Strength of silicon, sapphire and glass in the subthreshold flaw region

Strength properties are determined for monocrystalline silicon and sapphire and for soda-lime glass plates containing subthreshold flaws. i.e. flaws without detectable microcracks. Nanoindentations covering a wide range of loads are used to introduce flaws of predetermined Sizes into highly polished or etched surfaces, to enable systematic study of the transition from the post-threshold to subthreshold regions. Strengths of the indented plates are measured using a simple bilayer test configuration in which the plates are bonded - indentations downward - onto a polycarbonate support base and loaded centrally at their top surfaces to failure. Failure occurs from flaw sites over the entire range of diminishing loads, up to a strength cutoff at >2 GPa associated with natural surface flaws. In none of the materials do the postthreshold strength/indentation-load data extrapolate unconditionally into the subthreshold region. However, whereas the glass data show an abrupt increase in strength at the threshold, the silicon and sapphire data show only a slight discontinuity in slope. A simple model for subthreshold flaws, based on the critical conditions to initiate radial cracking from embryonic shear faults within the contact damage sites, accounts for the main size trends in the data. The disparities between the glass and crystalline responses are attributed to differences in the precursor shear fault geometries. General applicability of the scaling concepts to a broad range of alternative flaw types, e.g. associated with microstructural defects, is discussed. Published by Elsevier Ltd on behalf of Acta Materialia Inc.