GB1108473A - Surface-devitrified glasses - Google Patents

Abstract

A strong surface-devitrified glass having a microcrystalline surface layer is made by heating a glass to 700-900 DEG C., the glass containing, in weight per cent: ZnO, 24-56; Al2O3, 3-25; SiO2, 32-54; and these constituents, with the nucleation catalyst, if any, total at least 80%, the balance, if any, consisting of other compatible constituents. If the proportion of alkali oxides is small, then ZnO\sF 32%, if Al2O3<10%, then 0%\sELi2O<2.0%. Also present may be: Li2O, Na2O, or K2O, up to 6% in total; MgO, CaO, SrO, or BaO, up to 10% in total; PbO up to 10%; B2O3, up to 10%; and, as nucleation catalysts one or more of P2O5, up to 6.0%; MoO3, up to 5.0%; and ZrO2, up to 7.5%. Preferably, the catalysts is P2O5 above, or MoO3 with P2O5 or ZrO2. The P2O3 may be added as a metallic phosphate, e.g. zinc orthophosphate Zn3(PO4)2. When the batch is melted, the atmosphere must be oxidizing or neutral if a reducible oxide, e.g. PbO, is present. The glass is formed by conventional processes into articles which may be annealed at 650-800 DEG C. and cooled to room temperature, or which may be immediately subjected to the following heat treatment process. The articles are heated at \sO 10 DEG C./min. to 700-900 DEG C., held for \sP 15 min.; the temperature is then optionally further raised to \sO 900 DEG C. and held for \sP 15 min. The atmosphere during treatment preferably contains gaseous O2 or H2O. The treated glasses have clear centres with opaque or opalescent surface layers. The crystalline phase in the surface contains zinc orthosilicate 2ZnO.SiO2 ("willemite") or an MgO/Al2O3/SiO2 crystal bearing structural resemblances to petalite (Li2O.Al2O38SiO2). The thermal expansion coefficients are 30-42 x 10-7/ DEG C. The products can be cut, ground and drilled with diamond drills. The surface compression stress is very small-about 10 lb./ sq. in.