Surface Review and Letters

Plasma electrolytic saturation (PES) technique which holds the advantages of short treating time and limited heating influence and immediate quenching effect is conducted under high voltage power supply in some electrolyte has been extensively applied to enhance the surface performance of metallic materials. Steel is widely used in various fields thanks to its promising merits of easy workability, plasticity, toughness and weldability. It accounts for a large proportion in the application scope of the metal materials. Steel surfaces with good corrosion resistance, promising wear resistance and high hardness would be obtained by PES. Meanwhile, uniformed coatings can be formed without special requirements for substrate geometries using the PES. This paper first presents a brief introduction of the technological principle of PES. The status on studies and applications of PES for improving surface performance of steels has been reviewed.

Magnesium (Mg) alloys have been increasingly used in industries and biomaterial fields due to low density, high specific strength and biodegradability. However, poor surface-related properties are major factors that limit their practical applications. This paper mainly focuses on laser-based anti-corrosion technologies for Mg alloys, beginning with a brief review of conventional methods, and then demonstrates the feasibility of laser surface technologies including laser surface melting (LSM), laser surface alloying (LSA), laser surface cladding (LSC) and laser shock peening (LSP) in achieving enhancement of corrosion resistance. The mechanism and capability of each technique in corrosion resistance is carefully discussed. Finally, an outlook of the development of laser surface technology for Mg alloy is further concluded, aiming to serve as a guide for further research both in industry applications and biomedical devices.

0.4C–13Cr stainless steel was alloyed with niobium using double glow plasma surface alloying and tribological properties of Nb-alloyed steel such as hardness, friction and wear were measured. Effects of the alloying temperature on microstructure and the tribological behavior of the alloyed steel were investigated compared with untreated steel. Formation mechanisms of Nb-alloyed layers and increased wear resistance were also studied. The result shows that after surface Nb-alloying treatment, the 0.4C–13Cr steel exhibits a diffusion adhesion at the alloyed layer/substrate interface and improved tribological property. The friction coefficient of Nb-alloyed steel is decreased by about 0.3–0.45 and the wear rate after Nb-alloying is only 2–5% of untreated steel.

In this paper, AlCrFeNi high entropy alloy coating was fabricated on the surface of Q235 steel using hot pressing sintering process. The coating has the controlled thickness size and excellent mechanical properties. Scanning electron microscopy (SEM), XRD and hardness testing method were used to study the morphology, phase structure and hardness of high entropy alloys coating. The lattice distortion plays a significant role in increasing the hardness. Coating formation mechanism caused by the element diffusion under the hot pressing effect is also discussed in the paper. Simultaneously, the dry sliding and oil lubrication wear tests, wear morphology observation and wear mechanism discussion were completed. As the result shows, AlCrFeNi high entropy alloys coating exhibits superior wear resistance either at dry sliding or oil lubrication tests owing to its hard high entropy solid solution structure.

In situ synthesized VC–VB particles reinforced Fe-based composite coatings were produced by laser beam melting mixture of ferrovanadium (Fe–V) alloy, boron carbide (B₄C), CaF₂ and Fe-based self-melting powders. The results showed that VB particles with black regular and irregular blocky shape and VC with black flower-like shape were uniformly distributed in the coatings. The type, amount, and size of the reinforcements were influenced by the content of FeV₄₀ and B₄C powders. Compared to the substrate, the hardness and wear resistance of the composite coatings were greatly improved.

Different organometallic nanostructures on highly oriented pyrolytic graphite (HOPG) have been synthesized by different metal ions coordinating with 1,4-Dibromo-2,5-diiodobenzene (C₆H₂Br₂I₂). Scanning tunneling microscopy (STM) images directly demonstrated the transformation of the nanostructure from self-assembled nanostructures formed by C₆H₂Br₂I₂ through halogen bond into organometallic network, formed by the dehalogenated C₆H₂Br₂I₂ molecules covalent bonded with metal ions. Moreover, by varying the concentrations of C₆H₂Br₂I₂ molecules or valence states of metal ions, organometallic structures with different shapes and sizes have been fabricated, which illustrates that the concentrations and valence states of the metal ions play important roles in the organometallic nanostructures.

One of the most important hardening methods of tool steel is the use of carbide coatings. During this process, vanadium atoms diffuse the specimen’s surface at high temperature and reacts with the available carbon in steel and create vanadium carbide with high hardness. During the plasma electrolytic saturation (PES) process, the vanadium element diffuses with the help of plasma and increases up to around 950${}^{\circ }$ as a result of the temperature, providing conditions for the creation of vanadium carbide. The best combination of electrolyte is 4g vanadium oxide and 50mL hydrochloric acid and different concern of sodium hydroxide. The results indicate that the formed coating is about 30$\mu$m. As the conductivity increases, the condition for diffusion is provided; however, with increase in the temperature, the diffusion decreases. The coating formation is observed in the current at 9–11 A and hardness of this coating is about 1400vickers.

In this paper, the wettability properties of coatings with hierarchical surface structures and low surface energy were studied. Hierarchically structured coatings were produced by using hydrophobic fumed silica nanoparticles and polytetrafluoroethylene (PTFE) microparticles as additives in polyester (PES) and polyvinyldifluoride (PVDF). These particles created hierarchical micro–nano structures on the paint surfaces and lowered or supported the already low surface energy of the paint. Two standard application techniques for paint application were employed and the presented coatings are suitable for mass production and use in large surface areas. By regulating the particle concentrations, it was possible to modify wettability properties gradually. Highly hydrophobic surfaces were achieved with the highest contact angle of 165${}^{\circ }$. Dynamic contact angle measurements were carried out for a set of selected samples and low hysteresis was obtained. Produced coatings possessed long lasting durability in the air and in underwater conditions.

In this work, a novel method to determine the point of zero net proton charge (PZNPC) of colloidal kaolinite in aqueous solutions was presented through the measurement of zeta potential of the particles in various concentrations of NaCl solution and the mathematic regression of the zeta potential versus pH curves. This method is based on the observation that there are two common intersection points (CIP) in the two curves of zeta potential versus pH of colloidal kaolinite with a low and a high NaCl concentration. The observation might be attributed to that NaCl would compress the electric double layer of kaolinite particles and affect the efficiency of the protonation/deprotonation of the HD-faces. The experimental result showed that the PZNPC of kaolinite is 5.37.

Anodized aluminum oxide (AAO) templates with an average diameter of $D_1\sim 20$nm and $D_2\sim 200$nm are synthesized by two-step anodization. Nickel nanowires are fabricated by AC electro deposition with less microstructure defects at low voltage in AAO templates. Magnetic properties of compact nickel (Ni) nanowires show that easy axis is parallel to nanowire axis for diameter $D_1\sim 20$nm while by varying diameter from $D_1\sim 20$nm to $D_2\sim 200$nm, easy axis shifts to perpendicular direction of nanowire axis. This shifting of magnetic easy axis from parallel to perpendicular direction is mainly due to shape anisotropy and interactive fields between the wires. The competition between shape anisotropy (due to individual wire) and interactive fields by varying diameter of nanowires could result in tailoring of the direction of magnetic easy axis of nanowires.

The effect of wet acid oxidation by means of sulfuric/nitric acid mixtures, and high-temperature treatment of commercial arc-discharge synthesized multi-walled carbon nanotubes (MWCNTs) was studied. In order to analyze the adsorption capacities of differently treated MWCNTs, we employed a multistep method that considers separately different pressure ranges (zones) on the experimentally obtained isotherms. The method is based on simple gas isotherm measurements (N₂, CO₂, CH₄, etc.). Low pressure ranges can be described using Dubinin’s model, while high pressure regimes can be fitted using different models such as BET multilayer and Freundlich equations. This analysis allows to elucidate how different substrate treatments (chemical and thermal) can affect the adsorbate–adsorbent interactions; moreover, theoretical description of adsorbate–adsorbate interactions can be improved if a combination of adsorption mechanisms are used instead of a unique model. The results hereby presented also show that, while MWCNTs are a promising material for storage applications, gas separation applications should carefully consider the effect of wide nanotube size distribution present on samples after activation procedures.

The semiconducting two-dimensional (2D) architectures materials have potential applications in electronics and optics. The design and search of new 2D materials have attracted extensive attention recently. In this study, first principle calculation has been done on 2D gallium nitride (GaN) monolayer with respect to its formation and binding energies. The electronic and optical properties are also investigated. It is found that the single isolated GaN sheet is forming mainly ionic GaN bonds despite a slightly weaker GaN interaction as compared with its bulk counterpart. The dielectric constant value of 2D GaN is smaller as compared to 3D GaN due to less effective electronic screening effect in the layer, which is accompanied by lesser optical adsorption range and suggested to be a promising candidate in electronic and optoelectronic devices.

Pre-annealing as part of a two-step thermal oxidation process has a significant effect on the growth of hematite ($\alpha$-Fe₂O₃) nanowires on Fe foil. High-density aligned nanowires were obtained on iron foils pre-annealed at 300${}^{\circ }$C under a dry air flow for 30min. The X-ray diffraction (XRD) patterns indicate that the nanowires are transformed from the small $\alpha$-Fe₂O₃ grains and uniquely grow in the (110) direction. The formation of a high-density of small grains by pre-annealing improved the alignment and density of the $\alpha$-Fe₂O₃ nanowires.

Ba_xSr${}_{1-x}$TiO₃ ($0.1\le x\le 0.5$) (BST) thin films were prepared on La${}_{1.1}$Sr${}_{0.9}$NiO₄ (LSNO)/SrTiO₃ (STO) structure by combinatorial pulsed laser deposition (comb-PLD). The capacitances of the Au/BST/LSNO capacitors exhibited strong frequency dependence especially when the applied frequency was higher than 10kHz. On the basis of an equivalent circuit model, we presented a theoretical simulation of the relationships between capacitance and frequency for the capacitors with different electrode serial resistances. Based on the fitting results, the observed strong frequency dependence of the measured capacitance at high frequency in our study could be ascribed to the large serial resistance of 750 $\mathrm{\Omega }$ for oxide electrode LSNO. Further simulation studies found that large serial resistance (1000 $\mathrm{\Omega }$) could result in an apparent deviation from the intrinsic dielectric properties especially at high frequencies ($>$100kHz) for capacitors with capacitances above 1nF. Our results provide useful information for the design of all-oxide electronic devices.

Composite coating was prepared on AZ91D magnesium alloy with a new method which combined silica sol with micro-arc oxidation (MAO). The MAO coating was prepared on the basis of MAO solution, and then coated by sol–gel process. The composite coating was obtained after second MAO treatment. Scanning electron microscopy coupled with X-ray diffraction (XRD), energy spectrum analysis and electrochemical testing was applied to characterize the properties of MAO coating and composite coating. The experimental test results indicated that the Si element derived from SiO₂ gel particle embedded into the MAO coating by second MAO treatment. The surface of composite coating became dense and the holes were smaller with silica sol sealing process. The corrosion resistance of composite coating was improved than the MAO coating.

In order to improve the protective performance of Zn coating on reinforcing steel in concrete, the electrochemical deposition of Zn–Mn coatings was conducted on steel surface. The morphology, chemical and phase compositions of Zn–Mn coatings obtained from sulfate–citrate bath were investigated in the first part of paper. In the second part, the obtained deposits were tested in solution simulating carbonated concrete, consisting of NaHCO₃ and Na₂CO₃. Data obtained from Tafel analysis showed higher corrosion resistance for Zn–Mn alloy deposits obtained at –1700 and –1800mV versus SCE, when compared to pure Zn deposit. Impedance spectroscopy investigations revealed that the total impedance of Zn–Mn coatings increased steadily with time, and was significantly higher as compared to pure Zn after 24h in corrosion solution. On the contrary, for pure Zn, the impedance increased in the first 12h, and then decreased during prolonged exposure time, which can be explained by rapid growth of nonprotective white rust and the degradation of zinc coating, as was confirmed by optical microscope after 24h of immersion in carbonated concrete pore solution.