Confinement Effects on the Spatially Inhomogeneous Dynamics in Metallic Glass Films

This work develops the elastically collective nonlinear Langevin equation theory to investigate, for the first time, the glassy dynamics in capped metallic glass thin films. Finite-size effects on the spatial gradient of structural relaxation time and glass transition temperature (Tg) are calculated at different temperatures and vitrification criteria. Molecular dynamics is significantly slowed down near rough solid surfaces, and the dynamics at location far from the interfaces is sped up. In thick films, the mobility gradient normalized by the bulk value obeys the double-exponential form since interference effects between two surfaces are weak. Reducing the film thickness induces a strong dynamic coupling between two surfaces and flattens the relaxation gradient. The normalized gradient of the glass transition temperature is independent of vitrification time scale criterion and can be fitted by a superposition function as the films are not ultrathin. The local fragility is found to remain unchanged with location. This finding suggests that one can use Angell plots of bulk relaxation time and the T-g spatial gradient to characterize glassy dynamics in metallic glass films. Our computational results agree well with experimental data and simulation.

Automated summary

This work develops a new theoretical model to investigate the glassy dynamics in metallic glass thin films. The study finds that the thickness of the films has a significant impact on the spatial gradient of structural relaxation time and glass transition temperature. Moreover, reducing the film thickness enhances the dynamic coupling between two surfaces and makes the gradient of the glass transition temperature more uniform. These findings are important for understanding the properties of metallic glass thin films.