Nondestructive Evaluation and Mechanical Characterization of a Defect-Embedded Ceramic Matrix Composite Laminate

A C/SiC composite panel with defects of known size and of familiar nature was manufactured successfully for nondestructive evaluation. A computed tomography (CT) system was used to detect embedded defects. The results show that CT imaging corresponds well with the designed defects. The defect-embedded composites undergo relatively greater loss in tensile strength and failure strain than the as-received samples, although their initial Young's moduli are almost identical. It is also observed that the embedded defects make a significant contribution toward the initiation and accumulation of the damage in the composites, which result eventually in the early failure of the composite.     

Study of material removal mechanisms in grinding of C/SiC composites via single-abrasive scratch tests

As one of the ceramic matrix composites (CMCs), carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are promising materials used in various engineering applications owing to their superior properties. Precision surface grinding has been widely applied in the machining of CMC composites; however, the material removal mechanisms of C/SiC composites have not been fully elucidated yet. To reveal the material removal mechanisms in the grinding of chemical vapor infiltration-fabricated C/SiC composites, novel single-abrasive scratch tests were designed and conducted in two typical cutting directions. The experimental parameters, especially the cutting speed, conformed to the actual grinding process. The results show that the grinding parameters (feed rate, spindle speed, depth of cut, and cutting direction) have significant influences on the grinding forces, surface integrity, and affected subsurface region. The tangential force is in general larger than the normal force at the same cutting depth. Furthermore, both the tangential and normal forces in the longitudinal cutting direction are larger than those in the transverse cutting direction. The impacts and abrasive actions at the tool tip mainly caused the material removal. The predominant material removal mode is brittle fracture in the grinding of unidirectional C/SiC composites, because the damage behaviors of the C/SiC composites are mainly the syntheses of matrix cracking, fiber breakage, and fiber/matrix interfacial debonding. These results are rationalized based on the composite properties and microstructural damage features.     

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。     

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites

Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated.     

Fabrication process and mechanical properties of C/SiC corrugated core sandwich panel

Carbon fiber reinforced SiC composite is a kind of promising high-temperature thermal protection structural material owing to the excellent oxidative resistance and superior mechanical properties at high temperatures. In this work, a novel design and fabrication process of lightweight C/SiC corrugated core sandwich panel will be proposed. The compressive and three-point bending of the C/SiC corrugated sandwich panels are conducted by experiment and numerical simulation. The relative density of as-prepared C/SiC sandwich panel and the density composite material are 1.1 and 2.1 g/cm³, respectively. As the density of the C/SiC sandwich panel is only 52.3% of the bulk C/SiC, suggesting that lightweight characteristic is realized. Moreover, the C/SiC sandwich panel manifests itself as linear-elastic behavior before failure in compression and the strength is as high as 15.1 MPa. The failure mode is governed by the core shear failure and panel interlayer cracking. The load capacity under the three-point bending C/SiC composite sandwich panel is 1947.0 N. The main failure behavior is core shear failure. The stress distribution under the compression and three-point bend was simulated by FE analysis, and the results of numerical simulations are in accordance with the experimental results.     

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.     

Study on ablation behavior and mechanism of C/SiC composite irradiated by a laser with various parameters

C/SiC composite has been widely used as a high-temperature material for engineering components due to its excellent thermal properties. Facing the rapid development and threat of high-energy laser, study on the ablation resistance under laser irradiation is strongly required. In this work, a continuous high-energy laser was applied to explore the laser ablation behavior and mechanism of C/SiC composite. From the results, C/SiC composite shows different morphologies when irradiated at various laser power densities for 500 and 700 W/cm². We divided the ablation area into three regions; the central, transition, and edge regions, where the formation of SiO₂, SiO, and the breakage of carbon fiber were observed. The generated highly reflective SiO₂ layer reduces the absorption of laser energy, which is beneficial to lower the back-surface temperature and reduce the damage of composite. In addition, we put forward the ablation physical models and ablation mechanisms irradiated at different power densities. The work provides a basis for the laser ablation resistance of C/SiC composites under different conditions.     

Etching of SiC–SiC-composites by a laser-induced plasma in a reactive gas

The precise machining of silicon carbide composite (SiC–SiC) as a high-tech material with extraordinary characteristics is required for different applications in aerospace, light weight construction and car industry. Laser machining enable new approaches for fabrication processes but the regularly applied ablation processes can cause damage to the SiC–SiC material. Here we propose and demonstrate a new approach for gentle SiC–SiC machining making use of a laser-induced plasma for reactive species generation enabling chemical material removal processes. A fs-laser (775 nm, 150 fs, 1 kHz) was focussed to a CF₄/O₂ gas mixture igniting a laser-induced plasma (LIP) approximately 100 μm in front of a SiC–SiC sample. This LIP initiate material removal processes of the textured, multiphase SiC–SiC sample without a mechanical damage of the SiC–SiC composite structure. Different surface features such as etching of the cover SiC layer, etching of the SiC matrix and exposure, thinning and sharpen of the SiC fibres, underetching of the fibres has been observed. Across the whole etched area, no mechanical damage such as cracks, delamination's, broken fibres were observed so that a gentle machining process can be expected.     

Microstructure and mechanical property of SiC whisker coating modified C/C composite/Ti2AlNb alloy dissimilar joints prepared by transient liquid phase diffusion bonding

SiC whisker coating was prepared on the surface of C/C composite successfully by CVD, and transient liquid phase (TLP) diffusion bonding was employed to realize the joining of SiC whisker coating modified C/C composite and Ti₂AlNb alloy using Ti–Ni–Nb foils as interlayer. The microstructure, shear strength and fracture behavior were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD) and universal testing machine. The results show that SiC has good compatibility with C/C composite, and gradient interface formed between SiC-modified C/C composite and Ti₂AlNb alloy. When the bonding experiment was carried out under bonding temperature of 1040 °C and holding time of 30min with 5 MPa pressure in vacuum, the joints formed well and no obvious defects can be observed. The typical microstructure of joints is C/C composite/SiC + TiC/Ti–Ni compounds + Ti–Ni–Nb solid solutions/residual Nb/diffusion reaction layer/Ti₂AlNb alloy. With the increasing of bonding temperature, the thickness of joining area increased due to sufficient element diffusion. However, when bonding temperature is elevated to 1060 °C, some defects such as cracks and slag inclusions exist in the interface layer between interlayer and Ti₂AlNb. The joints with maximum average shear strength of 32.06 MPa are bonded at 1040 °C for 30min. C, SiC and TiC can be found on the fracture surface of joints bonded at 1040 °C which indicated that fracture occurred at the interface layer adjacent SiC layer.     

Micromechanisms of Creep–Fatigue Crack Growth in a Silicide-Matrix Composite with SiC Particles

An experimental study has been conducted to examine the cyclic fatigue crack growth characteristics in 1200^oC air of a MoSi₂-50 mol% Wsi₂ alloy the unreinforced condition and with 30 vol% SiC particles. For comparison purposes, crack growth experiments under sustained loads were also carried out in the silicide-matrix composite. Particular attention is devoted to developing an understanding of the micromechanisms of subcritical crack-tip damage. The results indicate that enhanced viscous flow of glass films along interfaces and grain boundaries imparts pronounced levels of subcritical crack growth in the composite material; the composite exhibits a higher fatigue fracture threshold and a more extended range of stable fracture than the unreinforced alloy. The effects of glass phase in influencing fatigue crack growth in the silicide-based material are compared to the influence of in situ -formed and preexisting glass films on high-temperature cyclic fatigue crack growth in ceramics and ceramic composites. The paper concludes with a comparison of present results with the high-temperature damage tolerance of a variety of intermetallic alloys and ceramic materials.     

Development and tribological studies of a novel metal-ceramic hybrid brake disc

Ceramic matrix composite (CMC) friction materials show promising tribological properties. Typically, carbon ceramic brake discs consist of a C/SiC rotor which is joined to a brake disc bell. Within this work, a novel metal-ceramic hybrid brake disc, consisting of C/SiC friction segments which are mounted by screws onto an aluminum carrier body, was designed and investigated. A prototype was built which was tribologically tested with three different brake pad materials, LowMet reference, modified SF C/SiC as well as C/C. A constant starting sliding velocity of 20 m/s and braking pressures of 1, 2, and 3 MPa were investigated. To simulate emergency braking conditions 10 consecutive brake applications were carried out in close succession for each brake pad material and braking pressure. The C/C brake pad material showed the highest average coefficient of friction followed by the LowMet and C/SiC material. However, the wear rates of the C/C and LowMet material were orders of magnitude higher compared to the C/SiC material.     

Thermal shock response behaviour of carbon fibre reinforced silicon carbide matrix composites prepared by chemical vapour infiltration at different heating rates

The present work investigated the effects of thermal cycles in air on the tensile properties of a two-dimensional carbon fibre reinforced silicon carbide composite (2D C/SiC) prepared by chemical vapour infiltration at different heating rates. The composite was exposed to different cycles of thermal shock between 20 °C and 1300 °C in air. The damage mechanisms were investigated by AE online monitoring and fractured morphology offline analysis. The tensile strength of 2D-C/SiC decreases with increasing thermal cycles. However, the modulus only decrease within 40 cycles. Due to oxidation, with the decrease in heating rate, the residual properties of the material decrease more obviously. Meanwhile, the results of AE online monitoring and fracture analysis show that the matrix damage is more serious at higher heating rate and that more delamination occours in tensile fractures. The above results indicate that for the thermal shock of 2D C/SiC composites in air, oxidative damage plays a key role in the residual properties.     

Preparation and mechanical behaviors of SiOC-modified carbon-bonded carbon fiber composite with in-situ growth of three-dimensional SiC nanowires

Carbon-bonded carbon fiber (CBCF) composites are novel and important high-temperature insulation materials owing to their light weight, low thermal conductivity and high fracture tolerance. To further improve the mechanical property of CBCF composite, we propose a three-dimensional (3D) SiC nanowires structure, which is in situ grown on a CBCF matrix via directly annealing silicon oxycarbide (SiOC) ceramic precursor. The synthesized multiscale reinforcements including microscale SiOC ceramics and nanoscale SiC nanowires are mainly attributed to the initial phase separation of SiOC phase and subsequent solid-phase reaction of SiO and C phases. Compared to SiOC/CBCF composite, the resulting 3D SiC nanowires/SiOC/CBCF hybrid structure exhibited high flexural/tensile strength and fractured strain due to the pull-out and bridging behavior of SiC nanowires. This one-step process supplied a feasible way to synthesize 3D SiC nanowires to reinforce and toughen SiOC-modified CBCF composite.     

Thick Protective UHTC Coatings for SiC-Based Structures: Process Establishment

A new process to form thick and dense ultra-high-temperature ceramic (UHTC) composite coatings over SiC surfaces is described. Coatings of ZrB₂/SiC/(ZrC) thicker than 100 μm are formed by a reaction-bonded SiC (RBSC) approach based on Si infiltration into ZrB₂/C preform coating. The residual Si, typically found in RBSC, can be eliminated efficiently to provide a coating material that performs at temperatures above 1500°C. The process is performed at 1500°C in Ar at ambient pressure. The interface between the in situ formed SiC and the Zr phases is very tight, as is the interface with the substrate.
The ZrB₂ particles used in this process are rearranged in their morphology and an additional new phase containing Zr–C is formed. The coatings exhibit excellent integrity, hardness, and bonding to the tested substrates. A preliminary oxidation study indicates good protection of substrates at 1500°C under both passive and active oxidation conditions, provided that the coatings have sufficient thickness.     

Fatigue Damage Accumulation in 3-Dimensional SiC/SiC Composites

The effect of fatigue loading on the mechanical performance of 3-D SiC/SiC composites was investigated. A non-destructive macromechanical approach was applied which permits for the evaluation of the material damage state by monitoring its dynamic response as function of fatigue cycles. The correlation of the results provided by this method to that of other non-destructive techniques such as Acoustic Emission (AE), leads to a detail micromechanical-macromechanical monitoring of the material fatigue behaviour. The damage modes identification and their successive appearance, together with the evaluation of the material performance at the different stages of fatigue loading, is among the inspection capabilities that provides the above mentioned combination of non-destructive techniques. The proposed methodology applied in the case of a 3-D SiC/SiC ceramic matrix composite material and the effect of fatigue loading on the material integrity was evaluated by measuring the degradation of the dynamic modulus of elasticity and the increase of the material damping. Conclusions, concerning design aspects using these materials, as well as fatigue life prediction were provided. Finally, the sensitivity of the proposed methodology for the definition, the characterisation of the development and the separation of the different damage modes during fatigue loading has been discussed.     

Anisotropic thermal diffusivity and conductivity in SiC/SiC tubes studied by infrared imaging and X-ray computed tomography

Traditional flash diffusivity evaluation of thermal diffusivity/conductivity of composite tubes require machining of specimens. For a thin-wall tube, this method can only be used to obtain through-thickness transport property. A novel method to evaluate anisotropic thermal diffusivity in a composite tube has been developed. Braided SiC/SiC composite tubes were subjected to a xenon flash heating pulse. A high-speed, high-sensitivity infrared camera was used to measure surface temperature changes as a function of time and nondestructively detect subsurface defects/damages, such as macroscopic pores. Standard reference material (Pyroceram 9606) and curved SiC/SiC composite tube specimens were used to validate thermal diffusivity obtained from infrared imaging. Unlike the traditional method, there is no need prepare special specimens, and thermal diffusivity values in three orientations are obtained after a single flash. A finite element analysis model based on x-ray computed tomography scans was developed to simulate the heat transfer. This technique is significant in assessing thermal conductivity and inspecting the health of ceramic tubes during and after service.     

Design and preparation of composite coatings with increased reflectivity under high-energy continuous wave laser ablation

High-energy continuous wave (CW) laser ablation can cause severe damage to structural materials in an extremely short time, which generates considerable concern in terms of material safety. For the purpose of reducing or even eliminating such laser-induced damage, a novel composite coating consisting of a boron-modified phenolic formaldehyde resin incorporating ZrC and SiC has been designed and prepared. The experimental results reveal that ZrC and SiC are rapidly oxidized to ZrO₂ and SiO₂ respectively, leading to the formation of a white ceramic layer consisting of ZrO₂ particles and melted SiO₂. After ablation at 1000 W/cm² for 50 s, elemental analysis indicates that no Si can be found in the central ablation zone because of gasification. A relatively compact ZrO₂ layer is formed through the sintering of adjacent ZrO₂ particles, which effectively improves the reflectivity of the coating from 7.3% (before ablation) to 63.5% (after ablation). The high reflectivity greatly reduces the absorption of laser energy. In addition, no obvious ablation defects are observed in the composite coating. The excellent anti-laser ablation performance of the coating makes it a promising system for protecting a material against the effects of long-term CW laser ablation.     

反应熔体浸渗法制备C/SiC复合材料的显微结构与摩擦性能

C/SiC复合材料由于密度小、耐磨性好、耐高温等一系列优异性能,极有希望成为新一代的先进摩擦材料.反应熔体浸渗法由于工艺简单、成本低等优点最适合制造摩擦用C/SiC复合材料.本文采用反应熔体浸渗法制备C/SiC复合材料,进行显微结构和X射线衍射分析,测试试样的开气孔率、热扩散率及摩擦性能.结果表明材料致密度高,开气孔率为3.1～4.4%,热扩散率为0.089cm2/s.RMI工艺过程中有微小孔洞及裂纹产生.摩擦性能在后几次刹车实验时不稳定,还有待进一步提高.     

Fatigue of a SiC/SiC ceramic composite with an ytterbium-disilicate environmental barrier coating at elevated temperature*

Tension-tension fatigue performance of a SiC/SiC composite with an ytterbium-disilicate environmental barrier coating (EBC) was investigated at 1200°C in air and steam. The composite is reinforced with Hi-Nicalon™ SiC fibers and has a melt-infiltrated matrix processed by chemical vapor infiltration of SiC with subsequent infiltration with SiC particulate slurry and molten silicon. The EBC consists of a Si bond coat and an Yb₂Si₂O₇ top coat applied via air plasma spraying. Tensile properties were evaluated at 1200°C. Tension-tension fatigue was examined for maximum stresses of 110-140 MPa. To assess the efficacy of EBC, experimental results obtained for the coated composite are compared to those for a control uncoated composite. Surface grit-blasting inherent in the EBC application process degrades tensile strength of the composite. However, the EBC effectively protects the composite from oxidation embrittlement during cyclic loading in air or steam. Fatigue runout set to 200 000 cycles (55.6 hours at a frequency of 1.0 Hz) was achieved at 110 MPa in air and steam. Retained properties of pre-fatigued specimens were characterized. Composite microstructure, along with damage and failure mechanisms were investigated. Damage and failure of the composite are attributed to the growth of cracks originating from numerous processing defects in the composite interior.     

Oxidation Kinetics and Stress Effects for the Oxidation of Continuous Carbon Fibers within a Microcracked C/SiC Ceramic Matrix Composite

Carbon fiber-reinforced silicon carbide (C/SiC) composites have the potential to be utilized in many high-temperature structural applications, particularly in aerospace. However, the susceptibility of the carbon fibers to oxidation has hindered the composite's use in long-term reusable applications. In order to identify the composites limitations, fundamental oxidation studies were conducted to determine the effects of such variables as temperature, environment, and stress. The systematic studies first looked at the oxidation of the plain, uncoated carbon fiber, then when fiber was utilized within a C/SiC composite, and finally when a stress was applied to the C/SiC composite (stressed oxidation). The first study, oxidation of just the carbon fibers, showed that the fiber oxidation kinetics occurs in two primary regimes: chemical reaction control and diffusion control. The second study, oxidation of the C/SiC composite, showed the self-protecting effects from the SiC matrix at elevated temperatures when the composite was not stressed. The final study, stressed oxidation of the C/SiC composite, more closely simulated application conditions in which the material is expected to encounter thermal and mechanical stresses. The applied load and temperature will affect the openings of the as-fabricated cracks, which are an unavoidable characteristic of C/SiC composites. The main objective of the paper was to determine the oxidation kinetic regimes for the oxidation of carbon fibers in a cracked silicon carbide matrix under stressed and unstressed conditions. The studies help to provide insights in to the protective approaches, that could be used to prevent oxidation of the fibers within the composite.