Nondestructive evaluation of plasma-sprayed thermal barrier coatings
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Nondestructive Evaluation of Plasma-Sprayed Thermal Barrier Coatings D.J. Andrews and J.A.T. Taylor Acoustic emission has been used as a nondestructive evaluation technique to examine the thermal shock response of thermal barrier coatings. In this study, samples of partially stabilized zirconia powder were sprayed and acoustic emission (AE) data were taken in a series of thermal shock tests in an effort to correlate AE with a given failure mechanism. Microstructural evidence was examined using parallel beam x-ray diffraction and optical microscopy. The AE data are discussed in terms of cumulative amplitude distributions and the use of this technique to characterize fracture events.
Keywords ceramics, nondestructive evaluation, plasma spray, yttria, zirconia
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1. Introduction Nondestructive evaluation (NDE) has become a popular choice in the evaluation of materials and finished products in both the laboratory and the workplace. Time, money, and resources are saved through the use of NDE techniques in combination with destructive characterization tests. Methods such as ultrasonics, thermal wave imaging, and x-ray computed tomography provide information on existing microstructural defects (Ref 1). Acoustic emission (AE) is a nondestructive test that provides information on a structure while it is under stress. It helps describe the dynamic aspects of hidden defects that are contributing to microstructural deformation. The defects that produce A E may or may not be detectable by the other methods listed above. These features of AE make it attractive as a monitoring tool for quality control. In this study, AE was monitored during thermal shock tests of plasma-sprayed yttfia-stabilized zirconia. Thick films of this material are commonly used as thermal barrier coatings (TBCs) in engine applications. They have a lamellar microstructure composed of splats that are mechanically bonded. An intermediate layer, or bond coat, is used to reduce the thermal expansion mismatch between coating and substrate. After thermal cycling, TBCs develop vertical microcracks, which have been shown to improve the thermal shock fatigue of the material (Ref 2). Microcracking is a common AE source due to thermal stress gradients in plasma-sprayed zirconia (Ref 3). Additionally, horizontal crack propagation in the coating and delamination at the bond coat substrate interface can produce spalling, or coating failure. The focus of this study was to use AE to develop a preliminary database of the acoustic response o f a TBC. Supported by microstructural evidence, A E could be used as a quality control technique in TBC applications.
Nickel-chromium alloy straps that measured 300 by 25 by 2 mm were used as substrates. A NiCoCrAIY bond coat was used in conjunction with a yttria (8 wt%) partially stabilized zirconia top coat. The coating covered an approximate area of 100 m by 25 mm on one end of the strap. All the spraying was done in an air atmosphere using argon and hydrogen as the plasma constituents and argon as the powder c
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