Surface Review and Letters

We demonstrate a morphological change in $\text{Zn}{}_{1-x}\text{Cu}{}_x\text{O}$ ($x=$ 0–5%) samples from rod-like ($x\le$ 2%) to flower-like ($x>$ 2%) with increasing Cu concentration. This effect is attributed to Cu induced change in the ZnO growth mode from one-dimensional (forming rod-like) to two-dimensional (forming flower-like) structures. The optical properties of the samples were also found to be well correlated with the observed morphological changes in the samples. Optical bandgap decreases with increasing $x$, exhibiting a minimum (2.88eV) at $x=$ 2% and afterwards it increases slightly up to 3.00eV for $x=$ 5%. Our results show an intriguing possibility of bandgap tuning by the sample morphology.

Copper aluminium oxide (Cu–Al₂O₃) films were synthesized on Si(111) substrates through RF magnetron sputtering by using the layer stacking technique. Cu and Al₂O₃ targets were used to deposit Cu and Al₂O₃ thin films under Ar atmosphere, respectively and the deposited films were then annealed under N₂ environment at 350${}^{\circ }$C, 450${}^{\circ }$C and 550${}^{\circ }$C for 6h. The structural properties of the films were investigated by using X-ray diffraction (XRD) while the surface morphology and topography of the deposited films were examined through Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray (EDX) and Atomic Force Microscopy (AFM). XRD analysis revealed the existence of multiple phases of CuO, Al₂O₃ and CuAl₂O₄ in the deposited films on Si(111) substrates. As a result of the annealing effect, the peak intensities of CuO, Al₂O₃ and CuAl₂O₄ were found to be increased along with the shifting of peak positions. Williamson–Hall (WH) analysis was also implemented to analyze the structural properties such as crystallite size, stress, strain, and energy density. Based on the three models used in WH analysis, the changes in the crystallite size and strain of the films were indicated to be anomalous with the changes in the annealing temperature. Moreover, the strain of films was also showed to be changed from compressive strain into tensile strain. The FESEM results also indicated the formation of various surface morphologies under various annealing temperatures whereas EDX analysis showed an increased atomic percentage of Cu, Al, and O due to the effect of increase in annealing temperature. The AFM analysis showed that the surface roughness of the deposited films increased with the increase in the annealing temperature.

Biocompatibility is crucial for implants. In recent years, numerous researches were conducted aiming to modify titanium alloys, which are the most extensively used materials in orthopedic fields. The application of zirconia in the biomedical field has recently been explored. In this study, the biological ZrO₂ coating was synthesized on titaniumalloy (Ti6Al4V) substrates by a duplex-treatment technique combining magnetron sputtering with micro-arc oxidation (MAO) in order to further improve the corrosion resistance and biocompatibility of Ti6Al4V alloys. The microstructures and phase constituents of the coatings were characterized by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), the surface wettability was evaluated by contact angle measurements. The results show that ZrO₂ coatings are porous with pore sizes less than 2$\mu$m and consist predominantly of the tetragonal ZrO₂ (t-ZrO${}_2)$ and cubic ZrO₂(c-ZrO${}_2)$ phase. Electrochemical tests indicate that the corrosion rate of Ti6Al4V substrates is appreciably reduced after surface treatment in the phosphate buffer saline (PBS). In addition, significantly improved cell adhesion and growth were observed from the ZrO₂/Zr surface. Therefore, the hybrid approach of magnetron sputtering and MAO provides a surface modification for Ti6Al4V to achieve acceptable corrosion resistance and biocompatibility.

The material removal rate (MRR) during precision finishing/polishing is a key factor, which dictates the process performance. Moreover, the MRR or wear rate is closely related to the material/part reliability. For nanoscale patterning and/or planarization on nano-order thickness coatings, the prediction and in-process monitoring of the MRR is necessary, because the process is not characterizable due to size effects and material property/process condition variations as a result of the coating/substrate interactions. The purpose of this research was to develop a practical methodology for the prediction and in-process monitoring of MRR during nanoscale finishing of coated surfaces. Using a specially designed magnetic abrasive finishing (MAF) and acoustic emission (AE) monitoring setup, experiments were carried out on indium-zinc-oxide (IZO) coated Pyrex glasses. After a given polishing time interval, AFM indentation was conducted for each workpiece sample to measure the adhesion force variations of the coating layers (IZO), which are directly related to the MRR changes. The force variation and AE monitoring data were compared to the MRR calculated form the surface measurement (Nanoview) results. The experimental results demonstrate strong correlations between AFM indentation and MRR measurement data. In addition, the monitored AE signals show sensitivity of the material structure variations of the coating layer, as the polishing progresses.

Wear and friction hold an important place in engineering. Currently, scientific societies are struggling to control wear by means of studies on lubricants. Boric acid constitutes an important alternative with its good tribological properties similar to MO₂S and graphite alongside with low environmental impacts. Boric acid can be used as a solid lubricant itself whereas it can be added or blended into mineral oils in order to yield better mechanical and tribological properties such as low shear stress due to the lamellar structure and low friction, wear and surface roughness rates. In this study, distinguishing from the literature, boric acid addition effect considering the temperature was investigated for the conventional ranges of internal combustion engines. Surface roughness, wear and friction coefficient values were used in order to determine tribological properties of boric acid as an environmentally friendly additive and mineral oil mixture in the present study. Wear experiments were conducted with a ball on disc experimental setup immersed in an oil reservoir at room temperature, 50${}^{\circ }$C and 80${}^{\circ }$C. The evolution of both the friction coefficient and wear behavior was determined under 10N load, at 2m/s sliding velocity and a total sliding distance of 9000m. Surface roughness was determined using atomic-force microscopy (AFM). Wear rate was calculated utilizing scanning electron microscope (SEM) visuals and data. The test results showed that wear resistance increased as the temperature increased, and friction coefficient decreased due to the presence of boric acid additive.

Oxygen-deficient zinc oxides thin films with different levels of defects were prepared by using radio frequency magnetron sputtering method with sintered zinc oxide disk as target at different sputtering powers. The composition, structure and electrical properties of the prepared films were investigated. Under the present conditions, all the obtained films possessed würtzite structure, which were growing preferentially along the $c$-axis. The thickness of the films, the size of the zinc oxide grains and the content of Zn atoms increased with increasing sputtering power. In the films deposited at a sputtering power from 52W to 212W, the main defect was interstitial zinc. With increasing sputtering power, due to the enhanced number of interstitial zinc in the films, their room-temperature electrical resistivity would decrease, which was controlled by electron conduction. At increasing measurement temperature, their electrical resistivity would increase, owing to the decrease of defect concentration caused by oxidization.

This paper presents the results of an experimental work devoted to the synthesis and the characterization of tin dioxide (SnO${}_2)$ thin layers doped with group-IA elements (Li, Na and K). The materials were synthesized by the sol–gel method and deposited by dip-coating, using tin (II) chloride dihydrate as a source of tin and absolute ethyl alcohol as solvent. Thin films prepared were characterized by several techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), visible and ultraviolet spectroscopy and complex impedance method. The results obtained show that the materials kept their tetragonal rutile structure with preferred orientation of (101), whereas doping leads to a reduction of their energy band gap. The complex impedance analysis suggests that the different processes occurring at the electrode interface are modeled by an electrical circuit not affected by the doping.

In the present work, we apply wurtzoids nanocrystals with density functional theory to explain the sensitivity of ZnO nanostructures towards hydrogen and oxygen molecules. Present results of ZnO nanocrystals’ sensing to H₂ and O₂ molecules show a reduction in the energy gap and hence electrical resistivity of ZnO nanocrystals upon attachments of these molecules in agreement with experiment. The results also show that higher temperatures increase the sensitivity of ZnO wurtzoids towards H₂ and O₂ molecules with the maximum sensitivity approximately at 390${}^{\circ }$C and 417${}^{\circ }$C for H₂ and O₂ molecules, respectively, after which it begins to decline according to calculated Gibbs free energy. These temperatures are comparable with experimentally reported operating temperatures of 325${}^{\circ }$C and 350${}^{\circ }$C for the two gases, respectively. The main reaction mechanism is the dissociation of H₂ or O₂ molecules on ZnO nanocrystal surface in which hydrogen and oxygen atoms are attached to neighboring Zn and O surface atoms. The removal of these molecules from the surface is also performed by the formation of H₂ and O₂ molecules prior to their removal from the ZnO nanocrystal surface. Electronic charge transfers to the adsorbed atoms and molecules confirm and illustrate the mechanism mentioned above.

In order to improve the wear resistance of Ti alloys, different mass ratios of Ti-Si-Al powders were designed to fabricate hard phases reinforced intermetallic matrix composite coatings on the Ti-6Al-4V substrate by laser cladding. The corresponding coatings were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and high resolution transmission microscopy (HRTEM). The HV-1000 hardness tester and MM200 wear test machine were employed to test the hardness and the wear resistance of the composite coatings, respectively. The composite coatings mainly consisted of the reinforcements of Ti₅Si₃, Ti₃AlC₂ and Ti₇Al₅Si${}_{12}$ and the matrix of Ti₃Al, TiAl, TiAl₃ and $\alpha$-Ti. The micro-hardness of the Ti-35Al-15Si coating was from 956 HV${}_{0.2}$ to 1130 HV${}_{0.2}$, which was approximately 3–4 times of the substrate and the highest in the three samples. The wear rate of the Ti-35Al-15Si coating was 0.023cm³$\cdot$min${}^{-1}$, which was about 1/4 of the Ti-6Al-4V substrate. It was the lowest in the three samples.

Fe/(Zn–6%Al–$x$%Mg) solid–liquid diffusion couples were kept at various temperatures for different periods of time to investigate the formation and growth of the Fe–Al alloy layer. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) were used to study the constituents and morphology of the Fe–Al alloy layer. It was found that the Fe₂Al₅Zn_x phase layer forms close to the iron sheet and the FeAl₃Zn_x phase layer forms near the side of the melted Zn–6%Al–3%Mg in diffusion couples. When the Fe/(Zn–6%Al–3%Mg) diffusion couple is kept at 510${}^{\circ }$C for more than 15min, a continuous Fe–Al alloy layer is formed on the interface of the diffusion couple. Among all Fe/(Zn–6%Al–$x$%Mg) solid–liquid diffusion couples, the Fe–Al alloy layer on the interface of the Fe/(Zn–6% Al–3% Mg) diffusion couple is the thinnest. The Fe–Al alloy layer forms only when the diffusion temperature is above 475${}^{\circ }$. These results show that the Fe–Al alloy layer in Fe/(Zn–6%Al–$x$%Mg) solid–liquid diffusion couples is composed of Fe₂Al₅Zn_x and FeAl₃Zn_x phase layers. Increasing the diffusing temperature and time period would promote the formation and growth of the Fe–Al alloy layer. When the Mg content in the Fe/(Zn–6%Al–$x$%Mg) diffusion couples is 3%, the growth of the Fe–Al alloy layer is inhibited. These results may explain why there is no obvious Fe–Al alloy layer formed on the interface of steel with a Zn–6%Al–3%Mg coating.

In the present investigation, WC-10Co-4Cr coating was deposited by high velocity air-fuel (HVAF) process on CA6NM hydro turbine steel. A detailed microstructural and phase compositional study was carried out on the coating. Mechanical properties of the coating were also evaluated. WC-10Co-4Cr coating showed a homogeneous, well-bonded structure with low porosity, which is mainly attributed to less decarburization of WC. Erosion resistance of the coating was evaluated by air jet erosion tester at three different impingement angles (30${}^{\circ }$, 60${}^{\circ }$ and 90${}^{\circ }$) for 35 and 70m/s impact velocities. The FESEM micrographs were taken, before and after erosion tests, to determine the erosion mechanism. The test results revealed that the coating protects the substrate at 30${}^{\circ }$, 60${}^{\circ }$ and 90${}^{\circ }$ impingement angles. At 70m/s impact velocity, uncoated and coated steel showed higher cumulative volume loss than in the case of 35m/s impact velocity. It was observed that uncoated steel showed a ductile behavior during erosion and WC-10Co-4Cr coating showed mixed (ductile and brittle) mode of fracture during erosion.

The present work is concerned with the preparation of thin films of nanocrystalline porous silicon (PSi) by the method of electrochemical etching. CdTe nanoparticles (NPs) have been prepared by utilizing the pulsed laser ablation in liquid. The measurements of tunneling microscopy, X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR) and atomic force microscopy (AFE) were carried out and revealed that the PSi was nanostructured and the produced CdTe NPs were ball shaped, having good disposability. The diffusion of CdTe NPs on the properties of PSi solar cell assures that there was an improvement upon their properties. The relationship between $1∕C^2$ and the reverse bias voltage was observed to be linear. Values of the built-in potential were observed to be dependent on the laser fluence, current density and the etching time.

The inhibitive characteristics of aqueous extracts from mangrove (Rhizophora apiculata) bark and leaf on the corrosion of mild steel (MS) coupon in 1 M HCl were examined by means of the gravimetric measurement (weight loss), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The MS surface morphologie were investigated using field emission scanning electron microscopy (FESEM) with Energy Dispersive X-Ray Analysis (EDX) at 120h in the presence and absence of green inhibitors in corrosive medium. Respective inhibition efficiencies of 68.1% and 59.0% were obtained when exposed to 10% v/v Rhizophora apiculata solution for 72h. Examination of the adsorption behavior of the solutions indicated the Langmuir isotherm model as being the most likely relevant adsorption mechanism. In addition, polarization measurements showed that both bark and leaves extract inhibitors act via mixed type inhibition.

The present work investigates the effects of heat treatment on friction and wear behavior of electroless Ni–B coatings at elevated temperatures. Coating is deposited on AISI 1040 steel specimens and subjected to heat treatments at 350${}^{\circ }$C, 400${}^{\circ }$C and 450${}^{\circ }$C. Coating characterization is done using scanning electron microscope, energy dispersive X-Ray analysis and X-Ray diffraction analysis. Improvement in microhardness is observed for the heat treated deposits. Further, the effect of heat treatment on the tribological behavior of the coatings at room temperature, 100${}^{\circ }$C, 300${}^{\circ }$C and 500${}^{\circ }$C are analyzed on a pin-on-disc setup. Heat treatment at 350${}^{\circ }$C causes a significant improvement in the tribological behavior at elevated temperatures. Higher heat treatment temperatures cause deterioration in the wear resistance and coefficient of friction. The wear mechanism at 100${}^{\circ }$C is observed to be predominantly adhesive along with abrasion. While at 300${}^{\circ }$C, abrasive wear is seen to be the governing wear phenomenon. Formation of mechanically mixed layers is noticed at both the test temperatures of 100${}^{\circ }$C and 300${}^{\circ }$C for the coatings heat treated at 400${}^{\circ }$C and 450${}^{\circ }$C test temperature. The predominant wear mechanisms at 500${}^{\circ }$C are abrasive and fatigue for as-deposited and heat treated coatings, respectively.

Ni–W–Al₂O₃ nanocomposite coatings were deposited on the low-carbon steel substrates from aqueous sulfate-citrate electrolytes containing various amounts of Al₃O₂ nanoparticles. Their surface morphology, element and phase composition were carried out using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The corrosion resistance was carried out using the potentiodynamic polarization (Tafel) and electrochemical impedance spectroscopy (EIS). It is shown that incorporation of Al₂O₃ nano-particles into amorphous Ni–W coating would transform its structure to being crystalline and influence its properties. The hardness sharply increases and corrosion resistance performance significantly decreases even after few Al₂O₃ nanoparticles (2 g/L) were added into the bath. In addition, with increase of Al₂O₃ addition in the bath, a tendency of increase in hardness and corrosion resistance performance is observed.

In order to evaluate the left life of the aircraft tank in the storage stage and enhance the reliability of aircraft, the electrochemical corrosion characteristic of 2A14 aluminum alloy has been investigated by the polarization curve method, electrochemical impedance spectroscopy methods and soaking corrosion test in this paper. The corrosion equivalent relationship of 2A14 aluminum alloy at different conditions was established. Moreover, the corrosion mechanism and law of 2A14 aluminum alloy in the solution of N2O4-HNO3 were obtained. It is shown that the nitric acid with the volume fraction of 30% corrodes 2A14 aluminum alloy had the fastest speed. The corrosion intensity of the specimen increases with the increase of the environment temperature. In the corrosion program, the corrosion is begun with the pit with very small size. As the time passes by, the pits expand and the adjacent pits combines together to be a larger pit. Finally, the thickness of the specimen decreases uniformly as the corrosion product peels off from the surface of the specimen. Therefore, the corrosion evaluation of aluminum alloy structure can be performed by weighting or measuring the thickness in application.