Non-destructive thermal barrier coating (TBC) damage assessment using laser-induced luminescence and infrared radiometry

A new non-destructive evaluation (NDE) approach for assessing damage in thermal barrier coating (TBC) systems is described. It is based on a combined measurement of the optical and thermal response due to photo-excitation, through the TBC, of Cr3+ ions within the thermally grown oxide (TGO). This method has subsurface selectivity to the TBC/TGO interface, as it is based on using a laser wavelength to which the TBC is transparent but the TGO absorbs strongly, so that the energy of the laser is converted into both luminescence and heat within the TGO. Using mid-infrared detection, the subsequent change in temperature of the TGO is measured as heat is conducted away from the TGO into the adjacent alloy and TBC coating. Damage decreases the rate at which the heat decays away and can be quantified from measurements of the transient temperature changes. The luminescence signal may be used to quantify the compressive stress and the thickness of the TGO, using photoluminescence piezospectroscopy and luminescence lifetime measurements, respectively. Both the optical arrangement used for implementing this new approach and the results obtained from a series of thermally cycled TBCs are described.