| The exchange of small alkali ions in glass with larger ions from
a molten salt bath below T9 produces surface compression in
the glass surface, which results in glass strengthening. Two
competing processes occur: (1) generation of stress due to ion
stuffing and (2) relaxation of the same stress due to viscous
flow and network ductility. The economics of superior protection
of glass products under common challenging environments.
generally require a compromise between obtaining greater
depths of compression (case depth) versus aiming for higher
compression magnitudes. Control of process parameters such
as bath temperature and composition is essential. Methods of
quality control include measuring the modulus of rupture (MOR),
surface compression and case depth. The newly instituted ASTM
standard C1422-99 classifies chemically strengthened flat glass
products on the basis of the surface compression magnitude
and the case depth. Aircraft windshields, watch covers, autoinjector
glass syringes and high speed glass centrifuge tubes
are examples of chemically strengthened glass products wellaccepted
in the market place. Products such as laminated chemically
strengthened architectural glass for hurricane-prone
regions and borosilicate glass single-use needleless injectors
are being developed. |
