Residual stresses in plasma sprayed partially stabilised zirconia TBCs: Influence of the deposition temperature

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 (T-dep) 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 T-dep 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 T-dep. Surface and average (bulk) residual stresses were measured by two distinct techniques: XRD (sigma(XRD)) and the coating-length change after debonding from the substrate (sigma(D)), respectively. The surface was always in tension, approaching the value of quenching stress (30-40 MPa) at low T-dep, whereas the bulk was always in compression. Assuming a linear gradient model, sigma(XRD) and sigma(D) were used to calculate the residual stress trend inside the ceramic as a function of T-dep.