Proactive control of the metal-ceramic interface behavior of thermal barrier coatings using an artificial alpha-Al2O 3 layer

The reliability and life of thermal barrier coatings (TBCs) used in the hottest sections of advanced aircraft engines and power generation systems are largely dictated by: (1) the ability of a metallic bond coating to form an adherent thermally grown oxide (TGO) at the metal-ceramic interface and (2) the rate at which the TGO grows upon oxidation. It is postulated that a thin α-Al₂O₃ layer, if it could be directly deposited on a Ni-based alloy, will guide the alloy surface to form a TGO that is more tenacious and slower growing than what is attainable with state-of-the-art bond coatings. A chemical vapor deposition (CVD) process was used to directly deposit an α-Al₂O₃ layer on the surface of a single crystal Ni-bases superalloy. The layer was 150 nm thick, and consisted of small columnar grains (∼100 to 200 nm) with α-Al₂O ₃ as the major phase with a minute amount of &thetas;-Al₂O ₃. Within 0.5 h of oxidation at 1150°C, the resulting TGO formed on the alloy surface underwent significant lateral grain growth. Consequently, within this time scale, the columnar nature of the TGO became well established. After 50 h, a network of ridges was clearly observed on the TGO surface instead of equiaxed grains typically observed on uncoated alloy surface. Comparison of the TGO morphologies observed with and without the CVD-Al₂O ₃ layer suggested that the transient oxidation of the alloy surface was considerably reduced. The alloy coated with the CVD-Al₂O ₃ layer also produced a much more adherent and slow growing TGO in comparison to that formed on the uncoated alloy surface. The CVD-Al₂O ₃ layer also improved its spallation resistance. Without the CVD-Al ₂O₃ layer, more than 50% of the TGO spalled off the alloy surface after 500 h in oxidation with significant wrinkling of the TGO that remained on the alloy surface. In contrast, the TGO remained intact with the CVD-Al₂O₃ layer after the 500 h exposure. Furthermore, the CVD layer significantly reduced the degree of internal oxidation of Ta-rich areas which were present in the alloy as casting defects. The present study demonstrated that this artificial α-Al₂O₃ coating concept could be used as a novel means of favorably altering TGO growth and morphological characteristics, consequently increasing the oxidation resistance of Ni-based superalloys, and improving the reliability and life of next generation TBCs.