STRENGTHENING OF SODA LIME SILICA GLASS BY GEL-DERIVED ZINC BOROSILICATE AND SODIUM ALUMINO-BOROSILICATE GLASS COATINGS

Glass rods and slides were coated with gels of compositions (mol%) 40ZnO-30B2O3-30SiO2 (ZBS) and 15Na2O-15Al2O3-10B2O3-60SiO2 (NABS) by dipping in solutions containing metal alkoxides and salts. After drying and firing at 600-degrees-C transparent crack-free glassy coatings of ZBS up to 0.6-mu-m thick were obtained for a single dipping and up to 4-mu-m thick for multiple dipping. The NABS coatings were about 0.3-mu-m for one dipping. The modulus of rupture of rods after one coating of ZBS fired at 600-degrees-C was 577 MPa, an increase of 87% over uncoated rods. Strengthening was attributed to flaw filling and to compressive stresses in the coating because of its low thermal expansion relative to the rods. NABS coated rods fired at 600-degrees-C showed a 39% increase in strength over uncoated rods. This was attributed mainly to flaw filling rather than compressive stresses because the expansion coefficient of the coating was similar to that of the rods. Significant but smaller increases in strength were obtained for abraded rods after one coating of ZBS or NABS because of incomplete filling of flaws on the severely damaged surfaces. After coating, Weibull plots demonstrated a narrowing of the distribution of rod strengths and a reduction in the low strength tail.