IRANIAN JOURNAL OF SURFACE SCIENCE AND ENGINEERING
Year:2008 | Volume:- | Issue:5|Papers Count:9

Mechanical properties of plasma-sprayed HA coatings deposited on Ti-6AI-4V substrates by varying two spray parameters were evaluated using the indentation technique. The content of amorphous phase (ACP) at coating interface or surface as measured by XRD increased both with increasing spray distance and/or plasma power and also from the coating surface toward the interface. The microhardness value, elastic response, and Young's modulus (E) were found to be controlled by the compromise between porosity level and ACP content. The indentation fracture toughness (KC) of coatings mainly depended on the contents of recrystallized HA (RHA) and ACP, as well as on porosity level. The Knoop and Vickers indentation methods were used to estimate E and KC, respectively. The critical point at which no crack appears at the interface was determined by the interface indentation test, which was used to define the interfacial toughness (KCa) representative of the crack initiation resistance of the interface. KCa was found not to change in a simple manner with the two plasma parameters. The results indicate that the optimized values of ACP and RHA contents at the interface were the factors most influencing KCa values.

Electropolymerization of Pyrrole on metals has been carried out in different electrolytes despite shortcomings prevailing due to defects formed in the coating during polymerization. Surface passivation before polymerization may reduce such defects but optimization of the effects and elucidation of its mechanism await further research. Conflicting reports exist on the mechanism of protection by Polypyrrole coating because of differences in its behavior due to structural variations in the presence of different ions in the polymerization solution. In this study, the effect of molybdate as a doping anion on the corrosion protection of Polypyrrole is investigated. The polymerization solution was 0.1M Pyrrole plus 0.1M Molybdate at pH5. Pretreatment in nitric acid improved coating integrity so that the defects, active pits appearing during polymerization, disappeared. Surface morphology showed a perfect defect-tree surface and exhibited a remarkable resistance in electrochemical corrosion test. It was found that pretreatment of the substrate and introduction of molybdate doping ions improve corrosion protection of Polypyrrole conducting polymer on steel.

Laser surface melting of Ti-6A1-4V alloy under a pure nitrogen environment of 30 lit/min gas flow rate was carried out with 200-600 mJ laser beams produced by a Nd- YAG pulsed laser. The Laser frequency and the pulse duration time were fixed at 10Hz and 200 mS, respectively. The microstructure and corrosion behavior of the nitrided samples were examined, using scanning electron microscope, X-ray diffraction (XRD), and anodic polarization tests in 2M HCl solution. The microstructures consisted mainly of a thin continuous layer of titanium nitride followed by nearly perpendicular dendrites, and below this, a mixture of small dendrites and large needles with a random orientation. The dendritic structure was the TiN phase, and the needle phase and the phase of the matrix between the dendrites were nitrogen-enriched a’-Ti. An improvement in corrosion behavior, associated with the presence of a good TiN coating, was observed. Also, the improved pitting corrosion resistance was obtained due to the microstructural changes after laser treatment.

Oxide layers produced by plasma electrolysis oxidation are nowadays receiving considerable attention because of their useful characteristics. The high-temperature ceramic layers thus produced enjoy such properties as high hardness and wear resistance among others. They include a-Al2O3 and g- Al2O3 phases and their porosity is much lower than that of the conventionally produced ceramic layers. There is a variety of current regimes applied with important effects on their properties. This study investigates the production of hard ceramic layers on aluminum using direct pulsed current in plasma electrolysis oxidation. The results reveal that, compared with the direct current method, the pulsed current improves the characteristics of the PEO coatings due to its better control over the coating mechanism.

M-type hexaferrite SrFe9Mn1.5 Ti1.5O19 and BaFe9Mn1.5 Ti1.5O19 were synthesized by usual ceramic sintering method. The powders of the 70% ferrites by weight were mixed with resin epoxy. The prepared composite was sprayed on a 1.8mm-thick aluminum 5083 substrate. Their structure and microwave adsorption property were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), and vector network analyzer. The results showed that the magnetoplumbite structure for both samples was formed and powders had the hexagonal or amorphous morphology with particle sizes within the range of 1-5mm. The maximum reflection loss for Sr-ferrite coating was 18.84 GHz equal to (-26.43 dB). The waveband of absorption was approximately 3GHz more than 15dB for reflection losses. The results for Ba-ferrite were 19.18GHzwith (-44.lldB) reflection loss and 3.2 GHz waveband. Both coatings can be good candidates for stealth radar targets.

In this paper, nanocrystalline nickel was electrodeposited using direct current and rotating electrodes. A Watt's bath containing saccharin as a grain refiner was used. The microhardness of deposits was studied and maximum hardness was found to be 620 HV. X-ray diffraction (XRD) tests were carried out to study grain size and texture. The texture of the deposits were changed from [220], [200], and [111] for microcrystalline nickel to [111] and [200] double fiber texture in nanocrystalline nickel. The minimum grain size of the deposit was 14 nm. Morphological studies by Scanning Electron Microscopy (SEM) exhibited a smooth morphology for nanocrystalline nickel, while it was rough for microcrystalline nickel. In addition, the effects of saccharin concentration, cathode rotation speed, and current density on microhardness, grain size, and morphology 0f deposits were studied.

This work deals with precipitation of silica phases on aluminum using DC current for establishment of plasma electrolytic coatings. High concentrations of aqueous sodium silicate solutions were used for preferential precipitation of silica phases on aluminum. SEM, EDS, XRD, and polarization curves were used to study the properties of the coatings thus produced. The results showed that it is possible to establish the growth of silica phases on aluminum surfaces using the plasma electrolysis technique. It was found that the optimum concentration of sodium silicate was 30 gr/lit. Moreover, lower corrosion resistance of the precipitated layer at very high concentrations was attributed to the porous nature of the silica coating. It was also found that increasing sodium silicate concentration in the electrolyte would lead to the substitution of amorphous compounds for crystalline phases.

Flatness of surface is of great importance in industrial products such as steel sheets. In many cases, the quality of the final product is directly linked to the flatness of its components. One possible defect in the production of hot strip mill steel sheets is the straightness defect which results in flatness asymmetries in the steel sheets, causing a certain curvature on its edges. This defect is more likely to occur at the final stages of the hot strip mill. The deformation may cause stoppages in the rolling process and damage to the machinery. Detection and measurement of these asymmetries can be fed back to the previous stages of the rolling process for proper adjustment of the roller's speed to avoid further defects. Nowadays, a family of methods for surface detection, called topometry, is available for measurement of surface flatness asymmetries. In this paper, a method is proposed based on Interferometric Fringe Projection (IFP) which performs the flatness measurement. The proposed method has the advantage of simplicity and can, hence, be used in real-time applications.

Low-temperature plasma nitriding effects on corrosion, wear and corrosion-wear properties of 17-4 PH martensitic and A286 austenitic precipitation-hardening stainless steels have been investigated over a wide range of treatment temperatures and durations. Experimental results have shown that the nitrided layer characteristics and properties are highly process-condition dependent and that there exits the possibility for considerable improvement upon surface properties depending on application purposes. The results have also indicated that 420oC/10h is the optimal condition for both 17-4PH and A286 steels improving wear resistance under unlubricated, as well as under fairly good corrosion resistance conditions. Under corrosion-wear conditions, plasma nitriding at 420oC/10h is also the optimal treatment condition for A286 steel. However, for 17-4PH steel, 500oC/10h plasma nitriding has shown to be the optimal treatment condition to combat corrosion-wear.