Evaluation of Thermal Damage of Thermal Barrier Coatings by Using Raman Microspectroscopy

Abstract

Damages in thermal barrier coatings (TBCs) exposured under high temperature are accumulated by various factors, and ultimately the spallation of top coats in TBCs is occurred. One of the damages is formation of thermally grown oxide (TGO). In the present study, measurements of crystallinity, microstructures and residual stresses in TGO layers grown under various thermal exposure conditions were conducted by using Raman microspectroscopy with sub-micro spatial resolution. In TGO layers, Raman spectrum of Alumina was obtained. Its crystallinity was varied with thermal exposure time. In crystallographic orientations, TGO layers had Alumina c-axis direction perpendicular to bond coat surface. It means that TGO layer grew in the direction perpendicular to interface between top and bond coat. TGO layer had high compressive residual stress, 1.0GPa at maximum value in TBCs exposured for 300h at 900°C and 1.2GPa for 500h at 900°C. On the other hand, in top coat, Raman peak shift of α-Chromia (Cr2O3) was obtained. Especially, isolated islands of α-Chromia were formed in top coat near TGO layers, and some micro cracks were observed near the region of Cr2O3. Therefore it was considered that Cr2O3 formation significantly affected micro crack initiation in top coat.