Plasma-sprayed duplex and graded partially stabilized zirconia thermal barrier coatings: Deposition process and properties
Musil, J.; Alaya, M.; Oberacker, R.
JOURNAL OF THERMAL SPRAY TECHNOLOGY 6 (1997), 4, 449–455
Plasma-sprayed duplex and graded partially stabilized zirconia thermal barrier coatings: Deposition process and properties.
Atmospheric plasma spraying of duplex and graded ZrO2 (8% Y2O3) thermal barrier coatings (TBCs) on Inconel 617 substrate with a NiCrAIY bond coat is described in terms of a deposition process of controlled coating structure, Special attention is devoted to the dominant spray parameters and the injector configuration for powder feeding, which play a fundamental role in graded coating deposition with controlled formation of a graded metal-ceramic (GMC) intermediate zone, The results of the graded coating spraying allow: (a) suppression of step-interface effects, (b) suppression of large differences (misfit) between physical and mechanical constants of the coating and those of the substrate material, and (c) favorable intergrowth of crystallites for a microstructurally integrated structure, Sprayed TBCs were investigated and compared with regard to their thermal cycling, oxidation behavior, and mechanical properties. The influence of crystal anisotropy changes on the resulting coating structure and properties is shown, On the basis of finite element (FE) calculations, the stress distribution within thermally cycled coating systems was analyzed, It is confirmed that the graded coating structure relaxes considerably the stresses resulting from the internal constraint due to thermal expansion difference between both metallic and ceramic materials, This stress distribution also decreases the gradient of elastic deformation and/or residual stresses between the metal bond coat and top ceramic coating, and hence leads to a better thermal cycling behavior of the graded TBC systems, However, this advantage is not practical in every case, since the rapid oxidation of the metallic lamellae causes the ceramic phase in the GMC zone to undergo tensile stresses within a short thermal exposure time, The lifetime of duplex TBC systems that are under steady-state thermal load conditions is much higher than that of graded ones.