GB1467459A - Glass ceramics and to the coating of metal articles with glass-ceramic - Google Patents

Abstract

1467459 Glass-ceramic ENGLISH ELECTRIC CO Ltd and CANNON INDUSTRIES Ltd 10 Oct 1975 [5 Dec 1974] 52638/74 Heading C1M A glass-ceramic consists of, in per cent by weight: SiO 2 55-67, Al 2 O 3 4-10, Li 2 O 10-15, Na 2 O 0-4, K 2 O 1-6, B 2 O 3 0-12, ZnO 0-12, P 2 O 5 1-4, TiO 2 0-2, ZrO 2 0-6, one or more colouring oxide(s) 0-4, and one or more other divalent metal oxides 0-10. The colouring oxide(s) may be one or more oxides of Co, Ni, Mn, Cr and Fe, and the divalent metal oxide(s) may be one or more of CaO, SrO and BaO. A coating of the glass-ceramic on a metal (e.g. mild steel) may be formed by: forming a suspension in a liquid (e.g. water or a wateralcohol mixture) of the corresponding devitrifiable glass in powder form; applying the suspension to the metal; drying; and firing in a single stage to coalesce the glass particles and to devitrify the glass, the maximum firing temperature being below that at which the metal may deform (preferably in the range 800- 870‹ C.) and the maximum temperature being maintained until a continuous glass-ceramic layer is formed (e.g. up to 30 minutes). The glass particles may be such as to pass a 200 mesh B.S. sieve, and the firing may be in air or in an air-nitrogen mixture. One or more glass or glass-ceramic ground coats may be applied to the metal (e.g. a glass consisting of, in weight per cent: SiO 2 , 48À5, Al 2 O 3 6À4, Na 2 O 14À7, K 2 O 4À4, B 2 O 3 16À0, CaO 4À0, F 2 3À7, Co 3 O 4 0À6 and MnO 2 1À7). A plurality of glass-ceramic coating layers may be applied, each layer being fired before the next layer is applied, and the firing temperature of any layer not exceeding that of the first layer. The glass-ceramic coating preferably has an expansion coefficient in the range 80-120 Î 10<SP>-7</SP>/‹ C. (20-500‹ C.), so that the expansion mismatch with steel creates a compressive stress coating layer.