Development of a new molecular dynamics method for tribochemical reaction and its application to formation dynamics of MoS2 tribofilm

Recently we have developed a novel molecular dynamics program NEW-RYUDO-CR, which can deal with chemical reactions. The developed method has been applied to the study of tribochemical reaction dynamics of MoS₂ tribofilm on iron surface. The initially amorphous MoS₂ layer self-organized its structure as result of the tribochemical reactions and formed layered MoS₂ tribofilm. The friction coefficient significantly decreased as the MoS₂ tribofilm was formed. Besides, sliding was observed between sulfur layers of MoS₂ tribofilms which occurred due to repulsive Coulombic interaction forces between sulfur atoms. This indicates that the formation of the layered MoS₂ tribofilm is important to achieve better lubrication properties.     

The effect of oxidized surface layers on the inner core of fine iron particles

Mössbauer measurements on fine iron particles with the size ranging from 150 to 350 Å with oxidized surface layer are carried out between 81 and 300K. The spectra consist of two components, a component due to metallic iron and a second component with a larger hyperfine field. The thickness of the oxide surface layer calculated from the area ratio of the subspectra is about 19 Å, and is independent of the average diameter of the particles. A slight enhancement of the hyperfine field of the iron core with a decrease of particle size has been observed. It results from the contribution of the atoms at the interface layer and the dipole interaction between the particles. The dipole interaction decreases owing to the effects of the oxide surface layer on the dipole field.     

Study of the interface formed between poly(2-methoxy-5-(2′-ethyl-hexyloxyl)-p-phenylene vinylene) and indium tin oxide in top emission organic light emitting diodes

X-ray photoelectron spectroscopy (XPS) technique have been used to investigate the interface formed between poly(2-methoxy-5-(2′-ethyl-hexyloxyl)-p-phenylene vinylene) (MEH-PPV) and indium tin oxide (ITO) layer in top emission organic light emitting diodes. A weak but noticeable diffusion of indium into the polymer film was observed. Interactions between the diffused metallic atoms with the polymer resulted in the formation of carbon-metal complexes at the interface region. Compared to the ITO/MEH-PPV interface, the penetration of indium into the polymer layer was less important and may be explained by the surface morphology of the polymer film. It was however, a probable factor for fast degradation of devices using this structure.     

Improvement of hot-dip zinc coating by enriching the inner layers with iron oxide

The performance of hot-dip galvanic coating formed on steel not only depends on the alloy composition of the superficial layer but also significantly, on the composition of the inner alloy layers at the coating/substrate interface. Further, the presence of barrier oxide layers, if any can also improve the performance of galvanic coating. In the present work, the effect of inner iron oxide barrier layer formed prior to hot-dip galvanization was investigated. A continuous and adherent iron oxide layer was formed on steel by anodic oxidation of the steel substrate. Although the wettability of oxide surface by liquid zinc was initially poor, the increase in dipping time and the transition of the oxide layer to unstable form due to the presence of Cl⁻ ion in the flux facilitated localized growth of Fe-Zn alloy phases. The inhibitive nature of the oxide layer was temporary, since the presence of Cl⁻ induces micro cracks on the oxide surface thereby facilitating better zinc diffusion. The modification of the substrate structure during galvanization was found to influence the galvanizing process significantly. The present study predicts scope for application of this process for protection of rusted steel specimens too.     

Praseodymium silicide formation at the Pr2O3/Si interface

The interface and layer structure of praseodymium (Pr) oxide layers grown on Si(0 0 1) from a high-temperature effusion cell are studied using grazing incidence X-ray diffraction. Due to the interdiffusion of praseodymium and silicon atoms, Pr silicide forms in the layers. We find that Pr silicide is the favorable structure under oxygen deficient growth conditions in the Pr oxide layer. To avoid the silicidation, additional oxygen must be supplied. The formation of Pr silicide is suppressed for layers grown with an oxygen partial pressure of 10⁻⁷ mbar at a substrate temperature of 700 °C.     

How metallic Fe controls the composition of its native oxide

We have studied in situ the oxidation of ultrathin iron layers and monitored the chemical changes induced by subsequent deposition of Fe metal using hard x-ray absorption spectroscopy. The site sensitivity of the technique allows us to quantify the composition of the layer throughout the oxidation or deposition process. It is found that the thin native oxide incorporates a significant fraction of Fe atoms remaining in a metallic configuration even in the saturated state. Subsequent deposition of Fe leads to a complete reduction of the oxide that adopts an FeO-like structure containing Fe2+ sites only.     

Photoemission studies of initial oxidation for ultra-thin zinc film on 6H-SiC(0 0 0 1) surface with synchrotron radiation

The thermal oxidation process of metallic zinc on 6H-SiC(0 0 0 1) surface has been investigated by using atomic force microscopy (AFM), synchrotron radiation photoelectron spectroscopy (SRPES) and XPS methods. The AFM images characterize the surface morphology of ZnO film formed during the thermal oxidation and SRPES record the valence band, Si 2p and Zn 3d spectra at different stages. The O 1s peak is recorded by XPS because of the energy limit of the synchrotron radiation. Our results reveal that the silicon oxides layer of SiC substrate can be reduce by hot metallic zinc atom deposition. The oxygen atoms in the silicon oxides are captured by the zinc atoms to form ZnO_x at the initial stage and as a result, the oxidized SiC surface are deoxidized. After the zinc deposition with the final thickness of 2.5 nm, the sample is exposed in oxygen atmosphere and annealed at different temperatures. According to the evolution of peaks integrated intensities, it is considered that the Zn/SiC system will lose zinc atoms during the annealing in oxygen flux at high temperature due to the low evaporation temperature of pure zinc. After further annealing in oxygen flux at higher temperature, the substrate is also oxidized and finally the interface becomes a stable SiC-SiO_x-ZnO sandwich structure.     

Elastic strain at the interface in Ge clusters on Si substrate—a Raman spectroscopic measurement

Ge clusters are grown on Si substrate at room temperature (Ge-RT) and also at liquid nitrogen temperature (Ge-LNT) by cluster evaporation technique. These clusters show blue luminescence. Raman measurement demonstrates the increase in strain with annealing in diffused disordered Si at the interface between Ge-LNT clusters and Si substrate. This manifests in strain-relaxation in the clusters as observed by Photoluminescence (PL) measurements. The decrease in PL intensity for Ge-RT with annealing has been attributed to reduction in surface oxide species, which is supported by Raman spectroscopic measurements. The objective of the paper is to understand the effect of thermal annealing on both interfacial strain and interdiffusion of elemental Si at the interface, together with luminescence characteristics of the clusters.     

Effects of temperature gradient on the interface microstructure and diffusion of diffusion couples:Phase-field simulation

The temporal interface microstructures and diffusions in the diffusion couples with the mutual interactions of the temperature gradient, concentration difference and initial aging time of the alloys are studied by phase-field simulation, and the diffusion couples are produced by the initial aged spinodal alloys with different compositions. Temporal composition evolution and volume fraction of the separated phase indicate the element diffusion direction through the interface under the temperature gradient. The increased temperature gradient induces a wide single-phase region on two sides of the interface.The uphill diffusion proceeds through the interface, no matter whether the diffusion direction is up or down with respect to the temperature gradient. For an alloy with short initial aging time, phase transformation accompanying the interdiffusion results in the straight interface with the single-phase regions on both sides. Compared with the temperature gradient,composition difference of diffusion couple and initial aging time of the alloy show greater effects on diffusion and interface microstructure.     

Au/GaP接触体系界面特性的XPS分析

利用X射线光电子能谱研究了Au／GaP接触体系界面结构，组分随着热处理温度的变化.实验结果表明，即使在室温下金属和半导体原子间的扩散与迁移也会发生.Au/GaP接触退火时导致界面上GaP的分解并伴随着原子间快速扩散.当热处理温度升高，Au-Ga原子界面反应增强，从而生成复相结构Au-Ga金属间化合物.如Ga₂ Au，GaAu等.Au/GaP接触的界面是一个含有金、半导体原子的合金再生长层.从金属学的观点对界面的特性进行了讨论. 关键词：     

Epitaxial growth of bcc Fe films on Al(1 0 0) surfaces using Ti as an interface stabilizer

An approach is described to promote epitaxial growth of thin metal films on single-crystal metal substrates by stabilizing the interface with an extremely thin metallic interlayer. A single atomic layer of a metal is deposited at the interface, Ti on Al(1 0 0) in this case, prior to the growth of the metal film of interest to produce an epitaxial interface in a system that is otherwise characterized by interdiffusion and disorder. The stabilized interface reduces interdiffusion and serves as a template for ordered film growth. Using Rutherford backscattering and channeling techniques along with low-energy electron diffraction and low-energy He⁺ scattering, it is demonstrated that an atomically thin layer of Ti metal deposited at the Fe-Al interface, a system well known for considerable intermixing at room temperature, reduces interdiffusion and promotes the epitaxial growth of Fe films on the Al(1 0 0) surface. The decrease in ion scattering yield for Al atoms, Fe-Fe shadowing and long-range order of the surface suggest that the epitaxial growth of Fe is greatly improved when the Ti interlayer is introduced prior to Fe deposition. Off-normal ion channeling experiments provide clear evidence for the bcc structure of Fe on the Ti/Al(1 0 0) template with the measured average (1 0 0) interplanar distance of 1.44 Å in the Fe overlayer.     

多尺度模拟中网格守恒重映算法

针对耦合微观分子动力学(MD)和宏观有限元方法(FE)的多尺度模拟,提出一类新的基于贡献单元法的网格守恒重映算法.由于物理量是由有限元节点以及相应区域的原子信息通过积分重构得到的,对结构和非结构网格都能适用.对于未知量定义在顶点的情形,引入辅助网格.数值例子验证了算法的准确性和有效性.     

Analysis of the interface between a pulsed laser deposited calcium phosphate coating and a titanium alloy substrate

Calcium phosphate coatings deposited on titanium alloy are intended to add a bioactive surface to medical implants. This work presents the characterisation of the interface between Ti-6Al-4V and a crystalline calcium phosphate coating obtained by pulsed laser deposition, with a KrF excimer laser, at 575 °C and under a 45 Pa water-vapour atmosphere. The coating–substrate system was studied by secondary-ion mass spectrometry, scanning electron microscopy, X-ray diffractometry, Raman spectroscopy and X-ray photoelectron spectroscopy. The results show that the deposition process promotes the interdiffusion of substrate elements into the coating and coating elements into the substrate oxide layer. Thus, a graded layer of mixed calcium phosphate and amorphous titanium oxide is formed. For the substrate, a hydroxyapatite coating acts more as a barrier for oxygen incoming from a gas than as an oxygen source during deposition. Moreover, oxygen diffusion into the substrate occurs. Thus, the content of oxygen of this oxide layer diminishes with depth. When the oxygen concentration is low enough it is incorporated in solid solution in the titanium alloy . PACS 81.15.Fg; 68.55.-a; 87.68.+z     

High-efficiency preparation of oil-dispersible MoS<Subscript>2</Subscript> nanosheets with superior anti-wear property in ultralow concentration

Molybdenum disulfide (MoS₂) nanosheets are a promising lubricant additive for enhanced engine efficiency in cars. However, high-cost production methods and poor dispersion have limited their application in industry. In this study, the ball milling process is demonstrated as a low-cost and high-efficient method for fabrication of oil-dispersible MoS₂ nanosheet, and the ball milling parameters are optimized. Moreover, the lubrication effectiveness of ball-milled MoS₂ nanosheet was also evaluated. Results indicated that well-dispersed MoS₂ nanosheets with a size of 250 nm can be manufactured with optimized surfactants of zinc dialkyldithiphosphates (ZDDP) and polyisobutylene succinimide (PIBS) after being ball milled for 36 h. Tribological results revealed that a friction coefficient of white oil with 0.25% MoS₂ nanosheets reached 0.075, much lower than that of lubricant without nanosheets (0.16). The wear scar radius of 0.015% MoS₂ nanosheets was similar with that of Hertz contact, and the wear scar radius reduction reached 20% compared with that of 1% ZDDP. In addition, EDS and XPS results indicated the formation of a MoS₂ and FeS tribofilm on the wear surface.     

ESCA-Studies of the structure and composition of the passive film formed on stainless steels by various immersion temperatures in 0.1 M NaCl solution

This paper deals with the structure and composition of the passive film formed on the surface of two commercial stainless steels during immersion in 0.1 M NaCl for 0.5 h at 25, 60, and 90°C. The data shows that the passive films formed at 60 and 90°C by short-time solution exposure are very similar to these formed at 25°C, and are consistent with the three-factor model described previously: a hydrated layer in contact with the solution, an oxide layer consisting of iron and chromium oxides having maxima at depths of 3 and 10 Å, respectively, and a metallic layer enriched in nickel. There is a smooth transition between the layers, with the thickness of the outer two layers being about 15 Å.     

Protection against corrosion of iron alloys by aluminized coatings produced using two different processes

Aluminized coatings were produced on iron by means of two different processes: electron beam deposition under UHV of Al on iron samples previously covered with ⁵⁷Fe films, and hot-dipping of iron samples in molten aluminium. Aluminized samples were submitted to thermal treatments in order to promote interdiffusion at the Fe–Al interface and favour the formation of Fe–Al intermetallic compounds of composition suitable to protect the underlying iron from oxidation. Phase composition, structure and morphology of both as deposited and thermally treated coatings were characterized by means of X-ray diffraction, Mössbauer spectroscopy and metallographic techniques. Significant differences among the effects of the Fe–Al interdiffusion occurring for Al layers produced with the two processes are pointed out and discussed.     

Difference between chemical structures of the interface at the Al-oxide tunneling barrier prepared by plasma or by radical oxidation

We have studied chemical structures of the interface between the Al-oxide tunneling barrier and the underlying Co₉₀Fe₁₀ layer in magnetic tunnel junctions when a 1-nm thick metallic Al barrier was oxidized by two different methods: plasma oxidation and radical oxidation. Our chemical analyses confirmed that the underlying CoFe layer was unavoidably attacked by oxygen during the oxidation and that this left different oxide states at the AlO_x/CoFe interface, depending on the oxidation method. The radical oxidation required long oxidation time for optimizing tunneling performance and resulted in a large amount of oxygen at the interface, which, in turn, resulted in the formation of mostly α-Fe₂O₃ and Al₂O₃. Conversely, the plasma oxidation required a relatively short oxidation time for optimization and left FeO as a dominant phase at the interface. Our results also show that the thermal treatment helped AlO_x, an oxygen-deficient phase, to be re-oxidized and transformed into Al₂O₃, the thermodynamically stable stoichiometric phase. The oxygen that diffused from the reduced CoFe layer into the barrier is likely responsible for this oxygen enrichment. We show that such differences in the chemical structure of the interface are critical clues to understanding what causes the change in tunneling properties of magnetic tunnel junctions.     

W/Fe/CuCrZr热等静压模块组织及性能分析

为实现面向等离子体材料钨(W)和热沉材料铜铬镐(CuCrZr)合金的可靠连接,对纯铁(Fe)作为W/CuCrZr合金热等静压连接中间层的可行性进行了探索性研究。在850℃/150MPa/135min的热等静压参数下制作了W/Fe/CuCrZr合金的实验模块,分别对连接界面进行了焊接界面、微观形貌、组织成分及剪切力学性能的检测分析。检测结果显示,两连接界面无明显缺陷,均发生了约0.5μm的短距离互扩散,且均未生成明显的反应相,连接界面呈现出较好的塑性,剪切强度大于186.5MPa。因此,纯铁作为中间层实现了W/CuCrZr合金的可靠连接。用ANSYS Workbench平台对模块进行的高热负荷下的瞬态传热模拟表明W/Fe/CuCrZr合金模块最高可承受16MW·m^-2的热负荷。     

Interaction of iron atoms with the oxidized silicon surface

The room-temperature interaction of iron atoms with the oxidized Si(100)2×1 surface at a coverage from a submonolayer to four monolayers is studied by core-level photoelectron spectroscopy using synchrotron radiation. Computer simulation of the Si 2p core electron spectra demonstrates that iron atoms penetrate beneath the silicon oxide even at room temperature. This process causes the initial silicon phases at the SiO_x/Si interface to disappear; gives rise to a complex ternary phase involving Fe, O, and Si atoms; and favors the formation of a Fe-Si solid solution at the interface.     

Iron oxide thin film growth on Al2O3/NiAl(1 1 0)

The electronic structure and the growth morphology of iron oxide thin films were studied by means of Synchrotron Radiation Photoelectron Spectroscopy (SRPES) and Low Energy Electron Diffraction (LEED). A thin well-ordered alumina film on a NiAl(1 1 0) single crystal surface as a template for iron oxide growth was employed. Two different methods of iron oxide film preparation were applied. In the first attempt, iron deposited at room temperature was subsequently annealed in oxygen. Even though a whole layer of iron was oxidized, an expected long-range order was not achieved. The second attempt was to perform reactive deposition. For this reason iron was evaporated in oxygen ambient at elevated substrate temperature. This method turned out to be more efficient. Diffused but clear LEED patterns of six-fold symmetry indicating hexagonal surface atoms arrangement were observed. From the PES measurements, binding energies for Fe2p for grown iron oxide film were established as well as energy distribution curves for the valence band. Growth curves based on Fe3p core-level peak intensities for iron and iron oxide were plotted identifying type of film growth for both deposition methods. Based upon these results we have found evidence for interdiffusion in the interface between alumina and iron oxide at the early stages of growth. Further deposition led to formation of Fe₃O₄(1 1 1) (magnetite) overlayer. Moreover, the quality of the film could also be improved by long-time annealing at temperatures not exceeding 575 K. Higher annealing temperature caused disappearance of LEED pattern indicating loss of long-range ordering.