فصلنامه علوم و مهندسی سطح ایران
پیاپی 15 (پاییز 1391)

Aluminum alloys are one of the most attractive materials due to their high specific strength, low density, and very abundance reserves in the earth’s crust.Aerospace, automobile, building, and so on are some of the industrial fields where aluminum and its alloys are utilized. However, it has some disadvantages, namely softness, lower wear resistance and lower corrosion resistance, decreases the service life of machine components. An effective approach to defeat these above disadvantages is coating the aluminum components with alumina-based composites coatings. Among alumina-based composites coatings, Al2O3-ZrO2 system has demonstrated unique features. Micro Arc oxidation(MAO) technique was employed to grow zirconia–alumina porous layers. The layers were grown under alternative current in the electrolytes of ZrOCl2 salt. Considering XPS,XRD, and EDX results, the layers mainly consisted of a-Al2O3, γ-Al2O3, monoclinic ZrO2, tetragonal ZrO2. Increasing the voltage resulted in higher zirconium concentration. AFM studies revealed that the surface roughness increased with voltage. Morphological evaluations, performed by SEM, showed that the microstructure of the layers strongly depended on the applied voltage.

Aluminide coatings are great importance in protecting hot-section gas turbine components. Study the effects of pre-diffused layer on the final microstructure and properties of coating is useful in better exploiting of aluminide coatings potential. In this investigation the results of microstructural studies of modified aluminide coatings with pre-diffused titanium or silicon layers was presented and compared with a co-deposited titanium-modified aluminide coating and simple aluminide coating. It was found that silicon diffusion layer grows inwardly and increases the final aluminide coating thickness more than twice of the simple aluminide coating. About half of the silicon-modified aluminide coatings consisted the brittle Ni2Al3 phase. In contrast, titanium diffusion layer grew outwardly and didn’t have a considerable effecton the final thickness, but dilutes the areas close to the coating surface from alloying elements. This effect may be considered beneficial to the hot corrosionbehavior of the coating via the basic fluxing mechanism.

The electrical resistance of metal oxide semiconductor thin films varies with the composition of their surrounding gas. Titanium oxide nanotubes are sensitive to hydrogen gas and their electrical resistance are drastically decreased in hydrogen gas exposure. In this study, titanium oxide nanotubes were synthesized on a titanium foil by anodic oxidation method. Two types of electrolytes, containing aqueous and organic electrolytes, were used to obtain nanotube arrays with differentgeometries. The results demonstrated that the nanotube arrays with narrower wall thickness and shorter length were moresuitable choices for hydrogen gas detection and sensing by this nanoarchitecture. In this work, surface morphology was studied by field emission scanning electron microscopy (FESEM). Also, structural changes were evaluated by X-ray diffraction (XRD) analysis.

In this study the effect of addition YSZ on structural properties and oxidation resistance of CoNiCrAlY/YSZ composite were investigated. CoNiCrAlY/YSZ composite powders produced by mechanical alloying process. Various amounts of YSZ particles (0%, 5%, 10% and 15 wt.%) were mixed with CoNiCrAlY powder and milled for 12, 24 and 36 h, then composite and commercial coating were deposited with high velocity oxy -fuel method on the inconel 617 substrate. The structural and mechanical evolutions of the coatings were evaluted using X-ray diffractometry, scanning electron microscopy and microhardness test. It was observed that by increasing YSZ content, porosity of coatings were increased and hardness of coatings were increased too. Also, addition of YSZ to CoNiCrAlY decreased the rate of grain growth in thermal spray process and heat treatment. Increase of porosity result to decreasing oxidation resistance of coatings.

Gas Tungsten Arc Welding (GTAW) was applied to create a composite layer on the surface of cast aluminum alloy A380. Different mixtures of aluminum, silicon and silicon carbide powders with Sodium silicate solution were pasted on the substrate. After drying and preheating, surface melting was conducted by TIG process. A composite layer of Al-SiC was created on the surface by this process. Evaluation of microstructure and properties of obtained composite clad was done by X-ray diffraction (XRD), optical and scanning electron microscope (SEM), elemental microanalysis (EDS) and microhardness testing methods. The results showed uniform distribution of SiC particles in Aluminum dendritic matrix. SiC particles are dissolved partially during heating and melting of the preplaced layer and may lead to the formation of Si and Al4SiC4 phases with a Vickers hardness of about 1000 HV. Surface hardness can be increased by further addition of silicon in preplaced powders.

Titanium oxide nanotubes were successfully fabricated by the anodizing of titanium sheets Then, platinum nanoparticles were doped on titanium oxide nanotubes using micro-emulsion method. The morphology and surface analysis of the Pt -TiO2 electrodes were investigated using scanning electron microscopy and energy dispersive X-ray spectroscopy respectively.The electro-oxidation of alcohols and sugars on the Pt -TiO2 electrodes were studied using electrochemical methods. The results showed that the oxidation peak currents on the Pt -TiO2 electrode for alcohols and sugars oxidation were several times larger than those on a smooth platinum electrode. The photo-catalytic properties of the TiO2 made the Pt-TiO2 electrode reusable after a short UV treatment and the electro-oxidation current density of the Pt -TiO2 electrode after UV-cleaning could be reestablished.

In the present study, formation of the nanostructured NiAl intermetallic coatings on carbon steel by MA method was studied. The composition of Ni-Al powder with an atomic ratio of 1:1 with different ball size, ball-to powder weight ratio and milling duration were used. The substrate, fixed at the top of a vibration chamberd. During mechanical alloying process, the substrate surface was subjected to high-energy ball impacts. Powder particles trapped between the ball and cold welding occurred at surface. After that, synthesized samples were annealed. Thickness and hardness of coating optimized with factors ball size, ball-to-powder weight ratio and milling duration. The cross-section of the coated substrate was investigated using X-ray diffraction and scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the formation of Ni-Al intermetallic coatings were achievable. By increasing milling duration the thickness of the Ni-Al coatings significantly increased and the layered or pancake-type structure of the coating consolidate into a bulk material. Best coating was formed with ball 4mm diameter and ball to powder weight ratio of 10:1. The milling duration increases to 480 minutes and particle size reduced to 28nm.

The thermal spray parameters of WC-12%Co coatings deposited using HVOF System were investigate with the purpose of particle velocity optimization. The main purpose of this study was to determine whether the Taguchi approach could be used for optimization of the particle velocity. The peening stress, bonding of particles to substrate or previously coated layer andthe deposition efficiency were directly related to the impact velocity. This, in turn, makes possible the select of the key thermal spraying parameters to have HVOF sprayed coatings with desired residual stress states and deposition efficiency together. The particle velocity in impact has been studied using spray watch system. In this study the values of particle wit h four replications for each trial and the mean effect were recorded for the 9 runs (L9 array). The Taguchi approach for experiments with replication prefers the use of signal to noise ratio (SNR). The quality characteristic was chosen to be bigger the better. Analysis of variance (ANOVA) was used to identify the most significant factors and the contribution portion of each factor.

In this study, effects of three methods of chemical, electrochemical and electrochemical - chemical passivation on corrosion behavior of martensitic stainless steel type 420 in 3.5% NaCl have been evaluated by d.c. potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and analysis of Cr and Fe content in passivation solutions by Induction Coupled Plasma (ICP). The results showed that the impedance of surface was depend upon Fe/Cr ratio in passivation's solutions, inner and outer layers thickness of passive film and homogeneity of inner and outer layers. Also,The result indicated that electrochemical - chemical passivation provided the best performance in terms of increase corrosion resistance and the passive film formed by chemical and electrochemical passivation methods had the same functional. The result also suggested that the passive film formed with all three methods had higher corrosion resistance to passive film formed naturally (formed in moist air).

In the present study, Al2O3-DLC-MoS2-Au nanocomposite coating was deposited by physical vapor deposition (PVD) technique using two thermal sources including electron beam and resistant source on 440C steel as substrate to improve the wear behavior of this steel (one of the most widely used metals in themoving aerospace systems). To evaluation of tribological behavior of coating, Vickers micro hardness test, roughness test and analysis of wear in dry air (Nitrogen) and humid atmosphere by pin on disk test were performed. The results of surveys done by Energy Dispersive Spectroscopy and scanning electron microscope of abrasion of the coating, showed that the presence of all components of coating with suitable adhesion and toughness on substrate and also Roughness and hardness up much better than requirements for space coatings and improvement in tribological behavior of substrate by depositing the coating that was discussed above.

Corrosion behavior of Ni and Ni-ZrO2 composite coatings were studied using electrochemical impedance spectroscopy (EIS) technic. An equivalent circuit diagram based on blocked and partially corroded surface characterizations were proposed and a good agreement was observed between theoretical impedance spectra obtained on the basis of the equivalent circuit and spectra recorded during the measurements. The changes in the charge transfer resistance obtained from impedance measurements were comparable with the changes in polarization resistance obtained from potentiodynamic measurements and both of potentiodynamic and EIS tests showed that the corrosion resistance of NiZrO2 was higher than that of pure Ni. Investigation of corroded surfaces showed that the cluster boundaries in pure Ni and weak bonds between Ni matrix and ZrO2 particles in Ni-ZrO2 composite coating were the appropriate paths for corrosion to proceed. Changing of microstructure and corrosion proceeding paths were recognized as the reasons for higher corrosion resistance of Ni-ZrO2 with respect to pure Ni.