Modelled and measured residual stresses in plasma sprayed thermal barrier coatings

Thermal barrier coatings consisting of a NiCrAlY bond coating and a 1.4 mm thick partially stabilised zirconia top coating were air plasma sprayed onto grit blasted nickel-base substrates. Two samples were produced using different amounts of external cooling during spraying of the top coatings. The residual stress profiles in the samples were measured after each manufacturing process step with a laver removal technique. A finite element model including a thermal analysis and a stress-strain analysis of the deposition was developed to model the origin of the thermal stresses and to verify the measured residual stresses. The main components for the residual stresses in the sprayed coatings were identified as stresses developing during the rapid cooling of individual droplets (quenching stresses) and stresses formed during cooling from deposition temperature to room temperature. The quenching stresses were predicted to be low and tensile in the top coating, due to stress relaxation by formation of vertical microcracks. During cooling to room temperature, compressive stresses were superimposed on the top coating quenching stresses. The final residual stresses were predicted to be compressive in the top coating. This was confirmed by measurements (-15 MPa). In the bond coating, no stress relaxation by microcracking was observed and the residual stresses were found to be tensile (similar to 100 MPa). In the substrate, compressive residual stresses reaching -200 MPa were found in a zone to a depth of 0.3 mm into the substrate. The stresses were found to have originated during the grit blasting of the substrates prior to bond coating deposition. A correlation between modelled inelastic strain and measured densities of vertical microcracks in the top coating was obtained. High values were found close to the bond coating, which were correlated to a low substrate temperature during spraying of the top coating material. (C) 1997 Elsevier Science S.A.