Aging of vacuum plasma sprayed MCrAlY protective layers and their interaction with nickel- and cobalt-based γ/γ'-superalloys

γ/γ' single crystal superalloys with plasma-sprayed thermal barrier coating systems are used as turbine rotor blades in gas turbines if the blades are exposed to high temperatures and high mechanical loads. A bond coat (BC) is part of the thermal barrier coating system. It protects the substrate from oxidation and ensures good bonding of the ceramic coating that serves as a thermal insulator. MCrAlY (M=Ni,Co) alloys are commonly used as BCs. They form a protective Al₂O₃ layer. This study investigates four different vacuum plasma-sprayed MCrAlY BCs with and without Re after thermal treatment of up to 1000 h at 1044 C in air. The employed substrates are the Ni-based superalloy ERBO1 and the novel Co-based γ/γ' superalloy ERBOCo-1. Additionally, the ternary γ/γ' alloy Co-9Al-9W (in at.%) was aged with a BC for up to 500 h at 900 C. Up to now little is known about the interaction of the Co-based substrates and the BCs. Oxidation and Al depletion of the BC as well as the interdiffusion of BCs and substrates are analysed primarily on the basis of SEM/EDX and XRD. The effect of Y and Hf on the microstructure of the oxide scale is discussed. Rate constants show that Hf results in higher oxidation rates while Re slows down the oxidation. The influence of the alloying elements on the BC microstructure is described. For example, Co prevents the formation of γ' phase, Re slows down diffusion and results in the formation of brittle phases. The choice of substrate material has no measurable influence on the oxidation. Qualitative and quantitative analysis of the interdiffusion zone (IDZ) shows that the choice of substrate surface pre-treatment (grit blasting or grinding) has a major influence on the interdiffusion behaviour with the BC. Grinding results in a thinner IDZ and fewer topologically closed packed (TCP) phases. The reason for this is the recrystallisation of the single crystal substrate. A study of the influence of the substrate crystal orientation on the interdiffusion shows no correlation. However, microstructure and thickness of the IDZ are influenced by the choice of BC. Co-9Al-9W shows a better compatibility with a CoNiCrAlY BC compared to a NiCo-CrAlY BC. This is attributed to the Cr activity. μ phase and σ phase form in the IDZ. The reaction of Y and Hf with Al₂O₃ grit on the substrate surface is analysed. ERBOCo-1 shows particularly good results with MCrAlY BCs. No or few TCP phases form. Parabolic rate constants of the IDZ growth are similiar to those for ERBO1. Diffusion of substrate elements to the outer part of the BCs was found.