ICMCTF 2022 Session PL-MoPL: Plenary Lecture: The Fundamental Physics of Spray Coatings and Surface Impacts: Unit Process Studies of Hypersonic Particle Impacts

Many surface treatment processes involve impact events, including abrasive spray, peening methods, or spray coatings. The fundamental physics behind such processes, including deformation, bonding, and coating development, however, remain mysterious; the impacts are extremely fast and involve microscopic particles, so that they are challenging to resolve. This talk will review a new line of research aimed at understanding the unit process of particle impacts at velocities into the supersonic range—we study individual ~5-50 µm particles and record their approach and impact with a substrate using an all-optical single-particle test method with nanosecond time resolution. For hard particles, this method leads to quantitative measures of plasticity at extreme rates (>10⁷ s^-1). For metallic particles, it quantitatively reveals the changes in plasticity that occur as particles approach the threshold velocity for bonding, as well as other deleterious transitions such as impact-induced melting and erosion. When combined with post-mortem characterization, details on microstructural evolution in extreme conditions can be discerned, including, e.g., dynamic recrystallization by a new mechanism that emerges at high rates, or the fracture and delamination of nanoscopic surface oxide layers.