Atomic Force Microscopy Study of Ceramic Powder Compacts during Drying

Atomic force microscopy (AFM) was used to image the surface of ceramic powder compacts that contained polymer and solvent during drying. Topographical information was continuously collected as a function of drying time. Surface changes were directly observed during imaging. Surface crystals appeared on damp zirconia sheets after drying, subsequently contributing to secondary flaws in the form of holes in fired ceramic products. In cases where few surface crystals were observed in the green stage, fine grain size and high strength were observed after firing. The fired material was viewed via AFM and shown to be directly related to the drying process that was observed earlier.     

Adhesion Maps Using Scanning Force Microscopy Techniques

A technique has been developed that allows a real-time measurement of the lift-off force required to remove a scanning force microscope tip from a substrate. Both topography and adhesion maps are obtained simultaneously, allowing the correlation between topography and adhesion properties to be studied. Quantitative values of important adhesion parameters can be extracted from these data. A number of examples are given which illustrate the utility of this technique.     

Simulated Environments Testing System for Advanced Ceramic Matrix Composites

The present work aims toward the application of an innovative methodology for testing the environmental performance of advanced ceramic matrix composites in the presence of combined mechanical, thermal, and environmental applied conditions. To obtain a comprehensive understanding of how a composite might perform in certain application environments, a newly developed environmental performance testing platform, which includes simulated equivalent environments system (ES) and a wind tunnel environments system (WS), is proposed. A multiplication factor of 10.8 between the ES and WS at 1300°C has been obtained tentatively.     

连续纤维增强陶瓷基复合材料界面研究进展

在陶瓷基复合材料中引入高强陶瓷纤维的目的是为了增强陶瓷的断裂韧性,纤维与基体的界面是决定CMC韧性的关键因素。国内外许多专家和机构研究重点主要集中于连续纤维增强陶瓷基复合材料的界面,包括纤维与基体的化学相容性和热物理相容性,以及用TEM、HRTEM、SADP、AEM、声学显微法、EDX等微观测试手段研究不同体系的界面形成机理。本文对上述界面研究概况进行了综述,并简述了界面设计原则和近年来计算机技术在界面研究中的应用情况。指出,连续纤维增强陶瓷基复合材料界面研究将一直是复合陶瓷基复合材料界研究的重点和难点。     

Oxidation of BN-Coated SiC Fibers in Ceramic Matrix Composites

Thermodynamic calculations were performed to analyze the simultaneous oxidation of BN and SiC. The results show that, with limited amounts of oxygen present, the formation of SiO₂ should occur prior to the formation of B₂O₃. This agrees with experimental observations of oxidation in glassceramic matrix composites with BN-coated SiC fibers, where a solid SiO₂ reaction product containing little or no boron has been observed. The thermodynamic calculations suggest that this will occur when the amount of oxygen available is restricted. One possible explanation for this behavior is that SiO₂ formation near the external surfaces of the composite closes off cracks or pores, such that vapor phase O₂ diffusion into the composite occurs only for a limited time. This indicates that BN-coated SiC fibers will not always oxidize to form significant amounts of a lowmelting, borosilicate glass.     

In Situ Tensile Measurements of Single Fibers by Scanning Electron Microscopy

A newly developed tensile module allows tensile experiments of single fibers to be carried out under visual observation in the scanning electron microscope. This allows correlation of measured data with observed changes in the microstructure, such as surface irregularities and crack formation. With point heating, the thermal behavior of the fibers may be studied up to 2500°C. The results are presented with tensile elongation recordings and micrograph sequences of the structural changes. Carbon fibers with and without an aluminum coating were selected as testing specimens.     

Local Phenomena in Oxides by Advanced Scanning Probe Microscopy

In the last two decades, scanning probe microscopies (SPMs) have become the primary tool for addressing structure and electronic, mechanical, optical, and transport phenomena on the nanometer and atomic scales. Here, we summarize basic principles of SPM as applied for oxide materials characterization and present recent advances in high-resolution imaging and local property measurements. The use of advanced SPM techniques for solutions of material related problems is illustrated on the examples of grain boundary transport in polycrystalline oxides and ferroelectric domain imaging and manipulation. Future prospects for SPM applications in materials science are discussed.     

CVI法制备先进陶瓷基复合材料

化学气相渗透(CVI)是最具潜力的先进陶瓷基复合材料(CMC)制备工艺。本文概要阐述了CVI技术的基本原理和工艺特点，对不同类型的CVI工艺进行了简单论述和评价，并提出了解决制件中大量残余气孔率的新思路、新方法，最后还对CVI技术的今后发展方向进行了展望。     

Crack Bridging by Inclined Fibers/Whiskers in Ceramic Composites

Crack bridging by inclined fibers has been studied in a brittle fiber–brittle matrix model ceramic composite. Results of the fiber bridging force vs the crack opening displacement have been obtained for different fiber inclination angles using a fracture mechanics approach. Localized matrix cracking has been observed for inclined fibers and related to fiber inclination angle. The experimental results showing the influence of fiber inclination angle are discussed and compared with theoretical analyses to provide insight into crack bridging by inclined fibers/whiskers. Implications for toughening by whisker bridging are also discussed.     

BN纤维织物增强陶瓷透波材料的制备及其力学性能初探

以全氢聚硅氮烷(PHPS)为先驱体,采用聚合物浸渍裂解工艺制备BN纤维织物增强陶瓷透波材料,研究了复合材料的致密化工艺和力学性能。结果表明:先驱体PHPS在1637℃裂解产物主晶相为α-Si3N4。以PIP工艺制备BNf/Si3N4复合材料,经过4个浸渍裂解周期密度达到1·5g/cm3,复合材料的室温弯曲强度达到39·6MPa。裂解过程中,PHPS与BN纤维发生了强界面反应,导致复合材料力学性能不高。     

碳纤维增强碳化硅陶瓷基复合材料的研究进展及应用

碳纤维增强碳化硅陶瓷基复合材料具有密度低、高强度、高韧性和耐高温等综合性能,已得到世界各国高度重视.本文综述了碳纤维的研究进展,C_f/SiC复合材料的制备方法,并分析了各种制备方法的优缺点.概述了C_f/SiC复合材料作为高温热结构材料和制动材料的应用状况.最后,指出了有待解决的问题和今后的主要研究方向.     

Investigation of Grain-Boundary Segregation in Ceramic Oxides by Analytical Scanning Transmission Electron Microscopy

Analytical scanning transmission electron microscopy (AEM) was used to study grain-boundary solute segregation in the systems MgO-NiO, A1₂O₃-Y, O₃, NiO-Cr₂O₃, and NiO-Al₂O₃. Electron beam spreading within the specimen was incorporated into a model to quantitatively measure solute segregation. Grain-boundary segregation occurred in A1₂O₃-Y₂O₃ and NiO-Cr₂O₃ but was not detected in MgO-NiO and NiO-Al₂O₃ specimens. These results and the quantitative measurements agree with equilibrium solute segregation theories. Microhard-ness measurements indicate no difference in hardness between the grain boundary and the matrix for Cr-doped NiO, a system in which segregation was detected.     

先进陶瓷基复合材料制备技术-CVI法现状及进展

化学气相渗透法(CVI)是制备先进陶瓷基复合材料最赋潜力的技术.本文概要阐述了CVI法的原理与动力学机制,论述了CVI先进陶瓷基复合材料中纤维、基体、界面的研究现状,对不同类型的CVI工艺及目前的CVI模拟技术作了一定的评价,提出了CVI技术的发展方向和研究课题.     

Measuring the Adhesion Force on Natural Fibre Surface Using Scanning Probe Microscopy

This work has focused on measuring the adhesion forces on both untreated and atmospheric helium plasma treated single jute fibre surfaces using scanning probe microscopy (SPM). The measurements were conducted on three differently aged surfaces for one week, three weeks and six weeks using a standard silicon nitride tip in force-volume (f-v) mode. Up to 256 adhesion data points were collected from various locations on the surface of the studied fibres using in-house developed software and the resulting data were statistically analysed by the histogram method. Results obtained from this analysis method were found to be very consistent with a small statistical variation. The work of adhesion, W _a, was calculated from measured adhesion force using the Johnson–Kendall–Roberts (JKR) and Derjaguin–Muller–Toporov (DMT) models. Increases in both adhesion force and work of adhesion were observed on jute fibre with certain levels of atmospheric plasma treatment and ageing time.     

Cyclic Thermal Shock Resistance of Several Advanced Ceramics and Ceramic Composites

The cyclic thermal shock behavior of two Si₃N₄ ceramics, two SiC-whisker-reinforced alumina composites (Al₂O₃/SiC_w), a SiC-particulate-reinforced alumina (Al₂O₃/SiC_p), and an alumina continuously reinforced with SiC fibers (Al₂O₃/SiC_f) composite has been studied. Specimens were repeatedly quenched from 1473 K into a fluidized bed with a heat transfer coefficient of 1400 W/(Km²) [250 Btu/(hft²F)]. The thermal shock damage was assessed by room-temperature flexure strength measurements. Si₃N₄ and Al₂O₃/SiC_p showed no noticeable damage after 100 cycles, whereas Al₂O₃/SiC_w and Al₂O₃/SiC_f degraded substantially. The experimental results are discussed and rationalized in terms of finite element simulations and microstructural observations. Our analysis suggests that the thermal shock performance of other materials may be estimated from comparisons with the present work.     

助熔剂对原位转化碳纤维增韧陶瓷基复合材料性能的影响

本文以聚丙烯腈(PAN)预氧化纤维为先驱体,以氧化铝为主要原料,添加SiO2-MgO-CaO三系助熔剂,采用真空热压烧结法制备了原位转化碳纤维增韧氧化铝复合材料.主要探讨不同助熔剂添加量对复合材料微观结构和各项性能指标的影响.以体积密度、显微硬度和断裂韧性等性能指标为主要评价标准选择最佳的助熔剂添加量.并研究了原位转化碳纤维增韧氧化铝陶瓷的摩擦磨损行为与机制以及力学性能和微观结构对摩擦磨损特性的影响.结果表明:当助熔剂含量为3vol％时,复合材料的综合性能最优,此时体积密度为3.72 g·cm-3,显微硬度为1624 HV,断裂韧性为10.6 MPa·m1/2.在室温干摩擦条件下,复合材料的磨损率随着助熔剂含量的增加呈先升高后降低趋势.室温下原位转化碳纤维增韧氧化铝基复合材料的磨损机制以脆性剥落为主,并伴有疲劳磨损.     

Ultramicrotoming of Ceramic Powders for Electron Microscopy

Compacts of monosize TiO₂ particles and agglomerates of TiO₂ and γ-alumina were infiltrated with Spurr's epoxy resin, cured, and ultramicrotomed. Transmission electron microscopy of the thin sections revealed the packing of the TiO₂ spheres and the structure of the agglomerates.     

Study of the Ion‐Irradiation Behavior of Advanced SiC Fibers by Raman Spectroscopy and Transmission Electron Microscopy

6H–SiC single crystals and two types of SiC fibers, Hi‐Nicalon type S and Tyranno SA3, have been irradiated with 4‐MeV Au³⁺ up to 2 × 10¹⁵ cm^?2 (4 dpa) at room temperature, 100°C and 200°C. These fibers are composed of highly faulted 3C–SiC grains and free intergranular C. Stacking fault linear density and grain size estimations yield, respectively, 0.29 nm^?1 and 26–36 nm for the Hi‐Nicalon type S fibers and 0.18 nm^?1 and 141–210 nm for the Tyranno SA3 fibers. Both transmission electron microscopy and surface micro‐Raman spectroscopy reveal the complete amorphization of all the samples when irradiated at room temperature and 100°C and a remaining crystallinity when irradiated at 200°C. The latter observations reveal a multi‐band irradiated layer consisting in a partially amorphized band near the surface and an in‐depth amorphous band. Also, nanocrystalline SiC grains with high stacking fault densities can be found embedded in amorphous SiC at the maximum damage zone of the Hi‐Nicalon type S fibers irradiated at 200°C.