Measuring Hardness of Stable Glasses Using Nanoindentation*

It has been shown that the process of Physical Vapor Deposition, or PVD, can form glasses with enhanced stability compared to liquid-quenched glasses. This occurs when the substrate temperature is held close to 85% of the glass transition temperature and the deposition occurs sufficiently slowly. These stable glasses have been shown to exhibit higher density, lower enthalpy, and better kinetic stability over ordinary glasses. Given these exceptional properties, it is of interest to investigate other properties that may be affected by PVD. Some work has shown that the speed of sound and elastic moduli increase with increased stability for some glass formers. Here we directly measure mechanical properties of stable glasses using nanoindentation. We investigate the relationship between mechanical properties and stability of several stable glass-forming molecules with various shapes and aspect ratio. Elastic modulus and hardness of these molecules are measured as a function of deposition temperature. It is found that the molecular structure can play an important role in the stability and mechanics of PVD glasses.