Characterization of Ti cold spray coatings by indentation methods

The mechanical behavior of Ti cold spray coatings deposited on Ti substrates was determined and compared to that of a bulk Ti specimen. Hardness testing was performed over a wide range of loads with both nanoindentation and microindentation. A model for strain gradient plasticity was used to explain the indentation size effect observed for nanoindentation measurements and determine true hardness values for each sample. For microindentation, a plateau hardness was determined by averaging data at high loads in a region where the hardness values did not vary with indentation depth. The mechanism for the indentation size effect for nanoindentation was the formation of geometrically necessary dislocations. For microindentation, the mechanism was related to larger scale defects such as porosity and prior particle boundaries. As a result of the difference in mechanisms, the plateau hardness was always lower than the true hardness. Overall, it was demonstrated that the sprayed powders were harder than the bulk titanium specimen but higher particle velocities were necessary to achieve mechanical behavior approaching that of the bulk.

► Using a multi-scale indentation method, the mechanical behaviors of titanium cold spray coatings and bulk titanium were compared.
► For nanoindentation, hardness of the coatings was higher than bulk titanium.
► For microindentation, a decrease in the coating hardness was attributed to microscopic defects.
► Effect of the microscopic defects on hardness was less for coatings deposited at higher velocities.