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Yttria-partially stabilised zirconia (Y-PSZ) coatings were plasma sprayed in air on aluminium bars under controlled conditions of surface cooling. The effect of the deposition temperature (Tdep) on coating microstructure and residual stresses was studied. X-ray diffraction (XRD) line broadening analysis showed crystallites to be much smaller than the columnar grains visible on scanning electron microscopy micrographs. Moreover, the stabilising oxide (Y2O3) was not uniformly distributed among the various crystalline domains. By increasing Tdep the coating morphology was not visibly affected, while the lattice disorder decreased slightly because of a reduction of point defect density, mainly recognised as excess oxygen vacancies. On the contrary, the residual stress field was strictly connected to Tdep. Surface and average (bulk) residual stresses were measured by two distinct techniques: XRD (σXRD) and the coating-length change after debonding from the substrate (σD), respectively. The surface was always in tension, approaching the value of quenching stress (30–40 MPa) at low Tdep, whereas the bulk was always in compression. Assuming a linear gradient model, σXRD and σD were used to calculate the residual stress trend inside the ceramic as a function of Tdep.

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 state 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 × 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.