PyC Interphase Research Articles

Actively-controlled PyC interphase failure mechanisms in C/SiC composite revealed using micro-mechanical interfacial testing

Actively-controlled PyC interphase failure mechanisms in C/SiC composite revealed using micro-mechanical interfacial testing

Influence of the thickness of pyrolytic carbon interphase on the mechanical behavior of SiC/(BN/PyC)/SiC composites

The composition and thickness of the interphase are critical factors affecting the strength of SiC/SiC materials. Therefore, SiC/(BN/PyC)/SiC ceramic matrix composites with varying PyC thicknesses were fabricated, and the mechanism by which thickness influences the mechanical properties of the materials was investigated. During the fracture process of the composites, debonding and crack deflection predominantly occurred at the BN/PyC interface. The PyC interphase thickness emerged as a critical factor influencing the mechanical performance of composites. The residual stresses within the PyC and its adjacent SiC matrix were determined by analyzing shifts in Raman peak positions. The findings indicate that a reduction in PyC thickness leads to an increase in residual compressive stress within the PyC and a corresponding increase in residual tensile stress within the SiC matrix, which increases the interfacial shear stress and improves the mechanical properties of the composites.

PyC interphase growth mechanism and control models of C/SiC composites

PyC interphase growth mechanism and control models of C/SiC composites

Obtaining SiC Fibers–PyC interfacial properties through push-out FEM Models

Silicon carbide (SiC) has attractive properties for use in nuclear reactors. However, it is hampered by inherent brittleness that is overcome through a fiber-reinforced composite. A finite element model based on cohesive zone contact coupled to Coulomb friction is proposed to simulate the push-out response of isolated fibers in SiC/PyC fiber-reinforced composites. The model reproduces the measurements obtained during push-out experiments of isolated fibers in different configurations and predicts interfacial failure between the PyC interphase and fiber. The brittle properties predicted coincide with literature indicating that fiber–PyC interface could prevent the crack propagation through the fiber, fulfilling the function of the structure.

Fabrication process and growth mechanism of novel micro‐stack PyC interphase with different textures

AbstractPyrolytic carbon (PyC) interphase plays a crucial role in the mechanical properties of fiber‐reinforced ceramic matrix composites. In this research, a novel micro‐stack PyC interphase with different PyC textures was designed and fabricated by changing the deposition parameters during the chemical vapor infiltration process. The growth mechanism of the micro‐stack PyC interphase with different texture were also studied by experimental characterizations and kinetic calculations, and the results show that the content ratio of (C2H2 + C2H4) to C6H6 gas intermediate is a key parameter to control the texture types of PyC interphase. Furthermore, the value of orientation angle (OA) value, thickness, and modulus of the micro‐stack PyC interphase were further characterized by high resolution TEM (HRTEM), scanning electronic microscopy, and nanoindentation. Finally, the tensile testing of mini‐Cf/PyC/SiC composites was conducted, and the results showed that the tensile strength of mini‐Cf/PyC/SiC composites with micro‐stack PyC interphase is approximately 40% higher than that containing single high texture PyC interphase. The improvements on the tensile strength of Cf/PyC/SiC composites prove the significant advantages of micro‐stack PyC interphase.

Effects of single-phase and Co-deposited interphases on mechanical hysteresis behavior in T700TM mini-Cf/SiC composites

Hysteresis behavior in Cf/SiC composites is affected by the interface properties. The objective of this paper is to establish the relationship between the mechanical hysteresis behavior of mini-Cf/SiC composites and interphase type (i.e., single-phase PyC interphase and co-deposited (PyC + SiC) interphase) and thickness (i.e., 300 ∼ 2000 nm). Cyclic tensile loading/unloading experiments were conducted for different mini-Cf/SiC composites. Evolution of composite’s cyclic strain, hysteresis width, and secant modulus with increasing peak stress were analyzed. The composite’s thermal residual stress (TRS) was obtained through the common intersection compliance point (CIP) method. Micro damage mechanisms of surface cracking in the matrix and fiber’s pullout at the fracture surface were observed under the SEM to characterize composite’s internal damage state. A micromechanical first loading–unloading-reloading constitutive model was developed to predict cyclic mechanical response. The values of interface slip ratio (ISR) and interface shear stress (ISS) were obtained through the hysteresis analysis. For the same interphase thickness and similar fiber volume of PyC and (PyC + SiC) interphase, the ultimate tensile strength (UTS) of mini-Cf/SiC composites with the PyC interphase was higher than that of the samples with the (PyC + SiC) interphase. The values of ISS for the mini-Cf/SiC with the (PyC + SiC) interphase were much higher than those of mini-Cf/SiC with PyC interphase.

Key role of interphase in continuous fiber 3D printed ceramic matrix composites

During the continuous fiber (CF) 3D printing process, the high viscosity of the printing materials makes it difficult to penetrate into the fiber bundles, resulting in the lack of matrix and poor interfacial bonding. In this paper, the interfacial bonding of CF 3D printed ceramics was controlled by chemical vapor infiltration (CVI). A thermoplastic ceramic precursor and continuous carbon fibers were used as printing materials, and complex lightweight structures were printed. After cross-linking and pyrolysis, the ceramic precursor was converted into amorphous SiOC. Then CVI process was carried out to control the interphase and fill the fiber bundles with ceramic matrix. The flexural strength and modulus increased with the increase of CVI time. When the PyC interphase thickness was 150–250 nm, the flexural strength increased by about 203%. Each fiber was surrounded by the interphase and dense ceramic matrix, which was the reason for the improvement of mechanical properties.

Influence of Texture and Thickness of Pyrocarbon Coatings as Interphase on the Mechanical Behavior of Specific 2.5D SiC/SiC Composites Reinforced with Hi-Nicalon S Fibers

In the framework of SiC/SiC composite development for nuclear applications, the influence of pyrocarbon interphase texture and thickness on the mechanical behavior both on as-processed materials and on irradiated materials is a major concern. Thus, the PyC interphase influence has to be clearly addressed to define its optimal chemical vapor infiltration processing parameters. For this purpose, specific 2.5D SiC/SiC composites reinforced with Hi-Nicalon S fibers and with two kinds of PyC texture and thickness were produced. Transmission electronic microscopy allowed PyC thickness and microstructure/texture characterizations, whereas push-out and tensile tests were employed as experimental mechanical procedures. The original result is that PyC nature directly influences the interfacial shear stress and failure mode of the weakest interface, regardless of the PyC thickness within the studied range. Adhesive failure or cohesive failure are highlighted depending on the PyC CVI deposition mechanisms. Similar post-irradiation characterizations will be required to assess the role of irradiation on the PyC microstructure/texture evolution and mechanical behavior of these materials.

Molecular dynamics simulation on PyC interfacial failure mechanism and shear strength of SiC/SiC composites

In this paper, a molecular dynamics simulation model was established for SiC/pyrolytic carbon (PyC)/SiC composites manufactured through a chemical vapor phase infiltration (CVI) process based on microscopic observation results to investigate the shear strength and failure mechanism of PyC interphase. The simulation results showed that due to the PyC interphase’s disordered structure, the carbon layer underwent non-planar shear, overlap, flip, and deflection during the shearing process, which increased the shear strength of the composites. In addition, when the strength of the PyC interphase was lower than the bonding strength with a fiber and matrix, failure was caused by the internal breakage of the PyC interphase; when the strength of the PyC interphase was higher than the bonding strength with a fiber or matrix, failure was caused by the debonding of the PyC interphase with a fiber or matrix.

A multi-scale prediction model of elastic modulus for ceramic matrix composites considering oxidation damage

The oxidation damage evolution law of ceramic matrix composites prepared by chemical vapor infiltration (CVI) process was analyzed. Considering the effects of temperature and oxidation time, prediction models of elastic modulus about fiber and cell were established based on the distribution of microcracks in matrix and the oxidation process of interface, fiber and matrix. The prediction results show that the tensile elastic moduli of carbon fiber (Cf)/SiC and SiC fiber (SiCf)/SiC composites decrease more obviously with the increase of temperature and oxidation time. The elastic prediction model was verified by the tensile tests of the composites after high temperature oxidation. The error between the prediction results and the test results of SiCf/SiC composites with BN interphase after oxidation at 1000 ℃ for different time is no more than 2%, and the error between the prediction results and the test results of Cf/SiC composites with PyC interphase after oxidation at 700℃ for different time is no more than 7%.

Improvement of high‐temperature stability of PIP SiCf/SiC material through in situ grown BNNTs

AbstractIn this paper, the effect of in situ grown boron nitride nanotubes (BNNTs) and preparation temperature on mechanical behavior of PIP (Precursor Infiltration and Pyrolysis) SiCf/SiC minicomposites under monotonic and compliance tensile is investigated. In situ BNNTs are grown on the surface of SiC fibers using ball milling–annealing process. Composite elastic modulus, tensile strength, fracture strain, tangent modulus, and loading/unloading inverse tangent modulus (ITM) are obtained and adopted to characterize the mechanical properties of the composites. Microstructures of in situ grown BNNTs and tensile fracture surfaces are observed under scanning electronic microscopic (SEM). For SiCf/SiC minicomposites with BNNTs, the elastic modulus, tensile strength, and fracture strain are all lower than those of SiCf/SiC minicomposites without BNNTs, mainly due to high preparation temperature and the oxidation of the PyC interphase during the annealing process. Tensile stress–strain curves of SiCf/SiC minicomposites with and without BNNTs are predicted using the developed micromechanical constitutive model. The predicted results agreed with experimental data. This work will provide guidance for predicting the service life of SiCf/SiC composite materials and may enable these materials to become a backbone for thermal structure systems in aerospace applications.

In-situ tensile damage and fracture behavior of PIP SiC/SiC minicomposites at room temperature

In-situ tensile damage and fracture behavior of original SiC fiber bundles, processed and uncoated SiC fiber bundles, SiC fiber bundle with PyC interphase, SiC/SiC minicomposites without/with PyC interphase are analyzed. Relationships between load-displacement curves, stress-strain curves, and micro damage mechanisms are established. A micromechanical approach is developed to predict the stress-strain curves of SiC/SiC minicomposites for different damage stages. Experimental tensile stress-strain curves of two different SiC fiber reinforced SiC matrix without/with interphase are predicted. Evolution of composite’s tangent modulus, interface debonding fraction, and broken fiber fraction with increasing applied stress is analyzed. For the BX™ and Cansas-3303™ SiC/SiC minicomposite with interphase, the composite’s tangent modulus decreased with applied stress especially approaching tensile fracture; the interface debonding fraction increased with applied stress, and the composite’s tensile fracture occurred with partial interface debonding; and the broken fiber fraction increased with applied stress, and most of fiber’s failure occurred approaching final tensile fracture.

Micromechanical modeling cyclic loading/unloading hysteresis loops of 3D needle-punched C/SiC ceramic-matrix composites

ABSTRACT In this paper, the 3D needle-punched C/SiC composite was fabricated using the chemical vapor deposition and reactive infiltration method. Four different fiber preforms were formed using the needled-punch technology. The PyC interphase was deposited on the surface of the fibers to increase the toughness of the composite under tensile loading. Cyclic loading/unloading tensile tests were conducted at different peak stresses. A micromechanical hysteresis constitutive relationship was developed considering damage mechanisms of matrix cracking, interface debonding and slip, and fiber’s fracture and pullout. Using the developed hysteresis loops models and damage models, the hysteresis loops of four different types of 3D needle-punched C/SiC composites were predicted for different peak stresses. The hysteresis parameters of unloading residual strain, peak strain, hysteresis loops width, hysteresis loops area, interface slip ratio, and inverse tangent modulus were adopted to characterize the tensile damage evolution inside of composites. Relationship between composite’s hysteresis behavior and microstructure damage evolution is established. It was found that the composite’s hysteresis loops depend on the fiber’s preform and interface properties.

Sintering Temperature and Interphase Effects on Mechanical Properties of an Oxide Fiber-Reinforced Al2O3-SiO2 Composite Fabricated by Sol–Gel Method

Three-dimensional Nextel™ 440 fiber-reinforced Al2O3-SiO2 matrix (N440/Al2O3-SiO2) composites were fabricated by sol–gel method with sintering temperature from 1100 °C to 1400 °C. The interface engineering of the composites was carried out by employing PyC and FC interphase. The microstructure and mechanical properties of the composites were investigated. Results showed that the sintering temperature had remarkable effects on the mechanical properties of the composites. The average flexural strength of the composite without interphase decreased as the temperature increased, with fracture behaviour changing from ductile to brittle, and the highest flexural strength of 90.0 MPa was obtained for the composite fabricated at 1100 °C. However, the flexural strength of the composite with PyC interphase increased initially and then decreased, with fracture behaviour remaining ductile, and the highest flexural strength of 116.3 MPa was obtained for the composite fabricated at 1300 °C. Moreover, PyC and FC interphase could effectively weaken the interfacial bond strength and efficiently promote load transfer, thus improving the strength. For the composite fabricated at 1300 °C, the average flexural strength significantly increased after introducing the optimised PyC and FC interphase.

Mechanical Properties and Failure Behavior of 3D-SiCf/SiC Composites with Different Interphases.

Continuous silicon carbide fiber-reinforced silicon carbide ceramic matrix composites (SiCf/SiC) are promising as thermal structural materials. In this work, the microstructure and static mechanical properties of 3D-SiCf/SiC with PyC, SiC, and PyC/SiC and without an interface prepared via polymer infiltration and pyrolysis (PIP) were investigated systematically in this paper. The results show that the microstructure and static mechanical properties of SiCf/SiC with an interphase layer were superior to the composites without an interlayer, and the interface debondings are existing in the composite without an interphase, resulting in a weak interface bonding. When the interphase is introduced, the interfacial shear strength is improved, the crack can be deflected, and the fracture energy can be absorbed. Meanwhile, the shear strength of the composites with PyC and PyC/SiC interfaces was 118 MPa and 124 MPa, respectively, and showing little difference in bending properties. This indicates that the sublayer SiC of the PyC/SiC multilayer interface limits the binding state and the plastic deformation of PyC interphase, and it is helpful to improve the mechanical properties of SiCf/SiC.

The effect of heat treatment on tensile properties of 2D C/SiC composites

AbstractTwo‐dimensional (2D) carbon fiber reinforced silicon carbide (C/SiC) composites with different initial strength were prepared by chemical vapor infiltration (CVI). After tensile property testing, results exhibited that as the heat‐treatment temperature (HTT) increases to 1900°C, the tensile strength and toughness of the low strength specimen (LSS) increased by 110% and 530%, while the high strength specimen (HSS) increased by 5.4% and 550%, respectively. As observed from morphologies, the heat treatment increases the graphitization of the amorphous PyC interphase, which leads to the weakening of interfacial bonding strength (IBS). Meanwhile, the defects arising from heat treatment cause thermal residual stress relaxation. Therefore, the tensile strength and toughness of LSS with relatively high initial IBS increase significantly as HTT increases. For HSS with moderate initial IBS, the heat treatment slightly improves the tensile strength, but significantly improves the toughness. Consequently, the post‐heat‐treatment tensile properties of 2D C/SiC composites can be regulated by varying HTTs and different initial strength.

Temperature-dependent proportional limit stress of SiC/SiC fiber-reinforced ceramic-matrix composites

AbstractIn this paper, the temperature-dependent proportional limit stress (PLS) of SiC/SiC fiber-reinforced ceramic-matrix composites (CMCs) is investigated using the micromechanical approach. The PLS of SiC/SiC is predicted using an energy balance approach considering the effect of environment temperature. The relation between the environment temperature, PLS, and composite damage state is established. The effects of the fiber volume, interface properties, and matrix properties on the temperature-dependent PLS and composite damage state of SiC/SiC composite are analyzed. The experimental PLS and interface debonding length of 2D SiC/SiC composites with the PyC and BN interphase at elevated temperatures are predicted. The temperature-dependent PLS of SiC/SiC composite increases with the fiber volume, interface shear stress and interface debonding energy, and the matrix fracture energy and decreases with the interface frictional coefficient at the same temperature.

Preparation and tensile behavior of SiCf/SiC mini composites containing different types and thicknesses of interphase

SiC fiber-reinforced SiC matrix (SiCf/SiC) composites are promising materials for high-temperature structural applications. In this study, KD-II SiC fiber bundles with a C/Si ratio of approximately 1.25 and an oxygen amount of 2.53%, were used as reinforcement. PyC interphase, PyC-SiC co-deposition interphase I and II, with different thicknesses, and SiC matrix were deposited into the SiC fiber bundles by using chemical vapor infiltration (CVI) to form SiCf/SiC mini composites. When the thickness of the interphase is approximately 1000 nm, the ultimate tensile stress and strain of SiCf/SiC mini composites with PyC-SiC co-deposition interphase I can reach 1120.0 MPa and 0.72%, respectively, which are significantly higher than those of SiCf/SiC mini composites with a PyC interphase (740.0 MPa, 0.87%) and PyC-SiC co-deposition interphase II (645.0 MPa, 0.54%). The effect of thicknesses and types of interphase on tensile fracture behavior of mini composites and then the fracture mechanism are discussed in detail.

Enhancing thermal conductivity of C/SiC composites containing heat transfer channels

In this study, CNTs/SiC micro-pillars at controlled content ratios were introduced into C/SiC composites as heat transfer channels to improve the thermal conductivity in the thickness direction. The thermal conductivities and bending strengths before and after heat treatment at 1650 °C were investigated and the results were discussed. The theoretical calculations and finite element analyses confirmed that CNTs/SiC micro-pillars successfully worked as heat transfer channels. The theoretical thermal conductivity calculated by effective medium theory (EMT) model was 19.25 W/m⋅k and agreed-well with the experimental value. The measured thermal conductivity was estimated to 20.69 W/m⋅k and improved to 22.36 W/m⋅k after heat treatment. The latter was 3.56-fold higher than that of traditional C/SiC and attributed to increased grain growth during heat treatment. The optimal bending strength before heat treatment was recorded as 324.5 ± 23.74 MPa due to microstructure evolution caused by CNTs. After heat treatment, the bending strength improved by 138 % with ductile fracture mode attributed to ordered layer structure of PyC interphase and complex phase composition of the composites. These features benefited the abundant propagation of cracks and energy consumption. In sum, introduction of heat transfer channels into C/SiC composites provided a new way to improve the thermal conductivity in thickness direction of ceramic matrix composites.

The Effect of PyC/SiC Compound Interphase on the Properties of PIP SiC/SiC Composite

In order to improve the properties of SiC/SiC composite, chemical vapor infiltration (CVI) process was applied for fabricating the PyC/SiC compound interphase. Precursor infiltration and pyrolysis (PIP) process was used to fabricate SiC matrix. Then an investigation was carried out for the effect of PyC/SiC compound interphase thicknesses on the properties of the composites. According to the results, it was found that the mechanical properties of composites with the PyC/SiC compound interphase are much higher than those of composite with only PyC interphase. The flexural strength, flexural modulus, fracture toughness and interlaminar shear strength of the composites increase gradually when the thickness of CVI SiC interphase is lesser than 2 μm. Further increase of the thickness of CVI SiC interphase to 4μm leads to a continuous increase on the modulus of the composites, while a decrease trend occurs on the flexural strength, fracture toughness and interlaminar shear strength of the composites because of the different coefficients of thermal expansion of the CVI SiC interphase and the PIP SiC matrix. SiC/(PyC/SiCx)/SiC composites showed the better properties of oxidation resistance than those of SiC/PyC/SiC composite. It was due to the survived PyC interphase protected by CVI SiC interphase when the oxygen diffuses into the compound PyC/SiCx intherphase.