Acoustic Emission Techniques for Life Prediction of TBCs Under Thermal Cycling Conditions

Abstract

Abstract A thermal cycling test rig and procedure was designed in order to predict the life expectancy of Thermal Barrier Coatings (TBC) under thermal cycling conditions similar to those meet in combustion chambers. Two 2kW-halogen lamps highly focused on the TBC were used to expose the surface of the coating to an intense heat flux. A 25x100 mm TBC is Air Plasma Sprayed (APS) centered onto a substrate 25x370 mm. The thermal cycling can be done either under inert or oxidizing atmosphere in order to separate oxidation-induced acoustic emissions from that resulting from the mismatch of the Coefficient of Thermal Expansion (CTE) of the coating compared to that of the substrate. Two transducers located at each end of the substrate monitor the Acoustic Emission (AE) signals emitted by crack initiation and/or propagation, were recorded and analyzed in order to deduce available information about TBC behavior under thermal load. The use of two transducers with a time of flight approach provides a valuable means of identifying both the crack formation and its location. This thermal cycling test is adequate for the study of various samples, like welded substrates coated with TBC or TBC coated around holes. The presence of cracks is observed using metallography preparation and microscopic observation.