Analysis of Thermally Grown Oxide Developed by Thermal Barrier Coatings with the Aid of Selective Deep Etching

Abstract

The thermally grown oxide developed by a thermal barrier coating deposited on Al2O3-forming Ni-base superalloy containing Hf was analyzed with the aid of selective deep etching to partially expose the oxide in contact with the bond coat. Yttria-stabilized zirconia and a Pt-modified aluminide were used as top and bond coats, respectively. Thermal exposure tests were carried out at 1,150 °C in air with 24-h cycling period to room temperature. Scanning electron microscopy combined with energy dispersive x-ray spectroscopy and transmission electron microscopy were used to characterize the microstructure. Hafnium-rich oxide pegs enveloped by Al2O3 were observed to emanate from a continuous layer of Al2O3 and grow into the bond coat with continued thermal exposure. However, a correlation appeared to exist between the local structure and composition of the oxide and thermal stability of the bond coat. Ultimately, decohesion occurred between the continuous layer of Al2O3 and underlying bond coat involving fracture of the Hf-rich oxide pegs leading to spallation of the top coat. © 2013 Springer Science+Business Media New York and ASM International.