The CaSO4 phase in fully infiltrated electron-beam physical vapour deposited yttria stabilized zirconia top coats from engine hardware

Abstract

Thermal barrier coating (TBC) recession mechanisms caused by CaSO4 and Fe –Ti – rich CaO– MgO–Al2O3 –SiO2 (CMAS) (FTCMAS) particles on an in-service electron-beam physical vapour deposited (EB-PVD) yttria stabilized zirconia (YSZ) coated high-pressure turbine blade have been studied using analytical electron microscopy and concomitant laboratory experiments. A vapour phase deposition process is proposed for CaSO4 in order to rationalize the unique microstructure of pure CaSO4 infiltration through the full thickness of the YSZ top coat prior to deposition FTCMAS particles.While no CaSO4/YSZ interaction is observed in the bottom sections of the coating, distinct reactive interfaces develop at the surface due to YSZ interaction with both, CaSO4 and FTCMAS particles at working temperatures. Upon partial dissolution of the YSZ column tips the CaSO4/YSZ interface results in CaZrO3 formation, while the FTCMAS/YSZ interface consists of a thin double layer of CaZrO3 and a garnet-type Ca3Zr2(Fe, Al, Si)3O12 phase, also known as the mineral kimzeyite.Despite CaSO4 infiltration and an accumulated service life time of 17,000 h the top coat was still found to be intact. Bond-coat rumpling at the less efficiently cooled regions close to the trailing edge gives rise to strong surface undulations. This effect is particularly pronounced on the pressure surface and yields a crest/trough topography. The well adherent CaZrO3 layer from the CaSO4/YSZ interface may have a similar impact on TBC spallation as a glassy CMAS-type overlay.