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.     

SiC过渡层对SiC/Si-MOSi2涂层抗氧化性能的影响

采用液硅反应渗透和料浆烧结法在石墨表面制备SiC/Si-MoSi2抗氧化涂层.详细研究SiC过渡层对SiC/Si-MoSi2涂层抗氧化性能的影响.结果表明,SiC过渡层对SiC/Si-MoSi2涂层的抗氧化性能有很大影响,并从微观结构上分析和解释了SiC过渡层对所制备涂层抗氧化性能影响的原因.     

W-Mo-Si/SiC Oxidation Protective Coating for Carbon/Carbon Composites

A W-Mo-Si/SiC double-layer oxidation protective coating for carbon/carbon （C/C） composites was prepared by a two-step pack cementation technique. XRD （X-ray diffraction） and SEM （scanning electron microscopy）results show that the coating obtained by the first step pack cementation was a thin inner buffer layer of SiC with some cracks and pores, and a new phase of （WxMo1-x）Si2 appeared after the second step pack cementation. Oxidation test shows that, after oxidation in air at 1773 K for 175 h and thermal cycling between 1773 K and room temperature for 18 times, the weight loss of the W-Mo-Si/SiC coated C/C composites was only 2.06%. The oxidation protective failure of the W-Mo-Si/SiC coating was attributed to the formation of some penetrable cracks in the coating.     

Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating

A new kind of oxidation protection coating of Si-MoSi₂ was developed for three dimensional carbon fiber reinforced silicon carbide composites which could be serviced upto 1550 °C. The overall oxidation behavior could be divided into three stages: (i) 500 °C < T < 800 °C, the oxidation mechanism was considered to be controlled by the chemical reaction between carbon and oxygen; (ii) 800 °C < T < 1100 °C, the oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (iii) T > 1100 °C, the oxidation of SiC became significant and was controlled by oxygen diffusion through the SiC layer. Microstructural analysis revealed that the oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, the media layer is Si + MoSi₂ layer, and the inside layer is SiC layer. The coated C/SiC composites exhibited excellent oxidation resistance and thermal shock resistance. After the composites annealed at 1550 °C for 50 h in air and 1550 °C 100 °C thermal shock for 50 times, the flexural strength was maintained by 85% and 80% respectively. The relationship between oxidation weight change and flexural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failure starting point for oxidation protection coating.     

The role of oxidation in time-dependent response of ceramic–matrix composites

In this work, a model is constructed to account for the effect of oxidation of the fiber, fiber interface coating and surrounding matrix on the stress distribution and strain accumulation in ceramic–matrix composites. The model includes the role of the fabric architecture, the effect of porosity and the distribution of cracks in its formulation and utilizes oxidation rate constants and phenomenological models for the progress of oxidation as reported in literature.Dwell fatigue experiments were carried out for silicon carbide/silicon carbide nitride (SiC/SiNC) and Melt infiltrated silicon carbide/silicon carbide (MI SiC/SiC) composites to evaluate their time-dependent strain accumulation. Strain accumulation due to oxidation calculated by the model was compared to time-dependent strain obtained from experiment and showed that the rate of strain accumulation due to oxidation was low before the fibers were exposed to the environment but drastically increased after that. Such high rate of strain accumulation can be one of the main causes for failure of the composite.Model results showed that strain accumulation in both composites due to oxidation was dependent on the stress level with the SiC/SiNC accumulating more strain at similar stress levels. This can be explained by the higher modulus of the MI SiC/SiC that limits deformation, reducing crack density and accordingly decreasing the chance of oxygen to infiltrate the specimen and oxidize the fibers. Strain accumulation due to oxidation was also dependent on the fabric architecture and stress distribution within the unit cell. Additionally, comparing the effect of the value of the linear and parabolic oxidation rate constants reported by different researchers showed that not only is their absolute value important, but also their ratio to one another.     

XPS Studies on the Oxidation Behavior of SiC Particles

An X-ray photoelectron spectroscopic study has been carried out on both oxidized and as-received SiC specimens. Oxidation of SiC was performed in the temperature range 900-1100°C. It has been observed that a native oxide layer of silicon (SiO₂) exists on the surface of as-received SiC particles and that static oxidation of SiC increases the thickness of this oxide layer. It has also been observed that at a given temperature SiC particle surfaces are oxidized to a greater extent as the time of oxidation is increased. Chemical states are identified from the measured values of Auger parameters. Specimens studied were found to contain extraneous carbon in addition to carbide carbon.     

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.     

Oxidation Protective Multilayer CVD SiC Coatings Modified by a Graphitic B-C Interlayer for 3D C/SiC Composite

A layered graphitic CVD B-C coating was introduced between two CVD SiC coating layers. Microstructure and chemical characterization of the CVD B-C and the hybrid SiC/B-C/SiC multilayer coating was performed using SEM, EDS, XPS and XRD. Oxidation protection ability of the coating for the C/SiC composite was studied using a thermogravimetric analyzer (TGA) in the isothermal mode and by measuring residual flexural strength. The layered graphitic CVD B-C coating middle layer reduced the maximum crack width in the CVD SiC coating. The hybrid SiC/B-C/SiC multilayer coating provided a better oxidation protection for C/SiC composite than a three layer CVD SiC coating due to coating crack control and sealing effects at temperatures up to 1,300°C for 900 min.     

Tension–compression fatigue of a SiC/SiC ceramic matrix composite at 1200 °C in air and in steam

Tension–compression fatigue behavior of a non-oxide ceramic composite with a multilayered matrix was investigated at 1200 °C in laboratory air and in steam. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied. Tension–compression fatigue behavior was studied for fatigue stresses ranging from 80 to 200 MPa at a frequency of 1.0 Hz. Presence of steam significantly degraded the fatigue performance. Specimens that achieved fatigue run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Modern boron and SiC CVD filaments: A comparative study

Large diameter filaments (100–150 μm in diameter) made by chemical vapor deposition (CVD) of two ceramic materials (i.e. boron and SiC) on a heated tungsten or carbon core are compared from a mechanical and chemical standpoint. The most interesting of the filaments studied have received a rather thick surface coating (1–3 μm) which is made of boron carbide for B(W) filaments and a sequence of pyrocarbon and silicon carbide layers for SiC filaments. The mechanical behavior of the filaments in tension is explained on the basis of a Weibull statistics approach as well as a fracture analysis. Failure appears to be mainly controlled by surface defects, a feature which emphasizes the protective role played by the coating. Annealing at high temperatures (i.e. 800–950°C) in the presence of titanium shows that coated filaments have superior behavior. The coating acts in fact as a consumable sacrificial material, the strength of the filament remaining unchanged as long as the coating is not totally consumed by chemical reaction with titanium. Modern CVD filaments appear to be the most suitable ceramic reinforcements from a fundamental point of view.     

Fatigue Properties of Steel Coated with TiN by a PVD Process

Specimens of the steels S 6-5-2 (AISIM2) and 100 Cr 6 (AISI 52100) were coated with TiN by a reactive DC-magnetron-sputtering process. The fatigue behaviour of coated and uncoated specimens was investigated under cyclic bending. The fatigue limit of the uncoated steel S 6-5-2 is mainly governed by internal defects like carbide clusters and micropipes. Therefore, a coating has no influence on the fatigue limit. At high stress amplitudes the failure of the uncoated material is initiated at the specimen surface. Thus, coating of the surface causes higher mean lifetimes. The fatigue behaviour of the uncoated steel 100 Cr 6 is mainly governed by crack initiation at the surface of the specimens. At low stress amplitudes, a coating may shift the crack initiation place to the interior of the specimens. Hence, a slight improvement of the fatigue limit by coating is observed. At high stress amplitudes the coating has no influence on crack initiation and no improvement of the lifetime can be achieved by coating.     

Fatigue properties and fatigue fracture mechanisms of SiC whiskers or SiC particulate-reinforced aluminium composites

Fatigue properties and fracture mechanisms were examined for three commercially fabricated aluminium matrix composites containing SiC whiskers (SiC_w) and SiC particles (SiC_p) using a rotating bending test. The fatigue strengths were over 60% higher for SiC_w/A2024 composites than that for the unreinforced rolled material, while for the SiC_p/A357 composites, fatigue strengths were also higher than that for the unreinforced reference material. For the SiC_p/A356 composites at a volume fraction of 20%, the fatigue strength was slightly higher than that of the unreinforced material. Fractography revealed that the Mode I fatigue crack was initiated by the Stage I mechanism for the SiC_w/A2024 and SiC_p/A357 composites, while for the SiC_p/A356 composite, the fatigue crack initiated at the voids situated beneath the specimen surfaces. On the other hand, the fatigue crack propagated to the whisker/matrix interface following the formation of dimple patterns or the formation of striation patterns for SiC_w/A2024 composites, while for the SiC_p/A356 and SiC_p/A357 composites the fatigue crack propagated in the matrix near the crack origin and striation patterns were found. Near final failure, dimple patterns, initiated at silicon carbide particles, were frequently observed. Mode I fatigue crack initiation and propagation models were proposed for discontinuous fibre-reinforced aluminium composites. It is suggested that the silicon carbide whiskers or particles would have a very significant effect on the fatigue crack initiation and crack propagation near the fatigue limit.     

Interlaminar shear strength of SiC matrix composites reinforced by continuous fibers at 900 °C in air

To reveal the shear properties of SiC matrix composites, interlaminar shear strength (ILSS) of three kinds of silicon carbide matrix composites was investigated by compression of the double notched shear specimen (DNS) at 900 °C in air. The investigated composites included a woven plain carbon fiber reinforced silicon carbide composite (2D-C/SiC), a two-and-a-half-dimensional carbon fiber-reinforced silicon carbide composite (2.5D-C/SiC) and a woven plain silicon carbon fiber reinforced silicon carbide composite (2D-SiC/SiC). A scanning electron microscope was employed to observe the microstructure and fracture morphologies. It can be found that the fiber type and reinforcement architecture have significant impacts on the ILSS of the SiC matrix composites. Great anisotropy of ILSS can be found for 2.5D-C/SiC because of the different fracture resistance of the warp fibers. Larger ILSS can be obtained when the specimens was loaded along the weft direction. In addition, the SiC fibers could enhance the ILSS, compared with carbon fibers. The improvement is attributed to the higher oxidation resistance of SiC fibers and the similar thermal expansion coefficients between the matrix and the fibers.     

Effect of Particulate Silicon Carbide on Cyclic Plastic Strain Response and Fracture Behavior of 6061 Aluminum Alloy Metal Matrix Composites

In this paper, the cyclic stress response and cyclic stress–strain response characteristics, cyclic strain resistance and low-cycle fatigue life, and mechanisms governing the deformation and fracture behavior of aluminum alloy 6061 discontinuously reinforced with silicon carbide (SiC) particulates are presented and discussed. Two different volume fractions of the carbide particulate reinforcement phase in the aluminum alloy metal matrix are considered. The composite specimens were cyclically deformed using fully reversed tension–compression loading under total strain-amplitude-control. The stress response characteristic was observed to vary with strain amplitude. The plastic strain-fatigue life response was found to degrade with an increase in carbide particulate content in the metal matrix. The fracture behavior of the composite is discussed in light of the interactive influences of composite microstructural effects, cyclic strain amplitude and concomitant response stress, deformation characteristics of the composite constituents and cyclic ductility.     

C/SiC复合材料应力氧化失效机理

研究了干氧和湿氧两种气氛、疲劳和蠕变两种应力下C/SiC复合材料在1300℃的应力氧化行为. 试验结果和断口形貌SEM分析表明: C/SiC复合材料在疲劳应力下比在蠕变应力下具有更强的抗氧化能力和更长的持续时间; 干氧环境中的蠕变试样以C纤维氧化失效为主; 水蒸气的存在加剧了SiC基体的氧化, 并且使受蠕变应力的C/SiC复合材料以SiC基体氧化失效为主.     

The oxidation behaviour of two- and three-dimensional C/SiC thermostructural materials protected by chemical-vapour-deposition polylayers coatings

The oxidation behaviour of two- and three-dimensional C/SiC protected by a chemicalvapour-deposition (CVD) ceramic coating was studied. The elements used to achieve the surface protection were silicon, boron and carbon, preferably forming SiC, B or B₄C. The best results were obtained with the trilayer coatings, that is with, SiC as the internal layer, boron or boron carbide, as the intermediate layer and an external SiC layer. To get a good protection in a large temperature range, from 450 to 1500 °C, the total thickness of the trilayers must be higher than 160 m and the intermediate layer thickness must be higher than 5 m. Morphological characterization of oxidized samples has shown that, for intermediate oxidation temperatures, a glass was produced in the cracks. When the oxidation temperature was equal to or higher than 1300 °C, sealing of the cracks was rarely observed, but the oxidation resistance remained satisfactory.     

Tensile behavior of SiC/C and Rene'41 following isothermal exposure and thermal fatigue

The performance of a coated silicon carbide/carbon composite under isothermal and thermal fatigue conditions was investigated. The material studied is known as Ceracarb which consists of eight-harness satin weave Nicalon® silicon carbide cloth reinforcement, a carbonaceous matrix, and a silicon carbide composite coating. This advanced composite is being considered for replacing the nickel based superalloy Rene'41, as the exhaust nozzle components on military afterburning turbine engines. Thermal fatigue experiments, performed in the laboratory using a thermal cycling test system, were intended to roughly simulate the thermal excursions of an afterburning exhaust nozzle. Several thermal profiles were used to characterize the role of temperature, number of cycles, temperature range, and time at temperature, on the room temperature residual tensile strength of the material. The same thermal profiles were also conducted on test specimens of Rene'41 in order to compare its durability in the laboratory simulation test set-up to the composite. Both materials showed no loss in strength from the as-received condition following thermal testing. However, the Rene'41 showed evidence of microstructural instability at the maximum test temperature of 1093°C (2000°F) which did affect the toughness of the material. While the results from this study showed that both materials retained strength when thermally exposed in the laboratory under no loads, thermal testing under load may provide a more realistic view of how the materials perform in the afterburning exhaust nozzle application.     

Fatigue of SIC-based ceramics at ambient temperature

The effects of static and cyclic loading on monolithic sintered silicon carbide (SiC) and SiC reinforced with 16 vol.% particulate titanium diboride (SiC/16vol.% TiB₂) have been studied. Tests were carried out at ambient temperature in air on precracked specimens loaded in three- or four-point bending. No crack growth under cyclic loading has been observed in the monolithic silicon carbide. There is an additional cyclic contribution to crack growth after static crack growth has arrested in the composite material. Observations suggest that damage to the titanium diboride particles ahead of the crack tip occurs prior to crack extension through the SiC matrix.     

电子束沉积SiC/ZrO2热防护层的结构优化及其抗热震性能

高性能辐射热防护层是高超声速飞行器金属热防护系统的重要组成部分。为获得高性能的热防护层,利用L5 EB-PVD电子束物理气相沉积设备在Haynes 214镍基合金表面沉积了SiC/ZrO2防护层,测试了其在热循环条件下的抗热震性能;通过分析其沉积温度及厚度对残余热应力的影响,确定了热防护层的沉积工艺参数。结果表明:热防护层在800℃和900℃循环80次后未出现明显的宏观裂纹;1000℃循环60次后,SiC表面层应力集中区出现裂纹,在交变热应力作用下,裂纹不断扩展形成网状龟裂纹,最终导致热防护层剥落;热膨胀系数不匹配导致热防护层在急冷急热热震过程中产生热应力是导致其失效的主要原因。     

Fatigue-life prediction of SiC particulate reinforced aluminum alloy 6061 matrix composite using AE stress delay concept

A study of the residual fatigue life prediction of 6061-T6 aluminum matrix composite reinforced with 15 vol % SiC particulates (SiC_p) by using the acoustic emission technique and the stress delay concept has been carried out. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were stimulated at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. It was found that a lower stress level was required to reach a specified number of cumulative AE events for specimens fatigued to higher percentage of the fatigue life. This stress level is called stress delay. Approximately a linear relation was found between stress delay and fatigue damage. Using the procedure defined in this study, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. The number of the cumulative AE events increased exponentially with the increase of strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion and linkage of voids. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.