فصلنامه علوم و مهندسی سطح ایران
پیاپی 33 (پاییز 1396)

In this study, the influence of the sodium phosphate electrolyte concentration on the coating microstructure, phase composition and corrosion resistance of Ti-6Al-4V alloy coated by plasma electrolytic oxidation (PEO) was investigated. For this purpose, alkaline-phosphate electrolytes with different concentration were applied as follows: 8, 12, 16 g/L. The morphology and phase composition of these coatings were analyzed by a scanning electron microscope (SEM), and X-ray diffraction (XRD) technique. Corrosion properties of the coating were studied under simulated body fluid (Hanks) by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. Through the PEO process, an increase in the concentration of sodium phosphate led to increment in the electrolyte electrical conductivity, average pore size and thickness of the coatings. As a consequence of increasing the electrical conductivity, the breakdown voltage in V-t curves decreased. The XRD results showed that with increase in the concentration of sodium phosphate, the metastable anatase phase transformed to the stable rutile phase. The results of the electrochemical tests indicated that the corrosion resistance of the coating produced by PEO process, in the electrolyte with 12 g/L sodium phosphate, was significantly better in comparison to the other specimens. The maximum measured corrosion potential of the coating (391 mV) together with the minimum corrosion current density (6.18×10-8 A cm-2) in the electrolyte with 12 g/L sodium phosphate leads to the highest corrosion resistance of twofold layer, that is, 8.8 MΩ. Corrosion resistance of other samples is 2 to 6 orders of magnitude smaller than sample with 12 g/L sodium phosphate.

Tungsten carbide powders have been known as one of the hardness and wear resistance materials. The combinations of carbide with soft metals such as cobalt as a binder are widely used to improve the wear resistance, toughness and prevent brittle failure. In this research, WC-12Co powders were deposited on steel substrate by high velocity oxygen fuel (HVOF) spraying. Due to the existence of pores and microcracks in the coatings, the corrosion and wear resistance properties of the coatings could be improved by sealing the porosities and cracks via a post deposition treatment. A thin layer of Al2O3 were deposited on the HVOF thermally sprayed WC-Co coatings by Sol-Gel method. The phase composition of coatings was investigated by X-Ray Diffraction (XRD) analysis and the morphology and microstructure of the coatings as well as their thicknesses were investigated by Scanning Electron Microscopy (SEM) followed by image analysis. To study the wear resistance of the coatings a pin on disk wear tester was used. The results showed that the coefficient of friction was reduced from 0.6 to about 0.3 after sealing. The wear rates of the sealed coatings were decreased from 5.5x 10-3 in the as-sprayed coatings to 3.4 x 10-3.in the sealed coatings. The wear resistance of coatings after the sol-gel sealing process was improved due to the filling of the pores and cracks as well as formation of the aluminum and cobalt oxides on the surface during wearing test.

Among different methods which have been used for degradation of organic pollutants, photocatalytic process has received a remarkable attention due to its unique properties. In the present investigation, photocatalytic titanim dioxide (TiO2) and magnetite (Fe3O4) coatings were produced using HVOF thermal spraying process. Different weight percent of TiO2 (75vol. % anatase, 25vol. % rutile) and commercial Fe3O4 powders were mixed together and used as feedstock powder and precipitated on stainless steel 316 substrate. The structure, morphology, and photo-catalytic characteristics of TiO2-Fe3O4 coatings (using different Fe3O4 concentration) were investigated using scanning electron microscopy (SEM), X-ray diffractometer (XRD), and UV-Vis-NIR spectroscopy. Furthermore, the conversion rate of xylene under Vis and UV irradiation was also studied in order to evaluate the photo-catalytic efficiency of coatings in gas dynamic photoreactor. Results showed that the optimum value of Fe3O4 concentration affects the photocatalytic properties of TiO2-Fe3O4 coatings under UV and visible light irradiation. Addition of 7.5 weight percent of magnetite has resulted in an efficiency upper than 35 percent (under UV irradiation). Also addition of 10 weight percent of magnetite has resulted in an efficiency of approximate 4 percent, under visible light irradiation.

Friction is one of the important factors influencing metal forming processes e.g. forging. Besides considering a set of basic principles, efficient lubrication is the best way to control friction and wear. One way to evaluate the effect of lubricants on the friction in forming processes, is ring compression test. By using calibration curves in the forging process, friction coefficient is calculated. In this paper, the effects of nano- particles solution including copper oxide and alumina within paraffin and oil-10 as the base lubricants on the Coulomb friction model has been studied. In order to obtain the optimal design of experiments Taguchi method has been used and considering the number of investigating factors, L8 orthogonal array selected. After carrying out all suggested tests and using calibration curves for Al7xxx, Coulomb friction coefficients for different lubricants have been obtained. Weight percentage of nano-particles and the type of both base lubricant and nano-particles additives, considered as input variables, and the Coulomb friction coefficient selected as objective variable. Through variance analysis the effectiveness of each input variable on the objective evaluated and values of the input parameters to minimize friction coefficient by Taguchi signal-to-noise method were extracted. The results show that the greatest impact belongs to weight percentage of nanoparticles with a share of 62.15 percent on the friction coefficient and increasing the weight percentage of nanoparticles lead to increasing the friction coefficient. Also, the best combination to minimize friction coefficient is 0.8% solution of alumina nanoparticles with oil 10 as base lubricant.

Surface alloying by TIG process, is one of the techniques to improve the surface properties of the steels. This method can be conducted without changing the substrate properties. Alloying is carried out by adding certain alloying elements before or during surface melting. In this study, Surface alloying is done using nickel and Tungsten Carbide reinforcement particles, and by gas Tungsten arc welding process on steel St52. Surface melting was conducted with applied current of 130 mA and the effect of increasing the percentage of WC powder on surface hardening of the samples were evaluated. The range of increasing hardness the coatings made between 225 to 680 Vickers was measured and the amount hardness of the uncoated sample is about 180 Vickers. Infrastructure assessment of worn surfaces was performed by scanning electron microscopy SEM. Typical abrasion resistance was evaluated by means of reciprocating wear machine. The results of wear surfaces showed that by increasing the amount of WC reinforcement particles, the matrix strength against changing the form is increased, and eventually causes to improve the wear resistance of the samples. The wear test results showed that with increasing percentages of Tungsten Carbide reinforcement particles up to 80%, the amount of weight loss were significantly reduced, and the wear rate of the samples decreases up to 100 ⁄ .

Molybdenum disulfide (MoS2) is one of the most widely used solid lubricants applied in different ways on the surfaces under friction. In this work, AISI 316 austenitic stainless steel was coated with MoS2, using chemical vapor deposition (CVD) at four different temperatures (400, 500, 600 and 700°C). Coatings properties were investigated using SEM, EDX, XRD and FTIR, Hardness Tester and Roughness tester. The results showed that with simultaneous evaporation of sulfur and molybdenum trioxide (MoO3) in the CVD chamber, a uniform coating layer containing MoS2 and MoO2 phases is formed. Increase in the substrate temperature resulted in the rise in the proportion of MoS2 to MoO2 phases as well as the thickness of coating. The thickness, grain size and the hardness of the coating were 17-29 µm, 50-120 nm and 260- 480 HV respectively.

In this paper, electronic and optical properties of chalchogenid 2( , , ) CuSbX X Se S Te  compounds have been studied in the surface state at (001) direction. Calculations done using pseudo-potential method within the framework of density functional theory (DFT) by using Quantum-Espresso software with the generalized gradient approximation (GGA). Band gap of 2( , ) CuSbX X Se S  in the bulk state is 0.81 and 0.93 eV respectively but in the surface state these compounds are no band gap, which the surface dangling bonds are covered the band gap. Calculations showed that the 2 CuSbTe in the bulk and surface states is metal. The surface energy, work function, density of states and optical properties in the surface state in the (001) direction are calculated with the three times of duplication layers and a 15 angstrom vacuum respectively. The experimental and theoretical results of these compound in the surface state are not available to comparison this calculation with them.

In this study, amorphous silicon films were deposited by physical vapor deposition using electron beam (EBPVD). The process of deposition was carried out under high vacuum (torr 10-6) which led to the formation of an amorphous coating with controlled thicknesses. After sampling procedure, the samples were put under heat treatments at 800 oC under inert gas environment and their structural and electrical properties were analyzed before and after annealing conditions. The results obtained from microraman revealed the amorphous structure in the initial coatings which had been formed because of the increased thickness of the coating and due to the presence of more defects in nano-crystals. Moreover, we witnessed the development, growth, and integration of nano-crystals and formation of a poly crystal structure. Consequently, nano-crystals formation resulted in development of sp2 and sp3 bonds, which, in turn, increased the electrical conductivity of the coatings in that condition. Finally, as the thickness of the samples coatings increased, their electrical conductivity was also greatly enhanced.

In this research, coating of ZrO2 nano-particles with different thicknesses on the surface of Ti-6Al-4V sheet was used for surface melting using by TIG welding process and without filler metal. After the melting operation in the presence of zirconia nanoparticles, the depth of the melting surface and hardness was measured. Also, using of optical microscope and SEM microscope, the microstructure of the different melting surface areas was investigated. To defined the presence of nanoparticles in the melted area, chemical analysis of EDS and X-ray diffraction pattern (XRD) have been carried out to identify the phases formed in the samples from melting operations. Nano particles penetrate deeply from the melt surface, resulting in a acicular structure that is interlocked. The causes of the formation of this structure is the presence of nanoparticles during surface melting operations And the formation of nucleation centers to create appropriate buds in suitable places. The applied nanoparticles on the surface with the surface tension shift gradient mechanism have increased the depth of the penetration of the melt surface. The most important effect of nanoparticles on the surface was hardness. The hardness result is in the sample, which has the highest nanoparticles. The hardness of these samples is 2.5 times higher than the base metal. With the constant consideration of welding parameters and increasing the ZrO2 nanoparticles, the depth of the melting area and the hardness of the melting region increased compared to the base metal.