فصلنامه علوم و مهندسی سطح ایران
پیاپی 12 (زمستان 1390)

Cadmium sulfide thin films were fabricated in a vacuum (5 10-6torr) with 2. 5 nm/s coating rate and the approximate thickness of 800nm. In order to study the effects of the substrate on the structural properties of samples، two types of substrates (glass and ITO) were used. Structural properties of the samples were studied by X-ray diffraction. Calculated structural parameters revealed that the samples built on ITO substrate had a better crystal structure compared to those made on glass substrate. Moreover، the calculation of stress and crystalline defects in the samples showed that the samples with ITO as substrate had better crystal quality. The CdS layers made on glass substrate were annealed at 200°C and 300°C for one hour in vacuum in order to study the effects of heat treatment on their structural properties. X-ray diffraction spectrum of samples prepared on glass substrate showed improvement of crystalline quality upon annealing.

In this work، the optical properties of ZnO thin films were investigated experimentally. Zinc was sputtered by the magnetron sputtering method on the LiNbO3 crystal (as substrate)، followed by a thermal oxidation at 450oC of the obtained ZnO films. The variations of the transmittance، reflectance، and extinction coefficient were measured for temperatures (450-800 oC) and different periods of heating (15-120min) which were employed to study their effects on the optical characteristics of the ZnO samples. The optical band gap energy was determined and it showed reduction with temperature and an increase at 800 oC with period of heating.

In this study the effect of substrate temperature on the structural properties of zirconium nitride (ZrN) coatings prepared by ion beam sputtering on the glass and silicon substrates was investigated. Rutherford Back Scattering (RBS) and X-Ray Diffraction (XRD) analyses were performed to evaluate the coating thickness and structure respectively. The ZrN coatings presented a preferential crystalline orientation following the [111] axis at all temperatures. RBS results showed an increase in coating thickness with the increase of temperature up to 400˚C due to reaction kinetic for the formation of Zr-N monomers. The decrease of thickness at 450˚C is probably due to the increase in coating density and decrease in grain size at this temperature.

Ni-nano TiO2 composite coatings on low carbon steel have been prepared in Watts-bath using the electro deposition process. Characteristics of coatings such as micro-hardness، corrosion and hydrogen permeation were studied and compared with those of pure nickel coating. Micro-hardness results showed a noticeable increase in hardness of composite coatings in respect to the pure nickel coating. It was found that، an increase in TiO2 particle incorporation lead to the micro-hardness increment of the coatings. The corrosion study of specimens in %5 H2SO4 solution with pH=1 acquired through Tafel polarization، revealed several times reduction in the corrosion rate (CR) of composite coatings as compared to the pure nickel coating. Furthermore، hydrogen permeation test was performed on both pure nickel and Ni-TiO2 composite coating by the electrochemical approach (Devanthan-Stachursky) to compare their resistivity against hydrogen atoms through hydrogen charging by a power supply and recording permeation density versus time with a Potentiostat. Analysis of the results showed better resistivity of the composite coating against hydrogen permeation compared to the pure nickel coating.

The eddy current test is a non-destructive technique which can be performed rapidly. Since the eddy current response is sensitive to the chemical composition and microstructure of the materials، it can be used to determine case depth of induction hardened parts due to the difference in magnetic properties of the hardened layer and the core of the specimen. In the present study، identical rods of CK45 carbon steel were case hardened using induction hardening method. Plotting of hardness profile was done، and the effective and total case depths were also determined. In order to investigate the applicability of the eddy current technique، the relation between effective and total case depths and eddy current outputs (primary and secondary voltages، normalized impedance and phase angle) have been studied. Good correlations between these findings indicate an acceptable level of accuracy of this method in comparison with destructive methods.

This work’s aim is to investigate the dynamics of ions، electrons، and charged nanoparticles inside the dusty plasma sheath، in order to determine the appropriate conditions for the controlled and site selective deposition of plasma particles on a pre-patterned substrate of ordered microstructures. The approach used in this study، is the numerical simulation of the fluid equations، describing the dynamics of plasma particles inside the plasma sheath. The results obtained، show that the focusing effect of the electric field of nanostructures increases with the increasing of the electron number density، leading to the growth of high aspect ratio nanostructures. Increase in the electron temperature، results in the decreasing of the charge of nanoparticles، and consequently in the decreasing of the incident flux on the nanostructures. The enhancement of the number density and Mach number of the nanoparticles، leads to the enhancement of the flux on nanostructures and as a result their growth rate increases. Moreover، the incident flux grows with the increasing of the height and period of micro-pattern. From the present findings it can be concluded that to achieve high deposition rates and nanostructures with a large aspect ratio one should employ a high density، low temperature plasma، large impinging velocity of nanoparticles، and a micro pattern with a relatively large period.

This paper studies the microstructure and properties of stellite6 coatings produced by high velocity oxy-fuel (HVOF) presses on low carbon steel (St 37). The microstructure، porosity، coating thickness، phase formation and microhardness of the coating were investigated using the combined techniques of optical microscopy، X-ray diffraction (XRD) and scanning electron microscopy/ energy dispersive spectrometry (SEM/EDS). The coatings exhibited a layered microstructure due to deposition and re-solidification of molten powder particles. Porosity، unmelted and semi melted particles and inclusions were present in the microstructure of the coating. In addition، the coating showed porosity content of about 2% and a hardness of about 700 HV. The thickness of as-sprayed coatings was about 500 m. The adhesive strength of coating to substrate was more than 67 MPa. The result of XRD indicated that no phase transitions occur in feedstock powder after HVOF spraying and the coatings consist of a Co-rich matrix of face centered cubic (FCC).

In this investigation، NiCrAlY coatings were deposited on steel substrate using the HVOF process. Different spray parameters were applied such as Propane flow rate، spray distance and powder feed rate. The hot corrosion behavior of resulted samples was studied in salt mixture Na2SO4-60%V2O5 at 900˚C for 30 min to 22 h in an ambient atmosphere. After hot corrosion tests the microstructure and phase composition of these coatings were analysed using Optical microscope، Scanning electron microscopy/Energy dispersive spectroscopic analysis (SEM/ EDS)، and X-ray Diffraction (XRD) techniques. The analysis of the results indicates that the parameters variation have a significant effect on the properties of NiCrAlY coatings، especially on the oxide phase content present in the coating، and therefore on the hot corrosion resistance of this coating. It was observed that coating prepared by 60 lit/min Propane flow rate، 300 lit/min Oxygen flow rate، 20 cm spray distance and 32 gr/min powder feed rate has the best corrosion resistance، because of the smaller oxide contents inside the coating.

In the present study، due to the importance of nickel plating in the improvement of carbon nanofiber surface properties، the electroless nickel plating of electrospun carbon nanofibers was carried out using a commercial electroless bath. The electrospun carbon nanofibers were submerged into the electroless nickel bath and held for 20 minutes at 45 ˚C and a pH value of 9. Results obtained from scanning electron microscopy observations and X ray diffraction analysis indicated that a uniform nickel coating of about 184 nm thickness was formed on the whole surface of carbon nanofibers. The fiber fabrication process was performed in the presence of nickel as a graphitization catalyst. Raman spectroscopy and X-ray diffraction studies confirmed the presence of nickel nanoparticles on the surface of fibers and the accomplishment of graphitization at 1400 °C.