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

Puentes, S. M. and Bradshaw, J. and Briggs, G. Andrew D. and Kolosov, Oleg and Bowen, K. and Loxley, N. (1998) Characterization of near surface mechanical properties of ion-exchanged glasses using Surface Brillouin Spectroscopy. In: Nondestructive Characterization of Materials VIII :. Plenum Press, BOULDER, pp. 817-823. ISBN 0-306-45900-0

Abstract

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.