Tomographic Investigation of Bonding and Microstructure in Supersonic Microparticle Impacts

Abstract

The Laser Induced Particle Impact Test (LIPIT) enables particle-wise controlled launch of micron-scale metallic powders at velocities relevant to cold spray deposition. Many cold spray studies are empirical and discuss the effects of process parameters on porosity, density, and only aggregate material properties. As such, LIPIT is a powerful platform for probing the fundamental mechanisms governing high-velocity deformation and solid-state bonding. This method has been extensively used to study the subsurface features associated with bonding: the oxide layer thickness, jetting, and microstructural evolution; however, the true morphology of these features has remained largely inferred due to the limitations of conventional two-dimensional characterization. This thesis proposes a synergistic use of the LIPIT and FIB-SEM tomography characterization to study Cu-on-Cu impacts in 3D and provide detailed quantitative insights into fundamental aspects of a coating: its bonding and microstructure. First, the true geometry of bonded interfaces in single impacts is revealed for the first time, and a mechanistic analytical model for the onset of the bonding regime is developed, enabling a prediction of optimal bonding conditions. The 3D methodology is then applied to the study of microstructure to measure and model the extent of metadynamic recrystallization which occurs immediately following the high-velocity impact due to the presence of residual heat. The second half of the thesis discusses two-particle LIPIT stacks where individual impacts are aimed directly on top of one another. Such stacks enable careful study of the coating buildup step, characterized by particle-particle bonding and the evolution of the particle-substrate interface. Typically, isolating the impact conditions of any one particle during a coating application is impossible, and other “few-particle” experiments still lack the detail to decouple the variables. We first revisit our developed model for the onset of bonding and modify it for the new scenario to show that “peening”, the deformation of the underlying particles by the incoming high-velocity impacts typically associated with bonding enhancement in cold spray, is a two-step process. Finally, the effects of surface roughness and hardness on particle-particle adhesion are decoupled. These comprehensive 3D studies of several phenomena help inform the solid-state manufacturing process.