فصلنامه علوم و مهندسی سطح ایران
پیاپی 27 (بهار 1395)

The purpose of this work is to investigate the effect of deposition current density on superhydrophobicity characteristic and corrosion resistance of nickel electrodeposited coating. Nickel coating with hierarchical structure was fabricated on copper substrate by electrodeposition process at current density of (10 and 20) mA/cm2. The surface morphology of electrodeposited nickel coatings was examined by scanning electron microscope which showed that the coating structure consisted of micro and nano shaped cones and the size of micro cones decreased by increasing the current density. The relative texture of the coatings was also determined by X-ray diffraction pattern. The results showed that the relative texture direction changed from (220) to (311) by increasing the current density from 10 to 20 mA/cm2. The surface topography studies by atomic force microscope showed decrease in surface roughness by increasing the current density. The wet angle measurement results showed that the wettability of nickel coatings varied from super- hydrophilicity to super- hydrophobicity by exposing the coating in air at room temperature. The reason of this behavior is the formation of NiO on the surface of rough nickel film which decreased the surface energy. The corrosion behavior of the coatings was evaluated in 3.5wt% NaCl solution by electrochemical impedance spectroscopy and potentiodynamic polarization tests. Results revealed the positive effect of hydrophobicity on increase in corrosion resistance of coatings and it was shown that the coating formed at 20 mA/cm2 had higher polarization resistance compared to other coating.

Corrosion behavior of self-coated Nickel Aluminide IC221M alloys were examined in molten carbonate media with LiNaCO3 eutectic composition, to investigate the possibility of usage in wet-seal area in molten carbonate fuel cell. The desired temperature and time of oxidation process assigned as 1000֯ C and 25 hours respectively. Self-coating process resulted the formation of NiO NiAl2O4 ZrO2 and Al2O3 in order, from the outer interface. Under these compounds, in the substrate interface, an Aluminum depleted zone (ADZ) was observed. Oxide compounds together with this layer, perform as a barrier for alloying elements outward diffusion. After immersion test, more stable compounds such as NiCO3 LiAlO2 and CrO3, Na2CrO4 (formed by Cr diffusion), were observed above the ADZ. Weight-gain rate between the 100 to 215 hours of immersion test were estimated 2*10-5 g. cm-2. hrs-1 with a decreasing trend by time progress; which means that the mentioned stable compounds, provide a suitable corrosion resistance in Base alloy, after formation of required amount.

In this work, functionally graded coatings of HA/TiO2 nanoparticles were fabricated by electrophoretic deposition on Ti-6Al4V substrate. The functionally graded structure of HA/TiO2 coatings was formed by gradual addition of HA suspension into the deposition cell containing TiO2 nanoparticles under voltage of 20 V for 3 min. The X-ray diffraction was used for phase analyses of coatings before and after sintering. The microstructure and chemical composition of coatings were characterized by scanning electron microscope equipped with energy dispersive spectrometry. The micro-scratch test results showed that the adhesion strength of functionally graded coating was increased and critical contact pressures of Pc1=4.13 GPa and Pc2=5.28 GPa was obtained which would be due to the reduction of thermal expansion coefficient mismatch between Ti-6Al4V substrate and HA. The results of potentiodynamic polarization measurements showed that addition of TiO2 in the graded structure of coating could efficiently reduce the corrosion current density and improve corrosion resistance in simulated body fluid (SBF) solution which is consistent with lower porosities in FG coating as compared to HA coating.

In the present research, the effect of borohydride ion concentration existing in the electroless plating bath on the microstructure and hardness of electroless Ni-B coatings in the as-plated and heat treated conditions has been studied. In order to study the structure of electroless Ni-B coatings, the surface and cross-sectional images were examined. XRD analysis of the as-plated and heat-treated coatings showed that the former has an amorphous structure, while the latter has a crystalline one. EDS analysis confirmed the presence of Ni, B, Fe and Pb in the coatings. Microhardness test results revealed that the increasing of sodium borohydride concentration during plating enhanced the microhardness of the coatings in the asplated and heat-treated conditions. Also, at constant concentration of sodium borohydride, applying the heat treatment led to increase the microhardness. The maximum microhardness for the as-plated and heat treated coatings were 854 HV50g and 1121 HV50g respectively.

The aim of this work was the optimization of annealing time of silver chip silanized by aminopropyl try ethoxy silane. Silver chip with 50 nm thickness silanized by aqueous silane was annealed in different time at 100 °C. Silanized chips were analyzed via AFM, surface Plasmon resonance sensor and contact angle measurement. Result chips had good optical property for surface Plasmon resonance sensors. By increasing the annealing time, the roughness and chemical stability of silanized chips increased, while their surface plasmon resonance response weakened. Also, annealing increased amine accessibility of chips.

Alfa layer due to the hot forging of the titanium alloy is removed using the acid pickling operation. Because of the interaction between the effective parameters, investigation and optimization of the effective parameters needs experimentation and modeling methods. In this research effect of the temperature, operation time and percent of the hydrofluoric and nitric acids on the surface roughness and depth of cutting are modeled and optimized. First the experiments are designed using taguchi method and implemented. Then the process is modeled by an artificial neural network and the effect of the parameters on the surface roughness and depth of cutting are investigated. Next by combining the artificial neural network with genetic algorithm the process is optimized. Results show that a Multilayer Feed forward network with Levenberg-Marquardt backpropagation learning algorithm and ten nods in the hidden layer could model the operation precisely. Increasing the temperature and hydrofluoric acid percent cause to increase the depth of cutting. Nitric acid percent and operation temperature have interaction in affecting on the amount of depth of cutting. Operation time have no affect on the roughness. Increasing the temperature at low percent of hydrofluoric acid cause to decrease the roughness however it increases at high percent of hydrofluoric acid.

Silica-alumina composite coatings have been applied on metals, alloys and special steels to increase the wear resistance, corrosion resistance, and resistance to oxidation. In this regard, silica-alumina ceramic coating by sol-gel method was applied on Carbon Steel. Alumina sol was prepared by using Aluminum-isopropoxide as a precursor and Silica sol was prepared by using Tetraehyl Orthosilicate as a precursor. Then the sols with the different weight ratios of SiO2:Al2O3 were mixed together and steel samples after polishing and degreasing were coated by dipping method. Microstructure and Phase analysis of coated samples evaluated by using scanning electron microscopy and X-ray diffraction techniques. Corrosion resistance and adhesion strength evaluated by using electrochemical testing and pull off method. The results show that with increasing silica content in the silica-alumina composite coatings corrosion resistance and adhesion strength of the coating to the substrate increases .

In present contribution two main methods of Washburn capillary rise (WCR) and sessile drop technique on non-porous and smooth surfaces are to be compared in evaluating the surface energy of powder materials. Pristine silica and silane modified powders are used to assess the influence of silane nature and silane spacer length of 3-carbon and 16-carbon chain upon the surface energy of these powders. Having examined the surface modification of silica using infrared spectroscopy, an innovative method for preparing non-penetrable surface based on a combinatory press technique encompassing hydraulic and isostatic press is introduced. Using silane of high spacer length leads to an extremely hydrophobic surface where a contact angle of 47 degree for 3-carbon silane modified silica was varied to a value of 137 degree in 16-carbon silane modified one. In an attempt to clarify the significance of accurate measurement of surface energy, silane to silica ratio was adjusted to control surface properties of silica and then the silica dispersion was expected to be similar in specimens having identical surface energy.

Zirconium and its alloys are frequently used in orthopedic medicine and in comparison with stainless steel, titanium and other metals used in the manufacture of implants; the strength and corrosion resistance is more. In this research, ceramic glass SiO2- P2O5-CaO-ZrO2 was coated by sol gel method on zirconium. Bioactivity of the coatings was evaluated by immersion in SBF solution. The prepared samples before and after immersion in SBF solution was analyzed using X-ray diffraction techniques (XRD) and infra-red spectroscopy (FTIR). Morphology and quality of coatings were examined by scanning electron microscopy (SEM). The results show that by replacing calcium with zirconium the obtained apatite structure can be considered in the form of Ca10-xZrx(PO4)6(OH)2. Also, the coating consist of very fine particles and by increasing of zirconia content a uniform coating with lower porosity is obtained.