In-situ Observations of the Bonding Moment in Cold Spray

The current understanding of bonding in cold spray has been almost entirely gathered through post-mortem observations of full coatings or single splats produced by nozzle experiments. Such experiments largely obscure the kinetic and thermal histories of particles before impact, as well as transient extreme events occurring during impact, making a conclusive mechanistic understanding of bonding extremely challenging. In this work, we overview our alternative approach for studies of impact bonding that resolves these shortcomings. We conduct in-situ single-particle level studies of supersonic impact bonding using a ballistic test platform. We present precisely controlled impact bonding experiments with individual particles whose flight, impact, bonding or rebound are recorded in-situ. We, thus offer direct measurements of critical velocity for a number of structural metals. What is more, with our in-situ experiments we resolve the moment of bonding in supersonic particle impact with microscale-nanosecond spatiotemporal resolution, giving the first real-time observations of jetting, local material ejection and fragmentation during bonding. We demonstrate that these extreme phenomena are dominated by hydrodynamic pressure effects, and substantiate a proportionality between the critical adhesion velocity and the bulk speed of sound in matched metals impact.