Erosion of EB-PVD thermal barrier coatings

Thermal barrier coating manufacture is based on two different processing technologies, namely thermal spraying and electron beam physical vapour deposition (EB-PVD) of a yttria stabilised, zirconia ceramic. The former is widely used to deposit ceramics on combustor cans, ductwork, platforms and other hot gas path components. The latter, due to its unique columnar structure, is the only process that can offer satisfactory levels of spall resistance, erosion resistance and surface finish retention for aerofoil applications. By using a unique high velocity gas gun rig, that has been developed with the capability of impacting thermal barrier coatings at particle velocities upto 300m/s at test temperatures up to 920 degrees C, this paper contrasts the erosion behaviour of an APS and EB- PVD ZrO2-8wt%Y2O3 ceramic. When subjected to room temperature erosion using 100 mu m Al2O3, the erosion rate of an EB-PVD thermal barrier ceramic was observed to be an order of magnitude lower than its plasma sprayed counterpart. At high temperatures erosion rates increased over those measured at room temperature, consistent with the higher test velocities achieved at 910 degrees C. Post test examination showed that cracking occurs within the near-surface region of the EB-PVD ceramic, and that erosion results from material loss as cracks propagate parallel to the coating substrate interface, the cracks being arrested by the major columnar boundaries. In contrast, removal of material for plasma sprayed coatings occurs through poorly bonded splat boundaries, and hence larger volumes of material are easily lost resulting in higher erosion rates.