Portable Optical System for In-Situ Thermal Barrier Assessment of Service Operated Blades

Thermally Grown Oxide (TGO) layer develops and grows subjected to very high compressive residual stresses at the interface between metallic bond coat and EB-PVD thermal barrier coating. A significant decrease of such stress level is a useful indicator of incipient local detachment of the TBC. Photo-Stimulated Luminescence Piezo Spectroscopy (PLPS) provides information on the TGO stress state. In this paper a new portable PLPS instrument is described, developed to allow the in shop detection of PLPS signals on service operated blades with TBCs, to detect incipient failure. The instrument capability was verified comparing its results with the measurements performed with a commercial instrument on both TBC coated specimens aged in laboratory conditions and on new and operated components. The results confirm the system capability of prematurely detect and localise initial damage processes, before spallation of the TBC. A miniaturised probe was shaped to perform the PLPS measurement also at the leading edge. Finally the first in-shop application of the prototype is discussed starting from the results of the damage level assessment performed on operated first stage blades of a V94.3A2 gas turbine.