Study on Molecular Dynamics of Grain Regulation in Polycrystalline Tin under Plasma Modification and Annealing Processes

Soft polycrystalline tin, owing to its exceptional physical properties, finds various applications, such as energy and medical devices. However, achieving superior surface quality in machining of this material remains challenging due to the deleterious impact of grain boundary (GB) step effects on surface roughness. This study systematically investigates the mechanistic interplay between plasma modification and annealing treatments on GB evolution in polycrystalline tin, employing molecular dynamics simulations to elucidate atomic-scale behaviors. Complementary experimental comparisons of grain growth dynamics are conducted using electron backscatter diffraction and white-light interferometry for microstructural and topographical characterization. Results demonstrate that while thermal annealing induces moderate grain coarsening through temperature-driven GB migration and subsequent boundary reduction, plasma modification introduces a distinct transformation pathway. The localized thermal effects of plasma treatment enable directional grain growth via transient high-temperature melting-induced grain fusion, yielding more pronounced grain enlargement and significant GB density reduction. It is confirmed that single-point diamond turning of plasma-modified specimens is capable of achieving sub-nanometer surface roughness, surpassing the performance of conventionally annealed counterparts.