Mechanism Of Layer Formation Research Articles

Mechanism of Layer Formation during Gas Nitriding of Remelted Ledeburitic Surface Layers on Unalloyed Cast Irons

Unalloyed cast iron materials exhibit low tribological and corrosive resistance. In this respect, nitriding has a wide range of applications for steels. In the case of cast iron, the advantageous properties of nitrided layers are impaired by the presence of graphite. Electron beam remelting of cast iron surfaces prior to nitriding removes graphite. The homogeneous ledeburitic microstructure within the approx. 1 mm-thick remelted layer enables the formation of a dense compound layer during subsequent nitriding. The main objective of this study is to investigate the nitriding mechanism of unalloyed ledeburitic microstructures. Due to the complex relationships, investigations were carried out on both conventional ferritic and pearlitic cast irons and Fe-based model alloys containing one to four additional alloying elements, i.e., C, Si, Mn and Cu. The iron (carbo-)nitride composition (γ’, ε) of this compound layer depends on the gas nitriding conditions, the chemical composition of the substrates and the microstructural constituents. As a result, a schematic model of the nitriding mechanism is developed that includes the effects of the nitriding parameters and alloy composition on the phase composition of the nitriding layer. These findings enable targeted parameter selection and a further optimization of both the process and the properties.

Analysis of the Formed Protective Layer Inhibiting Alkali Corrosion in Aluminosilicate Refractory Castables

This article analyzes the mechanism of the protective layer formation under the action of alkali in a refractory castable when ground quartz sand (GQS) is used as an admixture to produce refractory conventional castables (CC) and medium cement castables (MCC). It was found that, independently of the castable composition, the addition of GQS (2.5%) reduces the degree of K2CO3 dissolution at high temperature, and the released potassium reacts with the silica and forms a viscous potassium silicate glass, which reduces the mobility of alkali. The liquid phase formed filled some of the open pores and hindered the penetration of potassium into the deeper layers of the refractory castable. The thickness of the formed protective layer, after three cycles of the alkaline corrosion test, varies from 700 µm up to 1300 µm, depending on the castable composition.

Experimental study on the mechanism of ice layer formation in fractured rock masses in cold regions

The formation and growth of ice layers in fractured rock masses is one of the main causes of frost weathering in bedrock in cold regions. The mechanism of ice layer formation in fractured rock masses was simulated in this study by conducting frost weathering tests on a simulated rock mass with a vertical through fracture, which was fabricated by splicing two cement blocks. The test results were used to propose a novel mechanism for ice layer formation in terms of the initial crack-frost effect in which moisture mainly migrates in vapor form. The continuous accumulation and growth of these frosts eventually forms an ice layer that expands the crack, and the fractured rock simulation model undergoes frost-heaving damage. The frost layer on the rock wall in the negative temperature region is similar to the frosting process on the cold surface of thermodynamic studies. The relative humidity (RH) gradient and temperature gradient were shown to be the factors affecting water vapor migration, where the former plays a direct and dominant role, and the temperature gradient has a very limited direct effect on vapor migration.

Flow characteristics of heavy oil-water flow during high water-content cold transportation

As the water content in heavy oil gathering systems increases to 70–90% and large-scale popularization of cold transportation. Understanding the characteristics of heavy oil-water flow during high water-content cold transportation has greatly gained in importance. Here, a comprehensive flow loop experiment and microscopic observation was employed to investigate the flow characteristics of oil-water flow at low temperatures. The flow patterns of cold transportation were studied experimentally. The flow pattern maps were also created in detail, taking into account the temperatures (60–20 °C), mixture velocities (0.1–1.0 m/s), and water contents (70–90%). The relevant influencing factors of the pressure gradient were then investigated. The lubrication coefficient and viscosity models were used to modify the pressure gradient models, and average relative deviations were within ±25%. Moreover, the distinction between the oil-sticking layer and wax deposition formation mechanisms was highlighted. According to microscopic observation, the major elements impacting the strength of the oil-sticking layer were asphaltene aggregation and water droplet distribution. Finally, an evaluation method for predicting the minimum heavy oil-gathering temperature during high water-content cold transportation was presented. The findings of this study greatly contribute to understanding the mechanisms of cold transportation.

Characteristics and Formation Conditions of Thin Phytoplankton Layers in the Northern Gulf of Mexico Revealed by Airborne Lidar

The thin layers in the ocean are temporally-coherent aggregations of phytoplankton with high concentrations at small vertical scales, presenting important hotspots of ecological activity. Lidar could identify thin phytoplankton layers at a large spatial scale due to its capabilities of profile detection with a high efficiency. However, studies that linked thin layers to environmental factors are few, which limits our understanding of the layer formation mechanism. This paper investigates the characteristics and formation conditions of thin phytoplankton layers in the northern Gulf of Mexico using airborne lidar. The results depict that the chlorophyll concentration determines the formation probability of the phytoplankton layer. The layer is mainly formed at concentrations less than 6 mg m−3 and mostly distributed at 2 mg m−3. In addition, layer thicknesses were within 5 m and layer depths were mainly in the range of 10–15 m. Layer depths in the nearshore region were shallower than those in the offshore region. We conclude that the characteristics and formation conditions of the thin phytoplankton layers depend on the nutrients and light that are related to the seabed topography, turbidity, eddies and upwelling. The findings of this paper will enhance the understanding of layer formation mechanisms.

Influence of the injection molding thermal boundary conditions on the filling flow of PET

During the filling phase of the injection molding process, the polymer in contact with the wall solidifies, developing the so-called skin layer. Although the no-flow temperature is often exploited in numerical simulations to model the skin layer formation mechanism, there is still an incomplete understanding of what happens for semi-crystalline PET. In fact, for this polymer, the high shear stresses near the cavity wall combined with the thermal gradient at the polymer-mold interface can cause the so-called flow-induced crystallization. This work aims to experimentally investigate the skin layer formation at different injection molding processing conditions. A full-factorial experimental plan was performed using a commercial PET injected into an open-cavity mold. The thermal and mechanical boundary conditions at the polymer-wall interface were varied using insulating coatings and different flow rates, respectively. The mold coatings were applied to different steel substrates to consider the substrate thermal conductivity contribution. Different flow rates were imposed, varying the cavity thickness and the injection speed. The in-line acquisition of the cavity pressure allows for assessing the contributions of the modified boundary conditions to the solidified skin layer formation. The results show that the insulating mold coating changes the thermal boundary conditions, leading to a maximum pressure decrease of 14 %. The cavity pressure vs. filling time curves for the uncoated inserts show an evident derivative discontinuity, suggesting the onset of flow-induced crystallization.

Aluminium wire anodisation: Controlling the thickness, porosity, and morphology of alumina layer for the application of solid-phase microextraction

ABSTRACTIn this study, the aluminium wire anodisation process to generate a cracked alumina layer with a high surface area for the application of solid-phase microextraction was studied. The effect of aluminium anodisation variables, including anodisation voltage, anodisation time, electrolyte temperature, and electrolyte concentration was investigated and the cracked alumina layer formation mechanism was revealed. Characterisation of the generated alumina layer was carried out by scanning electron microscope (SEM) and adsorption and desorption isotherm of nitrogen by Brunauer–Emmett–Teller (BET). Under optimised conditions, an adherent cracked alumina layer with high porosity and thickness was achieved. The highest surface area was obtained in anodisation voltage of 15 V, anodisation time 120 min, electrolyte temperature 25 °C, and electrolyte concentration 3.0 mol L−1. The fabricated fibres were used to headspace solid-phase microextraction of n-butanol from aqueous samples and then injected into a gas chromatography-flame ionisation detector (GC-FID) to evaluate their efficiency. Results showed that the fibres which were prepared in the optimum conditions have an extraction efficiency distinguishably higher than other types.

High-quality surface smoothening of laser powder bed fusion additive manufacturing AlSi10Mg via intermittent electrochemical polishing

Though the laser powder bed fusion (LPBF) additive manufacturing (AM) AlSi10Mg alloy (LPBF AlSi10Mg) has been utilized for weldability and fatigue resistant property, its high surface roughness limits widespread application. Here, we report the surface roughness reduction of the LPBF AlSi10Mg via electrochemical polishing (ECP). A novel viscous layer formation mechanism is found on LPBF AlSi10Mg surface during the ECP process. An environmentally friendly NaOH based electrolyte was utilized to passivate the LPBF AlSi10Mg surface, resulting in successful ECP of the LPBF AlSi10Mg. Furthermore, the Si phase and its reaction products with electrolyte formed on the polishing surface were found to be a barrier for improving the surface quality of LPBF AlSi10Mg. Therefore, an intermittent removal of the polishing products on AlSi10Mg further improves the polishing effect, i.e., an intermittent electrochemical polishing (IECP) method was utilized to improve the polishing quality of LPBF AlSi10Mg. The surface roughness (Sa) is reduced by 87.7 %, and the corrosion current density (icorr) is decreased by 17 folds. The significantly improved polishing effect of IECP is dominated by removing ECP products constantly to enhance dissolution of Al element in AlSi10Mg and fluent dispersion of electrolyte and Al3+ in the metal-electrolyte interface.

Formation mechanism of white layer in the high-speed cutting of hardened steel under cryogenic liquid nitrogen cooling

The coupled thermal–mechanical effect in the high-speed machining of hardened steels induces the formation of a surface white layer structure, which adversely affects the mechanical properties and service performance of the machined component. However, knowledge about the white layer formation mechanisms is in severe dearth, particularly for high-speed machining under a cryogenic cooling condition. This paper presents an experimental study of the formation mechanisms for surface white layers in high-speed machining of a hardened steel. Both machining with cryogenic liquid nitrogen (LN2) cooling and dry cutting conditions are considered and the characteristics of the white layers formed under the two cooling conditions are compared to reveal the effect of process parameters. It is shown that the hardness of the white layers is increased while their grain size is decreased under the cryogenic LN2 cooling condition as compared to dry cutting. Under both the cooling conditions, no material phase transformation or recrystallization is noticed alongside the white layer formation, but severe plastic deformation is found to be the dominant reason for white layer formation. Tool wear is noticed to increase the white layer thickness. It is shown that during the cutting process, the work material undergoes two stages of intense plastic deformation. In the first stage, the material slips and approaches the first deformation zone, then gradually bulges and produces significant dislocations, and eventually forms a dense dislocation center around the shear plane. In the second stage, part of the material slides toward the tool flank face. The material dislocation slip induced by friction and material shearing further stretches the surface martensite lath bundles to form dislocation tangles and cellular substructures. The highly stretched martensite lath bundles are finally transformed into white layer structure under the interweaving multi-dislocation movement.

A facile in situ Mg surface chemistry strategy for conditioning-free Mg[AlCl4]2 electrolytes

Developing a simple and reliable strategy for modulating overpotential of Mg plating/stripping is crucial for realizing overall high performance of rechargeable Mg batteries, which have been demonstrated to be rather challenging. Herein, we report a facile LiI-assisted Mg surface chemistry to reduce Mg plating/stripping overpotential via in situ solubilization mechanism and ion conductive surface layer formation mechanism. As a result, we successfully formulate a new conditioning-free Mg[AlCl4]2-based Mg electrolyte system that improves Mg plating/stripping from the very first cycle. The overpotential of Mg plating/stripping of the as-prepared electrolyte with a LiI electrolyte additive is remarkably reduced to 240/240 mV at a current density of 1000 µA cm−2. Using this electrolyte, a rechargeable Mg battery coupled with PANi intercalated V2O5 (V2O5-PANi) cathode and Mg anode delivers a stable discharge specific capacity of nearly 50 mA h g−1 after an impressively prolonged cycle life of 500 cycles. Our finding provides a new inspiration of in situ Mg surface chemistry to activate Mg anode surface with high Mg ion conductivity.

Anisotropic Phase Transformation Mechanism on Coarse-Grained and Fine-Grained Pure Titanium at Low-Temperature Plasma Nitriding

The nitriding process of the coarse-grained and fine-grained pure titanium proceeded by multidirectional forging technique has been investigated at temperatures of 623, 673, and 723 K. The process was carried out by high-density radiofrequency-direct current plasma combined with a rectangular hollow cathode device. The result obtained is a significant increase in surface hardness with increasing holding temperature. The surface hardness increases due to forming a surface layer composed of δ-Ti2N, ε-Ti2N and TixNx observed from x-ray diffraction results. This paper explains the mechanism of surface layer formation. We also observed anisotropic phase transformation of titanium nitride through the right shift of the x-ray diffraction peaks. Diffused nitrogen atoms during the nitriding process cause a change in crystal orientation through structural transformation of the metastable δ-Ti2N to the stable ε-Ti2N. The structural reconstruction will continue by forming TixNx to achieve stoichiometric equilibrium. More compacting of the surface microstructure is also obtained by increasing nitriding temperature.

Mechanism of gradient strengthening layer formation based on microstructure and microhardness of Inconel 718 grinding surface

Microstructure and microhardness are interrelated variables, which remarkably affecting the wear resistance and fatigue life of materials. This work aims to establish a relationship between the microstructure evolution and microhardness in the grinding surface layer of Inconel 718. The microstructure of the grinding surface layer was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron backscattered diffraction, while the microhardness was measured using the Vickers microhardness test. The experimental results show that the grinding surface layer consists of a refined grain region and a high-density dislocation region. Original grains in the refined grain region were refined to form nanograins and elongated grains, owing to the high temperature-induced dynamic recrystallization. The high-density dislocation region contained a large number of dislocations, resulting in the maximal hardness (438.6 ± 11.1 HV0.01) in this region. The microstructure increased the microhardness first and then decreased it to the inner matrix value, and the maximal value was obtained at the high-density dislocation. The microhardness variation is discussed in terms of two strengthening mechanisms (namely grain boundary strengthening and dislocation strengthening), and the strengthening mechanisms in the different regions of the grinding surface layer are revealed.

Diffusion kinetics and elemental transport in selective surface nitriding of CoCrFeNiTi0.5 high entropy powder

Ceramic coated metal powders have recently found prominence owing to their prospective application in 3D microstructured metal matrix composite synthesis. To ensure a perfect matrix-reinforcement interface in these composites, the coating process needs to be in situ. To that end, we recently developed a process for in situ nitride surface coating of CoCrFeNi high entropy alloy (HEA) powder. This approach offers various advantages over other processes including ease of controlling the surface morphology and compositional control by adjusting the nitriding parameters. In this study, we aimed to elaborate the nitride layer formation mechanism, growth kinetics, and compositional control. Mechanically alloyed CoCrFeNiTi0.5 HEA powder was used as the starting powder. The nitride layer growth on the powder was observed at different temperatures of 973, 1073 and 1173 K for 1–12 h under partial nitrogen atmosphere. The nitrogen preferentially reacted with Ti to form a continuous layer of TiN on the powder surface. Further growth was found to be diffusion-controlled by the growth rate increase with increasing nitriding temperature. Morphological investigation revealed a strong efflux of Ti through the TiN grain boundaries resulting in a scale growth at the TiN/gas interface. This growth mechanism was validated by a high TiN layer growth activation of 240 kJ.mol−1. Finally it was revealed that the Cr dissolution within the TiN layer can be achieved at higher temperature. However, to retain the integrity of the surface layer, the Cr dissolution process needs better control

Environmentally Friendly Layered Double Hydroxide Conversion Layers: Formation Kinetics on Zn-Al-Mg-Coated Steel.

Phosphate- or chromate-based industrially produced conversion layers, while effectively increasing adhesion for organic coatings and corrosion resistance, come at the cost of environmentally problematic and harmful treatment solutions and waste. In this respect, layered double hydroxide (LDH)-based conversion layers offer an environmentally benign alternative without toxicologically concerning compounds in the treatment solution. Here, we study an LDH conversion layer on Zn–Al–Mg-coated steel (ZM-coated steel), which was produced by immersion into a carbonate- and magnesium-containing alkaline solution. The mechanism and kinetics of the conversion layer formation were investigated with in situ open circuit potential measurements, cyclic voltammetry (CV), and scanning electron microscopy (SEM). Acceleration of the LDH layer formation through high convection in the treatment solution was found. This was attributed to a higher oxygen availability at the metal/solution interface because no diffusion-limited state during the layer formation is reached due to high convection. The importance of oxygen within the kinetics indicates a corrosion-like mechanism, with cathodic and anodic sites on the steel sample. The LDH formation happens by co-precipitation of ions present in the treatment solution and dissolved ions from the ZM-coated steel. With CV, SEM, and X-ray diffraction, the growth of the LDH conversion layer was investigated with respect to the immersion time. It was found that after 30 s, the sample surface was almost fully covered with an LDH layer, and with the increasing immersion time, the layer grows in thickness. Increased understanding on the kinetics and mechanism of the LDH conversion layer formation on ZM-coated steel gives rise to a targeted optimization of the treatment solution and process parameters.

Two-Phase-Interfaced, Graded-Permittivity Titania Electrical Insulation by Atmospheric Pressure Plasmas.

Functionally graded materials (FGMs) exhibit unique properties and are expected to deliver outstanding and stable performance under extreme conditions. High-voltage, high-power FGM-based electric insulation commonly fails because of inadequate surface charge control (flashover) performance and stability of stacked layers of dielectric materials with graded permittivity εr. Here, we address these issues by interfacing the rutile and anatase TiO2 layers on a ceramic with very different εr values of 110, 48, and 9, respectively, using scalable, environment-benign, and energy-efficient atmospheric pressure plasma processing. The FGM drastically reduces the maximum electric field along the optimized surface by 66% and increases surface flashover voltage by 36 %, while featuring a remarkable (120/180 days) long-term stability. The mechanisms of the plasma-enabled graded layer formation are presented, which can be used for precise engineering of FGMs for diverse applications in other fields.

Phase transition and dynamic recrystallization mechanisms of white layer formation during turning superalloy Inconel 718

White layer is commonly formed on the machined surface of nickel-based Inconel 718 superalloy, which probably induces the generation of cracks, leading to the fatigue life reduction of the workpieces. Nevertheless, the microstructure of the white layer is still not revealed clearly. The core issues, which this paper aims to reveal, are the transition of γ, γ′, γ" and δ phases and dynamic recrystallization mechanism within the white layer formed during turning Inconel 718. Therefore, the dry orthogonal cutting tests with worn tools were conducted to make the white layer first. After that, the morphology and microstructure of the white layer are studied with the aid of scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The results indicate that the thickness of the white layer is uneven and varies within three microns. Meanwhile, the amount of γ′ phase decreases, and δ phase is crushed and refined within the white layer. The lattice mismatch between γ and γ" phases decreases and the white layer is more homogeneous than the original material. Furthermore, the grains were refined to submicron, and continuous dynamic recrystallization (CDRX) occurs in the white layer. The CDRX results in a recrystallized structure with significantly reduced defect density in the white layer formed during the turning of Inconel 718.

Effect of chemical activity of bulk and pad materials on the redeposition layer during polishing of glass

A near-surface redeposition layer is formed in the chemical mechanical polishing process of silicate glasses. In this work, secondary ion mass spectroscopy analyzer and transmission electron microscope are used to investigate the effect of bulk and pad materials on the microstructure and contaminations of the redeposition layer. The results suggest that the polishing materials with higher chemical reactivity will generate a deeper redeposition layer with higher concentration of contaminant elements, which will improve the surface quality by mitigating the mechanical abrasion effect. Then, a more detailed chemical mechanism of the redeposition layer formation has been concluded according to the chemical reaction and arrhenius equation, which can explain the effect of bulk materials and polishing pad on the redeposition layer well. These findings are helpful to understand the chemical process of polishing and improve the lifetime and quality of optical components especially in UV or high-power laser systems.

Mechanism of Passive Layer Formation during Electrochemical Oxidation of Copper(I) Sulfide

Mechanism of Passive Layer Formation during Electrochemical Oxidation of Copper(I) Sulfide

Experimental study of ultrastructural mechanisms and kinetics of tegumental secretion in cestodes parasitizing fish (Cestoda: Diphyllobothriidea).

The structural response and plasticity of the cestode tegument in response to the influence of the host organism is not yet well understood. The main aims of our in vitro study were to analyse the ultrastructural mechanisms and kinetics of tegumental secretion in two cestode species, Dibothriocephalus dendriticus and Ligula interrupta, in response to the influence of fish host blood serum. The incubation of plerocercoids in the culture medium, which contained fish host blood serum, resulted in an increased number of secretory products on the tegumental surface. Our study is the first to experimentally demonstrate the formation of plerocercoid protective layers influenced by the host's internal environment factors. The mechanism of the generation of the protective layer included the following: the intensive formation of organelles in the tegumental cytons and their transfer to the distal cytoplasm of the tegument; increases in extracellular vesicles and vacuoles released on the tegumental surface; arrangement of secretory products and fine-dispersed extracellular matrix in layers; and formation of the protective layer. The structural tegumental response included increases in the glycocalyx layer and structural changes. Our study revealed that the universal mechanism of protective layer formation was intrinsic to different tapeworms. We hypothesize that plerocercoids of cestodes parasitizing fish may use tegumental secretion in the formation of a protective layer and in the release of immunoregulator molecules to evade the host's immune response.

Feasibility of artificial neural network on modeling laser-induced colors on stainless steel

Abstract It is indicated that a certain RGB color can be obtained by specific processing parameters in Laser Color Marking (LCM). However, these parameters do not necessarily provide the same color in different laser systems. In most of the conducted studies on LCM, the focus is on the oxide layer formation mechanism, reflectance spectra, composition, and thickness of the oxide layer, which are ultimately controlled by various factors such as laser processing parameters, sample substrate properties, and ambient conditions. However, there is a lack of study on the quantitative relationship between processing parameters (as the main influential factor) and the produced RGB color in LCM. In this work, for the first time, we propose a method to quantitatively match the processing parameters and the RGB colors in LCM using the artificial neural network (ANN). In this regard, two models have been developed based on experimental data to predict the resultant RGB color and the proper processing parameters for a desired color, respectively. The results show that the ANN has a superior ability to model the behavior of LCM in the fabrication of RGB colors in different laser systems. It is demonstrated that the ANN model can be employed to improve the color consistency and repeatability in LCM without the need of trial and error.