Reaction Diffusion Behaviors for Interface Between Ni-Based Super Alloys and Vacuum Plasma Sprayed MCrAlY Coatings

Abstract

The object of this study is overlay coatings of MCrAlY alloy sprayed by a vacuum plasma spray (VPS) process for the protection against high-temperature corrosion and oxidation in the field of gas turbine components. Reaction diffusion behaviors at the interface between the MCrAlY coatings and the substrate, which have an important effect on coating degradation, have not always been clarified. Three kinds of substrate, equiaxis 1N738LC, directional solidified CM247LC and single-crystal CMSX-2, and the four kinds of vacuum plasma sprayed MCrAlY coating have been selected for these experiments.The experimental results showed that the reaction diffusion layers consisted of aluminum compound layer and aluminum depleted layer, excepting that the aluminum depleted layer could not be observed in the case of CoNiCrAlY and NiCoCrAlY coatings. It also indicated that the diffusion thickness could be observed to follow a parabolic time dependence.. The order of reaction diffusion rate was NiCrAlY > CoCrAlY > CoNiCrAlY > NiCoCrAlY independent of the substrates. A convenient computer-aided system was developed for analyzing the reaction diffusion behaviors at the interface between coating and substrate. It was also clear that the estimated results of long time diffusion behaviors by simulation analysis was in good agreement with experiments.

Some aspects of elemental behaviour in HVOF MCrAlY coatings in high-temperature oxidation

“…The interdiffusion of the alloying elements drives the microstructure changes [9]. Itoh et al's research [8] indicated that the microstructures in the substrate were mainly controlled by the diffusion of Co (and Ni) through the CSI, which agrees with the results in this work. The microstructures in the GPDZ are different when the substrate is coated with different coatings (Fig.…”

“…For instance, due to the diffusion of Co into the substrate, secondary β phase (SBP) may form in the substrate [8]. Taylor et al's result [10] further showed that the Al in the SBP could diffuse back to the coating and be used for the oxidation at the coating surface.…”

“…the dots in the graphs, one can see that from 900 °C to 1100 °C, Co and Cr peaks form at the CSI where a Ni valley is found. In other words, Co and Cr replace Ni near the CSI, which causes the microstructure changes [8]. SBP forms in the substrate in short time, but gradually disappears with longer time (see Fig.…”

“…The microstructural evolution near the coating-substrate interface (CSI) was dependent on coating composition, substrate composition, and temperature [8,9]. For instance, due to the diffusion of Co into the substrate, secondary β phase (SBP) may form in the substrate [8].…”

“…TCP phases, like σ-CrW, may also form near the CSI due to the interdiffusion of alloying elements in some coating-superalloy systems [11]. In coating side, due to the diffusion of Al into the substrate, there usually formed a β-depletion zone [8]. With development of thermodynamic and kinetic databases in recent decades, the microstructural evolution in coating-superalloy systems can be simulated [9,12,13].…”