Influence of Pt concentration on structure of aluminized coatings on a Ni base superalloy

Abstract

Aluminum diffusion coatings have been processed by chemical vapor deposition techniques. Particularly, pack cementation is frequently used to process competitive oxidation resistance coatings on Ni based alloys. The superior oxidation resistance is associated with the presence of a NiAl layer that supplies Al to maintain a continuous and adherent α-Al2O3 film. Further enhancements on the performance are gained when a layer of Pt is deposited on the Ni based alloy before aluminization. Typically, the role of Pt on the performance of coatings has been characterized using aluminizing procedures by immersion of parts in the pack mixture. This work assessed the influence of Pt concentration on the structure of out-of-pack aluminized coatings aiming to clarify some contradictory results in the literature. Pt layers of 3 and 7μm thicknesses were electroplated on a Ni superalloy. Aluminization at 1100°C for 5h used out-of-the pack procedures avoiding direct contact between samples and the pack mixture. Analysis revealed that aluminized coatings are composed of two main regions: a layer of β-(Ni,Pt)Al and a interdiffusion zone (IDZ). Increasing Pt concentration increased the total thickness of coatings, smoothen the Al gradient in coatings and reduced the Kirkendall porosity. These effects are a consequence of Pt in reducing the activation energy for Al diffusion contributing to a higher atomic mobility of Al in the β-(Ni,Pt)Al layer. Richer Pt coatings enhanced oxidation performance due to the formation and maintenance of a continuous and adherent α-Al2O3 film and the lower density and dimensions of voids at the NiAl/Al2O3 interface.