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.

Keywords:

• thermal barrier coatings
• high-pressure turbine blade
• yttria stabilized zirconia
• formation mechanism
• reactive interfaces and damage assessment