Papers by Keyword: Columnar Structure

Abstract: In this work, the adhesion and corrosion resistance of TiN coating by magnetron sputtering on stainless steel substrates under different bias voltages conditions (-50 V and-100 V) was investigated. AFM was used for surface roughness and grain size analysis, XRD for phase identification, Rockwell C for adhesion and nanoindentation for hardness and elastic modulus. According to AFM the coating deposited at-100V bias had decreased surface roughness, the value decreased from 2.7 nm (for-50V sample) to 1.8 nm, this was due to an etching like process which occurs when ions with higher energies start hitting the surface. The coated samples came under HF1 adhesion parameter which is the highest class of adhesion in the model on the basis of Rockwell C adhesion test. nanoindentation hardness and elastic modulus results of-50 V and-100V were found to be 224 GPa and 182 GPa, respectively and the value of hardness, 16 GPa and 22 GPa, respectively. The corrosion behavior of TiN coatings were studied in 3.5wt. % NaCl solutions using Tafel Extrapolation, Cyclic Polarization and Open Circuit Potential. It was noted on the basis of these corrosion tests that, as bias voltage is increased, it leads to the formation of more densely packed, fine grained columnar structures with less pores, which decreases the chances of corrosion. .

Abstract: Columnar or equiaxed structure selection and particularly the C→E transition in the solidifying massive rolls is shown as the result of changes in heat transfer. The numerical treatment of heat transfer allows to separate the temperature field for columnar structure formation from equiaxed structure formation. An area where transition from columnar into equiaxed structure occurs, C→E≡CET is also distinguished. The current model requires the transformation of the calculated temperature field into the thermal gradients’ field. Thermal gradients are approximately constant during the examined C→E transition according to the numerical simulation. This result is in accordance with the Hunt’s theoretical predictions. The localization of the structural transition (CET) in space and in time is also shown within the map which yields from the temperature field.

Abstract: This paper describes Transmission Electron Microscopy studies of the structural changes of GaN1-xAsx alloys grown by Molecular Beam Epitaxy at low temperatures on Al2O3 substrate. We found that by lowering the growth temperature increasing amount of As can be incorporated in GaN1-xAsx forming a single phase alloy. For the low As content a columnar growth of wurtzite structure is observed but for increasing As in the range of 0.170.75 the layer becomes amorphous. Increase in Ga flux at low growth temperature (about 200°C) leads to columnar alloys with As content >75% with a cubic structure. In addition to the structural changes monotonic change of the band gap is also observed with the As content in the alloy. The amorphous alloy is stable up to annealing at temperatures not higher than 600°C. Annealing at higher temperature leads to phase separation of GaAs:N and GaN:As confirmed by Z-contrast electron microscopy.

Abstract: The mechanical properties of copper thin films deposited by sputtering and electroplating were compared using tensile test and nano-indentation. Both the Young’s modulus and tensile strength of the films were found to vary drastically depending on the microstructure of the deposited films. The Young’s modulus of the sputtered film was almost same as that of bulk material. However, the Young’s modulus of the electroplated thin film was about a fourth of that of bulk material. The micro structure of the electroplated film was polycrystalline and a columnar structure with a diameter of a few hundred-micron. The strength of the grain boundaries of the columnar grains seemed to be rather week. In addition, there was a sharp distribution of Young’s modulus along the thickness direction of the film. Though the modulus near the surface of the film was close to that of bulk material, it decreased drastically to about a fourth within the depth of about 1 micron. There was also a plane distribution of Young’s modulus near the surface of the film.

Abstract: The morphological changes and corresponding plume deflection effect during long laser (XeCl excimer laser, λ=308 nm, τ=30 ns) irradiation of Al target have been investigated,and results showed that, a the number of laser pulses per site increases, the target morphology changes and produces a visible deviation of the plasma plume up to 5º. Scanning electron spectroscopy analys of the target surface related the plume deflection angle to the target morphology and the number of laser pulses per site. Typically well-defined columnar structures oriented along the laser beam direction were observed on the target surface. The plume created during the laser ablation was clearly visible to the naked eye and was recorded by a digital camera. Detailed studies were also performed on the ablation rate at different laser fluences (4.6 J/cm2-12.5 J/cm2). The morphologies and the thickness of the Al thin films deposited on Si substrates during the present laser ablation experiments were also studied. Finally, the scenario explaining the formation of columnar structure on target surface responsible for the plume deflection effect will be discussed.