Determination of the Yield Stress of Borosilicate Glasses by Means of Spherical Nanoindentation

From the advent of mechanical tests conceived to measure material deformation characteristics by pressing an indenter into a surface, the desire to relate the prima facie quick and simple test back to more traditional testing methods and common material mechanical properties has persisted. For brittle materials, a particularly pernicious property to measure using conventional means is the yield stress, often neglected entirely in favor of fracture stress. Nevertheless, the yield stress is still an essential property necessary to describe the initial deformation of brittle materials, such as the response to a ballistic impact. The yield stresses of a suite of three very low alkali sodium borosilicate glasses, candidates for transparent armor, were determined by means of a novel methodology involving spherical nanoindentation. A theoretical description of this new method is provided alongside corroboration with uniaxial compression tests. The practical effect of water interaction during mechanical surface preparation of the glasses on measuring indentation-derived mechanical properties was analyzed. Connections between differences in yield stress for the three glasses were established on the basis of compositional and structural features elucidated through vibrational spectroscopy. The inclusion of high pressure double toroid diamond anvil cell compression of the glasses allowed extension of the yield stress analysis and correlated structural elements to instances of permanent compaction, a common deformation mode of amorphous materials. Together, this work represents a comprehensive inquiry into the nature of the yield stress for three borosilicate glasses and a process by which the yield stress itself may be determined using spherical nanoindentation.