Site-specific study of jetting, bonding, and local deformation during high-velocity metallic microparticle impact

In the cold-spray process, jetting is often considered a precursor to particle-substrate adhesion, but the conditions under which these phenomena occur remain unclear. This paper presents a systematic site-specific study of single-particle impact across a wide range of impact velocities for Cu particles impacting a Cu substrate. A high degree of velocity control enables identification of new behaviors that emerge in distinct velocity ranges. At the lowest velocities, we observe a fully plastic impact regime with rebound of the incident particle. As velocity increases, substrate jetting emerges even while the particle still rebounds. At the critical adhesion velocity, v_cr, jetting only occurs on the substrate, leading to poor metallurgical bonding. Bonding improves as velocity increases and the particles also begin to jet, but there is an apparent maximum in bonding extent at ~1.3 v_cr. This is followed by a drop in metallurgical bonding due to the ‘backward’ jetting of the particle that is associated with peeling forces on the interface. At ~1.6 v_cr and above, we observe hydrodynamic penetration of the particle and possible erosion, as indicated by petalling of the substrate and burial of the particle below the substrate impact plane. Taken together, this catalog of phenomena, correlated to specific velocities, points to an impact-structure-based approach to establishing processing windows for cold spray.