The creep damage behavior of the plasma-sprayed thermal barrier coating system NiCr22Co12Mo9-NiCoCrAlY-ZrO2/7 %Y2O3

During creep loading metallic substrates impose deformation on deposited ce ramic thermal barrier coatings (TBC). Strain accomodation of the TBC is not attained by plastic deformation, but by means of crack initiation, crack o pening, crack propagation or sliding of adjacent crack faces. In technical applications a distinction is mane between tolerated or desired cracks perp endicular to the surface, and detrimental cracks parallel to the substrate- coating interface. Thus, TBC can respond to creep deformation by segmentati on or spallation, the latter being referred to as failure. The parameters i nfluencing the probability of either segmentation or spallation are tempera ture, creep rate, magnitude of creep deformation, layer thickness, and micr ostructure of the TBC. It can be stated that spallation failure probability increases with increasing creep rate, creep deformation, and layer thickne ss. The presence of pores between single spraying layers also strongly augm ents the likelyhood of spallation. No significant influence of temperature on spallation failure probability can be found in the range from 850 degree s C to 1050 degrees C, Light microscopy and scanning electron microscopy in vestigations show drat the microstructure of the ceramic TBC changes during creep, and that the density of cracks detected on micrographs with low mag nification (x50) increases with increasing creep deformation. On the other hand the density of microcracks visible with high magnification (x500) is c onstant, or even decreases with increasing creep deformation. These finding s are explained by sintering processes enabled by stress relaxation due to formation of macroscopic crack perpendicular to the surface as a response t o creep deformation. A relationship between microstructural changes and the emission of acoustic signals recorded during creep is presented.