Isothermal oxidation behavior of cryomilled NiCrAlY bond coat: Homogeneity and growth rate of TGO

Isothermal oxidation behavior of thermal barrier coatings (TBCs) with cryomilled NiCrAlY bond coats, in comparison with equivalent TBCs with conventional bond coats was investigated. The growth rate and homogeneity of the thermally grown oxide (TGO) were quantified and characterized through rigorous statistical evaluation. The initial TGO growth follows parabolic growth kinetics in both systems, but the cryomilled coatings exhibit improved homogeneity in TGO thickness, thinner TGO layers after long-term exposure, slower TGO growth rate, and uniformity in TGO composition consisting primarily of α-Al2O3 with a limited presence of other oxides such as Ni(Cr, Al)2O4 spinel. Results indicate that the distinct bond coat structure derived from cryomilling, i.e., the creation of a thermally stable, uniform distribution of ultrafine Al- or Y- rich oxide dispersoids within the bond coat layer, may affect the Al outward diffusion from the bond coat to the bond coat/top coat interface, which ultimately influences the TGO growth behavior. In addition, computational simulation is applied using the commercially available Thermo-calc® software to provide further insight into the chemical composition of the TGO layers.

Research highlights
► TGO growth in both conventional and cryomilled coatings follows parabolic kinetics.
► Cryomilled coatings exhibit advantageous oxidation behavior over conventional ones.
► TGOs in cryomilled systems are more homogeneous in thickness and composition.
► TGO growth rate for cryomilled coatings is significantly slower and steadier.