Reedel, 1. märtsil 2019 kell 14.15 toimub Physicumis, W.Ostwaldi 1 seminariruumis D312
Laserspektroskoopia erakorraline seminar No 268
Esineb Prof Vincenzo M. Sglavo
Dipartimento di Ingegneria Industriale, Universita' degli Studi di Trento
Polo Scientifico e Tecnologico “ F. Ferrari”
Via Sommarive, 9 - I-38123 Trento (Italy)
Chemical strengthening of silicate glass: processing issues
Ion-exchange process has gained remarkable interest during the last years for chemical strengthening silicate glasses because of its suitability and flexibility in the reinforcement of components with different geometry and thickness. During a typical industrial process,sodium atoms contained in the glass are substituted by potassium ions diffusing from molten potassium nitrate at temperatures below the strain point of the glass, thus creating a bi-axial residual compressive stress in the material surface which strengthens the component.
The basics of the ion exchange process are initially discussed in this presentation: then, two main aspects regarding processing issues and innovative procedures are discussed. At first, an interesting aspect regarding the presence of impurities in the bath, introduced with the raw salt or accumulated during the process and responsible for the resulting chemical, physical and mechanical performance is pointed out. Analyses of commercial soda lime silicate float glass and sodium borosilicate glass are reported and the effect of variable sodium, magnesium and calcium concentration in the molten bath on the efficiency of the ion exchange process is analyzed. The performances of chemically strengthened samples in terms of potassium penetration profile, residual stress and mechanical strength are studied. The addition of limited quantities of silica is also advanced as possible remedial action for non-efficient salts.
Then, an innovative process where an electric field - assisted ion exchange is carried out to enhance sodium-potassium interdiffusion and improve the mechanical performances of soda-lime-silicate and soda-borosilicate glass is presented. Electric fields with variable intensity are applied cyclically in both direct and inverted polarization. Compression test and Vickers indentation are used to characterize the mechanical performances of the strengthened material. Energy Dispersion X-ray Spectroscopy are also carried out to determine the potassium concentration in the surface layers of the samples. It is shown that the application of the electrical field allows to enhance Na-K interdiffusion to a large extent, thus reducing the treatment time well below the hour. The cyclic process allows to reinforce both surfaces thus allowing to reach very high mechanical strength.