Microstructural analysis after furnace cyclic testing of pre-oxidized ReneN5/(Ni,Pt)Al/7YSZ thermal barrier coatings

Abstract

The effect of pre-oxidation treatments at various oxygen partial pressures (pO2) on the microstructure of ReneN5/(Ni,Pt)Al/7YSZ thermal barrier coatings systems and the number of furnace cycles at 1100 °C up to top coat spallation is here presented. (Ni,Pt) Al bond coats deposited on ReneN5 superalloy were pre-oxidized under different pO2 before top coat deposition. After pre-oxidation, a 7YSZ ceramic top coat was deposited on the samples using the electron beam physical vapor deposition. The pre-oxidized specimens in the range of 1 × 10−19 ≤ pO2 ≥ 1 × 10−1 atm showed a higher number of furnace cycles compared to those either pre-oxidized at pO2 = 2.1 × 10−1 atm and without pre-oxidation. The thermally grown oxide (TGO) scale after top coat deposition of pre-oxidized samples in the range of 1 × 10−19 ≤ pO2 ≥ 1 × 10−1 atm showed the lowest thickness and thus slow growth kinetics. Moreover, a microstructural analysis by scanning electron microscopy revealed the formation of a mixed zone within the TGO of non-pre-oxidized samples, whereas for all pre-oxidized conditions a dense continuous Al2O3 scale was observed. Upon furnace cyclic exposure, phase transformations from B2 to L10 and L12 were identified by electron backscattering diffraction in all bond coats, which are most likely a feature associated with high heating/cooling rates under furnace cyclic tests.