IRANIAN JOURNAL OF SURFACE SCIENCE AND ENGINEERING
Year:2018 | Volume:14 | Issue:37|Papers Count:9

Thermal barrier coatings (TBCs) as one of the high temperature coatings are widely used to improve the performance of the blades of turbine engines and in particular gas turbines. The purpose of this study was to investigate the thermo-mechanical properties, hot corrosion resistance and high temperature oxidation of conventional zirconia TBC coatings and improve their performance by adding cerium oxide stabilizer to the compound. For this purpose, CoNiCrAlY/YSZ and CoNiCrAlY/CYSZ coatings were applied to substrates of IN738LC by atmospheric plasma spraying. Thermomechanical behavior of the coatings was investigated by measuring the thermal expansion of the coatings. Hot corrosion behavior and oxidation of the coatings by furnace method were investigated at 1050 ° C and 1100 ° C with 4-hour cycles, with and without the applied of a mixture of corrosive salts (Na2SO4-V2O5). The results showed that CYSZ coating has a higher thermal expansion coefficient than YSZ coating. Microstructural and phase studies of coatings by SEM/EDS and XRD analysis before and after hot corrosion and oxidation tests showed better resistance to CYSZ coating than YSZ coatings.

In this research, creation of the composite coating containing tungsten carbide particles on the surface of Hadfield manganes e steel by using gas tungsten arc welding (GTAW) process has been investigated. For this purpose, tungsten carbide particles were prefigured on the surface of manganese steel. By changing welding current, surface melting of carbides and the base metal occurs. The microstructure of coating and phase analysis of hard layer was investigated through different characterization techniques such as light microscopy, Scanning Electron M icroscopy (SEM ), and X-ray Diffraction (XRD). The micro-hardness of the coating was evaluated. According to the characterization results, the created coating includes WC and CW3 amplifier of tungsten in a dendritic microstructure. Such carbides can significantly improve the hardness and wear behavior of the coating. The micro-hardness of the coated sample (650 HV) is more than 3 times the uncoated one (200HV).

Nowadays, HA attracts a lot of attention as a bio-ceramic coating on metallic implants due to the its desired biological properties. In order to improve the properties of HA coating, especially cracking induced by drying and sintering, using of HA composite coatings are in the spotlight. In this study composite coatings of HA-TiO2 containing of 0, 5, 10 and 20 wt% TiO2 were coated using the electrophoretic deposition process on the NiTi substrates. The used suspension solution was mix of n-butanol and triethanolamine. The electrophoretic deposition was carried out at 60 V for 120 seconds on the cathode. After deposition, specimens were dried at room temperature and then sintered. The XRD and SEM were used to study the phases and microstructure of coatings, respectively. The composition of coatings were investigated using EDX. The results show that increase of TiO2 nanoparticles up to 10 wt% improves the drying behavior of coatings and the bonding strength of HA10% TiO2 coating increases up to 21. 1 ± 2. 3 MPa in compared to that of HA coating (18. 1 ± 3. 1 MPa).

Pure and Cu doped ZnS (ZnS: Cu) nanoparticle thin films were synthesized by Successive Ion Layer Absorption and Reaction (SILAR) method. The obtained thin films were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), FESEM and absorption spectra. The photocatalytic degradation of methylene blue (MB) dye molecules was evaluated by controlling the concentration of MB solution using a UV-vis absorption spectrophotometer. Changes in Cu concentration in thin films of ZnS nanoparticles showed that a sample containing 2 wt% Cu is much more effective in degradation dye molecules. Experimental results showed that the degradation efficiency of photocatalytic thin films was 36% after 30 minutes of UV radiation and the highest degradation efficiency (56%) was obtained by increasing exposure time for 360 minutes.

The performance characteristics of the deep violet In0. 082Ga0. 918N/GaN double quantum well (DQW) laser diodes (LDs) has been numerically investigated by using ISE TCAD software. The role of thickness and Al composition of AlGaN Cladding layers on the different performance characteristics including output power, threshold current, and slope efficiency, DQE, and optical intensity has been studied. Simulation results indicated that DQE and slope efficiency increase, while output power and threshold current decrease on the average by increasing AlGaN composition of cladding layers. Output power increases with increasing AlGaN cladding thickness because of increasing electron and hole carrier densities in Quantum wells and consequently, increasing radiative recombination. The slight increase were observed in QW electron and hole carrier densities of deep violet In0. 082Ga0. 918N/GaN DQW LDs with increasing cladding layer thickness.

Hard anodizing, as one of the methods to improve surface properties, has always been considered for aluminum. But the difficulty of controlling the temperature and the formation of an anodic aluminum oxide layer at the bottom of the pores are two important problems in this method. In this investigation, by designing an anodizing reactor, precise control of the coating conditions and the use of a mild anodizing (at 40 volts) prior to hard anodizing (at 130 volts), current density changes during the coating of 1100 aluminum alloy in an oxalic acid solution were studied. T he current density-time curves and field emission scanning electron microscopy (FESEM) images showed an increase of current density and a transition from mild to hard anodizing, which was accompanied by an increase in interpore distance. The x-ray diffraction (XRD) results indicate a coating with an amorphous alumina structure. Energy dispersive spectroscopy (EDS) analysis also showed that the coating only contains oxygen and aluminum due to the formation of alumina. The results of the polarization test indicated a significant improvement in the alloy corrosion resistance with the application of aluminum oxide layer. Anodizing at three temperatures of zero, 10 and 17 ℃ showed that increasing the temperature would increase the charge density and thickness of the oxide layer, but there was no significant effect on the interpore distance, pore diameter, the thickness of the barrier layer and the pore density, indicating that these parameters are independent of current density. However, due to more corrosive intensity of solution, the range of changes in these parameters increases. T he porosity of the coat ing also increases from 15. 6% to 17. 3%. By comparing the anodizing of pure aluminum and 1100 aluminum alloy, it was found that the presence of alloying elements would lead to a reduction in the coating arrangement.

In this research, the effect of different parameters in Friction Stir Processing (FSP) on hardness and wear resistance of Al2024 is investigated. For this reason, five levels are considered for the three main parameters including pin rotating speed, pin traveling speedand pin angle using response surface methodology. The maximum hardeness and minimum wear rates are considered as the responses for optimization. For evaluating the hardness (under the 300 gr loading) and wear behaviour of processed samples (under the 500 gr loading and 1000m sliding distance), ASTM-E384 standard micro-vikers hardness tests and ASTM-G99 standard pin on disk tests are used, respectively. For investigating the FSP effect on micro-structure, grain size and morphology, optical microscopy is used. The results show that pin rotating speed and traveling speed have directly effect on the average hardness and also have a second power index relation on it. Furthermore it is obvious that these two parameters have interaction on each other and able to study them separately. Pin angle parameter had not any effect on the average hardness approximately and also no interaction by other two parameters. After optimizing process, the optimized values for pin rotating speed, pin traveling speed and pin angle for the Al-2024 were 28mm/min, 1274 rpm and 2. 5 degree, respectively. Hardness value and weight reduction in wear test for processed case in optimized case were 154 vikers and 9. 3mg.

In this study, ceramic coatings were formed successfully by plasma electrolytic oxidation process in aluminate-based electrolyte containing different concentrations of potassium hydroxide (3, 4 and 5 g/l) on pure titanium substrate. All processes were carried out at constant voltage of 420 v and for 180 s. The effects of the chemical composition of electrolyte on the sparking voltage, microstructure and corrosion behavior were studied. The surface and cross-sectional scanning electron microscopic (SEM) images and corrosion tests of coatings in a 3. 5 wt. % NaCl solution showed that the increasing of potassium hydroxide concentration from 3 to 4 g/l resulted in increasing the compactness and uniformity, decreasing the average size of surface micro-pores despite increasing the porosity, increasing the thickness and increasing the corrosion resistance. However, increasing of potassium hydroxide concentration in the electrolyte to 5 g/l due to the excessive electrolyte conductivity caused to formation of bigger and stronger sparks and thus increasing the average micro-pores size despite decreasing the porosity and increasing the thickness and decreasing the corrosion resistance of coating. Therefore, the coating formed in the electrolyte containing 4 g/l potassium hydroxide showed the most uniform and dense structure with the smallest micro-pores size and the best corrosion behavior. In fact, this sample with corrosion resistance about 22. 88×10 5 (Ω 2 cm ) increased the resistance of uncoated titanium, 36 times. Using X-ray diffraction pattern, it was determined that the coating was composed of rutile, anatase and TiAl2O5 phases.

Because of high strength, hardness and chemical stability of titanium carbide (TiC), it is known as an ideal material for manufacturing the different types of composite. In this study, surface of hot work H13 tool steel was composited with TiC particles using pulsed laser process. Hardness of the composited surfaces was measured with a microhardness tester. characterization of the composited area was done using field emission scanning electron microscope (FESEM), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The results showed that, laser surface compositing using TiC particles, increased the microhardness of composited layer compare to the untreated H13 tool steel. Increasing in laser beam scan rate, led to decreasing in the temperature of molten basin, so the amont of unmelted TiC particles and microhardness of composited layer increased.