Surface Review and Letters

The porous silicon (PSi), which is produced by the electrochemical etching, has been used as a substrate for the growth of the titanium oxide (TiO₂) thin films. By using the EBPVD method, TiO₂ thin films have been deposited on the surface of the PSi substrate. TiO₂/PSi layers were annealed at the temperature of 400${}^{\circ }$C, 500${}^{\circ }$C and 600${}^{\circ }$C for different tests. The morphology and structures of layers were investigated by the scanning electron microscopy (SEM) and X-ray diffraction (XRD). The current–voltage characteristic curves of samples and the ideality factor of heterojunction were studied. The results showed that the electrical properties of the samples change with increase in the annealing temperature. The optical properties of the prepared samples were investigated by using UV–Vis and photoluminescence (PL) spectroscopy. Green light emission of the PSi combined with the blue light and violet–blue emission obtained from the TiO₂/PSi PL spectra. The results showed that the optical band gap energy of the PSi has increased from 1.86eV to 2.93eV due to the deposition of TiO₂ thin film.

Recent advances in polymer nanocomposites open a wide range of applications in various industrial sectors. Due to their high potential properties, these materials are replacing the usage of metals for many heavier components in automobile industries. In this experimental work, the tribological performance of Graphene oxide (GO) — Polyamide is investigated against pristine polyamide by fabricating gears for the usage in engineering applications. A gear test rig was developed in-house for analysis to study the specific wear rate and temperature gradient at different conditions of load and speeds. The wear resistance of the polyamide gears with the addition of 0.03wt.% of graphene oxide is better than the pristine polyamide gears and the specific wear rate is reduced significantly. The reduced specific wear rate of these polymer nanocomposite gears is attributed to the superior properties of graphene oxide such as High specific surface area, good adhesion properties and enhanced glass transition temperatures. The GO nanocomposite gear seems to be a potential alternative against conventional gears for engineering applications. Finally, the wear mechanisms and the potential of GO-based polyamide nanocomposite gears were proposed tentatively in the development of transmission gears for engineering applications.

The CVD diamond film with favorable adhesion and relatively thinner thickness is essential facing for its application on drills for machining carbon fiber reinforced plastics (CFRP), with regard to either the tool lifetime or the machining quality. A 500-nm-thick CrN layer was deposited by the cathode arc technique on slight chemical etched WC–Co 6wt.% drill, and nano-crystalline diamond (NCD) is subsequently deposited by the hot filament chemical vapor deposition (HFCVD) technique. The same NCD film is also deposited on the drills pretreated only by the slight chemical etching or the CrN interlayer, which are adopted as comparisons in the present study. The nucleation and growth of diamond film and the cutting performance of the coated drills are systematically studied. The results show that the drill pretreated by the slight chemical etching and CrN interlayer can acquire highest nucleation density (ND) compared to the other pretreatment methods as it sufficiently prevents the Co diffusion. The diamond-coated drill with deep chemical etching was used for comparison to study the machining quality when drilling CFRP. During machining the CFRP, the failure mode of the diamond-coated drill is mainly the delamination and peeling off of the diamond film at areas with stress concentration, while the diamond-coated drill pretreated by slight chemical etching $+$ CrN interlayer can retard such failure. The exit hole quality of CRFP machined by drill pretreated with slight chemical etching $+$ CrN interlayer is better than that by drill pretreated with deep chemical etching, which is ascribed to the different cutting edges of the drills.

A kind of coral-like melamine formaldehyde resins/carbon nanotubes (MF/CNTs) composite particles were prepared via in situ polymerization, which showed good thermal stability under 450${}^{\circ }$C. To illustrate the mechanism of the polymer growth process, silica/CNTs were also prepared via the same method to achieve a contrast. Generally, two models were used to illustrate silica growth mechanism, the monomer addition one and the controlled aggregation one. We preferred that the monomer addition model could explain the growth of the novel coral-like MF/CNTs particles.

Amorphous Al–Cu–Ti metal foams were prepared by spark plasma sintering (SPS) process with the diameter of 10mm. The SPS process was conducted at the pressure of 200 and 300MPa with the temperature of 653–723K, respectively. NaCl was used as the space-holder, forming almost separated pores with the porosity of 65 vol%. The microstructure and mechanical behavior of the amorphous Al–Cu–Ti metal foams were systematically investigated. The results show that the crystallinity increased at elevated temperatures. The effect of pressure and holding time on the crystallization was almost negligible. The intermetallic compounds, i.e. Al–Ti, Al–Cu and Al–Cu–Ti were identified from X-ray diffraction (XRD) patterns. It was found that weak adhesion and brittle intermetallic compounds reduced the mechanical properties, while lower volume fraction and smaller size of NaCl powders improved the mechanical properties.

The microstructure resulting from diffusion reactions, at solid-state temperatures, in Ni/Sn couples has been studied by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The Ni₃Sn₄ phase was the only reaction product which formed in the couples at temperatures from 150${}^{\circ }$C to 220${}^{\circ }$C. The growth kinetics of Ni₃Sn₄ layer followed the parabolic law with an apparent activation energy of 104kJmol${}^{-1}$. The effect of the atmosphere on the microstructure of the interfacial layer was investigated at 220${}^{\circ }$C for a pressure of 10${}^{-8}$atm. A decrease in the growth rate of the phase layer was observed.

Tungsten-doped TiO₂ gas sensors were successfully synthesized using sol–gel process and spin coating technique. The fabricated sensor was characterized by field emission scanning electron microscopy (FE-SEM), ultraviolet visible (UV–Vis) spectroscopy, transmission electron microscopy (TEM), X-Ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Gas sensing properties of pristine and tungsten-doped TiO₂ nanolayers (NLs) were probed by detection of CO₂ gas. A series of experiments were conducted in order to find the optimum operating temperature of the prepared sensors and also the optimum value of tungsten concentration in TiO₂ matrix. It was found that introducing tungsten into the TiO₂ matrix enhanced the gas sensing performance. The maximum response was found to be (1.37) for 0.001g tungsten-doped TiO₂ NLs at 200${}^{\circ }$C as an optimum operating temperature.

The self-assembly structures of 2,2${}^{\prime }$:6${}^{\prime }$,2${}^{\prime \prime }$-terpyridine-4${}^{\prime }$-carboxylic acid (C${}_{16}$H${}_{11}$N₃O₂; $Y_{\text{N}}$) molecules and 3,5-diphenylbenzoic acid (C${}_{19}$H${}_{14}$O₂; $Y_{\text{C}}$) molecules on Ag(110) surface have been investigated by scanning tunneling microscopy (STM) and Density Functional Theory (DFT) calculation. The $Y_{\mathrm{\text{C}}}$ molecules form two different well-organized structures due to the $\pi$–$\pi$ stacking and dipole–dipole interactions. When three C atoms of $Y_{\mathrm{\text{C}}}$ molecules are replaced by three N atoms to form $Y_{\mathrm{\text{N}}}$ molecules, the main driving force to form ordered assembly structures of $Y_{\mathrm{\text{N}}}$ molecule is changed to metal–organic coordination bond and hydrogen bond. The dramatic changes of main driving force between $Y_{\mathrm{\text{C}}}$/Ag(110) and $Y_{\mathrm{\text{N}}}$/Ag(110) system demonstrate that the N atoms are apt to form metal–organic coordination bond and hydrogen bond but dipole–dipole interactions and $\pi$–$\pi$ stacking are relative to C atoms. These findings further reveal that the optimization design of organic molecules could vary the main driving force and then lead to the change of the molecular self-assembly structures.

A ZrO₂–Al₂O₃ ceramic coating and YSZ–Al₂O₃ coating were prepared on ZAlSil₂Cu₃Ni₂ alloys by a modified plasma electrolytic oxidation (PEO) in zirconium salt solution. The microstructure, heat-insulating and high temperature oxidation properties were investigated by SEM, XRD and a fabricated heat insulation temperature testing device. The results showed that the surface of the YSZ–Al₂O₃ coating was composed of fine particles and had lower roughness. Yttrium partially stabilized the zirconia solid solution (Y${}_{0.15}$Zr${}_{0.85}$O${}_{1.93}{\square }_{0.07})$ which was formed in zirconium yttrium salt solution with a higher reaction temperature. The growth rate of the YSZ–Al₂O₃ coating was greater than the ZrO₂–Al₂O₃ coating. Especially, the outward growth obviously improves. Furthermore, the heat insulation property of the coating enhances. The curves of the weight gains in the sample with YSZ–Al₂O₃ coating showed a logarithmic shape under the high temperature oxidation treatment of 400${}^{\circ }$C. The weight gains of the sample with YSZ–Al₂O₃ coating were relatively lower than that of the as-coated sample. The ceramic layer had good high temperature stability.

Film-forming effects of cathodic deposition on pure Mg substrate at constant DC in aqueous solutions of magnesium nitrate (Mg(NO₃)${}_2\cdot$6H₂O), magnesium chloride (MgCl${}_2\cdot$6H₂O) and magnesium sulfate (MgSO${}_4\cdot$7H₂O) respectively were investigated systematically. Typical processes were studied by potentiodynamic cathodic polarization and galvanostatic polarization and typical samples were analyzed by SEM and XRD. The results indicate that the depositing efficiency is not only the highest but stablest, and deposited coatings show the best uniformity with Mg(NO₃)${}_2\cdot$6H₂O solution employed as depositing medium and applied current density $\ge 1.0$mA cm${}^{-2}$. Cathodic deposition leads to regular mass loss of Mg substrate. The cathodic polarization curve of pure Mg in magnesium nitrate solution shows more obvious pseudo-passivation, several Tafel regions with different slopes appearing before diffusion-limited current density region, and oxygen consumption is the major cathodic reduction reaction under specified current density. However, hydrogen evolution reaction is dominant in both Mg chloride and Mg sulfate solutions. The deposition coatings are all composed of continuous and uniform mesh-like “basic layer” adjacent to substrate and discrete distributed snowball-like particles on the microscopic scale. The phase compositions are all crystal Mg(OH)₂, and the coatings deposited in Mg chloride solution have (011) preferred orientation.

(Cr, Fe)₇C₃/$\gamma$-Fe composite layer has been in situ synthesized on a low carbon steel surface by vacuum electron beam VEB irradiation. The synthesized samples were then subdued to different heat treatments to improve their impaired impact toughness. The microstructure, impact toughness and wear resistance of the heat-treated samples were studied by means of optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), microhardness tester, impact test machine and tribological tester. After heat treatment, the primary and eutectic carbides remained in their original shape and size, and a large number of secondary carbides precipitated in the iron matrix. Since the Widmanstatten ferrite in the heat affected zone (HAZ) transformed to fine ferrite completely, the impact toughness of the heat-treated samples increased significantly. The microhardness of the heat-treated samples decreased slightly due to the decreased chromium content in the iron matrix. The wear resistance of 1000${}^{\circ }$C and 900${}^{\circ }$C heat-treated samples was almost same with the as-synthesized sample. While the wear resistance of the 800${}^{\circ }$C heat-treated one decreased slightly because part of the austenite matrix had transformed to ferrite matrix, which reduced the bonding of carbides particulates.

Powder mixed electro discharge machining (PMEDM) is further advancement of conventional electro discharge machining (EDM) where the powder particles are suspended in the dielectric medium to enhance the machining rate as well as surface finish. Cryogenic treatment is introduced in this process for improving the tool life and cutting tool properties. In the present investigation, the characterization of the cryotreated tempered electrode was performed. An attempt has been made to study the effect of cryotreated double tempered electrode on the radial overcut (ROC) when SiC powder is mixed in the kerosene dielectric during electro discharge machining of AISI 304. The process performance has been evaluated by means of ROC when peak current, pulse on time, gap voltage, duty cycle and powder concentration are considered as process parameters and machining is performed by using tungsten carbide electrodes (untreated and double tempered electrodes). A regression analysis was performed to correlate the data between the response and the process parameters. Microstructural analysis was carried out on the machined surfaces. Least radial overcut was observed for conventional EDM as compared to powder mixed EDM. Cryotreated double tempered electrode significantly reduced the radial overcut than untreated electrode.

The present work is focused on the optimization of process parameters in cylindrical surface grinding of AISI 1050 steel with grooved wheels. Response surface methodology (RSM) and genetic algorithm (GA) techniques were merged to optimize the input variable parameters of grinding. The revolution speed of workpiece, depth of cut and number of grooves on the wheel were changed to explore their experimental effects on the surface roughness of machined bars. The mathematical models were established between the input parameters and response by using RSM. Then, the developed RSM model was used as objective functions on GA to optimize the process parameters.

In this study, the effect of contact pressure on fretting fatigue behavior of Al7075-T6 under cyclic normal contact loading is investigated. It is found that fretting fatigue life for the case of cyclic contact load was significantly less than that for constant contact load at the same axial and contact load levels, particularly for High Cycle Fatigue (HCF) conditions. The results showed that the fretting fatigue life decreased monotonically with the increase in normal contact load for all axial stresses. Examination of the fretting scars was performed using optical microscopy and numerical simulation was carried out using commercial finite element (FE) codes ABAQUS${}^{\text{®}}$ and FRANC2D/L${}^{{}^{\text{®}}}$ to calculate the crack propagation life. The crack initiation life was calculated by a combination of numerical and experimental results. Finally, the FE simulation was validated by a comparison between the numerical crack growth rate and the experimental measurement using replica.

Milling processing is an important way to obtain wood–polyethylene composite (WPC) end products. In order to improve the processing efficiency and surface quality of WPC and meet the practical application requirements, this paper focussed on morphology and roughness of the WPC-milled surface and studied surface quality changes under different cutting parameters and milling methods through multi-parameters milling experiments. The milling surface morphology and roughness of WPC were analyzed and measured during cut-in, cutting and cut-out sections. It also revealed the affect rule of different cutting parameters and milling methods on milled surface morphology and roughness. The results show that the milling surface roughness of WPC products with wood powder content of 70% is significantly larger than the one whose wood powder content is 60%, and defects such as holes are also relatively more. Finally, a surface roughness prediction model was established based on the mathematical regression method and its multi-factor simulation was carried out. A comparative analysis of predictive and experimental values was performed to verify the reliability of the model. It could also provide theoretical guidance and technical guarantee for high processing quality of WPC milling and cutting.

A TiAlSiN coating was deposited on H13 hot work mould steel using a cathodic arc ion plating (CAIP). The friction-wear performances of the obtained coating under different loads were characterized on an HT-1000 type high temperature tribological tester at 800${}^{\circ }$C. The morphologies and chemical compositions of the worn tracks were analyzed using a scanning electronic microscope (SEM) and its configured energy dispersive spectrometer (EDS), respectively, and the corresponding wear mechanisms were discussed. The results show that the average coefficient of friction (COF) of the TiAlSiN coating at the loads of 2, 4 and 6N is 0.3072, 0.3040, and 0.4114, respectively, the COFs of the coating are stable at the loads of 2 and 4N, while those at the load of 6N are obviously increased. Under the load of 2N, a small amount of large and scattered white debris disperses on the worn track, the wear mechanism is adhesive wear. While the white wear debris appears on the worn track under the load of 4N, the COFs are relatively stable, due to the effects of oxide layer on the worn track, and the wear mechanism is abrasive wear and oxidation wear. Under the load of 6N, a number of shallow grooves and cracks appear on the worn tracks, showing that the coating is basically worn out, the wear mechanism is abrasive wear and oxidation wear.