SiCf/O′-SiAlON composite: properties and oxidation retained properties

A composite of O′SiAlON (Si_2-xAl_xN_2-xO_1+x, with x 0.14) reinforced with 20 vol.% SiC monofilaments was fabricated by hot-pressing, at 1600°C, for 2 h under 34 MPa pressure. The mechanical and interfacial properties of the composites, as-fabricated as well as post-oxidized, were, investigated. The composite exhibited a significant improvement in ultimate flexure strength (640 MPa) and work of fracture (42 kJ m⁻²) compared with that (350 MPa and 1.8 kJ m⁻², respectively) of the monolithic material. These mechanical properties were slightly increased after the composite was heat treated for 24 h in air at 1200 and 1300°C. However, the composite exhibited a significant degradation in ultimate strength, while the work of fracture (WOF) remained unchanged after exposure in air at temperatures beyond 1400°C. The as-fabricated composite revealed a low interfacial shear strength (6.2 MPa) and a frictional sliding stress (3.2 MPa). After the composite was oxidized at elevated temperatures, the interfacial bonding and sliding stresses were reduced to noticeable extents, resulting from the degradation of the carbon coating layer of the SiC monofilaments.     

Effects of interface coating and nitride enhancing additive on properties of Hi-Nicalon SiC Fiber reinforced reaction-bonded silicon nitride composites

Strong and tough Hi-Nicalon SiC fiber reinforced reaction-bonded silicon nitride matrix composites (SiC/RBSN) have been fabricated by the fiber lay-up approach. Commercially available uncoated and PBN, PBN/Si-rich PBN, and BN/SiC coated SiC Hi-Nicalon fiber tows were used as reinforcement. The composites contained 24 vol% of aligned 14 m diameter SiC fibers in a porous RBSN matrix. Both one- and two-dimensional composites were characterized. The effects of interface coating composition, and the nitridation enhancing additive, NiO, on the room temperature physical, tensile, and interfacial shear strength properties of SiC/RBSN matrix composites were evaluated. Results indicate that for all three coated fibers, the thickness of the coatings decreased from the outer periphery to the interior of the tows, and that from 10 to 30 percent of the fibers were not covered with the interface coating. In the uncoated regions, chemical reaction between the NiO additive and the SiC fiber occurs causing degradation of tensile properties of the composites. Among the three interface coating combinations investigated, the BN/SiC coated Hi-Nicalon SiC fiber reinforced RBSN matrix composite showed the least amount of uncoated regions and reasonably uniform interface coating thickness. The matrix cracking stress in SiC/RBSN composites was predicted using a fracture mechanics based crack bridging model.     

Effects of Steam Environment on Fatigue Behavior of Two SiC/[SiC+Si<Subscript>3</Subscript>N<Subscript>4</Subscript>] Ceramic Composites at 1300°C

The fatigue behaviors of two SiC/[SiC+Si₃N₄] ceramic matrix composites (CMC) were investigated at 1,300°C in laboratory air and in steam. Composites consisted of a crystalline [SiC+Si₃N₄] matrix reinforced with either Sylramic™ or Sylramic-iBN fibers (treated Sylramic™ fibers that possess an in situ BN coating) woven in a five-harness satin weave fabric and coated with a proprietary boron-containing dual-layer interphase. The tensile stress–strain behaviors were investigated and the tensile properties measured at 1,300°C. Tension–tension fatigue behaviors of both CMCs were studied for fatigue stresses ranging from 100 to 180 MPa. The fatigue limit (based on a run-out condition of 2 × 10⁵ cycles) in both air and steam was 100 MPa for the CMC containing Sylramic™ fibers and 140 MPa for the CMC reinforced with Sylramic-iBN fibers. At higher fatigue stresses, the presence of steam caused noticeable degradation in fatigue performance of both composites. The retained strength and modulus of all run-out specimens were characterized. The materials tested in air retained 100% of their tensile strength, while the materials tested in steam retained only about 90% of their tensile strength.     

不同界面SiC纤维束复合材料的拉伸力学行为

利用国产三代SiC纤维通过化学气相渗透工艺(CVI)制备不同界面厚度和基体体积分数的SiC纤维束复合材料,并对其拉伸力学行为进行研究;同时,通过有限元方法研究界面厚度和基体体积分数对SiC纤维束复合材料热残余应力的影响。有限元分析结果表明:该纤维束复合材料的界面存在较为明显的径向和环向热残余应力,而且这两种应力均随着界面厚度增加而减小,随着基体体积分数的增加而增加。拉伸实验结果表明:随着界面厚度增加SiC纤维束复合材料的拉伸强度有增大趋势,且纤维拔出长度也相应增加;但在界面厚度相同的情况下,过高的基体体积分数将导致复合材料拉伸强度和韧性下降。     

CVD SiC fiber-reinforced barium aluminosilicate glass—ceramic matrix composites

Unidirectional CVD SiC (SCS-6) monofilament reinforced BaOAl₂O₃2SiO₂(BAS) glass—ceramic matrix composites have been fabricated by a tape lay-up method followed by hot pressing. The glass matrix flows around fibers during hot pressing resulting in nearly fully dense (95–98%) composites. Strong and tough composites having first matrix cracking stress of 250–300 MPa and ultimate flexural strength as high as 900 MPa have been obtained. Composite fracture surfaces showed fiber pullout with no chemical reaction at the fiber/matrix interface. From fiber push out, the fiber/matrix interfacial debond strength and the sliding frictional stress were determined to be 5.9 ± 1.2 MPa and 4.8 ± 0.9 MPa, respectively. The fracture surface of an uncoated SiC (SCS-0)/BAS composite also showed fiber/matrix debonding, fiber pullout, and crack deflection around the fibers implying that the SiC fibers may need no surface coating for reinforcement of the BAS glass-ceramic. Applicability of micromechanical models in predicting the first matrix cracking stress and the ultimate strength of these composites has also been examined.     

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.     

A functionally gradient coating on carbon fibre for C/Al composites

A functionally gradient coating on carbon fibre for casting C/Al composites with an ultimate tensile strength up to 1250 MPa (V _f=0.35) has been produced. The coating consisted of three layers: an inner pyrocarbon layer, an outer silicon layer and an intermediate gradient layer C/SiC/Si, and their optimum thicknesses were 0.1–0.15, 0.1 and 0.2 m, respectively. This coating was fabricated by chemical vapour deposition and the C/Al composite was performed by pressure-regulated infiltration. Auger electron spectroscopy and X-ray diffraction analyses confirmed that the structure of the coating was in keeping with its design. The excellent ultimate tensile strength of the C/Al composite also proves that the functionally gradient coating has many functions, including wetting agent, diffusion and reaction barrier, releaser of residual thermal stresses, and tailor of interfacial shear strength. According to the mechanical, physical and chemical coordination between fibre and matrix, the functionally gradient coating can solve nearly all the problems of the interface during fabrication and service.     

SEM环境下纤维推出技术结合电子束云纹技术表征复合材料界面细观力学性能

纤维推出技术是研究复合材料界面细观力学性能的常用方法。本文将该方法在SEM环境下与电子束云纹技术相结合开发一套基于SEM环境下的纤维推出实验系统。利用该系统测试了SiC/Ti-15-3复合材料的界面剪切强度、摩擦应力、摩擦系数及残余应力分布等细观力学性能。结果表明:对于厚度为500 μm的SiC/Ti-15-3复合材料界面剪切强度为35 MPa,摩擦应力为32.8 MPa,纤维与界面间的摩擦系数为0.082,径向残余应力为?400 MPa。该系统在SEM环境可以实现直径为几微米的纤维推出,扩展了纤维推出技术的应用范围,提高了纤维推出过程的对准精度,减小了测量误差。并且与电子束云纹技术相结合,实时测量纤维推出后界面残余应力分布情况,为复合材料界面的设计、评估及优化提供必要的实验方法。      

Synthesis and properties of model SiC-Si3N4 interfaces

Microscopic and macroscopic SiC-Si₃N₄ interfacial structures were synthesized and their properties examined. Microscopic interfaces were produced by hot isostatic pressing vapour-liquid-solid SiC whisker-polycrystalline Si₃N₄ matrix composites without densification aids. Macroscopic interfaces were formed by the chemical vapour deposited Si₃N₄ coating of large SiC single crystals. The characteristics of these model interfaces were investigated using transmission electron microscopy and indentation fracture. Results showed the microscopic interfaces to contain a small amount of second phase, while the macroscopic interfaces were pristine in nature with no second phase present. Pristine SiC-Si₃N₄ interfaces were strongly bonded at room temperature, but interfacial strength decreased at elevated temperatures.     

Preparation and characterization of multilayered ZrO<Subscript>2</Subscript> coatings on silicon carbide fibers for SiC/SiC composites

We have studied the surface morphology, phase composition, and oxidation resistance of multilayered tetragonal zirconia coatings produced on silicon carbide fibers by a sol-gel process and measured the tensile strength of individual fibers as a function of the number of layers in the coating. SiC-fiber-reinforced silicon carbide minicomposites have been prepared through pyrolysis of an organosilicon polymer, and their fracture surfaces have been examined. Using microindentation, we have determined the critical fiber-matrix debonding stress. The results demonstrate that the ZrO₂ coating on the fibers has the form of uniform, weakly bonded layers. The presence of a multilayered ZrO₂ interphase alters the fracture behavior of the SiC/SiC composites. The fiber debond stress in the composites markedly decreases with an increase in the number of layers in the interphase.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

Wear-mechanical properties of filler-added liquid silicon infiltration C/C–SiC composites

A carbon fiber reinforced silicon carbide matrix (C/C–SiC) composites material was manufactured by introducing a filler into the liquid silicon infiltration (LSI) process. The filler consisted of Si:Carbon black = 1:1 mixed with a phenol resin. Use of the filler resulted in a negligible reduction in the residual free Si of approximately 0.7% but increased 15% of reacted SiC amount. Dilatometer and X-ray diffraction (XRD) evaluations also confirmed improved formation of reaction-bonded silicon carbide (SiC) in the matrix. The wear rate was decreased more than 2.5-fold, indicating significantly improved wear-resistance properties. However, flexural strength gradually decreased and fiber damage was observed in fracture surface with increases in filler content.     

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.     

碳纤维表面改性增强Fe3Al-Al2O3复合材料的制备及组织性能

采用溶胶-凝胶分散和热压烧结制备了短切碳纤维（CF_s）/Fe₃Al-Al₂O₃复合材料。分别通过电化学镀Cu和化学气相沉积SiC对CF_s表面修饰和改性,研究了Cu镀层和SiC涂层对CF_s/Fe₃Al-Al₂O₃复合材料显微组织、相组成、力学性能及断裂行为的影响。结果表明,未修饰的CF_s在Fe₃Al-Al₂O₃基体中受到严重侵蚀,CF_s/Fe₃Al-Al₂O₃复合材料致密度低,抗弯强度仅为239.0 MPa,与Fe₃Al-Al₂O₃强度相当;表面镀Cu可有效保护CF_s不被侵蚀,同时提高了CF_s/Fe₃Al-Al₂O₃复合材料的烧结致密性和界面结合强度,从而明显提高了复合材料的断裂强度,但断裂过程中纤维拔出较短;CF_s表面沉积SiC的CF_s/Fe₃Al-Al₂O₃复合材料组织均匀致密,表面涂层完整,且与纤维及基体之间结合力相当,断裂过程中,涂层既可随纤维一起拔出基体,也可与CF_s分离而留在基体之中,SiC涂层与纤维及基体之间的弱相互作用很大程度上促进了纤维脱黏和拔出,从而促进CF_s/Fe₃Al-Al₂O₃复合材料韧化所需的渐进破坏机制。      

纤维涂层法制备SiCf/Cu复合材料及其性能分析

为研究纤维涂层法制备SiCf/Cu复合材料的性能特点,通过磁控溅射法先后将Ti6Al4V界面改性层和基体Cu涂层涂覆到SiC纤维表面,并通过真空热压法将被涂覆的纤维制备成SiCf/Cu复合材料.对Ti6Al4V涂层、Cu涂层以及复合材料进行了微观分析,并测试了复合材料的拉伸强度.研究表明,复合材料的Cu基体由致密而细小的晶粒组成;Ti6Al4V提高了纤维/基体界面结合强度,复合材料轴向抗拉强度高达500 MPa,界面脱粘主要发生在纤维表面的碳涂层与纤维之间.     

An investigation into a multilayered BN/Si3N4/BN interfacial coating

In order to prevent environmental degradation of the interface, a triplex coating was employed as the interface in ceramic matrix composites (CMC). This interface consists of an initial BN layer followed by a Si₃N₄ layer and lastly another BN layer. Single strand unidirectional mini-composites using BN/Si₃N₄/BN coated ceramic grade Nicalon? fibers as the reinforcement and chemical vapor infiltrated (CVI) SiC as the matrix were fabricated to understand the initial properties of the interfacial coating. Field emission scanning electron microscopy (FE-SEM) confirmed the thickness of the triplex coating before and after mini-composite fabrication. FE-SEM micrographs after mechanical and environmental testing of the single strand unidirectional mini-composites showed the consequences of using the triplex interfacial coating. Finally, eight ply continuous fiber reinforced (CFR) CMCs with the BN/Si₃N₄/BN triplex interface and the traditional BN/Si₃N₄ duplex interface were fabricated using the polymer impregnation and pyrolysis (PIP) process. The PIP process has gained popularity in recent years and this allows for the fabrication of larger CMC panels as compared with the CVI process. Mechanical testing for the PIP-fabricated CFR-CMC panels showed that the composites using the triplex interface had better mechanical properties than those fabricated with a BN/Si₃N₄ duplex interface after environmental testing.     

Micromechanical modeling of interface damage of metal matrix composites subjected to off-axis loading

A three dimensional micromechanics based analytical model is presented to investigate the effects of initiation and propagation of interface damage on the elastoplastic behavior of unidirectional SiC/Ti metal matrix composites (MMCs) subjected to off-axis loading. Manufacturing process thermal residual stress (RS) is also included in the model. The selected representative volume element (RVE) consists of an r × c unit cells in which a quarter of the fiber is surrounded by matrix sub-cells. The constant compliance interface (CCI) model is modified to model interfacial de-bonding and the successive approximation method together with Von-Mises yield criterion is used to obtain elastic–plastic behavior. Dominance mode of damage including fiber fracture, interfacial de-bonding and matrix yielding and ultimate tensile strength of the SiC/Ti MMC are predicted for various loading directions. The effects of thermal residual stress and fiber volume fraction (FVF) on the stress–strain response of the SiC/Ti MMC are studied. Results revealed that for more realistic predictions both interface damage and thermal residual stress effects should be considered in the analysis. The contribution of interfacial de-bonding and thermal residual stress in the overall behavior of the material is also investigated. Comparison between results of the presented model shows very good agreement with finite element micromechanical analysis and experiment for various off-axis angles.     

Thermal effects on the mechanical properties of SiC fibre reinforced reaction-bonded silicon nitride matrix composites

The elevated temperature four-point flexural strength and the room-temperature tensile and flexural strength properties after thermal shock were measured for ceramic composites consisting of 30 vol% uniaxially aligned 142 m diameter SiC fibres in a reaction-bonded Si₃N₄ matrix. The elevated temperature strengths were measured after 15 min exposure in air at temperatures upto 1400 ° C. The thermal shock treatment was accomplished by heating the composite in air for 15 min at temperatures up to 1200 ° C and then quenching in water at 25 ° C. The results indicate no significant loss in strength properties either at temperature or after thermal shock when compared with the strength data for composites in the as-fabricated condition.     

Preparation and interface modification of Si_3N_(4f)/SiO_2 composites

In order to modify the interface, SiON coating was introduced on the surface of silicon nitride fiber by perhydropolysilazane conversion method. Si₃N_4f/SiO₂ and Si₃N_4f/SiON_c/SiO₂ composites were prepared by sol-gel method to explore the influence of SiON coating on the mechanical properties of composites. The results show that with the protection of SiON coating, Si₃N₄ fiber enjoys a strength increase of up to 24.1% and Si₃N_4f/SiON_c/SiO₂ composites have a tensile strength of 170.5 MPa and a modulus of 26.9 GPa, respectively. After 1000 °C annealing in air for 1 h, Si₃N_4f/SiON_c/SiO₂ composites retain 65.0% of their original strength and show a better toughness than Si₃N_4f/SiO₂ composites. The improvement of mechanical properties is attributing to the healing effect of SiON coating as well as its intermediate coefficient of thermal expansion between Si₃N₄ fiber and SiO₂ matrix.     

Designing the fiber volume ratio in SiC fiber-reinforced SiC ceramic composites under Hertzian stress

Finite element method (FEM) analysis and experimental studies are undertaken on the design of the fiber volume ratio in silicon carbide (SiC) fiber-reinforced SiC composites under indentation contact stresses. Boron nitride (BN)/Pyrocarbon (PyC) are selected as the coating materials for the SiC fiber. Various SiC matrix/coating/fiber/coating/matrix structures are modeled by introducing a woven fiber layer in the SiC matrix. Especially, this study attempts to find the optimum fiber volume ratio in SiC fiber-reinforced SiC ceramics under Hertzian stress. The analysis is performed by changing the fiber type, fiber volume ratio, coating material, number of coating layers, and stacking sequence of the coating layers. The variation in the stress for composites in relation to the fiber volume ratio in the contact axial or radial direction is also analyzed. The same structures are fabricated experimentally by a hot process, and the mechanical behaviors regarding the load–displacement are evaluated using the Hertzian indentation method. Various SiC matrix/coating/fiber/coating/matrix structures are fabricated, and mechanical characterization is performed by changing the coating layer, according to the introduction (or omission) of the coating layer, and the number of woven fiber mats. The results show that the damage mode changes from Hertzian stress to flexural stress as the fiber volume ratio increases in composites because of the decreased matrix volume fraction, which intensifies the radial crack damage. The result significantly indicates that the optimum fiber volume ratio in SiC fiber-reinforced SiC ceramics should be designed for inhibiting the flexural stress.