation path of thermal stress-induced cracks is determined which emerges from the center ablation region, extending through gas holes to the normal ablation region.

Ablation behavior of microwave absorbing SiCf/Si3N4 composites containing ZrOC–SiCN matrix

The thermal protection materials of ultra-high speed aircraft need the stealth performance, and it require the microwave absorbing materials should have high ablation resistance to serve under extreme conditions (>2000 °C). In this work, amorphous ZrOC–SiCN and SiCN were introduced into the inter-bundle matrix of SiC fiber reinforced Si3N4 matrix composites to improve the ablation resistance while retaining the wave-absorbing properties. For SiCf/Si3N4, the SiC nanocrystallines in SiC fiber are oxidized into SiO2 and accumulate on the surface, and the Si3N4 matrix is first transformed into SiC grains and then the SiC grains are gradually ablated into spherical SiO2 in the ablation process. After the introduction of ZrOC–SiCN and heat-treated at 1500 °C, the ZrCN can be formed to increase the scouring resistance of each woven layer, and the oxide film with high viscosity also can be formed, preventing the fiber bundles from being blown away in the central region, leading to the decrease of mass and line ablation rate from 12.2 mg/s and 27.3 μm/s to 7.3 mg/s and 16 μm/s, respectively. Meanwhile, the minimum reflection coefficient of the composite decreases from −4.9 to −10.6 dB.

Effect of Different Skew Angle on Tensile Fracture Behavior of SiC Fiber Monofilament

Abstract In fiber reinforced composites (FRP), silicon carbide fiber is indispensable. Its mechanical properties determine the performance of ceramic matrix composites. Therefore, the tensile and fracture behavior of silicon carbide fiber monofilaments should be studied in depth. In order to make the test results of fiber properties more accurate, this paper takes SiC fiber monofilament as the research object, carries out tensile tests at different deflection angles, and carries out WEIBULL statistical analysis and comparison of the data. The microscopic morphology of the tensile fracture of each single fiber was observed by SEM, and the change rule of the mechanical properties and fracture behavior of the fiber with the deflection angle was summarized, and whether the slight deflection affected the test results was explored. The results showed that: (1) The mechanical properties of SiC fiber monofilament decreased gradually with the increase of deflection angle, and the extent of decline gradually increased. (2) With the increase of deflection angle, the proportion of stepped fracture gradually increases, and that of flat fracture gradually decreases.

Failure envelope prediction of 2D SiCf/SiC composites based on XGBoost model

In this paper, a multiple failure mechanisms-based progressive damage model is developed to capture the mechanical response and defect-induced nonlinear behavior of 2D SiCf/SiC composites. This model is then adopted to simulate the stress–strain response of a representative volume element model under uniaxial loading, showing excellent agreement with experimental results. The effects of the volume fraction of SiC fibers, as well as those of the porosity and microcrack density of SiC matrix, on the deformation and damage behaviors of SiCf/SiC composites are predicted. Based on this, the failure envelopes related to micro-mesoscopic characteristics under combined loadings are generated as the database for the eXtreme Gradient Boosting (XGBoost) model training. Finally, a data-driven failure model is established for 2D SiCf/SiC composites, whose prediction is compared with the formal failure criteria. The results demonstrate that the data-driven 2D SiCf/SiC composite failure model is reliable in constructing the failure criteria related to micro-mesoscopic features.

Effect of long-time annealing at high temperature on the microstructure and mechanical properties of different types of SiC fibers

SiC fibers play a crucial role in ceramic-matrix composites due to their excellent high-temperature resistance. To analyze the high temperature stability of different types of SiC fibers, this work investigated three typical SiC fibers (KD-II, KD-S, KD-SA) after being annealed at 1600 °C and 1800 °C for 1 h ∼ 200 h, respectively. For annealing at 1600 °C, the decomposition of the SiCxOy phase and grain growth were the main reasons for strength degradation in KD-II and KD-S fibers, while the KD-SA fiber maintained good thermal stability due to its large grain size. For long-time annealing at 1800 °C, obvious Si sublimation in the three fibers, leading to appearance of skin-core structure, resulted in obvious strength decline. This phenomenon was more pronounced over 1800 °C. These results in this work could be helpful guidance for the preparation of higher performance SiC fibers and SiCf/SiC composites.

Oxidation of SiC fibers in water vapor

AbstractThe current study focuses on the role of composition in the oxidation behavior of three commercial SiC fibers (Hi‐Nicalon Type S (HNS), Tyranno ZMI, and Tyranno SA) in a steam‐containing environment at 1000°C. Oxide scales that form on each of the three fiber types differ in morphology, microstructure, and composition. Differences are attributed to O and C contents, crystallinity, and minor alloying elements. Oxidation kinetics of the HNS fibers follow linear/parabolic (Deal–Grove) predictions over the entire range of exposure times (to 96 h). In the other fibers, deviations from the initial linear/parabolic trend occur when either the scale crystallizes in situ during oxidation (as on the ZMI fibers) or when structural changes internal to the fiber alter the chemical environment at the scale/fiber interface (on the SA fibers). Adaptations of the linear/parabolic oxidation model are used to rationalize the growth kinetics in the latter two cases. Collectively, the data and analyses indicate a two‐fold difference in permeabilities of crystalline and amorphous silica scales. Inferred interface reaction rates further suggest faster oxidation for fibers with greater amounts of amorphous Si–O–C. Moreover, crystalline scales are more prone to cracking, because of their higher viscosity at elevated temperatures and a deleterious phase transformation upon cooling.

Microstructure and mechanical properties of multilayer SiC nanofiber paper‐reinforced SiC composites by the NITE method

AbstractIn this work, stacked SiC fibers paper‐reinforced SiC ceramic matrix composites (SiCnf/SiC CMCs) were prepared by nanoimpregnation and transient eutectic (NITE) process using ultra‐long single‐crystal SiC nanofibers. The effect of the SiCnf surface modification via boron nitride (BN) interphase deposition as well as the sintering additive (Al2O3–Y2O3) content on the density, microstructure, and mechanical properties of SiCnf/SiC CMCs has been investigated systematically. The results revealed that the laminated SiCnf/SiC CMCs feature a significant saw‐tooth‐like curve on the load–displacement test, which indicates that the obtained laminated structure with the weak interlayer binding owning to the BN coating treatment possesses significant toughening to the SiC ceramics. Specifically, the SiCnf/SiC CMCs with the sintering additive content of 12 wt% revealed a fracture toughness of over 10 MPa m1/2 after 60 min of BN interface deposition, which is 104.31% higher than that of the composites prepared without BN coating.

Oxidation and mechanical properties of SiC fibers after high temperature exposure in air and steam

SiC fibers are widely used in composites exposed to high temperature oxidizing environments. The present study compares the oxidation behavior of SiC fibers and the associated changes in tensile strength upon their exposure to dry air and steam for up to 50 hours at temperatures from 950°C to 1350°C. In both cases, the scale thickness and oxide microstructure were analyzed by scanning electron microscopy. When the SiC fibers were oxidized in air, a crack-free amorphous oxide layer formed at temperatures up to 1200°C, whereas a cracked, crystallized oxide layer was found at 1350°C. By contrast under steam environment, a crystalline oxide forms at 950°C for oxidation times longer than 30 h. In both environments, the fiber strength decreased with increasing exposure time and temperature, the degradation being faster after oxidation in steam compared to dry air. This appears to relate to residual stress in the oxide scale and a reduction in the cross section of the load-bearing unoxidized SiC. An increase in defect size may also contribute to the loss of strength. The oxide scale follows parabolic growth kinetics in both dry air and in steam from 950°C to 1350°C, except at 1350°C for over 30 hours. The oxidation rate in steam is faster than that in dry air, with activation energies of 79.8±1.4 kJ/mol in steam and 153.6±3.4 kJ/mol in dry air, respectively. This work is relevant to the application of SiC fibers and SiC fiber reinforced composites in air and steam environments at high temperature.

Improving mechanical and thermal expansion properties of aluminum matrix composites reinforced by silicon carbide fiber

Combining the colloidal dispersion method with the squeeze melt infiltration technique, aluminum matrix composites with different volume fractions of silicon carbide fiber were fabricated at 680 °C under vacuum. Microstructure analysis found that composite presented a dense structure and fibers were homogeneously distributed in matrix. The hardness, yield strength and elastic modulus of the composite monotonically increased with increasing the volume fraction of fiber. The highest flexural strength for composite with 10 vol% SiC fiber was achieved. The improved mechanical properties could be attributed to the dispersion strengthening and enhanced load transferring ability by fibers as well as the dislocation strengthening. On the other hand, the coefficient of thermal expansion of composites was measured at temperature up to 400 °C. It was found that the uniform introduction of silicon carbide fiber could significantly low the CTE of aluminum matrix composite, which could be attributed to the low CTE of SiC fiber and good fiber–matrix interface bonding.