The role of elastic barriers in glass vitrification

Glasses produced by physical vapor deposition (PVD) onto rigid temperature-controlled substrates have been shown to use this surface equilibration mechanism to access low-energy states, producing glasses with high density and improved stability. This process can also imprint structural features that don’t otherwise exist in liquid-cooled glasses. The most optimized stable PVD glasses have been shown to have properties analogous to glasses aged for ~1-100 million years.



We have recently shown that, upon deposition on soft substrates, the surface mobility is further enhanced up to 200 nm away[1]. The mobility enhancement and increased thickness of the mobile surface have a cumulative effect of slowing down the deposition rate by ~7 orders of magnitude. For a similar degree of equilibration, it would take 3000 years to deposit a 200-nm layer on a rigid substrate, as opposed to 1 hour on the soft substrate. To investigate the role of elasticity in this phenomenon, we studied the interplay between aging and transformation kinetics in bilayer PVD glasses (150 nm SG deposited on 50 nm LQG) deposited on rigid and soft substrates. The results showed that the kinetics of the bilayer interface are significantly influenced by the rigidity of the substrate 100 nm away. These effects become increasingly pronounced at lower temperatures or for thinner LQG layers. By comparing the activation energy for relaxationon on soft and rigid substrate, we directly measure the elastic barrier and show that the barrier size crosses that of the local barrier just below T_g.