November 7, 2024 10:30 am Wu and Chen Auditorium
Dean, Robert R. McCormick School of Engineering and Applied Science, Northwestern University
“Supersonic collisions of microparticles on metal: In-situ studies at the nanosecond and micrometer scales”
Abstract: There are many situations in which small particles impact metals at high speeds, even above the speed of sound. Sometimes these are unintentional (as for foreign object damage or micrometeorite strikes), and sometimes they occur by design (as for surface treatment processes like abrasive spray, peening, or spray coatings). The fundamental physics behind supersonic impacts, 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). In this range, conventional plasticity gives way to ballistic dislocation motion, with counterintuitive “anti-thermal” behavior in which hotter metals are stronger. For metallic particles, our approach 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.