Stock material or miscellaneous articles – Surface property or characteristic of web – sheet or block – Surface modified glass
Patent
1995-09-29
1998-06-30
Speer, Timothy M.
Stock material or miscellaneous articles
Surface property or characteristic of web, sheet or block
Surface modified glass
428426, 65 3013, 65 3014, B32B 1700
Patent
active
057731489
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to chemically toughened panes, that is to say panes in which the surface has been brought into compression by alkaline ion exchange. More specifically, the invention concerns panes having high compressive stresses over a large depth and intended, more especially, for aeronautical applications.
For aeronautical applications, notably panes for aeroplanes or helicopters, the demands regarding mechanical strength are such that usually an operation is carried out for strengthening by chemical means and not simply by thermal means as is usual, for example, for automobile panes. Chemical toughening may also be used for other highly demanding applications, such as panes for armoured vehicles, railway vehicles or ships.
As in the case of thermal toughening, chemical toughening consists of bringing the surface of the glass into compression, the breaking strength of the glass being increased by a value substantially identical to the intensity of the surface compressive stress generated by the treatment, in the present case generated by the replacement of a portion of the alkaline ions of the surface layers of the glass by other ions of larger size, which enter the vitreous network.
For loadings applied over the entirety of the pane, in the manner of the pressure applied by the air in a pressurized aircraft cockpit, the quality of the mechanical toughening is essentially dictated by the value of the surface compressive stress. Nevertheless, for more dynamic loadings, for example impact with a bird, the depth treated is also critical, because the impact creates a very high load which can cause breakage of the glass from a face subject to tensile stress and having surface defects. Ideally, the chemical toughening operation thus has as its objective to place the surface layers of the glass object treated under a very high compressive stress, to a depth which is very great and at least equal to that of the largest possible defect.
For a given glassmaking composition, the depth exchanged depends upon the duration of the ion exchange treatment and/or the temperature at which it in carried out. However, a rise in the temperature leads to an increase in the relaxation rate of the stresses and, consequently, to low levels of breakage stress. To prolong the treatment excessively likewise leads to an unsatisfactory degree of toughening, because the stresses then have the time required for relaxing.
SUMMARY OF THE INVENTION
These considerations have led to the development of new glassmaking compositions that are more favourable to ion exchanges than the glass compositions of conventional sheet glass, and make it possible, notably, to obtain greater depths of exchange for treatment durations that do not exceed a few hours. Thus, Patent Application FR-A-2 128 031 proposes silico-sodic glasses that make use of oxides commonly encountered in conventional industrial glasses, complying with the following composition, defined by its percentages by weight: and 0.35--these limits being inclusive.
The above defined compositions enable a depth of strengthening from 1.8 to 3.3 times greater than the depth achieved with ordinary sheet glass to be obtained after 24 hours.
Nevertheless, in Patent Application FR-A-2 128 031, the ion exchanges are relatively brief, systematically limited to durations of at most 24 hours, which enables a thickness of the strengthened layer of at most approximately 100 microns to be obtained (for a treatment temperature of 450.degree. C.). Now for aeronautical applications, in particular, it is necessary for this thickness to be considerably greater and, for example, to be approximately 300 microns, which raises once again the problem recalled above for conventional glass compositions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows results of conductivity versus time tests of toughened glass according to the invention and comparative non-toughened glass.
FIG. 2 shows the typical form of a curve representing the value of the failure stress in bending as a fun
REFERENCES:
patent: 4053679 (1977-10-01), Rinehart
patent: 4156755 (1979-05-01), Rinehart
patent: 4671814 (1987-06-01), Aratani
patent: 5084328 (1992-01-01), Fine
Charrue Herve
Crepet Renee
Guillemet Claude
Rifqi Francoise
Saint Gobain Vitrage
Speer Timothy M.
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