Strength gradient in impact-induced metallic bonding

Solid-state bonding can form when metallic microparticles impact metallic substrates at supersonic velocities. While the conditions necessary for impact-induced metallic bonding are relatively well understood, the properties emerging at the bonded interfaces remain elusive. Here, we use in situ microparticle impact experiments followed by site-specific micromechanical measurements to study the interfacial strength across bonded interfaces. We reveal a gradient of bond strength starting with a weak bonding near the impact center, followed by a rapid twofold rise to a peak strength significantly higher than the yield strength of the bulk material, and eventually, a plateau covering a large portion of the interface towards the periphery. We show that the form of the native oxide at the bonded interface-whether layers, particles, or debris-dictates the level of bond strength. We formulate a predictive framework for impact-induced bond strength based on the evolution of the contact pressure and surface exposure.