On the Evolution of Microstructure of Gamma Prime + Gamma Platinum Bond Coats Deposited on Ni-Based Superalloys and Their Oxidation Resistance: Role of Superalloy Substrate

The sequence of events leading to the development of γ′ + γ Pt bond coats by diffusing a 10-μm electroplated Pt layer into Ni-based superalloys is elucidated. Most evidence points out that a Pt₃Al layer is initially formed at the Pt–superalloy interface during the diffusion heat treatment at 1150 °C. With continued interdiffusion through the Pt₃Al layer, the Pt–Pt₃Al interface is displaced outward, and the Pt₃Al–superalloy interface is displaced inward. This leads to formation of a surface layer of Ni-rich Pt₃Al-based γ′-phase, which is eventually transformed into Pt-rich Ni₃Al-based γ′-phase leaving behind inner layers depleted in γ′-forming elements and consisting of various mixtures of γ′- and γ-phases. It is shown that the composition of the superalloy particularly the Ti content influences both the thickness and oxidation resistance of the bond coat. Although a higher Ti content promotes a thicker coating, it can have an adverse effect on oxidation resistance.