Chemical Strengthening of Low-Expansion Glass Ceramics

With glasses chemical strengthening can be performed by an exchange of ions for larger ions below Tg or smaller ions above Tg. 

Chemical Strengthening of Crystallized Glass Ceramics with Large Ions Below Tg 

Low-expansion glass ceramics have a crystalline as well as a vitreous phase. In this vitreous phase normal ion exchange can take place. But for the chemical strengthening of low-expansion glass ceramics at transformation temperatures above 700 °C, salt melts of potassium nitrate cannot be used, because the salt melts of nitrates decompose at temperatures above 500 °C and produce the toxic NO . Other potassium salts such as sulphates and chlorides and their eutectics have melting temperatures above 700 °C. 

Therefore, special potassium pastes have been developed which do not decompose. They consist of a potassium salt, such as potassium sulphate (K2SO4), or a mixture of potassium sulphate and chloride (e.g., 52% in weight K2SO4 + 48% in weight KCl) and different organic solvents [3.39]. 

Chemical Strengthening of Glass Ceramics by Li Exchange Before Crystallization 

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