Comparison of Fatigue Life Between C/SiC and SiC/SiC Ceramic-Matrix Composites at Room and Elevated Temperatures

In this paper, the comparison of fatigue life between C/SiC and SiC/SiC ceramic-matrix composites (CMCs) at room and elevated temperatures has been investigated. An effective coefficient of the fiber volume fraction along the loading direction (ECFL) was introduced to describe the fiber architecture of preforms. Under cyclic fatigue loading, the fibers broken fraction was determined by combining the interface wear model and fibers statistical failure model at room temperature, and interface/fibers oxidation model, interface wear model and fibers statistical failure model at elevated temperatures in the oxidative environments. When the broken fibers fraction approaches to the critical value, the composites fatigue fracture. The fatigue life S–N curves and fatigue limits of cross-ply, 2D and 3D C/SiC and SiC/SiC composites at room temperature, 550 °C in air, 750 °C in dry and humid condition, 800 °C in air, 1000 °C in argon and air, 1100 °C, 1300 °C and 1500 °C in vacuum, have been predicted. At room temperature, the fatigue limit of 2D C/SiC composite with ECFL of 20 % lies between 0.78 and 0.8 tensile strength; and the fatigue limit of 2D SiC/SiC composite with ECFL of 20 % lies between 0.75 and 0.85 tensile strength. The fatigue limit of 2D C/SiC composite increases to 0.83 tensile strength with ECFL increasing from 20 to 22.5 %, and the fatigue limit of 3D C/SiC composite is 0.85 tensile strength with ECFL of 37 %. The fatigue performance of 2D SiC/SiC composite is better than that of 2D C/SiC composite at elevated temperatures in oxidative environment.     

Simulation of Mechanical Behaviors of Ceramic Composites Under Stress-Oxidation Environment While Considering the Effect of Matrix Cracks

This article proposes a model which takes the effect of matrix cracking into consideration and analyzes the mechanical behaviors of unidirectional ceramic matrix composites under stress-oxidation environment. The change in the rules of mass loss ratio, residual modulus and residual strength of unidirectional C/SiC composite under different stress, oxidation time, temperature and fiber volume fraction with the temperature varying from 400 to 900 °C have been discussed in this paper. The comparison between the predicted residual mechanics properties and the experiment results demonstrates that the predicted results have a good agreement with the experiment results, which means that the model is feasible to simulate mechanical behaviors of unidirectional C/SiC composite under stress oxidation environment.     

Numerical Modeling of Oxidized 2D C/SiC Composites in Air Environments Below 900 °C: Microstructure and Elastic Properties

A numerical model is presented for simulation of the oxidation-affected behaviors of two dimensional carbon fiber-reinforced silcon carbide matrix composite (2D C/SiC) exposed to air oxidizing environments below 900 °C, which incorporates the modeling of oxidized microstructure and computing of degraded elastic properties. This model is based upon the analysis of the representative volume cell (RVC) of the composite. The multi-scale model of 2D C/SiC composites is concerned in the present study. Analysis results of such a composite can provide a guideline for the real 2D C/SiC composite. The micro-structure during oxidation process is firstly modeled in the RVC. The elastic moduli of oxidized composite under non-stress oxidation environment is computed by finite element analysis. The elastic properties of 2D-C/SiC composites in air oxidizing environment are evaluated and validated in comparison to experimental data. The oxidation time, temperature and fiber volume fractions of C/SiC composite are investigated to show their influences upon the elastic properties of 2D C/SiC composites.     

Synergistic Effects of Temperature and Oxidation on Matrix Cracking in Fiber-Reinforced Ceramic-Matrix Composites

In this paper, the synergistic effects of temperatrue and oxidation on matrix cracking in fiber-reinforced ceramic-matrix composites (CMCs) has been investigated using energy balance approach. The shear-lag model cooperated with damage models, i.e., the interface oxidation model, interface debonding model, fiber strength degradation model and fiber failure model, has been adopted to analyze microstress field in the composite. The relationships between matrix cracking stress, interface debonding and slipping, fiber fracture, oxidation temperatures and time have been established. The effects of fiber volume fraction, interface properties, fiber strength and oxidation temperatures on the evolution of matrix cracking stress versus oxidation time have been analyzed. The matrix cracking stresses of C/SiC composite with strong and weak interface bonding after unstressed oxidation at an elevated temperature of 700 °C in air condition have been predicted for different oxidation time.     

Effects of Temperature,Oxidation and Fiber Preforms on Fatigue Life of Carbon Fiber-Reinforced Ceramic-Matrix Composites

In this paper, the effects of temperature, oxidation and fiber preforms on the fatigue life of carbon fiber-reinforced silicon carbide ceramic-matrix composites (C/SiC CMCs) have been investigated. An effective coefficient of the fiber volume fraction along the loading direction (ECFL) was introduced to describe the fiber architecture of preforms. Under cyclic fatigue loading, the fibers broken fraction was determined by combining the interface wear model and fibers statistical failure model at room temperature, and interface/fibers oxidation model, interface wear model and fibers statistical failure model at elevated temperatures in the oxidative environments. When the broken fibers fraction approaches to the critical value, the composites fatigue fracture. The fatigue life S–N curves and fatigue limits of unidirectional, cross-ply, 2D, 2.5D and 3D C/SiC composites at room temperature, 800 °C in air, 1100, 1300 and 1500 °C in vacuum conditions have been predicted.     

Simulation of Degraded Properties of 2D plain Woven C/SiC Composites under Preloading Oxidation Atmosphere

In this paper, a numerical model which incorporates the oxidation damage model and the finite element model of 2D plain woven composites is presented for simulation of the oxidation behaviors of 2D plain woven C/SiC composite under preloading oxidation atmosphere. The equal proportional reduction method is firstly proposed to calculate the residual moduli and strength of unidirectional C/SiC composite. The multi-scale method is developed to simulate the residual elastic moduli and strength of 2D plain woven C/SiC composite. The multi-scale method is able to accurately predict the residual elastic modulus and strength of the composite. Besides, the simulated residual elastic moduli and strength of 2D plain woven C/SiC composites under preloading oxidation atmosphere show good agreements with experimental results. Furthermore, the preload, oxidation time, temperature and fiber volume fractions of the composite are investigated to show their influences upon the residual elastic modulus and strength of 2D plain woven C/SiC composites.     

Oxidation behavior of 3D Hi–Nicalon/SiC composite

Oxidation behavior of a three dimensional (3D) Hi–Nicalon/SiC composite with CVD SiC coating was investigated in the simulated air using a thermogravimetric analysis (TGA) device. Below 1100 °C, the oxidation kinetics was controlled by gas diffusion through the defects in the SiC matrix and coating and resulted in the consumption of PyC interphase. The residual flexural strength did have not a remarkable fluctuation and the relationship between the residual strength to temperature and weight change to temperature of the 3D Hi–Nicalon/PyC/SiC composite indicated the same regularity. Above 1200 °C, the oxidation kinetics was controlled by oxygen diffusion through the SiO₂ scale formed on the SiC coating and matrix. And the residual flexural strength of the composites was governed by the strength degradation of the Hi–Nicalon fiber. After oxidation, the fracture displacement in flexural tests increased with the weight loss increasing and the fracture mode showed a non-brittle pattern.     

Analysis of Residual Performance of UD-CMC in Oxidation Atmosphere Based on a Notch-like Oxidation Model

Experimental observation indicates unidirectional ceramic matrix composites (UD-CMC) will react with oxygen under high-temperature atmosphere inhomogeneous. As a result of the oxidation on fiber surface, fiber shows a notch-like morphology. Stress concentration near by the fiber notch causes a decline of the mechanic performance of UD-CMC. In this paper, the change rule of fiber notch depth is fitted by circular function. Based on this formula the residual strength and modulus of UD-CMC under 400–900 °C atmosphere are derived. The mechanical performance of unidirectional C/SiC composite is simulated by finite element method. The stress distribution of fiber, matrix and interface are obtained. The residual properties of unidirectional C/SiC composite are predicted by theoretical method and finite element method. And the predicting results are compared with the experiment data. The predicting results show a good accordance with experiment data, which means the notch-like oxidation model can analyze the mechanic performance of UD-CMC efficiently.     

Synergistic Effects of Temperature,Oxidation and Multicracking Modes on Damage Evolution and Life Prediction of 2D Woven Ceramic-Matrix Composites under Tension-Tension Fatigue Loading

In this paper, the synergistic effects of temperature, oxidation and multicracking modes on damage evolution and life prediction in 2D woven ceramic-matrix composites (CMCs) have been investigated. The damage parameter of fatigue hysteresis dissipated energy and the interface shear stress were used to monitor the damage evolution inside of CMCs. Under cyclic fatigue loading, the fibers broken fraction was determined by combining the interface/fiber oxidation model, interface wear model and fibers statistical failure model at elevated temperature, based on the assumption that the fiber strength is subjected to two-parameter Weibull distribution and the load carried by broken and intact fibers satisfy the Global Load Sharing (GLS) criterion. When the broken fibers fraction approaches to the critical value, the composite fatigue fractures. The evolution of fatigue hysteresis dissipated energy, the interface shear stress and broken fibers fraction versus cycle number, and the fatigue life S–N curves of SiC/SiC at 1000, 1200 and 1300 °C in air and steam condition have been predicted. The synergistic effects of temperature, oxidation, fatigue peak stress, and multicracking modes on the evolution of interface shear stress and fatigue hysteresis dissipated energy versus cycle numbers curves have been analyzed.     

无应力氧化下C/SiC复合材料弹性性能模拟及验证

结合复合材料氧化质量损失率模型和混合率公式, 发展了单向C/SiC复合材料在无应力氧化下的弹性模量预测方法。对400~700 ℃和700~900 ℃两种氧化机制下C/SiC复合材料的弹性模量进行了预测, 分析了氧化温度、氧化时间和纤维体积含量对C/SiC复合材料弹性模量的影响。开展了单向C/SiC复合材料在650 ℃和800 ℃空气环境下的无应力氧化试验, 建立了复合材料质量损失率与氧化时间的变化关系, 得到了氧化后材料拉伸应力-应变曲线。同时, 将理论预测值与试验结果进行对比, 发现理论值与试验值基本吻合, 从而验证了该方法能够有效地预测无应力氧化下陶瓷基复合材料的弹性性能。     

Fabrication and mechanical properties of oriented SiC short-fiber-reinforced SiC composite by tape casting

SiC fiber-reinforced SiC composites with nearly unidirectionally and randomly aligned SiC short fiber were prepared by tape-casting and hot-pressing (HP). Volume fractions of fibers were 10 and 20 vol.%. Three-point bending test was carried out at room temperature. The SiC short-fiber-reinforced SiC composites showed completely brittle fracture for any fiber volume fraction and orientation. The maximum strength increased with increasing sintering temperature regardless of orientation of short fiber. In the unidirectionally and randomly aligned composites sintered at 1700 °C containing 20 vol.% fiber, the maximum bending strength was about 390 and 280 MPa, respectively.     

Failure Modeling of SiC/SiC Mini-Composites in Air Oxidizing Environments

An iterative method was presented for simulation of the failure process of SiC/SiC mini-composites with pyrolytic carbon interphase exposed to air oxidizing environments under a constant load at 900 °C. This method was based on the possibility fracture strength of SiC fibers caused by random defects and the fiber stress distribution in mini-composites. The fiber strength probability model and Monte Carlo simulation were combined to generate the fracture strength along SiC fibers at 900 °C. The influence of fiber arrangement on fiber stress distribution was assessed to simplify the geometry model which was used to calculate the fiber stress distribution in the mini-composites. The failure process of the mini-composites was simulated, and the calculated oxidation life of the mini-composites matches the experimental data well with an error of ?9.40%.     

High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.     

Oxidation behaviour of three-dimensional woven C/SiC composites

Abstract

The oxidation behaviour of a three-dimensional woven C/SiC composite protected with an SiC seal coating and with an SiC coating combined with an SiO₂–B₂O₃ glassy coating have been respectively investigated through an experimental approach based on mass and flexural strength changes. Three main temperature domains exist for C/SiC composites protected with an SiC seal coating. At low temperatures (<700°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics. At an intermediate temperatures (between 700 and 1100°C), the oxidation kinetics are controlled by gas phase diffusion through a network of microcracks in the SiC matrix and coating. At high temperatures (>1100°C), the oxidation kinetics are controlled by oxygen diffusion through the SiO₂ scale formed on the SiC coating. Composites of C/SiC with an SiC/(SiO₂–B₂O₃) coating exhibit better oxidation resistance. The filling of the pores and the microcracks and the flow of the glassy coating at higher temperatures result in a global decrease of mass loss in the composites. By researching the relationship between the residual flexural strength and the mass variation in different temperature ranges, it is shown that the change in the residual flexural strength is dominated by the degradation of carbon phase.     

SiC<Subscript>f</Subscript>/SiC composites reinforced by randomly oriented chopped fibers prepared by semi-solid mechanical stirring method and hot pressing

SiC short fibers, with an average diameter of 13 μm, length of 300–1,000 μm and chopped from SiC continuous fibers, were surface modified by the semi-solid mechanical stirring method to produce a discrete coating of aluminum particles. Then the starting mixtures, which consist of SiC short composite fibers, aluminum powder less than 50 μm and α-SiC powder of an average diameter of 0.6 μm, were mechanically mixed in ethanol for about 3 h, dried at 80 °C in air, and hot pressed under 30 MPa pressure at 1,650, 1,750 and 1,850 °C with 1 h holding time to prepare SiC_f/SiC composites. Volume fraction of SiC short fibers in the starting powder for SiC_f/SiC composites was about 25 vol.%. The composites were characterized in terms of bulk density, phase composition, and mechanical properties at room temperature. In addition, the distribution of SiC short fibers in the matrix and the cracking pattern in the composites were examined by optical microscope. Fracture surface of the composites were performed by a scanning electron microscope (SEM). The effect of hot-pressing temperature on bulk density and mechanical properties was investigated. The results indicated that SiC short fibers were uniformly and randomly distributed in the matrix, bending strength and bulk density of the composites increased with increasing sintering temperature. The composite, hot-pressed at 1,850 °C, exhibited the maximum bulk density and bending strength at room temperature, about 3.01 g/cm³ and 366 MPa, respectively. SEM analyses showed that there were a few of fiber pullout on the fracture surface of samples sintered at 1,650 °C and 1,750 °C, which was mainly attributed to lower densities. But few of fiber pullout was observed on the fracture surface of sample sintered at 1,850 °C, the combined effects of high temperature and a long sintering time were considered as a source of too severe fiber degradation because of the large amount of oxygen in the fibers.     

Effects of microstructure on flexural strength of biomorphic C/SiC composites

Biomorphic C/SiC composites were fabricated from different kinds of wood by liquid silicon infiltration (LSI) following a two-step process. In the first-step, the wood is converted into carbon preforms by pyrolysis in a nitrogen atmosphere. The carbon preforms are then infiltrated by silicon melt at 1,560°C under vacuum to fabricate C/SiC composites. The mechanical properties of the C/SiC composites were characterized by flexural tests at ambient temperature, 1,000, and 1,300°C, and the relationship between mechanical properties and microstructure was analyzed. The flexural strength of the biomorphic composites was strongly dependent on the properties of the carbon preforms and the degree of silicon infiltration. The flexural strength increased with increasing SiC content and bulk density of composite, and with decreasing porosity in the C/SiC composite. An analysis of fractographs of fractured C/SiC composites showed a cleavage type fracture, indicating brittle fracture behavior.     

SiCf/TC17复合材料界面剪切强度测试与有限元分析

界面强度对钛基复合材料的性能有重要影响。采用纤维顶出实验(push-outtest)对连续SiC纤维增强TC17复合材料的界面剪切强度进行了测试,采用SEM观察了样品的形貌。以纤维/基体完全分离后的摩擦力为出发点,采用有限元方法确定了复合材料成型过程中残余应力的产生温度,并计算了残余应力的分布,比较了顶出实验样品制备前后残余应力的变化情况及样品厚度、体积分数对残余应力分布的影响;采用内聚力模型(CZM)分析了界面的化学结合强度。结果表明:SiCf/TC17复合材料高温成型后的冷却过程中开始产生残余应力的温度为775℃;顶出实验样品制备后界面处生成了残余剪切应力,其大小和分布与样品的体积分数和厚度相关,界面处的残余剪切应力造成了界面剪切强度的测试结果与界面化学结合强度的差异;室温下SiCf/TC17复合材料的界面化学结合强度约为450MPa。     

Mechanical properties and damping capacity of SiCp/TiNif/Al composite with different volume fraction of SiC particle

SiC_p/TiNi_f/Al composite with 20 Vol.% TiNi fibers were fabricated by pressure infiltration method. The effect of volume fraction of SiC particle on the mechanical properties and damping capacity of the composite were studied. Four different volume fractions of SiC particle in the composite were 0%, 5%, 20% and 35% respectively. The microstructure and damping capacity of the composites was studied by SEM and DMA respectively. As the gliding of dislocation in the Al matrix was hindered by SiC particle, the yield strength and elastic modulus of the composites increased, while the elongation decreased with the increase in volume fraction of SiC particle. Furthermore, the damping capacity of the composites at room temperature was decreased when the mount of strain was more than 1 × 10⁻⁴. In the heating process, the damping peak at the temperature of 135 °C was attributed to the reverse martensitic transformation from B19′ to B2 in the TiNi fibers.     

Manufacture of fibrous β-Si3N4-reinforced biomorphic SiC matrix composites for bioceramic scaffold applications

The manufacturing of the Si₃N₄ reinforced biomorphic microcellular SiC composites for potential medical implants for bone substitutions with good biocompatibility and physicochemical properties was performed in a two step process. First, wood-derived porous Si/SiC ceramics with various porosities were produced by liquid silicon infiltration (LSI) at 1550 °C with static nitrogen atmosphere protection (0.1 MPa), followed by subsequent partial removing of the Si in vacuo at 1700 °C for different periods of time. Secondly, the final porous Si₃N₄ fiber/SiC composite was obtained by further chemical reaction of nitrogen with the infiltrated residual silicon at 1400 °C for 4 h under high concentration flowing nitrogen atmospheres (0.5 MPa). The bending strengths of the porous Si₃N₄ fiber/SiC composite at axial and radial direction were measured as 180.03 MPa and 90 MPa respectively. The improvement in bending strength was primarily attributed to grain pull-out and bridging enhanced by the elongated β-Si₃N₄ grains cross-linked in the depth of the pore channels. The TG analysis showed an obvious improvement in oxidation resistance of the nitride specimens.     

Effect of chopped Si–Al–C fiber addition on the mechanical properties of silicon carbide composite

Silicon carbide (SiC) composites containing 0–50 mass% of chopped Tyranno^® Si–Al–C (SA) fiber (mean length: 214 μm (SA(214)), 394 μm (SA(394)), and 706 μm (SA(706)) were fabricated using the hot-pressing technique at 1800 °C for 30 min under a uniaxial pressure of 31 MPa in Ar atmosphere. The maximum flexural strength of the SiC composite was 344 MPa for 30 mass% of SA(706) fiber addition, whilst the maximum fracture toughness was 4.7 MPa m^1/2 for 40 mass% of SA(706) fiber addition. Increasing the mean fiber length from 214 to 706 μm decreased the flexural strength from 380 to 281 MPa for 30 mass% of fiber addition, whilst the fracture toughness increased from 3.4 to 4.7 MPa m^1/2 for 40 mass% of fiber addition. Through use of a treated SA(706) fiber containing an approximately 100 nm surface layer of carbon, the fracture toughness further increased to 6.0 MPa m^1/2 for 40 mass% of fiber addition; this value was more than twice that of the monolithic SiC ceramic and is believed to be the highest so far achieved for this type of SiC/SiC composite containing chopped fibers.