STRENGTHENING OF SODA LIME SILICA GLASS BY SOL GEL-DERIVED AND MELT-DERIVED COATINGS

Soda-lime-silica glass rods were coated with sol-gel-derived 0.3-0.6 mum thick lead silicate and lead borosilicate glass layers. The modulus of rupture of the rods increased with decreasing thermal expansion coefficient (TEC) of the coating. The largest strength of 545 MPa, a 77% increase over uncoated rods, was obtained with the lowest TEC (40 X 10(-7)degrees-C-1) coating, strengthening being attributed to a combination of flaw filling and to compressive stresses in the coating arising from its TEC which is lower than that of the rods. A strength increase of 29% for a coating with a TEC (104 X 10(-7)degrees-C-1) higher than that of the rods was attributed to flaw filling minus the effects of tensile stresses in the coating arising from the TEC difference. The lowest expansion coatings showed an increase in Weibull modulus, indicating narrower strength distributions. Coatings, 200 mum in thickness, of a 48ZnO.10PbO.42B2O3 glass were prepared by melt dipping. Strength increases of 62 and 148% were obtained for coatings on as-received and on abraded rods, respectively. The mean strength of melt-coated rods given a final abrasion was 294 MPa, 131% higher than that of uncoated abraded rods. The larger strength retention compared with that of sol-gel coatings may be attributed to the much thicker melt-derived coatings. Significant improvements in bursting strength were obtained for sol-gel coated soda-lime-silica and Pyrex test tubes.