IRANIAN JOURNAL OF SURFACE SCIENCE AND ENGINEERING
Year:2007 | Volume:- | Issue:3|Papers Count:8

Nanostructured and nanoporous TiO2-Ga2O3 films and powders with various TiO2-Ga2O3 molar ratios with high specific surface area (SSA) were prepared by a new straightforward particulate sol-gel route. Titanium isopropoxide and gallium (UI) nitrate hydrate were used as precursors along with hydroxypropyl cellulose (HPC) as a polymeric fugitive agent (PF A) to increase the SSA. X-ray diffi'action (XRD) revealed that the powders contained rhombohedral a- Ga2O3 and monoclinic b- Ga2O3 phases as well as anatase and rutile. It was also observed that the Ga2O3 formed by the nitrate precursor, retarded anatase-to-rutile transformation. Furthermore, TEM analysis showed that Ga2O3 hindered the crystallization and crystal growth of powders. The analysis indicate that one of the smallest crystallite size and one of the highest SSA already reported in the literature were obtained, which can be used in many applications in areas ranging from optical electronics to gas sensors.

Hydroxyapatite coating that is the most important mineral part of human bone was applied on Ti6A14V using pulse electrode position process in an electrolyte consisting of Ca (NO3)2 and NH4H2P04. The effect of electrode position parameters such as temperature, current density, duty cycle, and number of cycles on the morphology of the deposit were examined. Fine-grain deposits with spongy structure were obtained at 12°C at voltages more than 4V. Optimum deposition and coating structure were obtained at 4.5 V while higher operating voltage resulted in a dendritic and flake type deposits. With respect to the fact that the duty cycle and pulse electrode position can change the electro nation and diffusion of acting species, finer grains toward nano sizes were obtained by controlling electrochemical parameters. TEM and XRD studies showed the presence of three phases of calcium phosphates. Selected area diffraction pattern of the deposits were indexed and confirmed the presence of those three calcium phosphate phases which are biocompatible. Corrosion resistance of the deposits was checked using polarization and Electrochemical Impedance Spectroscopy (EIS) techniques in which both methods confirmed better resistance of the nano HA coating.

In this study, corrosion resistance of ordinary (non-skin passed) and skin passed hot-dip galvanized steel sheets produced in Mobarakeh steel company was compared with each other. For this reason, lead content of the zinc bath was changed from 0.01 wt.% to 0.11wt.% and the corrosion resistance of the sheets was analyzed. Also, to study the effects of skin pass rolling on the corrosion resistance of the sheets, two skin passed specimens obtained from zinc baths of 0.045 and 0.065 wt.% of lead content were studied. Effect of lead content of the zinc bath and skin pass rolling on (00.2) basal texture component were also investigated. Relative intensity of (00.2) basal texture component was determined using XRD whilst the corrosion behaviour was analyzed employing Tafel polarization test. Results showed that increasing the lead content of the zinc bath would decrease the corrosion resistance and (00.2) basal texture component relative intensity of ordinary coating and conversely, increase the spangle size and darkness of the coating surface. Investigation on the effects of skin pass rolling on the galvanized sheets indicated that in this case, relative intensity of (00.2) basal texture component and spangle size have been decreased and some cracks created in the coating. In addition, it was seen that the corrosion resistance of skin passed specimens, in comparison with ordinary specimens, has been decreased.

The analysis results show that by the selection of appropriate conditions, a metastable Ni- Ti-C powder could be obtained by nominal compositions of 50wt. %Ni, 40wt.% Ti, and 10wt.% C. A powder, suitable for HVOF spraying, with particle sizes ranging from -38 to 8 mm was sieved and classified. Coatings with an approximate thickness of 250 mm were deposited by HVOF spraying on to mild steel substrates, and the resultant microstructures were investigated. XRD showed that a self-propagating high-temperature synthesis (SHS) reaction had occurred in the powder particles during spraying. It also revealed that the principal phases involved in the coating were TiC and a Ni-rich solid solution; small quantities of NiTi, TiO2, and NiTiO3 were also present. SEM revealed that the coating had a splat-like morphology characteristic and that TiC was formed by nanoscale dispersion, with particle sizes ranging from 50 to 200 nm, with in solidified splats.

This paper considers the effects of various active screen and conventional plasma nitriding parameters, on the corrosion behavior of low alloy steel, DIN 1.6580. Both nitriding processes were carried out at 550 and 580°C for 5 hrs in atmospheres having nitrogen and hydrogen with volume ratio (N2/H2) of 1/3 and 3/1. The corrosion test results show that, the corrosion resistance of active screen samples increases with increasing treatment temperature, screen hole size diameter (when using screen top lid) and with screen top lid. The effect of treatment gas in similar condition, depends on the top lid type, namely corrosion resistance decreases with increasing nitrogen in the treatment gas mixture (when using screen top lid) and with using simple top lid. This can be attributed to the thickness of the compound layer. It was indicated that the corrosion resistance of conventional plasma nitriding samples with the lowest value is 20 times of the corrosion resistance of active screen samples (with the lowest corrosion resistance value).

In this research the corrosion behavior of zinc-manganese phosphate coating on AISI 4130 subjected to various heat treatments has been investigated. Prior to coating, different heat treatments were applied to the steel substrates. The analytical results showed that the microstructure of steel substrate has a significant effect on the coating morphology. Decreasing the substrate grain size, reduces the coating porosity and increases the polarization resistance (Rp) in EIS test. In addition, the corrosion resistance of the coating depends on its morphology such that better corrosion resistance has been achieved with finer coatings.

The tribological behavior of A12024/Grp composites containing 5-20 wt.% Graphite produced by in situ powder metallurgy method was investigated. The age-hardened composite samples were tested under dry and lubricated conditions by a pin-on-disk wear test machine. In order to investigate the effect of graphit content on the quantity of the mild to severe transition load different load were used (0.5-3.5MPa). The worn surface of samples as well as the wear debrises was studied by Scanninng Electron Microscopy (SEM). It was found that in the composite samples, due to the formation of a tribolayer, the wear rate and friction coefficient decrease dramatically as compared with the matrix alloy. The composites containing 5wt.% graphite exhibited the minimum wear rate. In composite samples lower wear rate together with a higher friction coefficient in dry wear test were observed as compared with the lubricated conditions.