RESIDUAL-STRESSES IN PLASMA-SPRAYED PARTIALLY-STABILIZED ZIRCONIA TBCS - INFLUENCE OF THE DEPOSITION TEMPERATURE

Yttria-partially stabilised zirconia (Y-PSZ) coatings were plasma spra yed in air on aluminium bars under controlled conditions of surface co oling. The effect of the deposition temperature (T-dep) on coating mic rostructure 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 micrograph s. Moreover, the stabilising oxide (Y2O3) was not uniformly distribute d among the various crystalline domains. By increasing T-dep the coati ng morphology was not visibly affected, while the lattice disorder dec reased 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 (bul k) residual stresses were measured by two distinct techniques: XRD (si gma(XRD)) and the coating-length change after debonding from the subst rate (sigma(D)), respectively. The surface was always in tension, appr oaching the value of quenching stress (30-40 MPa) at low T-dep, wherea s the bulk was always in compression. Assuming a linear gradient model , sigma(XRD) and sigma(D) were used to calculate the residual stress t rend inside the ceramic as a function of T-dep.