Application of thermography in the evaluation of early signs of failure of thermal barrier coating systems

Abstract

For a number of years piezospectroscopy (Cr fluorescence) has been used to monitor the stress levels in the thermally grown oxide (TGO) that forms between the bondcoat and the thermal barrier coating (TBC) in TBC systems. The purpose of that work has been to observe early signs of failure and thus allow operators to schedule service intervals before failure of the TBC system occurred. This paper reports the use of thermography as an additional tool that can be used to assess the “health” of TBC systems. The technique consists of imaging the surface of the TBC coated specimen with a high spatial resolution infra-red camera while the specimen is heated, and monitoring the temperature of the outer surface of the TBC. Conductive heating through the substrate and radiative heating incident on the TBC have been studied.Early results are encouraging, revealing a clear correlation between thermograms obtained using the conductive and radiative forms of heating, some of the stress maps obtained using piezospectroscopy and direct metallographic evidence. Examples of electron beam physical vapour deposited (EB PVD) and air plasma sprayed (APS) TBC systems have been studied as they were progressively aged. Cracking and disbonding associated with the TGO and/or TBC have been observed in places where thermography showed differential heating.