The compatibility of TiB2 protective coatings with SiC fibre and Ti-6Al-4V

TiB₂ coatings have been studied as prospective protective layers to inhibit the interfacial reaction between SiC fibres and Ti-alloy matrices. This protective coating has been deposited onto SiC monofilament fibres using a chemical vapour deposition (CVD) technique. The fibre-matrix compatibility of these TiB₂-coated SiC fibres in Ti-6Al-4V composites was evaluated by incorporating the coated fibres into Ti-6Al-4V using a diffusion bonding technique. The interfaces of this composite were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis, to evaluate the interfacial microstructures, chemical stability and the efficiency of TiB₂ as a protective coating for SiC fibres in Ti-alloy matrices, and to study the effects of deposition temperature on the interface of the coated fibre. Results show that stoichiometric TiB₂ coatings are stable chemically to both SiC fibres and Ti-6Al-4V and hinder the deleterious fibre-matrix reactions effectively. Boron-rich TiB₂ coatings should be avoided, as they lead to the formation of a needle-like TiB phase at the fibre–matrix interface. These findings provide promising evidence for the value of further exploration of the use of stoichiometric TiB₂ as a protective coating for SiC fibre in Ti-based composites.     

Microstructural characterization in diffusion-bonded SiC/Ti–6Al–4V composites

The microstructural evolution during the diffusion bonding consolidation of a Ti–6Al–4V/SiC fibre composite was investigated by optical, scanning and transmission electron microscopy. The effects of processing parameters, particularly temperature, on the microstructures of the matrix and the fibre and their bonding were considered. Processing at too high a temperature can result in growth of SiC crystals in the fibre in addition to rapid interfacial reaction, while interfacial bonding cannot be established if the temperature is too low. Various defects can be caused by inadequate fabrication practices. These include micro-pores, matrix-cracking, cracking, bending and impingement of fibres, and heterogeneous fibre distribution. Methods for avoiding these are discussed. A defect-free and uniformly distributed fibre composite can only be achieved by optimizing the processing parameters (such as temperature, pressure, time and cooling rate) and adequately combining fibre spacing and matrix thickness with accurate fibre alignment.     

Carbon-fibre reinforced magnesium alloys: nanostructure and chemistry of interlayers and their effect on mechanical properties

The micromechanical fracture behaviour of C/Mg–Al composites of varying interface reactivity was investigated by scanning electron microscope bending tests. Structure and chemistry of fibre/matrix interlayers were studied down to the atomic scale by imaging and spectroscopical transmission electron microscope techniques (high-resolution electron microscopy, energy dispersive X-ray spectroscopy, parallel-recording electron energy loss spectroscopy and energy-filtered transmission electron microscopy). The chemical reactions at the fibre/matrix interfaces of the C/Mg–Al composites were found to form plate-shaped carbidic precipitates, mainly Al₂MgC₂, which strongly influence the composite's mechanical properties by changing the fibre/matrix bonding strength.     

Electron energy-loss spectroscopy study of thin film hafnium aluminates for novel gate dielectrics

We have used conventional high‐resolution transmission electron microscopy and electron energy‐loss spectroscopy (EELS) in scanning transmission electron microscopy to investigate the microstructure and electronic structure of hafnia‐based thin films doped with small amounts (6.8 at.%) of Al grown on (001) Si. The as‐deposited film is amorphous with a very thin (～0.5 nm) interfacial SiO_x layer. The film partially crystallizes after annealing at 700 °C and the interfacial SiO₂‐like layer increases in thickness by oxygen diffusion through the Hf‐aluminate layer and oxidation of the silicon substrate. Oxygen K‐edge EELS fine‐structures are analysed for both films and interpreted in the context of the films’ microstructure. We also discuss valence electron energy‐loss spectra of these ultrathin films.     

The microstructure of experimental SiC fibre-reinforced yttrium magnesium aluminosilicate (SiCf-YMAS) materials

Two experimental SiC fibre-reinforced yttrium magnesium aluminosilicate (SiCf-YMAS)-type ceramic-matrix composite (CMC) materials fabricated (i) by the glass process and (ii) by chemical precursor infiltration have been studied by light microscopy, transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) and energy-dispersive X-ray spectroscopy (EDS). The distribution of the fibres inside the composite as well as the average diameter of fibres have been determined by image analysis. The microstructure of the YMAS matrices has been characterized by TEM observations. YMAS matrices are formed of two main phases, cordierite and β-yttrium silicate (Y₂Si₂O₇). Two minor phases (mullite and spinel) have been found to crystallize inside the cordierite and the yttrium silicate crystals. Fibre-matrix interfaces have been observed in HREM. A thin turbostratic carbon layer (20–30 nm) has been imaged in both composites at the fibre-matrix interface. It crystallizes along the matrix interface and grows inside the fibre, forming a diffuse interphase. The carbon layer is believed to be the consequence of reaction between oxygen in the matrix and SiC nanocrystals of the Nicalon fibres.     

Review of the principal contrast effects observed at interphase boundaries using transmission electron microscopy

This review is concerned with the interpretation of contrast features observed at interphase interfaces using conventional transmission electron microscopy. The principal features which arise are interfacial dislocations and steps, four types of fringes, namely thickness, Δw, displacement and moiré fringes, and lattice images. The contrast mechanisms leading to these features are discussed in the framework of the dynamical theory of electron diffraction. In particular, the influence on image contrast of the cyrstal composition and structure, the interfacial configuration and the crystallographic relationship of the two phases in the specimen is considered. In general, electron diffraction in bi-crystalline specimens is complicated, and reliable interpretation can only be carried out when the diffraction conditions have been carefully established. The diffraction conditions most widely used are discussed, and their relative merits compared.     

Successful application of spatial difference technique to electron energy-loss spectroscopy studies of Mo/SrTiO3 interfaces

The electron energy‐loss near‐edge structure (ELNES) of Mo/SrTiO₃ interfaces has been studied using high spatial resolution electron energy‐loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope. Thin films of Mo with a thickness of 50 nm were grown on (001)‐orientated SrTiO₃ surfaces by molecular beam epitaxy at 600 °C. High‐resolution transmission electron microscopy revealed that the interfaces were atomically abrupt with the (110)_Mo plane parallel to the substrate surface. Ti‐L_2,3 (～460 eV), O‐K (～530 eV), Sr‐L_2,3 (～1950 eV) and Mo‐L_2,3 (～2500 eV) absorption edges were acquired by using the Gatan Enfina parallel EELS system with a CCD detector. The interface‐specific components of the ELNES were extracted by employing the spatial difference method. The interfacial Ti‐L_2,3 edge shifted to lower energy values and the splitting due to crystal field became less pronounced compared to bulk SrTiO₃, which indicated that the Ti atoms at the interface were in a reduced oxidation state and that the symmetry of the TiO₆ octahedra was disturbed. No interfacial Sr‐L_2,3 edge was observed, which may demonstrate that Sr atoms do not participate in the interfacial bonding. An evident interface‐specific O‐K edge was found, which differs from that of the bulk in both position (0.8 ± 0.2 eV positive shift) and shape. In addition, a positive shift (0.9 ± 0.3 eV) occurred for the interfacial Mo‐L_2,3, revealing an oxidized state of Mo at the interface. Our results indicated that at the interface SrTiO₃ was terminated with TiO₂. The validity of the spatial difference technique is discussed and examined by introducing subchannel drift intentionally.     

The scratch behaviour of aluminium composite coatings

Wear data obtained from single and multiple scratch passes, combined with the examination of the reinforcement-matrix interfacial structure by transmission electron microscopy (TEM) and toughness measurements made on the reinforcement phase, wereused to evaluate the wear behaviour of aluminium-based composite coatings. Reinforcement properties such as reinforcement volume fraction (V_f ), size, interfacial bonding and toughness appear to be important variables in deciding the abrasive wear resistanceof composite coatings. Scratch testing appears to be a valuable tool in providing information regarding the nature of the reinforcement-matrix interfacial structure, which is a deciding parameter in the abrasive wear resistance of metal matrix composite (MMC)coatings.     

Achieving High Tribological Performance of Graphite-like Carbon Coatings on Ti6Al4V in Aqueous Environments by Gradient Interface Design

A duplex treatment involving nitrogen ion pre-implantation and gradient interfacial transition was performed to obtain a high-performance graphite-like carbon (GLC) coating on a Ti6Al4V alloy. Characteristics of the as-deposited coating systems were systemically investigated by Raman spectrometry, scanning electron microscopy, atomic force microscopy, nano-indentation, and scratch tests. The friction and wear behaviors in distilled water and sea water environments were evaluated by a ball-on-disk tribometer. The results showed that the GLC multilayer coating on nitrogen ion-implanted Ti6Al4V possessed a greater hardness and adhesion strength than to that on un-implanted Ti6Al4V. The tribological performances of these duplex process systems showed a great improvement in both the distilled water and sea water environments. In particular, the Cr/CrN/GLC coatings on nitrogen ion-implanted substrates demonstrated the best friction and wear behaviors. These striking improvements were attributed to the greatly enhanced interface strength between substrate and coating by the nitrogen ion implantation process and improved adhesion strength between gradient layers by the appropriate gradient interlayers with a similar thermal expansion coefficient.     

Spatially resolved electron energy-loss studies of metal–ceramic interfaces in transition metal/alumina cermets

Composites consisting of an alumina matrix and 20 vol.% transition metal (Ni or Fe) particles, prepared by hot pressing powder blends, have been studied using spatially resolved transmission electron energy-loss spectroscopy (EELS), and, to a lesser extent, by high-resolution electron microscopy (HREM). Particular attention was paid to the elucidation of the chemical bonding mechanisms at the metal-ceramic interface; EELS spectra from interfacial regions being obtained via a spatial difference technique. From both qualitative and quantitative interpretation of EELS near-edge structures, as well as observed HREM images, the data appear to be consistent with the presence of an Al-terminated alumina at the interface and the formation of direct transition metal – aluminium bonds in Al(O₃M) (M = Ni or Fe) tetrahedral units, possibly as a result of the dissolution and interfacial reprecipitation of Al during processing. These results correlate well with similar model studies on diffusion-bonded Nb/Al₂O₃ interfaces and may, in the light of recent theoretical electronic structure calculations, have implications for the resultant interfacial bond strength in such materials.     

High Tc ceramic superconductors: Introduction,background, and challenges to the electron microscopist

This introductory article reviews the recent developments in high T_c superconducting ceramics. Particular emphasis is given to the structural and chemical aspects of these compounds that may be of interest to the electron microscopist. Included are some of the contributions made by electron microscopy techniques over the last 10 months. The applications of electron microscopy to solving practical problems associated with these exciting new materials are also discussed.     

TEM and AES investigations of the natural surface nano‐oxide layer of an AISI 316L stainless steel microfibre

The chemical composition, nanostructure and electronic structure of nanosized oxide scales naturally formed on the surface of AISI 316L stainless steel microfibres used for strengthening of composite materials have been characterised using a combination of scanning and transmission electron microscopy with energy‐dispersive X‐ray, electron energy loss and Auger spectroscopy. The analysis reveals the presence of three sublayers within the total surface oxide scale of 5.0–6.7 nm thick: an outer oxide layer rich in a mixture of FeO.Fe₂O₃, an intermediate layer rich in Cr₂O₃ with a mixture of FeO.Fe₂O₃ and an inner oxide layer rich in nickel.     

The Effect of Hot Extrusion on Mechanical and Tribological Behavior of Ag-Cu/MoS2 Composites

Silver–copper/molybdenum disulfide (Ag-Cu/MoS₂) composites, prepared by powder metallurgy and hot press sintering, were extruded at a temperature of 680°C with extrusion ratios of 10 and 70. Mechanical tests and tribotests were carried on both the hot-pressed and hot-extruded composites. The tribological properties of the composites against a silver coin disc were investigated on a pin-on-disc tester with normal load and sliding speed of 5 N and 0.27 m/s, respectively. The microstructure, wear morphology, and cross section of the worn subsurface were observed by scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analyses were performed on the worn surfaces of Ag-Cu/MoS₂ composites. The results indicated that the distribution of the MoS₂ particles in the composites was improved and the interfacial strength of Ag/MoS₂ was enhanced during the process of hot extrusion. The hardness, bending strength, and wear resistance of hot-extruded composites increased remarkably due to the presence of the continuous matrix skeleton and the stronger interfacial bonding of Ag/MoS₂. XPS revealed that a chemical reaction had occurred at the worn surface due to the friction heat. Although the dominant wear mechanism was fatigue wear for both the hot-pressed and hot-extruded composites, finer debris and a lower wear rate were observed in hot-extruded composites due to the fact that the nucleation and growth of cracks in the worn subsurface were restrained in the process of tribotest.     

Mapping of valence energy losses via energy-filtered annular dark-field scanning transmission electron microscopy

The advent of electron monochromators has opened new perspectives on electron energy-loss spectroscopy at low energy losses, including phenomena such as surface plasmon resonances or electron transitions from the valence to the conduction band. In this paper, we report first results making use of the combination of an energy filter and a post-filter annular dark-field detector. This instrumental design allows us to obtain energy-filtered (i.e. inelastic) annular dark-field images in scanning transmission electron microscopy of the 2-dimensional semiconductor band-gap distribution of a GaN/Al₄₅Ga₅₅N structure and of surface plasmon resonances of silver nanoprisms. In comparison to other approaches, the technique is less prone to inelastic delocalization and relativistic artefacts. The mixed contribution of elastic and inelastic contrast is discussed.     

Chemical reactions and morphological stability at the Cu/Al2O3 interface

The microstructures of diffusion‐bonded Cu/(0001)Al₂O₃ bicrystals annealed at 1000 °C at oxygen partial pressures of 0.02 or 32 Pa have been studied with various microscopy techniques ranging from optical microscopy to high‐resolution transmission electron microscopy. The studies revealed that for both oxygen partial pressures a 20–35 nm thick interfacial CuAlO₂ layer formed, which crystallises in the rhombohedral structure. However, the CuAlO₂ layer is not continuous, but interrupted by many pores. In the samples annealed in the higher oxygen partial pressure an additional reaction phase with a needle‐like structure was observed. The needles are several millimetres long, ～10 µm wide and ～1 µm thick. They consist of CuAlO₂ with alternating rhombohedral and hexagonal structures. Solid‐state contact angle measurements were performed to derive values for the work of adhesion. The results show that the adhesion is twice as good for the annealed specimen compared to the as‐bonded sample.     

Characterization and microanalysis of interfacial reactions in metal–matrix composite systems

Understanding the solid-state reactions involved in metal/ceramic systems is important when combining the two types of materials into a composite. In this investigation, the solid-state reaction between Al₂O₃ (alumina) and a β-Ti alloy has been characterized by transmission electron microscopy (TEM), scanning electron microscopy, parallel-acquisition electron energy-loss spectroscopy and X-ray energy-dispersive spectroscopy. Two different systems were used to investigate this reaction. The first system utilizes a controlled reaction geometry and involved diffusion bonding single-crystal α-alumina and a β-Ti alloy. Here, three interfacial regions were found to form: a region of intermetallics (Ti₃Al and TiAl) located near the alumina interface, an α-Ti region, and a β-Ti region (rich in Mo, the β-phase stabilzer). Analysis of cross-section TEM samples of this reaction revealed the presence of both Ti₃Al and TiAl at the alumina interface. Orientation relationships between the intermetallics and the alumina are discussed. In the second, system, interfacial reactions inside metal–matrix composites that contain alumina and a β-Ti alloy were investigated. Here, different coatings used in the MMCs are investigated for their ability to prevent the reaction between the matrix and fibres. Reaction products inside the MMCs are compared with those found in the model reaction geometry.     

CF/Al复合丝的超声浸渗制备工艺与性能

探讨了超声液相浸渗法在制备碳（石墨）／铝复合丝过程中的适应性,通过选取适当的工艺参数和,包括纤维去胶及预热温度、铝液温度、浸渗速度和引入的超声能量等,可以得到复合质量较好、具有较高力学性能的复合丝。复合丝拉伸断口的扫描电镜（ＳＥＭ）观察可怕面有一定起伏,部分纤维拔出但长度较短,表明纤维与基体具有适当的界面结合。     

The microstructure of silicon carbide (Nicalon) fibre reinforced silicon carbide ceramic-matrix composites heat treated in vacuo and in pure oxygen

Silicon carbide (Nicalon) fibre reinforced SiC composites have been heat treated in vacuo and in pure oxygen environments at 1400°C for 100 h. The response of the microstructure and, in particular, of the interface between fibre, carbon interlayer and SiC matrix components has been studied. Microstructural modifications were observed by transmission electron microscopy, using imaging, electron energy-loss spectroscopy and electron diffraction techniques, and fibre stoichiometries were determined using a scanning Auger microprobe. Recrystallization of Nicalon fibres within composites heat treated in vacuo was found to result from decomposition of the metastable silicon oxy-carbide phase found in the fibres. No significant changes to the pyrolytic carbon interlayer were observed. Fibre recrystallization was considered to embrittle the composite. Samples heat treated in oxygen showed no appreciable fibre recrystallization. Study of interlayers in such aged samples often revealed decohesions, holes and narrow silica layers. In the most extreme cases, complete displacement of carbon by SiO₂ was found and such interfaces were identified as silica and α-cristobalite. Interfacial modifications were considered to be responsible for the retention of the small β-SiC grain size in Nicalon fibres and were also considered to be deleterious to the mechanical properties.     

Revealing local variations in nanoparticle size distributions in supported catalysts: a generic TEM specimen preparation method

The specimen preparation method is crucial for how much information can be gained from transmission electron microscopy (TEM) studies of supported nanoparticle catalysts. The aim of this work is to develop a method that allows for observation of size and location of nanoparticles deposited on a porous oxide support material. A bimetallic Pt‐Pd/Al₂O₃ catalyst in powder form was embedded in acrylic resin and lift‐out specimens were extracted using combined focused ion beam/scanning electron microscopy (FIB/SEM). These specimens allow for a cross‐section view across individual oxide support particles, including the unaltered near surface region of these particles. A site‐dependent size distribution of Pt‐Pd nanoparticles was revealed along the radial direction of the support particles by scanning transmission electron microscopy (STEM) imaging. The developed specimen preparation method enables obtaining information about the spatial distribution of nanoparticles in complex support structures which commonly is a challenge in heterogeneous catalysis.     

SiC_f/Ti-6Al-4V复合材料的界面反应及其影响

采用扫描电镜和透射电镜研究了SiCf/Ti-6Al-4V复合材料的界面反应,并借助纤维顶出试验和有限元方法,分析了界面反应层厚度对界面剪切强度和残余热应力的影响.结果表明:SiCf/Ti-6Al-4V复合材料界面反应产物主要为TiC,其又分为二层,晶粒较粗大的TiC层靠近基体钛合金,晶粒细小的TiC层靠近SiC表面的碳涂层;随着反应层厚度的增加,界面抗剪强度提高,纤维和反应层内的径向残余热应力有所增大,反应层内的环向残余热应力分布有所改变.