A comparison between the erosion behaviour of thermal spray and electron beam physical vapour deposition thermal barrier coatings

Abstract

Thermal barrier coating (TBC) technology is used in the hot sections of gas turbines to extend component life. To maximise these benefits, the TBC has to remain intact through the life of the turbine. High velocity ballistic damage can lead to total thermal barrier removal, while erosion may lead to progressive loss of thickness during operation. This paper compares the erosion behaviour of thermally sprayed and electron beam physical vapour deposited (EB-PVD) TBCs. A unique high velocity gas gun has been developed with the capability of impacting TBCs at particle velocities up to 300 m/s at test temperature up to 920°C. Using this facility it was found, both at room temperature and 910°C, that EB-PVD ZrO 2–8 wt.% Y 2O 3 thermal barriers are significantly more erosion-resistant than the equivalent plasma spray coating, when impacted with either alumina or silica in the particle size range 40–100 μm. Examination of tested hardware reveals that cracking occurs within the near surface region of the columns for EB-PVD ceramic and that erosion occurs by removal of these small blocks of material. In stark contrast, removal of material for plasma sprayed ceramic occurs through poorly bonded splat boundaries. This difference in material removal mechanism accounts for the seven- to 10-fold improvement, in erosion resistance observed for EB-PVD TBCs over those produced by plasma spraying. At both room temperature and 910°C, erosion rates are linear with velocity for both types of coating. When impacted with hard particles (alumina at room temperature and 910°C, silica at room temperature) peak erosion rates occur for normal impact, with impact angle dependence scaling similarly for the air plasma sprayed (APS) and EB-PVD coatings. At high temperatures, erosion rates are increased over those measured at room temperature, consistent with the higher test velocities achieved at 910°C.