Thermal stability of atmospheric pressure plasma jet-deposited ZrO2 top coats and sputtered RuAl/RuAlZr/Ru bond coats on Inconel 617

In this study, the feasibility of using ZrO2 coatings, fabricated through the atmospheric pressure plasma jet technique, as top coats in thermal barrier systems was explored. Inconel 617 was used as the substrate material, and sputtered laminated RuAl/RuAlZr/Ru was used as the bond coat. >20-μm-thick ZrO2 coatings were deposited through several iterations of deposition at 500 °C and annealing at 600 °C; here, ZrO2 existed in the tetragonal phase. The tetragonal ZrO2 was stable after annealing at up to 700 °C but transformed into a mixture of tetragonal and monoclinic ZrO2 phases when annealed at >800 °C. The thermal stability of the ZrO2/RuAl/RuAlZr/Ru/Inconel 617 assembly was evaluated at 800–900 °C in air. The variations in volumetric fractions of the tetragonal and monoclinic ZrO2 phases were recorded; 44% of the tetragonal phase remained after 100 h of annealing at 800 °C, whereas 80% of the tetragonal phase had transformed into the monoclinic phase after 4 h of annealing at 900 °C. Observation of BC after annealing at 800 °C indicated that a dense thermally grown oxide scale, comprising O and Al, formed between the ZrO2 and RuAl layers, which impeded the inward diffusion of O. The inhibition of O diffusion declined with the crystallization of the thermally grown oxide scale into the δ-Al2O3 phase when the annealing time was extended.