Evolution of microstructure and mechanical properties of NiAl-Diffusion coatings after thermocyclic exposure

High temperature materials require both, high oxidation resistance and mechanical strength. Owing to their excellent high temperature strength, Ni-based superalloys are extensively used in turbine engines. In order to enhance the oxidation resistance and thereby extend the lifetime, their surface has to be modified by the enrichment of stable oxide formers. Mainly NiAl-diffusion coatings are used to protect turbine components, serving as an Al-reservoir for the formation of a thin, protective scale. Besides their oxidation resistance, the mechanical behavior of such coatings is crucial for the integrity of the system. Under service conditions, compositional changes will occur due to two mechanisms: outward-diffusion of Al to form the oxide and interdiffusion with the substrate. Such chemical changes lead to a change in the mechanical behavior of the coating and thus the coated system. In this study, the compositional and microstructural changes, which occur during the thermocyclic exposure, are correlated with the mechanical properties of NiAl diffusion coatings. Prior to and after thermocyclic exposure at 900, 1050 and 1100 °C for durations up to 1000h four-point bending flexural tests with in-situ acoustic emission measurement are used to determine the fracture strain of the coating. The fracture strain increases due to Al-depletion during cyclic exposure and can be correlated with the Al-concentration in the diffusion zone. Moreover, elastic modulus and hardness of the coating zones are determined by nanoindentation. Both show a decrease in the single-phase sub-stoichiometric β-NiAl with lower Al-content, increasing again within the two-phase stability region.