Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.     

Thermal stability of CVI and MI SiC/SiC composites with Hi-Nicalon™-S fibers

The influence of high-temperature argon heat-treatment on the microstructure and room- temperature in-plane tensile properties of 2D woven CVI and 2D unidirectional MI SiC/SiC composites with Hi-Nicalon?-S SiC fibers was investigated. The 2D woven CVI SiC/SiC composites were heat-treated between 1200 and 1600 °C for 1- and 100-hr, and the 2D unidirectional MI SiC/SiC composites between 1315 and 1400 °C for up to 2000 hr. In addition, the influence of temperature on fast fracture tensile strengths of these composites was also measured in air. Both composites exhibited different degradation behaviors. In 2D woven CVI SiC/SiC composites, the CVI BN interface coating reacted with Hi-Nicalon?-S SiC fibers causing a loss in fast fracture ultimate tensile strengths between 1200 and 1600 °C as well as after 100-hr isothermal heat treatment at temperatures > 1100 °C. In contrast, 2D unidirectional MI SiC/SiC composites showed no significant loss in in-plane tensile properties after the fast fracture tensile tests at 1315 °C. However, after isothermal exposure conditions from 1315° to 1400°C, the in-plane proportional limit stress decreased, and the ultimate tensile fracture strain increased with an increase in exposure time. The mechanisms of strength degradation in both composites are discussed.     

Stressed-oxidation behavior of 2D CVI SiC/BN/SiC at elevated temperature in air

The stressed-oxidation behaviors of 2D woven SiCf/BN/SiC composites were investigated at 950 °C and 1100 °C in air. The different proportions (60%–90%) of the ultimate tensile strength (UTS) at corresponding temperatures were chosen as constant stress. The stressed-oxidation experiments were taken to failure or interrupted (240h). The UTS decreases by 20.75% at 950 °C and 30.71% at 1100 °C. The composites did not fail during stressed oxidation when subjected to constant stress corresponding to the initial linear and the beginning of nonlinear segments of the tensile curve, above which the composites failed with a maximum failure life of about 10h. Fiber degradation due to the thermal exposure and the fiber cracks caused by the oxidation of BN interface coating and SiC fiber could be responsible for the strength degradation and failure of the composites during stressed oxidation.     

Fatigue resistance and damage mechanisms of 2D woven SiC/SiC composites at high temperatures

Fatigue resistance and damage mechanisms of 2D woven SiC/SiC composites at high temperatures were investigated in this research. Fatigue behavior tests were performed at 1200℃ and 1000°C at 10 Hz and stress ratio of 0.1 for maximum stresses ranging from 80 to 120 MPa, and the fatigue run-out could be defined as 10⁶ cycles. Evolution of the cumulative displacement and normalized modulus with cycles was analyzed for each fatigue condition. Fatigue run-out was achieved at 80 MPa and 1000°C. It could be found that the cycle lifetimes of the composites decreased sharply with the increasing maximum stress and temperature conditions significantly affected the fatigue performance under matrix cracking stress. The cumulative displacement showed no noticeable increase before 1000 cycles and the modulus of the failed specimens decreased before fracture. The retained properties of composites that achieved fatigue run-out, as well as the microstructures, were characterized in order to understand the fatigue behavior and failure mechanisms. The composites exhibited similar fracture morphology with matrix crack extension and glass phase oxidation formation under different conditions. In general, the high-temperature fatigue damage and failure of composites could be affected by combination of stress damage and oxidative embrittlement.     

Tensile creep behavior of SiCf/SiC ceramic matrix minicomposites

Single fiber-tow minicomposites represent the major load-bearing element of woven and laminate ceramic matrix composites (CMCs). To understand the effects of fiber type, fiber content, and matrix cracking on tensile creep in SiC_f/SiC CMCs, single-tow SiC_f/SiC minicomposites with different fiber types and contents were investigated. The minicomposites studied contained either Hi-Nicalon™ or Hi-Nicalon™ Type S SiC fibers with a boron nitride (BN) interphase and a chemical-vapor-infiltrated-silicon-carbide (CVI-SiC) matrix. Tensile creep was performed at 1200 °C in air. A bottom-up creep modeling approach was applied where creep parameters of the fibers and matrix were obtained separately at 1200 °C. Next, a theoretical model based on the rule of mixtures was derived to model the fiber and matrix creep-time-dependent stress redistribution. Fiber and matrix creep parameters, load transfer model results, and numerical modeling were used to construct a creep strain model to predict creep damage evolution of minicomposites with different fiber types and contents.     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Compressive strength and damage mechanisms of 2D-C/SiC composites at high temperatures

Compressive strength of 2D-C/SiC composite was investigated from room temperature(RT) to 1600?°C at present work. Damage evolution was investigated by conducting loading/unloading tests at RT and the damage mechanisms were elucidated by observing the fracture morphology. It is found that compressive strength of 2D-C/SiC was retained until 1200?°C and then decreased with increasing temperature. The variation of compressive strength is closely related to the degradation in matrix modulus. The compressive damage of 2D-C/SiC starts at the buckling of 0° fiber and is followed by opening and closing of original pores, initiation and growth of longitudinal interbundle cracks, separation of 90° fiber bundles by longitudinal cracks, matrix cracking from intrabundle pores, propagation of matrix cracks into 0° fiber bundles, connection of cracks in 0° fiber bundles and longitudinal cracks in 90° fiber bundles.     

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.     

Effect of off-axis angle on mesoscale deformation and failure behavior of plain-woven C/SiC composites with digital image correlation

In this study, the deformation response and failure behavior of a plain-woven C/SiC composite were investigated under on-axis and off-axis tensile loading. Digital image correlation (DIC) was utilized to characterize the full-field deformation and mesoscale strain distribution. The test results indicate a strong influence of the woven architecture on the mechanical properties and strain distribution, and the materials exhibit failure modes dependent on the loading directions or off-axis angles: the fracture positions of different layers are the same under off-axial load, while for on-axil loading, the fracture positions of different layers do not affect each other. SEM results provide direct evidence that the width of the off-axis specimen has a great influence on the mechanical properties. The reduction of the modulus and strength of off-axis specimen, is not only due to the off-axis loading, but also due to the reduction of effective bearing area or effective bearing fiber.     

The effects of preparation temperature on the SiCf/SiC 3D4d woven composite

Continuous silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites have promising applications in aero-engine due to their unique advantages, such as low density, high modulus and strength, outstanding high temperature resistance and oxidation resistance. As SiC fibers are main reinforcements in SiC_f/SiC composites, the crystallization rate and initial damage degree of SiC fibers are seriously influenced by preparation temperatures of SiC_f/SiC composites, namely mechanical properties of SiC fibers and SiC_f/SiC composites are influenced by preparation temperatures. In this paper, KD-II SiC fibers were woven into 3D4d preforms and SiC matrix was fabricated by PIP process at 1100 °C, 1200 °C, 1400 °C and 1600 °C. Digital image correlation (DIC) method was adopted to measure the uniaxial tensile properties of these SiC_f/SiC composites. In addition, finite element method (FEM) based on representative volume element (RVE) was adopted to predict the mechanical properties of SiC_f/SiC composites. The good agreements between numerical results and experimental results of uniaxial tensile tests verified the validity of the RVE. In last, the transverse tensile, transverse shear, uniaxial shear properties were predicted by this method. The predicted results illustrated that axial tensile, transverse tensile and axial shear properties were greatly influenced by the preparation temperatures of SiC_f/SiC composites while transverse shear properties were not significantly various. And the mechanical properties of SiC_f/SiC composites peaked at 1200 °C among these four temperatures while their values reached their lowest points at 1600 °C because of thermal damage and brittle failure of SiC_f/SiC composites.     

Temperature Dependence of Tensile Strength for a Woven Boron-Nitride-Coated Hi-Nicalon™ SiC Fiber-Reinforced Silicon-Carbide-Matrix Composite

The temperature dependence of tensile fracture behavior and tensile strength of a two-dimensional woven BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composite fabricated by polymer infiltration pyrolysis (PIP) were studied. A tensile test of the composite was conducted in air at temperatures of 298 (room temperature), 1200, 1400, and 1600 K. The composite showed a nonlinear behavior for all the test temperatures; however, a large decrease in tensile strength was observed above 1200 K. Young's modulus was estimated from the initial linear regime of the tensile stress–strain curves at room and elevated temperatures, and a decrease in Young's modulus became significant above 1200 K. The multiple transverse cracking that occurred was independent of temperature, and the transverse crack density was measured from fractographic observations of the tested specimens at room and elevated temperatures. The temperature dependence of the effective interfacial shear stress was estimated from the measurements of the transverse crack density. The temperature dependence of in situ fiber strength properties was determined from fracture mirror size on the fracture surfaces of fibers. The decrease in the tensile strength of the composite up to 1400 K was attributed to the degradation in the strength properties of in situ fibers, and to the damage behavior exception of the fiber properties for 1600 K.     

Creep properties of melt infiltrated SiC/SiC composites with Sylramic™-iBN and Hi-Nicalon™-S fibers

Isothermal tensile creep tests were conducted on 2D woven and laminated, 0/90 balanced melt infiltration (MI) SiC/SiC composites at stress levels from 48 to 138 MPa and temperatures to 1400°C in air. Effects of fiber architecture and fiber types on creep properties, influence of accumulated creep strain on in-plane tensile properties, and the dominant constituent controlling the creep behavior and creep rupture properties of these composites were investigated. In addition, the creep parameters of both composites were determined. Results indicate that in 2D woven MI SiC/SiC composites with Sylramic™-iBN or Hi-Nicalon™-S fibers, creep is controlled by chemical vapor infiltration (CVI) SiC matrix, whereas in 2D laminated MI SiC/SiC composites with Hi-Nicalon™-S fibers, creep is controlled by the fiber. Both types of composites exhibit significant variation in creep behavior and rupture life at a constant temperature and stress, predominantly due to local variation in microstructural inhomogeneity and stress raisers. In both types of composites at temperatures >1350°C, residual silicon present in SiC matrix to reacts with SiC fibers and fiber coating causing premature creep rupture. Using the creep parameters generated, the creep behaviors of the composites have been modeled and factors influencing creep durability are discussed.     

Fatigue of three advanced SiC/SiC ceramic matrix composites at 1200°C in air and in steam

High‐temperature mechanical properties and tension‐tension fatigue behavior of three advanced SiC/SiC composites are discussed. The effects of steam on high‐temperature fatigue performance of the ceramic‐matrix composites are evaluated. The three composites consist of a SiC matrix reinforced with laminated, woven SiC (Hi‐Nicalon?) fibers. Composite 1 was processed by chemical vapor infiltration (CVI) of SiC into the Hi‐Nicalon? fiber preforms coated with boron nitride (BN) fiber coating. Composite 2 had an oxidation inhibited matrix consisting of alternating layers of silicon carbide and boron carbide and was also processed by CVI. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Composite 3 had a melt‐infiltrated (MI) matrix consolidated by combining CVI‐SiC with SiC particulate slurry and molten silicon infiltration. Fiber preforms had a CVI BN fiber coating applied. Tensile stress‐strain behavior of the three composites was investigated and the tensile properties measured at 1200°C. Tension‐tension fatigue behavior was studied for fatigue stresses ranging from 80 to 160 MPa in air and from 60 to 140 MPa in steam. Fatigue run‐out was defined as 2 × 10⁵ cycles. Presence of steam significantly degraded the fatigue performance of the CVI SiC/SiC composite 1 and of the MI SiC/SiC composite 3, but had little influence on the fatigue performance of the SiC/SiC composite 2 with the oxidation inhibited matrix. The retained tensile properties of all specimens that achieved fatigue run‐out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Joining of SiC ceramic by using the liquid polyvinylphenylsiloxane

ABSTRACT

The joining of SiC ceramic using the liquid polyvinylphenylsiloxane at the high temperature was investigated. The characteristic evolution of polyvinylphenylsiloxane during heating process, shear strength and microstructure of joint were especially discussed. The results show that active groups Si-OH and CH=CH₂ of polyvinylphenylsiloxane through cross-linking at low temperature (200°C) form the macromolecular structure, crosslinked polyvinylphenylsiloxane possess the higher ceramic yield and structure stability at high temperature. Shear strength of SiC joints increase with the joining temperature from 1000 to 1200°C, and then decrease when the joining temperature reaches to 1350°C. Combination with microstructure of fine grains of SiO₂ and SiC dispersion in the Si–O–C ceramic of the join layer and new phase SiC formation on the joint interface through the gas–solid reactions, the shear strength of joint achieves the maximum at 1200°C. The defects of joint increase with temperature higher than 1200°C, and the shear strength of joint begin to decrease.     

A chemo-mechanical coupled constitutive model applied to the oxidation simulation of SiC/SiC composite

Herein, a chemo-mechanical coupled constitutive and failure model is proposed to predict the tensile behavior of SiC/SiC composites under oxidizing environments. The diffusion of O₂ through the oxide scale and the oxidation reaction of SiC/O₂ are modeled and implemented in finite element software, through a user-defined element. Numerical validation studies and tests are conducted on a domestic SiC fiber. An orthotropic constitutive model for reinforcements, which considers modulus reduction due to oxidation damage, and a continuum damage model associated with O₂ diffusion along the micro-cracks in the SiC matrix are subsequently presented. The developed framework is used to simulate the mechanical behavior and oxidation process of a single fiber SiC/SiC composite.     

Novel SiC/C composite targets for the production of radioisotopes for nuclear applications

A partially porous SiC ceramic, reinforced with 30 vol% short carbon fibers, was hot pressed and characterized as potential ISOL target for nuclear applications. Powder milling and hot pressing were effective for the realization of a ceramic with about 40% interconnected porosity in the 0.6–0.8 µm size range. A fiber-free porous SiC material was also synthesized for the sake of comparison. Compression strength of the fiber-rich SiC passed from about 200 MPa at room temperature to about 120 MPa upon testing at 1200 °C. The thermal conductivity was higher than the fiber-free SiC and other state-of-art ISOL target materials and was 48 W/m·K at 600 °C and decreased to 17 W/m·K at 1400 °C, owing to the porosity. Remarkably, this fiber-rich ceramic in form of thin disk, possessed suitable thermo-mechanical behavior to successfully withstand a 350 °C thermal gradient without failure.     

Tensile creep properties and damage mechanisms of 2D-SiCf/SiC composites reinforced with low-oxygen high-carbon type SiC fiber

Tensile creep properties of 2D-SiC_f/SiC composites reinforced with low-oxygen high-carbon type SiC fibers were studied in vacuum at 1300°C∼1430°C. The fracture morphology was observed by scanning electron microscopy and the damage of fiber in 2D-SiC_f/SiC composites was characterized by nanoindentation. Moreover, the microstructure of the composite was investigated by high-resolution transmission electron microscopy. The results show that rupture time is much shortened and steady-state creep rate increase three orders of magnitude when creep temperature is higher than 1400°C. There are two different creep damage mechanisms due to the decrease of interfacial bonding strength at high temperature. The amorphous SiO_xC_y phase in the fibers can crystallize into SiC and C and the SiC grain grows in the fiber. The microstructural changes lead to the decrease of fiber strength and degrade the creep properties of the composite above 1400°C.     

Evaluation of mechanical properties of a dense SiC/SiCN composite produced via PIP process

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.     

Study on crosslinking of polymers and joining mechanism of SiC ceramic

ABSTRACT

Based on crosslinking of polymers with different vinyl contents at low temperature and pyrolysis of the polymer at high temperature, joining temperature and impregnation cycles of an SiC joint were discussed. Polyvinylphenylsiloxane as the polymer with active groups of Si–OH and CH=CH₂ by crosslinking enhances ceramic yield and thermal stability. The microstructure of the polymer changes from amorphous ceramic into grains of SiO₂ and SiC, grains can dispersion strengthening enhances strength of joint layer. Shear strength of SiC joints reaches the maximum at 1200°C. A relatively good interface between the SiC substrate and pyrolysis product of the polymer is formed, but there exist loose cracks or voids in the joint layer which affect the shear strength of the joint. The shear strength of the SiC joint reaches 69.2?MPa through seven times of vacuum impregnation/pyrolysis enhancement. According to the microstructure and properties of the SiC joint, the pyrolysis mechanism of the joining layer as part of the joint by using polyvinylphenylsiloxane is explained.     

Tensile properties of two-dimensional carbon fiber reinforced silicon carbide composites at temperatures up to 2300 °C

Carbon fiber reinforced silicon carbide (C/SiC) composites are of the few most promising materials for ultra-high-temperature structural applications. However, the existing studies are mainly conducted at room and moderate temperatures. In this work, the tensile properties of a two-dimensional plain-weave C/SiC composite are studied up to 2300 °C in inert atmosphere for the first time. The study shows that C/SiC composite firstly shows linear deformation behavior and then strong nonlinear characteristics at room temperature. The nonlinear deformation behavior rapidly reduces with temperature. The Young’s modulus increases up to 1000 °C and then decreases as temperature increases. The tensile strength increases up to 1000 °C firstly, followed by reduction to 1400 °C, then increases again to 1800 °C, and lastly decreases with increasing temperature. The failure mechanisms being responsible for the mechanical behavior are gained through macro and micro analysis. The results are useful for the applications of C/SiC composites in the thermal structure engineering.