Evolution of Photostimulated Luminescence During Thermal Cycling of Electron Beam Physical Vapor Deposited Thermal Barrier Coatings

Abstract

Thermal barrier coatings (TBCs) are widely used for thermal protection of hot section components in turbines for propulsion and power generation. Development of a robust non-destructive evaluation (NDE) technique for TBCs is essential for quality control, life assessment and health monitoring that will facilitate reliable application, efficient maintenance and prevention of catastrophic failure. In this study, degradation of TBCs was non-destructively evaluated by photostimulated luminsecence (PSLS) and microstructurally examined as a function of furnace thermal cycling carried out in air with 10-minute heat-up, 1-, and 10-hour dwell duration at 2050°F (1121°C), and 10-minute forced-air quench. TBCs examined in this study consisted of electron beam physical vapor deposited (EB-PVD) yttria-stabilized zirconia (YSZ) on grit-blasted (Ni,Pt)Al or as-coated (Ni,Pt)Al or shot-peened NiCoCrAlY bond coats and various superalloy substrates. Characteristics of subcritical-subsurface damage near the thermally grown oxide (TGO) were documented by cross-sectional scanning electron microscopy. Mechanisms of damage varied as a function of TBC type and thermal cycling dwell time, and included preferential grain boundary oxidation after ridge-induced micro-cracking, racheting and undulation of TGO/bond coat interface, internal oxidation of bond coats, and formation of Ni/Co-rich oxides. These microstructural observations are correlated to the evolution in compressive residual stress in the TGO scale determined by photostimulated luminescence shift, including stress-relief associated with subcritical cracking in the TGO scale, and stress-relaxation associated with racheting of the TGO/bond coat interface. Correlations between the microstructural development and the photostimulated luminescence from the TGO scale are discussed as a function of TBC type and thermal cycling dwell time.