Effects of Fiber Architecture on Matrix Cracking for Melt-Infiltrated SiC/SiC Composites

The matrix cracking behavior of slurry cast melt-infiltrated SiC matrix composites consisting of Sylramic-iBN fibers with a wide variety of fiber architectures were compared. The fiber architectures included 2D woven, braided, 3D orthogonal, and angle interlock architectures. Acoustic emission was used to monitor in-plane matrix cracking during unload–reload tensile tests. Two key parameters were found to control matrix-cracking behavior: the fiber volume fraction in the loading direction and the area of the weakest portion of the structure, that is, the largest tow in the architecture perpendicular to the loading direction. Empirical models that support these results are presented and discussed.     

Creep and Stress–Strain Behavior after Creep for SiC Fiber Reinforced, Melt-Infiltrated SiC Matrix Composites

Silicon carbide fiber (Hi-Nicalon Type S, Nippon Carbon) reinforced silicon carbide matrix composites containing melt-infiltrated silicon were subjected to creep at 1315°C at three different stress conditions. For the specimens that did not rupture after 100 h of tensile creep, fast-fracture experiments were performed immediately following the creep test at the creep temperature (1315°C) or after cooling to room temperature. All specimens demonstrated excellent creep resistance and compared well to the creep behavior published in the literature on similar composite systems. Tensile results on the after-creep specimens showed that the matrix cracking stress actually increased, which is attributed to stress redistribution between composite constituents during tensile creep.     

Multiple Cracking and Tensile Behavior for an Orthogonal 3-D Woven Si-Ti-C-O Fiber/Si-Ti-C-O Matrix Composite

This paper presents experimental results for the multiple microcracking and tensile behavior of an orthogonal 3-D woven Si-Ti-C-O fiber (Tyranno™ Lox-M)/Si-Ti-C-O matrix composite with a nanoscale carbon fiber/matrix interphase and processed using a polymer impregnation and pyrolysis route. Based on microscopic observations and unidirectional tensile tests, it is revealed that the inelastic tensile stress/strain behavior is governed by matrix cracking in transverse (90°) fiber bundles between 65 and 180 MPa, matrix cracking in longitudinal (0°) fiber bundles between 180 and 300 MPa, and fiber fragmentation above 300 MPa. A methodology for estimation of unidirectional tensile behavior in orthogonal 3-D composites has been established by the use and modification of existing theory. A good correlation was obtained between the predicted and measured composite strain using this procedure.     

Tensile Creep Behavior and Thermal Stability of Orthogonal Three-Dimensional Woven TyrannoTM ZMI Fiber/Silicon-Titanium-Carbon-Oxygen Matrix Composites

This article presents experimental results for tensile creep behavior of orthogonal three-dimensional woven Tyranno™ ZMI fiber/Si-Ti-C-O ceramic matrix composites at 1300°–1450°C in air. The composite contained Tyranno ZMI (56% silicon, 1% zirconium, 34% carbon, and 9% oxygen) fibers with a BN coating layer to improve interface properties, and it exhibited excellent tensile properties at elevated temperature in air. For creep stresses between 60 and 140 MPa, the creep rate decreased continuously with time, with no apparent steady-state regime observed at 1300°–1450°C. Under the test conditions, the microstructure of the Tyranno ZMI fiber and Si-Ti-C-O matrix was unstable, resulting in weight loss and SiC grain growth. As a result, the viscosity of the fiber and matrix increased, because increased viscosity caused a creep rate that continuously decreased, which made steady-state creep impossible under these conditions.     

三维正交机织铜丝／玻璃纤维层间混杂复合材料的拉伸和弯曲性能研究

本文设计和制作了两种层间混杂结构的三维正交机织铜丝／玻璃纤维复合材料,分别为铜丝单面混杂和双面混杂复合材料。两种复合材料的拉伸性能和弯曲性能测试结果表明,单面铜丝／玻璃纤维混杂复合材料的归一化拉伸强度和模量分别为1214MPa和83GPa;高于双面铜丝／玻纤混杂复合材料44％和51％。单面铜丝／玻璃纤维混杂复合材料的归一化弯曲强度为964NPa,高于双面铜丝／玻纤复合材料27％。两者的弯曲模量比较接近,均为60GPa左右。由于铜丝的混杂效应,三维正交机织铜丝／玻璃纤维层间混杂复合材料的拉伸和弯曲性能与相同结构的玻璃纤维复合材料相比有一定的下降。     

Kinetics and Mechanisms of Oxidation of 2D Woven C/SiC Composites: I, Experimental Approach

The oxidation behavior of a 2D woven C/SiC composite partly protected with a SiC seal coating and heat-treated (stabilized) at 1600°C in inert gas has been investigated through an experimental approach based on thermogravimetric analyses and optical/electron microscopy. Results of the tests, performed under flowing oxygen, have shown that the oxidation behavior of the composite material in terms of oxidation kinetics and morphological evolutions is related to the presence of thermal microcracks in the seal coating as well as in the matrix. Three different temperature domains exist. At low temperatures (<800°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics and are associated with a uniform degradation of the carbon reinforcement. At intermediate temperatures, (between 800° and 1100°C), the oxidation kinetics are controlled by the gas-phase diffusion through a network of microcracks in the SiC coating, resulting in a nonuniform degradation of the carbon phases. At high temperatures (>1100°C), such diffusion mechanisms are limited by sealing of the microcracks by silica; therefore, the degradation of the composite remains superficial. The study of the oxidation behavior of (i) the heat-treated composite in a lower oxygen content environment (dry air) and (ii) the as-processed (unstabilized) composite in dry oxygen confirms the different mechanisms proposed to explain the oxidation behavior of the composite material.     

Kinetics and Mechanisms of Oxidation of 2D Woven C/SiC Composites: II, Theoretical Approach

A model for the oxidation kinetics of SiC-coated 2D C/SiC composites is developed on the basis of mechanisms derived from TGA data (between 700° and 1500°C in dry oxygen or air, P = 100 kPa). Carbon/oxygen reaction, gas phase diffusion through microcracks present in the external SiC coating, and silica growth are the main phenomena taken into account in the modeling. A morphological characterization of the microcracks network based on a compression test and SEM observations has been developed. The differential equations of the model are solved according to an iterative procedure. The mass variations of the composite during an oxidation test, as derived from the model, are in good agreement with the experimental data. The model is then used to predict the oxidation rate variations when (i) the oxygen partial pressure is decreased, and (ii) the state of damage of the external SiC coating is increased by mechanical loading.     

Mechanical Behavior of SiC(f)/SiC Composites and Correlation to in situ Fiber Strength at Room and Elevated Temperatures

Mechanical properties of Nicalon-fiber-reinforced silicon carbide matrix composites were evaluated, in flexure, at various temperatures ranging from ambient to 1300°C. First matrix cracking stress ranged from 250 to 280 MPa and was relatively insensitive to test temperature. The measured ultimate strength showed a small increase from a room-temperature value of 370 to 460 MPa at 800°C. Beyond 800°C, however, strength dropped to as low as 280 MPa at 1300°C. This decrease in ultimate strength at elevated temperatures is believed to be partly due to degradation of in situ Nicalon fiber strength. Scanning electron microscopy was employed to evaluate the in situ Nicalon fiber strengths via fracture mirror size measurements. Degradation of Nicalon fiber strength is attributed to thermal damage and to structural changes to the fiber at elevated temperatures. Measured values of ultimate strength of the composites were compared with predictions made on the basis of in situ fiber strength characteristics and an available analytical model.     

纤维丝束大小对Mini-C/SiC拉伸性能与强度分布的影响

为了探究碳纤维丝束大小对纤维束复合材料碳/碳化硅(Mini-C/SiC)拉伸性能和强度分布的影响,采用化学气相浸渗(CVI)法制备了1k Mini-C/SiC和3k Mini-C/SiC复合材料.测试了C纤维束以及Mini-C/SiC复合材料的拉伸性能,并采用两参数Weibull分布模型分析了强度分布,同时还观察了拉伸...     

Correlating Electrical Resistance Change with Mechanical Damage in Woven SiC/SiC Composites: Experiment and Modeling

Silicon carbide (SiC) fiber‐reinforced SiC matrix composites are inherently multifunctional materials. In addition to their primary function as a structural material, the electric properties of the SiC/SiC composites could be used for the sensing and monitoring of in situ damage nucleation and evolution. To detect damage and use that information to further predict the useful life of a particular component, it is necessary to establish the relationship between damage and electrical resistance change. Here, two typical SiC/SiC composites, melt infiltrated (MI), and chemical vapor infiltrated (CVI) woven SiC/SiC composites, were tested to establish the relationship between the electrical response and mechanical damage in unload–reload tensile hysteresis tests. Compared to the 55% resistance increase seen for CVI composites, the MI SiC/SiC composites exhibit a maximum resistance change in 450% in response to mechanical loading (damage), which is the highest sensitivity known among various composites. An analytic model accounting for fiber breakage and matrix cracks was developed to link the electrical resistance to mechanical damage in the composites. The predictions from the models agree well with the experimental data for both composites with high and low conductive matrices. The residual resistance change after unloading is also correlated to the loading history by the analytical relationship. This study demonstrates that resistance change is sensitive to damage in a predictable manner and can be used to improve the reliability of damage assessment of SiC/SiC composites.     

Tensile and stressed-oxidation behavior of 2D SiC/BN/SiC composites at Intermediate Temperatures in Air

The stressed-oxidation behavior of 2D CVI SiC/BN/SiC composites was studied at intermediate temperatures (800 °C) in air. The ultimate tensile strength (UTS) was acquired to determine the constant stress. The results show that the UTS at intermediate temperature is 14.3 % lower than that at room temperature. The strain-time curves at all stress levels show a deceleration stage and a stable stage. The stressed-oxidation rupture life decreases from 5.4 h to 0.9 h when the stress increases from 60 % to 90 % of the UTS. The element composition and fracture morphologies of the composites were also analyzed. The results show that the oxidation degree increases as the rupture time increases or constant stress decreases. Fiber degradation and interface defects caused by component oxidation induced local fiber failure and ultimate rupture of the composites, which may be attributed to strength degradation at intermediate temperatures and rupture of the composites during stress oxidation.     

Tensile Creep and Creep-Strain Recovery Behavior of Silicon Carbide Fiber/Calcium Aluminosilicate Matrix Ceramic Composites

The tensile creep and creep strain recovery behavior of 0° and 0°/90° Nicalon-fiber/calcium aluminosilicate matrix composites was investigated at 1200°C in high-purity argon. For the 0° composite, the 100-h creep rate ranged from approximately 4.6 × 10⁻⁹ s⁻¹ at 60 MPa to 2.2 × 10⁻⁸ s⁻¹ at 200 MPa. At 60 MPa, the creep rate of the 0°/90° composite was approximately the same as that found for the 0° composite, even though the 0°/90° composite had only one-half the number of fibers in the loading direction. Upon unloading, the composites exhibited viscous strain recovery. For a loading history involving 100 h of creep at 60 MPa, followed by 100 h of recovery at 2 MPa, approximately 27% of the prior creep strain was recovered for the 0° composite and 49% for the 0°/90° composite. At low stresses (60 and 120 MPa), cavities formed in the matrix, but there was no significant fiber or matrix damage. For moderate stresses (200 MPa), periodic fiber rupture occurred. At high stresses (250 MPa), matrix fracture and rupture of the highly stressed bridging fibers limited the creep life to under 70 min.     

High-Temperature Strength and Creep Behavior of Melt-Infiltrated SiC-Mo≤5Si3C≤1 Composites

Molybdenum carbosilicide composites (SiC-Mo_≤5Si₃C_≤1) were fabricated via the melt-infiltration process. The fracture behavior of the composites was studied from room temperature up to 1800°C in 1 atm (∼10⁵ Pa) of argon. The bend strength of the composites slightly increased at ∼1200°C, because of the brittle-ductile transition of the intermetallic phase. The composites retained ∼90% of their room-temperature strength, even at 1700°C. Compressive creep tests were performed over a temperature range of 1760°-1850°C and a stress range of 200–250 MPa. The creep rate of the SiC-Mo_≤5Si₃C_≤1 composites was approximately an order of magnitude higher than that of reaction-bonded SiC.     

Tensile creep and rupture of 2D-woven SiC/SiC composites for high temperature applications

The tensile creep and rupture behavior of 2D-woven SiC fiber-reinforced SiC matrix composites with potential for advanced high temperature structural applications was determined in air at 1315 °C. The results are compared to similar SiC/SiC data in the literature in order to understand the underlying creep and rupture mechanisms. Focus was placed on three different near-stoichiometric SiC fiber-types and three SiC-based matrix systems produced by different process routes. In general, the creep and rupture properties of the tested composites were primarily dictated by the creep resistance of the fiber-type, with the Sylramic-iBN fiber typically showing the best behavior. However, the type of matrix did have an effect on the composite creep and rupture lives due to load-sharing differences for the different matrix types and due to stoichiometry in the case of chemical vapor infiltration SiC matrices.     

Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites and Its Effect on Fiber Strength

Microstructural changes at the interface were analyzed in two Nicalon-fiber ceramic-matrix composites with a dual BN/SiC coating on the fibers after thermal exposure at different temperatures (in the range 800°-1400°C) and in different environments (air and argon). The outer SiC coating acted as a barrier to oxygen, which penetrated into the composite via pipeline diffusion along the BN/fiber interfaces. Oxygen penetration led to the formation of an SiO₂ layer by oxidation of the fiber surfaces. The in situ fiber strength at different temperatures, as determined from the radius of the mirror region on the fiber fracture surface, indicated that this SiO₂ layer severely degraded the fiber strength. Oxidation was highly dependent on the nature of the BN/fiber interface. The presence of a thin carbon-rich interlayer, which burned out rapidly at high temperature, favored the entry of oxygen and accelerated oxidation of the fibers.     

Strength Measurements of Monolithic SiC and A12O3/SiC Composites after Exposure to Coal Slag

Four Si-based ceramics—siliconized SiC (Si-SiC), sintered α-SiC, and two grades of SiC-particulate-reinforced A1₂O₃—were exposed to a typical western coal ash with high calcium content (Wyodak) and a typical eastern coal ash rich in iron (Illinois #6) at two temperatures for 300 h in a muffle furnace. After exposure, the coupons were cut into flexure bars and remaining strength was measured in four-point flexure at room temperature. The residual strength was compared to the as-received strength, and the fracture originating flaws were identified. The standard grade of the particulate-reinforced A1₂O₃ showed no corrosive attack under any of the exposure conditions, and a slight strength reduction was attributed to aging effects. The three other materials showed varying degrees of strength reduction, with corresponding fracture-initiating flaws being pits produced during the exposure. A general trend was that the eastern coal seemed to cause more corrosion than the western, and that 1260°C was a harsher exposure condition than 1093°C.     

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.     

Optimization of Parameters for Maximum Tensile Strength of Friction Stir Welded AA6082/Si3N4 and AA6082/SiC Composite Joints

Silicon - Weldability, microstructure evolution and tensile strength of an aluminium alloy (AA6082) and of two category of composites with AA6082 matrix reinforced with Si3N4 and SiC particles...     

有机前驱体中添加ZrB2和SiC微粉制备Cf/SiC-ZrB2复合材料

通过在有机前驱体溶液中加入惰性填料ZrB_2和SiC微粉,制备了C_f/SiC-ZrB_2复合材料。分别研究了三组不同含量的料浆对复合材料浸渍裂解效果的影响,并借助扫描电镜(SEM)和能谱分析(EDS)对制备的Cf/SiC-ZrB2复合材料的微观结构和元素组成进行了分析。研究表明:在有机前驱体溶液中添加无机粉体浸渍复合材料能够起到缩短制备周期、提高材料强度的目的。当加入惰性填料的质量分数为15%时,能够在8个制备周期内将复合材料的密度快速提升到2.0g?cm~(-3),而且制备的材料内部结构致密,力学性能优异。     

Mechanical Strength Behavior of Hybrid Composites Tailored by Glass/Kevlar Fibre-Reinforced in Nano-Silica and Micro-Rubber Blended Epoxy

Silicon - The present work aims to investigate the mechanical properties and fracture toughness of woven fabric Glass/Kevlar based hybrid composite tailored using modified epoxy with micro rubber...