جستجوی مقالات مرتبط با کلیدواژه "پوشش های سد حرارتی" در نشریات گروه "مکانیک"

Hot corrosion and thermally grown oxide (TGO) formation are destructive factors in thermal barrier coatings (TBCs) that lead to coating failure under operational conditions. In the present study, the hot corrosion behavior and TGO evolution for TBCs whose Bond coat by deposited by atmospheric plasma spraying (APS) and high velocity oxygen fuel (HVOF) thermal spray methods were evaluated. Both types of coatings were subjected to cyclic hot corrosion testing at a temperature of 1100°C in the presence of molten salts of Na2SO4 and V2O5 under identical conditions. Subsequently, their microstructures were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD) images. Additionally, changes in TGO thickness were measured across different cycles using Image J software and SEM images. The results indicate that TBCs deposited using the HVOF method for the bond coat exhibit better performance compared to those deposited using the APS method. The results show that a phase change from tetragonal to monoclinic has occurred for zirconia with the penetration of corrosive salt melt and its reaction with the YSZ layer, and also with the depletion of yttria from the coating structure, YVO4 reaction products have been formed for TBCs. The endurance of hot corrosion cycle of TBCs and the growth behavior of TGO show that the coatings whose interface layer is applied by the HVOF method show better performance than the APS method.

Exposure of the TBCs layers to high temperatures for a long time causes sintering in the ceramic layer, resulting in changes in the porosity. Since the thermal conductivity of air in the porosity of the layers is much lower than the ceramic material in the coating, a decrease or increase in the percentage of porosity will end up with changes in the conductivity of the layer, which subsequently affect the overall efficiency. In this study, a TBC was first applied to the prepared samples from Inconel738 turbine blades. The samples were subjected to an aging process in the furnace for a specified time. The porosity changes in the ceramic layer of the coating during the aging process were investigated using image processing of the SEM images. The porosity of the images obtained from SEM and thermal conductivity in different porosities were determined by OOF2 and ImageJ software, respectively. To validate the numerical results, temperature distribution on the thermal barrier coatings is obtained by the analytical solution of the Fourier equation. According to the analysis performed on the SEM images gathered from the samples during different aging hours, it is observed that by increasing the aging hours during 48 hours, the percentage of porosity of the whole ceramic layer decreased by 8%. According to the results, by decreasing porosity in the ceramic layer, an increase in thermal conductivity has been observed, followed by an increase in the substrate's temperature and a decrease in the overall performance over time.

The need for a high-temperature operation to increase efficiency has led to the use of protective coatings called Thermal Barrier Coatings (TBCs) to protect turbine equipment against destructive conditions and relatively high thermal loads. However, the coatings themselves are also gradually affected by these destructive conditions. In the present study, based on the microstructural characteristics of thermal barrier coatings, first, the modeling of the deposition process and the fabrication of these coatings was done using the finite element method. In this modeling, the deposition process of TBCs for studying the thermal and mechanical behavior of these coatings simulated. Then the temperature field and the residual stresses obtained during and after the deposition process and cooling to ambient temperature has been studied. Subsequently, by applying thermal loading and simulating the actual working conditions of the turbine, the thermal and mechanical behavior of TBCs was evaluated in working conditions and the distribution of the temperature and stress field in the system was calculated. The results show that the maximum residual stress after cooling is at the interface between the substrate and the bond coat. The obtained results and the accuracy of the proposed model validated by experimental reports.

In this paper, residual thermal stresses caused by quenching of layers as well as the thermal expansion mismatch between the coating layers due to a sharp drop in temperature during the deposition process in the thermal barrier coating system is evaluated using an analytical model, experimental tests and a finite element model. Due to the inherent complexity of these coatings, the problem is solved analytically after some simplification. The residual stresses of the coated samples were measures using XRD method. In order to calculate the stress distribution in the whole system, a finite element model is proposed. In this model, the interface profile of the coating layers was extracted based on SEM images so it was considered to be resemble the real case. Also the boundary conditions were applied to this sub model in a way that it is in a good accordance with the experimental tests loading. The analytical results showed that the quenching residual stresses of the layers have a minor amount compared to the thermal mismatch stresses between the layers in the tests. The average value of the measured residual stresses is -150 MPa, which has a good agreement with the results obtained from the analytical and finite element methods.

In this study, the effects of thermal cyclic loading in the presence of a constant mechanical load were evaluated experimentally on failure of thermal barrier coatings. For this purpose, specimens of Inconel 617 with approximate dimensions of 100×10×6.2 mm that coated by two-layered thermal barrier coatings include a Ni22Cr10Al1Y bond coat and ZrO2.8wt%Y2O3 using air plasma spraying (APS), were considered. These specimens were tested under low cycle thermal fatigue experiment with maximum temperatures of 1000oC, 1100oC and 1170oC for thermal cycles and constant bending loads of 4500 Nmm, 6000 Nmm, 7500 Nmm and with no load by using a made test rig with the ability of four point bending load. In a thermal cycle, a specimen heating time was 10 minutes and cooling time was 5 minutes approximately. The results obtained in this study, showed that different loads change thermal barrier coatings failure mechanisms and service life cycles of coating reduce exponentially by rising maximum temperature of thermal cycle. Also increases in mechanical load have significant effect on the reduction of thermal barrier coatings life during thermal fatigue loading.