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.

Tensile Mechanical Behavior and Failure Mechanism of a Plain-Woven SiCf/SiC Composites at Room and Elevated Temperatures

Ceramic matrix composites (CMCs) are the preferred materials for solving advanced aerospace high-temperature structural components;it has the comprehensive advantages of higher temperature (~1500˚C) and low density. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso-architecture and inhere defects. In this paper, the in-plane tensile mechanical behavior of a plain-woven SiCf/SiC composite at room and elevated temperatures was investigated, and the factors affecting the tensile strength of the material were discussed in depth. The results show that the tensile modulus and strength of SiCf/SiC composites at high temperature are lower, but the fracture strain increases and the toughness of the composites is enhanced;the stitching holes significantly weaken the tensile strength of the material, resulting in the material is easy to break at the cross-section with stitching holes.

Compositional and Hollow Engineering of Silicon Carbide/Carbon Microspheres as High-Performance Microwave Absorbing Materials with Good Environmental Tolerance

Microwave absorbing materials(MAMs)characterized by high absorption efficiency and good environmental tolerance are highly desirable in practical applications.Both silicon carbide and carbon are considered as stable MAMs under some rigorous conditions,while their composites still fail to produce satisfactory microwave absorption performance regardless of the improvements as compared with the individuals.Herein,we have successfully implemented compositional and structural engineering to fabricate hollow Si C/C microspheres with controllable composition.The simultaneous modulation on dielectric properties and impedance matching can be easily achieved as the change in the composition of these composites.The formation of hollow structure not only favors lightweight feature,but also generates considerable contribution to microwave attenuation capacity.With the synergistic effect of composition and structure,the optimized SiC/C composite exhibits excellent performance,whose the strongest reflection loss intensity and broadest effective absorption reach-60.8 dB and 5.1 GHz,respectively,and its microwave absorption properties are actually superior to those of most SiC/C composites in previous studies.In addition,the stability tests of microwave absorption capacity after exposure to harsh conditions and Radar Cross Section simulation data demonstrate that hollow SiC/C microspheres from compositional and structural optimization have a bright prospect in practical applications.

Synergistically Toughening Effect of SiC Whiskers and Nanoparticles in Al_2O_3-based Composite Ceramic Cutting Tool Material

In recent decades, many additives with different characteristics have been applied to strengthen and toughen Al2O3-based ceramic cutting tool materials. Among them, SiC whiskers and SiC nanoparticles showed excellent performance in improving the material properties. While no attempts have been made to add SiC whiskers and SiC nanoparticles together into the ceramic matrix and the synergistically toughening effects of them have not been studied. An Al2O3-SiCw-SiC np advanced ceramic cutting tool material is fabricated by adding both one-dimensional SiC whiskers and zero-dimensional SiC nanoparticles into the Al2O3 matrix with an effective dispersing and mixing process. The composites with 25 vol% SiC whiskers and 25 vol% SiC nanoparticles alone are also investegated for comparison purposes. Results show that the Al2O3-SiCw-SiCnp composite with both 20 vo1% SiC whiskers and 5 vol% SiC nanoparticles additives have much improved mechanical properties. The flexural strength of Al2O3-SiCw-SiCnp is 730＋ 95 MPa and fracture toughness is 5.6 ± 0.6 MPa.m1/2. The toughening and strengthening mechanisms of SiC whiskers and nanoparticles are studied when they are added either individually or in combination. It is indicated that when SiC whiskers and nanoparticles are added together, the grains are further refined and homogenized, so that the microstructure and fracture mode ratio is modified. The SiC nanoparticles are found helpful to enhance the toughening effects of the SiC whiskers. The proposed research helps to enrich the types of ceramic cutting tool and is benefit to expand the application range of ceramic cutting tool.

Studies on High Temperature Oxidation of Electrodeposited RE-Ni-W-P-SiC Composite Materials

The oxidation of the Electrodeposited RE-Ni-W-P-SiC Composite materials at high temperature is investigated. The results show that during high temperature oxidation the relationship between the mass change of pure Ni, Ni-W-P, Ni-W-P-SiC or RE-Ni-W-P-SiC coatings and the oxidation time follows a mixed curve, i.e. it is approximately a linear relationship when the oxidation time is less than 60 mins while it is a power function relationship when the oxidation time is over 60 mins. The order for the oxidation rate of the four coatings is Ni> Ni-W-P> Ni-W-P-SiC>RE-Ni-W-P-SiC. The mass change of Ni-W-P, Ni-W-P-SiC or RE-Ni-W-P-SiC coatings increases exponentially with a rise of oxidation temperature. The high temperature-oxidation resistance of RE-Ni-W-P-SiC composite material is 3-4 times than that of Ni-W-P alloy coating. The cross section morphologies and X-ray diffraction patterns indicate that the high temperature-oxidation resistance of RE-Ni-W-P-SiC composite coating is better than any other coatings.

Electromagnetic wave absorption and mechanical properties of SiC nanowire/low-melting-point glass composites sintered at 580°C in air

Si C nanowires are excellent high-temperature electromagnetic wave (EMW) absorbing materials. However, their polymer matrix composites are difficult to work at temperatures above 300℃, while their ceramic matrix composites must be prepared above 1000℃ in an inert atmosphere. Thus, for addressing the abovementioned problems, SiC/low-melting-point glass composites were well designed and prepared at 580℃ in an air atmosphere. Based on the X-ray diffraction results, SiC nanowires were not oxidized during air atmosphere sintering because of the low sintering temperature. Additionally, SiC nanowires were uniformly distributed in the glass matrix material. The composites exhibited good mechanical and EMW absorption properties. As the filling ratio of SiC nanowires increased from 5wt%to 20wt%, the Vickers hardness and flexural strength of the composite reached HV 564 and 213 MPa, which were improved by 27.7%and 72.8%, respectively, compared with the low-melting-point glass. Meanwhile, the dielectric loss and EMW absorption ability of SiC nanowires at 8.2–12.4 GHz were also gradually improved. The dielectric loss ability of low-melting-point glass was close to 0. However, when the filling ratio of SiC nanowires was 20wt%, the composite showed a minimum reflection loss (RL) of-20.2 dB and an effective absorption (RL≤-10 dB) bandwidth of2.3 GHz at an absorber layer thickness of 2.3 mm. The synergistic effect of polarization loss and conductivity loss in SiC nanowires was responsible for this improvement.

SYNTHESIS OF SiC WHISKER AND ITS APPLICATION IN COMPOSITE MATERIALS

The SiC whiskers (SiCw) synthesized from rice hulls is studied in this paper. The properties of the application in the composite materials are tested, and also compared with the SiCw produced in US and Japan, the results indicate that the SiCw produced by this method is mainly straight crystals with multi nodes on face, the main type is P-SiC. lt has many advantages such as high strength and excellent oxidation resistance to high temperature. The Si3N4 ceramic composite materials reinforced and toughened with the SiCw, δfRT of the material is (856±22)MPa, δf300℃ is approximately (418. 5±14. 2) MPa and Klc is approximately (11. 3±1. 0) MPa m1/2. Besides, the application of the SiCw in the engineering materials of mining is forecasted.

Residual Stress Distribution of Si_(3)N_(4)/SiC Gradient Material and the Effect of Residual Stress on Material Properties

The Si_(3)N_(4)/SiC gradient material with a gradient composition structure was prepared by a hot pressing sintering.The sinterability,distribution of residual stress and the effect of residual stress on mechanical properties of Si_(3)N_(4)/SiC gradient materials were studied.The research results show that,at 1750℃,Si_(3)N_(4)/SiC gradient materials with different ratios can achieve co-sintering,and the overall relative density of the sample reaches 98.5%.Interestingly,the flexural strength of Si_(3)N_(4)/SiC gradient material is related to its loading surface.The flexural strength of SiC as the loading surface is about 35%higher than that of Si_(3)N_(4)as the loading surface.The analysis of the residual stress of the material in the gradient structure shows that the gradient stress distribution between the two phases is a vital factor affecting the mechanical properties of the material.With the increase of SiC content in the gradient direction,the fracture toughness of each layer of Si_(3)N_(4)/SiC gradient materials gradually decreases.The surface hardness of the pure SiC side is lower than that reported in other literature.

Damping performance of SiC nanoparticles reinforced magnesium matrix composites processed by cyclic extrusion and compression

This work dealt with the damping performance and its underlying mechanism in SiC nanoparticles reinforced AZ91D composite(SiC_(np)/AZ91D)processed by cyclic extrusion and compression(CEC).It was found that the CEC process significantly affects the damping performance of the composite due to alterations in the density of dislocations and grain boundaries in the matrix alloy.Although there would be dynamic precipitation of the Mg17Al12 phase during processing which increases the phase interface and limits the mobility of dislocations and grain boundaries.The results also showed that the damping capacity of 1%SiC_(np)/AZ91D composite continuously decreases with adding CEC pass number and it consistently increases with rising the applied temperature.Considering the first derivative of the tanδ-T curve,the dominant damping mechanism based on test temperature can be divided into three regions.These three regions are as follows(i)dislocation vibration of the weak pinning points(≤T_(cr)),(ii)dislocation vibration of the strong pinning points(T_(cr)∼T_(V)),and(iii)grain boundary/interface sliding(≥T_(V))

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/SiC复合材料层板低速冲击及其剩余强度试验研究

高速飞行器中的陶瓷基复合材料结构在服役过程中不可避免地会遇到低速冲击问题,低速冲击后的损伤形式以及剩余承载能力是影响飞行器结构安全的关键问题。本研究以二维编织SiC/SiC复合材料板件为研究对象,在不同能量下开展了低速冲击试验,分析了低速冲击载荷下试验件的表面损伤状态,通过计算机断层扫描技术观察了试验件内部的损伤形貌,结合冲击过程中的冲击响应曲线以及应变历史曲线,分析了SiC/SiC复合材料低速冲击过程的损伤机理。针对含勉强目视可见损伤的试验件开展了冲击后剩余强度试验,研究了勉强目视可见损伤对SiC/SiC复合材料剩余承载性能的影响。结果表明,在低速冲击载荷的作用下,试验件的表面损伤主要包括无表面损伤、勉强目视可见损伤、半穿透损伤以及穿透损伤,试验件的内部损伤主要有锥形体裂纹、纱线断裂以及分层损伤。低速冲击损伤会严重影响SiC/SiC复合材料的剩余性能,虽然试验件损伤勉强目视可见,但其剩余压缩强度为无损件81%,剩余拉伸强度仅为无损件的68%。

SiC_(f)/Ti65复合材料的拉伸行为研究

采用磁控溅射先驱丝法结合热等静压工艺制备SiC_(f)/Ti65复合材料,研究了SiC_(f)/Ti65复合材料室温、高温拉伸行为,揭示了SiC_(f)/Ti65复合材料拉伸断裂机制。研究结果表明:SiC_(f)/Ti65复合材料的室温、高温抗拉强度相比于Ti65合金的抗拉强度分别提升29%和164%,验证了复合材料的增强效果。SiC_(f)/Ti65复合材料的室温拉伸断裂机制为反应层断裂、纤维/基体界面脱粘、基体脆性断裂、纤维断裂、包套韧性断裂;高温断裂机制为反应层断裂、纤维/基体界面脱粘、纤维断裂、W/SiC界面脱粘、基体和包套韧性断裂。

Ti预处理的SiC_(f)/SiC与镍基高温合金复合铸件的界面组织与强度

由SiC_(f)/SiC复合材料与K403镍基高温合金熔体制备的一体化铸件,冷却到室温时会出现自行断裂。通过采用Ti粉埋覆包渗工艺在1100℃下对SiC_(f)/SiC表面进行预处理,并在适当工艺下与K403镍基高温合金熔体进行陶瓷型精密铸造,成功实现SiC_(f)/SiC与K403镍基高温合金的一体化成形和界面的牢固结合。结果表明:Ti预处理层平均厚度为17μm左右,Ti向SiC_(f)/SiC渗透、扩散和反应,形成含TiC,Ti3SiC2,Ti5Si3Cx,SiC相的显微组织;经过与高温镍基金属液复合铸造后,预处理层演变成厚约120μm的界面反应层,其典型界面组织为Ni2Si+C+Al4C3+MC(M主要含Ti及少量的Cr,Mo,W)。预处理层的存在减轻Ni与SiC的有害石墨化反应,缓解高温金属液对SiC_(f)/SiC的热冲击,形成的界面反应层降低热膨胀系数失配造成的热应力,使得SiC_(f)/SiC与K403一体化铸件结合界面的室温剪切强度达到63.5 MPa。

浸锑石墨与无压烧结SiC和3D打印SiC密封材料配对副的摩擦磨损特性研究

为研究干气密封用密封材料的摩擦学特性,采用销盘旋转试验台研究3种浸锑石墨在干摩擦条件下分别与无压烧结SiC和3D打印SiC配对时的摩擦磨损特性,探讨了速度、载荷对密封材料摩擦磨损特性的影响,揭示了配对副材料表面的磨损机理。结果表明:随着载荷p与速度v的乘积(pv值)的增大,摩擦因数和磨损率均呈现逐渐减小趋势;在pv值较小时磨损机理为严重的磨粒磨损和轻微的黏着磨损,在pv值较高时磨损主要发生在碳化硅摩擦表面黏附的石墨转移层间,磨损机理主要表现为黏着磨损;在研究pv值变化对浸锑石墨与SiC密封材料配对副摩擦磨损特性的影响时,首选定载荷变速度试验方法。

环境温度对二维编织SiC/SiC复合材料拉伸性能的影响研究

为了研究环境温度对陶瓷基复合材料拉伸性能的影响,在室温和800℃,1000℃,1200℃惰性气体保护环境下开展了二维编织SiC/SiC复合材料的拉伸试验。采用数字图像相关技术采集了高温环境下试件的变形数据。通过光学显微镜和扫描电子显微镜拍摄了试件的断口形貌。结果表明:800~1200℃内,二维编织SiC/SiC复合材料的拉伸应力-应变响应同样具有明显的双线性特征,初始线性段的弹性模量与室温测试结果相近,高温环境下第二线性段弹性模量低于室温环境;800~1200℃惰性气体环境下材料拉伸强度较室温环境低20%左右;温度主要影响材料中纤维与基体的结合状态和SiC纤维的强度。一方面,温度越高断口纤维拔出情况越严重;另一方面,温度越高SiC纤维强度越低,二维编织SiC/SiC复合材料强度也有所下降。

C/SiC复合材料数据库及机器学习性能预测平台的设计与开发

以MySQL Workbench 8.0为数据库平台,采用三层浏览器/服务器架构,利用PyCharm 2022开发了一套基于Web界面和机器学习技术的C/SiC复合材料数据库系统,该系统包括材料数据库和机器学习模型2部分,具有对材料组分结构、制备工艺、试验性能等材料研发各阶段的信息查询、添加、修改、删除、搜索以及支持信息管理、设计分析视图、案例推理辅助设计、用户与系统管理等功能。基于神经网络回归算法训练机器学习模型,利用材料微观结构参数预测材料力学性能,对不同微观结构参数的影响权重进行评价,并部署在系统平台中;通过Web用户界面调用机器学习模型,对预留的验证集数据进行拉伸模量和弯曲模量预测。结果表明:机器学习模型预测材料力学性能的精度达到95%左右,训练出来的预测模型具有良好的精度与泛化能力。

浆料浸渍辅助PIP工艺制备C/HfC-SiC复合材料的微观结构及性能研究

针对高速飞行器对于防热/承载一体化超高温陶瓷基复合材料的迫切需求,以及现有反应型HfC先驱体存在的成本高、效率低和致密效果差等不足,本研究将HfC亚微米粉体配制成稳定的陶瓷浆料,利用浆料加压浸渍辅助先驱体浸渍裂解(PIP)工艺制备了HfC基体均匀分布的C/HfC-SiC复合材料,探讨了HfC含量对于复合材料微观结构、力学与烧蚀性能的影响。结果表明,当HfC实际体积分数为13.1%~20.3%时,复合材料密度为2.20~2.58 g·cm^(-3),开孔率约为5%。通过单层碳布加压浸渍陶瓷浆料,HfC颗粒能够分散到纤维束内部,且在复合材料中分布比较均匀。提高HfC含量会降低复合材料纤维含量,其力学性能也呈现出降低趋势。当HfC体积分数为20.3%时,复合材料的密度、拉伸强度和断裂韧性分别为2.58 g·cm^(-3)、147 MPa和9.3 MPa·m^(1/2);经氧乙炔焰烧蚀60 s后,复合材料的线烧蚀率和质量烧蚀率分别为0.0062 mm/s和0.005 g/s,烧蚀过程中形成的熔融相Hf_(x)Si_(y)O_(z)能覆盖在材料表面,起到良好的保护作用。

2D-SiC_(f)/SiC复合材料层间Ⅰ型断裂试验及表征

二维编织碳化硅纤维增强碳化硅复合材料(2D-SiC_(f)/SiC)在航空领域中得到广泛使用,然而该材料层间强度低,使其易萌生层间裂纹,引起分层破坏。为此,本工作采用楔形双悬臂梁法(W-DCB)和悬臂梁法(DCB)开展了层间Ⅰ型断裂试验,获得了2D-SiC_(f)/SiC的层间裂纹驱动的加载数据,得到了其裂纹端口张开力及张开位移变形曲线。在试验加载过程,通过光学显微镜监测了视觉裂纹扩展过程,探究了2D-SiC_(f)/SiC的层间I型裂纹扩展规律。结合理论分析和裂纹视觉特征解释了加载曲线拐点及其他特征点的断裂力学含义。利用扫描电子显微镜分析了2D-SiC_(f)/SiC的层间断面特征,揭示了断面分层裂纹扩展机制。结果表明:W-DCB方法测量的2D-SiC_(f)/SiC层间Ⅰ型初始能量释放率与DCB方法等效;2D-SiC_(f)/SiC层间Ⅰ型断裂过程中,裂纹端口变形曲线的多峰性不符合经典线弹性断裂力学预测的加载峰后特征,反映了2D-SiC_(f)/SiC层间约束关系的复杂性;层间断面为结构性非完全损伤,发生了局部纤维桥连现象。

国产三代2.5D SiC_(f)/SiC复合材料的界面力学性能

连续碳化硅纤维增强碳化硅复合材料(SiC_(f)/SiC)是下一代航空发动机的关键结构材料,其界面性能是决定材料力学性能的重要因素之一。为此,本研究表征了国产三代2.5D SiC_(f)/SiC的界面性能,并探究其与材料拉伸性能的关系。利用拉伸加/卸载过程中的迟滞特性定量分析了2.5D SiC_(f)/SiC中各组元残余应力和界面滑动应力(IFSS),根据断口拔出纤维的断裂镜面半径得到了纤维就位强度(σ_(fu))的统计分布,通过纤维推入法得到界面剪切强度(ISS)和界面脱黏能(G_(i))。结果表明:利用宏观结合细观的方法能够较全面地描述SiC_(f)/SiC从初始裂纹萌生到最终脱黏不同阶段的界面力学性能,2.5D SiC_(f)/SiC的IFSS、ISS和Gi分别为56 MPa、(28±5)MPa和(2.7±0.6)J/m2。ISS和Gi较低,表明界面结合较弱,在剪应力作用下易产生裂纹,而IFSS较大,表明界面脱黏后纤维与基体间相对滑动较为困难,阻碍了纤维拔出,不利于发挥纤维的增强作用。根据获得的界面性能和经典ACK模型,较好地预测出比例极限应力,并结合σ_(fu)预测了2.5D SiC_(f)/SiC的拉伸强度。拉伸强度预测值高于实验值,这与界面处径向残余压应力以及纤维承受的残余拉应力有关。

电压对15%SiC_(p)/2009铝基复合材料微弧放电析出气体成分的影响

在硅酸盐溶液中,对15%SiC_(p)/2009铝基复合材料进行不同电压下的微弧氧化试验。通过分析微弧放电过程中复合材料析出气体的组成,评估电压对阳极析出气体成分的影响,并阐释其产生机制。结果表明:不同电压下阳极析出的气体均由H_(2)、O_(2)及少量CO组成,不同于传统电化学理论认为的阳极析出氧气。在360~440 V范围内,随着电压的升高,放电火花强度增大,微弧氧化膜生长速率加快,H_(2)含量随之增加,O_(2)含量相应降低;电压超过440 V后,H_(2)含量约为80%(体积分数),O_(2)含量约为20%,两者含量保持稳定。阳极火花放电区水热分解释放大量H_(2)和O_(2),且放电越剧烈,水分解量越多。放电区中铝和水氧化反应及高能电子轰击电离水分子也是过量H_(2)析出的重要原因。此外,SiC增强体参与成膜过程,并在放电区高温环境中被氧化,生成了少量的CO气体。