STRENGTHENING SODA-LIME-SILICA GLASS BY A LOW-EXPANSION COATING APPLIED BY MELT DIPPING

A method of strengthening glass by application of a low-expansion glass coating is described. Soda-lime-silica glass rods were dipped vertically into a low-viscosity borate glass melt at 1140 to 1200-degrees-C and rapidly withdrawn. Uniform well-bonded coatings, 100 to 200-mu-m in thickness, were produced. The modulus of rupture (MOR) of unabraded rods coated with a 60 ZnO-40 B2O3 (wt%) glass (glass 1) was 548 MPa, compared with 225 MPa for a control set of uncoated rods; coated rods after abrasion gave 343 MPa compared with 110 MPa for abraded uncoated rods. Rods coated with glass 1 fractured uniformly into small pieces in a manner similar to thermally toughened glass. Approximate calculated axial stresses, using thermal expansion and other data, were 233 MPa (compressive) in the coating (glass 1) and 56 MPa (tensile) in the substrate rod. The magnitude of the calculated axial compressive stress in the coating was in good agreement with the increase in average strength of the rods after melt dipping. The results indicate that the fracture origin was probably at the outer coating/air surface and not at the substrate/coating interface, and that the flaw length was less than the coating thickness. Severe flaws on the original rod surfaces were probably filled during melt dipping, rendering them inoperative.