EB–PVD TBCs of zirconia co-doped with yttria and niobia, a microstructural investigation

Turbine blades of airplanes and thermoelectric plants work in adverse conditions, with corrosive environment and high temperature and pressure. One way to improve the life and/or the working temperature of the blades is the use of special coatings over metallic material applied by Electron Beam–Physical Vapor Deposition (EB–PVD). The most usual material for this application is yttria doped zirconia. Addition of niobia, as a co-dopant in the Y2O3–ZrO2 system, can reduce the thermal conductivity and improve mechanical properties of the coating. The purpose of this work is to show the influence of the addition of niobia on the microstructure of ceramic coatings by using X-ray diffraction (XRD) techniques and scanning electron microscopy (SEM) observations. SEM on fractured cross-section shows a columnar structure and the results of XRD show only zirconia tetragonal phase in the ceramic coating for the chemical composition range studied. As the difference (NbO2.5–YO1.5) mol% increases, the ratio c/a (tetragonality) increases. Considering that the t-ZrO2 solid solutions begins unstable when the relation c/a exceeds 1.020, it is possible to evaluate the maximum niobia content that can be added to the coating without losses in its mechanical properties. SEM on ceramic coatings polished cross-section shows color bands associated with chemical composition changes due to the differences in saturation vapor pressure of the individual components. As the niobia content increases, there is a tendency to reduction of the ceramic coating theoretical density.