Ion-exchange induced mechanical properties enhancement and microstructural modifications of transparent lithium aluminosilicate glass-ceramics

Alkali-aluminosilicate glass and glass-ceramics can be effectively strengthened by ion-exchange if compression is introduced into the surface, which have been widely used in many electronic devices, especially for mobile phones. In this work, the transparent lithium aluminosilicate glass-ceramics with eucryptite crystalline phase were ion-exchanged in the mixed x NaNO3 + (100－x) KNO3 (wt%, x = 60, 70, 80, 90, 100) molten salts at 420 °C for 4 h. With the increase of NaNO3 concentration in the molten salts from 60 to 100 wt%, the depth of stress layer increase from 98 to 104 μm, and the surface compressive stress increase to the maximum ∼500 MPa at x = 80 and then decrease. In addition, there is also a K+-rich layer observed with the depth of ∼25 μm. The ion-exchange takes place in both crystalline and residual glassy phases, and thus leads to the microstructural modifications, including the amorphization of eucryptite and the presence of lithium phosphate. Upon ion-exchange, the Vickers hardness of glass-ceramics increase from 7.03 GPa to the maximum ∼7.62 GPa (x = 80), and the bending strength from 198 MPa to the maximum ∼510 MPa (x = 90). The improvement in mechanical properties is mainly related to the generation of surface compressive stress layer. Meanwhile, the amorphization and transformation of crystalline phase during the ion-exchange process are probably conductive for the reduction of surface cracks and defects, leading to the enhancement of mechanical properties, especially for the bending strength.