Experimental investigation of the failure of air plasma sprayed thermal barrier coatings

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2015
Kütükoğlu, Başar
In this thesis, an experimental method is developed to determine the interfacial fracture toughness of air plasma sprayed thermal barrier coatings. The thesis is composed of three major parts. In the first part, nanoindentation is used to measure the elastic modulus and hardness of the APS TBC specimens composed of three major layers: Inconel 718 substrate, NiCrAlY based metallic bond coat (BC) and YSZ based ceramic top coat (TC). Microscopy, XRD and EDS are used for metallurgical characterization. In the second part, a model material system of bonded Al/PMMA composite beam is used to develop a 4-pt bending test to measure the critical interfacial energy release rate as proposed by Charalambides et al. in conjunction with digital image correlation (DIC) method. In the third part, the properties measured in the first part and the experimental method developed in the second part are used to carry out 4-pt microbending tests on an APS TBC system to determine interface toughness for three different coating thicknesses. The failure progress is recorded in-situ using a microscope in conjunction with a DIC system. Failure mechanism is comprised of nucleation and growth of vertical cracks in the TC followed by interfacial crack growth. The critical interfacial energy release rate is measured and is found to increase with thickness. The developed experimental method can be used by jet turbine engine companies to develop standardized quality assessment procedures for coatings.

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Citation Formats
B. Kütükoğlu, “Experimental investigation of the failure of air plasma sprayed thermal barrier coatings,” M.S. - Master of Science, Middle East Technical University, 2015.