Microstructure of gas flow sputtered thermal barrier coatings

This work investigates the influence of substrate temperature and applied bias voltage on the resulting microstructure and crystallographic properties of gas flow sputtered (GFS) partially yttria stabilized zirconia coatings (PSZ).On a FeCrAlY-alloy substrate PSZ coatings were deposited at substrate temperatures of 500 °C, 650 °C and 800 °C with bias voltages up to − 100 V and were subsequently analyzed utilizing SEM, FIB and XRD. Without bias voltage, all coatings were columnar and composed of small stacked platelets, but they varied in their porosity and morphology. Higher substrate temperatures lead to higher adatom mobility decreasing the porosity and the deposition rate. For all microstructures adistinct growth direction of the columns is observed changing from 〈111〉 (500 °C and 650 °C) to 〈100〉 (800 °C). For the 〈111〉 morphology, three ridges at intervals of 120° are observed throughout the whole column. In addition to the tetragonal/cubic phase, monoclinic fractions are found at the two lower substrate temperatures. The application of a negative bias voltage enhances the surface mobility of the film-forming adatoms and can cause densification due to atom displacements, resputtering or channeling phenomena. Moderate bias voltages up to − 40 V result in more regular columns and a lower porosity. For high bias voltages (− 100 V), the densification effects seem to dominate leading to fully dense coatings with residual stresses up to − 2.5 GPa and no preferential grain orientation, rendering this bias value unsuitable for thermal barrier coatings.