GB1247243A - Glass-ceramic articles - Google Patents

Abstract

1,247,243. Strengthened glass ceramics. PPG INDUSTRIES Inc. 26 Feb., 1969 [1 March, 1968], No. 10316/69. Heading CIM. A strengthened bulk crystallized glassceramic article has substantially the same composition throughout, and has a surface zone in a state of compression and an interior zone in a state of tension. Such an article may be formed substantially craze-free by maintaining the absorbed water of the surface of the article at a substantially constant concentration during the crystallization period. This is thought to prevent the reaction occurring at the surface of the article. The water concentration may be maintained during the crystallization heating by adding water to the atmosphere surrounding the article so as to substantially maintain an equilibrium. The atmosphere preferably contains about 0À1 moles water per mole of gas. The furnace may be flushed with an inert gas (e.g. N 2 ) saturated with water, or water may be added directly to the furnace. An alternative way of maintaining the surface water concentration substantially constant is to coat the article before crystallization with a water-impermeable substance. Suitable coating materials are the oxides of Si, Ti, Al, Cr, Sn, In and Fe, and the metals Ti, Cr and Pt. The coatings may be applied by any suitable technique; low temperature coating or vacuum coating are preferred. By coating with TiO 2 or ZrO 2 these materials may also act as nucleating agents. Preferably the crystallizable glass composition is one which gives a crystalline phase whose coefficient of thermal expansion is lower than the vitreous phase. A preferred composition comprises (in wt. per cent) SiO 2 45-72; Al 2 O 3 15-35; Li 2 O 3-10; preferably with 1-5 each of ZrO 2 and TiO 2 .