Structure of electrodeposited graded composite coatings of Ni–Al–Al2O3

The structure of electrodeposited composite coatings of Ni–Al–Al₂O₃, with Ni as the matrix and Al and Al₂O₃ as second-phase particles, was investigated using light microscopy and scanning electron microscopy. Ni coatings with no particles, which were used as reference samples, had progressively coarser structures with increasing current density. Co-deposition with Al resulted in refinement of the Ni matrix structure at high (>10 A dm⁻²) current densities. For single-particle baths, the co-deposition of Al₂O₃ was more strongly affected by current density and bath particle content than was the co-deposition of Al. However, for baths containing both Al and Al₂O₃ the amount of incorporated Al₂O₃ no longer depended on current density. With the choice of appropriate conditions, coatings of Ni with up to 39 vol.% Al₂O₃ were made. Similar experiments with Al yielded a maximum of 17.5 vol.% only. Uniform and graded mixed-particle coatings were also produced. When coatings containing Al were annealed, the reaction of the two elements resulted in the formation of either single-phase γ or two-phase γ–γ' alloys, in agreement with the equilibrium phase diagram.     

In situ observation of superdislocation motion in pre-strained Ni3Ge single crystals

Dislocation structures and their effect on the superdislocation motion in Ni₃Ge single crystals have been studied by two-step deformation. In these tests either octahedral or cube slips were induced by prestraining. A difference in the induced dislocation structure is found to cause a notable change in the second deformation step depending on the combination of the stress axes. In order to understand the orientation sensitive hardening, in-situ deformation experiments have been made on the prestrained specimens in a high voltage electron microscope. Besides observation of the structural change due to dislocation motion, electron irradiation, which decorates antiphase boundary tubes, is utilized to trace the history of the rapid dislocation motion. Based on these observations, the origin of the anomalous strengthening in Ni₃Ge is discussed with particular interest in the fine and non-planar dislocation structures induced by cross slips and dislocation–dislocation interactions.     

The effect of an Ni–Cr protective layer on cyclic oxidation of Ti3Al

The effect of an 80Ni?20Cr (at.%) metallic coating on the cyclic oxidation behaviour of a Ti₃Al‐based alloy with the composition Ti?25Al?11Nb (at.%) was investigated in this study. Cyclic oxidation tests were carried out in air at 600 °C and 900 °C for 120 h. For one cycle test, the specimens were held for 24 h at test temperature and then furnace‐cooled to room temperature. The oxidation rate was determined by plotting the mass gain per unit surface area of the specimen vs. exposure time. The morphology and composition of the oxidation products were characterized on the cross‐section of the specimens by scanning electron microscopy, energy‐dispersive X‐ray spectroscopy and atomic force microscopy. The oxidation scale forms during exposure at both 600 °C and 900 °C. TiO₂ is the main oxide component, whereas the Al₂O₃ layer appears only discontinuously. The remarkable improvement in oxidation resistance at 900 °C was attributed to the chemical composition and structure of the scale formed on the 80Ni?20Cr coating.     

TEM and HREM study of Al3Zr precipitates in an Al-Mg-Si-Zr alloy

The structure of Al₃Zr precipitates in Al‐1.0Mg‐0.6Si‐0.5Zr (in wt.%) alloy was investigated using conventional transmission electron microscopy (TEM) and high‐resolution TEM (HREM). After annealing of the alloy in the temperature range 450–540 °C, spherical precipitates of metastable L1₂‐Al₃Zr phase appeared nearly homogeneously within the matrix, and elongated particles were found at grain boundaries. L1₂‐structured Al₃Zr were about 20–30 nm in diameter and coherent with the matrix. Inside some of them, planar faults parallel to {100} planes were revealed by use of HREM. Most probably, these faults are an indication of the transition stage of transformation to the stable D0₂₃‐type Al₃Zr phase. The elongated precipitates (about 100 nm) were identified as D0₂₂‐type Al₃Zr. Energy‐dispersive X‐ray analysis showed that they contain, apart from Al, mainly Zr with small amounts of Si. The substitution of Al by Si increased the stability of the D0₂₂‐Al₃Zr as compared with D0₂₃‐Al₃Zr.     

A study of the initial growth of PtxCo(1−x) thin films on Si3N4

Pt _x Co_{(1− x )} thin films where x  =0.24 have been deposited onto Si₃N₄ windows and studied using transmission electron microscopy. The films are used in ultrahigh-density recording studies and it was found that the surface of the substrate had a strong influence on the microstructure, crystallography and magnetic properties of the film. An investigation of the early growth of the film was made by studying films of different thickness between 100 and 300 Å. It was found that the grains were hexagonal in structure with a strong c -axis orientation perpendicular to the surface.     

A study of charge distribution in Bi2Sr2CaCu2O8+δ superconductors using novel electron-diffraction and imaging techniques

We report our study of the distribution of valence electrons in Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductors using novel electron-diffraction and imaging techniques. The former method was based on quantitative analyses of the diffraction intensity of many reflections as a function of crystal thickness to determine, with an unprecedented accuracy, the Fourier components of the electron distribution in Bi₂Sr₂CaCu₂O_8+δ. The latter was based on examining the effect of charge transfer on many-beam imaging by comparing the observed and calculated low- and high-resolution images of long-period displacive and charge modulation of the cuprate. Our study demonstrates that fast electrons have greater sensitivity than X-rays to valence electrons distribution at small scattering angles, and that electron microscopy is a powerful tool in revealing charge distribution in materials.     

3D reconstruction of prior β grains in friction stir–processed Ti–6Al–4V

The prior β grain structure and orientations in the central stir zone of friction stir–processed Ti–6Al–4V were reconstructed from measured α phase orientations obtained by three‐dimensional serial sectioning in a dual‐beam focused ion beam scanning electron microscope. The data were processed to obtain the α colony and β grain size distributions in the volume. Several β grains were individually analysed to determine the total number of unique α variants and the respective volume fractions of each. The analysis revealed that some grains experienced overwhelming variant selection (i.e. one variant dominated) whereas other β grains contained a more evenly distributed mixture of all 12 variants.     

Near-field second harmonic imaging of the c/a/c/a polydomain structure of epitaxial PbZrxTi1–xO3 thin films

Near-field optical second harmonic microscopy has been applied to imaging of the c/a/c/a polydomain structure of epitaxial PbZr _x Ti_{1– x} O₃ thin films in the 0 <  x  < 0.4 range. Comparison of the near-field optical images and the results of atomic force microscopy and X-ray diffraction studies show that an optical resolution of the order of 100 nm is achieved. Symmetry properties of the near-field second harmonic signal allow us to obtain good optical contrast between the local second harmonic generation in c- and a-domains. Experimentally measured near-field second harmonic images have been compared with the results of theoretical calculations. Good agreement between theory and experiment is demonstrated.     

In situ observations of unusual dislocation mechanisms in the intermetallic alloy Ti3Al

In situ straining experiments in a transmission electron microscope have been carried out on a Ti₃Al intermetallic alloy, with the aim of determining the microscopic mechanisms controlling glide in prism, basal and pyramidal planes. Five different antiphase boundary energies have been measured and compared with the corresponding densities of incorrect first nearest neighbour atoms. The determination of a tension–compression asymmetry in pyramidal slip, and the detailed analysis of the complex microscopic mechanisms involved illustrate the efficiency of in situ experiments to solve complex problems in plasticity. A comparison between the properties of the different slip systems shows that they are controlled by different microscopic mechanisms, none of them being of covalent origin.     

Extended energy-loss fine structure spectroscopy of structural modifications in Nd2CuO4−δ at the oxygen K edge

The discovery of the superconducting electron-doped compound Nd₁₈₅Ce₀₁₅CuO_4?δ has stimulated great interest in its micro- and crystal structure, since the superconducting properties depend on parameters such as nonstoichiometry, phase composition, heat treatment and microstructure. The work presented herein is focused on the determination of the oxygen environment in the undoped parent compound Nd₂CuO₄ and in the structural modification Nd₂CuO₃₅ The analysis of the oxygen K (O 1s) edge extended electron energy-loss fine structure (EXELFS) of the tetragonal parent compound Nd₂CuO₄ and of the orthorhombic modification Nd₂CuO₃₅ is reported by using electron energy-loss spectroscopy in combination with transmission electron microscopy. Nd₂CuO₃₅ is produced by in situ heating and reduction of Nd₂CuO₄ in the transmission electron microscope. The EXELFS of the O 1s electron energy-loss edges is analysed with the classical extended X-ray absorption fine structure (EXAFS) treatment and compared with ab initio multiple scattering EXAFS calculations for both structural modifications. Highly accurate information on the local atomic environment of the oxygen atoms in Nd₂CuO₃₅ is obtained from EXELFS analysis using Nd₂CuO₄ as a standard. The results are in accordance with the structural data gained from X-ray diffraction analysis. This applies especially to the more complicated structure of Nd₂CuO₃₅ determined recently.     

Friction and wear of self-mated SiC and Si3N4 sliding in water

The friction and wear of self-mated SiC and Si₃N₄ with different initial roughness sliding in water were investigated with pin-on-disk apparatus at normal load of 5 N and sliding speed of 120 mm/s in ambient condition. It was found that, for self-mated Si₃N₄, the wear mechanism for surface smoothening to obtain low friction was tribochemical wear, but for self-mated SiC, it changed from mechanical wear into tribochemical wear with increasing sliding cycles. After running-in in water, self-mated Si₃N₄ exhibited lower steady-state friction coefficient than self-mated SiC did. For these two ceramics, initial and steady-state friction coefficients were hardly dependent on initial roughness. Initial roughness mainly affected the running-in period. The larger the initial roughness, the longer the running-in period, but the running-in period was much shorter for self-mated Si₃N₄ at each initial roughness than that for self-mated SiC.     

Interface study by dual-beam FIB-TEM in a pressureless infiltrated Al(Mg)–Al2O3 interpenetrating composite

This paper considers the microstructures of an Al(Mg)–Al₂O₃ interpenetrating composite produced by a pressureless infiltration technique. It is well known that the governing principle in pressureless infiltration in Al–Al₂O₃ system is the wettability between the molten metal and the ceramic phase; however, the infiltration mechanism is still not well understood. The objective of this research was to observe the metal–ceramic interface to understand the infiltration mechanism better. The composite was produced using an Al-8 wt% Mg alloy and 15% dense alumina foams at 915°C in a flowing N₂ atmosphere. After infiltration, the composite was characterized by a series of techniques. Thin-film samples, specifically produced across the Al(Mg)–Al₂O₃ interface, were prepared using a dual-beam focussed ion beam and subsequently observed using transmission electron microscopy. XRD scan analysis shows that Mg₃N₂ formed in the foam at the molten alloy–ceramic infiltration front, whereas transmission electron microscopy analysis revealed that fine AlN grains formed at the metal–ceramic interface and MgAl₂O₄ and MgSi₂ grains formed at specific points. It is concluded that it is the reactions between Al, Mg and the N₂ atmosphere that improve the wettability between molten Al and Al₂O₃ and induce spontaneous infiltration.     

Local analysis of the edge dislocation core in BaTiO3 thin film by STEM-EELS

The a <100> edge dislocation core formed in an epitaxial BaTiO₃ (BTO) thin film grown on a substrate was investigated by scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Elemental analysis using core-loss spectrum indicates that the atomic ratios of O/Ti and Ba/Ti are decreased at the dislocation core. The near-edge fine structure of the oxygen K-edge recorded from the dislocation core differs slightly from that of relaxed BTO region, which suggests that Ba-O bonding is decreased at the dislocation core. The structure of the dislocation core is discussed using a high-angle annular dark-field image and the electron energy-loss spectroscopy results.     

Quantification of the Ti oxidation state in BaTi1−xNbxO3 compounds

The Ti oxidation state of a series mixed-valence BaTi_1−xNb_xO₃ compound (where x=0.002, 0.004, 0.02, 0.10, 0.20 and 0.50) is investigated using high resolution electron energy loss spectroscopy (EELS). The energy loss near edge structure (ELNES) of the Ti-L_2,3 and O-K edges was recorded with high energy resolution. The fraction of Ti⁴⁺ and Ti³⁺ components is determined in each compound by linear profile fitting with Ti⁴⁺ and Ti³⁺ standard spectra obtained from reference compounds within the series. The fitting results indicate an increase in the fraction of the Ti³⁺ component as the Nb content increases. A deviation from the expected Ti³⁺ valence fraction based on the charge balance across the series was detected and discussed. By considering all detailed features on the spectra obtained with high energy resolution, this linear fitting method can be used to determine the oxidation state of transition metal oxides, especially for the early transition metals where conventional methods based on the L_2,3 edge ratio have shown to fail. The potential of this method to provide insight to mixed valence systems, vacancies and properties of oxides is discussed.     

PDMS/GO@Fe3O4海绵顶空固相萃取结合GC-MS分析薰衣草中挥发性成分

建立了一种基于PDMS/GO@Fe₃O₄海绵复合材料结合GC-MS的新型微波辅助顶空(HS)固相萃取技术,实现了对薰衣草中微量挥发性成分的快速提取及分析。成功制备了PDMS/GO@Fe₃O₄海绵复合材料。通过单因素实验法确定HS-PDMS/GO@Fe₃O₄的最佳实验参数,同时对该方法进行方法学考察。获得的最佳萃取条件为:GO@Fe₃O₄负载量2.4 mg,微波功率600 W,微波时间10 min,萃取溶剂为正己烷。薰衣草精油中6个代表性化合物(芳樟醇、萜品烯-4-醇、乙酸芳樟酯、乙酸薰衣草酯、石竹烯和石竹烯氧化物)的线性范围为7.5~120 ng,相关系数(R²)大于0.998 8,检出限(LOD)为0.14~0.33 ng,定量限(LOQ)为0.50~0.80 ng,相对标准偏差(RSD)小于6.36%。所建立的HS-PDMS/GO@Fe₃O₄方法具有较高的灵敏度,每次分析仅需要10 mg薰衣草样品。在最佳萃取条件下,分析2个采摘批次的18个薰衣草样品的挥发性成分,共鉴定出52个化合物,并采用主成分分析法(PCA)对不同采摘批次的薰衣草样品进行分析。结果表明,微波辅助HS-PDMS/GO@Fe₃O₄集萃取及富集过程一步完成,是一种快速、简单、灵敏的分析方法,适用于分析天然产物中的挥发性成分。     

Lubrication of Si3N4 and Al2O3 in water with and without addition of silane coupling agents in the range of 0.05–0.10 mol/l

To improve water lubrication of ceramics at a lower sliding velocity, the effect of the addition of silane coupling agents was investigated. Si₃N₄ and Al₂O₃ were slid against themselves in water with and without the addition of silane coupling agents in amounts ranging from 0.05 to 0.10 mol/l. Silane coupling agents containing one or more amino groups were effective in reducing the friction of Si₃N₄ and Al₂O₃ in water. Si₃N₄ also showed significant wear reduction but not Al₂O₃. However, the addition of a silane coupling agent containing an epoxy group increased both friction and wear of Si₃N₄. Improved lubricative characteristics of Si₃N₄ in water and in silane coupling agent solutions were obtained when Si₃N₄ contained smaller amounts of sintering additives. The adsorption behaviour of a silane coupling agent on ceramics was examined using both Fourier transform infrared spectroscopy and thin layer chromatography to clarify the interaction between the silane coupling agent and the ceramics. The role of polysiloxane film formation on ceramics is discussed to demonstrate the lubrication properties of ceramics.     

Lubrication at 750°C in a vacuum by a transfer film from self‐lubricating composites for roll/slide contact of Si3N4

Roll/slide friction tests were carried out at a temperature of 750°C in a vacuum. Disc specimens were made of Si₃N₄ with or without a sputtered MoS₂ film. A pin specimen was rubbed against one disc to supply a lubricating transfer film. With a pin made of an MoS₂‐based composite, the friction coefficient was around 0.3 and almost no wear of the discs was observed after 24 h of operation at a load of 50 N, a rotating speed of 0.5 m/s, and a slip ratio of 10%. Transferred patchy MoS₂ films were observed on the friction track. With a pin made of Ni‐based composite containing BN and graphite, the friction coefficient increased from 0.2 to 0.7 over a test time of about 8 h and severe disc wear was found. In an additional test using Si₃N₄ discs with a sputtered MoS₂ film without a pin, the friction coefficient was about 0.3, and no wear of the discs was found after 24 h of operation. The appearance of the friction track was similar to that in the test using the MoS₂‐based composite pin. It seems that the sputtered MoS₂ film wore, but wear particles reattached on the friction path to develop an effective lubricating film. These results demonstrate the effectiveness of transfer film lubrication for long‐term operation in a high‐temperature vacuum, and the superior ability of MoS₂ to develop an effective transfer film.