Investigation of the formation of an amorphous film at the ZrO2-Y2O3/NiCoCrAlY interface of thermal barrier coatings produced by plasma spraying

The interface between metallic bonding layers (NiCoCrAlY) and ceramic coatings of 8% yttria partially stabilized zirconia (YPSZ), produced by air plasma spraying (APS) at different deposition temperatures, was investigated by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM) on transverse thin foils. The presence of an Al-rich amorphous film was ascertained, probably originating during the deposition of the bond coat as a crystalline alumina surface layer. This layer successively remelts, grows and solidifies in the amorphous stare due to the rapid heat extraction during plasma spraying. The thickness of this interlayer is a function of the deposition temperature: the higher the substrate temperature during spraying, the thinner the interfacial amorphous film. Columnar YPSZ grains, elongated in the direction perpendicular to the surface and having typical dimensions of 50 nm x 500 nm, are superimposed on the amorphous layer. Equiaxed grains of comparable size extend above the columnar structure. Both columnar and equiaxed grains consist of a metastable tetragonal phase t', characterized by a higher toughness than the face-centred cubic equilibrium phase. The negative influence of this brittle interlayer, in particular on the adhesion strength of the thermal barrier coating (TBC), was also evaluated by means of four-point bending tests. Results confirm a low adhesion strength between the ceramic and the bond coat in the case of TBCs produced at low deposition temperatures.