Ion exchange processes on float glass surfaces

Abstract: Glass can be strengthened by ion exchange and this process is presently used inspecial applications e.g. aircraft windshields, displays and spectacle lenses allowinga higher production cost. Chemically strengthened float glass is moreexpensive than thermally strengthenened, but will likely find applications in futurebuilding and interior constructions where strength demands, design andshape prevent the use of thermal strengthening. The aim of this work is tostudy ion exchange on float glass surfaces. In longer terms, the chemicalstrengthening is planned to be applied to specific critical area e.g. around adrilled hole which without treatment deteriorates the overall strength of theglass.Strengthening the glass through ion exchange can be done in several ways butis most often referred to as the replacement of smaller ions in the glass structureby larger ions from the salt used for treatment. By determining concentrationvs. depth profiles of ion exchanged float glasses, it is possible to calculate thediffusion coefficients and activation energy for different ions. In this study, theless frequently studied approach single-side ion exchange of different ions ofcommercial float glass is described. The concentration vs. depth profiles weredetermined either by the use of the Surface Ablation Cell (SAC), which allowsthe continuous removal of the material from a flat glass surface by slow controlledisotropic dissolution or SEM-EDX.The results of the work are that similar diffusivities and concentration vs. depthprofiles are achieved with single-side ion exchange as from the traditional wayof immersing glass in molten salt bath. Ion exchange of Ag+ stains the floatglass on both sides giving it a yellow or amber-brownish colour. Unlike Ag+ ionexchange of Cu+ stains the float glass on the tin-side only, giving it a yellow,red or red-brown colour. Determining the concentration vs. depth profiles ofion exchanged float glasses with the SAC was convenient except for Ag+ whichwas determined with SEM-EDX. The work confirms that the procedure andequipment of the SAC are very cheap, easy to use and gives data similar tothose gained by much more expensive equipment. Calculated diffusion coefficientsof K+, Ag+ and Rb+ are in accordance with literature data while Cu+ and Cs+ diffusion coefficients were slightly lower. The diffusion coefficients of the different ions follow the order Ag+>K+>Cu+>Rb+>Cs+ and ranges between9.4E-10 and 4.8E-13 cm2s-1. The calculated activation energies for diffusion of K+, Ag+ and Cu+ corresponds with reported literature data and were calculated to: Ag+(air-side) 152 kJ/mol, Ag+(tin-side) 185 kJ/mol, K+ 108 kJ/mol and Cu+115 kJ/mol.