Chemical tempering of soda lime silicate glasses by ion exchange process for the improvement of surface and bulk mechanical strength

The aim of this work is improving mechanical properties of soda-lime glass by imparting compression stress on the surface through ion exchange and investigate the effect of process parameters on chemical strengthening. KNO3 salt bath was used for ionic exchange at varying temperatures and durations. Treated samples were investigated for their optical transmittance, surface potassium ionic concentrations and concentration profiles from surface to bulk using EDS and line scan EDS techniques. Effect of process temperature and time on the amount of mechanical strengthening was determined through the bending strengths (four-point), scratch resistances and indentation behaviors as micro hardness, crack formation load limit and cracking probability. Results were evaluated regarding ionic concentration variations. Transmittance decreased by 2% after the treatment. Surface potassium concentration increased with increasing temperature and was increased to become saturated in a short period. Potassium concentrations decreased from surface to bulk, and diffusion reached to deeper layers with increasing time and temperature. Ionic exchange improved micro hardness by 8-17%, threshold loads for indentation crack initiation 2-10 times, cracking probability down to 50%, strength up to 3.5-4 times and scratch resistance 2-4 times. Crack types changed between radial, median and lateral; depending on the degree of chemical tempering.