Examples of the use of optical spectroscopy to detect damage of thermal barrier coatings during cyclic oxidation

This paper describes some examples of the use of two optical spectroscopy techniques to study thermal barrier coating (TBC) degradation preceeding failure. The first part describes photoluminescence piezospectroscopy (PLPS) results obtained on a series of specimens with EB-PVD TBC and Pt -aluminised bond coats. The monotonic decrease of the alumina compressive stress level with ageing and thermal cycling confirms that TGO compressive stress levels can be used as residual life indicators in this type of coating. The automatic mapping system implemented by RSE (Ricerca sul Sistema Energetico) provides precise and reliable results about the level of damage at the BC/TBC interface, well before failure; mapping provides data regarding the precise positions where the first macroscopic detachment (a few millimeters wide) occurs. As PLPS is not applicable to thermal-sprayed APS TBCs, the second part of the paper describes some examples of the contribution that Raman spectroscopy can provide to detect phase changes due to degradation preceeding failure of the TBCs. Possible problems relating to the presence of undesired RE elements in the ceramic layer due to strong fluorescence are also described; solutions are proposed. Finally, examples of how innovative confocal microRaman produces maps evidencing areas where high temperature exposure and thermal cycling-produced phase transformation of the Yttria partially stabilised Zirconia from tetragonal to monoclinic (which typically occurs during cracking processes preceeding final TBC failure) are provided.