Particle flattening during cold spray: Mechanistic regimes revealed by single particle impact tests

The flattening ratio, epsilon, is an important diagnostic measurement of particle deformability and coating quality assessment in cold spray. However, the experimental determination of epsilon and correlation of it to experimental parameters (particle size, velocity) is not straightforward from observations of cold-sprayed coatings. Here we present a series of direct site-specific measurements of epsilon for well-known particle sizes and velocities, without the many complications involved in a full cold spray experiment. This approach permits us to identify mechanistic transitions separating three apparent regimes of behavior. In regime I, at lower velocities just high enough to cause bonding, an impacting particle flattens largely unhindered. In regime II, at somewhat higher velocities, the substrate develops a significant crater, which in turn constrains lateral particle flattening as the particle burrows more deeply into the substrate. In regime III, at very high velocities, hydrodynamic particle penetration is observed, distinguished by deep burrowing of the particle, which adopts a concave-saucer shape, with large petals of substrate ejected backwards out of the surface. While direct measurements in regimes I and II show that deformed particles can be reasonably modeled as oblate spheroids, that assumption fails in regime III. To the extent that particle flattening is deemed desirable, this analysis suggests a more nuanced view of the velocities used in cold spray.