Mechanical behavior of Ti cold spray coatings determined by a multi-scale indentation method

The effect of the indentation load on the hardness measurements on cold spray Ti coatings was evaluated. Varied load indentation was carried out with nanoindentation at loads between 1 and 20 mN and microindentation at loads between 0.1 and 5 N. Nanoindentation measurements showed an indentation size effect and were fit to the Nix–Gao model for strain gradient plasticity. The microindentation measurements depended on the indentation size in a manner different from the Nix–Gao model and were strongly affected by the material porosity and cohesive strength between cold sprayed particles. By comparing results from the two indentation techniques, a hardness loss parameter was formulated that helped explain the mechanical behavior at all length scales and its relation to the various defects within the coating. Comparisons of the hardness loss parameter between cold spray coatings and a bulk Ti plate showed that this new parameter provided a good measure of when the mechanical behavior of the cold spray coatings approached that of a traditionally manufactured material. Evaluation of the hardness loss parameter of coatings and the bulk material, combined with microstructural characterization before and after indentation, was used to examine the relationships between processing conditions, coating microstructure and mechanical behavior for coatings fabricated with spherical and non-spherical Ti powders.