In-situ Investigation of Splat Sliding Mechanisms in Cold Sprayed Metallic Coatings

The mechanical properties of cold sprayed deposits, with splats as their building blocks, are affected by splat sliding and de-bonding. The role of processing gas type (He vs. Air), splat geometry and particle oxidation on ‘splat sliding’ behavior is evaluated by in-situ mechanical characterization. High porosity and poor splat bonding in air-sprayed coating result in inferior microhardness and flexural strength, as compared to denser He-sprayed coating. In-situ cyclic bending of coatings is performed inside a scanning electron microscope to examine the splat sliding behavior. Digital image correlation (DIC) analysis of the real-time deformation videos is performed to examine the localized microstructure strains developed in the coating. The strain maps demonstrate relative sliding between the splats. This splat sliding phenomenon is accentuated in the air-sprayed coating, evidenced by 70% permanent residual strain, as against only 20% in the He-sprayed coating. A splat sliding factor (SS_f) is introduced, which compares cold sprayed microstructure deformation with bulk structures, providing quantitative information about inter-splat bonding. The SS_f is found to be higher for air-sprayed coating (~2) as compared to He-sprayed coating (~1.4). The correlation between splat sliding and cold spray processing will enable development of coatings with predictable and improved mechanical properties.