Applying molecular dynamics simulations to the study of micro cold spray of yttria



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Micro cold spray (MCS) is a novel manufacturing process with proven success depositing dense, thick films of technical ceramics without the need for elevated processing temperatures or chemical precursors. The process transforms a loose submicrometer powder into an adherent film with mechanical properties approaching that of the bulk material through a phenomenon known as room temperature impact consolidation. While this phenomenon has been demonstrated for a wide variety of ceramic materials and film substrates, yttria stands out as a material that is both well-characterized in experiments and of significant interest for industrial applications due to its excellent refractory properties. This dissertation seeks to investigate the fundamental mechanisms that enable room temperature impact consolidation by using molecular dynamics (MD) simulations of yttria particle impacts and characterizing the deformation behavior of the system. The behavior of yttria nanoparticles impacting yttria substrates is first explored by systematically varying the impact velocity (300-1200 m/s), particle diameter (5-8 nm), and orientations of both the particle and substrate. Once the key variables have been identified, further studies were performed using much larger particles (12-50 nm), which unlike the smaller particles, exhibited shear localization. This is a key phenomenon responsible for particle sticking during cold spray of micron-scale particles used in the cold spray process. Thereafter, a plane strain geometry was developed to further extend the particle sizes accessible to MD simulations (25-200 nm), which showed that for particles with similar sizes to those used in experiments, the deformation behavior is dominated by shear localization and fragmentation. Finally, a novel technique for equilibrating ionic MD simulations that requires ∼1250 times less computational resources than the previous methodology is presented.


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