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

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.