| Thermal barrier coatings (TBCs) have attracted ever-increasing attention for aircraft and industrial gas-turbine applications due to their excellent wear resistance, corrosion resistance, and thermal insulation as engine components. During engine operation, ceramic coating failure always takes place at an unpredicted time, which severely limits the full exploitation of thermal insulation and protection functions of TBCs. Therefore, the characterization of damage evolution and the quantitative evaluation of interfacial properties for TBCs have become an important subject of research. In this thesis, we mainly present an application of digital image correlation (DIC) on the study of interfacial damage of TBCs under mechanical or thermal loading. The main contents in this thesis are listed as follows,Firstly, the evolutions of full and local field strain in TBCs during tension have been successfully monitored by DIC technique. Based on the analysis of strain distribution, the failure mechanism of air plasma sprayed TBCs under tension was discussed. The fracture strength of coating and interfacial shear strength were estimated as 35.0±4.6 and 14.1±3.2 MPa, respectively, which are consistent well with the available experimental data. In addition, we have obtained an explicit and concise solution by a phenomenological shear-lag model to evaluate the tensile and interfacial shear stress fields in a coating segment under uniaxial tensile load. The validity of model has been confirmed by comparing the results with those determined by in situ DIC and the literature.Secondly, a measurement method combined with both DIC and acoustic emission (AE) techniques was proposed to non-destructive in-situ monitor strain field and internal damage of TBCs under bending tests. We pay more attention on accurately judging cracking formation and coating delamination during bending by analyzing DIC data, AE data and DIC images. During the failure process, it was found that there are two main types of cracking patterns, i.e. surface vertical surface crack and interface crack. The appearance time and location of cracking nucleation were accurately discerned. Moreover, the critical experimental data at the crucial transition points were obtained and used to predict the interface performances and mechanical properties of the system. The interfacial fracture toughness of TBCs was evaluated as about 64.5-91.2 J/m2 by a modified mechanical model of local failure.Thirdly, based on DIC, acoustic emission (AE) and electrochemical impedance spectroscopy (EIS), the design of experimental simulation system of TBCs has been introduced in this part. Full/local field strain evolution, surface/interface crack nucleation and propagation, temperature field, interfacial oxidation growth law of TBCs can be in-situ obtained by means of data acquisition and analysis system of the device. Moreover, the function of experimental simulation system was verified through debugging.Fourthly, the hollow cylindrical TBCs samples with interface defect were prepared by APS technique and residual stresses in as-sprayed TBCs were predicted by finite element method (FEM). Then, thermal shock tests were conducted to study the failure process and failure mode of TBCs using the above experimental simulation system. During the tests, the full field evolution of TBCs were monitored successfully. It was found that the failure location started at the coating when thermal cycle ranged within 182-185. And the local fracture strength was in-situ determined to be 93.8-102.2 MPa. Based on DIC and SEM analysis, the failure process and failure mode of TBCs under thermal shock were also studied.In general, with the advanced DIC technique, the thesis has systematically studied strain field evolution and detected the interfacial fracture of TBCs under mechanical or thermal loads. The results will be of great advantage to provide an important experimental basis of reliability and life prediction of TBCs in the future, and also have expanded the application of DIC technique. |
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