فصلنامه علوم و مهندسی سطح ایران
پیاپی 9 (تابستان 1389)

Nanocrystalline and mesoporous TiO2–Er2O3 films and powders with various TiO2:Er2O3 molar ratios have been prepared by a sol–gel route. X-ray diffraction and Fourier transform infrared spectroscopy revealed that erbium oxide formed in the range 50–100 mol.-%Er2O3, whereas erbium dititanate formed in the range 25–100 mol.-%Er2O3. Oxygen deficient titania phases (TiO2-x), such as Ti7O13 and Ti2O3, were observed for TiO2:Er2O3=25:75 (molar ratio) system annealed at 800ºC. It was observed that Er2O3 retarded anatase to rutile transformation. Furthermore, TEM analysis also showed that Er2O3 hindered the crystallisation and crystal growth of powders. Atomic force microscope (AFM) analysis confirmed that TiO2:Er2O3=75:25 (molar ratio) system annealed at 600ºC produced the smallest grain size (17 nm) and the highest roughness. Moreover, TiO2:Er2O3=50:50 (molar ratio) system annealed at 800ºC showed the smallest grain size (32 nm) and the highest roughness. The sensing properties of TiO2 films were improved by introducing Er2O3 phase into the sensing film. The fabricated sensors showed stable, reliable and reproducible response with small response and recovery times towards CO and NO2 gases. The working temperature of the sensors was reduced down to 200ºC, and therefore they can be used for industrial application.

In the present research, electroless Ni-P deposits were obtained on Al5083 substrate and the effects of the coating time on morphology, phosphorus content and corrosion behavior of the deposits were investigated. The scanning electron microscopy (SEM) of specimens obtained in various coating times, were studied and compared in their surface morphology, thickness and uniformity of the coatings. Surface morphology of the coatings exhibited a nodular feature with a typical cauliflower-like structure. Corrosion resistance of the coatings was evaluated in 3.5 wt.% NaCl solution by potentiodynamic polarization and electrochemical impedance (EIS) methods. The results indicated that by increasing thickness of coatings, the number of porosities decreases and the corrosion resistance and coating thickness increases. The corrosion resistance of electroless deposits on a smooth substrate in comparison to a rough substrate is higher, probably due to the fact that by increasing substrate surface roughness, the number of defects and porosities of the coatings increas.

Plasma electrolytic oxidation (PEO) is a plasma assisted electrochemical treatment which can provide or is usedto change the coating of some metals to an oxide ceramic layer on the surface of some metals. In PEO technology, spark or arc plasma micro-discharges in an aqueous solution are utilized to ionize gaseous media in the solution to form complex compounds on the metal surface through the plasma chemical interactions. This work deals with the study on the effect and impression mechanism of caustic soda on the microstructure and thickness of the silica-alumina ceramic coatings formed on the aluminum substrate by the plasma electrolytic technique. Morphology, surface roughness profile and surface elemental analysis were studied by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Also, the phase composition of the coatings was detected using X-ray diffraction (XRD). The results showed that increasing the NaOH content of the electrolyte causes thickness reduction and increasing of the coating micro cracks. The results of surface roughness profiles, reported as mean value of the surface roughness (Ra), showed that Ra increases from 0.73 to 1.08 m by increasing the NaOH content of the electrolyte up to 20g/l However, Ra decreased in higher concentrations. Also, reducing of the silica phases and increasing of the amorphous phases were observed in more concentrated electrolytes. Moreover, the results revealed that by increasing the NaOH content of the electrolyte, the coating porosity decreases and the distribution of the Al, Si and O elements becomes more homogeneous in the coating cross section.

In this paper the microstructure, phase formation and Vickers hardness profile of the hardfaced layer of Stellite-6 filler metal on carbon steel were investigated without and with martensitic stainless steel and austenitic stainless steel interlayer. Gas Tungsten Arc Welding (GTAW) cladding was carried out for deposition. The specimens were investigated by the X-ray diffractometer (XRD), energy dispersion spectroscopy (EDS), scanning electron microscopy (SEM) and microhardness test. The results show that the microstructure of these claddings includes chromium carbide phases dispersed in the matrix of the Co-based alloy with a dendritic structure. With the increasing of Stellite layers and interlayer, hardness increased and dilution decreased. The dilution of the clad layer by Fe from the substrate decreases hardness, wear and corrosion resistance. The interlayer resulted in a decrease in the dilution of Fe and increase in hardness.

Plasma nitriding is one of the effective methods for the improvement of hardness, wear and corrosion resistance of iron alloys. In this research, AISI H11 steel was plasma nitrided with various gas mixtures, times and temperature The effect of process parameters on the morphology and hardness of nitride layers has been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and microhardness measurements. The results showed that by increasing the nitriding time, the depth of nitrogen diffusion into the sample and consequently, the size of nitride precipitates formed on the surface increased. therefore, the density of these particles and hence, the surface hardness decreased. By increasing the nitrogen percentage in the gas composition, the surface hardness increased due to the formation of more ε phase at the surface. Meanwhile, an increase in process time and/or temperature resulted in a decrease in the amount of ε phase which decreased the surface hardness.

Bio metals have been used for curing bone and join illnesses for years. 316L stainless steel has been used comprehensively as a bio metals because of its lower price in comparison to other bio metals, however its lower corrosion resistance in biological media caused some problems in the body. This studie’s aim is to design and prepare a novel HA-Nb composite coating and study the effect of different electrical current plasma spray intensities to produce coating on 316L stainless steel, in order to find the optimum condition for the plasma spray process. Heat treatment has been done in humid and pure argon media for eliminating unsuitable phases and increasing crystalinity of the coating which was produce at 600A by the plasma spray process. X-ray diffraction and scanning electron microscopy techniques were utilised to investigate the characteristics of coatings. Electrochemical polarization tests were carried out in the physiological ringer solution in order to evaluate the corrosion behavior of coated and uncoated specimens which were done in 400,600 and 800 A plasma spray current intensities. Results indicated that the optimum electrical current intensity for deposition of the HA–Nb composite coating on 316L SS substrate was 600 A. In this condition, corrosion intensity was 120 nA/Cm2 and XRD and SEM results exhibited good structural properties which in turn increased biocompatibility of, 316L SS substrate. In addition heat treatment at 600 oC in the humid media caused the eliminating of tricalcium phosphate which is considered to be an unsuitable phase in the coating.

The tribological aspects of contact systems are greatly affected by the friction and contact pressure distribution throughout the surface of the contact interface. Generally the contact of deformable bodies is a nonlinear problem and for the viscoelastic bodies it has a time-dependent response, since their viscous characteristics depends on time. The introduction of friction with its irreversible characteristic in contact surface makes the contact problem more difficult. The objective of this study is to develop a general augmented Lagrangian finite element formulation associated with an incremental adaptive procedure which established for analysis of frictional contact problems in viscoelastic systems. The contact behavior has been studied through an improved augmented Lagrangian approach. A generalized Maxwell model has been used to model the viscoelastic constitutive equations in which bulk and shear relaxation functions are represented by the sum of a series of decaying exponential functions of time. Based on the principle of virtual work, an effective finite element formulation associated with an incremental relaxation procedure has been developed. As an application of formulation, the numerical example has been presented to evaluate the numerical approach.