An Impedance Spectroscopy Study And Applications Of High-temperature Property Of Thermal Barrier Coatings

Thermal Barrier coatings (Thermal Barrier Coatings, TBCs) is a kind of thermalprotection material, spraying or deposited on the metal surface of high temperatureparts, main effect is to reduce the working temperature of metal parts, to improve theeffect of thermal efficiency. Because of the high temperature resistance and corrosionresistance of the TBCs, and excellent performance, so the TBCs are widely used inaerospace and other fields. But TBCs is multilayer structure and coefficient of thermalexpansion mismatch between layer and layer, and the service environment is usuallyfor high temperature and high pressure, so the high temperature oxidation and CMAS(CaO-MgO-Al2O3-SiO2) corrosion occurred in TBCs frequently, eventually leading todisbonding failure. It is difficult to test and life prediction of thermal barrier coatingsdue to the coating spalling failure caused by many factors, so it has importantsignificance to use advanced testing technology to detect the failure process of thethermal barrier coatings. In this paper, the author discussed the effects of differentconditions in the process of the impedance spectroscopy(IS) measurement, then ISmethod was used to study the property of high temperature oxidation and CMAScorrosion of the Plasma Spraying TBCs. The main research content is as follows:Firstly, discussed the effect of different measuring parameters and conditions onthe process of the impedance spectroscopy measurement, mainly including measuringvoltage and temperature, the size of the electrode, and sample geometry, etc., and theresult is different from the affect of the samples which are as-received or after thermalaging. The four measuring factors were discussed in the process of the impedancespectroscopy measurement, respectively. Results show that the electrode response inlow frequency reduced with the measurement of the voltage increase, the electricalconductivity of YSZ and TGO increase with increase in the measurement temperature,so the impedance response becomes smaller, the response of YSZ in the highfrequency independent of the electrode size, but the peak of TGO increases with theincrease of electrode size, and the impedance characteristics of TBCs independent ofthe curvature change, so the detection method for the plate shape is consistent withthe turbine blade.Secondly, we measured the high temperature oxidation process of TBCs bymeans of impedance spectroscopy, observed the microstructure of cross section ofTBCs after oxidation using scanning electron microscopy. The results show that there are a lot of pores and micro cracks internal the Plasma Spraying TBCs, when exposedto950°C in the air, the ceramic coating begins to sinter, and the porosity becomelower, the structure become dense, so the impedance response of ceramic layerdecreases. After8hours, there are TGO present between the bond and top coats, aTGO layer is seen at the interface After8hours, and the thickness of the TGO is about3um. Then the impedance response of the ceramic layer and pore decrease slowly,while the TGO thickness thicker with the increase of oxidation time, the impedanceresponse of TGO also will increase.Thirdly, we measured the impedance of YSZ and CMAS, and calculate theresistivity. Then we discussed the variation of the impedance of YSZ when it wassubjected the CMAS corrosion. The results show that the Plasma Spraying YSZsintered when subjected the CMAS corrosion, the porosity of YSZ become lower, themelting CMAS infiltration YSZ internal along with the temperature rise, and reactwith YSZ, make the YSZ phase change and formation of mixing layer. The mixinglayer thickening with the increase of oxidation time, the serious influence the life ofthe ceramic layer because of the structure and stability of YSZ is destroyed.