Development and Prospect of Two-step Chemically Strengthened Cover Glass Composition and Process

Alkali alumina silicate glass (R₂O-Al₂O₃-SiO₂, R is Li/Na/K either or combination) is an important screen protected material for display terminals. To further improve the scratch resistance and impact resistance of cover glass, the two-step chemically strengthened process has become a research direction. In this paper, the characteristics of surface compressive stress (CS) and depth of layer (DOL) were compared with different glass systems and chemical compositions. Results show that after two-step chemical strengthened process of Li₂O-free glass, the CS value on the surface is reduced compared with the first step, and the maximum stress value migrates into the interior of the glass, preventing the glass microcrack from expanding to the inside. After two-step chemically strengthened process of Li₂O-containing glass, the surface CS and DOL of the glass increase simultaneously, indicating that the principles of the two chemically strengthened methods are different. The Li₂O-free glass, when in the first step of chemically strengthened process, the K⁺ in the pure KNO₃ molten salt exchanges Na⁺ on the surface of the glass, at that time CS reaches its maximum, and in the second step, added some NaNO₃ in the molten salt. It reduces the K⁺ concentration gradient, so that the K⁺ accumulated on the glass surface in the first step is exchanged by Na⁺ in the molten salt. Li₂O-containing glass, used KNO₃ and NaNO₃ mixed molten salt when in the first step of chemically strengthened process, the main purpose is to exchange Na⁺ in molten salt with Li⁺ in glass, and then the molten salt with higher KNO₃ ratio or pure KNO₃ is used in the second step of chemically strengthened process, so that K⁺ can be exchanged with Na⁺ entering the glass surface. The main purpose is to increase CS and DOL. At the same time, this paper describes that the chemically strengthened process of alkali alumina silicate glass should return to the one step ion exchange process because of complex process and cost of production.