جستجوی مقالات مرتبط با کلیدواژه "اکسیداسیون دما بالا" در نشریات گروه "مواد و متالورژی"

The purpose of this research is to investigate the performance of vertical cracks along the thickness of YSZ-40%wtAl2O3 coating and to improve the resistance to oxidation and thermal shock of thermal barrier coatings by creating a layer of YSZ-Al2O3 applied on the YSZ by atmospheric plasma spraying. In this research, YSZ-40%wtAl2O3 powder was synthesized using co-precipitation process and then applied on the YSZ by atmospheric plasma spraying process. High temperature oxidation test at 1100 ˚C and thermal shock test at 1000 ˚C were performed. The structural and phase characteristics of the coatings were investigated using optical microscope, field emission scanning electron microscope (FE-SEM) and X-ray diffractometry (XRD). The structural comparison of the samples showed that the use of the YSZ-40%wtAl2O3 eutectic compound due to the decrease in the melting temperature of the compound and the increase in the melting which brings the proper contact between the splats. This coating showed 18.6% increase in oxidation resistance compared to conventional TBC. Also, the findings showed that the decomposition of the un-pyrolysed precursor in the coating structure of YSZ-40%wtAl2O3 leads to the creation of vertical cracks in the coating structure, which as the oxidation time increases, the number of vertical cracks with a certain distance in the coating structure will also increase. Also, the suitability of the vertical cracks created in the YSZ-40%wtAl2O3 coating, in terms of stress tolerance, increases the ability of the coating to release the stresses during thermal cycles and finally leads to increase in the durability of the coating.

The present study investigates the oxidation behavior of GTD-111 nickel base superalloy at 1000 °C in air. To this aim, several techniques including thermal gravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), grazing incidence X-ray diffraction (GI-XRD) and glow discharge optical emission spectroscopy (GDOES) were employed. The oxidation kinetics of this superalloy alters from linear to parabolic after 10 h exposure which is accompanied by the formation of a uniform surface oxide layer. Based on the cross-sectional analysis of FE-SEM and GDOES, Ti can most probably be considered as the first species which enters the reaction front when oxidation begins. However, as a result of its oxidation, Ti is depleted just beneath the oxide scale, and conditions changes in favor of the formation of other oxides, namely, Cr2O3. Subsequently, due to the internal diffusion of O2- species, Al is internally oxidized, and islands of Al2O3 form underneath the oxide layer. After 100 h of oxidation, the chemical composition of the oxide film consists of an Al-rich inner region, a Ti-rich outer region, and a Cr-rich intermediate layer. GI-XRD results also confirmed the formation of mentioned oxide phases. No indication of oxide spallation and formation of nickel oxides were observed even after 100 h of oxidation.

In this research, firstly amorphous Alumina powder was produced by co-precipitation method. Then YSZ/Al2O3 coatings were applied by plasma spraying process in two types of pyrolyzed and crystalline nano-alumina. High temperature oxidation and thermal shock resistance test were done at 1100˚C. Microstructure and phase analysis of coatings were studied by optical and electron microscopes and XRD method. Comparison of the microstructure of coatings showed that the use of crystalline nano-alumina powder in the YSZ/Alumina layer composite upgrades the thermal properties. High temperature oxidation and thermal shock resistance of plasma sprayed YSZ/Al2O3 with un-pyrolysed nano-alumina and coatings with same composition with crystalline nano-alumina to created by plasma spraying were studied. Findings showed that the use of un-pyrolyzed nano-alumina powder in YSZ/Al2O3 layer composite resulted in increased porosity and shrinkage cavities in the coating, which increased the diffusion of O2 that causes the TGO growth rate. Also, high density and proper contact between the splats made of crystalline nano-alumina powder results in higher resistance of thermal cycles.

In this research, a double layer thermal barrier coating was applied and then an alumina diffusion barrier layer was deposited on the YSZ by two solution precersore plasma and solution precersore flame spraying. High temperature oxidation and thermal shock resistance tests were done at 1100˚C. Microstructure of coatings were studied by optical Microscopy and Field Emission Scanning Electron Microscopy. Comparison of the microstructures of coatings showed that applying of Alumina with the solution precursor flame spray process upgrades the thermal properties. High temperature oxidation and thermal shock resistance of YSZ/Al2O3 coatings with Alumina applied by the solution precersoure thermal spray with the same compound were studied. Findings showed that applying alumina with the solution precursor flame spray process leads to increase the amount of the deposited splats and proper contact between them, causes to decrease the diffusion of O2 and as a result TGO thickness decreases and also thermal shock resistance increases.

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 study the effect of addition YSZ on structural properties and oxidation resistance of CoNiCrAlY/YSZ composite were investigated. CoNiCrAlY/YSZ composite powders produced by mechanical alloying process. Various amounts of YSZ particles (0%, 5%, 10% and 15 wt.%) were mixed with CoNiCrAlY powder and milled for 12, 24 and 36 h, then composite and commercial coating were deposited with high velocity oxy -fuel method on the inconel 617 substrate. The structural and mechanical evolutions of the coatings were evaluted using X-ray diffractometry, scanning electron microscopy and microhardness test. It was observed that by increasing YSZ content, porosity of coatings were increased and hardness of coatings were increased too. Also, addition of YSZ to CoNiCrAlY decreased the rate of grain growth in thermal spray process and heat treatment. Increase of porosity result to decreasing oxidation resistance of coatings.