Microstructure and tensile behavior of (BN/SiC)n coated SiC fibers and SiC/SiC minicomposites

Interphase plays an important role in the mechanical behavior of SiC/SiC ceramic-matrix composites (CMCs). In this paper, the microstructure and tensile behavior of multilayered (BN/SiC)_n coated SiC fiber and SiC/SiC minicomposites were investigated. The surface roughness of the original SiC fiber and SiC fiber deposited with multilayered (BN/SiC), (BN/SiC)₂, and (BN/SiC)₄ (BN/SiC)₈ interphase was analyzed through the scanning electronic microscope (SEM) and atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. Monotonic tensile experiments were conducted for original SiC fiber, SiC fiber with different multilayered (BN/SiC)_n interfaces, and SiC/SiC minicomposites. Considering multiple damage mechanisms, e.g., matrix cracking, interface debonding, and fibers failure, a damage-based micromechanical constitutive model was developed to predict the tensile stress-strain response curves. Multiple damage parameters (e.g., matrix cracking stress, saturation matrix crack stress, tensile strength and failure strain, and composite’s tangent modulus) were used to characterize the tensile damage behavior in SiC/SiC minicomposites. Effects of multilayered interphase on the interface shear stress, fiber characteristic strength, tensile damage and fracture behavior, and strength distribution in SiC/SiC minicomposites were analyzed. The deposited multilayered (BN/SiC)_n interphase protected the SiC fiber and increased the interface shear stress, fiber characteristic strength, leading to the higher matrix cracking stress, saturation matrix cracking stress, tensile strength and fracture strain.     

Electrical Resistance of SiC Fiber Reinforced SiC/Si Matrix Composites at Room Temperature during Tensile Testing

The implementation of Ceramic Matrix Composites necessitates the understanding of stress‐dependent damage evolution. Toward this goal, two liquid silicon infiltrated SiCf reinforced SiC composites were tensile tested with electrical resistance (ER) monitoring as well as acoustic emission to quantify matrix cracking. ER was modeled using a combination of resistors in series and parallel to model transverse matrix cracks and fiber/matrix segments between matrix cracks. It is shown that resistance change is sensitive to transverse matrix crack formation and stress‐dependent debonding length. The model appears to be accurate up to the stress for matrix crack saturation.     

Matrix cracking of 2D SiC/SiC composite characterized by in situ SEM and nano-CT

Two-dimensional plain-woven silicon carbide (SiC) fiber-reinforced SiC matrix (2D SiC/SiC) composite was prepared by polymer infiltration-pyrolysis (PIP). Matrix cracking mechanisms of the composite were investigated by in situ SEM and nano-CT to grasp tensile damage evolution. Results showed that PIP-SiC matrix possessed low-fracture energy with non-homogeneous distribution, leading to simultaneous initiation of matrix cracking outside transverse fiber bundles and in unreinforced regions. Cracks then got deflected along weak fiber/matrix interface, which accelerated crack proliferation within the composite. With an increase in the stress, cracks subsequently deflected along plain-woven layers and converged to form longitudinal macrocracks. The composite was finally delaminated via sliding.     

Matrix cracking onset stress and strain as a function of temperature,and characterisation of damage modes in SiCf/SiC ceramic matrix composites via acoustic emission

The complex damage mechanisms that accumulate within SiC_f/SiC ceramic matrix composites (CMCs) subject to thermal and mechanical stress are being investigated in anticipation of the material’s introduction into high performance gas turbine engines. Acoustic emission (AE) is recognised as a leading non-destructive evaluation (NDE) tool to this end, and was used in this study to determine the so-called matrix cracking onset stress under tensile load as a function of temperature up to a maximum of 1100 °C. Onset stress was interpreted using three traditional measurements based on AE energy characteristics during monotonic tests to failure. Pattern recognition (PR) analysis was performed on the AE data, revealing a specific cluster of signals that correlated closely with the initial matrix cracking region of the stress-strain curve. Taken in isolation, the onset stress of this activity was significantly lower than the conventional value. PR results were investigated further, and isolated clusters were linked to damage modes anticipated at other specific regions of the stress history. A secondary series of experiments was performed on specimens representing the individual constituents of the CMC (single-phase SiC flexural bars, Hi-Nicalon? fibre bundles and SiC_f/SiC mini-composites) in attempts to further validate the corresponding AE signal characteristics. Matrix cracking and interphase debonding/sliding damage modes could be identified consistently, while fibre breaks remained difficult to isolate under the current experimental conditions.     

Study on interfacial debonding stress and damage mechanisms of C/SiC composites using acoustic emission

Among ceramic matrix composites (CMCs), carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are widely used in numerous high-temperature structural applications because of their superior properties. The fiber–matrix (FM) interface is a decisive constituent to ensure material integrity and efficient crack deflection. Therefore, there is a critical need to study the mechanical properties of the FM interface in applications of C/SiC composites. In this study, tensile tests were conducted to evaluate the interfacial debonding stress on unidirectional C/SiC composites with fibers oriented perpendicularly to the loading direction in order to perfectly open the interfaces. The characteristics of the material damage behaviors in the tensile tests were successfully detected and distinguished using the acoustic emission (AE) technique. The relationships between the damage behaviors and features of AE signals were investigated. The results showed that there were obviously three damage stages, including the initiation and growth of cracks, FM interfacial debonding, and large-scale development and bridging of cracks, which finally resulted in material failure in the transverse tensile tests of unidirectional C/SiC composites. The frequency components distributed around 92.5 kHz were dominated by matrix damage and failure, and the high-frequency components distributed around 175.5 kHz were dominated by FM interfacial debonding. Based on the stress and strain versus time curves, the average interfacial debonding stress of the unidirectional C/SiC composites was approximately 1.91 MPa. Furthermore, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) were used to observe the morphologies and analyze the chemical compositions of the fractured surfaces. The results confirmed that the fiber was completely debonded from a matrix on the fractured surface. The damage behaviors of the C/SiC composites were mainly the syntheses of matrix cracking, fiber breakage, and FM interfacial debonding.     

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

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

Oxidation effects on failure mechanism of plain-woven SiC/SiC Z-pinned joints subjected to push-out loading

Plain-woven SiC/SiC Z-pinned joints were prepared using chemical vapour infiltration. The oxidation behaviours and failure mechanism were investigated using push-out tests before and after oxidation. Microstructural analysis indicated that the degree of oxidation of the pin near the surface was more severe than that in the middle region. Internal damage was monitored using an acoustic emission (AE) system. Pattern recognition was introduced to study the evolution of failure during the test. Combined with the failure morphology, all AE signals were labelled as matrix cracking, fibre–matrix debonding, matrix peeling, and fibre break. The results suggest that the failure mechanism of the Z-pinned joints under axial loading is dominated by fibre–matrix debonding and matrix cracking.     

Modeling the effect of damage on electrical resistivity of melt-infiltrated SiC/SiC composites

Electrical resistivity (ER) measurements are a possible health monitoring technique for ceramic matrix composite components in future aerospace applications. In order to use ER measurements to detect and identify damage, it is necessary to understand how each specific damage state will affect the ER response. In this study, finite element models are developed and applied to quantify the effect of specific damage states on the ER response in a melt-infiltrated silicon carbide (SiC) fiber-reinforced SiC composite. The ER of several damage states are calculated by simulating the electric current flow through the damaged microstructure. This is achieved by performing the numerical solution of the steady-state conservation of charge density equation. Numerical results reveal the effect of various cracking features on the ER response such as type of cracking, extent of cracking, crack density and fiber/matrix debonding.     

Effect of multiple loading sequence on time-dependent stress rupture of fiber-reinforced ceramic-matrix composites

In this paper, the effect of multiple loading sequence on time-dependent stress rupture of fiber-reinforced ceramic-matrix composites (CMCs) at intermediate temperatures in oxidative environment is investigated. Considering multiple damage mechanisms, a micromechanical constitutive model for time-dependent stress rupture is developed to determine damage evolution of matrix crack spacing, interface debonding and oxidation length, and fiber failure probability under single and multiple loading sequences. The relationships between multiple loading sequence, composite strain evolution, time, matrix cracking, interface debonding and oxidation, and fiber fracture are established. The effects of fiber volume, matrix crack spacing, interface shear stress in the slip and oxidation region, and environment temperature on the stress/time-dependent strain, interface debonding and oxidation fraction, and fiber broken fraction of SiC/SiC composite are analyzed. The experimental stress rupture of SiC/SiC composite under single and multiple loading sequences at 950°C in air atmosphere is predicted. Compared with single loading stress, multiple loading sequence affects the interface debonding and oxidation fraction in the debonding region, leading to the higher fiber broken fraction and shorter stress-rupture lifetime.     

基于深度学习的平纹Cf/SiC复合材料原位拉伸损伤演化与断裂分析

为揭示平纹C_f/SiC复合材料的拉伸损伤演化及失效机理,开展了X射线CT原位拉伸试验,获得材料的三维重构图像,利用深度学习的图像分割方法,准确识别出拉伸裂纹并实现其三维可视化。分析了平纹C_f/SiC复合材料损伤演化与失效机理,基于裂纹的三维可视化结果对材料损伤进行了定量表征。结果表明:平纹C_f/SiC复合材料的拉伸力学行为呈现非线性,拉伸过程中主要出现基体开裂、界面脱黏、纤维断裂及纤维拔出等损伤;初始缺陷易引起材料损伤,孔隙多的部位裂纹数量也多;纤维束外基体裂纹可扩展至纤维束内部,并发生裂纹偏转。基于深度学习的智能图像分割方法为定量评估陶瓷基复合材料损伤演化与失效机理提供了有效分析手段。     

Combining in-situ synchrotron X-ray microtomography and acoustic emission to characterize damage evolution in ceramic matrix composites

In-situ synchrotron X-ray microtomography and acoustic emission (AE) were combined to study the behavior of ceramic matrix composite laminates subjected to in-plane tensile or flexural loading at room temperature. A detailed characterization of the initiation and progression of two key damage modes (matrix cracking and fiber breaks) is obtained from microtomography, and the relationship between damage and AE is directly observed. A graphical representation of AE data, which has potential for real-time use, is employed to reveal differences in damage progression due to fiber architecture or loading mode. In addition, strong empirical relationships are observed between matrix crack area and AE energy, as well as between fiber breaks and number of AE events.     

Interfacial properties of two SiC fiber–reinforced polycarbonate composites using the fragmentation test and acoustic emission

Interfacial shear strengths (IFSS) between the fiber and the matrix in two SiC fiber–reinforced polycarbonate (PC) composites (TFC) were investigated through the fragmentation method and the acoustic emission (AE) technique. Statistical analysis of SiC fiber tensile strength was performed mainly in terms of a Weibull distribution. The tensile strength and elongation for SiC fiber decreased with increasing gauge lengths, because of the heterogeneous distribution of flaws on the fiber surface. Using an amino-silane coupling agent, the IFSS showed significant improvement, in the range of 150%, under dry conditions. On the other hand, in the aspect of the environmental effect, the IFSS was improved about 170% under wet conditions (immersed in hot water at 85°C for 75 min). This is probably due to chemical and hydrogen bonds in the two different interphases in the SiC fiber/silane coupling agent/PC matrix system. In-situ monitoring of AE during straining of microspecimens showed the sequential occurrence of two distinct groups of AE data. The first group may result from SiC fiber breakages, and the second probably results from mainly PC matrix cracking. Characteristic frequencies coming from the failures of the fiber and the PC matrix were shown via fast Fourier transform (FFT) analysis. By setting an appropriate threshold level, a one-to-one correspondence between the number of AE events and fiber breakages was established. This AE method could be correlated successfully to the IFSS via the fragmentation technique, which can also applied to nontransparent specimens.     

Thermal Cycling Damage Mechanisms of C/SiC Composites in Displacement Constraint and Oxidizing Atmosphere

A constraint stress of 62.5 MPa is created on a three-dimensional C/SiC composite specimen whose both ends are fixed when temperature is cycled between 900° and 1200°C. The cyclic stress results in a maximum damage strain of 0.06% within 50 cycles owing to coating and matrix cracking, fiber debonding, sliding, and breaking in the composite. This constrained specimen elongation also leads to a final compressive stress of 14 MPa on the composite through a decrease in the baseline constraint stress. Wet oxygen atmosphere at a high cyclic temperature, concomitant with stresses, can aggravate the damage situation by alternate oxidation between internal and external fibers in composites.     

Failure mechanism and tensile constitutive model of the unidirectional-laminated Cf/SiC–Al composites

The unidirectional-laminated C_f/SiC–Al composites were prepared by using precursor infiltration and pyrolysis (PIP) and vacuum pressure infiltration processes. Bulk density and open porosity of as-prepared C_f/SiC–Al composites were characterized which showed a large number of pores in the unidirectional-laminated carbon fiber preform were filled with SiC and Aluminum alloy matrix. The uniaxial tensile tests were conducted to study the mechanical properties. The fracture surface and cross-section of tensile specimens were characterized to clarify the failure mechanism. The results showed that under the action of load, the propagation of microcracks in matrix led to interface debonding, fiber fracture and pull-out. According to the stress-displacement behavior and analysis of damage process, the prediction formulas of the linear proportional limit stress value and the tensile strength value were proposed. A bilinear constitutive model was established based on the assumption of the damage process which well characterized constitutive response of the composites.     

Effects of interface bonding properties on cyclic tensile behavior of unidirectional C/Si3N4 and SiC/Si3N4 composites

In this paper, the effect of fiber/matrix interface bonding properties on the cyclic loading/unloading tensile stress?strain hysteresis loops of 2 different ceramic‐matrix composites (CMCs), ie, C/Si₃N₄ and SiC/Si₃N₄, has been investigated using micromechanical approach. The relationships between the damage mechanisms (ie, matrix multicracking saturation, fiber/matrix interface debonding and fibers failure), hysteresis dissipated energy and internal frictional damage parameter have been established. The damage evolution processes under cyclic loading/unloading tensile of C/Si₃N₄ and SiC/Si₃N₄ composites corresponding to different fiber/matrix interface bonding properties have been analyzed through damage models and interface frictional damage parameter. For the C/Si₃N₄ composite with the weakest fiber/matrix interface bonding, the composite possesses the lowest tensile strength and the highest failure strain; the hysteresis dissipated energy increases at low peak stress, and the stress?strain hysteresis loops correspond to the interface partially and completely debonding. However, for the SiC/Si₃N₄ composite with weak interface bonding, the composite possesses the highest tensile strength and intermediate failure strain; and the hysteresis dissipated energy increases faster and approaches to a higher value than that of composite with the strong interface bonding.     

Phase-field simulation of microscale crack propagation/deflection in SiCf/SiC composites with weak interphase

To understand the microscale toughening mechanism, the crack propagation, and stress–strain response of unidirectional SiC_f/SiC composites with h-BN interphase under transverse and longitudinal tension are investigated by a promising micromechanical phase field (PF) method along with representative volume element. Of much interest, the calculation results are well consistent with the available experimental results. With a strong dependence on the interphase strength, the toughening mechanisms during crack propagation are well presented, for example, fiber pull-out, crack deflection, and interphase debonding. Furthermore, the longitudinal tensile strength of SiC_f/SiC composites increases with decreasing the interphase strength, where only a weak enough interphase can result in a significant crack deflection by its cracking. In particular, the ratio of the interphase strength along fibers to the matrix strength should be less than 1.254 to ensure crack deflection in the interphase and fiber pull-out. Moreover, the transverse tensile strength of SiC_f/SiC composites reaches a maximum with increasing the interphase thickness into the range of 0.25–0.5 µm.     

Multiple Cracking of Unidirectional and Cross-PlyCeramic Matrix Composites

This paper examines the multiple cracking behavior of unidirectional and cross-ply ceramic matrix composites. For unidirectional composites, a model of concentric cylinders with finite crack spacing and debonding length is introduced. Stresses in the fiber and matrix are found and then applied to predict the composite moduli. Using an energy balance method, critical stresses for matrix cracking initiation are predicted. Effects of interfacial shear stress, debonding length and bonding energy on the critical stress are studied. All the three composite systems examined show that the critical stress for the completely debonded case is lower than that for the perfectly bonded case. For cross-ply composites, an extensive study has been made for the transverse cracking in 90° plies and the matrix cracking in 0° plies. One transverse cracking and four matrix cracking modes are studied, and closed-form solutions of the critical stresses are obtained. The results indicate that the case of combined matrix and transverse crackings with associated fiber/matrix interfacial sliding in the 0° plies gives the lowest critical stress for matrix cracking. The theoretical predictions are compared with experimental data of SiC/CAS cross-ply composites; both results demonstrated that an increase in the transverse ply thickness reduces the critical stress for matrix cracking in the longitudinal plies. The effects of fiber volume fraction and fiber modulus on the critical stress have been quantified. Thermal residual stresses are included in the analysis.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

Effect of a Boron Nitride Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites

Typically, the debonding and sliding interface enabling fiber pullout for SiC-fiber-reinforced SiC-matrix composites with BN-based interphases occurs between the fiber and the interphase. Recently, composites have been fabricated where interface debonding and sliding occur between the BN interphase and the matrix. This results in two major improvements in mechanical properties. First, significantly higher failure strains were attained due to the lower interfacial shear strength with no loss in ultimate strength properties of the composites. Second, significantly longer stress-rupture times at higher stresses were observed in air at 815°3C. In addition, no loss in mechanical properties was observed for composites that did not possess a thin carbon layer between the fiber and the interphase when subjected to burner-rig exposure. Two primary factors were hypothesized for the occurrence of debonding and sliding between the BN interphase and the SiC matrix: a weaker interface at the BN/matrix interface than the fiber/BN interface and a residual tensile/shear stress-state at the BN/matrix interface of melt-infiltrated composites. Also, the occurrence of outside debonding was believed to occur during composite fabrication, i.e., on cooldown after molten silicon infiltration.     

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.