Efficiency of α-alumina as diffusion barrier between bond coat and bulk material of gas turbine blades

Gas turbine blades are often covered with protective coatings to withstand the high thermal and corrosive loads during operation. One of the most effective coating systems used consists of a MCrAlY bond coat and a YSZ thermal barrier coating. At high operating temperatures, a depletion of aluminum in the bond coat occurs not only due to the formation of thermally grown oxide (TGO), but also due to interdiffusion between bond coat and substrate. Other elements, e.g. Ti, may diffuse outwards and lead to spallation of TGO. Exclusion of these interdiffusion effects could prevent premature coating failure. a-alumina was deposited as a diffusion barrier by CVD on a nickel-based superalloy (CMSX-4) and then coated with NiCoCrAlY and ZrO2. The thickness of the coatings was 3, 20 and 500 gm, respectively. The samples were annealed at 1373 K for 100 h in argon atmosphere. All samples were analyzed by XRD, SEM and WDX to investigate the efficiency of the diffusion barrier. No delamination or cracks were observed except vertical segmentation cracks in the TBC. The diffusion barrier remained intact suppressing any noticeable interdiffusion, while the adhesion still needs to be improved. (C) 2003 Elsevier Science Ltd. All rights reserved.