Effect of SiC content and orientation on the properties of Si/SiC ceramic composite

Silicon/silicon carbide ceramic composites are made by infiltrating carbonaceous material with liquid silicon to form SiC crystallites dispersed in a silicon matrix. The present study was conducted to determine the effects on the room temperature properties of density, elastic modulus, strength, and fracture toughness by varying the amount and distribution of the SiC crystallites. Most of the work involved uniaxially aligned SiC crystals of varying volume fraction tested both longitudinally and transversely to the converted fibre axis.     

轻质碳化硅平面反射镜超光滑表面加工

介绍了对角线为110mm六边形反应烧结轻质碳化硅平面反射镜超光滑光学加工工艺流程。详细阐述了各个工序所使用的磨具、磨料和抛光机床工艺参数,对实际加工的轻质碳化硅平面反射镜超光滑表面进行检测,检测结果为:面形精度均方根值(RMS)为0.011λ(PV值为0.071,λ=632.8nm),表面粗糙度RMS达0.75nm。     

Single step fabrication of N-doped graphene/Si3N4/SiC heterostructures

In-plane heteroatom substitution of graphene is a promising strategy to modify its properties. The ability to dope graphene with electron-donor nitrogen heteroatoms is highly important for modulating electrical properties of graphene. Here we demonstrate a transfer-free method to directly grow large area quasi free-standing N-doped graphene bilayers on an insulating substrate （Si3N4）. Electron-bombardment heating under nitrogen flux results in simultaneous growth of N-doped graphene and a Si3N4 layer on the SiC surface. The decoupling of N-doped graphene from the substrate and the presence of Si3N4 are identified by X-ray photoemission spectroscopy and low-energy electron diffraction. The substitution of nitrogen atoms in the graphene planes was confirmed using high resolution X-ray photoemission spectroscopy which reveals several atomic configurations for the nitrogen atoms： Graphitic-like, pyridine-like, and pyrrolic- like. Furthermore, we demonstrated for the first time that N-doped graphene could be used to efficiently probe oxygen molecules via nitrogen atom defects.     

碳模板转化法制备管状SiC/Si复合陶瓷

利用纸作为原材料,通过卷曲、树脂浸渍、碳化预制备出具有层状结构的管状碳模板,之后在1550℃通过原位反应液相渗Si0.5-1h,在常压烧结条件下制备出具有层状结构特征的SiC/Si管状陶瓷复合材料.采用XRD、SEM对碳模板反应前后的物相变化和显微结构进行了研究.结果表明了该材料的最终产物为β-SiC和Si,且两者分布表现出明显的交替成层现象,呈现出层状陶瓷的结构特征.     

Elevated-temperature stress rupture in interlaminar shear of a Hi-Nic SiC/SiC ceramic matrix composite

Assessments of stress rupture in a gas-turbine grade, melt-infiltrated Hi-Nicalon^TM SiC continuous fiber-reinforced SiC ceramic matrix composite (CMC) were made in interlaminar shear at 1316 °C in air. The composite exhibited appreciable life limiting behavior with a life susceptibility parameter of n_s = 22–24, estimated based on a proposed phenomenological life prediction model together with experimental data. The phenomenological life model was in good agreement in prediction between the stress rupture and the constant stress-rate data, validating its appropriateness in describing the life limiting phenomenon of the CMC coupons subjected to interlaminar shear. The results of this work also indicated that the governing mechanism(s) associated with failure in interlaminar shear would have remained almost unchanged, regardless of the type of loading configurations, either in stress rupture or in constant stress rate.     

Si-B-C陶瓷涂敷2D C/SiC复合材料的抗氧化性能

利用化学气相沉积(CVD)法制备了Si-B-C陶瓷涂敷改性的2D C/SiC复合材料,研究了其在700～1200℃氧化10 h性能和结构的演变规律以及自愈合机制,同时获得了Si-B-C涂层在不同温度氧化后的形貌、组分和物相转变规律.结果表明:涂敷在复合材料表面的Si-B-C陶瓷随温度的升高氧化加快,但氧化程度较低,不深于7μm;随温度的升高,氧化形成的硅硼玻璃黏度降低,挥发增强;当温度达到1200℃时,硅硼玻璃析出SiO2晶体;Si-B-C陶瓷涂敷改性的C/SiC具有优良的抗氧化性能,随氧化温度的升高,复合材料失重率增加,但在1200℃氧化10h后失重率仅为0.47％;此外材料在1000℃氧化后的强度保持率最高,达到91.6％,Si-B-C陶瓷氧化形成的硅硼玻璃可以有效封填裂纹,这是材料具有优良抗氧化性能的主要机制.     

Si-B-C陶瓷涂敷2D C/SiC复合材料的抗氧化性能

利用化学气相沉积(CVD)法制备了Si-B-C陶瓷涂敷改性的2D C/SiC复合材料, 研究了其在700～1200 ℃氧化10 h性能和结构的演变规律以及自愈合机制, 同时获得了Si-B-C涂层在不同温度氧化后的形貌、 组分和物相转变规律。结果表明: 涂敷在复合材料表面的Si-B-C陶瓷随温度的升高氧化加快, 但氧化程度较低, 不深于7 μm; 随温度的升高, 氧化形成的硅硼玻璃黏度降低, 挥发增强; 当温度达到1200 ℃时, 硅硼玻璃析出SiO₂晶体; Si-B-C陶瓷涂敷改性的C/SiC具有优良的抗氧化性能, 随氧化温度的升高, 复合材料失重率增加, 但在1200 ℃氧化10 h后失重率仅为0.47%; 此外材料在1000 ℃氧化后的强度保持率最高, 达到91.6%, Si-B-C陶瓷氧化形成的硅硼玻璃可以有效封填裂纹, 这是材料具有优良抗氧化性能的主要机制。     

Precipitation sequence in a SiC/Al-Mg2Si alloy composite material

The precipitation sequence of an Al-1.0mass%Mg₂Si composite material having 8 vol.% SiC particles was investigated by Vickers micro hardness and specific electrical resistivity measurements, and by TEM observation. The formation of GP zones was suppressed in the composite material and its age-hardenability was reduced. Distribution of precipitates in the composite material was coarser and their size was larger than that in the Al-1.0mass%Mg₂Si alloy (base alloy). Some types of precipitates in the composite material were not similar to those found in the base alloy but were similar to those in an Al-1.0mass%Mg₂Si alloys with excess silicon (the excess Si alloy). Especially, the metastable phases in the composite material aged at 473 K belonged to the type-A, type-B and type-C precipitates, which are typical metastable phases in the excess Si alloy, instead of the phase that is a typical metastable phase in quasi-binary Al-Mg₂Si alloys. The dislocations had little effect on the aging process of this composite material, because of the small number of dislocations introduced by the quenching after solution treatment.     

Energy absorption in calcium aluminosilicate/SiC glass ceramic matrix composite tensile tests

Abstract

An analysis is presented of the energy absorbed in a tensile test of a calcium aluminosilicate/SiC glass ceramic matrix composite based on the mechanical properties of the interface and the constituents. The results of the model are compared with those obtained in a set of 21 tensile tests. The strain energy density and the permanent strain have been measured in the tests, and the results are in good agreement with the theoretical approach. The relationship between the variability of micromechanical properties and the variability observed for the composite is studied, and the influence of the different fracture mechanisms is also discussed. It is concluded that the main failure mechanism, in terms of its contribution to the total energy absorption, is the interfacial debonding. Finally, the model permits the identification of the main causes responsible for the material variability, some of which might be controlled during the manufacturing process.     

Si3N4／SiC纳米复合陶瓷的微观结构

利用ＪＥＭ２０００ＥＸⅡ高分辨电镜和ＨＦ２０００冷场发射枪透射电镜对Ｓｉ３Ｎ４ＳｉＣ纳米笔合陶瓷材料的微观组织，结构和成分进行了研究。结果表明，ＳｉＣ颗粒弥散分布基体相β－Ｓｉ３Ｎ４晶内和晶界，晶内ＳｉＣ颗粒与基体相的界面结构有三种类型；１）直接结合的的界面；２）完全非晶态的界面；３）混合型的界面，晶间ＳｉＣ颗粒与基体相的界面大部分是直接结合的。     

Preparation and dielectric properties of Si3N4/SiCw composite ceramic

Gelcasting was employed to fabricate Si₃N₄/SiC whisker (SiCw) composite ceramics, and the effects of heat-treatment temperature on the length-to-diameter ratio of the whiskers and SiCw content on microwave dielectric properties were studied. Compared with pure SiCw of spherical structure obtained at temperature of 1,750 °C(Ar), pure SiCw treated at 1,600 °C(Ar) showed rod-like structure, higher dielectric properties and more evenly distribution in Si₃N₄/SiCw composite ceramics. Both the real (ε′) and imaginary (ε″) permittivity of Si₃N₄/SiC whisker (SiCw) composite ceramics decreased with increasing frequency and increased as the whisker content raised owing to the interface and SiCw playing a role of dipole in the frequency range of 8.2–12.4 GHz. In addition, comparing the ceramics with lower content of SiCw, the reflectivity of the composite ceramics moved to a lower frequency; the maximum absorption peak reached ?22.4 dB at the whisker content of 15 wt%.     

Slurry processing for fabrication of SiC whisker-reinforced Si3N4 composites

In order to solve the major problems of processing whisker-reinforced ceramic composites, such as agglomeration of whiskers, correlation between pH and viscosity has been carefully investigated in a mixed slurry of whiskers and matrix powder. SiC whiskers and Si₃N₄ powder were dispersed homogeneously by controlling pH in aqueous suspension, and the state was successfully fixed by a sudden change of pH to make the slurry more viscous. The slurry was then filtrated rapidly and dried. The strength of hot pressed composites obtained by this procedure was scarcely lowered, with increased whisker loading in the range 0–30 wt% and fracture toughness increased more than 75%.     

碳纤维复合材料空间反射镜制造技术研究进展

碳纤维复合材料(CFRP)具有比刚度高、热稳定性好,可设计性强等特点,成为轻量化反射镜制造的新型理想材料,特别适合大口径、高分辨率空间反射镜的制造。简单介绍空间光学反射镜的国内外研究应用的现状。根据反射镜的特点介绍了碳纤维复合材料的材料选择要求,以及国外常用碳纤维复合材料预浸料体系。碳纤维复合材料反射镜制造方法与传统光学材料反射镜的制造不同,一般采用高效、快速、制造成本低的复制工艺来实现。总结了碳纤维复合材料反射镜的复制工艺和反射镜复制精度的影响因素,影响因素包括复制模具、预浸料铺层、反射镜变形、纤维印痕现象、尺寸稳定性以及反射镜尺寸等,并对前五项影响因素提出相应的控制措施来减小影响程度。重点阐述了反射镜纤维印痕现象的成因和解决方法,以及控制尺寸稳定性的关键。     

Synthesis and pyrolysis of polysilazane as the precursor of Si3N4/SiC ceramic

By co-ammonolysis of MeHSiCI₂ and Me₂SiCl₂ and following thermal polymerization, polysilazanes as the precursors to Si₃N₄/SiC ceramic fibre were synthesized. The tendency to cross-link in thermal polymerization is suppressed as the result of the introduction of Me₂SiCl₂ as a starting material. The reactions occurring during thermal polymerization of ammonolysis products are discussed based on infrared (i.r.) and ¹H nuclear magnetic resonance (NMR) spectra analysis. The pyroiysis process of polysilazanes is examined in nitrogen and in an NH₃ atmosphere. It is shown that the formation of SiC results from the pyrolysis of Si-CH₃ groups in polysilazanes by means of the formation of Si-CH₂-Si intermediate bonds. The structure and properties of ceramics derived from the pyrolysis of polysilazane and polycarbosilane in N₂ or NH₃ atmosphere are compared and discussed. Si-N-C ceramic, which was proved to be a composite of -Si₃N₄ and -SiC, exhibits better thermal stability at temperatures higher than 1300°C.     

C/SiC陶瓷基复合材料螺栓连接件的振动响应特性及防松性能

通过对C/SiC陶瓷基复合材料螺栓连接件的模态进行计算和试验分析,确定了其模态参数和振动响应特性;然后通过对C/SiC陶瓷基复合材料螺栓连接件进行正弦扫频振动试验,研究了拧紧力矩对螺栓连接件振动性能的影响规律;最后研究了液态聚硼硅氮烷（L-PBSZ）对C/SiC陶瓷基复合材料螺栓连接件防松性能的影响。研究结果表明：一阶和三阶固有计算模态频率与试验模态频率一致,因此可以采用有限元分析方法对C/SiC陶瓷基复合材料螺栓连接件进行振动响应特性分析;采用正弦扫频振动频谱信号差值曲线分析方法可以检测螺栓连接件是否松动;SEM观察表明,L-PBSZ改性的C/SiC陶瓷基复合材料螺栓连接件的螺纹副间形成陶瓷填充体,使得螺纹与螺母间有效摩擦系数和有效摩擦面积增加,因此螺母松脱退出的力矩增大,提高了C/SiC陶瓷基复合材料螺栓连接件的防松可靠性能。     

Preparation and characterization of nano-structured monolithic SiC and Si3N4/SiC composite by hot isostatic pressing

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal stability of ceramic fibre in a CVI-processed SiC matrix composite

The thermal stability of the HPZ fibre in a chemical vapour infiltration (CVI)-processed SiC matrix composite was studied. The mechanical properties and fracture behaviour of the untreated and SiC-coated fibres after thermal exposure at different temperatures and atmospheres were characterized. The results show that, below 1000 °C, the strength degradation is negligible. However, severe degradation occurs at temperatures above 1000 °C due to the evolution of CO, SiO, and other gaseous species. Also, pyrolytic C-coating is needed to tailor the interfacial bond strength in the HPZ/SiC composite.     

SiC纤维增强钛基复合材料空心结构成形研究

采用箔材刻槽法制备连续纤维增强钛基复合材料(SiC/Ti)面板,通过分析不同复合工艺参数条件下的纤维/基体界面和基体微观组织,获得了优化的制备工艺:925℃/100 MPa/1 h.利用SiCf/Ti复合材料沿垂直纤维方向具有大变形的能力,将超塑成形/扩散连接技术(SPF/DB)与SiCf/Ti复合材料的复合技术相结合...     

Non-destructive testing of satellite nozzles made of carbon fibre ceramic matrix composite,C/SiC

Carbon fibre ceramic matrix composite materials, C/SiC, are excellent candidates as lightweight structural materials for high performance hot structures such as in aerospace applications. Satellite nozzles are manufactured from C/SiC, using, for instance, the Liquid Polymer Infiltration (LPI) process.In this article the applicability of different non-destructive analysis methods for the characterisation of C/SiC components will be discussed. By using synchrotron and neutron tomography it is possible to characterise the C/SiC material in each desired location or orientation. Synchrotron radiation using tomography on small samples with a resolution of 1.4 μm, i.e. the fibre scale, was used to characterise three dimensionally fibre orientation and integrity, matrix homogeneity and dimensions and distributions of micro pores. Neutron radiation tomography with a resolution of about 300 μm was used to analyse the over-all C/SiC satellite nozzle component with respect to the fibre content. The special solder connection of a C/SiC satellite nozzle to a metallic ring was also successfully analysed by neutron tomography. In addition, the residual stress state of a temperature tested satellite nozzle was analysed non-destructively in depth by neutron diffraction. The results revealed almost zero stress for the principal directions, radial, axial and tangential, which can be considered to be the principal directions.