  Amorphous silica (a-SiO2) exhibits unique thermo-mechanical behaviors that
set it apart from other glasses. However, there is still limited understanding
of how this mechanical behavior is related to the atomic structure and to the
preparation conditions of a-SiO2. Here, we used electron beam (e-beam) physical
vapor deposition (PVD) to prepare a series of a-SiO2 films grown at different
substrate temperatures and then combined molecular simulations with Positronium
Annihilation Lifetime Spectroscopy and nanoindentation experiments to establish
relations among processing, structure, and mechanical response of the films.
Specifically, we found that increase in the growth temperature leads to
increase in the elastic moduli and hardness of the films. The relative porosity
in the films also increases while the a-SiO2 network itself becomes denser,
resulting in an overall increase in density despite increased porosity. In
addition, we found that the a-SiO2 films exhibit the same anomalous temperature
dependence of elastic modulus as bulk a-SiO2. However, the rate of increase in
the elastic modulus with the measurement temperature was found to depend on the
density of the a-SiO2 network and therefore on the growth temperature. Our
findings provide new insights into the influence of the atomic network
structure on the anomalous thermomechanical behavior of a-SiO2 and in turn
guidance to control the mechanical properties of a-SiO2 films.
