Structure formation and interdiffusion in vacuum plasma sprayed CoNiCrAlY coatings on IN738LC

The MCrAlY-type alloys used as overlay coatings have proved successful in a variety of gas turbine applications in protecting against oxidation and hot corrosion. Since the properties and behaviour of the coatings are related to the microstructure, it is essential to investigate structure formation during deposition, superalloy heat treatment and service. In this study the phase transformations during heat treatment and aging in the CoNiCrAlY-IN738LC system were investigated. The CoNiCrAlY coatings were produced by vacuum plasma spraying (VPS) in a full-scale production unit and subsequently vacuum heat-treated (at 1120°C for 2 h and at 845°C for 24 h) and aged in air at 850°C for up to 500 h. The microstructural features were revealed by scanning and transmission electron microscopy and the diffusion profiles were determined by energy dispersive X-ray analysis.

In the as-deposited condition, the CoNiCrAlY coating has a dual-phase structure of β-NiAl (B2, ordered body-centred cubic) and γ (face-centred cubic). As a consequence of interdiffusion during heat treatment at 1120°C, a three-phase equilibrium, γ + γ′ + β, forms in the interface region between the CoNiCrAlY coating and the substrate. Furthermore, yttria (Y₂O₃) and chromium carbides (M₂₃C₆) are present as dispersed particles in the CoNiCrAlY coating. It is proposed that phase transformations during heat treatment of the CoNiCrAlY-IN738LC system can be predicted by using the simplified NiCoAl three-phase equilibrium diagram.