Investigation on Failure in Thermal Barrier Coatings on Gas Turbine First-Stage Rotor Blade

Abstract

Failure in turbine blades can affect the safety and performance of the gas turbine engine. Results of coating decohesion, erosion and cracking at the first-stage high-pressure (HPT) blade working in gas turbine engine are being reported in this paper. This investigation was carried out for the possibility of various failure mechanisms in the thermal barrier coating exposed to high operating temperature. The blade was made of nickel-based superalloy, having directionally solidified grain structure coated with thermal barrier coatings of yttria-stabilized zirconia with EB-PVD process and platinum-modified aluminum (Pt–Al) bond coat with electro-deposition. The starting point of analysis was apparent coating decohesion close to the leading edge on the suction side of blade. The coating decohesion was found to be widening of interdiffusion zone toward the bond coat at higher operating temperature which could change the composition and induce thermal stresses in the bond coat. The erosion, cracking and decohesion of the coating on the pressure side was also observed during failure investigation. The erosion of the coating was coupled by two factors: one by increase in temperature as demonstrated by change in microstructure of the substrate and second by increase in coating inclination toward the trailing side. As a result of high operating temperature, swelling and thickening of TGO was observed due to outward diffusion of aluminum from the bond coat to form alumina (non-protective oxide) which causes internal stresses that leads to top coat decohesion and cracking. The possibility of hot corrosion was also investigated, and it was found that top coat decohesion did not involve this failure mechanism. Visual inspection, optical microscopy, scanning electron microscopy and energy-dispersive spectroscopy have been used as characterization tools.