Thermal Shock Behavior of a Soda Lime Glass Treated by Ion Exchange

Abstract

In this work, we have submitted a soda lime glass to thermochemical treatment by ion exchange in order to introduce compressive stresses in their surface. Subsequently, we studied the hot-cold thermal shock resistance of ion-exchange reinforced glass. A potassium ion from the molten salt bath replaces the sodium ion of the glass surface by a superficial diffusion process. We have determined the potassium diffusion profile by two techniques: X-ray fluorescence spectroscopy XRF and X-rays microanalysis coupled with Scattering Electronic Microscopy. In addition, for the same purpose, we used X-ray fluorescence spectroscopy. Then, we determined by micro-indentation the profile of the compressive stresses induced in the glass surface by the exchange of Na+ ions by larger ones of K+. The compressive stress profile induced by chemical tempering depends on potassium ion concentration. It reaches a maximum value of 988 MPa for a treatment done during 50 hours at 480°C. We have subjected the glass specimens treated by ion exchange to a hot-cold thermal shock technique; we realized the cooling by air jet at room temperature. The heat transfer coefficient was about 600 W/m2 °C (Biot number β = 0.3). We have determined the residual strengths of the ion exchange treated and thermal shocked specimens by a three-point bending test. The strength is improved; it passes from 90 MPa of the as received glass to 380 MPa for the chemically treated glass. We noticed that the critical temperature difference ΔTc, which caused the thermal shock fracture, depends on the glass samples states. Consequently, because of the thermochemical treatment, ΔTc has shifted to higher temperatures. Its value was 254°C for raw glass and reached 455°C for the case of reinforced glass by ion exchange at 480°C during 50 hours. The thermochemical ion exchange treatment improve the glass thermal shock resistance.