Glass manufacturing – Processes – With coating
Patent
1994-12-09
1998-02-10
Ahmad, Nasser
Glass manufacturing
Processes
With coating
65 172, 65 22, 65 2914, 65 337, 65 37, 65 605, 65 6051, 65 6053, 65 608, C03C 1700
Patent
active
057164240
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a process for the manufacture of glass substrates with improved long term stability at raised temperatures, in particular a process, in which a glass substrate is provided with a coating, which exhibits a certain residual porosity and, therefore, experiences a volume contraction subject to the influence of high temperatures.
It is well-known that glass can be provided with a coating comprising Si-, Ti-, Zr-, Al-based materials, etc. in order to improve the different properties of the base glass. In principle, a covering layer is deposited on the base glass via a wet-chemical process (e.g. sol-gel process); and this layer is compacted into a film without pores by means of heating. It is also well-known that such coatings increase; e.g., the strength of the glass, especially when their coefficient of expansion is less than that of the base glass. In so doing, one can distinguish between two mechanisms. The one consists of reducing the number of defects on the glass surface and the other consists of forming compressive strains in the region near the surface when said layer is cooled following condensation owing to the layer's lower thermal coefficient of expansion. Since such compressive strains generally increase the strength of glass, it also affects the thermal stability, since the strains that occur at raised temperatures do not result in a break until there are higher stresses. One has had similar experiences with thermally or chemically hardened glasses; they, too, are characterized by a higher stability to thermal shock. Therefore, such glasses are used in fields, where they must resist a thermal stress, e.g. during fire proofing.
Whereas there already exist methods for improving the thermal shock property of glasses, no one has succeeded to date in providing glasses whose long-term stability at raised temperatures is satisfactory, thus, developed, for example, a glass pane that does not start to flow away when it is subjected to a raised temperature over a prolonged period of time. Especially glasses that are easy and inexpensive to manufacture (and are preferred, therefore, for most applications), such as sodium lime glasses, usually exhibit, on account of their chemical composition, a low glass transition temperature or a low viscosity at temperatures, to which this glass is subjected within the framework of a specific application. Even at comparatively mild temperatures, such glasses can present a problem with respect to softening and flowing away. Examples of glasses that can be subjected to high temperatures (optionally in combination with corrosive conditions) are furnace windows, fire proof glazing, optical glasses and chemical containers.
Thus, the present invention is based on the problem of providing a process, with which, starting from the most inexpensive base glass one can obtain in an inexpensive manner glass substrates with improved flow behavior, i.e. improved long-term stability at raised temperatures.
This problem is solved according to the invention by producing, not as in the case of the conventional process for improving the stability to thermal shock a compressive strain, but rather a tensile stress, with the aid of a coating on the surface of the glass substrate, and in particular during the heating up or heating period with the high temperature stress. This process takes place when layers, which have a tendency toward volume reduction subject to the effect of high temperatures owing to their thermomechanical or thermochemical properties and thus produce tensile stresses on the surface of the glass substrate, are deposited on the glass substrate. Such layers can be produced if a coating is produced by the conventional methods on the glass substrate, but said coating is not completely compacted. In other words, the coating still exhibits a certain residual porosity after its manufacture.
DETAILED DESCRIPTION
Correspondingly the invention provides a process for manufacturing glass substrates with improved
REFERENCES:
patent: 2474061 (1949-06-01), Moulton
patent: 4596745 (1986-06-01), Chao
Jonschker Gerhard
Mennig Martin
Schmidt Helmut
Ahmad Nasser
Institut fur Neue Materialien gemeinnutzige GmbH
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