Characterization of near surface mechanical properties of ion-exchanged glasses using Surface Brillouin Spectroscopy.

The technique of Surface Brillouin Spectroscopy is attracting considerable attention for the non-destructive characterization of near-surface properties that depend upon the elastic behaviour. These include surface strengthening, delamination in metallised layers and structural changes near surfaces. We here present a study of the chemical strengthening of glass. SBS measurements of surface acoustic wave (SAW) velocity in glasses strengthened by the exchange of K(+) for Na(+) ions were carried out using the Bede BriSc instrument. The study of surface acoustic waves on a transparent material such as glass was made possible by depositing a thin layer of aluminium on the glass surface. At a working frequency of 20 GHz, the optimum thickness of the layer was found to be 30 nm. The replacement of Na(+) by K+ ions that takes place during the treatment of the glass causes near-surface modifications of both density and elastic constants. An increase of 3.8% in density due to the replacement of light Na(+) ions by heavier K(+) ions at the surface could account only for a 1.9% decrease in the surface acoustic wave velocity. Residual stresses were estimated to have a negligible effect on surface acoustic wave velocity. At the same time a 2.8+/-0.4% decrease in the surface acoustic wave velocity due to the ion-exchange process was observed by SBS. This result suggests that a significant part of the surface acoustic wave (SAW) velocity change should be attributed to variations in the elastic constants (approximately 1.8%) and agrees well with independent estimates of the Young's modulus (E) and shear modulus (C(44)) changes caused by replacing the Na(+) ions by K(+) ions during the process.