| Thermal barrier coatings(TBCs) have been widely used to protect the critical metallic parts in the hot sections of aeroengine, thus reducing the surface temperature of hot section components and improving the engine efficiency. Since the YSZ coating has the advantages of high melting point, low thermal conductivity, and good wear resistance, the TBCs are considered as an effective approach to enhance the service temperature of aeroengine. However, as gas turbine engines are driven to operate at higher temperatures, components become susceptible to attack by deposites of calcium magnesium alumino silicate(CMAS). CMAS can interact with the coatings via thermal-mechanical and thermal-chemical mechanisms. But it is difficult to analyze the damage mechanisms of TBCs caused by the molten CMAS, especially the local strain field of the coating during CMAS corrosion. Hence, it is important to develop a method to evaluate the failure mechanism of TBCs caused by the CMAS attack. Digital image correlation(DIC) techniques are powerful tools for measuring the strain in materials, and hence can real-time monitor the strain evolution in the sample surface under different micro or nano scales along with experimental temperatures. In this study, the TBCs prepared by air plasma spraying(APS) is considered, and the local strain of the coating caused by CMAS corrosion during the cooling process is measured using DIC techniques. The main research results are as follows:1. The artificial speckle pattern is designed with the mixture specific ratio of high-temperature inorganic adhesive and CoO on the thermal barrier coatings cross section of sample using high-pressure spray gun. The artificial CMAS powder is made in the laboratory at the similar constituents with volcanic ash. Then it is grind with a mortar and constantly makes CMAS powder particle size distribution is concentrated at about 20 ?m. CMAS powder is weighed and deposited uniformly on the surface of the thermal barrier coatings sample with a brush. The sample with good random speckle pattern and CMAS powder deposited are put into DIC equipment constant furnace at 1250°C heating 30 min. The experiments of thermal barrier coatings attacked by CMAS corrosion is finished and with the furnace cooling. The digital images are acquired during cooling process with DIC. The ARAMIS software finally analysis strain field evolution of thermal barrier coatings subjected to hot CMAS corrosion during cooling process.2. The results show that the strain of ceramic coating is compressive and it increases during the cooling process from 1250°C to 25°C. The CMAS makes the cracks form at the YSZ/bond coat interface when the temperature drops to 300-400°C. These cracks can even be observed without CMAS corrosion as the temperature decreases to 100°C. In addition, spallation of the entire ceramic coating occurs when the TBCs is further cooled down to room temperature. The digital image correlation tests indicate that the critical strain values for the failure of the coatings with and without the interaction of CMAS are approximate-0.7% and-1.6%, respectively. And it found that the strain values of the coatings without substrate subjected to CMAS are approximate-0.5%.3. The theoretical model of thermal barrier coatings attacked by CMAS corrosion is established in order to analyze the solution of stress and strain field, respectively. The appropriate material parameters were selected to calculate strain values in the cooling process(from 1250°C to 25°C) of different temperatures. It found that the theoretical values of the stress and strain are increasing with the temperature dropping increased. The stress maximum reaches-200 MPa without CMAS corrosion, and strain maximum is of-1.7%. The stress maximum value is-900 MPa with CMAS corrosion, and strain maximum is about-1.5%. The theoretical results are in a good agreement with experimental results. |
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