Surface Review and Letters

In recent years, nanoparticles are increasingly used in scientific research and have attracted the attention of many scholars. In this paper, ceramic coatings were prepared on the surface of magnesium and its alloys using the plasma electrolytic oxidation (PEO) technique. We investigated different nanoparticles added to the electrolyte and explored the mechanism of nanoparticle effects on the formation and protection mechanism, morphology and structure, thickness and roughness, and electrochemical corrosion behavior of the coatings. The results show that the coating morphology changes significantly and the surface is more uniform and dense due to the addition of nanoparticles in the electrolyte. The addition of nanoparticles increases the thickness of the coating to some extent, but as its addition to the electrolyte increases, the coating thickness decreases. Since the prepared coatings inevitably produce micropores and microcracks, which may have an impact on the corrosion resistance of the coatings, how to improve the corrosion resistance of the coatings has become a common concern. Nanoparticles can participate in the growth of the coating and will enter the micropores under discharge conditions. On the one hand, they can play a role in closing the porous layer, and on the other hand, they will form some special structures on the surface, thus improving the corrosion resistance of the coating. Finally, we outlook the problems and challenges of the PEO technique in practical applications.

Pure copper, copper-based alloys, brass, graphite and steel are the most common tool materials for the electrical discharge machining (EDM) process. The electrode material, which exhibits good conductivity to heat and electricity and possesses better mechanical and thermal properties, is preferred for EDM applications. The major problem with conventional electrodes like copper and graphite is their low wear resistance capacity. This study is on the fabrication of Cu–SiC composites as an electrode of EDM with improved wear characteristics for machining of hardened D2 steel which is widely used in die formation. Powder metallurgy route was used to fabricate the samples which was further followed by three-step sintering process. Copper metal was used as a matrix element which was reinforced with SiC in volume fractions of 10%, 15% and 20%. Based on the desirable properties for the EDM tool, the best composition of Cu–SiC composite tool tip was suggested and was further used for machining of D2 steel. The performance of newly developed Cu–SiC composite electrode in terms of surface roughness (SR), material removal rate (MRR) and tool wear rate (TWR) was explored, and it was compared with the pure copper electrode. Pulse on-time, pulse off-time and the input current were selected as the input process parameters. Result reveals that the TWR and SR were decreased by 12–18% and 10–12%, whereas the MRR was increased by 9–28% for Cu–SiC composite tool as compared to the pure Cu electrode. Adequacy of the results was checked by statistical analysis. The surface texture of tool and machined surface was analyzed using scanning electron microscopy (SEM). SEM micrograph revealed that surface cracks on composite tool tip were lesser than pure Cu tool tip, whereas the work surface was rough while machining with the copper tool surface. Therefore, the result indicates that the newly developed Cu–SiC composite tool can be used for machining of hard materials with EDM.

The aim of this in-vitro study was to evaluate the influence of Ag${}^{+}$-ion-coated conditions on the corrosion, tribocorrosion and antibacterial properties of Ti15Mo alloy. The mean wear volume losses of all test specimens after tribocorrosion test procedures were determined using a noncontact 3D profilometer. The specimens’ hardness, roughness values and microstructures were measured using the microhardness tester, surface profilometer, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The mean wear volume loss of 30-min Ag${}^{+}$-ion-coated Ti15Mo alloy was lower than the other specimens. In this study, correlations between the hardness, surface roughness and wear volume loss were found to be significant. The PVD coating process enhanced the antibacterial activity of Ti15Mo alloy owing mainly to the formation of silver film on the substrates.

The implementation of the fused deposition modeling (FDM) technique in the production system is mainly due to its flexibility and ability to fabricate complex 3D prototypes and geometries. However, the mechanical strength of the printed parts needs to be investigated which was influenced by the process parameters such as layer thickness (LT), raster angle (RA), and Infill Density (ID). Therefore, these process parameters need to be optimized to attain better mechanical strength from the FDM printed parts. In this research, ePA-CF filament material was used to fabricate the specimens based on the selected process parameters such as LT (0.07, 0.14, and 0.20mm), RA (0^∘, 45^∘, and 90^∘) and ID (50%, 75%, and 100%). The artificial neural network (ANN) method was implemented to determine the influential printing process parameters. Tensile, flexural, and impact tests were considered as the response parameters based on the various combination of the input parameters. It was concluded that the printing of nylon carbon parts using $\mathrm{\text{LT}}=0.14$mm, $\mathrm{\text{ID}}=100\%$, $\mathrm{\text{RA}}=90^{\circ }$ retains improved tensile strength of 66 MPa, flexural strength of 87MPa and impact strength of 12.5KJ/m². Further, the propagation of cracks and the mode of failure were examined using SEM fractography. These observations substantiate that the selection of an optimal combination of FDM parameters assists in enhancing the mechanical strength of the printed nylon carbon parts.

The ternary PtNiCo catalyst grafted by sulfonic group on reduced graphene oxide (RGO–SO₃H) was prepared by a simple solvothermal method. The sheets of nanostructure were stacked in the shape of near-sphere by field-emission scanning electron microscope (FESEM). X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were carried out to explore the phase structure, element analysis and carbon hybridization, respectively. The ternary PtNiCo alloys were evenly distributed on the supports of RGO–SO₃H with size ranging from tens of nanometer in thickness and hundreds of nanometer in length. The electrocatalysis of PtNiCo/RGO–SO₃H was superior to that of PtNiCo/RGO and PtNiCo/GO catalyst for ORR. The stability of PtNiCo/RGO–SO₃H catalysts was characterized by the electrochemical surface area (ECSA) with 35% loss of the hydrogen adsorption/desorption after repeating 5000 cycles. The –SO₃H groups grafted on RGO were in favor of ORR and anchoring site for PtNiCo nanoparticles. The high lattice contraction will support the retention of Ni and Co to enhance the catalyst activity in the ternary PtNiCo alloy. The synergistic effect of –SO₃H groups and alloying elements can improve the catalytic efficiency and stability of PtNiCo/RGO–SO₃H in the potential application of proton exchange membrane fuel cells.

Hydrogen has received widespread attention as a new clean energy in order to reduce the carbon emissions of fuel vehicles. This paper studies a tubular microreactor based on methanol steam reforming. Methanol and steam are mixed in proportion and the chemical reaction takes place in a porous catalytic bed. For heating purposes, hot gas from the burner penetrates the reactor bed through heating tubes. Energy is supplied through the heating tubes to drive the endothermic reaction system. The microreactor is enclosed in an insulated jacket. In this paper, parameters such as methanol conversion and hydrogen concentration are evaluated by considering microreactor materials, heating gas temperature and flow direction, heating tube distribution, pressure drop and reaction channel length. First of all, choosing a microreactor material with a smaller thermal conductivity can avoid excessive heat loss, and improve heat transfer performance. Increasing the heating gas temperature leads to an increase in the temperature of the reaction zone, thereby increasing the CH₃OH conversion rate and H₂ mass fraction. Changing the flow direction of the heating gas affects the reaction rate, but has little effect on the reaction result. Through the research on the distribution of the heating tubes, the results show that the hydrogen production rate is higher when the contact area between the heating tubes and the reaction zone is larger. Secondly, through the comparison of the data under different pressure drops, the best parameter $P=50$pa is obtained, and the CH₃OH conversion rate is 80.6% at this time. Finally, increasing the length of the reaction channel can make the reaction more complete. For example, when the reaction channel length $L=0.39$m, the CH₃OH conversion rate is as high as 83.7%.

In order to improve the interfacial bonding at coating-substrate, a graphene oxide-hydroxyapatite (GH) coating was designed and constructed. The microstructure, bonding strength and in vitro bioactivity of the coating were analyzed. The results showed that GH coating presented uniform and crack-free structure with a flake morphology. The bonding strength of GH coating was 7.81MPa, which is about 1.2 times that of pure hydroxyapatite coating. In vitro bioactivity test using simulated body fluid exhibited that GH coating processes the ability to induce apatite formation. The GH coating should have potential application in surface modification for biomedical materials.

As a material widely used in aerospace and energy development, the surface quality of nickel and its alloy will need to be improved urgently. Electrochemical polishing, as a surface treatment method, can smoothen the metal surface and improve its corrosion resistance. Strict environmental regulations have given rise to new electrochemical polishing techniques. In this paper, the electrochemical polishing of nickel was carried out using resin particles wetted by deep eutectic solvent (DES) as polishing medium. The surface morphology and roughness of polished nickel were characterized by scanning electron microscopy (SEM) and optical profilometer. In addition, electrochemical polarization curve and electrochemical impedance spectroscopy (EIS) were used to test the corrosion behavior of polished specimens in 3.5wt.% NaCl solution. The results show that the surface morphology of polished nickel shows grain boundary characteristics. The surface roughness Ra can be reduced from 0.612$\mu$m to 0.0913$\mu$m (under 30V voltage polishing 1h), and the corrosion current density can be reduced from 27.30$\mu$Acm${}^{-2}$ to 12.15$\mu$Acm${}^{-2}$. Pitting potential in the polarization curve indicates that the pits at the grain boundaries are corroded due to the influx of corrosive chloride ions. This polishing method combines the resin with DES which can reduce the production of polishing liquid waste while avoiding the use of harmful acid-base electrolytes and effectively reduce the surface geometry uneven degree, improving the surface corrosion resistance.

In this paper, a high-throughput droplet method is presented for screening corrosion inhibitors, particularly for those metals that are subjected to pitting. To this objective, AA5083 was used as a case study as it is subject to pitting corrosion in saline solution containing different corrosion inhibitors. This paper outlines how critical parameters are measured and calculated (average pit depth and maximum pit depth) and the errors and consistency of the method, and the definition of corrosion inhibition via the method. The results from this method were then compared to the results of inhibition efficiency derived from potentiodynamic polarization scans for inhibitors with a range of performance. The method was conducted at temperature $T=22\pm 2^{\circ }$C inside a humidity chamber, and concentration 10${}^{-3}$ M of seven chemical compounds that have similarities in their structures. The discrepancies were profound for the cases where the inhibitor was of low efficiency, which is ascribed to the fact that these pits’ growth is dominated by that of a few large pits.