Modified indentation techniques to probe inelasticity in Ni-5%Al coatings from different processes

In this study, two types of indentation experiments were performed on metallic (Ni-5%Al) coatings prepared by cold spray, high velocity oxy-fuel and air plasma spray. In the first type, spherical tips were used with increasing loads, and subsurface deformation was observed using a modified bonded interface technique. In the second type, cyclic loading was imposed with a sharp tip, and tip displacement was continuously recorded. Results suggest that cold spray coatings are brittle under contact loads in their as-sprayed condition, and that they exhibit a size effect that is quite different from those of the other coatings. That is to say, heterogeneities in mechanical behavior exist not as much on the single particle level as expected, but on a much larger scale of order 100 microns. This is attributed to long unbonded regions between particles, in a coating of otherwise high density. Fracture mechanics arguments support this hypothesis.