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.     

Shear Damage Mechanisms in a Woven, Nicalon-Reinforced Ceramic-Matrix Composite

The shear response of a Nicalon-reinforced ceramic-matrix composite was investigated using Iosipescu tests. Damage was characterized by X-ray, optical, and SEM techniques. The large inelastic strains which were observed were attributed to rigid body sliding of longitudinal blocks of material. These blocks are created by the development and extension of intralaminar cracks and ply delaminations. This research reveals that the debonding and sliding characteristics of the fiber—matrix interface control the shear strength, strain softening, and cyclic degradation of the material.     

Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites

The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.     

Assessment of the Interfacial Properties of Ceramic-Matrix Composites Using Strain Partition under Load

The interfacial sliding stress is a key parameter in the global behavior of ceramic-matrix composites. An ultrasonic characterization, through the complete determination of the stiffness tensor along a tensile test, detects all the damage mechanisms: transverse matrix microcracking and the existence of longitudinal cracks at the fiber/matrix interface. It also allows a strain partition under load. A micromechanical model is established and gives access to the value of the interfacial sliding stress during the entire tensile test, whether the damage occurs at the mesostructural or at the microstructural level of the composite.     

Role of the Interfacial Thermal Barrier in the Effective Thermal Diffusivity/Conductivity of SiC-Fiber-Reinforced Reaction-Bonded Silicon Nitride

Experimental thermal diffusivity data transverse to the fiber direction for composites composed of a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon-coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix-fiber interface. Calculations of the interfacial thermal conductances indicate that at 300°C and 1-atm N₂, more than 90% of the heat conduction across the interface occurs by gaseous conduction. The magnitude of the interfacial conductance is decreased significantly under vacuum or by removal of the carbon surface layer from the fibers by selective oxidation. Good agreement is obtained between thermal conductance values for the oxidized composite at 1 atm calculated from the thermal conductivity of the N₂ gas and those inferred from the data for the effective composite thermal conductivity.     

Role of Interfacial Gaseous Heat Transfer in the Transverse Thermal Conductivity of a Uniaxial Carbon Fiber-Reinforced Aluminoborosilicate Glass

The transverse thermal conductivity of an aluminoborosilicate glass uniaxially reinforced with carbon fibers was found to be lower under near-vacuum than in nitrogen, whereas no such difference was found for the longitudinal thermal conductivity. This effect was attributed to the existence of an interfacial gap resulting from the thermal expansion mismatch between the matrix and fibers. The presence of this gap permits the gaseous environment access to the fiber-matrix interface and thereby contributes to the interfacial heat transfer. Its presence does not affect the longitudinal thermal conductivity, however, because the gap is aligned parallel to the fibers and, therefore, the direction of heat flow. Analysis of the experimental data indicates that, in nitrogen at atmospheric pressure, the gaseous conductance constitutes about one-third of the total interfacial conductance.     

Thermal Conductance of Delamination Cracks in a Fiber-Reinforced Ceramic Composite

The thermal conductance of delamination cracks in a unidirectionally reinforced ceramic composite is investigated. A phase-sensitive photothermal technique is used to measure the crack conductance in situ under load. Special emphasis is given to the effects of the local crack opening displacement (δ). A crack conductance model that considers the contributions from both the air and the fibers within the crack is developed and compared with the measurements. Despite considerable scatter in the experimental data, the model adequately predicts the increased conductance that is associated with fiber bridging, as well as the overall trend that is observed with δ.     

基体中含有随机分布裂纹的纤维增强复合材料的总体弹性常数

本文应用自洽方法计算了含随机分布裂纹的基体的等效弹性常数,然后利用GMC方法计算了复合材料的总体弹性常数.结果表明,随着裂纹密度的增加,基体的等效弹性模量和泊松比会降为零;而同时,复合材料的纤维方向的弹性模量的下降,但是仍然能达到没有裂纹时的90%.这表明当基体完全破坏的时候,纤维仍能够在纵向承受载荷.另外,当基体的等效弹性模量和泊松比会降为零时,复合材料的横向弹性模量和剪切模量都接近为零,表明这时复合材料无法承受横向的拉压力和剪切力.     

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.     

Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts Following Ground Testing on an F110 Turbofan Engine

Four ceramic-matrix composite flap inserts were evaluated following ground testing on a General Electric F110 turbofan engine. Three of the composites accumulated ∼117 h of engine time. The fourth composite, a Nextel^TM 720 material with aluminosilicate matrix, accumulated ∼40 h. Large through-thickness cracks developed along the longitudinal edges of a Nicalon™/Al₂O₃ insert and the Nextel 720/aluminosilicate insert. The cracks developed because of high tensile stresses caused by the steep in-plane thermal gradients induced across the flap width during afterburner lights. The Nextel 720/aluminosilicate insert also exhibited severe surface wear associated with the acoustic environment and contact with the adjacent divergent seals. Neither a Nicalon/silicon nitrocarbide insert nor a Nicalon/C insert exhibited significant signs of distress.     

Thermal and mechanical properties of crack-designed thick lanthanum zirconate coatings

Vertical cracks are beneficial in thermal barrier coatings due to enhanced thermo-mechanical compliance. Accordingly, an aqueous nitrate based precursor solution was atomized on stainless steel substrates by spray pyrolysis to deposit thick crack-designed lanthanum zirconate coatings. Coatings with designed crack patterns were deposited and characterized by electron microscopy, tribology, Vickers indentation, and thermal diffusivity. The crystallization of the coatings was investigated by in situ high temperature X-ray diffraction. The green coatings crystallized from 600 °C and the pyrochlore structure was formed after heat treatment at 1000 °C. Crystalline lanthanum zirconate multilayered coatings with small crack spacing and crack opening exhibited a higher density, a higher hardness, lower thermal diffusivities, and higher thermal conductivities compared to crystalline monolayered coatings of similar thickness with large crack spacing and crack opening. The thermal diffusivity of the coatings, ∼28 mm²/s at room temperature, was similar to the values reported for yttria-stabilized zirconia plasma sprayed coatings.     

Effect of Microcracking on the Thermal Diffusivity of Fe2TiO5

The effect of microcracking on the thermal diffusivity of polycrystalline Fe₂TiO₅ subjected to a range of annealing treatments was investigated. At fine grain size (∼1 μm), the thermal diffusivity exhibited the decrease with increasing temperature common for dielectrics. Extensive microcracking in the larger-grain-sized materials significantly decreased their thermal diffusivity. On heating, the microcracked materials exhibited increased thermal diffusivity at elevated temperatures which can be attributed primarily to microcrack closure and healing; on cooling, they exhibited a pronounced hysteresis, attributable to irreversible crack opening and closing. Thermal cycling closed the hysteresis curves, which suggests permanent changes in microcrack morphology. It appears that microcracking is a promising technique for tailoring ceramic materials to a combination of high thermal shock resistance and good insulating capability.     

Mechanical Behavior at 20° and 1200°C of Nicalon-Silicon-Carbide-Fiber-Reinforced Alumina-Matrix Composites

Tensile and fracture tests were conducted at 20° and 1200°C on a ceramic-matrix composite that was composed of an alumina (Al₂O₃) matrix that was bidirectionally reinforced with 37 vol% silicon carbide (SiC) Nicalon fibers. The composite presented nonlinear behavior at both temperatures; however, the strength and toughness were significantly reduced at 1200°C. In accordance with this behavior, matrix cracks were usually stopped or deflected at the fiber/matrix interface, and fiber pullout was observed on the fracture surfaces at 20° and 1200°C. The interfacial sliding resistance at ambient and elevated temperatures was estimated from quantitative microscopy analyses of the saturation crack spacing in the matrix. The in situ fiber strength was determined both from the defect morphology on the fibers and from the size of the mirror region on the fiber fracture surfaces. It was shown that composite degradation at elevated temperature was due to the growth of defects on the fiber surface during high-temperature exposure.     

Effects of Grain Size and Microcracking on the Thermal Diffusivity of MgTi2O5

The laser flash technique was used to study the effect of grain size on the thermal diffusivity of polycrystalline MgTi₂O₅ from 25° to 800°C. Microcracking decreased the thermal diffusivity by as much as a factor of two with the decrease increasing with increasing grain size. When specimens were heated then cooled, the thermal diffusivity exhibited a horizontal flat figure-eight hysteresis. This hysteresis, which appeared to be a function of the thermal history, was attributed to a balance between crack healing, or closure, at high temperatures and the growth of existing cracks or the formation of new cracks during cooling.     

Thermal diffusivity of 3D C/SiC composites from room temperature to 1400 °C

A 3D C/SiC composite and a bulk CVD SiC material were prepared. The effects of the CVD SiC coating and the heat treatment on the longitudinal and transverse thermal diffusivity of the C/SiC composites were investigated. The thermal diffusivity of the C/SiC composites could be well fitted by a multinomial function from room temperature to 1400 °C which includes a power term, an exponential term and a constant term. The exponential term affected the thermal diffusivity and led to its increase above 1200 °C with activation energy of 77 kcal/mol. The microstructure change in the composites was the reason that the thermal diffusivity was increased above 1200 °C. The longitudinal thermal diffusivity of the composite was twice or more than the transverse one and increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it. The heat treatment could increase the thermal diffusivity and make the exponential term disappeared in the functions. The functional curve before the treatment intersected that after the treatment at the treatment temperature.     

Push-Out Tests on a New Silicon Carbide/Reaction-Bonded Silicon Carbide Ceramic Matrix Composite

Fiber push-out tests have been performed on a ceramic matrix composite consisting of Carborundum sintered SiC fibers, with a BN coating, embedded in a reaction-bonded SiC matrix. Analysis of the push-out data, utilizing the most complete theory presently available, shows that one of the fiber/coating/matrix interfaces has a low fracture energy (one-tenth that of the fiber) and a moderate sliding resistance τ∼ 8 MPa. The debonded sliding interface shows some continuous but minor abrasion, which appears to increase the sliding resistance, but overall the system exhibits very clean smooth sliding. The tensile response of a full-scale composite is then modeled, using data obtained here and known fiber strengths, to demonstrate the good composite behavior predicted for this material.     

Role of Interfacial Carbon layer in the Thermal Diffusivity/Conductivity of Silicon Carbide Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites

The role of an interfacial carbon coating in the heat conduction behavior of a uniaxial silicon carbide nitride was investigated. For such a composite without an interfacial carbon coating the values for the thermal conductivity transverse to the fiber direction agreed very well with the values calculated from composite theory using experimental data parallel to the fiber direction, regardless of the ambient atmosphere. However, for a composite made with carbon-coated fibers the experimental values for the thermal conductivity transverse to the fiber direction under vacuum at room temperature were about a factor of 2 lower than those calculated from composite theory assuming perfect interfacial thermal contact. This discrepancy was attributed to the formation of an interfacial gap, resulting from the thermal expansion mismatch between the fibers and the matrix in combination with the low adhesive strength of the carbon coating. In nitrogen or helium the thermal conductivity was found to be higher because of the contribution of gaseous conduction across the interfacial gap. On switching from vacuum to nitrogen a transient effect in the thermal diffusivity was observed, attributed to the diffusion-limited entry of the gas phase into the interfacial gap. These effects decreased with increasing temperature, due to gap closure, to be virtually absent at 1000°C.     

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.     

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.     

Effects of gradual matrix crack closure on the constitutive behavior of SiC/SiC composites upon unloading

Gradual matrix closure and its effects on the constitutive behavior of SiC/SiC composites are examined in the present study. Real-time matrix crack detection and a macroscopic loading–unloading tensile test are performed on SiC/SiC minicomposites. To verify the effects of matrix crack closure, stress-strain responses under loading and unloading with and without crack closure are discussed. The experimental and numerical results show that matrix cracks close gradually upon unloading, and gradual matrix closure greatly reduces the unloading stiffness.