PURPOSE: To obtain a function imparted glass sheet having improved mechanical strength by forming a compressive layer by tempering the surface of a glass substrate.
CONSTITUTION: A glass substrate is immersed in a molten alkali salt bath at a temp. below the transition temp. of the glass to exchange alkali ions small in ionic radius in the surface of the substrate for alkali ions large in ionic radius in the bath. By this exchange, a layer having complessive stress with densely filled alkali ions large in ionic radius is formed in the surface of the glass substrate. molten potassium nitrate or the like is used as the molten alkali salt. A glass sheet having such strength as to well withstand residual tensile stress produced at the time of forming a functional film having a low coefft. of expansion on the surface of the glass substrate and microflaws and strain due to surface roughening is obtd.
OBARA KATSUMI
KINUGAWA KIYOSHIGE
TSUKIZAKI YOSHIYUKI
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