Uniaxial Fatigue Behavior of Freestanding Platinum-Aluminide (PtAl) Bond Coat

Abstract

Pt-aluminide (PtAl) bond coat is used as a part of Thermal Barrier Coating (TBC) system on Ni-base superalloy components in advanced gas turbine aeroengines. The thickness of B2-NiAl intermetallic based diffused coating varies in the range of 60-100 µm. The coating provides excellent protection against oxidation to the superalloy components experiencing extreme temperatures of 1000°C during the operation of aeroengine. High temperatures combined with fluctuating mechanical loads can induce fatigue damage in the coating during service. Damage initiated in diffused coating propagates into the superalloy substrate and causes failure of the components. Therefore, evaluation of micro-mechanical properties of the PtAl bond coat is essential from the scientific and engineering application standpoints. The present study evaluates fatigue behavior of freestanding PtAl coating in the temperature range of ambient to 1000°C. Testing of the coating in freestanding form provides scientific understanding of inherent fatigue behavior and the evolution of fatigue damage in the coating without any influence from the substrate. Detailed microstructural characterization of the fatigue tested coating microsamples using SEM-EBSD, XRM and TEM has been carried out to ascertain the fatigue damage mechanisms in the coating. The fatigue behavior of the coating can be categorized into two temperature regimes, i.e., ambient to 800°C and above 900°C. In the temperature regime of ambient to 800°C, high dislocation activity, formation of dislocation cells and dislocation-precipitate interactions induce strain hardening are observed in the coating. The failure of the coating in the ambient to 800°C range is caused by the formation of micro-voids due to de-cohesion of precipitate/B2-matrix interface in the heavily precipitated interdiffusion zone, subsequent formation of micro-cracks by coalescence of micro-voids, and final failure by propagation of micro-cracks. Fatigue tested samples above 900°C exhibit extensive voiding preferentially in the outer layer of the coating. The role of dynamic recrystallization in the transition of the preferential layer for fatigue damage initiation with the increase in temperature is ascertained. The formation of voids above 900°C results from the creep damage in the coating. Cyclic loading leads to the formation of microcracks from these voids, and final failure is caused by the propagation of cracks. Deflection of crack and blunting of the crack tip govern the resistance to the propagation of cracks.