فصلنامه علوم و مهندسی سطح ایران
پیاپی 24 (تابستان 1394)

The purpose of this article of depositing and structural characterization of Ni(5Al)-Al2O3-MoS2 composite coating on the CK45 substrate by atmospheric plasma spray process. For this purpose, Powdering materials by combining the 60%Ni(5Al)-40%Al2O3, 60%Ni(5Al)-35%Al2O3-5%MoS2 and 60%Ni(5Al)-25%Al2O3-5%MoS2(wt%) were prepared by mechanical working mill. The spray dryer process was performed to increase the uniformity of density and granulation of the powder particles. Spraying operation by a plasma spray with gun equipped model PS50 at a distance of 100 mm, a current of 350 A and a voltage of 35 kV was done. The powder and coated surface were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), roughness and powder flow tests. Fluidity of powder was investigated by powder flow test according to Agglomerated powder exit from outlet of Hall flow meter. In order to investigate the substrate roughness and the effect of depositing composite coating on the average substrate roughness, roughness test with ultrasonic detector by contact method was performed. The results of the roughness test showed that with the increasing amount of MoS2, average coating roughness was reduced from 5.95 to 4.08 μm. According to the results of XRD, brittle NiAl2O4 phase in the coating without lubricant compared to lubricant coatings the more was formed. Composite powder structure and micrometer interfaces were led to the combination of Ni and S. According to the results can be expressed that depositing of Ni(5Al)-Al2O3-MoS2 composite coating with different amounts of MoS2 lubricant by APS method is possible.

In this report a facile and optimized method for polishing of fluorine doped tin oxide (FTO) was presented in which nanoparticles in toothpaste was used as polishing material. Due to being more chemically and physically stability relative to another TCO such as indium tin oxide (ITO), FTO is used in optoelectronic devices. More thickness in FTO due to have a good conductivity resulting a rough surface. Toothpaste can make smooth FTO surface due to have polisher nanoparticles. Here, FTO surface was smooth using toothpaste so that according to AFM data, the RMS surface roughness was changed from 12 nm for FTO to 3 nm for FTO after polishing. The comparing of the transmittance spectra FTO before and after polishing process showed the FTO thickness was not changed dramatically. The X-Ray photoelectron spectroscopy showed there were no effects on the chemical states of FTO surface before and after polishing process. The hybrid light emitting diode was fabricated using polished FTO that it was more efficient comparing to those fabricated with FTO and ITO.

Cr2O3 feedstock of nano and micron size particles were coated on low carbon steel by APS process. Ni-Al bond coat was used to improve the adhesion of coatings to substrate. Microstructural investigations were conducted by FESEM and X-ray diffractometery (XRD) was utilized for structural analysis. Wear behavior of the coatings was studied by ball on disk testing using alumina balls at room temperature and 300 and 500˚C under a load of 30N. Experimental results showed that both coatings consist of a layered structured of melted and semi-melted splats. However the nanostructured coating contained more semi-melted splats resulting more porosity compared to the microstructured coating. Tribological evaluation of the coatings revealed that the nanostructured coating exhibits lower friction coefficient in comparison to the microstructured Cr2O3 coating. Wear rate of the coatings decreased by increasing the temperature so that at 300 and 500˚C. The nanostructured coating presented better wear resistance. Improvement of wear resistance and decreasing the friction coefficient in nanostructured coating at different temperatures can be attributed to the effect of nanoparticles, higher hardness and also formation of compacted and adhesive tribofilm on the surface of the coating.

In this study, it was attempted to design a composite coating based on Alumina - Titania, and evaluate the effect of spraying current on coating properties in order to obtain the best conditions to improve the in-flight behavior of Particles, mechanical properties and morphology of plasma sprayed coatings. Alumina - Titania composite coatings were prepared via plasma spray technique using different input powers (600, 700 and 800 A). Characterization of prepared coatings was performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Also microhardness and strength adhesion of the composite coatings was studied. Three dimensional (3D) numerical models were developed to study the temperature of fed particles. The investigation showed that the composite coating sprayed at a current of 700 A it possessed the best quality in terms of amount of porosity and the surface morphology and the most value of mechanical properties. In this current, Particles with Appropriate and optimal velocity of 139 m/s has collided on the surface of coating. Also microhardness and adhesion strength of the coating is increased to the value of 1409 HV1.9 and 28 MPa Respectively.

In the present study, a new process for the production of nano-structured composite coating of metal - oxide has been presented. Novel electroless composite combines sol-gel and electroless plating process to prepare highly dispersive oxide nano-particle reinforced composite coatings. Transparent TiO2 sol was added into the standard electroless plated Ni–P solution at a controlled rate to produce Ni P–TiO2 nano-composite coatings on AZ31 substrate. The coating was found to have a crystalline structure. The nano-sized TiO2 particles (∼15 nm) were well dispersed into the Ni–P coating matrix during the co-deposition process. This technique can effectively avoid the agglomeration of nano-particles in the coating matrix. As a result, the micro-hardness of the composite coatings were significantly increased to∼1025 HV200 compared to∼710 HV200 of the conventional electroless coatings produced with solid particle mixing methods.

Plasma Electrolytic Oxidation (PEO) method was employed to enhance corrosion resistance of 7075 Aluminum alloy. The coating process carried out in a silicate based electrolyte by DC constant current density. Characteristics of coating were evaluated by X-ray diffraction and scanning electron microscopy methods. Analysis showed the formation of γ-alumina oxide film, which has a porous surface and some pores in its structure. Coating corrosion behavior was evaluated by electrochemical tests after 1 hour immersion in 1M H2SO4. Tafel Polarization test revealed that PEO decreased the corrosion current density as ten orders of magnitude, and increased the charge-transfer resistance as twenty five order of magnitude, and thus improved corrosion resistance of 7075 Al alloy. Electrochemical Impedance spectroscopy (EIS) test revealed that the coating formed a physical barrier against the charge transfer, and decreased the diffusion rate of the corrosive solution due to its non-columnar structure.

In this study, effect of grain size reduction from micrometer to ultrafine on phases and thickness of nitrided layer of AISI 304L austenitic stainless steel after plasma nitriding was evaluated. To produce of fine grain size samples (11 μm) and ultrafine samples (135 nm) used cold rolling and reversion annealing of strain induced martensite. Plasma nitriding was performed at 450 °C for 5h and the gas composition was nitrogen and hydrogen with a ratio of 1 to 3. The evaluation of grain size and observation of microstructure was performed by scanning electron microscopy and identification of phases was performed by X-Ray diffractometer. An increase in the thickness of nitrided layer from 4.8 to 10.6 μm was experienced by decreasing of grain size from 11 μm to 135 nm. In addition to the increase of nitrided layer and percentage of precipitates in ultrafine grain samples, the type of phases was significantly affected by the grain size. It was observed that retained martensite after reversion annealing caused formation of iron nitride (- γ' Fe4N) in ultrafine grain samples.

The Effect of Fe addition (0.2-2wt%) on the wear behavior of Al-(5-13)Si alloys was investigated. The pin-on-disk wear tests were conducted at room temperature, under normal load of 45N, sliding speed of 0.13 m/s for sliding distance of 1000m. The results showed that bellow a critical Fe content (1.2wt% in this research), Si addition improved the wear resistance. In this regard, the wear rate of Al-13Si-0.8Fe alloy was observed to be less than 50% of Al-5Si-0.8Fe. This could be attributed to the fine distribution of hard AlFeSi platelets in interdendritic regions that increased the hardness and decreased the subsurface deformation and asperities adhesion. So, controlled delamination within tribolayer as well as abrasive wear could be proposed as the main wear mechanisms. Formation of large primary AlFeSi platelets at the iron content of more than critical value, however, facilitates the initiation and propagation of subsurface cracks and instability of tribolayer. This led to the higher delamination and abrasive wear with higher Si containing alloys.