Study of the Effect of Chemical Tempering on Flexural Strength of Soda Lime Silica Glass

Abstract

In the present work an endeavour has been made to enhance the flexural properties of glass thereby imparting higher fracture strength. The work mainly focusses on increasing the strength through the process of chemical tempering which involves the exchange of Sodium ions at the glass surface with Potassium ions to generate compressive stresses which restricts crack growth and crack propagation thereby strengthening the surface. The compressive stress generated also imparts crack bridging effect for the existing cracks in the glass surface leading to further strengthening effect. In this study soda lime silica glasses were treated in a molten bath of Potassium Nitrate (KNO3) at two different temperatures ranges of 4500C and 5000C for a treating time of 2,4,6,8,16 and 24 hours. The treated glass pieces were subjected to Three Point Bending studies to determine flexural properties and to study the effect of tempering time and temperature on flexural behaviour of glass. It has been observed that flexural strength increase with temperature as well as with increase of treatment time (Fig 1.1)
UV-Visible spectroscopy was conducted to check the effect of tempering on optical properties. EDX/Flame Photometry studies were done to confirm the presence of potassium ions in glass thereby accounting for the strength increment. Apart from the characterization studies Finite Element Modelling was done to determine the stress and stress distribution profiles in glass subjected to loading conditions. A Model has been developed with the concept of 3 layered glass structure. During the modelling the glass was considered to be composed of three different layers, the top and bottom surfaces and the bulk being potassium and sodium rich respectively. The modelling was done based on assumption of 1:1 replacement of sodium and potassium and uniform penetration depth of potassium ions at the surface. The depth of the potassium layer was calculated according to the case depth obtained through different tempering times. The penetration depth was determined according to the complementary error function formula:
Where x represents the penetration depth, D the inter-diffusion coefficient and t is the tempering time. The schematic of the designed model and variation in the data obtained from the theoretical and experimental values are depicted in the figure below:
In addition the effect of double tempering (thermal tempering at 600oC followed by chemical tempering at 500oC on flexural properties of soda lime silica glasses has also be investigated. The obtained data were compared with that of single ion exchange and it has been found an increase in flexural strength in all samples.