During creep loading metallic substrates impose deformation on deposited ceramic thermal barrier coatings (TBC). Strain accomodation of the TBC is not attained by plastic deformation, but by means of crack initiation, crack opening, crack propagation or sliding of adjacent crack faces. In technical applications a distinction is made between tolerated or desired cracks perpendicular to the surface, and detrimental cracks parallel to the substrate-coating interface. Thus, TBC can respond to creep deformation by segmentation or spallation, the latter being referred to as failure.
The parameters influencing the probability of either segmentation or spallation are temperature, creep rate, magnitude of creep deformation, layer thickness and microstructure of the TBC. It can be stated that spallation failure probability increases with increasing creep rate, creep deformation and layer thickness. The presence of pores between single spraying layers also strongly augments the likelyhood of spallation. No significant influence of temperature on spallation failure probability can be found in the range from 850°C to 1050°C.
Light microscopy and scanning electron microscopy investigations show that the microstructure of the ceramic TBC changes during creep, and that the density of cracks detected on micrographs with low magnification (x50), increases with increasing creep deformation. On the other hand, the density of microcracks visible with high magnification (x500) is constant, or even decreases with increasing creep deformation. These findings are explained by sintering processes enabled by stress relaxation due to formation of macroscopic cracks perpendicular to the surface as a response to creep deformation.
A relationship between microstructural changes and the emission of acoustic signals recorded during creep is presented.