Chemical Strengthening of Alkali-Free Glass via Pressure Vessel Ion Exchange

Ion exchange is a popular technique for chemically strengthening alkali-containing glass articles, such as Corning^® Gorilla^® Glass. The ion exchange process is based on a replacement of small alkali ions in the glass with larger alkali ions from a molten salt bath through inter-diffusion. As the larger alkali ions from the salt bath supplant the smaller alkali ions in the glass, a compressive stress profile is generated near the surface of the glass, which increases its strength and damage resistance. However, certain applications of high-tech glasses require alkali-free environments, such as glasses used as substrates for flat panel display applications. In this paper, we report the first successful chemical strengthening of an alkali-free glass. This is achieved via an aqueous ion exchange of barium salts under high pressure and temperature. X-ray photoelectron spectroscopy reveals that Ba²⁺ replaces both Ca²⁺ and B³⁺ in the glass, producing surface compressive stress values near 200 MPa. This technology may enable chemical strengthening for a wide range of applications, including flat panel display substrates.