Chemical vapor deposition of smooth α-Al2O3 films on nickel base superalloys as diffusion barriers

With rising operating temperatures of gas turbines, interdiffusion may occur between the structural material of the blades (superalloy) and the corrosion-resistant layer (MCrAlY) leading to premature coating failure. Depositing a diffusion barrier between the base material and the layer is a conceivable means to suppress this diffusion. alpha-Al2O3 appears to be well suited as a diffusion barrier since it passes through no phase transition at high temperatures and possesses a hcp crystal structure with a very small number of defects. However, chemical vapor deposition of alpha-Al2O3 is known to be difficult because of preferred whisker formation due to the high contents of Ni and Co in the substrate. In this paper two ways to suppress the growth of whiskers and to deposit smooth alpha-Al2O3 films on nickel base superalloy substrates are presented. Furthermore, the efficiency of the diffusion barrier was tested by annealing experiments at 1100 degrees C for 100 h. The alpha-Al2O3 films remained dense and very effective as a diffusion barrier. (C) 1999 Elsevier Science S.A. All rights reserved.