Indentation fracture behavior of plasma-sprayed nanostructured Al2O3–13wt.%TiO2 coatings

Indentation crack growth resistance of nanostructured Al2O3–13wt.%TiO2 coatings plasma sprayed using nanosized powders was investigated. Comparisons were made between the nano-coatings and a commercial baseline coating of the same composition, Metco 130. In Metco 130 coatings that contain only the single-phase splat microstructure, long cracks initiate at the indent corners and propagate along splat boundaries. In contrast, the nano-coatings are composed of a bi-modal microstructure (a fully melted splat structure and a partially melted particulate structure), and the partially melted particulate region serves to trap and deflect the splat boundary cracks. The interface between the fully melted region and the partially melted region also provides additional crack arrest mechanisms. At optimized conditions, these toughening mechanisms can produce an approximately 100% improvement in the crack growth resistance. The optimized microstructure for the nano-coatings is the microstructure containing 15–20% of the partially melted particulate region, which can be systematically controlled by changing the plasma flame temperature.