IRANIAN JOURNAL OF SURFACE SCIENCE AND ENGINEERING
Year:2006 | Volume:1 | Issue:2|Papers Count:8

Measurement of thermal properties for some coatings as potential selective surfaces is reported and parameters affecting the preparation of the coatings are indicated. Although Mo-Black oxide electroplated on aluminum substrate among the coatings did not show the selectivity desired, but its resistance and tolerance at moderate temperature (250°C for 36 hours at atmospheric pressure) are considerable and should be taken as a possible candidate for color coatings in decoration and as a resistant coating against oxidation. Dip coating on galvanized steel and zinc shows some order of selectivity (a/e » 5); better selectivity which is not far to reach would make this coating a good and suitable material. From the economical point of view and the simplicity of production it is also a promising coating as selective surfaces for flat plate solar collectors as well as concentrated collectors.

The Si surface charge sheet density of the p-Si/Si0.81 Ge0.19/Si inverted remote doped structures is evaluated in this paper. Owing to the existence of a quantum well (QW) in the valance band in the alloy layer (SiGe) of this structure, a two-dimensional hole gas (2DHG) is formed near the Si/SiGe/Si lower (Inverted) interface. The area (sheet) density nh of 2DHG, which depends on the Ge content and other structural parameters in the alloy, is strongly affected by the surface charge density nsur on the Si cap layer thus, increasing in proportional to the cap thickness. It can be concluded, through theoretical and experimental comparison, that the Si surface charge density in the structures under study, increases within the range of [(1-2.5)±0.2] x 1011/cm2 as the cap becomes thinner (400-150nm). Moreover, the position of Fermi level with respect to valance band edge at the surface is estimated to be DEFV=(0.5±0.05) eV.

In the present work the effect of diverse volume percentages of silicon-carbide particles as well as affecting parameters on particle wetting by molten matrix in an Al-SiC(p) composite were investigated. Wear behavior of the specimens was taken into account as criteria for the effectiveness of the variable parameters. Several specimens were prepared from the A356 alloy as the composite matrix by stir casting system and the effects of the addition of magnesium to the melt as well as those of the heat treatment of SiC particles on particle wetting were studied. All specimens experienced the pin-on-disk wear test for comparison. The results showed that the addition of magnesium to the molten matrix improved wear properties by 28 folds compared to that of the raw (none-composite) A356 specimen. The highest wear resistance was attained for the specimens with simultaneous addition of magnesium and heat-treated particles.

In the paper the corrosion behaviour of the Magnesium-based rare alloy has been studied by potentiodynamic and weight loss techniques. These methods were used for the calculation of corrosion rates in solutions containing various types of ions such as sulfates, chlorides and carbonates over the pH range of 1.5-11.5. Surface examinations and analytical studies were carried out by optical and scanning electron microscopy. The results showed that alkaline solutions with no chloride ions had no influence on the alloy. While the alloy was resistant to solutions of neutral pH (with no heavy metal and chlorine ions), they were corroded in strong acidic solutions containing such ions as well as alkaline ones. It was found that although intergranular corrosion was the predominant mode of corrosion in all the specimens, pitting was also observed on some regions of the grain facets. This became vigorous in the final stages of weight loss testing in chloride containing solutions. Comparison of the results revealed that the corrosion rates obtained by weight loss method were always slightly higher as compared to those obtained by potentiodynamic methods.

The aim of this study is to investigate the tribological behavior of DIN 1.4871 steel in plasma nitriding. Plasma nitriding treatment was carried out by entering nitrogen and hydrogen with the ratio of N2/H2: 1/4 and increasing pressure to about 10 torr. Nitriding cycles were performed at 500, 550 and 570°C respectively, for 12 hours each. For removing the oxide layer and better diffusion, sputtering treatment was applied for 60 minutes. The structure and property of the samples were examined by metallography, microhardness, XRD, and SEM tests. The pin-on-disk test was employed to evaluate the wear characteristics of the samples.The results revealed that the layer formed with the most wear resistance had a thickness of 80 micron and a hardness of about 800 HV. While at 500°C a white layer with no CrN was formed, at higher temperatures a white layer with CrN was produced. The sample showed the best wear behavior at 570°C after 12 hours of heating. The wear mechanism of the unnitrided sample showed severe adhesion wear, while for the nitrided samples, under optimum conditions, it was mild.

MCrAIY and YSZ coatings, and combinations of both, with voluminal percentages of 30%, 50%, and 70% in metallic phases, were prepared using APS and HVOF processes. In this study, the thermal expansion of the coatings was measured in a vacuum of 10-6mmHg ranging from room temperature to 1100°e. The results showed that, the expansion of each pure metal and ceramic specimen increased differently as the temperature increased. In fact, at high temperatures the difference was more than 70%. On the other hand, the porosity percentage decreased from 11.4 without metallic phase down to 4.3 in 70% of the content of the metallic phases. In contrast, the coefficient of the thermal expansion increased more than 68%. Therefore, it is recommended that, by adjusting the variables, coatings with appropriate thermal and expansion properties can be designed and employed for target working conditions.

Sol gel technique was used to deposit nano-structured HA coatings on different titanium implants. In this study HA nano-powders and nanostructured coatings were prepared by sol-gel method using phosphoric pentoxide (P20S) and calcium nitrate tetrahydrate (Ca(NO3h 4HzO). The effect of sintering time and temperatures on the composition of HA phase, crystal size of HA powder, and also the effect of calcination temperature and aging time on coating properties were studied by XRD, SEM, and TEM techniques. It was found that at sintering temperatures ranging from 600°C to 700°C, the dominant phase of the powders was HA with small amounts of calcium oxide and tricalcium phosphate (b-TCP) at 700°C, and at 600°C only the HA phase was observed. The 25-32nm HA powders were obtained using this technique. This sol-gel process produced nano-structured HA coatings with a 35-nanometer grain size. The optimum aging time for the production of the single phase sol-gel HA coatings was 24 hours.

Very smooth thin films of iridium were deposited on super-polished fused silica (SiO2) substrates employing D.C. magnetron sputtering in argon plasma. The influence of deposition process parameters on film micro-roughness was investigated. In addition, film optical constants were determined by variable angle spectroscopic eilipsometery within a spectrum ranging from vacuum ultraviolet to middle infrared (140mm-35mm). The surface roughness was also measured by atomic force microscopy (AFM).