Strengthening of Soda-Borosilicate Glasses by Ion Exchange Processes Ali Talimian

175 Pages·2017·4.51 MB·English

by  Ali Talimian

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Doctoral School in Materials, Mechatronics
and Systems Engineering

Strengthening of Soda-Borosilicate Glasses by Ion
Exchange Processes

Ali Talimian

December 2016

STRENGTHENING OF SODA-BOROSILICATE GLASSES BY ION
EXCHANGE PROCESSES

Ali Talimian
E-mail:[email protected]

Approved by: Ph.D. Commission:
Prof. Vincenzo M. Sglavo, Advisor Prof. Gian Domenico Soraru,
Department of Industrial Engineering Department of Industrial Engineering,
University of Trento, Italy. University of Trento, Italy.

Prof. Rodrigo Moreno,
Instituto de Ceramica y Vidrio CSIC,
Madrid, Spain.

Prof. Enrico Bernardo,
Department of Industrial Engineering
University of Padova, Italy.

University of Trento,
Department of Industrial Engineering

December 2016

University of Trento - Department of
Industrial Engineering

Doctoral Thesis

Ali Talimian 2016
Published in Trento (Italy) – by University of Trento

ISBN: - - - - - - - - -

To my family
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Abstract:

Flexible electronics and displays rely on strong thin borosilicate glasses. Furthermore,
borosilicate glass play a vital role in pharmaceutical packaging, particularly for the
container of liquid medicine in the auto-injectors. Because of the risk of failure due to
the fracture of the glass ampule the costumers need to purchase several units of auto-
injector; also, the injector price increases dramatically. This renewed the interest in the
strengthening of soda-borosilicate glass. Moreover, the applications of strong thin
borosilicate glasses in flexible electronics attracted attention. Chemical strengthening
is a practical means of improving the mechanical performance of soda borosilicate
glasses.
The chemical strengthening process involves the immersion of an alkali-silicate glass
in a molten nitrate salt containing potassium ions at temperatures below the glass-
transition temperature where the replacement of small alkali ions in the glass with
larger potassium ions from the molten salt occurs. The glass composition, salts
impurities, temperature and time, are crucial factors of the treatment. Furthermore,
applying an electric field can speed up the process and improves the efficiency. This
study investigates the impact of potassium for sodium ion exchange and its
parameters on the final strength of alkali borosilicate glasses.
Alkali borosilicate tubes, used in pharmaceutical packaging, were subjected to ion
exchange in potassium nitrate salts containing different impurities. The initial surface
flaws have a significant impact on the final strength due to the limited case depth of
surface compression in alklai borosilicate glass subjected to ion exchange.
Nonetheless, the results revealed that the sodium poisoning of salt has a limited
influence on strengthening; conversely, even a small amount of calcium spoils the
strengthening. The replacement of sodium ions with calcium is thermodynamically
favoured with respect to Na/K ion-exchange. Calcium can penetrate into the glass
surface and prevent the replacement of sodium with potassium and, consequently, the
generation of compressive stress.
Interestingly, performing electric field assisted ion exchange, EF-IE, for 10 min
produces an ion-exchanged layer as deep as conventional strengthening for 4 hours
in soda borosilicate glass. Electric field assisted ion exchange also augments the glass
strength and makes the glass more damage resistant; however, the initial defects on
the glass surface have an adverse influence on the efficiency, as expected.
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Applying an electric field changes the governing mechanism of ion exchange and
accelerates the penetration of potassium ions into the glass; furthermore, the glass
structure of the layer undergone electric field assisted ion exchange is modified during
the process. Although EF-IE generates a strong surface compression in glass, the
inhomogeneous distribution of residual stress is a drawback. Performing EF-IE using
AC E-fields produces homogenous ion-exchanged layers in glass and is, probably, a
practical approach to balancing the residual stress in glass.
Chemical strengthening of a thin alkali-borosilicate glass ,D 263 Teco®, is also
investigated in the present work. Na/K ion exchange improves the glass strength three
times. Although the surface compression generated by ion-exchange in alkali
borosilicate glasses is not as strong as typically used glasses for chemical
strengthening ( alklai aluminosilicate), it can be used to improve the mechanical
properties of borosilicate glass. Annealing prior to the ion exchange increases the
compressive stress generated on the surface; however, its effect on strengthening is
trivial.
The compressive stress produced by Na/K ion exchange in thin alkali borosilicate
glass improves the damage resistance and the bending strength of glass. Due to the
limited thickness of samples, heat treatments with high heating and cooling rates can
be conducted. Such heat treatments can be used to carry out surface relaxation and
improve the strength of samples by “surface relaxation”. A fast heat treatment after ion
exchange improves the finals strength samples about 40%.

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