COMPARISON OF FATIGUE AND CREEP BEHAVIOR BETWEEN 2D AND 3D-C/SiC COMPOSITES

The differences of tension-tension fatigue and tensile creep characters of 2D-C/SiC and 3D-C/SiC composites have been scrutinized to meet the engineering needs. Experiments of tension-tension fatigue and tensile creep are carried out under vacuum high temperature condition. All of the high temperature fatigue curves are flat; the fatigue curves of the 2D-C/SiC are flatter and even parallel to the horizontal axis. While the tension-tension fatigue limit of the 3D-C/SiC is higher than that of the 2D-C/SiC, the fiber pullout length of the fatigue fracture surface of the 3D-C/SiC is longer than that of the 2D-C/SiC, and fracture morphology of the 3D-C/SiC is rougher, and pullout length of the fiber tows is longer. At the same time the 3D-C/SiC has higher tensile creep resistance. The tensile curve and the tensile creep curve of both materials consist of a series of flat step. These phenomena can be explained by the non-continuity of the damage.

FRACTURE RESISTANCE OF 3D-C/SiC COMPOSITES AT 1300℃

Based on the energy conservation, the elastic energy linked to the compliance change, non-elastic energy dissipated by irreversible deformation and the resistance for crack propagation were quantitatively characterized by evaluation the load/load point displacement curves tested by three points bend experiment with single notch beam at 1300℃. The cracks length was determined by compliance calibration curves. It is shown by experimental results that the compliance of 3D-C/SiC composites changes with the cracks can be described by third order polynomial. The variation of crack advancing resistance with non-dimensional equivalent crack length presents a convex curve. The crack advancing resistance increases firstly and then decreases with the non-dimensional equivalent crack length, finally is in comparatively low level. The maximum values of crack advancing resistance are 269.73kJ/m2 for non-dimensional equivalent crack length of 0.318 and original notch length of 0.35mm, and 138.65kJ/m2 for non-dimensional equivalent crack length of 0.381 and original notch length of 2.06mm, respectively.

Multi-scale Modeling and Finite Element Analyses of Thermal Conductivity of 3D C/SiC Composites Fabricating by Flexible-Oriented Woven Process

Thermal conductivity is one of the most significant criterion of three-dimensional carbon fiber-reinforced SiC matrix composites(3D C/SiC).Represent volume element(RVE)models of microscale,void/matrix and mesoscale proposed in this work are used to simulate the thermal conductivity behaviors of the 3D C/SiC composites.An entirely new process is introduced to weave the preform with three-dimensional orthogonal architecture.The 3D steady-state analysis step is created for assessing the thermal conductivity behaviors of the composites by applying periodic temperature boundary conditions.Three RVE models of cuboid,hexagonal and fiber random distribution are respectively developed to comparatively study the influence of fiber package pattern on the thermal conductivities at the microscale.Besides,the effect of void morphology on the thermal conductivity of the matrix is analyzed by the void/matrix models.The prediction results at the mesoscale correspond closely to the experimental values.The effect of the porosities and fiber volume fractions on the thermal conductivities is also taken into consideration.The multi-scale models mentioned in this paper can be used to predict the thermal conductivity behaviors of other composites with complex structures.

Effects of Co_(2)O_(3)Addition on Microstructure and Properties of SiC Composite Ceramics for Solar Absorber and Storage

SiC composite ceramics for solar absorber and storage integration are new concentrating solar power materials.SiC composite ceramics for solar absorber and storage integration were fabricated using SiC,black corundum and kaolin as the raw materials,Co_(2)O_(3)as the additive via pressureless graphite-buried sintering method in this study.Influences of Co_(2)O_(3)on the microstructure and properties of SiC composite ceramics for solar absorber and storage integration were studied.The results indicate that sample D2(5wt%Co_(2)O_(3))sintered at 1480℃exhibits optimal performances for 119.91 MPa bending strength,93%solar absorption,981.5 kJ/kg(25-800℃)thermal storage density.The weight gain ratio is 12.58 mg/cm2after 100 h oxidation at 1000℃.The Co_(2)O_(3)can decrease the liquid phase formation temperature and reduce the viscosity of liquid phase during sintering.The liquid with low viscosity not only promotes the elimination of pores to achieve densification,but also increases bending strength,solar absorption,thermal storage density and oxidation resistance.A dense SiO_(2) layer was formed on the surface of SiC after 100 h oxidation at 1000℃,which protects the sample from further oxidation.However,excessive Co_(2)O_(3)will make the microstructure loose,which is disadvantageous to the performances of samples.

SiC/SiBCN-Si_(3)N_(4)复合材料力学性能研究

为了深入了解SiC/SiBCN-Si_(3)N_(4)材料微观形貌与高温力学行为,建立科学可靠的定量表征方法,本文使用多种表征手段对SiC/SiBCN-Si_(3)N_(4)材料进行定量观测,首先进行材料孔隙率及密度的测试,随后进行材料高温原位力学性能测试并对材料损伤机理进行了分析,最后基于试验数据构建了一种可解释的深度学习模型,实现了材料高温非线性本构关系预测。样件力学性能分析结果表明:平均应力预测误差为0.27%~0.59%、平均应变预测误差为1.96%~3.41%;同时通过量化分析明确了影响力学性能的因素依次为温度、偏轴角度、孔隙率及密度。本文实现了不同环境温度、偏轴角度与外载荷作用下SiC/SiBCN-Si_(3)N_(4)宏观力学性能的预测,可为陶瓷基复合材料高温本构模型的建立提供新思路。

Al_(2)O_(3)膜对高温环境下NiCrAlY-SiC复合镀层内部反应的阻隔及涂层耐磨性研究

为了解决以SiC颗粒为磨料的叶尖耐磨复合镀层在高温工况下SiC与涂层中Ni元素反应导致涂层耐磨性下降等问题,采用化学气相沉积(chemicalvapor deposition,CVD)工艺方法在SiC颗粒表面制备了Al_(2)O_(3)膜以阻隔SiC与涂层中Ni元素的反应。研究了0.5~1.0μm和5.0~10.0μm这2种厚度范围的Al_(2)O_(3)膜对NiCrAlY-SiC@Al_(2)O_(3)叶尖耐磨镀层中SiC颗粒与NiCrAlY镀层在1100℃高温环境下反应的阻隔效果,并测试了2种Al_(2)O_(3)膜厚度的NiCrAlY-SiC@Al_(2)O_(3)叶尖耐磨镀层在1100℃高温环境下保温500 h后的耐磨性。结果显示,5.0~10.0μm厚度的Al_(2)O_(3)膜可有效阻隔SiC颗粒与NiCrAlY镀层的高温反应,使NiCrAlY-SiC@Al_(2)O_(3)涂层在1100℃高温环境下保持良好的耐磨性,并与ZrO2陶瓷封严涂层形成良好的对磨匹配效果。

Copper-Ti_3SiC_2 composite powder prepared by electroless plating under ultrasonic environment

In this article, a new type of Cu-Ti3SiC2 composite powder prepared using the electroless plating technique was introduced. The initial Ti3SiC2 particles are 11 μm in diameter on an average. The Cu plating was carried out at middle temperature （62-65℃） with the application of ultrasonic agitation. The copper deposition rate was determined by measuring the weight gain of the powder after plating. It has been found that the pretreatment of Ti3SiC2 powder is very important to obtain copper nanoparticles on the surface of Ti3SiC2 The optimum procedure before plating aimed to add activated sites and the adjustment of the traditional composition of the electroless copper plating bath could decelerate the copper deposition rate to 0.8 gm/h. X-ray diffraction （XRD） indicates that the chemical composition of the plating layer is copper. SEM images show that the surface of the Ti3SiC2 particles is successfully coated with continuous copper layer. The wetting property between the copper matrix and Ti3SiC2 can be improved so as to increase the interfacial strength.

Microstructure and Mechanical Properties of Si_3N_4 Composites Containing SiC Platelet

Hoppressed Si3N4/SiC platelet composites had been investigated with respect to their microstructure and mechanical properties. The results indicate that Vickers hardness, elastic modulus and fracture toughness of the composites were increased by the addition of SiC platelet until the content up to 20 vol pct. A slight decrease in flexural Strength was measured at room temperature with increasing SiC platelet content. The high temperature flexural strength tests at 1150, 1250, and 1350℃ were conducted. It was found that the flexural strength at elevated temperature was degraded with the rising temperature, and the downward trend of flexural strength for the composite containing 10 vol. pct SiC platelet was less. The results indicate that SiC platelet had a positive influence on the high temperature strength. Effects of SiC platelet reinforcement were presented

Investigation on The Properties of Fe-Si_3N_4 Bonded SiC Composite

The mechanical properties of pressureless sintering Fe-Si_ 3N_ 4 bonded SiC and Si_ 3N_ 4 bonded SiC with same manufacture process have been compared in this paper. The oxidizing mechanism of Fe-Si_ 3N_ 4 bonded SiC ceramic matrix composite has been investigated especially through TG-DSC (thermo gravimetric analysis-differential scanning calorimeter) experiment. During oxidation procedure the main reaction is the oxidation of SiC and Si_ 3N_ 4, SiO_ 2 which form protecting film to prevent further oxidizing. And residual iron in the samples become Fe_ 2O_ 3 and Fe_ 3O_ 4,_ the oxidation kinetics at 1100～1300℃ of Fe-Si_ 3N_ 4 bonded SiC has been studied especially. The weight gain per unit area at initial stage changes according to beeline rule, in the middle according to conic, and in the last oxidation period follows parabola rule.

Mechanical Properties and Microstructure of Al_(2)O_(3)/SiC Composite Ceramics for Solar Heat Absorber

Al_(2)O_(3)/SiC composite ceramics were prepared fromα-Al_(2)O_(3) and SiC by a pressureless sinter method in this study.The effect of SiC contents on the mechanic properties,phase compositions and microstructure is studied.Experimental results show that the vickers hardness,wear resistance and thermal conductivity of the samples increase with the increase in the SiC content,and the hardness of the sample reaches 16.22 GPa,and thermal conductivity of the sample reaches 25.41 W/(m.K)at room temperature when the SiC content is 20 wt%(B5)and the sintering temperature is at 1640℃.Higher hardness means higher scour resistance,and it indicates that the B5 material is expected to be used for the solar heat absorber of third generation solar thermal generation.The results indicate the mechanism of improving mechanical properties of Al_(2)O_(3)/SiC composite ceramics:SiC plays a role in grain refinement that the grain of SiC inhibits the grain growth of Al_(2)O_(3),while the addition of SiC changes the fracture mode from the intergranular to the intergranular-transgranular.

The Microstructures and Properties of SiC/Al_2O_3/Al-Si Composites Prepared by Reactive Penetration

The composition, microstructures and properties of SiC /Al-2O-3/Al-Si composites formed by reactive penetration of the molten aluminum into the preforms of SiO-2 and SiC were investigated. The composition of the composites was measured by X-ray diffraction (XRD). The microstructures of the composites were also measured by scanning electron microscopy (SEM) and optical microscopy. In addition, the factors affecting the properties of the composites were discussed.The experiments show that the mechanical properties of the composites depend on their relative densities and the sizes of the fillers“SiC grains".The denser the SiC/Al-2O-3/Al-Si composites,the higher their bending strength.As the filler “SiC grains" become fine,the bending strength of the composites increases.

Fabrication and Properties of Ti_3SiC_2/SiC Composites

Ti3SiC2/SiC composites were fabricated by reactive hot pressing method. Effects of hot pressing temperature, the content and particle size of SiC on phase composition, densification, mechanical properties and behavior of stress-strain of the composites were investigated. The results showed that ： （ 1 ） Hot-pressing temperature influenced the phase composition of Ti3SiC2/SiC composites. The flexural strength and fracture toughness of composites increased with hot pressing temperature. （2） It became more difficult for the composites to densify when the content of SiC in composites increased. It need be sintered at higher temperature to get denser composite. The flexural strength and fracture toughness of composites increased when the content of SiC added in composites increased. However, when the content of SiC reached 50 wt%, the flexural strength and fracture toughness of composites decreased due to high content of pore in composites. （3） When the content of SiC was same, Ti3SiC2/SiC composites were denser while the particle size of SiC added in composites is 12. 8 μm compared with the composites that the particle size of SiC added is 3 μm. The flexural strength and fracture toughness of composites increased with the increase of particle size of SiC added in composites. （4） Ti3SiC2/SiC composites were non-brittle fracture at room temperature.

Preparation of SiC_p/Al_2O_3-Al Composites by Directed Metal Oxidation

SiCp/Al2O3-Al composites were synthesized by means of direct metal oxidation method. The composition and microstructures of the composites were investigated using X-ray diffraction （XRD）, scanning electron microscopy （SEM） and metallurgical microscope. The effects of technical parameters on the properties of the product were analyzed. The results indicate that the composite possesses a dense microstructure, composed of three interpenetrated phases. Of them, SiO2 layer prohibits the powdering of the composites; Mg promotes the wetting and infiltration of the system and Si restricts the interfacial reaction while improving the wetting ability between reinforcement and matrix.

Identification of Growth Promoter to Fabrication SiCp/Al₂O₃Ceramic Matrix Composites Prepared by Directed Metal Oxidation of An Al Alloy

SiC particulates reinforced alumina matrix composites were fabricated using Directed Metal Oxidation (DIMOX) process. Continuous oxidation of an Al-Si-Mg-Zn alloy with different interlayers (dopents) as growth promoters, will encompasses the early heating of the alloy ingot, melting and continued heating to temperature in the narrow range of 950°C to 980°C in an atmosphere of oxygen. Varying interlayers (dopents) are incorporated to examine the growth conditions of the composite materials and to identification of suitable growth promoter. The process is extremely difficult because molten aluminum does not oxidize after prolonged duration at high temperatures due to the formation of a passivating oxide layer. It is known that the Lanxide Corporation had used a combination of dopents to cause the growth of alumina from molten metal. This growth was directed, i.e. the growth is allowed only in the required direction and restricted in the other directions. The react nature of the dopants was a trade secret. Though it is roughly known that Mg and Si in the Al melt can aid growth, additional dopents used, the temperatures at which the process was carried out, the experimental configurations that aided directed growth were not precisely known. In this paper we have evaluated the conditions in which composites can be grown in large enough sizes for evaluation application and have arrived at a procedure that enables the fabrication of large composite samples by determining the suitable growth promoter (dopant). Scanning electron microscopic, EDS analysis of the composite was found to contain a continuous network of Al2O3, which was predominantly free of grain-boundary phases, a continuous network of Al alloy. Fabrication of large enough samples was done only by the inventor company and the property measurements by the company were confirmed to those needed to enable immediate applications. Since there are a large number of variable affecting robust growth of the composite, fabrication large sized samples for measurements is a difficult task. In the present work, to identify a suitable window of parameters that enables robust growth of the composite has been attempted.

3D-C/SiC复合材料的损伤机理

陶瓷基复合材料在热结构中潜在着许多用途 ,但对 3D C/SiC材料高温损伤尚不完全清楚。本工作用T3 0 0碳纤维编织为三维四向编织体 ,编织角 2 2° ,用CVI化学气相渗法在 95 0℃～ 10 0 0℃沉积热解碳界面层、SiC基体。最终得到纤维体积分数约为 40 %、热解碳界面层厚度约 0 .2微米和空隙率为 17%的复合材料 ,表面SiC涂层厚度为 5 0 μm。基体由于热应力和外力会产生许多微裂纹 ,用单向陶瓷基复合材料裂纹计算公式可大致估算出 3D C/SiC的基体开裂应力和裂纹间距。纤维束间的孔隙在蠕变中变形 ,孔隙表面基体易产生微裂纹 ,而且纤维束间的夹角不断改变。蠕变是损伤引起的 ,属于损伤蠕变机理。弯曲、断裂韧度、蠕变及疲劳等试验中 ,纤维束力图沿拉应力方向伸直 ,纤维束间相对滑动并产生损伤是细观主要的损伤机理。室温及疲劳循环应力低、循环周次多的断口粗糙度大 ,纤维拔出较长 ;高温及高应力、循环周次少的断口相对齐平 ,纤维拔出较短。纤维束与基体界面和纤维与基体界面的脱粘和滑动产生损伤中 ,以纤维束与基体之间的磨损产生的损伤为主要的 。

3D-C/SiC复合材料的高温拉伸性能

研究了 3D C/SiC复合材料从室温到 15 0 0℃真空条件的拉伸性能。试验材料用T30 0碳纤维编织为三维四向编织体 ,编织角为 2 2° ,用CVI法在 95 0℃～ 10 0 0℃沉积热解碳界面层、SiC基体。最终得到纤维体积分数约为4 0vol%、热解碳界面层厚度约 0 .2 μm和空隙率为 17vol%的复合材料 ,表面SiC涂层厚度为 5 0 μm。试验在超高温拉伸试验机上进行 ,真空度为 10 -3 Pa ,夹头位移速率为 0 .5 95mm/min。结果表明 ,拉伸应力 应变曲线是非线性的 ,大部分拉伸曲线基本由三段折线组成 ,对应着三段模量。第一阶段的模量和基体裂纹饱和应力对应的应变εsa 基本不随温度的升高而改变 ;第二和第三阶段的模量、损伤开始应力σmc、基体裂纹饱和应力σsa、断裂应力σf 和损伤开始应变εmc随温度有相似的变化规律 ,即随温度升高而增加 ,在 110 0℃ ～ 130 0℃范围内出现最大值 ,尔后随温度增加而下降 ;但是断裂应变的变化规律正好与此相反。试样机械加工后 ,由于残余应力部分得到松弛 ,并去除了表面SiC涂层开裂后引起的应力集中 ,因此材料断裂强度和断裂应变明显升高。高温和室温的拉伸断裂应变小于0 .6 % ,不能有效地松弛材料切口处的应力集中。测量了拉伸过程中试样的电阻相对变化率 。

3D-C/SiC复合材料在室温和1300℃的拉-拉疲劳行为

采用应力比为 0 .1,频率为 6 0 Hz的正弦波在室温和 130 0℃ ,10 - 4Pa真空中对 3D- C/Si C复合材料进行了拉 -拉疲劳试验。同时用 SEM分析了疲劳断口特征。结果表明 :若取循环基数为 10 6 ,130 0℃疲劳极限为 2 85 MPa,约为抗拉强度的 94 % ;室温疲劳极限为 2 35 MPa,约为抗拉强度的 85 %。 130 0℃疲劳断口的纤维拔出长度比室温短。疲劳损伤主要起源于纤维束编织交叉部位 ,随着疲劳循环次数的增加 。

3D-C/SiC的高温弯曲性能和后处理对弯曲性能的影响

3D C SiC用T3 0 0碳纤维编织为三维四向编织体 ,编织角 2 2°。CVI(chemicalvaporinfiltration)法致密 ,纤维体积分数 40 %～ 45 % ,热解碳界面层厚度约 0 .2微米、密度为 2 .0 1g cm3和空隙率为 17% ,最终在试样表面形成 5 0 μm的SiC涂层。在YKM 2 2 0 0超高温试验机上进行三点弯曲试验 ,真空度为 10 - 3Pa ,夹头位移速率为 0 .5mm min。结果表明 ,随试验温度升高 ,弯曲强度在 90 0℃和 170 0℃出现了两个峰值 ;弯曲模量在 110 0℃出现了最大值 ;而弯曲应变在低于 15 0 0℃基本保持不变 ,高于 15 0 0℃不断升高 ,出现了明显的不可逆变性。试样 180 0℃真空保温 1小时后处理 ,弯曲模量明显降低和强度降低 ,而变形能力明显升高。激光拉曼分析得出 :制备的 3D C SiC碳纤维存在残余压应力 ,13 0 0℃处理后碳纤维存在残余拉应力 ,而 2 0 5 0℃处理后碳纤维没有残余应力。高温下因基体和纤维膨胀系数不同 ,易产生界面脱粘和分离 ,使得基体中的裂纹不易扩展到纤维中 ,保持了纤维强度 ,同时纤维易于滑动。这是很高温度下弯曲模量降低、变形能力增加和弯曲强度略增加的原因。

界面相对3D-C/SiC复合材料热膨胀性能的影响

利用减压化学气相浸渗(LPCVI)技术制备了3D C/SiC复合材料,从热解碳(PyC)界面相厚度对界面结合强度和热应力的影响出发,研究了界面相对复合材料热膨胀性能的影响。结果表明:①界面相厚度对3D C/SiC复合材料热膨胀性能的影响主要归因于其对界面结合强度和脱黏面上的滑移阻力的影响。在一定厚度范围(约70～220nm)内,材料的热膨胀系数随热解碳厚度的增加而逐渐降低;②热处理可提高材料的热稳定性,并通过改变材料内部结构,使热应力重新分布,对复合材料的高温热膨胀产生显著影响,但是,并没有改变基体裂纹的愈合温度(900℃)。

高温热曝露对3D-C/SiC复合材料弯曲性能的影响(英文)

3D-C/SiC复合材料试样在空气介质中600℃、900℃和1300℃热曝露不同时间后,采用三点弯曲法测试了以室温弯曲弹性模量表征的损伤变化规律,并进行了SEM和EDS分析。结果表明:3D-C/SiC在热曝露15h后,损伤变化可分为急剧上升(阶段Ⅰ)和平稳上升(阶段Ⅱ)两个阶段。阶段Ⅰ归因于炭纤维和炭层界面在空气中的直接氧化,阶段Ⅱ由复合材料内部氧的扩散所致。在复合材料制备过程的冷却阶段,因基体和炭纤维热膨胀系数不同所产生的基体微裂纹提供了氧化反应的表面与氧扩散的途径。在同一热曝露时间下,损伤随温度的上升而减少的原由可能是由于高温下裂纹收缩导致氧化表面减少,并降低氧向复合材料内扩散所致。