Surface Phase Structure Research Articles

Surface Structural–Phase Modification of Structural Steels during Gas Nitriding

The formation of a composite nitride layer in an ammonia atmosphere with the addition of carbon– and oxygen–containing gases during gas nitriding is considered. The conditions for the formation of cementite during nitriding of steel and the influence of carbon in the matrix on the release of cementite are given. After oxidation of the nitride layer, modified nitride compositions with stable gradient structural–phase states were obtained.

Surface Structural–Phase Modification of Structural Steels during Gas Nitriding

Surface Structural–Phase Modification of Structural Steels during Gas Nitriding

How the duty cycle affects wear and corrosion: A parametric study in the Ni–B–TiN composite coatings

TiN-reinforced Ni–B composite coatings were synthesized in the watts bath using pulse electrodeposition method at different duty cycles of 20%, 40%, 60%, and 80%. The surface morphology, phase structure, hardness, wear, and corrosion behavior of the coatings produced at different duty cycles were examined. The effect of the duty cycle on the microstructure of the composite coating was investigated using scanning electron microscopy (SEM) analysis. Depending on the duty cycle, a coating thickness of ∼69.7μm was obtained on the cathode surface. Crystallite size and lattice distortion of coatings were calculated with X-ray diffraction (XRD) data. The nano-indentation hardness results showed that the high nucleation rate at 40% duty cycle increased the coating hardness to 1170 HV. The wear test results indicated that the distribution of TiN particles in the coating content reduced the wear rate of the coating to 2.121 × 10−5mm3/Nm. Corrosion tests performed in 3.5 wt.% NaCl solutions were analyzed by Tafel curves and electrochemical impedance spectroscopy (EIS). Depending on the duty cycle/reinforcement particle distribution relationship, icorr and Ecorr values were measured as 2.27x10 −6 (A/cm 2) and -0.427 V, respectively.

Synthesis and characterization of calcium carbonate/carbon from water lettuce via hydrothermal carbonization process

The conversion of biomass into value-added products has recently received much attention for a broad range of applications. In this work, water lettuce was converted into calcium carbonate (CaCO3)/carbon via hydrothermal carbonization (HTC) and pyrolysis at 900 °C. The HTC temperature and time varied in the range of 160–200 °C for 6–18 h, respectively. X-ray diffraction analysis indicated that the samples consisted of a mixture of calcite and vaterite phases of CaCO3 and amorphous carbon. The ratio of calcite and vaterite phases varied with HTC time. The Fourier transform infrared spectroscopy (FTIR) result showed the characteristic absorption bands confirming the presence of CaCO3. Scanning electron microscopy (SEM) images revealed the large crystal of CaCO3 and fine carbon particles. From the N2 sorption analysis, the sample prepared from the HTC at 200 °C for 6 h had the highest specific surface area of 95 m2/g due to the development of micropores. The results presented in this work demonstrated that both HTC temperature and time play critical roles in altering the surface area and phase structure of CaCO3/carbon. The CaCO3/carbon derived from water lettuce can potentially be used and adapted for many applications.

Effect of Y2O3 nanoparticles on the microstructure and corrosion resistance of electrodeposited Ni-Mo-Y2O3 nanocomposite coatings

In this study, a composite coating for corrosion protection was successfully prepared on Q235 steel under neutral conditions by means of the DC electrodeposition technique. According to the results of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electrochemical tests, the effects of different additions of yttrium oxide nanoparticles in the electrolyte on the surface morphology, phase structure and corrosion resistance of nickel-molybdenum alloys were analyzed, and the deposition mechanism of yttrium oxide nanoparticles in the bath was proposed. The average grain size of the composite coating was reduced from 6.9 nm to 4 nm due to the grain refining effect of yttrium oxide nanoparticles in the coatings. The content of yttrium oxide in the coating increased with its concentration in the electrolyte, while the content of Mo atoms increased first and then decreased, reaching a peak when the addition of Y2O3 reached 2 g/L. To facilitate the study of the corrosion behavior of the coating after the introduction of the added phase, an equivalent series circuit was also proposed to fit the EIS test results, and it was hypothesized that a corrosion microcell was formed between the nanoparticles and the Ni-Mo alloy when the electrolyte solution uniformly penetrated the film. Based on the results of the electrochemical tests in 3.5 wt% NaCl solution, it was shown that it was not the case that the higher the amount of yttrium oxide added was, the better the corrosion resistance of the coating. Among all samples, Ni-Mo-Y2O3-2 g/L had the most positive corrosion potential (−0.54 V) and the best corrosion current density of 2.452 μA cm−2, which was 52.8 % lower than that of the Ni-Mo alloy coating.

Fabrication of a durable superhydrophobic surface with corrosion resistance on copper

In order to fabrication a durable superhydrophobic surface with corrosion resistance, we have constructed a copper oxide layer with a hierarchical structure in situ on the copper surface by using a chemical treatment. The corrosion resistance and superhydrophobic durability of the surface are enhanced due to the chemical stability of the oxide and the stability of the hierarchical structure. The surface morphology, composition and phase structure were characterized by SEM, EDS, XPS, FT-IR and XRD. The superhydrophobicity of the surface was investigated by wettability tests. The superhydrophobicity durability was investigated by acidic and alkaline solution contact angle test, water immersion test, and UV irradiation test. Electrochemical tests were conducted to examine the potentiodynamic polarization curves and electrochemical impedance spectra to investigate the corrosion resistance of the superhydrophobic specimen. The experimental results are as follows. The oxide formed by chemical treatment is CuO. In the oxide layer, the nano-needles are aggregated into bundles, and the bundles are intertwined with each other to form a stable pyramidal hierarchical structure. The superhydrophobic surface has a water contact angle of 160° and a sliding angle of less than 2°. The superhydrophobic surface is chemically stable. The contact angle is greater than 150 ° for both acidic and alkaline test liquids. The superhydrophobic surface retains its hydrophobicity for a long time when immersed in water. After 20 days of exposure to UV light, the surface remains superhydrophobic. The superhydrophobic specimen has a low corrosion rate and high impedance. The superhydrophobic surface is corrosion resistant and can provide corrosion protection to the substrate.

Effect of Condensation Product on the Electrochemical Behaviour and Corrosion Resistance of Zn and Zn-Ni Deposits

Electrodeposition of Zn and Zn-Ni deposits were performed on steel substrates under various conditions from acid sulphate baths in presence of condensation product of 3, 4, 5-trimethoxybenzaldehyde and serine (TMBS). The electrochemical behavior of deposits was studied by polarization and electrochemical impedance techniques. The Zn-Ni-bright deposits obtained from the acid sulphate bath at 4 Adm-2 showed a lower corrosion current (Icorr) and a less negative corrosion potential (Ecorr) compared to Zn-dull, Zn-Ni-dull and Zn-bright deposits. The surface morphology and phase structure of the deposits was studied by Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis.

Insight into the Surface Reconstruction-Induced Structure and Electrochemical Performance Evolution for Ni-Rich Cathodes with Postannealing after Washing.

Ni-rich layered LiNixCoyAlzO2 (NCA, x ≥ 0.8) oxides have attracted wide attention as cathode materials for lithium-ion batteries due to their higher energy density and lower cost. However, the increase in the capacity for Ni-rich cathodes can cause faster capacity decay and increase sensitivity to ambient air exposure during the storage process. Especially, the residual lithium on the surface of Ni-rich cathodes will cause severe flatulence during cycling which greatly reduces the safety performance of the battery. Washing is an effective method to reduce residual lithium, but it will seriously damage the surface phase structure of Ni-rich materials. Here, we introduce a designed method involving two steps, washing and high-temperature annealing, which can ingeniously modify the surface phase structure of Ni-rich cathodes. The results show that the residual lithium content can be significantly reduced. The thin NiO-like rock-salt phase formed on the surface of Ni-rich cathode annealed at 600 °C improves the diffusion kinetics of Li+, reduces the polarization, and improves the electrochemical performance of Ni-rich materials, while the thick spinel-like phase formed at 400 °C hinders the diffusion kinetics of Li+, significantly increases the polarization, and eventually leads to the structural degradation of Ni-rich materials. As a result, the discharge capacity of the cathode annealed at 600 °C still retains 174.48 mA h g-1 after 100 cycles, with a capacity retention of 92.04%, much larger than the cathode annealed at 400 °C, for which the discharge capacity drops to 107.77 mA h g-1, with a capacity retention of 65.78%.

A two-stage approach to Ni-P-SiC coatings and their friction against SiC in water

Inspired by the low friction of self-mated SiC in water, a two-stage technique is proposed to deposit Ni-P-SiC coating with a high particle content to achieve low friction against SiC in water. The process comprises the electrophoretic deposition of SiC particles layer on a mild steel substrate followed by the electroless plating of NiP coating. The effects of SiC content on the surface appearance, phase structure and cohesive strength were studied. Much attention was paid on the friction behavior of the coatings in water. Experiment results show that compared with the traditional electroless plating method, the two-stage technique can enhance the implanted SiC particle content effectively. Notable, with the increasing of SiC content, the friction coefficient decreases correspondingly and the lowest friction coefficient of 0.015 can be achieved for the Ni-P-SiC coatings with the highest SiC particle content. The major surface degradation mode is abrasive wear during running-in then tribochemical wear dominates at the low friction stage.

Electrodeposition of Co-P/TiO2 composite and its electrochemical properties and photocatalytic application for degradation of methyl orange in simulated wastewater

Co-P/TiO2 composite was prepared on 45 steel substrate by electrodeposition technology. The influence of the concentration of TiO2 nano-particles in the plating solution on the surface morphology, phase structure and corrosion resistance in simulated wastewater was studied by scanning electron microscopy, energy dispersive spectrometer, X-ray diffractometer and electrochemical testing. Then the ultraviolet-visible spectrophotometer was used to determine the maximum absorption wavelength of methyl orange and evaluate the photocatalytic degradation rate of methyl orange in presence of different composite in simulated wastewater. The results show that the grains of the composite are cauliflower-like, and the surface structure is dense. Changing the concentration of TiO2 nano-particles in the plating solution has a certain effect on the content of TiO2 nano-particles in the composite and the corrosion resistance, but it has no significant effect on the phase structure of the composite. TiO2 nano-particles are uniformly dispersed in the composite. The higher the content of TiO2 nano-particles in the composite, the stronger the photocatalytic activity and the better the corrosion resistance of the composite possess. When the concentration of TiO2 nano-particles in the plating solution is 15 g/L, the content of TiO2 nano-particles in the composite reaches to 9.08%, and the photocatalytic degradation rate of methyl orange in simulated wastewater is the highest (about 13.3%).

Effect of Magnetic Field on Corrosion Performance of Ni–Co Alloy Coatings

The corrosion protection efficacy of Ni–Co alloy coatings was tried to improve by magnetoelectrodeposition (MED) approach. The magnetic field of varying strength (B) was applied in perpendicular and parallel to the direction of diffusion of metal ions, simultaneously to the process of deposition. The corrosion behaviour of the deposited coatings was studied through electrochemical DC method and results revealed that Magneto-electrodeposited (MED) Ni–Co alloys coatings were found to be more corrosion resistant than their conventionally electrodeposited (ED) counterparts. Moreover, the effect of magnetic field is more pronounced in perpendicular field direction and was explained by Lorentz force. Under optimal condition, MED Ni–Co alloy coating obtained at a magnetic field intensity of B = 0.3 T (Perpendicular) was found to be less prone to corrosion than its ED alloy (B = 0 T) counterpart. The increased limiting current density (iL) of Co2+ ions in turn increases the corrosion resistant properties of MED Ni–Co alloy coatings. The effect of magnetic field on improved corrosion resistance of the deposited coatings have been investigated in terms of their changed surface morphology, composition, phase structure and surface roughness using Scanning electron microscopy (SEM), Energy dispersion spectroscopy (EDS), X-Ray diffraction (XRD) technique and Atomic Force Microscopy (AFM) respectively.

Development of Ni-B electroless coating from stabilizer free bath and characterization of high temperature tribological behaviour, scratch and corrosion resistance

The current work investigates the effects of variation of coating bath temperature on friction and wear behaviour of electroless Ni–B (ENB) coatings developed from stabilizer free bath. Coating is applied to specimens made up of AISI 1040 steel. Coatings were deposited at three different coating bath temperatures (85 °C, 90 °C and 95 °C). Field emission scanning electron microscopy, inductively coupled plasma-optical emission spectrometer, and x-ray diffraction were used to characterize the coating for surface morphology, chemical composition, and phase structure respectively. Pin-on-disc tribo-tester was used to estimate the friction and wear behaviour of ENB coatings at room temperature (25 °C), 100 °C, 200 °C and 300 °C. The coefficient of friction was higher at high temperature due to higher roughness of the coatings obtained from stabilizer free bath, adhesion and ploughing. The wear rate at 200 °C or 300 °C was lower compared to 100 °C. Additionally, the ENB coatings were subjected to thermogravimetric analysis which reveals higher thermal stability of coatings obtained at 95 °C. A scratch tester at constant (6 N) and progressive load (5–24 N) was used to estimate the coatings scratch hardness and adhesion. The corrosion behaviour of ENB coatings in 3.5% NaCl was studied using potentiodynamic polarization tests. The Ni-B coated specimens could efficiently provide barrier protection to steel substrate. But the corrosion potential was lower compared to lead stabilized bath.

Constructing high performance Li-rich Mn-based cathode via surface phase structure controlling and ion doping

Li-rich Mn-based cathode materials are one kind of the promising potential candidates to electric vehicles powered by high-energy density lithium-ion batteries due to its much higher theoretical energy density. Unfortunately, the rapid capacity fading and voltage decay are the most critical factors affecting its practical application. Herein, Li1·17Na0·02Mn0·54Ni0·13Co0·13O2 (PN-LMNCO) is prepared via surface phase structure controlling and ion doping through an architecture strategy of surface lithium deficiency. It is found that the existence of lithium deficiencies can induce surface phase transformation, and thus resulting in an in-situ spinel surface conversion film, which can restrain the structure degradation during subsequent charge/discharge process. In addition, because of the larger ion radius than Li+, Na+ doping can effectively increase the spacing between Li layers, and thus improve the rate capacity. Accordingly, the as-prepared sample displays as a significantly higher initial coulombic efficiency (91.2%). After 200 cycles at 1 C, the PN-LMNCO can retain 94.7% discharge specific capacity. Furthermore, PN-LMNCO can still show a good discharge capacity of 214 mA h g−1 even at a high current rate of 5 C. Therefore, this work can preferably meet the need of the development of electric vehicle for high-energy density Lithium-ion battery.

Contributions of Mn-doping in CuO/Al2O3 sorbent for enhancement of H2S removal at low and wide temperature range

In this study, the sorbents of Mn doped CuO loading on alumina particles have been successfully synthesized by hydrothermal method for capturing H2S at low-temperature in a dry and anaerobic environment. Effects of the calcination temperature and Cu/Mn molar ratio on the desulfurization performance of CuO/Al2O3 sorbents are investigated. The low temperature desulfurization performances of CuO/Al2O3 are detected in the fixed-bed reactor and the surface morphologies, phase structures and desulfurization mechanism are systematically characterized using XRF, FESEM, XRD, XPS and HRTEM techniques. It is found that the sorbent prepared under optimum conditions is 300Cu-Mn-(1:1) sorbent and Mn-doping successfully modified the CuO microstructure into fibers. The average breakthrough H2S capacity of 300Cu-Mn-(1:1) sorbent is above 49.7 mg/g from 150 °C to 60 °C with over 62.3 % CuO active component utilization. Mn doping promotes CuO’s breakthrough H2S capacity by 176 %, because the doping of Mn ions may introduce more oxygen vacancies in CuO lattice, resulting to the decrease of the mass-transfer resistance of S ions in the CuO lattice at low temperature. This study provides a reference for low temperature desulfurization during iron production.

Surface chemical heterogeneous distribution in over-lithiated Li1+xCoO2 electrodes

In commercial Li-ion batteries, the internal short circuits or over-lithiation often cause structural transformation in electrodes and may lead to safety risks. Herein, we investigate the over-discharged mechanism of LiCoO2/graphite pouch cells, especially spatially resolving the morphological, surface phase, and local electronic structure of LiCoO2 electrode. With synchrotron-based X-ray techniques and Raman mapping, together with spectroscopy simulations, we demonstrate that over-lithiation reaction is a surface effect, accompanied by Co reduction and surface structure transformation to Li2CoO2/Co3O4/CoO/Li2O-like phases. This surface chemical distribution variation is relevant to the depth and exposed crystalline planes of LiCoO2 particles, and the distribution of binder/conductive additives. Theoretical calculations confirm that Li2CoO2-phase has lower electronic/ionic conductivity than LiCoO2-phase, further revealing the critical effect of distribution of conductive additives on the surface chemical heterogeneity evolution. Our findings on such surface phenomena are non-trivial and highlight the capability of synchrotron-based X-ray techniques for studying the spatial chemical phase heterogeneity.

Bulk vs. surface structural phases in Fe-27Ga alloy

The crystal structure of the bulk quenched Fe-27Ga alloy has been studied using high-resolution neutron and X-ray diffraction to determine the phase distribution over the volume of the material. Neutron diffraction results have shown that the bulk sample consists of completely ordered D03 phase. X-ray diffraction (XRD) with a long acquisition time has allowed investigating precisely the surface layer with a thickness of ∼ 4–16 µm after surface preparation by different methods. The obtained results have confirmed the presence of the ordered D03 phase. Additionally, the required mechanical and/or chemical treatments of the sample for the XRD experiments led to the formation of extra phases with less ordered structures, B2 and A2, in the surface layer. It has been found that the ratio and volume fraction of the additional phases depend on contact duration of the alloy with the air atmosphere. Existence of significant amounts of m-D03 phase, which is considered by several authors to be responsible for the giant magnetostriction of Fe-Ga alloy, is excluded by both neutron and XRD data.

Enhancement of magnetic sensing performance of diamond resonators coupling with magnetic-strictive FeGa films by various interlayers

The authors report the enhancement of the magnetic sensitivity of single-crystal diamond (SCD) MEMS magnetic sensor, which is composed of a SCD cantilever integrated with a magneto-strictive Galfenol (FeGa) thin film. To improve the magnetic sensing properties from room temperature to high temperatures, different interlayers of Ti film, WC film, and WC/Ti film are inserted between the SCD cantilevers and the FeGa thin films in the SCD-MEMS magnetic sensors. The thermal-stability of the microstructures, phase structures, and magnetic properties of the FeGa thin films are greatly enhanced by these interlayers. The FeGa thin films within the FeGa/SCD, FeGa/Ti/SCD, FeGa/WC/SCD, and FeGa/Ti/WC/SCD structures maintain high thermal-stability in surface morphologies, roughness, phase structures, and magnetic properties under the annealing temperature up to 573 K, 773 K, 573 K, and 673 K, respectively. The magnetic sensor with the FeGa/SCD structure has the weakest magnetic sensitivity of 1.96 Hz/mT @573 K and the FeGa/WC/Ti/SCD structure hosts the highest magnetic sensitivity of 75.40 Hz/mT @673 K. The present work provides a strategy to tailor the magnetic sensing properties of SCD MEMS for the applications under extreme conditions.

Fluidization behavior and reduction kinetics of pre-oxidized magnetite-based iron ore in a hydrogen-induced fluidized bed

The influence of different pre-oxidation temperatures and pre-oxidation degrees on the reduction and fluidization behaviors of magnetite-based iron ore was investigated in a hydrogen-induced fluidized bed. The raw magnetite-based iron ore was pre-oxidized at 800 and 1000°C for a certain time to reach a partly oxidation and deeply oxidation state. The structure and morphology of the reduced particles were analyzed via optical microscope and scanning electron microscopy (SEM). The reaction kinetic mechanism was determined based on the double-logarithm analysis. The results indicate that the materials with higher oxidation temperature and wider particle size range show better fluidization behaviors. The lower oxidation temperature is more beneficial for the reduction rate, especially in the later reduction stage. The pre-oxidation degree shows no obvious influence on the fluidization and reduction behaviors. Based on the kinetic analysis, the reduction progress can be divided into three stages. The reduction mechanism was discussed combing the surface morphology and phase structure.

Anti-corrosion performance of Si-surface-alloying NdFeB magnets obtained with magnetron sputtering and thermal diffusion

Si alloying in the surface layer of NdFeB magnets was realized by thermal diffusion combined with magnetron sputtering. The surface composition, phase structure and morphology of NdFeB(S–Si) specimens were characterized by an X-ray diffractometer, an X-ray photoelectron spectrometer and a field emission scanning electron microscope, respectively. The corrosion resistance of bare NdFeB(S–Si) was analyzed by static full immersion corrosion test and electrochemical experiments. Effects of sputtering and thermal diffusion on the microstructure and corrosion resistance of the surface layer were studied. Results show that surface alloying layer can effectively improve the corrosion resistance of bare NdFeB with the optimized static total immersion corrosion test time in NdFeB(1S–Si)-800 of 36 h, which is much longer than that of the pristine NdFeB (less than 0.5 h). The Ecorr of NdFeB(1S–Si)-800 positively shifts from −1.05 to −0.92 V, indicating that the corrosion tendency is obviously lower. The Jcorr is 1.45 × 10−6 A/cm2 which is 2 orders of magnitude lower than that of the pristine NdFeB (5.25× 10−4 A/cm2). The intergranular composite oxides existing in Nd-rich phase contribute to the enhancement of corrosion resistance of Si-surface-alloying NdFeB.

Micromorphology, Microstructure, and Wear Behavior of AISI 1045 Steels Irregular Texture Fabricated by Ultrasonic Strengthening Grinding Process

In this study, the tribological properties of three AISI 1045 steel samples were investigated. Two samples were treated with ultrasonic shot peening (USP) and ultrasonic strengthening grinding process (USGP), respectively, while the other one was only treated with a polishing process. Sample properties, such as surface morphology, roughness, microhardness, elastic modulus, frictional coefficient, and phase structures were analyzed. Results show that the sample treated with USGP had the best tribological properties. It realized the highest surface roughness, microhardness, and elastic modulus. Compared with a polished sample, the roughness of the sample treated with USGP increased by 157%, and the microhardness and elastic modulus improved by 32.8% and 21.3%, respectively. Additionally, USGP provided an average frictional coefficient of 0.4, decreasing approximately 45% compared to polishing. The possible mechanisms of USGP surface texturing were discussed. The findings denote that USGP could be an efficient approach to improve the fatigue life of some mechanical components.