Influence of the oxygen flow rate on gas flow sputtered thermal barrier coatings

Porous thermal barrier coatings (TBC) reduce the thermal load of gas turbine components. State-of-the-art TBCs consist of partially yttria-stabilized zirconia (PSZ) and are deposited by means of thermal spray techniques or electron beam physical vapor deposition. In this paper, an alternative, innovative deposition technique (reactive gas flow sputtering - GFS) is investigated and the influence of process parameters on the microstructure and the suitability of GFS coatings as TBCs is discussed. PSZ coatings were deposited on polished FeCrAlY-alloy substrates, with varying substrate temperature and oxygen flow rate, and characterized by means of SEM, and XRD. The substrate temperature is the crucial parameter. Between 500 and 800 °C, four types of columnar microstructures are found based on XRD pattern and morphology. The growth direction of the columns changes from <111> to <100>, accompanied by a change in shape from triangular to four-sided. Varying the oxygen flow rate at a given substrate temperature alters the microstructure defined by the substrate temperature. While oxygen flow rates above a certain level do not have an effect, low oxygen flow rates lead to further densification and compressive stresses, rendering these conditions unsuitable for TBC manufacturing. In conclusion, promising microstructures are presented accompanied by guidelines for process parameters.