Three-dimensional studies of siC polytype transformations

Scanning electron microscopy technique in colour cathodoluminescence mode is proposed for two-dimensional (2-D) and three-dimensional (3-D) studies of polytype structures of SiC epitaxial layers grown by the sublimation sandwich method. It allows us to perform layer-by-layer structure analysis of a polytype on depths up to 2.5 μm with high resolution. An effect of polytype instability under growth on singular substrate (0001) is revealed. It leads to the fact that the grown SiC layer represents the set of alternating sublayers of different polytypes such as 4H, 6H, and 3C. Some mechanisms of polytype syntaxy in SiC are discussed.     

Polytype transformations in SiC-epitaxial layers: The color cathodoluminescence-sem studies

The polytype transformation in SiC-epitaxial layers grown by the sublimation “sandwich method” have been studied by means of a real color SEM cathodoluminescence (CL) technique. The variation of polytypes grown on 6H-, 4H-, 15R-, and 21R-polytype substrates is demonstrated. Different spectral composition of local CL emission is observed on smooth and profiled substrate surfaces. The double polytype transformation effect was fixed. The nature of the polytype transformation in SiC epitaxial layers is discussed.     

Molecular beam epitaxy of BaF<Subscript>2</Subscript>/CaF<Subscript>2</Subscript> buffer layers on the Si(100) substrate for monolithic photoreceivers

This paper describes the study of the surface morphology of BaF2 epitaxial films grown by means of molecular beam epitaxy in various growth regimes on a CaF₂/Si(100) surface, which is performed by means of atomic force microscopy. The CaF₂ layers were obtained on a Si(100) substrate in a low-temperature growth regime (T _s = 500 °C). The technological regimes of growth of BaF₂ continuous films with a smooth surface on CaF₂/Si(100), suitable as buffer layers for the subsequent growth of PbSnTe layers or other semiconductors, such as A₄B₆, and solid solutions based on them.     

Microstructure,chemical composition,wear, and corrosion resistance of FeB–Fe<Subscript>2</Subscript>B–Fe<Subscript>3</Subscript>B surface layers produced on Vanadis-6 steel using CO<Subscript>2</Subscript> laser

The paper presents the study results of laser modification of FeB–Fe₂B surface layers produced on Vanadis-6 steel using pack cementation method. Microstructure, x-ray phase analysis, chemical composition study using wave dispersive spectrometry method, microhardness, corrosion resistance as well as surface condition, roughness, and wear resistance were investigated. The diffusion boronizing processes were performed at 900 °C for 5 h in the EKabor® powder mixture. The boronized layers had a dual-phase microstructure composed of two types of iron borides, FeB and Fe₂B, and their microhardness ranged from 1800 to 1400 HV. The laser surface modification was carried out on specimens after diffusion boronizing process using CO₂ laser with a nominal power of 2600 W. Laser beam power used in this experiment was equal to 1040 W and was constant. While the three values of scanning speed were used: 19, 48, and 75 mm/s. During laser modification, the multiple tracks were made where distance between of axis tracks was equal to 0.5 mm. As a result of this process, microstructure consisted of remelted zone, heat-affected zone, and substrate was obtained. In remelted zone, the boron-martensite eutectic was observed. Boronized layers after laser modification were characterized by the mild gradient of microhardness from surface to the substrate and their value was dependent on the scanning speed used and was between 1700 and 1100 HV. Corrosion resistance tests revealed reducing the current of corrosion in case of laser modification process. Wear resistance of laser modified specimens was improved in comparison to diffusion boronized layers.     

Intermediate transformation structures in silicon carbide

In order to examine the detailed structural origins of the confusing diffraction patterns often exhibited by partially transformed polytypic materials, HREM lattice imaging techniques have been used to examine directly the layer-by-layer stacking sequences in a range of samples of cubic (β) SiC partially transformed to α polytypes by high temperature annealing. A number of intermediate structures have been characterized, one of which provides the first direct evidence supporting an earlier model of the β→α transformation (Jagodzinski, 1971) in which three-layer twins, separated by multiples of three layers, propagate through the cubic structure to produce a faulted haxagonal (6H) polytype. These observations, together with considerations of twin geometry, also suggest a means of co-ordinating the sideways nucleation of 6H unit-cell high steps, whose lengthways growth has been previously observed to effect the solid-state β→α transformation (Jepps & Page, 1979).     

Nanotribology of Si oxide layers on Si by atomic force microscopy

Atomic force microscopy (AFM) has been used for tribological studies of Si surfaces covered by oxide layers of various kinds: chemical oxides prepared by the SC1 (NH4OH/H2O2/H2O) and the SC2 (HCl/H2O2/H2O) treatments and a thermal oxide. In the case of the SC1 chemical oxide, the oxide layer was scratched and the underlying Si substrate was ploughed by the Si3N4 AFM tip. On the other hand, no wear of the sample was noted on the other surfaces: the AFM often produced elevated patterns in the shape of the scanned area, which were no longer visible after HF etching. By annealing the SC1-treated surface in N2 gas at above 200 degrees C for 30 min, the oxide layer could not be scratched any more. By soaking the thermal oxide in KOH, the oxide layer was then scratched. It is concluded that the presence of OH bases is the necessary condition for the nano-scratching of the oxide layers.     

A high-resolution electron microscopic study of defects in sodium β′″-alumina

A high-resolution electron-microscopic study of sodium β′″-alumina, a polytype of the more-widely-studied sodium β- and β″-alumina, has been undertaken using the 600 kV instrument at Cambridge University. Images revealed the loss of sodium-containing planes, which had caused crystals to collapse and shear into defect layers. A model for the structure of these defects is proposed, based on the use of computed images and by comparison with high-resolution images of silver β″-alumina.     

Microstructure characterization of advanced protective Cr/CrN+a‐C:H/a‐C:H:Cr multilayer coatings on carbon fibre composite (CFC)

Studies of advanced protective chromium‐based coatings on the carbon fibre composite (CFC) were performed. Multidisciplinary examinations were carried out comprising: microstructure transmission electron microscopy (TEM, HREM) studies, micromechanical analysis and wear resistance. Coatings were prepared using a magnetron sputtering technique with application of high‐purity chromium and carbon (graphite) targets deposited on the CFC substrate. Selection of the CFC for surface modification in respect to irregularities on the surface making the CFC surface more smooth was performed. Deposited coatings consisted of two parts. The inner part was responsible for the residual stress compensation and cracking initiation as well as resistance at elevated temperatures occurring namely during surgical tools sterilization process. The outer part was responsible for wear resistance properties and biocompatibility. Experimental studies revealed that irregularities on the substrate surface had a negative influence on the crystallites growth direction. Chromium implanted into the a‐C:H structure reacted with carbon forming the cubic nanocrystal chromium carbides of the Cr₂₃C₆ type. The cracking was initiated at the coating/substrate interface and the energy of brittle cracking was reduced because of the plastic deformation at each Cr interlayer interface. The wear mechanism and cracking process was described in micro‐ and nanoscale by means of transmission electron microscope studies. Examined materials of coated CFC type would find applications in advanced surgical tools.     

A method for preparing cross sections of films on wear surfaces for transmission electron microscopy study

A method for preparing cross sections of surface layers which exist on bulk metal substrates for transmission electron microscopy (TEM) study is described. The surface layer or film is protected by a vacuum-deposited or sputtered coating of a suitable metal. A mask is placed over the surface and non-masked areas are subjected to ion beam etching until the substrate is exposed. A thick electroplated layer is then applied to the surface. This layer adheres well to the ion-etched substrate and seals the coated surface film against damage during the usual slicing and grinding steps which are required for the preparation from bulk materials of thin foils for TEM study. The method was developed specifically for the analysis of boundary and extreme pressure lubrication films on wear surfaces together with the near-surface region of the substrate. However, it is also applicable to the investigation of oxide, corrosion and other surface films.     

弧比变化对电火花沉积Ni201修复层界面行为的影响

再制造修复设备工艺参数选择是决定修复层质量的关键因素。利用电火花沉积技术,在Q235钢表面制备Ni201修复改性层,对比分析弧比取不同值时对修复层质量的影响,并利用扫描电镜(SEM)、能谱仪(EDS)、X射线衍射(XRD)等检测方法,研究了修复层与基体结合界面的微观结构、元素分布及相组成。结果表明：弧比取值在35%～45%之间时,修复层组织均匀致密,基体元素与修复层元素相互扩散形成冶金结合,修复层主要由Fe19Ni、Co3Fe7、Fe0.9Si0.1和C0.055Fe1.945相组成。     

梯度覆层体受粗糙面滑动作用时的弹塑性分析

运用大限元软件分别虽对单覆层体及梯度体受多微凸体粗糙面滑动使用时产生的应力在变进行了计算和研究，着重比较两种覆层体爱相同表面载荷作用下，在基体及表面出现部分塑性变形时，表面层、基体内及表面 基体界面处的应力、应变分北度层在防止其本产生塑性变形及改善界面应力等方面比单纯层体具有明显的优点。本文的研究结果表为表南 选一览表主加工提供参考。     

HgCdTe structures for dual-band photodetectors operating in the 3–5 and 8–12 µm spectral ranges

A bilayer cadmium-mercury-tellurium (CMT) heterostructure was designed consisting of photosensitive layers of compositions x _CdTe = 0.29–0.32 and x _CdTe = 0.220–0.230, sensitive in the spectral ranges of 3–5 and 8–12 µm, a barrier layer between them, and wide-band variable-gap layers on the heterojunction and the surface grown on a GaAs substrate with ZnTe and CdTe buffer layers. The molecular beam epitaxial (MBE) growth of the heteroepitaxial structure (HES) was controlled by real time ellipsometry. After the growth, the composition distribution throughout the thickness was measured by reflection spectra with layer-by-layer chemical etching. There is good agreement between the results of composition measurements using ellipsometry and reflection spectra. P-type conductivity of bilayer MBE CMT HESs was obtained after thermal annealing at 220–240 °C in an inert gas (helium) for 24 h. The concentration of holes in the photosensitive layers is (4–10) ·10¹⁵ cm^?3 and (8–20) · 10¹⁵ cm^?3 at 78 K.     

Reverse current in <Emphasis Type="Italic">p-n</Emphasis> junctions with a field electrode on heteroepitaxial CdHgTe/GaAs structures

Results of measuring the current-voltage characteristics of diodes with a control electrode are presented. The diodes are based on graded energy-gap Cd_0.22Hg_0.78Te (MCT) layers grown by molecular beam epitaxy (MBE) on a semi-insulating GaAs substrate. The diodes are designed for the IR radiation photodetectors with the cutoff wavelength λ_c = 10 μm. It is shown that the surface currents contribute substantially to the reverse currents of MBE MCT photodiodes during enrichment and depletion. A critical built-in charge density value for which the surface leakage level is under 20% of bulk current is obtained.     

低稳恒磁场对锡锌基合金液与铜片界面反应的影响

利用SEM、XRD等方法研究了0.1 T低稳恒磁场对锡锌合金液/铜片和锡锌铜舍金液/铜片的界面反应过程中金属间化合物(IMC)层的生长、晶体取向以及形貌的影响.结果表明:界面IMC层的厚度随反应时间的延长而增加;在0.1 T磁场下,锡锌合金液/铜片和锡锌铜合金液/铜片的界面IMC层的生长均受到抑制;同时磁场抑制了IMC晶粒的粗化,使IMC颗粒更加细小致密;0.1 T磁场抑制了锡锌合金液/铜片界面处Cu5Zn8(330)的取向度,但对锡锌铜合金液/铜片的界面晶粒取向度影响并不很明显.     

Characterizing inorganic crystals grown on organic self‐assembled bilayers with scanning probe and electron microscopies

Combined microscopy techniques are used to establish the usability of phosphonic acid layers as promoters of hydroxyapatite (HAp) growth. Using spread coating, octadecylphosphonic acid (OPA) self‐assembled bilayers are delivered to the thin natural oxide layer of a titanium film surface with no prior treatment. These bilayers aggregate two major advantages of phosphonic moieties to titanium surfaces: nucleation of hydroxyapatite crystals from ionic solution and affinity for both titanium oxide surface and HAp crystals. The functionalized substrates and bare titanium (control) samples are immersed in an aqueous solution containing calcium and phosphorus ions. Over a 4‐week immersion time, OPA‐functionalized substrates present numerous large agglomerates of inorganic crystals, in contrast to control samples, with no significant amount of deposits. Initial sample characterization was performed with atomic force microscopy (AFM). Compositional and structural characterization of these agglomerates (using TEM, EDS, and electron diffraction), revealed that they are indeed HAp, the main component of the inorganic bone matrix. Microsc. Res. Tech. 76:1278–1283, 2013. © 2013 Wiley Periodicals, Inc.     

The ELF®-97 phosphatase substrate provides a sensitive, photostable method for labelling cytological targets

We compared fluorescent signals obtained with fluorescein conjugates and the ELF‐97 (enzyme‐labelled fluorescence) phosphatase substrate [2‐(5′‐chloro‐2‐phosphoryloxyphenyl)‐6‐chloro‐4(3H)‐quinazolinone] in labelling cytological structures requiring high spatial resolution. Enzymatic cleavage of the ELF‐97 phosphatase substrate yields an extremely fine precipitate that remains well localized to the site of enzymatic activity. This precipitate fluoresces bright yellow‐green, with maximal excitation at ~360 nm and maximal emission at ~530 nm. The ELF substrate was used with streptavidin–alkaline phosphatase, to fluorescently label site‐specific probes bound to their targets, including cell‐surface sites, cytoplasmic organelles, nuclear antigens and cytoskeletal networks. All targets were labelled successfully with both the ELF substrate and fluoresceinated probes or protein conjugates. However, the ELF method was frequently more sensitive, with lower background fluorescence, allowing detection of more lysosomes, actin filaments, microtubules and nuclear targets than were visible with corresponding fluoresceinated probes. The ELF substrate was also used with antifluorescein–alkaline phosphatase to amplify fluorescein signals. We found that the ELF signal was in all cases brighter and more photostable than fluorescein signals, permitting shorter film exposures and allowing more time for examining samples. Surprisingly, relative brightness and photostability depended on the target, rather than being a general phenomenon related to the choice of dye alone.     

Surface stresses in coated steel surfaces—influence of a bond layer on surface fracture

Thin hard coatings in the thickness range of only a few micrometers deposited by physical vapour deposition (PVD) on components or tools can improve the friction and wear properties by several orders of magnitude. A 2 μm thick TiN (E=300 GPa) coating on a high-speed steel substrate with a bond layer at the interface between the coating and the substrate was modelled by micro-level three-dimensional finite-element method (3D FEM) in order to optimise a coated surface with regard to coating fracture. Both compliant low modulus (E=100 GPa) and stiff high modulus (E=500 GPa) bond layers at the coating/substrate interface of 200 and 500 nm thickness were investigated. First principal stresses were simulated for scratch test geometry in the load range of 7.5-15 N. Very high stress concentrations of above 5700 MPa tensile stresses were observed in the bond layer just behind the contact zone for the stiffer bond layer. The stiff bond layer generated 5 times higher tensile stress maxima compared to the compliant bond layer. There was approximately 3.5 times larger strain in the compliant bond layer compared to the stiff bond layer. The general coating design advice based on this exercise is that when a bond layer is used e.g. for coating/substrate adhesion improvement should the bond layer be less stiff than the coating not to generate high and critical tensile stresses. The thickness of the bond layer may vary and is not critical with respect to generated stresses in the surface.     

Evaluation of morphological and tribological properties of nanocrystalline nitrocarburised coating produced on AISI H13 carbon steel by pulsed plasma electrolytic saturation (PPES) technique

Abstract

The formation of superficial protective layers produced by pulsed plasma electrolytic saturation techniques appears to offer attractive properties in terms of wear resistance. In order to intensify the beneficial characteristics, a profound understanding of how they are affected by the processing parameters and the obtained nanocrystalline microstructure is required. In the present study, a nitrocarburised coating was produced on AISI H13 steel, and it was characterised using scanning electron microscopy and X-ray diffraction. The tribological properties of the produced coatings were evaluated using the pin on disc method. The results showed that the surface characteristics of the produced layer strongly depend on how the pulse related parameters have been applied. It was also realised that the obtained tribological properties were affected by the morphological characteristics of the produced coatings. The layers on the substrate were in the form of spherical and compact nanocrystalline nitrocarburised coatings.     

纳米碳管对MWPCVD过程增强金刚石形核的影响

利用石英钟罩式微波等离子体化学气相沉积（ＭＷＰＣＶＤ）实验装置,研究了硅基体表面沉积金刚石薄膜时纳米碳管对金刚石形核过程的影响。扫描电子显微镜（ＳＥＭ）形貌分析结果显示,纳米碳管处理能够促进金刚石形核。对非研磨基体而言,这是一种有效的增强金刚石形核的表面预处理方式。     

Effects of liquid aluminium on hot-work tool steel

The effects of liquid aluminium on hot-work tool steel H13 have been studied using specimens in three different surface conditions: rough, polished and oxidized. The specimens were rotated around their own axes in molten aluminium at different temperatures and for different exposure times. Three intermetallic phases of Fe---Al which formed on the surface of the steel when submerged in liquid aluminium were identified. These were shown to grow as continuous layers or as cone-shaped dendrites, depending on the surface condition of the steel. The growth of these layers/cones was related to time and temperature. The formation of cones and their subsequent breaking is assumed to be an important factor in the erosion of die-casting dies.