Interfacial Mechanical Characterization of Nicalon SiC Fiber/Alumina-Based Composites

Interfacial mechanical properties of both Nicalon SiC/aluminum borate and Nicalon SiC/aluminum phosphate with various fiber coatings and heat treatments were evaluated using a commercially-available indenter to induce fiber sliding during load cycling experiments. Varying degrees of sliding due to different coating materials were found. The interfacial characteristics including the shear, the residual axial fiber, and debond stresses were estimated by matching the experimental stress-displacement curves with curves predicted from an existing model. The elastic modulus and hardness of the interphase/interface in ceramic matrix composites were also evaluated. These results provided important insights into the ultimate mechanical performance of fiber-reinforced ceramic-matrix composites.     

Interfacial Mechanical Characterization of Nicalon SiC Fiber/Alumina-Based Composites

Interfacial mechanical properties of both Nicalon SiC/aluminum borate and Nicalon SiC/aluminum phosphate with various fiber coatings and heat treatments were evaluated using a commercially-available indenter to induce fiber sliding during load cycling experiments. Varying degrees of sliding due to different coating materials were found. The interfacial characteristics including the shear, the residual axial fiber, and debond stresses were estimated by matching the experimental stress-displacement curves with curves predicted from an existing model. The elastic modulus and hardness of the interphase/interface in ceramic matrix composites were also evaluated. These results provided important insights into the ultimate mechanical performance of fiber-reinforced ceramic-matrix composites.     

Mechanical Properties of Thin Pyrolitic Carbon Interphase SiC–Matrix Composites Reinforced with Near-Stoichiometric SiC Fibers

Tensile properties of Tyranno™-SA near-stoichiometric silicon carbide (SiC)-fiber–reinforced chemically vapor-infiltrated SiC-matrix composites with pyrolytic carbon interphases were experimentally studied. The influence of interphase thickness in a range of 60–300 nm on the tensile properties of the materials appeared to be generally minor. Thin interphase (<100 nm) did not have a significant deteriorating effect on composite properties, which has commonly been reported for conventional SiC-fiber composites. For very thin interphase (<60 nm) composites, a slight decrease in fracture strain and a substantial increase in interfacial sliding stress were noted. Increases in ultimate tensile strength and fracture strain were observed at a much thicker interphase (>600 nm) at the expense of composite stiffness.     

纤维类型对Cf/SiC复合材料力学性能的影响

本工作以AIN和Y2O3为烧结助剂,采用先驱体转化－热压烧结的方法制备出了Cf/SiC复合材料,研究了纤维类型影响复合材料力学性能的本质原因,由于T300纤维的制备温度明显低于M40JB纤维的制备温度,因此,与M40JB纤维相比,T300纤维的石墨化程度较低且含有较多的杂质,从而导致T300纤维表面的活性强,而M40JB纤维表面的活性较弱,正是这种结构和成分的差别,使T300纤维与基体的结合较强,而M40JB纤维与基体的结合较弱,因此以T300纤维为增强的复合材料呈现脆性断裂,而以M40JB纤维为增强相的复合材料则呈现韧性断裂,谈复合材料具有较好的力学性能。     

Effect of a Boron Nitride Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites

Typically, the debonding and sliding interface enabling fiber pullout for SiC-fiber-reinforced SiC-matrix composites with BN-based interphases occurs between the fiber and the interphase. Recently, composites have been fabricated where interface debonding and sliding occur between the BN interphase and the matrix. This results in two major improvements in mechanical properties. First, significantly higher failure strains were attained due to the lower interfacial shear strength with no loss in ultimate strength properties of the composites. Second, significantly longer stress-rupture times at higher stresses were observed in air at 815°3C. In addition, no loss in mechanical properties was observed for composites that did not possess a thin carbon layer between the fiber and the interphase when subjected to burner-rig exposure. Two primary factors were hypothesized for the occurrence of debonding and sliding between the BN interphase and the SiC matrix: a weaker interface at the BN/matrix interface than the fiber/BN interface and a residual tensile/shear stress-state at the BN/matrix interface of melt-infiltrated composites. Also, the occurrence of outside debonding was believed to occur during composite fabrication, i.e., on cooldown after molten silicon infiltration.     

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.     

纤维类型对C/SiC复合材料性能的影响

利用化学气相浸渗法制备了Cf-C/SiC复合材料,借助SEM、TEM等研究了纤维类型对Cf-C/SiC复合材料力学性能的影响.实验证明T300碳纤维增韧补强效果优于M40碳纤维,利用T300碳纤维制备出弯曲强度为459M,断裂韧性为20.0MPa*m1/2,断裂功为25170J/m2的Cf-C/SiC复合材料.2种碳纤维增韧效果的差异是由纤维的原始强度、热膨胀系数和弹性常数的不同决定的.     

连续碳化硅纤维增强碳化硅陶瓷基复合材料研究进展

碳化硅纤维增强碳化硅陶瓷基(SiC/SiC)复合材料具有轻质、耐高温、抗氧化的优异特性,在航空领域,如航空发动机的热端构件、高温结构功能一体化构件,航天及空天飞行器热防护结构部件、动力系统热端部件等领域具有广泛的应用前景,受到美国、欧洲、日本等国研究人员的广泛关注。本文从组成、制备工艺、加工工艺和考核应用等方面,综述了SiC/SiC复合材料的国内外研究进展,并指出了目前面临的问题和机遇。     

Mullite Coatings on SiC and C/C–Si–SiC Substrates Characterized by Infrared Spectroscopy

Mullite (3Al₂O₃·2SiO₂) coatings on SiC substrates and SiC precoated carbon/carbon composite (C/C-Si-SiC) substrates were produced by pulsed laser deposition (PLD) using pressed mullite powder targets. The layers can be characterized efficiently by IR reflection spectroscopy in the spectral range between 650 and 5000 cm⁻¹. The deposited coatings turn into mullite upon oxidation in air at temperatures between 1400° and 1600°C. Fabry-Perot interferences indicate a high quality and homogeneity of the mullite coating/SiC substrate interface. Amorphous SiO₂ gradually forms during prolonged heating or at higher temperatures.     

Mechanical Characterization of Annealed ZrB2–SiC Composites

Composites consisting of 70 vol% ZrB₂ and 30 vol% α‐SiC particles were hot pressed to near full density and subsequently annealed at temperatures ranging from 1000°C to 2000°C. Strength, elastic modulus, and hardness were measured for as‐processed and annealed composites. Raman spectroscopy was employed to measure the thermal residual stresses within the silicon carbide (SiC) phase of the composites. Elastic modulus and hardness were unaffected by annealing conditions. Strength was not affected by annealing at 1400°C or above; however, strength increased for samples annealed below 1400°C. Annealing under uniaxial pressure was found to be more effective than annealing without applied pressure. The average strength of materials annealed at 1400°C or above was ~700 MPa, whereas that of materials annealed at 1000°C, under a 100 MPa applied pressure, averaged ~910 MPa. Raman stress measurements revealed that the distribution of stresses in the composites was altered for samples annealed below 1400°C resulting in increased strength.     

Interfacial Characterization of Silicon Carbide Fiber/Lithia-Alumina-Silica Glass Matrix Composites

The development of the carbon-rich interphase in Nicalon SiC fiber/Li₂O-Al₂O₃–SiO₂ glass matrix composites was examined as a function of processing parameters with the use of high-resolution scanning electron microscopy and Auger electron spetroscopy. Specifically, hot-pressing temperatures (1000°, 1100°, and 1200°C) and times (15, 30, 60, and 240 min) were systematically varied in such a manner so as to fabricate dense composites suitable for evaluation of reaction kinetics. Carbon-rich interphase thickness, which ranged from 1400 to 5400 Å (140 to 540 nm), was observed to increase with either increasing times at constant temperature or increasing temperatures at constant time. The kinetics of formation of the carbon-rich interphase followed a diffusion-controlled model, with an activation energy of 25.4 kcal/mol.     

Electrical Resistance of SiC Fiber Reinforced SiC/Si Matrix Composites at Room Temperature during Tensile Testing

The implementation of Ceramic Matrix Composites necessitates the understanding of stress‐dependent damage evolution. Toward this goal, two liquid silicon infiltrated SiCf reinforced SiC composites were tensile tested with electrical resistance (ER) monitoring as well as acoustic emission to quantify matrix cracking. ER was modeled using a combination of resistors in series and parallel to model transverse matrix cracks and fiber/matrix segments between matrix cracks. It is shown that resistance change is sensitive to transverse matrix crack formation and stress‐dependent debonding length. The model appears to be accurate up to the stress for matrix crack saturation.     

Model of Oxidation‐Induced Fiber Fracture in SiC/SiC Composites

Stress rupture of SiC/SiC composites at intermediate temperatures in oxidizing environments is the result of a series of internal chemical and thermomechanical processes that lead to premature, localized fiber fracture. This article presents analytical models for two potentially critical steps in this process. The first involves the generation of tensile stresses in the fibers due to SiO₂ scale formation (following removal of fiber coatings) and the associated reduction in the applied stress required for fiber fracture. The second occurs once the gaps produced by coating removal are filled with oxide and subsequent oxidation occurs subject to the constraints imposed by the matrix crack faces. In this domain, the failure model is couched in terms of the stress intensification within the fibers caused by constrained oxidation. The models incorporate the combined kinetic effects of oxide growth and viscous flow. The competing effects of increased oxidation rate and accelerated stress relaxation with increasing temperature on fiber stress feature prominently in the results. The results suggest that, in dry air environments, the highest risk of fiber fracture occurs at temperatures in the range 840°C–940°C. In this range, the oxide scales grow at appreciable rates yet the resulting growth stresses cannot be mitigated sufficiently rapidly by viscous flow.     

Modulus-Graded Interphase Modifiers in E-Glass Fiber/Thermoplastic Composites

A process for coating E-glass fibers with polystyrene–polyethyleneimine (PEi) core–shell particles was developed, and uniform monolayers of particles of 143 and 327 nm diameter were covalently bonded to the glass surface. The effect of the particle coatings on the mechanical properties of fiber-reinforced composites of poly(vinyl butyral) (PVB) was investigated. The interfacial shear strength (IFSS) was measured for specimens containing one to 20 fibers each using the tensile fiber fragmentation test, and significant enhancements were found, in particular for samples containing larger numbers of fibers. The smaller-particle (143 nm) coatings in the 20-fiber specimens produced approximately a 100% enhancement in IFSS over equivalent specimens with bare or aminosilane-treated fibers, while the 327 nm particle coatings produced only approximately a 25% enhancement. The greater effectiveness of the smaller particles was attributed, at least in part, to the larger effective interfacial area they provide and their relatively greater shell-to-core ratio, providing greater interphase stiffness. The greater enhancements achieved for the multi-fiber vs single-fiber specimens suggest that the coatings produce a more uniform fiber–fiber spacing and, therefore, a more thorough wetting of the fibers by the resin in the multi-fiber samples. Composites formed using fiber tows of 3200 fibers each showed more than a 100% increase in composite toughness and 35% increase in ultimate tensile strength as compared to samples with bare fibers due to the presence of the 143 nm particle coatings, and somewhat more modest increases for the 327 nm particle coatings.     

Ultrasonic Characterization of Oxidation Mechanisms in Nicalon/C/SiC Composites

The nonbrittle fracture of composites consisting of ex polycarbosilane SiC (Nicalon) fibers in a SiC matrix prepared by chemical vapor infiltration is strongly dependent on the presence of a pyrocarbon layer at the fiber/matrix interface (Nicalon/C/SiC composites). The mechanical properties of such materials are known to be influenced by oxidation reactions. Elastic modulus measurements, using ultrasonic wave propagation in the "long bar" mode, have been used to show the influence of the environmental parameters temperature, atmosphere, and pressure on the mechanical behavior of bidirectional Nicalon/C/SiC. In situ , measurements of elastic modulus performed in parallel with thermogravimetric analysis allow examination of oxidation mechanisms which affect interfacial properties. Results showed the modulus to be affected by two interfacial oxidation mechanisms: (1) oxidation of pyrocarbon coating and (2) closure of the resulting interphase gap by silica formation.     

Modeling the Effects of Reactor Inlet Configuration on Isothermal CVI Process of C/SiC Composites

Two comparative models were proposed to simulate the effects of the reactor configuration on the isothermal chemical vapor infiltration (ICVI) process of C/SiC composites. The difference in the two models is that there is an expansion zone near the reactor inlet in one model while no expansion zone exists in another model. Calculation results show that the existence of the expansion zone has rather negligible effects on the ICVI process. It is accordingly suggested that the simplification of the reactor configuration by neglecting the expansion zone of the reactor is reasonable and acceptable for the ICVI process of C/SiC composites.     

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.     

Stress-Rupture of Nicalon/SiC Continuous Fiber Ceramic Composites in Air at 950°C

The stress-rupture behavior of plain-weave CG-Nicalon/enhanced SiC was studied in air at 950°C. It was found that this material exhibits delayed failure and that the lives of specimens subjected to constant stress levels of 80, 100, and 120 MPa were 21.6, 9.6, and 2.7 h, respectively. The strain histories of these tests revealed a continuous increase of the specimen compliance and accelerated deformation prior to failure. It is shown that both the shape of the strain vs time curves and the time dependence of the loss of strength can be explained using a simple model based on the oxidation-induced stress-rupture of the reinforcing fiber bundles.     

Tailoring Carbon Fiber/Carbon Nanotubes Interface to Optimize Mechanical Properties of Cf‐CNTs/SiC Composites

C_f/SiC composites were fabricated using fiber coatings including CNTs and matrix infiltration using the polymer impregnation and pyrolysis process. Interface between fiber and CNTs (CF/CNTs) was tailored to optimize mechanical properties of hybrid composites. The tailored interphases, such as Pyrocarbon (PyC) and PyC/SiC, protect fibers from degradation during the growth of CNTs successfully. Hybrid composites with well‐tailored CF/CNTs interface displayed significantly increased mechanical strength (352 ± 21 MPa) compared with that (34 ± 3 MPa) of composites reinforced with CNTs, which grown on carbon fibers directly. The interfacial bonding strength of hybrid composites was improved and optimized by tailoring the CF/CNTs interface. Interfacial failure modes were studied, and a firm interface bonding at the joint where CNTs grown was observed.     

Steady-State Creep Behavior of Si–SiC C-Rings

Because of the ease of experimental setup as well as economics in sample preparation, C-ring specimens are sometimes chosen for the evaluation of mechanical behavior. In this paper, the long-term creep of siliconized silicon carbide (Si–SiC) C-rings is investigated. Creep tests on a number of Si–SiC C-rings were carried out under constant compressive loads at 1300°C in air. Load-point displacements were continually monitored as a function of time, thereby establishing the steady-state regime as a function of load and ring geometry. Optical micrography on the postcrept specimens was performed to obtain damage zone sizes. A simple curved beam theory was employed to analyze the stress state developed throughout the body during steady-state creep. Loadpoint displacement rates were numerically calculated using both geometric and energy methods. Observed damage zone sizes and shapes within the specimen agreed with those predicted theoretically. Results obtained on the stress solutions are useful as local loading parameters in the study of high-temperture fracture behavior of a cracked C-ring.