Europâisches Patentamt ê 0 076 692 ® European Patent Office (ÏÏ) Publication number: Office européen des brevets B1 ® EUROPEAN PATENT SPECIFICATION (§) Date of publication of patent spécification: 04.06.86 ® Intel.4: C 03 C 10/04, C 03 C 3/078, C 03 C 3/1 12, C 03 B 32/00 (3) Application number: 82305288.1 (22) Date offiling: 05.10.82 (54) Alkali métal, calcium fluorosilicate glass-ceramics and production thereof. ® Priority: 05.10.81 US 308143 (73) Proprietor: Coming Glass Works Houghton Park Corning New York 14831 (US) (43) Date of publication of application: 13.04.83 Bulletin 83/15 ® Inventor: Bead, George Halsey 106 Woodland Drive (45) Publication of the grant of the patent: Big Flats New York (US) 04.06.86 Bulletin 86/23 (74) Representative: Smith, Sydney etal (84) Designated Contracting States: Elkington and Fife High Holborn House 52/54 DE FR GB IT High Holborn London WC1V6SH (GB) (58) References cited: GB-A-1 019 667 GB-A-1 370 324 CÛ CM O) (O (O O o Note: Withm nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall CL be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been LU paid. (Art. 99(1) European patent convention). Dourier Press, Leamington Spa, England. This invention relates to alkali metal, calcium fluorosilicate glass-ceramics and to the production thereof. Glass-ceramic articles, i.e. articles produced by subjecting glass articles to a controlled heat treatment to effect crystallization in situ, are well known. The production of such articles customarily involves three basic steps: firstly, a glass-forming batch is melted; secondly, the melt is simultaneously cooled to a temperature at least below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and, thirdly, the glass body is heated to temperatures above the transformation range of the glass in a controlled manner to generate crystals in situ. Frequently, the glass body is exposed to a two-stage heat treatment. Hence, the glass will be heated initially to a temperature within or somewhat above the transformation range for a period of time sufficient to cause the development of nuclei in the glass. Thereafter, the temperature will be raised to levels approaching or even exceeding the softening point of the glass to cause the growth of crystals on the previously-formed nuclei. The resultant crystals are commonly more uniformly fine-grained and the articles are typically more highly crystalline. Because glass-ceramic articles are generally highly crystalline, viz. greater than 50%, by volume, crystalline, they are normally mechanically stronger than the precursor glass articles from which they were derived. Hence, annealed glass bodies conventionally demonstrate modulus of rupture values in the range of from 352 to 703 Kg/sq.cm. (from 5,000 to 10,000 psi), while the glass-ceramic product will exhibit moduli of rupture over the range of from 703 to 1406 Kg/sq.cm (from 10,000 to 20,000 psi). Although the latter values represent a significant improvement, numerous investigations have been undertaken to enhance the mechanical strength of glass-ceramic bodies. Two methods for achieving that goal have been utilized commercially. The first has involved applying a glaze to the surface of the glass-ceramic, which glaze has a coefficient of thermal expansion lower than that of the body. The second has comprehended subjecting the body to chemical strengthening via an ion exchange reaction. Both of those techniques are effective in increasing the mechanical strength of glass-ceramic articles, but both also have two practical disadvantages. Hence, both methods require the body to be subjected to a further process which adds to the cost of the product. More importantly, however, both procedures increase the internal tension in the body such that, upon breakage, the body tends to fragment into a large number of small pieces. This phenomenon is especially significant when the product is designed for consumer goods where "gentle" breakage is desired with a resultant few large pieces. Accordingly, a glass-ceramic having high intrinsic body strength would be most desirable. GB-A-1,019,667 relates to crystalline glass-ceramic articles exhibiting a bending strength of 2000 to 3000 Kg/cm2 and having as the predominant crystal phase solid solution of a Na2O-CaO(MgO)-Al2O3- Si02 type. The articles are produced by melting a composition comprising, in weight percent on the oxide basis, 40 to 70% Si02, 5 to 15% AI203, 15 to 35% CaO, 2 to 12% MgO, 2 to 12% Na20 and 0.5 to 10% of a nucleating agent chosen from the group Cr203, MnO, CaF2, P205, Ti02; moulding the fused mixture and subjecting the moulding articles to heat treatment to crystallize the structure. In example 2, the composition to be melt consists of 6 parts Na20, 6.5 parts AIZ03, 15 parts CaO, 3 parts MgO, 66.5 parts Si02, 10 parts CaF2 and 3 parts of impurities. The present invention is directed to the production of translucent to opaque glass-ceramic articles which, as formed, may exhibit moduli of rupture in excess of 1406 Kg/sq.cm (20,000 psi) and, frequently, greater than 2812 Kg/sq.cm (40,000 psi). The present materials have compositions consisting essentially, expressed in weight percent on the oxide basis of: In the most general terms, the present products are prepared via a three-step process: firstly, a batch of a predetermined composition is melted; secondly, the melt is simultaneously cooled to a temperature at least below the transformation range and a glass body of a desired configuration shaped therefrom; and then, thirdly, the glass article is exposed to a temperature within the range of from 500 to 1000°Cfor a period of time sufficient to cause the growth of crystals in situ. It has frequently been observed that more uniformly fine-grained crystallization will be generated if the heat treatment of the glass article is undertaken in two stages. Moreover, subjecting the glass to a nucleation treatment inhibits the possibility of the thermal deformation thereof as the temperature approaches and, perhaps, exceeds the softening point of the glass. Thus, prior nucleation leads to more rapid subsequent growth of crystals, these crystals being more refractory than the glass and thereby acting to render the article dimensionally stable to thermal deformation. Consequently, the glass may initially be heated to from 500 to 750°C to develop nuclei and initiate the generation of crystals therein, after which the temperature will be raised to effect the growth of crystals on the nuclei. With the present compositions, fluorite (CaF2) constitutes the nucleating phase which separates out of the glass at about 550°C. As is well known, crystallization proceeds more rapidly as the temperature is increased. Accordingly, exposures of no more than, say, 0.25 hour may be necessary at from 900 to 1000°C, while up to 12 hours and more may be required to achieve a very highly crystalline article at 750°C. Where a two-step heat treatment is employed, nucleation for from 0.5 to 6 hours, followed by crystallization for from 0.5 to 8 hours has been found suitable to produce highly crystalline articles wherein the crystals are uniformly fine-grained. The predominant crystal phases developed in situ in the present materials appear to be canasite, Ca5Na4K2[Si12O30]F4 with probable solid solution to Ca5Na3K3[Si12O30]F4, and/or agrellite, NaCa2Si4O10F and/or fedorite, the exact composition of which is not known, but which is believed to approximate K0.3CaNa0.9-Al0.2Si3.8O9F, the presence of aluminium being deemed to be optional. Canasite is described by M. D. Dorfman, D. D. Rogachev, Z. I. Goroschchenko and E. I. Uspenskaya in "Canasite, a New Mineral", Trudy Minera/og. Muzeya Akad. Nauk S.S.R., No.9, 156-166 (1959) and is further analyzed structurally by M. I. Shiragov, K. S. Mamedov and N. V. Belov in "The Crystal Structure of Canasite-Ca5Na4K2[Si12O30] (OH, F)4", Dokl. Akad. Nauk. S.S.R., 185, 672-674 (1969). Agrellite is disclosed by J. Gittius, M. G. Brown and B. D. Sturman in "Agrellite, a New Rock-Forming Mineral in Regionally Metamorphosed Agpaitic Alkalic Rocks", Can. Mineral., 14, 120-126 (1976). Fedorite is discussed by A. A. Kukharenko et a/. in "The Caledonian Ultrabasic Alkalic Rocks and Carbonatites of the Kola Peninsula and Northern Karelia", /zd. "Nedra", Moscow, 479-489 (1965). X-ray diffraction analyses conducted upon the present glass-ceramics have yielded diffraction patterns closely approximating those of the naturally-occurring minerals. Therefore, although the identity of the crystals appearing in the present products has not been rigorously established, because of the apparent close similarity existing between them and the naturally-occurring minerals, the mineral names have been applied thereto and are used throughout this specification. Canasite is a multiple chain silicate exhibiting an anisotropic, blade-like crystal habit. Structurally, the crystals are composed of parallel silicate chains cross-linked to make a long box-like backbone in which the potassium ions rest. These complex chain units are cross-linked into groups of four and are separated by networks composed primarily of Na(O,F)6 and Ca(O,F)6 octahedra. Some articles wherein canasite comprises essentially the sole crystal phase have displayed moduli of rupture in excess of 3516 Kg/sq.cm (50,000 psi). The interlocking blade-like morphology of the crystal is assumed to account for the high strength of the final product.