فصلنامه علوم و مهندسی سطح ایران
پیاپی 7 (تابستان 1388)

In this article, the effect of Ni2+ addition was studied on phosphate coating porosities. The phosphating bath in this research was included of 5gr/lit zinc oxide, 12 mlit/lit phosphoric acid and 2gr/lit sodium nitrate as an accelerator. Zero to 8gr/lit nickel nitrate was added to the phosphating bath and the effects were studied. All of the samples were phosphated at 55°C and 210 seconds conditions. Coating porosities were studied by an electrochemical linear polarization method, coating morphology was studied by Scanning Electron Microscopy (SEM) and coating phases were studied by X-Ray Diffraction (XRD). The results were shown that nickel content increasing, causes to porosities decreasing of phosphate coating.

In this work, electroless Ni-P deposits, of approximately 20 μm in thickness and a medium phosphorous content, of approximately 6.73 wt.%, were obtained on Al5083 substrate and the effects of heat treatment on structure, microhardness and corrosion behavior of the deposits were investigated. Deposits were heated at temperatures ranging from 200 to 400 °C for 1 h. The structural characterization was done by means of X-ray diffraction studies. As-deposited coating exhibit broadened X-ray reflections indicative of a semi-amorphous structure. Heat treatment decreased the amorphous phase and after heat treatment at 400 °C for 1 h, the final equilibrium phases were Ni3P and Ni crystals with size of 32 and 18 nm, respectively. The results of Vickers microhardness testing showed that after heat treatment at 200 °C, the hardness decreases and the maximum hardness was achieved for the samples which have been heat treated at 400 °C. This is caused by the formation of intermetallic Ni3P stable phase at this temperature, acting as a function of precipitation hardening. Corrosion resistance of the coatings was evaluated in 3.5 wt.% NaCl solution by potentiodynamic polarization and electrochemical impedance (EIS) methods. The fitting results indicated that on heating to 240 °C, the corrosion resistance increased and then reduced at higher temperatures. Heat treatment at 400 °C leads to crystallization of the amorphous phase, making available more number of grain boundaries which are highly prone to corrosion attack. Therefore, the heat treatment of electroless specimens at 400 °C significantly decreases their corrosion resistance.

Effect of boriding and borochromizing treatments on microhardness, wear properties and corrosion resistance of austenitic low carbon stainless steel (AISI 316L) has been studied. Coating was applied by using of pack cementation method. The pin on disk test was employed at two loads of 75 and 115N to evaluate the wear behavior of the samples. Corrosion test was performed by using of immersion method in (10 vol.%)HCl and (10 vol.%)H2SO4 acid solutions on uncoated, borided and borochromized specimens.The results reveals that specimen with borochromized coating provides higher surface hardness and more desirable wear behavior in respect to uncoated and borided specimens. Fatigue- oxidation wear mechanism is also found as the main deterioration mechanisms of surface layers in coated specimens. Borochromized specimen has Also higher corrosion resistance in comparision with the borided specimen, although, it has, in some extend, similar corrosion behavior to that of the substrate.

LM13 alloy is widely used for piston production due to its low thermal expansion coefficient, high fluidity and good hot tear resistance. Optimization of production process and heat treatment are two effective factors in improvement of produced piston properties. In this research effect of change of casting process from gravity casting to squeeze casting on wear behavior of LM13 alloy was investigated. First the samples were cast using the two casting processes and heat treated. Then wear behavior of the alloy in two states of squeeze and gravity casting under dry sliding condition was examined and compared.Results of hardness and strength tests indicate that squeeze cast specimens have higher mechanical properties. The wear experiment results show that in both squeeze and gravity cast specimens, the amount of weight loss increases as the sliding distance increase, but the wear rate and the friction coefficient reduces. Comparing the properties of the two samples indicate that squeeze sample has better tribological behavior. Change of production process causes a decrease in friction coefficient from 0.4 to 0.2 for squeeze cast samples and also decreases the weight loss of the samples. Furthermore squeeze cast samples have better surface quality. In squeeze cast samples, abrasive, adhesive and delamination and in gravity cast samples, abrasive, adhesive and surface fatigue were recognized as the dominant wear mechanisms.

Light refractive index is an important constant in design of optical thin films and its value vary with variations of film thickness and density, crystal structure and internal defects. Accident electromagnetic waves on surface layer in proportion with various amounts of refractive index, absorb, reflect and or trasmit through the layer and some are diffracted because of surface and internal defects. Silica thin films in pure form or in combination with other oxides have considerable applications for optical properties.In this research, the effects of thickness and porosity in thin films of silica on refractive index and also the degree of light transmission through deposited layers was evaluated. With the aid of Sol-Gel method, silica was deposited on soda- lime glass specimens. Structural examinations of the layers were carried out, using Scanning Electron Microscopy (SEM), Energy Dispersion Analysis of X- ray (EDAX) and X-ray Diffraction technique (XRD). Also, by using light spectrophotometry, the spectrum of transmitted and absorbed rays, from the surface layers were prepared and compared. The results indicate that the layer thickness and porosity, have an effect on the degree of transmitted and absorbed waves. As the layer thickness is reduced, the amount of refractive index is reduced and the ability of transmitted light increase.

In this research, effects of some production parameters such as lead content of the zinc bath, jet wipers distance from sheet surface and galvanizing line speed on corrosion behaviour and structure of hot-dip galvanized coating produced in Mobarakeh Steel Company was investigated. Lead content of the zinc bath was changed in the range of 0.01 to 0.11Wt.% and its effects on the texture and corrosion behaviour and spangle size was studied. Coating texture was determined using X-ray diffraction and Corrosion behaviour of the coating was analyzed employing salt spray and Tafel polarization test. Then, influences of increasing the lead content of the zinc bath, variations of jet wipers distance from the sheet surface and galvanizing line speed on the coating structure were assessed. Results showed that increasing the lead content of the zinc bath, has decreased the relative texture coefficient of basal component and conversely, has increased the relative texture coefficient of high angle pyramids, low angle pyramids and prism plane components. Also, it was discerned that increasing the lead content of the zinc bath would result in the increase of spangle size and number of dull spangles in the surface. After investigation of the effects of increasing the lead content of the zinc bath, increasing the jet wipers distance and decreasing the galvanizing line speed, it was found that gamma layer thickness, the detrimental layer for corrosion resistance and formability, has been increased.

In this study, an attempt was made to synthesize Mo-Si-B multiphase alloy coatings using a combination of mechanical alloying (MA) and air plasma spraying (APS). Mo-14Si-10B (at%) elemental powders were milled using an attritor mill. Mechanically alloyed (MAed) powders as compacted buttons were annealed in an atmosphere controlled furnace at 1100ºC for different times. The annealed Mo–Si–B buttons were crushed, sieved and prepared for coating. Then, powders of Mo-Si-B as alloyed and agglomerated, were plasma sprayed under different conditions onto plain carbon steel. Metallurgical characteristics of powders and coatings were evaluated by SEM, OM, XRD and AAS. The results did not show any related intermetallics after MA. However, Molybdenum silicides were identified when the MAed powders were subjected to high temperature annealing over 10 h. Also, the critical parameters in APS to maintain the starting stoichiometry were identified; this included thermal spraying in an inert environment to minimize the oxidation of coatings. No intermetallic compounds could be identified when alloyed powders were sprayed.

this paper, the wear behavior of Hadfield steel has been investigated by using the grinding wheel in the pin on disk test method. The main parameters of this test method such as normal load on pin, sliding speed and sliding distance have been studied. In this test, the main and interaction effects of parameters on the weight loss and its regression models have been obtained. The results show that chipping and surface cracking are two basic mechanisms of wear of the steel. In addition, the normal load is more effective than sliding speed in weight loss of the steel. In addition, the role of normal load on the weight loss increases with increasing the sliding distance, while the role of disk velocity fixes. The normal pressure (load) must also be lower than 0.5 MPa for low stress wear condition.

Excellent wear resistance, impact resistance together with good thermal stability of carbide coatings that created by Thermo Reactive Diffusion(TRD), makes it favorable in technological respect. This research investigated kinetics of vanadium carbide coating on DIN:1.2367 hot work steel. These processes carried out in solid medium containing powders of ferro-vanadium, ammonium chloride and alumina To investigation the kinetics of these processes, coating layer thickness measured for 0.5, 1.5, 3 and 4.5 hours at different temperatures of 950, 1050 and 1150 °C. According to time and temperature, layer thickness obtained between 2.3 to 32.2 X-ray diffraction analyses, optical and scanning electron microscope with EDX spot analyser employed to characterize coating layer properties. Relation between temperature and activation energy obtained as Arrhenius equation and growth rate constant and activation energy obtained, 7.98x10-4 Cm2/s and 173.2 KJ/mol correspondingly. Finally characterized that to obtain minimum porosity in coating layer, processes must carry out in low temperature as far as possible.