Residual stress evolution of thermally grown oxide in thermal barrier coatings deposited onto nickel-base superalloy and iron-base alloy with thermal exposure ageing

The objective of this work is to understand the influences of different chemical compositions of iron-based and nickel-based substrates on the TGO growth rate, the distribution of residual stress in TGO, the Quantity of the Spinel Cap Presence (QSCP) at the convex of undulated TGO, as well as the θ-Al2O3 to α-Al2O3 phase transformation. A thermal barrier coatings (TBCs) system consisting of the NiCrAlY bond coat and the 8YSZ topcoat (500 μm thickness) were successfully produced onto the substrates of nickel-based superalloy and iron-based alloy by atmospheric plasma spraying (APS), respectively. The results suggest that, compared with the iron-based substrate, both the TGO growth rate and the aluminum depletion in the bond coat with the nickel-based substrate develop significantly slower as the thermal exposure proceeds. It is proposed that the Cr3+ and Al3+ with small radii may considerably influence the mechanisms of phase transformation in the TGO, and these small ions can promote the θ-Al2O3 to α-Al2O3 phase transformation. Furthermore, the TBCs system with iron-based substrate was prone to have much shorter stage of θ-Al2O3 to α-Al2O3 phase transformation and a dramatic reduction of θ-Al2O3 content in the TGO, which had indirect effect on the thermo-dynamic behavior of TGO during the subsequent stage of isothermal oxidation. Furthermore, the apex of the convex of undulated TGO usually has the largest compressive residual stress. The spinel oxides can be inclined to form at the apex region of the convex of the undulated TGO with a lower value of b/a geometric parameter. However, only few spinel oxides appeared at the flank region of the convex. Nevertheless, the 'spinel cap' feature usually formed at the area of the apex and the flank of the convex of undulated TGO with a higher value of b/a geometric parameter. Compared with the nickel-based substrate, the more presence of 'spinel cap' at the area of the convex of undulated TGO with a iron-based substrate can be found at the convex of undulated TGO, and its earlier end of the θ-Al2O3 to α-Al2O3 phase transformation may result in the lower value of compressive residual stress in average.