Optical NDT techniques for ceramic thermal barrier coatings

Thermal barrier coatings, usually comprised of yttria-stabilized zirconia, are used in gas turbine engines to reduce the operating temperature Of the superalloy base metal. A typical coating system is composed of a superalloy substrate, a bond coat and the top coat, or thermal barrier coating. Thermal barrier coatings are critical in high performance propulsion turbines and low-emission electric power gas turbines because if these are lost during operation, damage to the substrate material may occur. Reliable noncontact, nondestructive techniques are required to monitor the condition of the thermal barrier coating and predict coating failure. Several nondestructive testing (NDT) techniques are in various stages of development for determining the status of these critical coatings. Optical techniques take advantage of the fact that thermal barrier coatings are optically translucent. This means that surfaces within and below the thermal barrier coating can be examined using appropriate optical wavelengths. Optical NDT techniques presently under study include luminescence spectroscopy, laser backscatter, optical coherence tomography, thermal imaging and mid-infrared backscatter. The choice of the NDT technique depends to some extent on the type of thermal barrier coating of interest. Most thermal barrier coatings are being applied with electron beam physical vapor deposition or air plasma spraying. This paper discusses the various NDT techniques and provides examples of experimental results.