Phenomena of Foreign Object Damage by Spherical Projectiles in EB-PVD Thermal Barrier Coatings of Turbine Airfoils

Thermal barrier coatings (TBCs), attributed to their inherent brittleness, are vulnerable to damage by impacting foreign objects when kinetic energy of the objects surpasses certain limits. The damage is termed foreign object damage (FOD) and results in various issues to coatings as well as to substrates, from plastic impression to delamination to spallation to cracking, depending on the severity of impact. The FOD experiments were conducted utilizing a ballistic impact gun for vane airfoil components coated with 220 mu m-thick, 7% yttria-stabilized zirconia (7YSZ) by electron beam physical vapor deposit (EB-PVD). The testing was performed with impact velocities ranging from 150 m/s to Mach 1 using 1.6-mm hardened chrome-steel ball projectiles. The resulting FOD was in the forms of impact impressions, cone cracking, and delamination of the coatings/substrates. Prediction of delamination crack size as a function of impact velocity was made based on an energy-balance approach through a quasistatic, first-order approximation. The prediction was in reasonable agreement with experimental data considering a presumable compaction of the TBCs upon impact.