IRANIAN JOURNAL OF SURFACE SCIENCE AND ENGINEERING
Year:2012 | Volume:- | Issue:16|Papers Count:10

Anodic oxidation can be done in order to obtain a porous structure, and to obtain TiO2 nanotubes. In the current research, first, titanium dioxide nanotubes were fabricated by anodic oxidation and then hydroxyapatite nanoparticles have been deposited onto nanotublar TiO2 by electrophoretic deposition at 30V for 10min and then sintered at 400oC for 2hr. Corrosion behavior in SBF at 37oC was evaluated. The results show that formation of TiO2 nanotubes and sintering improve corrosion behavior in SBF.

Anodizing, as an electrochemical process, has been used for enhancing the corrosion resistance of aluminum and its alloys. The type of current (direct and pulse) strongly affects the properties of anodic films, which are obtained by the anodizing process. In this research, The 1070 aluminum alloy is anodized either by direct current (DC) or by pulse current (PC) (with different frequencies and duty cycles), in 10%wt acid sulfuric at 10oC temperature. Scanning electron microscopy (SEM) and Potentiodynamic Polarization method (in 0.5 M NaCl solution), respectively, are used for characterization of the surface morphology and corrosion resistant of samples, respectively. The results indicate that the pulse current improves the morphology of the anodized samples, but the corrosion resistance of these samples is lower than the sample anodized by direct current. The results also showed that in PC anodizing, Ecorr and icorr of samples are strongly affected by duty cycle. Among the PC samples, the sample is anodized at 200 Hz and duty cycle 70% showed the best corrosion resistance.

In this study, pulsed plasma nitriding treatment was performed on hot work tool steel AISI H11 samples and plastic mould steel AISI P20 samples at different process temperatures of 500oC and 550oC for 5 and 10 h in gas mixture of 75% N2 - 25% H2. Diffusion zones and compound layers were studied by optical and scanning electron microscopy and the surface phases were determined by X-ray diffraction analysis. Moreover, micro hardness changes from surface to core of the samples were determined. Results have shown that, at any specific plasma nitriding temperature and time, due to the microstructure and appropriate paths for diffusing of nitrogen in AISI H11 in comparison with AISI P20, the diffusion and compound layers of AISI H11 are thicker than that of AISIP20. Regarding the results of EDS, upward diffusion of carbon occurred during the ion nitriding of AISI P20, but EDS analysis of diffusion zone of AISI H11 does not show carbon. XRD results have shown that nitriding layer in AISI H11 iscomposed of g-nitride and e-nitride, while e-nitride was the most forming phase in AISI P20 at all the process parameters. The results of micro hardness profiles at the same depth, have shown that AISI H11 is harder than AISI P20.

Thermal barrier coatings (TBCs) in gas turbine engines are extensively used to produce thermal insulation for hot sections and to increase the turbine efficiency. The purpose of this investigation is the production and characterization of a two layer thermal barrier coating consisting of ceria and yttria stabilized zirconia (CYSZ) top coat and NiCoCrAlY bond coat on the superalloy substrate. For this scope, NiCoCrAlY bond coat followed by CYSZ coating were deposited on the substrates of IN 738 coupons by atmospheric plasma spraying (APS). Adhesion strength test was done similar to ASTM C633-01. To evaluate thermal insulation capability, the thermal insulation capacity test was performed. For coating characterization, microstructural investigation using field emission scanning electron microscope (FESEM) and phasic analysis using x-ray diffractometer (XRD) were performed. Results revealed that using atmospheric plasma spray process can fabricate the Thermal barrier coating with high thermal insulation capability.

Thermally sprayed coatings are inherently associated with residual stresses in the coatings. These stresses vary in nature and magnitude, and have a pronounced effect on the mechanical behavior of the system. This study develops the EDM drilling process to measure the residual stress in WC-Co thermally sprayed coatings, the stress state of which is difficult to obtain by a widely used method called high-speed hole-drilling method (ASTM Standard E837). The experimental results reveal that the stress curves are not uniform throughout the coating thickness. It has been found that the mean residual von Mises stress is of approximately 126MPa. Obtained results were confirmed by the value of the residual stress measured by postmortem curvature method in this study (-164MPa). This technique confirms the feasibility of residual stress measurement on HVOF thermally sprayed WC-Co coatings using ED-hole drilling method. Morphological and crystallographical studies were conducted using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction, respectively, to evaluate the powder and coating characteristics.

Lead zirconate titanate (PZT), with the general composition of Pb (ZrxTi1-x) O3, 0<x<1, is a well-known ferroelectric and piezoelectric ceramic material with high spontaneous polarization, dielectric permittivity, and piezoelectric coefficients peaking at compositions near the morphotropic phase boundary or MPB (x=0.52). In the current research, porous anodic alumina (AA) membranes were used for the preparation of the deposition template in order to provide a mold. The alumina membranes were fabricated through two-step anodizing of the aluminum foils in a cooled anodizing cell at 1ºC, using phosphoric acid (10 wt.%) as the electrolyte. Stabilized PZT sol, with the composition of Pb1.1 (Zr0.52Ti0.48) O3, was prepared through the chemical modification of the organo-metallic precursors of zirconium, and titanium with glacial acetic acid. The precursor solution was then driven into the template channels under the influence of a DC electric field. The filled mold were dried at 100oC, and subsequently annealed at 700oC to develop the desired perovskite structure. Scanning and transmission electron microscopy (SEM and TEM) investigations showed that PZT nanotubes have efficiently grown in template channels. Energy dispersive X-ray (EDX) investigations confirmed the composition of the grown nanotubes. X-ray diffraction (XRD) investigations indicated that the PZT nanotubes possess mainly the desired perovskite phase. The electron diffraction patterns also demonstrated the polycrystalline nature of the grown nanotubes.

In the present study, NiCr coating was created by the High Velocity Oxy-Fuel (HVOF) thermal spray technique under different conditions in respect to powder feed rate and fuel/oxygen ratio on the steel samples. Electro chemical corrosion behaviour of the NiCr coated samples was investigated by the potentiostatic polarization in NaCl 3.5% solution. In order to determine the structural traits of the coatings, scanning electron microscopy (SEM/EDX) was used before and after the corrosion testing. The analysis of the results indicates that the fuel/oxygen and the powder feed rate have a significant effect on the porosity, existing oxide phases, and corrosion resistance, and un/melted powders of these coatings. corrosion behaviour of the sample, which has coated by fuel/oxygen of 1.25 and powder feed rate of 18 gr/min, is the best due to lower porosity and oxide zones.

In this paper the effect of input parameters of the EDM process (pulse on-time, duty cycle and tool polarity), on the surface integrity (surface roughness, density of surface crack, and white layer thickness) in the machining AISI H13 tool steel has been studied. The result of this research shows that in the positive tool polarity the white layer thickness and density of surface cracks in the low pulse on-time are higher than that of the tool polarity chosen to be negative. However in the negative polarity in the high pulse on-time, density of surface cracks and the thickness of the white layer are higher on the machined surface. Moreover, according to the obtained results, density of surface cracks and the average of the white layer thickness is reduced when the duty cycle increases in the two conditions of tool polarity. By the increase in the pulse on time, the amount of surface roughness is increased.

Polymer coating is an effective way for modifying he particle surface. Polymer adsorption by nanoparticles improves the stability of the suspension against aggregation. In order to prevent nanoparticles from aggregation, in this work, zinc oxide nanoparticles and polyethylene glycol (PEG) are used as nano-sized metal oxide powder and adsorbed polymer, respectively. The adsorption process is analyzed by the infrared spectroscopy (IR) technique. The size and structure of zinc oxide nanoparticles have been characterized by X-ray diffraction (XRD). The amount of the adsorbed polymer was measured by thermogravimetry analysis (TGA) method. Moreover, the aggregation and accumulation of the nanoparticles was studied using scanning electron microscopy (SEM). We found that, using the polymers with lower average molecular weight made nanoparticles with higher adsorption, lower size, and a more spherical shape. Our results show that the coating process is an effective way of avoiding aggregation, sticking, and clumping of nanoparticles.