Damage mechanisms of coated systems under thermomechanical fatigue

Abstract

The damage mechanisms of several kinds of coatings on a single crystal nickel base superalloy under thermomechanical fatigue (TMF) are described. The systems investigated were diffusion platinum aluminide coatings, Co–Ni–Cr–Al–Y overlay coatings, and thermal barrier coatings (TBCs). The TMF experiments were carried out on hollow specimens over a temperature range from 300 to 1050°C, at strain ranges Δɛ = 0·5 and 0·7%, and at a strain ratio R = -∞. No coating cracking was found for the platinum aluminide coating. Instead, specimens failed owing to oxidation induced crack initiation from the uncoated inner surface of the hollow testpieces, although coating surface roughening caused by non-homogeneous oxidation was observed. For the overlay coating, roughening in terms of coating rumpling and coating cracking occurred, resulting in reduced TMF life. For TBC specimens with a thin ceramic coating processed by electron beam–physical vapour deposition (EB–PVD), TMF life was comparable with that of specimens with the overlay coating. Failure once again occurred owing to Co–Ni–Cr–Al–Y bond coat cracking and propagation into the substrate. In this system, some bond coat cracks penetrated through the top ceramic coat although others did not. In contrast with specimens coated with the overlay alone, no significant rumpling on the bond coat surface was observed and the crack density was low.