Scaling laws governing the erosion and impact resistance of thermal barrier coatings

Airfoils coated with columnar thermal barriers have been removed from aero-engines and characterized. Observations of deformation and cracking have been used to identify three material removal mechanisms. These are: (i) sub-surface, trans-columnar cracking attributed to the initial elasto-dynamic response (nanosecond timeframes), (ii) segmented cone cracks when the ensuing penetration (millisecond timescale) remains elastic, (iii) local densification, accompanied by either kink bands or lateral cracks when the projectile penetrates plastically, especially at high temperature. Basic impact mechanics, combined with fracture criteria, have been used to devise scaling laws for these mechanisms. The results are combined to derive relations that characterize the thresholds for material removal and the transitions between major mechanisms (expressed in terms of a mechanism map). Some aspects of the material removal at kinetic energies above the thresholds are examined.