Ultralow expansion (ULE) glass, a
binary TiO2–SiO2 glass with 5.67 mol
% TiO2, was exposed to microindentation.
Vitreous silica was similarly treated and used as a reference material,
including the characterization of mechanical properties by means of
ultrasonic echography and nanoindentation. The structural modifications
induced by indentation were analyzed by micro-Raman spectroscopy.
The observed structural changes are consistent with an anomalous,
densification-driven, deformation mechanism similar to those observed
for vitreous silica or commercially relevant low alkali borosilicate
glasses like Duran. As for these fully polymerized glasses, the Raman
spectra of indents in the ULE glass are characterized by an upshift
of the 407 cm–1 band and an increase in the intensity
of the D1 and D2 defect bands, all consistent
with structural rearrangements from mostly larger five- and six-member
rings to a larger population of smaller four- and three-member rings
and an overall lowering of the free volume in the glass. However,
contrary to silicon, titanium may change its coordination number under
the impact of microindentation. Raman spectra of selected reference
materials such as TiO2 and BaTiO3, with known
octahedral titanium coordination and known connectivity, as well as
fresnoite Ba2TiSi2O8 with known fivefold
Ti4+ coordination, are therefore included in this study
in support of assigning the new activity appearing in the Raman spectra
after an indentation of the ULE glass sample.