Top Surface Layer Research Articles

Fabrication of composite modified layer on aluminium alloy by surface mechanical nano-alloying combined with nitriding

A composite modified layer was fabricated on 6061 aluminium alloy by means of surface mechanical nano-alloying treatment (SMNAT) combined with nitriding treatment. The microstructure, mechanical properties and interfacial bond of composite modified layer were investigated systematically. The experimental results showed that a Fe coating with thickness of 40–65 μm could be fabricated on the surface of 6061 aluminium alloy by means of SMNAT for 1 h. The mean grain size of top surface layer in Fe coating is about 15 nm. After nitriding, a composite modified layer was generated and its microstructure is depended on the nitriding temperature. The phase structure of modified layer on samples after nitriding at 450 °C, 550 °C and 640 °C is Fe2-3N/Fe(N), Fe2-3N/Fe(N) and Fe2-3N/Fe2Al5, respectively. The properties measurements showed that the surface hardness of 6061 aluminium alloy was greatly enhanced by SMNAT combined with nitriding treatment. The hardness increased with increase of nitriding temperature. Additionally, the mechanical properties match and interfacial bond strength of layer-substrate on the N450 and N550 samples is better than those on the N640 sample. This paper offers a novel method to prepare a gradient modified layer on aluminium alloy for improving surface properties.

Enhancement of surface integrity by cryogenic diamond burnishing toward the improved functional performance of the components

17-4 precipitation-hardenable (PH) stainless steel is one of the widely used materials in various applications of engineering practices owing to their excellent corrosion resistance and high strength. The components such as automotive body, aerospace compressor blades, turbine blades and molds demand higher load carrying capacity and improved fatigue strength, which is possible to achieve by surface severe plastic deformation. Diamond burnishing process is an appropriate technique to produce such components which improves the surface integrity characteristics of the material. This article presents a comprehensive examination of the surface integrity of cryogenic diamond burnished 17-4 PH stainless steel using a novel diamond burnishing tool. The impact of diamond burnishing control factors on subsurface microhardness, surface roughness, surface hardness, surface topography, residual stress and surface morphology has been analyzed. The optimal control factor setting ensures the least surface roughness of 0.03 µm by the application of one factor at a time approach. Cryogenic diamond burnished surface achieves the exceptional surface finish and the surface hardness in tool-tip of 8 mm and 6 mm, respectively. The maximum surface hardness of 413 HV was attained using 6-mm tool-tip diameter. The subsurface microhardness improvement of 2% and 4% has been observed while using a tool-tip diameter of 6 mm in contrast to 8 mm and 10 mm. Compressive residual stresses have been generated at the top surface layer of the specimen. The attained experimental results prove that cryogenic diamond burnishing can be successfully applied to 17-4 PH stainless steel to enhance its surface integrity characteristics.

Grain refinement mechanism of nickel-based superalloy by severe plastic deformation - Mechanical machining case

This paper studied the formation mechanism of white layer of a next generation nickel-based superalloy formed under severe plastic deformation induced by a mechanical material removal process. A graded microstructure of the white layer in the nickel-based superalloy has been revealed for the first time, which is composed of (i) a “dynamic recrystallisation” layer formed by nanocrystalline (∼200 nm) grains at the vicinity of the surface and (ii) a “dynamic recovery” layer with subgrain microstructures extending further into the subsurface. The mechanism of surface grain refinement was identified based on the results obtained via crystallographic and chemical analysis, as well as in-situ micro-mechanics experiments in the scanning electron microscope. It is found that in the top surface layer not only grain refinement but also the γ′ phase dissolution occurs, changing drastically from the bulk material. Furthermore, it is shown how the high plastic strain and cutting temperature along the subsurface causes grain refinement in the white layer and grain elongation in the subsurface. The γ′ precipitates in the recrystallisation layer are dissolved during the machining process, while the ultra-high cooling rate suppresses the further precipitation of this phase, resulting in the supersaturation of γ grains or minimized γ′ precipitates in the top surface layer. Hence, the grain refinement does not result in an increase of mechanical stiffness but a deterioration of mechanical properties due to the dissolution of the strengthening phase γ’, which leads to a lower strength and increased ductility. Machining is generally treated as a cold-working process. However, according to our findings hot-working with dynamic recrystallisation and recovery, as well as phase evolution, occurs in the white layer of nickel-based superalloys.

(Invited) Creating High-Performance Pt-Based ORR Catalysts through Surface Engineering

Pt-based nanocatalysts have played important roles in PEMFC due to their high activity and chemical stability. Owing to the large surface to volume ratio at nanoscale slight changes in the surface structure of catalytic materials can have large impacts on the catalytic stability and activity. The composition, facet, and the topology of the top surface layers of a nanocatalyst eventually determine its performance in catalytic reactions. In this presentation, I will share our recent efforts on designing the surface compositions, structures and the overall morphology of noble metal catalysts to improve both catalytic activity and stability of nanocatalysts.

Effect of Polymer Capping Layers on Initial Fracture Mechanism of Silicon Anodes

Using silicon nanomaterials is a widely accepted strategy for mitigating the extreme volume expansion associated with the lithiation of silicon and the accompanying capacity fade. However, the high surface area of silicon nanomaterials provides more opportunity for electrolyte decomposition which leads to high irreversible capacity loss and low Coulombic efficiency. Silicon microparticles area available in large scale at a low cost and offer an alternative with lower surface area that have been shown to have high initial Coulombic efficiency. However, to make silicon microparticles viable, the drastic volume change and high stress of these larger particles must be mitigated. In order to develop a more thorough understanding of the initial lithiation mechanics of the silicon crystalline microparticles, this work studies the initial lithiation mechanism through model crystalline electrodes. We have discovered that the thickness of the crystalline silicon wafer itself affected the initial crack formation mechanism. The next step for this study was to modify the mechanical properties of the top surface layer of the crystalline silicon substrates by adding a polymer capping layer and observe how these polymer layers affect the electrochemically induced fractures in crystalline silicon anodes through TEM, SEM and ellipsometry analysis. Polymer films of typical binder materials such as PVDF, PAA and CMC as well as novel modified PAA polymer binders were deposited on crystalline silicon substrates and lithiated to the same extent as an uncoated silicon anode to directly compare crack formation under the same level of silicon lithiation. The results of this work provide important analysis targeted at the initial crack formation of silicon and how polymer capping layers affect the lithiation depth and cracking formation of crystalline silicon. These findings may be applied to create a viable capping layer to reduce cracking in larger, more economical, silicon particles. Figure 1

Influence of Shot Peening Treatment in Erosion Wear Behavior of High Chromium White Cast Iron

High alloy white cast irons (WCI) play an important role in many industrial fields such as mining, cement industry, or grinding due to their high hardness and wear resistance. In all these processes, white cast iron components must work under erosion and abrasion conditions. Many investigations have been carried out with the aim of improving the mechanical properties of this type of alloys. Wear resistance depends on the mechanical properties, mainly hardness. Thus, the WCI are typically heat treated in order to modify its microstructure, improving its tribological and wear behavior. The aim of this study is to propose a mechanical surface treatment, shot peening, as an alternative to global heat treatments, due to its capacity to induce phase transformation and microstructural modification, at the same time that it improves the mechanical properties of materials. Characterization of different treated samples was performed by means of microstructural characterization, X-ray diffraction analysis, SEM observation, hardness and roughness measurements, and erosion tests. The results show that shot peening treatment is able to transform residual austenite and increase hardness in the top surface layer of the material. Both effects contribute to improve the erosion wear behavior of the WCI.

HAADF-STEM Image Resolution Enhancement Using High-Quality Image Reconstruction Techniques: Case of the Fe3O4(111) Surface.

From simple averaging to more sophisticated registration and restoration strategies, such as super-resolution (SR), there exist different computational techniques that use a series of images of the same object to generate enhanced images where noise and other distortions have been reduced. In this work, we provide qualitative and quantitative measurements of this enhancement for high-angle annular dark-field scanning transmission electron microscopy imaging. These images are compared in two ways, qualitatively through visual inspection in real and reciprocal space, and quantitatively, through the calculation of objective measurements, such as signal-to-noise ratio and atom column roundness. Results show that these techniques improve the quality of the images. In this paper, we use an SR methodology that allows us to take advantage of the information present in the image frames and to reliably facilitate the analysis of more difficult regions of interest in experimental images, such as surfaces and interfaces. By acquiring a series of cross-sectional experimental images of magnetite (Fe3O4) thin films (111), we have generated interpolated images using averaging and SR, and reconstructed the atomic structure of the very top surface layer that consists of a full monolayer of Fe, with topmost Fe atoms in tetrahedrally coordinated sites.

Impact of single atomic defects and vacancies on the magnetic anisotropy energy of CoPt thin films

The impact of surface vacancies and single adatoms on the magnetic properties of tetragonal L10 CoPt thin films is investigated from first principles. We consider Co and Fe single adatoms deposited on a Pt-terminated thin film while a Pt adatom is assumed to be supported by a Co-terminated film. The vacancy is injected in the top-surface layer of the films with both types of termination. After finding the most stable location of the defects, we discuss their magnetic properties tied to those of the substrate and investigate the magnetocrystalline anisotropy energy (MAE). Previous simulations (Brahimi et al 2016 J. Phys.: Condens. Matter 28 496002) predicted a large out-of-plane surface MAE for the Pt-terminated CoPt films (4 meV per f.u.) in contrast to in-plane surface MAE for Co-terminated films (−1 meV per f.u.). Here, we find that the surface MAE is significantly modified upon the presence of the atomic defects. All investigated defects induce an in-plane MAE, which is large enough for Fe adatom and Pt vacancy to switch the surface MAE from out-of-plane to in-plane for the Pt-terminated films. Interestingly, among the investigated defects Pt vacancy has the largest effect on the MAE in contrast to Co vacancy, which induced the smallest but still significant effect. This behavior is explained in terms of the orbital moment anisotropy of the thin films.

Study of microstructure evolution, mechanical properties and hot corrosion behavior of friction stir processed boiler steel

In this investigation, commonly used boiler steel (SA210) was friction stir processed (FSPed) using pinless tool with plunge depth of 1 mm, transverse speed as 30 mm min−1 and tool-spindle rotational speeds of 600 revolution per minute (rpm), 300 rpm, and 150 rpm respectively with three number of passes in each case. Scanning Electron Microscopy (SEM) and Optical Microscopy (OM) analyses were done in order to characterize the Friction Stir Processed specimens. Prediction of the grain size distribution after friction stir processing under different rotational speeds was done using Electron Back Scattered Diffraction (EBSD). The initial grain sizes of the selected boiler steel (25 μm) has been decreased to 7.3 μm, 4.3 μm, and 1.75 μm respectively with decrease in rotational speed. The microhardness and tensile strength was evaluated of all the FSPed samples. The microhardness of the FSPed samples was enhanced by factor of 2, 2.5 and 3 in comparison to base steel respectively with the decrease in rotational speeds; this may be due to phase transformation (austenite to martensite and ferrite) verified by Image processing software "Image-J" and refinement in grain size as per Hall-Petch equation. The ultimate tensile strength was improved by 8%, 16% and 25% respectively in comparison to base metal, which may be accredited to enlarge in grain boundaries which hinder the dislocation movement. FSPed steel having grain size of 1.75 μm offered 68% resistance to hot corrosion in comparison to base steel. The enhanced corrosion resistance might be attributed to the increased microhardness, fine grain structures and the presence of protective oxides on the top surface layer.

Estimating evapotranspiration over vegetated surfaces based on wet patch patterns

Abstract Evapotranspiration (ET) is a critical component of the hydrological cycle and natural water-energy nexus. The dynamics of soil water content (θ) in the top surface layer, regulated by local climate, predominates the surface energy exchange and ET behavior. In this study, we proposed a novel ET-θ relation using a physically based wet patch radius coupling the near surface turbulent transfer and soil water availability. The model is tested against the dataset from eddy covariance (EC) sites in the AmeriFlux network. The results show that ET rate is supply-driven under low soil moisture conditions since the plant controls the transpiration rate to conserve water due to water stress. While in energy-limited condition, increasing soil moisture will not promote ET rate as it is bounded by the lower atmospheric demand. The proposed method is practically designed to calculate ET using variables readily measured by standard EC towers such as soil moisture and meteorological measurements. The method can also potentially be extended to predict the spatial and physical patterns of ecosystem services under different hydroclimatic conditions.

Effect of Plastic Deformation and Damage Development during Friction of fcc Metals in the Conditions of Boundary Lubrication

The main goal of the presented work was the analysis of the interaction between deformed microstructures and friction and the wear properties of four face centered cubic (fcc) metals. Pure fcc metals such as Ag, Cu, Ni, and Al with different values of stacking fault energy (SFE) were chosen for pin-on-disk tests in lubricated conditions. Friction properties of the four fcc metals are presented herein as their Stribeck curves. The transition from elasto-hydrodynamic lubrication (EHL) to boundary lubrication (BL) regions depends mainly on the values of SFE, hardness, and contact temperature. The acoustic emission (AE) parameters were analyzed in the transition from the EHL to the BL region. The models of friction in different lubricant conditions related to the AE waveforms were proposed. The nanocrystalline top surface layers characterized the deformed structure during friction of Ag in the BL region. The lamellar cross-sectional microstructure was formed in the subsurface layers of Ag, Cu, and Ni in the friction direction. Steady state friction and wear in the BL conditions were explained by a balance between the hardening and the dynamic recovery, which was strongly dependent on the SFE and the temperature. The interaction between the deformed structure, the friction, and the wear properties of the studied metals rubbed in the BL region is discussed herein.

Effect of laser shock peening and annealing temperatures on stability of AA2195 alloy near-surface microstructure

The thermal stability of surface microstructure as well as mechanical property induced by laser shock peening (LSP) in AA2195 alloy was investigated in the present work. Samples of the AA2195 alloy were firstly LSPed in once or three times and then annealed at 250, 300 and 350 °C for 1 h, respectively. The surface microstructures of LSPed and annealed samples were investigated by transmission electron microscopy (TEM), and the thermal stability of the mechanical properties was characterized by the changes of the residual stresses and microhardness at the top surface layer. Results showed that the critical temperatures of the sample with 3-times LSP leading to the notable grain growth and changes of mechanical properties were 350 °C and 300 °C respectively, and the corresponding temperatures of the sample with 1-time LSP were 300 °C and 250 °C respectively. Precipitates (eg. T1) in the surface layer of AA2195 alloy were dissolved after LSP, and the thermal stability of the structure and mechanical property was improved by the secondary precipitation of T1 phase during following annealing. The experimental results exhibited that the thermal stability of the sample with 3-times LSP was better than that of the 1-time LSPed one, which indicated that multiple times LSP enable benefit of the stability of structure and mechanical property of AA2195 alloy.

Corrosion behaviour of nanocrystallines 304 stainless steel in simulated secondary side environment

Surface nanocrystallisation (SNC) was prepared on the top-surface layer of 304 stainless steel by ultrasonic surface rolling treatment (USRT). Meanwhile, martensite transformation also occurred during grain refinement, and the surface microhardness increased more than tripled after USRT. The corrosion resistance of nanocrystallines (NC) 304 stainless steel was notably better than that of untreated samples in simulated pressurised water reactor (PWR) secondary water by potentiodynamic polarisation curves, electrochemical impedance spectroscopy and Mott-Schottky curves. Meanwhile, the corrosion mechanism was discussed before and after USRT by X-ray photoelectron spectroscopy (XPS). The main reason is due to the formation of denser Cr(OH)3 protective film and the reduction of the porous structure Fe2O3 content in the NC 304 stainless steel.

A kinetic study on the mechanisms of metal leaching from the top surface layer of copper aluminates and copper ferrites.

Recent studies have reported the potential copper immobilization in aluminates (CuAl2O4 and CuAlO2) and ferrites (tetragonal CuFe2O4 and cubic CuFe2O4) and suggested a reliable method to stabilize metals in reusable ceramic products. In this study, copper immobilization effect was further analyzed in the leaching solutions with pH close to environmental conditions. The results from the chemical equilibrium model Visual MINTEQ illustrated that almost all copper, aluminum, and iron formed complexes with CH3COO- ions in the leachates. The dissolution behavior on sample surface was further explicated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The weight percentage of leached copper was lower than 0.1% even after 22-day leaching, indicating the successful copper stabilization in aluminates and ferrites. The results showed the highest copper concentrations in CuAlO2 leachates and the smallest leached copper amount from tetragonal CuFe2O4, respectively. The incongruent dissolution with Al-O or Fe-O bonds still remaining on the solid surface may be beneficial for preventing further leaching of copper. Furthermore, the modeling of reaction kinetics found that copper leaching from the CuAl2O4 and CuAlO2 obeyed the second-order reaction with correlation coefficients higher than 0.99. Moreover, the shrinking core model was chosen to analyze the leaching mechanisms of both CuFe2O4 ferrites, and the diffusion through product layer model acted as the rate-controlling step in their leaching process.

Microstructure and dispersive optical parameters of iron films deposited by the thermal evaporation method

In the present study, the structure, microrelief and optical properties of the iron films deposited onto silica and silicon substrates by thermal evaporation have been evaluated. The film top surface layer is a textured iron polycrystal, as it has been verified by RHEED. When iron is deposited onto silicon, the microrelief magnitude is below 12–15 nm, as measured by AFM. The dispersive optical constants n(λ) and k(λ) of the iron films have been defined over the spectral range of λ = 250–1100 nm by spectroscopic ellipsometry. The n(λ) and k(λ) parameters open a possibility of precise thin semi-transparent iron film thickness measurement by laser ellipsometry.

Vertical profile of soil/sediment pollution and microbial community change by e-waste recycling operation

The present study aims to assess the effect of electronic waste (e-waste) recycling on microbial community and the underlying modulation mechanism. Core soil/sediment samples were collected from an abandoned e-waste burning site and neighboring farmland/stream sites in Guiyu, China. High concentrations and health risks of toxic heavy metals, particularly, Sb and Sn, and halogenated flame retardants (HFRs), including decabromodiphenyl ether (BDE 209) and decabromodiphenyl ethane (DBDPE) were mostly retained at the top surface layers of soils/sediments (0–30cm) after more than one year of natural vertical diffusion and microbe-facilitated biodegradation. Heavy metals, such as Ag, Cd, Cu, Pb, Sb, and Sn, played a critical role for the reduction of microbial diversity. This is the first study reporting the open burning of e-waste caused an obvious heat effect and enriched thermophilic/mesophilic microbes in local area. The acid washing during e-waste recycling process may result in the enrichment of acidophilic microbes. This investigation showed that e-waste processing operation resulted in not only severe pollution of the soils/sediments by various pollutants, but also reduction of microbial diversity that was difficult to self-store by the local ecosystem.

Synergetic deformation-induced extraordinary softening and hardening in gradient copper

A gradient-structured Cu sample composed of grain-size gradient surface layer (GSL) and homogeneous coarse-grained (CG) core was fabricated by surface severe plastic deformation. Microhardness measurements revealed that both tension-induced softening in the top nanograined surface layer and hardening in the subsurface layer of integrated gradient sample were far more pronounced than that of a freestanding GSL, i.e. extraordinary softening and hardening occurred in gradient sample. The synergetic deformation between GSL and CG core produced accumulation of geometrically necessary dislocations and thereby back stress strengthening, resulting in extra flow stress and hardening. The extraordinary softening was attributed to the extra flow stress-assisted grain coarsening in nanograined layer. Furthermore, strain measurements proved that the CG matrix plays critical roles in stabilizing the grain coarsening and homogenizing the strain distribution in nanograined layer, which were favorable for the uniform ductility.

Characterization and analysis on micro-hardness and microstructure evolution of brass subjected to laser shock peening

This paper is concerned with the effect of laser shock peening on micro-hardness and microstructure of brass. In order to study the plastic deformation behaviors of brass, laser shock peening with 1, 2, and 3 impacts at pulse energy of 6.5 J were performed in this experiment respectively. The micro-hardness distribution in the near-surface layer is performed by hardness tester, and the microstructure in the surface layer is determined by X-ray diffraction and transmission electron microscopy. The results indicate that higher surface micro-hardness of brass after laser shock peening impacts is induced comparing with untreated brass. Additionally, increasing the impact times of laser shock peening, the surface micro-hardness improves correspondingly. Combined with transmission electron microscopy analysis, the determinate factors for variation of surface micro-hardness owes to the twins spacing and grain refinement induced by laser shock peening. The formation and interaction of surface mechanical twins are accompanied by improving in surface strain and strain rate. The equiaxed grains with an average size of 70–100 nm are generated in the top surface layer after 3 laser shock peening impacts. Furthermore, the grain refinement mechanism in the top surface layer subjected to laser shock peening is summarized as follows: (i) parallel mechanical twins are generated due to the effect of strain deformations induced by laser shock peening; (ii) mechanical twins are subdivided into secondary twins with the formation of dense dislocation tangles at the mechanical twins boundaries; (iii) twins interaction in different directions refine the grain into nano-scale equiaxed grains.

Modelling of III-Nitride Epitaxial Layers Grown on Silicon Substrates with Low Dislocation-Densities

Large lattice and thermal expansion coefficients mismatches between III-Nitride (III N) epitaxial layers and their substrates inevitably generate defects on the interfaces. Such defects as dislocations affect the reliability, life time, and performance of photovoltaic (PV) devices. High dislocation densities in epitaxial layer generate higher v-shaped pits densities on the layer top surface that also directly affect the device performance. Therefore, using an approach such as the embedded void approach (EVA) for defects reduction in the epitaxial layers is essential. EVA relies on the generation of high densities of embedded microvoids (~108/cm2), with ellipsoidal shapes. These tremendous number of microvoids are etched near the interface between the III N thin-film and its substrate where the dislocation densities present with higher values. This article used a 3-D constitutive model that accounts the crystal plasticity formulas and specialized finite element (FE) formulas to model the EVA in multi-junction PV and therefore to study the effect of the embedded void approach on the defects reduction. Mesh convergence and 2-D analytical solution validation is conducted with accounting thermal stresses. Several aspect and volume ratios of the embedded microvoids are used to optimize the microvoid dimensions.

First-principles study of the structural and electronic properties of the clean and O-deficient ZnAl2O4(111) surfaces

In this study, the GGA-PBE method was used to study the structural and electronic properties of the ZnAl2O4(111) surface with and without oxygen vacancies. The ZnAl2O4(111) surface has six possible surface terminations called Zn(O2), Zn(Al), Al(Zn), Al(O2), O2(Zn), and O2(Al). The results obtained in this work show that the surface energy, electronic structure and work function of the ZnAl2O4(111) surface depend greatly on the nature of atoms in the top surface layer. The calculated surface energies indicate that the Zn(O2) termination is the most stable surface termination in O-rich and Al-poor environments, while the Al(Zn) termination is the most stable termination in Al-rich and O-poor environments. On the other hand, our results show that the formation of oxygen vacancies can greatly affect the stability of the ZnAl2O4(111) surface. We found that the O-deficient surface is more stable than the perfect surface in the case of O2(Zn) and O2(Al) surface terminations. In the case of other surface termination, the formation energy of VO is positive under both O-rich and O-poor conditions except the case of the Zn(O2) termination under O-poor and Al-rich conditions. Moreover, we found that the formation of oxygen vacancies can greatly affect the stability, electronic structure, and work function of the ZnAl2O4(111) surface.