Strong time-dependence for strengthening a lithium disilicate parent glass and the corresponding glass-ceramic by Li+/Na+ exchange

Chemical strengthening of a lithium disilicate (LD) parent glass with stoichiometric composition and the corresponding glass-ceramic by Li⁺/Na⁺ exchange was investigated. The ion-exchange was performed in a molten NaNO₃ salt bath at 380 °C below the glass transition temperature over a wide time range from 3.75 to 480 min. It was found that strength of both the glass and glass-ceramic could be significantly enhanced after very short time treatment (3.75-15 min); however, significant strength decreases occurred similarly for the glass and glass-ceramic with further increasing the treatment time to obtain a greater ion-exchanged depth. Morphology analysis of the ion-exchanged surfaces revealed that the over-long treatment would induce surface failure for the glass, or LD crystal micro-cracking in the ion-exchanged region for the glass-ceramic. The strong time-dependence of the strengthening was attributed to dual effects of the ion-exchange. Besides the conventional "stuffing effect" of ion-exchange, the thermal expansion increase of surface layer induced by Li⁺/Na⁺ exchange might be a critical factor to be considered for strengthening the LD glass and glass-ceramic. With prolonging the processing time, the dual effects would change the surface residual stress from compressive into tensile, resulting in surface failure and undesirable strength degradation.