Influence of Cryomilling on Microstructure, Phase Stability and Oxidation Behavior of NiCrAlY Bond Coat in Thermal Barrier Coatings: Experimentation and Mechanistic Investigation

Abstract

Improved thermal cycling lifetime has been observed in thermal barrier coatings (TBCs) with cryomilled NiCrAlY bond coat. To understand this improved behavior, a robust experimental investigation is coupled with mechanistic explanations to describe the influence of cryomilling on microstructure, phase stability and oxidation behavior of the bond coat. It is found that cryomilling results in two significant changes in the NiCrAlY bond coat: unintentional Fe additions and creation of a homogeneous distribution of ultrafine oxide/nitride dispersoids. Through extensive microstructural analysis combined with computational simulation using Thermo-Calc® software, it is determined that the presence of Fe stabilizes the high temperature γ and β phases in the NiCrAlY bond coat, corresponding to a decrease in the transformation temperature. The results are explained on the basis of the Gibbs free energy for the individual phases. Characterization of the thermally grown oxide (TGO) in TBCs after isothermal oxidation with rigorous statistical evaluation indicates that the TGOs in the TBCs with the cryomilled bond coats are more uniform in thickness and slower growing. Both behaviors are attributed to the more homogeneous distribution of oxide dispersoids, which are a direct result of the cryomilling, yet remain stable after extensive thermal exposure.