Chemical toughening of glass by potassium diffusion: how non-bridging oxygen and a surface calcium barrier limit the process

Abstract

Chemical toughening of soda-lime glass by in-diffusion of potassium from a molten salt to replace sodium is currently restricted to applications where thermal tempering is not suitable, such as toughening of thin glass and complex shaped objects. Chemical toughening would be more attractive as a commercial process if it could be made more rapid by elevating the temperature, accelerating the diffusion process. However, when the temperature is increased, stress generation processes are accompanied by stress relief through structural relaxation, limiting the stress achieved. Here we use cross sectional microscopy with energy dispersive X-ray spectroscopy analysis and a study of the vibrational spectrum using fourier transform infrared to show that there is another effect that also plays a role in determining the in-diffusion rate of potassium. Temperatures that initiate stress relaxation also accelerate the formation of excessive amounts of non-bridging oxygen which is accompanied by an increase in the surface concentration of calcium. We hypothesize that this calcium layer creates a barrier to the migration of potassium into the glass.