Influence of a PVD-Al interlayer on the properties of thermally sprayed thermal barrier coating system after thermal cycling

Thermal barrier coatings (TBC) generally consist of a metallic bond coat (BC) and a ceramic top coat (TC). Co-Ni-Cr-Al-Y metallic super alloys and Yttria stabilised zirconia (YSZ) have been widely used as bond coat and top coat for thermal barrier coatings systems, respectively. As a result of long-term exposure of thermal barrier coatings systems to oxygen-containing atmospheres at high temperatures, a diffusion of oxygen through the porous ceramic layer occurs and consequently an oxidation zone is formed in the interface between ceramic top coat and metallic bond coat. Alloying components of the BC layer create a so-called thermally grown oxides layer (TGO). One included oxide type is alpha -Al sub(2)O sub(3). alpha -Al sub(2)O sub(3) lowers oxygen diffusion and thus slows down the oxidation process of the bond coat and consequently affects the service life of the coating system positively. The distribution of the alloying elements in the bond coat layer, however, generally causes the formation of mixed oxide phases. The different oxide phases have different growth rates, which cause local stresses, micro-cracking and, finally, delamination and failure of the ceramic top coat layer. In the present study, a thin Al inter-layer was deposited by DC-Magnetron Sputtering on top of the Co-Ni-Cr-Al-Y metallic bond coat, followed by thermal spraying of yttria-stabilised zirconia (YSZ) as a top coat layer. The deposited Al inter-layer is meant to transform under operating conditions into a closed layer with high share of alpha -Al sub(2)O sub(3) that slows down the growth rate of the resulting thermally grown oxides layer. Surface morphology and microstructure characteristics as well as thermal cycling behaviour were investigated to study the effect of the intermediate Al layer on the oxidation of the bond coat compared to standard system. The system with Al inter-layer shows a smaller thermally grown oxides layer thickness compared to standard system after thermal cycling under same conditions.Original Abstract: Waermedaemmschichtsysteme (WDS) bestehen in der Regel aus einer metallischen Haftschicht und einer keramischen Deckschicht. CoNiCrAlY-Superlegierungen finden als Haftschichtmaterial weit verbreiteten Einsatz, fuer die Deckschichten wird oft Yttriumoxid-stabilisiertes Zirkondioxid verwendet. In der Folge der Langzeiteinwirkung von sauerstoffhaltigen Atmosphaeren bei hohen Temperaturen auf die Waermedaemmschichtsysteme kommt es zur Diffusion von Sauerstoff