فصلنامه علوم و مهندسی سطح ایران
پیاپی 4 (تابستان 1386)

In this paper, the inclined impact of a droplet on a solid surface in a spray coating process is studied using both numerical and analytical models. The numerical simulation is based on a previously developed model that includes the solution of Navier-Stokes equations along with an equation for the liquid free surface. The contact angle is modeled using two advancing and receding angles at the liquid front on the solid surface. The close agreement between the results of the numerical model with those of the experiments shows that the model can accurately predict the droplet impact behavior. Using the balance of droplet energy before and after the impact, a simple analytical model is presented for the maximum spread of a droplet during an inclined impact. This model is an extension of a previously developed model for the normal impact of a droplet on a solid surface. A comparison between the results of the analytical model with those of the simulations and experiments verifies the accuracy of the analytical model predictions for the maximum spread of a droplet in a spray coating process. Based on the analytical model, for high Reynolds numbers and small angles of impact, the effect of Weber number on the maximum spread is increased. For low Reynolds numbers and impact angles close to 90o, the maximum spread remains nearly constant

FFT techniques were employed to develop a full numerical solution for analyzing the elastic dry contact of two flat rough surfaces. The study of the quality of surfaces produced by different production methods required the simulation of the roughness of contacting surfaces and also the kind of contact. By assuming that the roughness and distribution of the contacting surfaces were random and Gaussian, the FFT method was used to simulate them. Considering the penetration of asperities of contacting surfaces to each other, parameters such as pressure distribution, surface deflection, real contact area, its distribution as well as mean pressure on the surface were computed. The effect of simulated surface specifications such as average roughness Rq, correlation length β* and the amount of equivalent modulus of elasticity on average pressure and real contacting area was investigated. A comparison was carried out to evaluate the results. Finally, it was found that FFT techniques for solving surface and contact simulation problems were rapid and time-saving with accurate results.

Heat treated electroless Ni-P coatings have high hardness and wear resistance. Traditional heat treatment (1 hr at 400 oC under neutral atmosphere), results in full crystallization of electroless Ni-P coatings. In the present research, electroless Ni-P coatings were treated under conventional as well as plasma conditions. Coating thickness was 5 and 10 micron. Temperature rates and time of the processes were decided to be at 400 oC and 600 oC for 1 and 5 hours, respectively. Thickness, structure, and composition of the coatings were evaluated by optical microscopy, XRD and EDS methods in both heat treatment (HT) and plasma treatment (PT) conditions. It was shown that coating thickness as well as its XRD spectrum (i.e. intensity and breadth of peaks) was different. These disparities were related to sputtering phenomenon, special transformation behaviour, and residual strain in the plasma treated electroless Ni-P coatings. It was also found that chemical composition of the coatings could be altered during plasma process

The WO3 film is the most important material for electrochromic (EC) layer especially in electrochromic display (ECD). In this project WO3 films were deposited on indium thin oxide (ITO) glass substrates. For this propose peroxotungstic acid derivative (APTA) was synthesized by sol-gel method and was used as a precursor solution for depositing by dip coating technique. Whereas many properties of the films, including some of the EC properties, they were dependent on microstructure of the films, were found to be modified by the addition of small amount of oxalic acid dihydrate which is an organic compound, ranging from 0% to 10% by weight in PTA solution. In this report, The morphology of the film structure, and EC response were investigated. Heat treatment of APTA and transparency of layer were carried out in different temperatures and concentrations. The findings showed that the addition of oxalic acid to the precursor solution on the films resulted in excellent EC properties without micro cracks.

Electrophoretic deposition has been used by establishing an electric field in a stable suspension, a perfectly smooth and uniform surface is obtained. In this study a coating is applied on a Inconel substrate by the ionizations of alumina single crystals in a suspension and in an electric field. Then, with sintering treatment, a dense coating is obtained. The effect of time, applied voltage, processing, sintering temperature, and the pH of the solution on the thickness of deposited layer are also investigated. To determine the thickness and analysis of coating layer, scanning electron microscopy was used.It was found that by increasing voltage, applied current, sintering temperature and also applying optimum processing of sintering treatment and increasing pH, the thickness of deposited layer was increased. Each of the mentioned factors had different effects on the thickness of coating layer. With due attention to the effective factors on the thickness of the deposited layer, optimum conditions for applying coating was achieved.

In this research, EIS technique was utilized to study the corrosion behaviour of AA5083-H321 aluminum-magnesium alloy in 3.5% NaCl solution. Impedance spectra were obtained during 240 hours of exposure of the sample to the test solution. The surface and cross section of the samples were studied by scanning electron microscopy (SEM) and EDAX analysis.The results indicated the the impedance of the surface is controlled by the reactions which occur in the passive layer on top of intermetallic particles and substrate. Intensification of cathodic and anodic reactions at the first 24 hours decreases the impedance of the surface. However accumulation of corrosion products inside the pits and subsequently suspension of cathodic reactions on top of intermetallic particles tends the impedance of the surface to decrease. Meanwhile pitting corrosion provides inhomogeneous surface which prevents the capacitors to behave ideally. Therefore capacitors are substituted by constant phase element.