The damage mechanisms of several kinds of coatings on a single crystal nick
el base superalloy under thermomechanical fatigue (TMF) are described. The
systems investigated were diffusion platinum aluminide coatings, Co-Ni-Cr-A
l-Y overlay coatings, and thermal barrier coatings (TBCs). The TMF experime
nts were carried out on hollow specimens over a temperature range from 300
to 1050 degrees C, at strain ranges Delta epsilon = 0.5 and 0.7%, and at a
strain ratio R = -infinity. No coating cracking was found for the platinum
aluminide coating. Instead, specimens failed owing to oxidation induced cra
ck initiation from the uncoated inner surface of the hollow testpieces, alt
hough coating surface roughening caused by non-homogeneous oxidation was ob
served. For the overlay coating, roughening in terms of coating rumpling an
d coating cracking occurred, resulting in reduced TMF life, For TBC-specime
ns with a thin ceramic coating processed by electron beam-physical vapour d
eposition (EB-PVD), TMF life was compatible with that of specimens with the
overlay coating. Failure once again occurred owing to Co-Ni-Cr-Al-Y bond c
oat 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 lo
w. MST/4233.