Impact-bonding with aluminum, silver, and gold microparticles: Toward understanding the role of native oxide layer

Bonding during cold spray has been phenomenologically associated with the breakup of the native surface oxide layer to produce intimate metallic contacts. Here, we resolve the instant of bonding for metals both with and without a significant native oxide, through direct in-situ observation of individual microparticles. We show that fracture of the native oxide layer is not the only physical barrier to impact adhesion; even noble gold must exhibit unstable jetting upon impact in order to attain bonding. However, surface oxide certainly presents an additional barrier to bonding and can increase the critical velocity by a large factor.