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Theoretical Analysis and Experimental Characteristics of Thermal Barrier Coating Systems under Tensions

Author: ZhouMeng
Tutor: MaoWeiGuo
School: Xiangtan University
Course: Materials Science and Engineering
Keywords: Thermal barrier coatings Digital image correlation Shear-lag model Residual stress Acoustic emission
CLC: TG174.4
Type: Master's thesis
Year: 2013
Downloads: 26
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Abstract

Thermal barrier coatings (TBCs) systems have attracted an increasing attentionin aircrafts and gas-turbines owing to their excellent wear resistance, corrosionresistance and thermal insulation. However, it is very difficult to validly appraise thedelamination and sapllation of the top coating due to the complicated structures andservice environments of TBCs, which strongly affects their life and reliabilities. Inthis work, the evolution of many important properties of TBCs were monitored andobtained by the aid of digital image correlation technique, acoustic emissiontechnique and tensions. The developed measurement methods and results would beuseful to predict the operation life and durability of TBCs system. On the basis of theconventional shear lag models, we have deduced a new shear lag model to analyze theinterface failure of air plasma sprayed TBCs system under tensile loads at room andhigh temperature conditions. The main conclusions are summarized as follows,Firstly, a three-layer shear lag model has been established under considering theeffect of residual stresses. An analytic solution of stress variation in the coatingsystem was obtained based on the interface stress transferring form under externaltensile loads, which was applied to predict the stress evolution of TBCs system duringthe fracture process. When normal strain in the substrate increase up to0.08%and0.11%, shear stress along the substrate/bond coat, bond coat/top coating interfacessuddenly changes, respectively. On the other hand, as normal strain in the substrate isequal to0.1%and0.24%, respectively, both normal stresses in the top coating andbond coat vary from compressive to tensile state. When normal strain in the substratecontinues to0.39%, the first crack in the top coating occurs due to the interface shearstress transfer under external tensile loads. Similarly, as normal strain in the substrateapproaches to0.43%, the first crack in the bond coat starts to occur. The relevantprediction results would provide important guide in the subsequent chapters.Secondly, by using the determined relationship of Raman frequency shift andresidual stress, residual stress near the coating surface region was measured bymicro-Raman spectrum method. When the coating thickness increases from200μmup to400μm, the related residual stress changes from-30.55±1.98MPa to-8.37±1.98MPa, respectively. These results would be useful for discussing the fracture strength,interface shear strength of the coating system by tensions in the next chapters. Thirdly, the fracture process of TBCs system has been successfully monitoredand analyzed under tensile loads by using universal tester and three-dimensionaldeformation of optical nondestructive testing system and acoustic emission technique.With strain data in the substrate and the analytic solution in the first chapter, bothcritical fracture strengths in the coating and bond coat, respectively, were estimated as100.6±8.9MPa and1397.2±44.9MPa. Similarly, both interface shear strengths atthe top coating/bond coat and bond coat/substrate were obtained about13.2±3.6MPaand51.7±16.5MPa, respectively. After strain data at the first fracture point in thecoating were discussed, the critical fracture strengths of the coating and bond coatwere, respectively, evaluated about94.9±8.9MPa and1832.3±44.9MPa. Thecharacteristics spectra of acoustic emission for vertical crack mode in the coating,interface crack mode along the top coating/bond coat, vertical crack mode in the bondcoat was, respectively, estimated as0.30-0.32MHz,0.06-0.08MHz and0.15-0.17MHz by using fast-Fourier-transform spectrum method.The failure mechanism ofTBCs system under tension was summarized by using these strain mappings andscanning electron microscope observations.Fourthly, all full-field strain and acoustic emission signals deduced by crackingnucleation and propagation were successfully monitored and recorded by computers.With strain data in the substrate and the analytic solution in the first chapter, bothcritical fracture strengths in the coating and bond coat, respectively, were estimated as64.5±6.9MPa and540.5±15.2MPa. Similarly, both interface shear strengths at thetop coating/bond coat and bond coat/substrate were obtained about26.7±2.3MPaand125.6±12.5MPa, respectively. After strain data at the first fracture point in thecoating were discussed, the critical fracture strengths of the coating is about61.2±3.9MPa under500centi degree. The characteristics spectra of acoustic emission forvertical crack mode in the coating, interface crack mode along the top coating/bondcoat, vertical crack mode in the bond coat was, respectively, estimated as0.28-0.30MHz,0.07-0.09MHz and0.22-0.24MHz by using fast-Fourier-transform spectrummethod. Similarly, the possible failure modes of TBCs system under tension at hightemperature were summarized by using these strain mappings and scanning electronmicroscope observations.

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