GB2217317A

GB2217317A - Strengthened glass

Info

Publication number: GB2217317A
Application number: GB8827033A
Authority: GB
Inventors: Brian Laurance Ward
Original assignee: BRITISH GLASS Manufacturing
Current assignee: BRITISH GLASS Manufacturing
Priority date: 1987-11-18
Filing date: 1988-11-18
Publication date: 1989-10-25
Also published as: GB8827033D0; GB8726940D0

Abstract

A method of strengthening glass comprises deliberately introducing and/or inducing and/or distributing fine bubbles into the glass of less than 100 mu m in diameter. The invention also includes glass or a glass article manufactured in accordance with the method defined above.

Description

STRENGTHENED GLASS This invention relates to a method of strengthening glass, ceramics and other relatively brittle materials (but hereinafter only the term "glass" will be used) and to glass strengthened by this method.

In theory, glass should be a very strong material (the tensile strength is usually calculated as about 1P13kg/mm2) but in practice it cracks and fails relatively easily (the design strength of annealed glass is only about 5 kg/mm2). There is no shortage of theories to explain this large difference, the most popular being that proposed by Griffith who assumed that glass contains numerous sub-microscopic cracks or flaws, often elliptical in shape, about 5 microns long which act as stress raisers.

The Griffith flaw theory also accounts for the fact that glass in the form of thin fibres is much stronger than in the form of large articles of glass; flaws of about 5 microns in size can exist only in the longitudinal direction in such fibres and their weakening effect will be much less provided the stress is applied in the same direction.

The origin of the flaws is usually attributed to surface damage caused both by mechanical contact with other objects and by chemical attack from components of the surrounding atmosphere. Explanation of Griffith flaws in terms of structure is largely irrelevant except at strength values well outside the range normally encountered in practice.

To date, the main methods of strengthening glass that have been tried, with varying degress of success, have been based on removing the surface layer containing the induced defects, placing the surface under compression by physical or chemical means (toughening) or adding a layer of another material to the surface (typically a metal oxide or organic coating) to protect the glass from further damage.

Removing the defective surface, by use of hydrofluoric acid for example, can increase the strength levels dramatically but no satisfactory way of preserving the "new" revealed surface from damage has yet been found. Coatings may protect the surface for a time but they are themselves subject to wear and sooner or later become ineffective.

Compressive stresses produced in the surface compensate for the weaking effects of microcracks but there are inherent limitations in the depth of the toughened layer and degree of compression that can be achieved in practice.

None of the strengthening procedures suggested or applied previously has succeeded in preventing the initiation of flaws and, since glass behaves essentially as an ideal elastic solid, failure results from extension of the flaws by the application of a tensile stress to the stage where the binding forces between the atoms are finally overcome. This happens with little previous deformation and the crack, which propagates in a direction normal to the main tensile stresses, develops rapidly and in a very unstable manner. The catastrophic nature of this so-called "brittle" fracture mode is the main reason why glass is frequently excluded as an engineering material for many applications.

According to a first aspect of the invention, there is provided a method of strengthening glass comprising deliberately introducing and/or inducing and/or distributing fine bubbles into the glass of less than 1BOLcm in diameter.

According to a second aspect of the invention, there is provided glass or a glass article manufactured in accordance with the method defined above.

The theory behind the strength improvement resulting from the invention is that- cracks are effectively blunted when they run into bubbles with the result that that section of the crack cannot propagate further. If there are sufficient bubbles, then propogation of all cracks will be effectively stopped. The introduction or inducement of bubbles into glass runs of course quite contrary to normal theory and practice, for in normal glass production a great deal of time is taken in refining glass to remove bubbles from the melt.

The "bubbles" may be of any gas, or mixture of gases or voids. They may be spherical or non-spherical, e.g. of ovoid shape.

Preferably, the bubbles should be less than 113/Clm. in diameter. For good transparency glass, the bubbles should be preferably < lXlAm in diameter.

The bubble distribution is also important in determining the magnitude of the strengthening effect.

Typically, an inter-bubble spacing or density of less than 18 bubble diameters is preferred. However, there is a relationship between bubble size and inter-bubble spacing such that the larger the bubbles, the smaller the inter-bubble spacing that is required.

The bubbles may be introduced throughout the glass/glass product to be strengthened. Alternatively, bubbles can be introduced simply into the surface layer(s).

Typically, the surface layer can vary in thickness from 0.01 to 5 mm. A preferred range would be 0.1 to 1 mm thick.

Bubbles may be introduced and/or induced and/or distributed into glass by a variety of techniques depending upon the product and its application. Typical techniques are: (a) Modifications to the conventional glass melt refining method; (b) Application of vacuum to the glass melt or to a glass article, causing dissolved gases to nucleate and grow into bubbles; (c) Thermal treatments to the glass melt or to a glass article to nucleate and grow bubbles; (d) Production of a glass melt from frit, followed by iso sta tic pressing and annealing; (e) As 4 but by melting the frit in a controlled fashion; (f) Physical vapour deposition techniques onto a glass article e.g. sputtering so to entrap gases; (g) Chemical vapour deposition techniques onto a glass article;; (h) Diffusion of gas into the glass melt or a glass article, followed by suitable treatment; (i) Use of a condensable phase in the glass melt or a glass article; (j) Neutron irradiation of the glass melt or a glass article; (k) Ion implantation of gases into the surface of a glass article; and (1) Ultrasonics/Ultrasound.

Furthermore, any suitable combination of the above techniques may be employed.

Typical applications of the invention are. in the strengthening of glass containers, "flat" glass products, ceramic products, e.g. stabilised zirconia, alumina and other brittle materials. The invention can also be used to improve the thermal shock resistance of same. It is ideally suitable for "high technology" applications such as in the aerospace industry. Furthermore, apart from increased strength, glass in accordance with the invention may have advantageous optical properties such as the ability to reflect laser light incident on its surface.

Claims

1. A method of strengthening glass comprising deliberately introducing and/or inducing and/or distributing fine bubbles into the glass of less than 10Brm in diameter.

2. A method as claimed in Claim 1, wherein the bubbles are introduced and/or induced and/or distributed into glass by any one or more of the following techniques: (a) Modifications to the conventional glass melt refining method; (b) Application of vacuum to the glass melt or to a glass article, causing dissolved gases to nucleate and grow into bubbles; (c) Thermal treatments to the glass melt or to a glass article to nucleate and grow bubbles; (d) Production of a glass melt from frit, followed by is static pressing and annealing; (e) As 4 but by melting the frit in a controlled fashion; (f) Physical vapour deposition techniques onto a glass article e.g. sputtering so to entrap gases; (g) Chemical vapour deposition techniques onto a glass article; (h) Diffusion of gas into the glass melt or a glass article, followed by suitable treatment;; (i) Use of a condensable phase in the glass melt or a glass article; (j) Neutron irradiation of the glass melt or a glass article; (k) Ion implantation of gases into the surface of a glass article; and (l) Ultrasonics/Ultrasound.

3. Glass, or a glass article, manufactured in accordance with the method of Claim 1 or Claim 2.

4. Glass, or a glass article, as claimed in Claim 3, wherein the bubbles are of any gas, or mixture of gases or voids.

5. Glass, or a glass article, as claimed in Claim 3 or Claim 4, wherein the bubbles are spherical.

6. Glass, or a glass article, as claimed in Claim 3 or Claim 4, wherein the bubbles are non-spherical.

7. Glass, or a glass article, as claimed in Claim 3 or Claim 4, wherein the bubbles are less than l /m in diameter.

8. Glass, or a glass article, as claimed in Claim 3 or Claim 4, wherein the bubbles are < lXlm in diameter.

9. Glass, or a glass article, as claimed in any one of Claims 3 to 8, wherein the bubbles are distributed at an inter-bubble spacing or density of less than l bubble diameters.

l. Glass, or a glass article, as claimed in any one of Claims 3 to 9, wherein the bubbles are introduced throughout the glass/glass product to be strengthened.

11. Glass, or a glass article, as claimed in any one of Claims 3 to 9, wherein the bubbles are introduced into the surface layer(s).

12. Glass, or a glass article, as claimed in Claim 11, wherein the surface layer has a thickness from 0.01 to 5 mm.

13. Glass, or a glass article, as claimed in Claim 11, wherein the surface layer has a thickness of 0,1 mm to 4 mm.