U.S. patent application number 15/600241 was filed with the patent office on 2017-11-23 for asymmetric chemical strengthening.
The applicant listed for this patent is Apple Inc.. Invention is credited to Victor Luzzato, Dale N. Memering, Christopher D. Prest, Matthew S. Rogers.
Application Number | 20170334770 15/600241 |
Document ID | / |
Family ID | 58794227 |
Filed Date | 2017-11-23 |
United States Patent
Application |
20170334770 |
Kind Code |
A1 |
Luzzato; Victor ; et
al. |
November 23, 2017 |
ASYMMETRIC CHEMICAL STRENGTHENING
Abstract
Asymmetrically strengthened glass articles, methods for
producing the same, and use of the articles in portable electronic
device is disclosed. Using a budgeted amount of compressive stress
and tensile stress, asymmetric chemical strengthening is optimized
for the utility of a glass article. In some aspects, the
strengthened glass article can be designed for reduced damage, or
damage propagation, when dropped.
Inventors: |
Luzzato; Victor; (Cupertino,
CA) ; Prest; Christopher D.; (San Francisco, CA)
; Memering; Dale N.; (San Francisco, CA) ; Rogers;
Matthew S.; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Apple Inc. |
Cupertino |
CA |
US |
|
|
Family ID: |
58794227 |
Appl. No.: |
15/600241 |
Filed: |
May 19, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62339062 |
May 19, 2016 |
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62362578 |
Jul 14, 2016 |
|
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62368787 |
Jul 29, 2016 |
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62368792 |
Jul 29, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C03C 2204/00 20130101;
C03C 21/008 20130101; C03C 17/002 20130101; C03C 4/18 20130101;
C03C 17/001 20130101; C03C 3/083 20130101; C03C 21/005 20130101;
C03C 21/00 20130101; C03C 23/0025 20130101; C03C 3/076 20130101;
C03C 17/225 20130101; C03C 2217/213 20130101; C03C 21/003 20130101;
C03C 17/23 20130101; C03C 2217/282 20130101; C03C 21/002 20130101;
C03C 2218/34 20130101 |
International
Class: |
C03C 17/00 20060101
C03C017/00; C03C 4/18 20060101 C03C004/18; C03C 3/076 20060101
C03C003/076; C03C 3/083 20060101 C03C003/083; C03C 21/00 20060101
C03C021/00 |
Claims
1. A cover glass for an electronic device comprising: a first zone
formed in the cover glass and having a first stress pattern; a
second zone formed in the cover glass and having a second stress
pattern; and a third zone formed in the cover glass and having a
third stress pattern; wherein: the first stress pattern, the second
stress pattern, and the third stress pattern differ from one
another; and the first zone and the second zone are contiguous with
one another and surround the third zone.
2. The cover glass of claim 1, wherein: the first zone corresponds
to one or more corner areas of the cover glass; the second zone
corresponds to one or more peripheral edge areas of the cover
glass; and the third zone corresponds to a center area of the cover
glass.
3. The cover glass of claim 2, wherein: at least a portion of the
cover glass in the first zone defines a curved edge.
4. The cover glass of claim 3, wherein: the stress pattern of the
first zone has a first surface stress; the stress pattern of the
second zone has a second surface stress; and the first surface
stress exceeds the second surface stress.
5. The cover glass of claim 4, wherein: the stress pattern of the
third zone has a third surface stress; and the second surface
stress exceeds the third surface stress.
6. The cover glass of claim 5, wherein: a portion of an external
surface in the third zone is substantially flat.
7. The cover glass of claim 1, wherein: the first zone defines a
first edge; the second zone defines a second edge; and the first
edge and the second edge form an oblique angle.
8. An electronic device, comprising: a housing; a display
positioned at least partially within the housing; and a cover glass
positioned above the display and defining at least three zones,
each of the at least three zones having different stress patterns;
wherein: the different stress patterns cooperate to inhibit crack
propagation across the cover glass.
9. The electronic device of claim 8, wherein: each of the different
stress patterns have tensile to compressive stress profiles that
are asymmetric.
10. The electronic device of claim 9, wherein: a first zone of the
at least three zones corresponds to one or more corners of the
cover glass; a second zone of the at least three zones corresponds
to a perimeter area of the cover glass; and the first zone and the
second zone form a contiguous area around a periphery of the cover
glass.
11. The electronic device of claim 10, wherein: a third zone of the
at least three zones is surrounded by, and shares a common plane
with, the contiguous area formed by the first zone and the second
zone; and the third zone has an external surface that is
substantially flat.
12. A method, comprising: asymmetrically strengthening a glass
article in a first zone; asymmetrically strengthening the glass
article in a second zone; and asymmetrically strengthening the
glass article in a third zone, wherein: the first zone has a first
stress pattern resulting in a first stress in the glass article;
the second zone has a second stress pattern resulting in a second
stress in the glass article; the third zone has a third stress
pattern resulting in a third stress in the glass article; the first
stress exceeds the second stress; and the second stress exceeds the
third stress.
13. The method of claim 12, wherein: asymmetrically strengthening
the glass article in the first zone comprises applying a potassium
salt at a greater rate to the first zone than a rate of applying
the potassium salt to the second zone.
14. The method of claim 12, wherein: asymmetrically strengthening
the glass article in the second zone comprises applying a potassium
salt at a greater rate to the second zone than a rate of applying
the potassium salt to the third zone.
15. The method of claim 14, wherein: asymmetrically strengthening
the glass article in the third zone comprises applying potassium
salt such that an external surface of the third zone remains
substantially flat.
16. The method of claim 15, wherein: the external surface of the
third zone is a top surface of the glass article.
17. The method of claim 12, further comprising: thermally heating
the first zone of the glass article to a temperature, such that:
the first stress is increased due to the thermal heating, as
compared to a stress induced by asymmetrically strengthening the
glass article in the first zone in an absence of the thermal
heating.
18. The method of claim 17, wherein: the temperature is below a
densification temperature of the glass article.
19. The method of claim 17, wherein: the temperature is above a
densification temperature of the glass article.
20. The method of claim 19, wherein: the thermal heating is
performed by microwave heating or laser heating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a nonprovisional patent application of,
and claims the benefit to, U.S. Provisional Patent Application No.
62/339,062, filed May 19, 2016 and titled "Asymmetric Chemical
Strengthening," U.S. Provisional Patent Application No. 62/362,578,
filed Jul. 14, 2016 and titled "Asymmetric Chemical Strengthening,"
U.S. Provisional Patent Application No. 62/368,787, filed Jul. 29,
2016 and titled "Asymmetric Chemical Strengthening," U.S.
Provisional Patent Application No. 62/368,792, filed Jul. 29, 2016
and titled "Asymmetric Chemical Strengthening," the disclosures of
which are hereby incorporated herein in their entirety.
FIELD
[0002] The described embodiments relate generally to asymmetric
chemical strengthening of a glass article. More particularly, the
present embodiments relate to calibrating the strength and safety
of a cover glass for use in a portable electronic device.
BACKGROUND
[0003] The cover window and display for small form factor devices
are typically made of glass. Glass, although transparent and
scratch resistant, is brittle and prone to impact failure.
Providing a reasonable level of strength in these glass parts is
crucial to reducing the likelihood of glass part failure, and hence
device failure.
[0004] Chemical strengthening has been used to increase the
strength of glass parts. Typical chemical strengthening relies on a
uniform and symmetric increase of the compression stress over the
entire surface of the glass part. Such strengthening processes have
proven effective at reducing some level of failure in glass parts.
However, there continues to be significant pressure on forming
thinner glass for use in small form factor devices, where symmetric
chemical strengthening is insufficient to prevent impact failure in
a reliable fashion.
[0005] As such, while conventional chemical strengthening is
effective, there is a continuing need to provide improved and
alternative ways to strengthen glass, particularly, thin glass.
SUMMARY
[0006] Various embodiments described herein encompass
asymmetrically strengthened glass articles. Asymmetrically
strengthened glass articles have enhanced reliability and safety as
compared to symmetrically strengthened glass articles. An
asymmetrically strengthened glass article has a first zone with a
first stress pattern, and a second zone with a second stress
pattern. The first stress pattern and second stress pattern differ
from one another. The differences in the first stress pattern and
second stress pattern result in an overall stress imbalance in the
asymmetrically strengthened glass article. The overall stress
imbalance may cause the glass article to exhibit warpage.
[0007] In additional embodiments, a material can be operatively
attached to the glass article to counterbalance the glass article's
stress imbalance and warpage, or alternatively, additional zones
can be formed in the glass having stress patterns useful in
counterbalancing the first zone and second zone stress imbalance.
It is also envisioned that the first zone and second zone can be
patterned to counterbalance each other, and limit or avoid stress
imbalance in the glass article.
[0008] In some aspects, the first zone has a first stress pattern
and first density, the first density being greater than a second
density found in the second zone, which has a second stress
pattern. In other aspects, ion-diffusion barriers and ion-inclusion
coatings can be coated on the first zone and/or second zone so as
to permit formation of the stress patterns. One ion-diffusion
barrier is composed of silicon nitrate. Another ion-diffusion
barrier is composed of silicon dioxide.
[0009] Various embodiments described herein also encompass an
asymmetrically strengthened cover glass for use with an electronic
device, where the cover glass is designed to reduce or limit damage
resulting from an impact, for example, a drop. The cover glass
includes three different stress patterns resulting from asymmetric
strengthening, a first stress pattern corresponding to corner zones
of the cover glass, a second stress pattern corresponding to
straight edge(s) or straight perimeter zones of the cover glass,
and a third stress pattern corresponding to the remainder or center
zone of the cover glass. The first zone has been strengthened the
most, the second zone to a lesser extent than the first zone, and
the third zone the least, as compared to the first and second
zones. In order to maintain a stress budget that corresponds to a
useful cover glass for an electronic device, all of the stress
budget is typically spent on the first and second zones, allowing
little or no strengthening of the third zone. This pattern of
asymmetric strengthening causes the corners, where most impacts
occur, to be most strengthened and resistant to impact, the second
zone having adequate strengthening for impact protection, and the
third zone to remain substantially flat.
[0010] Embodiments also include portable electronic devices that
include glass articles in accordance with the disclosure, as well
as to methods of manufacturing the same portable electronic
devices. In some aspects, the glass articles can undergo monitoring
and testing to identify conforming asymmetrically strengthened
glass articles for use in electronic devices.
[0011] In method embodiments, a glass article is asymmetrically
strengthened to calibrate the glass for use in a portable
electronic device. The glass article can be calibrated to have a
target geometry or provide one or more flat surfaces.
[0012] Some methods of asymmetric strengthening include immersing a
sodium-infused glass article in a potassium ion bath, while
preferentially transporting the potassium ions at a predetermined
surface of the glass article. In some aspects the immersing of the
sodium-infused glass article in the potassium ion bath is
accompanied by submitting microwave radiation to the same
predetermined surface of the glass article.
[0013] In additional method embodiments, a stress relationship is
identified and implemented using chemical strengthening. In some
aspects, glass forming is combined with asymmetric chemical
strengthening to provide a glass article having an appropriate
geometry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The disclosure will be readily understood by the following
detailed description in conjunction with the accompanying drawings,
wherein like reference numerals designate like structural elements,
and in which:
[0015] FIG. 1 shows a diagram of a glass article in accordance with
embodiments herein.
[0016] FIG. 2 is a flow diagram of a glass strengthening process in
accordance with embodiments herein.
[0017] FIG. 3 shows a glass strengthening system in accordance with
embodiments herein.
[0018] FIG. 4A is a cross-sectional diagram of a glass cover which
has been symmetrically chemically treated in accordance with
embodiments herein.
[0019] FIG. 4B is a cross-sectional diagram of a glass cover which
has been symmetrically chemically treated, as shown to include a
chemically treated portion in which potassium ions have been
implanted in accordance with embodiments herein.
[0020] FIG. 5A is a diagram of a lattice structure for glass.
[0021] FIG. 5B is a diagram of a lattice structure for
corresponding densified glass.
[0022] FIG. 6 is a diagram of a partial cross-sectional view of a
glass cover, which shows two zones of densified glass.
[0023] FIG. 7A is a diagram of a partial cross-sectional view of a
glass cover, which shows a tension/compression stress profile in
accordance with embodiments herein.
[0024] FIG. 7B is a diagram of a partial cross-sectional view of a
glass cover, which shows a reduced tension/compression stress
profile in accordance with embodiments herein.
[0025] FIG. 7C is a diagram of a partial cross-sectional view of a
glass cover, which shows an asymmetric tension/compression stress
profile in accordance with embodiments herein.
[0026] FIG. 8 is a flow diagram of asymmetric glass strengthening
in accordance with embodiments herein.
[0027] FIG. 9 is a cross-sectional diagram of a glass cover which
has been asymmetrically chemically treated.
[0028] FIG. 10 is a cover glass having a SiN coating applied to the
center portion, while the edge and corner portions remain
uncoated.
[0029] FIG. 11A is a cross-sectional diagram of a glass cover
having a combination of coatings applied to the top and bottom
surfaces.
[0030] FIG. 11B is a cross-sectional diagram of a glass cover that
illustrates the coating embodiments described in FIG. 11A.
[0031] FIG. 12A and 12B illustrate the use of high ion
concentration pastes on the front and back surfaces of a cover
glass.
[0032] FIG. 13 shows an alternative glass strengthening system in
accordance with embodiments herein.
[0033] FIG. 14A-14E illustrates processing to chemically strengthen
a pre-bent glass in accordance with embodiments herein.
[0034] FIG. 15 shows a glass strengthening system for clad layered
glass articles in accordance with embodiments herein.
[0035] FIG. 16 is a flow diagram of glass article production using
asymmetric glass treatment.
[0036] FIG. 17A and 17B illustrate chemically strengthening at
potential fracture spots to minimize fracture propagation.
[0037] FIG. 18 is a fracture pattern stress plot in accordance with
embodiments herein.
[0038] FIG. 19 is a flow diagram of glass article production where
the glass article has at least three zones of different chemical
strengthening.
[0039] FIG. 20 is a flow diagram of cover glass production where
the glass article has the greatest amount of chemical strengthening
in its corners, a lesser amount of chemical strengthening along its
perimeter side edges and the least amount in the remainder of the
glass.
[0040] FIG. 21 shows a diagram of a cover glass in accordance with
embodiments herein.
[0041] FIG. 22 shows a cross-sectional view of the corner in FIG.
19 to illustrate asymmetric chemical strengthening.
[0042] FIG. 23 is a flow diagram for compensating for asymmetric
chemical strengthening with glass forming techniques in accordance
with embodiments herein.
[0043] FIG. 24 is a graphical representation of compressive stress
to depth of compression for three illustrative glass articles in
accordance with embodiments herein.
[0044] FIG. 25 illustrates a glass article formed to a
predetermined geometry in accordance with embodiments herein.
[0045] FIG. 26 illustrates a glass article, after forming,
subjected to CNC and polishing in accordance with embodiments
herein.
[0046] FIG. 27 illustrates the glass article, after forming and
CNC, locally coated with a diffusion barrier (SiN) in accordance
with embodiments herein.
[0047] FIG. 28A and 28B illustrate asymmetric chemical
strengthening of the glass article of FIG. 12 in accordance with
embodiments herein.
[0048] FIG. 28C is a stress profile in accordance with the glass
article shown in FIG. 23A.
[0049] FIG. 29A and 29B illustrate oxidation of the SiN layer on a
glass article to SiO.sub.2 in accordance with embodiments
herein.
[0050] FIG. 30A and 30B illustrate asymmetric chemical
strengthening to a formed glass article in accordance with
embodiments herein.
[0051] FIG. 30C is a stress profile in accordance with the glass
article shown in FIG. 25A.
[0052] The use of cross-hatching or shading in the accompanying
figures is generally provided to clarify the boundaries between
adjacent elements and also to facilitate legibility of the figures.
Accordingly, neither the presence nor the absence of cross-hatching
or shading conveys or indicates any preference or requirement for
particular materials, material properties, element proportions,
element dimensions, commonalities of similarly illustrated
elements, or any other characteristic, attribute, or property for
any element illustrated in the accompanying figures.
[0053] Additionally, it should be understood that the proportions
and dimensions (either relative or absolute) of the various
features and elements (and collections and groupings thereof) and
the boundaries, separations, and positional relationships presented
therebetween, are provided in the accompanying figures merely to
facilitate an understanding of the various embodiments described
herein and, accordingly, may not necessarily be presented or
illustrated to scale, and are not intended to indicate any
preference or requirement for an illustrated embodiment to the
exclusion of embodiments described with reference thereto.
DETAILED DESCRIPTION
[0054] Reference will now be made in detail to representative
embodiments illustrated in the accompanying drawings. It should be
understood that the following descriptions are not intended to
limit the embodiments to one preferred embodiment. To the contrary,
they are intended to cover alternatives, modifications, and
equivalents as can be included within the spirit and scope of the
described embodiments as defined by the appended claims.
[0055] The following disclosure relates to glass articles, methods
of producing glass articles, and to the utility of such glass
articles in an electronic device. Embodiments also relate to the
asymmetric increase in the strength of glass, especially related to
asymmetrically strengthening a glass article to further calibrate
the reliability and safety of the glass article in an electronic
device. In some embodiments the electronic device can include a
housing, a display positioned at least partially within the housing
and a glass article, for example a cover glass, in accordance with
embodiments herein.
[0056] In one example, the glass article may be an outer surface of
an electronic device. The glass article may correspond to a glass
article that helps form part of a display area or, in some
instances, be involved in forming part of the housing. The
embodiments herein are particularly relevant for use in portable
electronic devices and small form factor electronic devices, e.g.,
laptops, mobile phones, media players, remote control units, and
the like. Typical glass articles herein are thin, and typically
less than 5 mm in thickness, and in most cases are between about
0.3 and 3 mm, and between 0.3 and 2.5 mm, in thickness.
[0057] FIG. 1 is a perspective diagram of a glass article in
accordance with one embodiment. The glass article 100 is a thin
sheet of glass with a length and width consistent with the
application. In one application as shown in FIG. 1, the glass
article is a cover glass for a housing of an electronic device 103.
The glass article 100 can have a front surface 102, back surface
(not shown), top surface 104, bottom surface 106, and side surfaces
108. The various surfaces and sides can be composed of zones and/or
portions. One zone of a glass article could be the entire front
surface, while the back surface would be considered a different
zone, for example. Another zone of a glass article could be an area
corresponding to one or more corners of the glass. A zone does not
have to be continuous, for example all four corners of the glass
article may be representative on a single zone. The strength
requirements for the surfaces and zones may differ on the use, for
example, a front surface 102, exposed to the outside environment,
may require a different strength than the back surface, enclosed
away from the environment. As discussed in more detail below, the
edges 110 of the glass article 100 can have predetermined
geometries.
[0058] These and other embodiments are discussed below with
reference to FIGS. 2-30. However, those skilled in the art will
readily appreciate that the detailed description given herein with
respect to these Figures is for explanatory purposes only and
should not be construed as limiting.
Chemical Strengthening
[0059] Embodiments herein may utilize a glass strengthening process
where a glass article is first enhanced by immersion in a first ion
solution (sodium, for example) and then strengthened by immersion
in a second ion solution (potassium, for example).
[0060] FIG. 2 is a flow diagram of a glass strengthening process
200 according to one embodiment. The glass strengthening process
200 includes obtaining a piece of glass 202, enhancing the glass
article through chemical processing 204, and strengthening the
glass article through further chemical processing 206.
[0061] FIG. 3 illustrates one embodiment for strengthening a glass
article in accordance with embodiments herein 300. A glass article
302 in need of glass strengthening is immersed in a first bath 304
that contains a sodium solution 306. The enhanced strength glass
article is then removed from the first bath 304 and immersed in a
second bath 308 that contains a potassium solution 310. At this
stage, the glass article 302 is symmetrically strengthened, meaning
that all exposed surfaces of the glass article have been equally
enhanced and strengthened through the immersion in the sodium and
then potassium solutions. In some embodiments, the strengthened
glass article can be quenched to eliminate further exchange of ions
from the treated glass article.
[0062] The level of glass article enhancement is generally
controlled by the type of glass (glass articles can, for example,
be alumina silicate glass or soda lime glass, and the like); the
sodium concentration of the bath (sodium or sodium nitrate,
typically 30%-100% mol); the time the glass article spends in the
bath (typically 4-8 hours); and temperature of the bath
(350-450.degree. C.).
[0063] Strengthening of the glass article in the second bath is
controlled by the type of glass, the potassium ion concentration,
the time the glass spends in the solution, and the temperature of
the solution. Here, the potassium or potassium nitrate is in the
range of 30-100% mol, but the glass article would remain in the
bath for about 6-20 hours at a solution temperature of between
about 300-500.degree. C.
[0064] The chemical strengthening process relies upon ion exchange.
In each solution bath the ions therein are heated to facilitate ion
exchange with the glass article. During a typical ion exchange, a
diffusion exchange occurs between the glass article and the ion
bath. For example, sodium ions in the enhancement process diffuse
into the surface of the exposed glass, allowing a build-up of
sodium ions in the surface of the glass by replacement of other
ions found in a silicate or soda lime glass. Upon immersion of the
enhanced glass article into the potassium bath, the sodium ions are
replaced by potassium ions in surface areas to a greater extent
than sodium ions found more toward the interior or middle of the
glass. As a result, the potassium ions replacing the sodium ions
form a compression layer near the surface of the glass article
(essentially the larger potassium ions take up more space than the
exchanged smaller sodium ions). The sodium ions that have been
displaced from the surface of the glass article become part of the
potassium bath ion solution. Depending on the factors already
discussed above, a compression layer as deep as about 10-100
microns, and more typically 10-75 microns, can be formed in the
glass article.
[0065] FIG. 4A is a cross-sectional diagram of a glass article 400
which has been chemically treated such that a symmetrical
chemically strengthened layer 402 is created according to
embodiments described herein. The glass article 400 includes a
chemically strengthened layer 402 and a non-chemically strengthened
inner portion 404. While discussed in greater detail throughout,
the effect of chemically strengthening the glass article is that
the inner portion 404 is under tension, while the chemically
strengthened layer 402 is in compression. The chemically
strengthened layer has a thickness (Y) which may vary depending
upon the requirements of a particular use.
[0066] FIG. 4B is a diagrammatic representation of a chemically
strengthened process. Note that some amount of sodium 405 diffuses
from the enhanced glass article to the ion bath, while potassium
(K) ions 406 diffuse into the surface of the glass article, forming
the chemically strengthened layer 402. Alkali metal ions like
potassium, however, are generally too large to diffused into the
center portion of the glass, thereby leaving the interior portion
404 only under tension and not in compression. By controlling the
duration of the treatments, temperature of the treatments, and the
concentration of the various ions involved in the treatments, the
thickness (Y) of a strengthening compression layer 402 may be
controlled, as well as the concentration of ions in the compression
layer. Note that the concentration of the ions involved in the
chemically strengthening process may be controlled by maintaining,
during glass article treatment, a substantially constant amount of
ion in each of the two baths (for example, as the potassium ions
diffuse into the glass, a controller would add more potassium ions
into the ion bath--thereby encouraging the potassium to continue to
diffuse into the glass). The relationship between the chemically
strengthened compression level (both ion concentration at the
surface and depth) and inner tension portion forms a stress pattern
for a chemically treated glass article.
[0067] Additional ion bath immersions may be added to the basic
glass chemical strengthening process. For example, a third bath
including sodium or sodium nitrate can be used to immerse the
strengthened glass so as to exchange potassium ions out of the
compression layer for sodium ions in the third bath. This is
referred to as a back-exchange or toughening process. The
toughening process is used to further control the depth and
strength of a compression layer, and in particular, to remove some
compression stresses from near the top surface regions, while
allowing the underlying potassium ions to remain in the lower
regions of the compression layer. In addition, the toughening
process reduces the central tension from the glass article (see
below).
[0068] Although sodium enhancement and potassium strengthening is
described herein, other ion combinations are within the scope of
the present disclosure, for example, use of lithium instead of
sodium, or cesium instead of potassium, e.g., sodium-potassium,
sodium-cesium, lithium-potassium, lithium-cesium treatment
combinations. Any ion combination can be used herein that provides
an increase in the glass article surface compression and
compression depth.
[0069] Chemical strengthening is applied to glass surfaces, and
relies upon exposure of the glass surface to the chemical
strengthening process. Where a glass article is immersed such that
all aspects of the article have equal exposure to the ion bath, the
glass article surface will be symmetrically strengthened, allowing
for a glass article with a uniformly thick and composed compression
layer (Y). As embodiments herein will show, where a glass article
surface is not equally exposed to chemical strengthening, the
surface will be asymmetrically strengthened, allowing for a glass
article with a non-uniform compression layer. As above,
asymmetrically strengthened glass articles have a stress pattern;
however, the stress pattern is modified based on the asymmetry of
the chemical treatment.
Pre-Heating to Increase Glass Density Prior to Chemical
Strengthening
[0070] Chemical strengthening may be enhanced or facilitated by
various thermal techniques that are performed prior to the chemical
strengthening process. Chemical strengthening is limited by the
saturation limit of the glass for an amount or volume of ions. The
size, depth and concentration of ions within a glass article
directly relates to the characteristic strengthening for that
glass, which as described herein, can be modified and calibrated
throughout the glass to optimize the glass for a particular
use.
[0071] At saturation, no additional compression layer or depth
modifications may be accomplished (via diffusion). However,
modification of thermal input to a glass article, prior to chemical
strengthening, can allow for enhancement of the glass surface
density, which will directly contribute to the concentration and
depth of the strengthened compression layer.
[0072] Where a significant amount of thermal energy is added to a
glass article prior to chemical strengthening, the glass density of
the article can be increased. Glass density in these embodiments
results in the glass lattice being heated to a point of
densification.
[0073] As shown in FIG. 5A and FIG. 5B, denser glass (5B) 500
provides a more limited lattice structure (more restricted and less
flexible) and is less able to undergo ion diffusion to deeper
levels than non-treated glass (5A) 502.
[0074] In FIG. 5A and 5B, the glass has a starting glass lattice
structure 502, which when heated to a densification temperature, is
densified and provides a smaller volume 506 for ions to move
through than the volume 508 of the non-densified glass 502. The
restriction on the glass lattice allows for fewer ions to diffuse
inwardly, while the concentration of ions in the chemical
strengthening bath remains high (as compared to an ion bath used
for non-densified glass). Also, although the glass lattice has been
densified, embodiments herein do not result in thermal input to the
point of crystal lattice collapse (not shown), rather heat is
applied to the point of lattice limitation, some ions are able to
diffuse into the glass. The ions that do diffuse into the glass are
tightly packed at the surface of the densified glass and thereby
provide a superior surface compression layer of shallow depth.
[0075] As such, the increase in glass density at the start of the
chemical strengthening process limits ion diffusion into the glass
surface, allowing the glass to exchange a greater amount of ions at
the surface of the glass, but only allowing the exchange to a
shallow depth. Glass articles treated prior to chemical
strengthening by initial thermal input typically express a higher
chemical stress at the surface, but to a shallower depth. These
glass articles are most useful for high compressive stress but to a
shallow depth, e.g., an article where polishing or other like
procedure is likely required on the chemically strengthened glass,
or where the glass may be exposed to increased risk of scratching
but not wear and tear (impact).
[0076] One such thermal technique is annealing a glass article
prior to chemical strengthening. Annealing includes subjecting the
glass article to a relatively high temperature in an annealing
environment for a predetermined amount of time, and then subjecting
the glass article to a controlled cooling for a second
predetermined amount of time. Once annealed and chemically
strengthened, the glass article will have a modified compressive
stress as compared to similar glass articles not annealed prior to
chemical strengthening. As noted above, annealing is particularly
important where the glass article is in need of high surface
compressive stress (but to a shallower depth).
[0077] The annealing process requires that the glass article be
heated to a temperature between the glass's strain point
temperature and softening temperature, also known as the glass's
annealing temperature (for aluminosilicate glass the annealing
temperature is between about 540-550.degree. C.). The time required
to anneal a glass article varies, but is typically between 1-4
hours, and cooling times typically are on the order of 1/2.degree.
C./min for up to about 5 hours.
[0078] Typically, glass articles that have been annealed may be
taken straight from a controlled cooling, and immersed in the
enhancement ion bath (sodium), or, alternatively, the article may
be further air cooled, and then immersed in the first ion bath.
Once annealed, the glass will resist deeper ion diffusion but allow
some diffusion at the surface. The diffusion into the surface
allows for high compression stress (with shallow depth).
[0079] A second thermal technique used to raise a glass article's
density prior to chemical strengthening is hot isostatic pressing
or HIP. HIP includes simultaneously subjecting the glass article to
heat and pressure for a predetermined amount of time in an inert
gas. The glass article is allowed to remain in the HIP pressure
vessel until the glass article is denser, where internal voids in
the glass are limited. As for annealing, the increase in glass
density prior to chemical strengthening by HIP allows for the
production of a higher compression stress at the glass article
surface, but to a shallower depth (than would be expected for a
glass article that does not undergo HIP).
[0080] HIP parameters vary, but an illustrative process would
involve placing the glass article to be chemically strengthened in
a HIP pressure vessel, drawing a vacuum on the vessel, and applying
heat to the glass article in the vessel. Under pressure, the vessel
may be heated to 600-1,450.degree. C., depending on the type and
thickness of the glass. Heat and pressure are typically maintained
for about 10-20 minutes, after which the processed glass is allowed
to cool. In some embodiments, a suitable inert gas can be
introduced in the vessel to facilitate heating of the glass
article. HIP is another tool for modifying or enhancing the
chemical strengthening process.
[0081] As shown in FIG. 6, the pre-heating of the glass article 600
can be localized (and not across the entire surface(s) of the glass
article), such that target or predetermined zones 602 of the glass
article are densified. In this embodiment, localized heating (shown
as arrows 604) is performed prior to chemical strengthening and to
a point between the glass's strain point temperature and softening
temperature. Laser or inductive coil heating can be used to
pre-heat the location and thereby provide a glass article that
includes both densified 608 and non-densified glass surfaces 610.
FIG. 6 shows a simple cross section of a glass cover 600 where the
sides have been locally pre-heated to form densified glass 608,
while the center of the glass article exhibits non-densified glass
610.
[0082] Embodiments herein include glass articles pre-treated by
heating techniques to form densified glass over an entire surface,
or in predetermined zones or locales, leaving zones of different
glass density. When a glass article so treated is chemically
strengthened 612, the article will be asymmetrically strengthened
and have an asymmetric stress pattern, where densified glass
exhibits a higher surface compression stress, but to a shallower
depth, than corresponding non-densified glass. It is envisioned
that the timing and placement of the pre-heating can be used to
optimize a glass surface compressive stress and the depth of the
compressive stress.
[0083] Although not explicitly noted in all embodiments herein, all
glass article embodiments herein may include the use of glass
articles that have been pre-heated to densify the glass prior to
chemical strengthening.
Chemical Strengthening of Preferred Edge Geometries
[0084] Certain glass article edge geometries can also be used to
strengthen a glass article for a particular utility in combination
with chemical strengthening. For example, embodiments herein
provide predetermined geometries useful in the strengthening of
glass covers. Edge manipulation can be accomplished, for example,
by machining, grinding, cutting, etching, molding or polishing.
[0085] Illustrative rounded edge geometries for a glass cover
useful in an electronic device include manipulation of an edge to
an edge radius of 10% of the thickness of the cover glass, e.g.,
0.1 mm edge radius for a 1.0 mm thick glass cover. In other
embodiments, the manipulation to the edge can include an edge
radius of 20%-50% of the thickness of the cover glass, for example,
0.2 mm edge radius for a 1.0 mm thick glass cover, 0.3 mm edge
radius for a 1.0 mm edge radius, etc.
[0086] In general, some embodiments herein show that rounding of
the edges of a glass cover increases the strength of the glass
cover. For example, rounding an otherwise sharp edge on a glass
cover improves the strength of the edges, which thereby strengthens
the glass cover itself. In general, the larger the edge radius, the
more uniform the strengthening can be over the surface of the glass
cover.
[0087] As such, in some embodiments herein, useful edge geometry
can be combined with chemical strengthening to produce a more
reliable and durable glass cover. For example, chemically
strengthening to increase the compressive stress layer depth along
the perimeter of a glass cover, combined with the four edges of the
glass cover having an edge radius of 30%.
[0088] Although not explicitly noted in all embodiments herein, all
chemically strengthened glass article embodiments herein may
include 1, 2, 3 or 4 of its edges machined to a useful geometry.
For cover glass designs the rounding may be from 10-50% of the
thickness of the cover glass.
Stress Profiles
[0089] Chemically treating a glass article in accordance with
embodiments herein effectively strengthens the exposed or treated
surfaces of the glass. Through such strengthening, glass articles
can be made stronger and tougher so that thinner glass can be used
in portable electronic devices.
[0090] FIG. 7A is a diagram of a partial cross-sectional view of a
glass article, for example a glass cover. The diagram shows an
initial tension/compression stress profile according to one
embodiment. The initial tension/compression stress profile may
result from an initial exchange process to symmetrically strengthen
the surface region of the glass. A minus sigma legend indicates a
profile region of tension, while a plus sigma legend indicates a
profile region of compression. The vertical line (sigma is zero)
designates crossover between compression and tension.
[0091] In FIG. 7A, thickness (T) of the glass cover is shown. The
compressive surface stress (CS) of the initial tension/compression
stress profile is shown at the surface of the cover glass. The
compressive stress for the cover glass has a compressive stress
layer depth (DoL) that extends from surfaces of the glass cover
towards a central region. Initial central tension (CT) of the
initial tension/compression stress profile is at the central region
of the glass cover.
[0092] As shown in FIG. 7A, the initial compressive stress has a
profile with peaks at the surfaces 700 of the glass cover 702. That
is, the initial compressive stress 704 is at its peak at the
surface of the glass cover. The initial compressive stress profile
shows decreasing compressive stress as the compression stress layer
depth extends from surfaces of the glass cover towards the central
region of the glass cover. The initial compressive stress continues
to decrease going inwards until crossover 706 between compression
and tension occurs. In FIG. 7A, regions of the decreasing profile
of the initial compressive stress are highlighted using
right-to-left diagonal hatching.
[0093] The peaks at the surface of the glass cover provides an
indication of the bending stress a glass article can absorb prior
to failure, while the depth of the compressive layer provides
protection against impact.
[0094] After crossover between compression and tension, a profile
of the initial central tension 708 extends into the central region
shown in the cross-sectional view of the glass cover. In the
diagram, FIG. 7A, regions of the decreasing profile of the initial
central tension (CT) extending into the central region is
highlighted using hatching.
[0095] Typically the combination of stresses on a glass article are
budgeted to avoid failure and maintain safety, i.e., if you put too
much stress into a glass article, the energy will eventually cause
the article to break or fracture. Therefore, each glass article has
a stress budget, an amount of compressive versus tensile strength
that provides a safe and reliable glass article.
[0096] FIG. 7B is a diagram of a partial cross-sectional view of a
glass cover, which shows a reduced tension/compression stress
profile according to one embodiment. The reduced
tension/compression stress profile may result from a double
exchange process. Reduced compressive surface stress (CS') of the
reduced tension/compression stress profile is shown in FIG. 7B. The
compressive stress layer depth (D) now corresponds to the reduced
compressive stress. In addition, reduced central tension (CS') is
shown in the central region.
[0097] In light of FIG. 7B, it should be understood that the
reduced compressive surface stress (CS') shows increasing profiles
as the compressive surface layer depth extends from surfaces of the
glass cover and towards the submerged profile peaks. Such
increasing profiles of compressive stress may be advantageous in
arresting cracks. Within a depth (DoL) of the submerged peaks, as a
crack attempts to propagate from the surface, deeper into the cover
glass, it is met with increasing compressive stress (up to DP),
which may provide crack arresting action. Additionally, extending
from the submerged profile peaks further inward toward the central
region, the reduced compressive stress turns to provide a
decreasing profile until crossover between compression and tension
occurs.
[0098] FIG. 7A and 7B show a symmetric stress profile, where both
sides of the cover glass have equal compressive stress, compressive
stress layer depth, and central tension.
[0099] FIG. 7C shows an asymmetric stress profile for a glass
article 714 where the top surface 716 shows a more significant
compressive stress CS and compressive stress layer depth (DoL) than
the bottom surface 718. Note that the top surface 716 would, in
this case, be more durable and impact resistant than the bottom
surface. Also note that there is a stress budget, the inclusion of
additional compressive stress on the surface may be compensated for
by a much shallower depth of compression on the bottom surface. In
the absence of the compensation, the tensile force 720 would be
extended to the left and ultimately result in a highly unsafe glass
cover (tensile strength would overcome compressive strength).
[0100] As will be discussed in greater detail below, design and
production of glass cover articles having modified stress profiles
like FIG. 7C for calibrated utility, are accomplished by using the
asymmetric chemical strengthening processes described herein. By
asymmetrically strengthening a glass article, calibrated and highly
useful glass articles may be produced. In such instances, the
stress budget for any piece of glass may be used to provide a
stress profile, and therefore glass article, having an optimized
surface for its utility.
Asymmetric Chemical Strengthening
[0101] Embodiments herein result in the production of
asymmetrically strengthened glass articles. Asymmetrically
strengthened glass articles, for example cover glass, can be
designed to be more reliable, damage resistant, and safer than
corresponding symmetrically strengthened glass articles.
[0102] FIG. 8 shows an illustrative flow diagram for asymmetrically
strengthening a glass article 800. A glass article is identified
for a desired utility based on its dimensions, its thickness, and
its inherent composition 802. A budget for how much stress the
identified glass can withstand is determined based on the glass's
utility 804, and a budget determined for optimal reliability and
safety for the glass, i.e., the stress in the glass is balanced to
provide both strength and safety 806. The glass article is then
calibrated to exhibit a useful stress pattern so as to maximize the
stress budget and utility through use of asymmetric chemical
strengthening 808.
[0103] For example, a piece of thin cover glass used on a portable
electronic device optimally requires different properties over its
surface. Asymmetry of the chemical strengthening may be required on
the front- versus the back-side of a glass article, on the
perimeter versus the center of a glass article, around features in
a glass article, or in hard to polish areas in a glass article.
However, as discussed above, each glass article has a stress
pattern to avoid failure, where the compressive stress and tensile
stress must be roughly balanced. As such, asymmetric chemical
strengthening is used to optimize the properties of a particular
glass article, within the glass article's stress budget, for a
particular use.
[0104] In general, asymmetric chemical strengthening can be used to
provide a higher (or lower) surface compression layer or a deeper
(or shallower) stress layer, for a particular region, while
maintaining the safety of the glass by not overstressing the
tensile stress within the glass article. Where a surface of glass
requires additional strength, the compression of the layer may be
increased, where the glass requires protection from wear and tear,
the depth of the compression layer may be modified, and the like.
The ability to maximize the stress within a glass article for a
zone or portion of a glass article, allows for the design of
reliable and safe glass parts. In general, the relationship of the
compressive stress (amount and depth) on the top and bottom surface
of a glass article in relationship to the resultant tensile stress
gives a stress pattern for the glass article. The stress pattern
can be along the X, Y or Z axis of the glass article.
[0105] In embodiments herein, asymmetric chemical strengthening of
a glass article is provided to: increase the reliability of a glass
article for a particular use; to increase the safety of a glass
article for a particular use; to facilitate target shapes or forms
(flat or substantially flat) of a glass article for a particular
use; to be used in combination with other techniques to facilitate
a glass article's target shape or form; and other like
utilities.
[0106] FIG. 9 shows that asymmetric chemical strengthening is
dependent on differentially incorporating ions into a surface of a
glass article. As noted above, a glass article 900, along any
surface area 902, can exchange and incorporate ions to a particular
depth and concentration based on the glass articles' density and
overall ion saturation point, i.e., there is only so much volume in
the glass that can be involved in exchange to larger sized ions, so
to increase the articles compression (see 901 versus 903). The
change in ion concentration along the surface, and to particular
depths, modifies the glass internal stress relationship, this
relationship extends across the thickness of the glass 904, as well
as throughout the glasses interior portion (how the internal
tension/compression stress changes across the middle of the glass
article) 906. As such, and as discussed previously, a stress
pattern can be across the thickness of a glass article
(vertical--top to bottom surface) 904 as well as across or
throughout the glass article (horizontal--side to side) 906.
[0107] Embodiments herein utilize these stress relationships to
calibrated utilities to provide modified glass articles for use in
portable electronic devices and small form factor devices.
Asymmetric Strengthening via Masking or Coating
[0108] Embodiments herein include the application of masking or
ion-diffusion barriers to portions of a glass article prior to
immersion in the ion containing baths. For example, a portion of
the glass surface can be physically masked from the ions in the
chemical strengthening process via a diffusion impermeable
material, such as a metal or ceramic, sealed over the region where
diffusion is not wanted. This type of physical masking completely
limits ion-diffusion into that surface and provides asymmetric
strengthening, i.e., the masked surface will receive no ion
exchange as compared to the other exposed surfaces of the glass
article. Once chemically treated, the physical barrier would
typically be removed from the glass article. Here you would have
treated and untreated surfaces.
[0109] In another embodiment, as shown in FIG. 10, a coating or
film composed on silicon nitrate (SiN), or other like material, is
used instead of a physical mask. In FIG. 10, a coating 1000 is
applied to the central portion of a glass cover 1002, while the
edges and corners 1004 are left uncoated. Such a coating would
limit or eliminate ion diffusion at the center zone or portion of
the cover glass, while allowing chemical strengthening at the
non-coated zones (edges and corners).
[0110] The coating is first applied to the glass article prior to
the enhancement treatment to block substantially all ion diffusion
through the coated portion of the glass article. Coatings can have
a thickness of from about 5-500 nm, although other thicknesses may
be used where appropriate. In this illustration, the coated surface
of the glass article, upon completion of the chemical strengthening
process, would not include a compression layer, whereas the
remainder of the glass article would exhibit a compression layer.
Upon completion of the chemical strengthening process, the coating
could be removed via polishing from the glass article, providing a
surface having asymmetric strengthening, or could be left on the
surface of the glass, as part of the finished glass article. In
this aspect, the coating would be tailored to an appropriate
thickness and composition in order to remain part of the glass
article.
[0111] In other embodiments, the SiN coating can be oxidized after
the chemical strengthening process is complete to provide a more
ion-permeable barrier. The same glass article may now be
re-immersed and processed through chemical strengthening, such that
some ion diffusion occurs through the silicon dioxide barrier, and
thereby some compression layer is formed at the locale (while the
remainder of the glass article has been treated twice).
[0112] As just noted, a coating composed of alternative materials,
silicon dioxide for example, can also be used to limit, rather than
eliminate, ion diffusion to the surface of the glass article. For
example, a coating composed of silicon dioxide would only limit ion
diffusion to the glass article surface, allowing some level of
compression layer formation in the coated region, but not the
complete strengthening contemplated by the ion exchange baths. As
above, the coating would be either removed upon completion of the
chemical strengthening process, or left in place as part of the
finalized article. In either case, the glass article would have a
surface with asymmetric strengthening.
[0113] FIG. 11 shows combinations of coating types (1100, 1102,
1104 . . . ) and thicknesses can be used in designing an
asymmetrically strengthened glass surface. In FIG. 11A, a series of
coatings (1100, 1102, 1104) are applied to both the top and bottom
surface (1106 and 1108, respectively) of a glass cover 1110. Each
combination of coating material is meant to control ion diffusion
to the target glass surface, and thereby modify the chemical
strengthening of that surface 1112.
[0114] The glass article can exchange and incorporate ions to a
particular depth and concentration based on the ion diffusion
through coatings 1100, 1102 and 1104. As described previously, the
change in ion concentration along the surface, and to particular
depths, modifies the glass internal stress relationship. The stress
pattern shown in FIG. 11B illustrates that the edges 1114 of the
top surface 1106, having no coating, receives the most robust ion
concentration along the surface, and to the greatest depth. The
remainder of the top surface 1106 shows some reduced ion
incorporation, but to a lower extent than at the edges 1116. The
bottom surface 1108, being internal, for example, has multiple
zones defining three areas of ion incorporation 1116, 1118, 1120,
based on the layered coatings. The center zone 1120 of the bottom
surface has little or no ion incorporation due to coatings 1100,
1102, and 1104. The combined coatings eliminate almost all ion
diffusion into the center zone. The other zones show some ion
diffusion that results from either the single coating or the
combination coating. Thus, a stress relationship where multiple
coatings (ion barriers) have been applied to prepare an
asymmetrically strengthened glass article is achieved.
[0115] It is further envisioned that multiple layers of coating can
also be used to control the ion diffusion process into the target
glass surface. For example, a thin coating that limits sodium and
potassium ion diffusion from a chemical strengthening process by
25%, could be layered across a first thicker coating that limits
sodium and potassium ion diffusion by 50%. The glass surface region
would potentially have a region limited of ion diffusion by 0%
(uncoated), 25% (first coat), 50% (second coat), and 75% (layered
coat); other embodiments may have different percentages for each
coat. As above, the finished glass article surface could include
each of the coating layers, or could be treated to remove the
coatings, leaving only the underlying asymmetrically strengthening
surfaces. It is also envisioned that the ion-diffusion barrier
coatings can be combined with the ion-barrier masks to further
allow for calibrated glass article surface strengths--for example,
physically mask a bottom surface of the glass cover and coat
patterns or locales with a 25% ion diffusion barrier on the top
surface of the cover.
Thermal Assisted Asymmetric Chemical Strengthening
[0116] Embodiments herein include asymmetric glass strengthening
during the chemical strengthening process through the targeted
application of heat. Preferential heating of a glass surface locale
can be used to facilitate stress relaxation in that locate, and
thereby allow for an increase in ion diffusion at that locale
during the chemical strengthening process. Note that the heat is
below the amount required to densify the glass as discussed above.
An increase in ion diffusion allows for the exchange of additional
ions into the glass, thereby changing the stress profile for the
heated surface compared to the non-heated surface. For example, a
localized region of a glass article can be heated through the use
of heating coils, laser, microwave radiation, and the like, while
the glass article is immersed in a chemical strengthening ion
bath.
[0117] As noted above, the increase in heat at the target locale
allows for an increase in ion diffusion in the glass surface at the
heated locale. Enhanced heating of target locales on the glass
surface provides asymmetric chemical strengthening at the heated
locales as compared to non-heated surfaces. Asymmetric chemical
strengthening using modified thermal profiles is of particular
value where a laser or microwave beam can be directed to modify the
chemical strengthening for parts having known failure spots. For
example, cover glass that requires additional chemical
strengthening at the corners to limit breakage as a result of
impact.
[0118] Heating temperatures are appropriate where the heat is
sufficient to relax the glass lattice, but not cause densification
of the glass, or to cause boiling of the ions in the ion bath.
[0119] In one embodiment, a glass article is chemically enhanced by
immersion in a first and second ion bath. While immersed in the
first and/or second ion baths the thermal profile of some
predetermined portion of the glass article is increased through use
of directed heating (coils, laser, microwave, etc.). The targeted
locale on the glass article undergoes additional ion exchange given
the relaxed and expanded lattice of the glass. Once the thermal
input is deemed sufficient, the asymmetrically strengthened locale,
now having additional ions packed into the surface, can be quenched
to inhibit exchange of the ions back out of the locale. Increasing
the thermal profile during chemical strengthening can be used to
both increase the compressive stress of the glass surface and
compressive stress layer depth of the glass surface.
Local Asymmetric Strengthening via Paste and Heat
[0120] As discussed in more detail below, it is often important to
form a glass article where the stress in the glass article is
matched to provide a particular shape, for example, provide a flat
surface.
[0121] In one embodiment, localized chemical strengthening
techniques can be used to promote ion diffusion into specific
regions or zones of the glass article. These high concentration
chemical strengthening zones can be used to instill higher surface
ion concentration and/or deeper compression layers with target
patterns or spots on the glass article. The inclusion of the
enhanced chemical strengthening can be used to provide slight
curvatures to the glass surface where required, or can be used to
counteract each other on opposite sides of the glass surface (front
and back surface, for example).
[0122] Pastes that include high concentrations of potassium, for
example, can be used in combination with heat to enhance or promote
ion diffusion directly from the paste into the localized surface of
the glass article. This high concentration and direct ion diffusion
is superior to the ion diffusion accomplished by immersion in ion
baths. In one embodiment, a glass article, requiring an increased
amount of ion diffusion in a predetermined pattern, is coated with
a high ion concentration paste in the predetermined pattern. The
paste can be 30-100% molar sodium or potassium nitrate for example,
and more typically 75-100% molar. The paste layer thickness is
determined by how much ion is required for diffusion into the glass
article surface. The coated glass article is then placed in an oven
and heated, for a predetermined amount of time, to increase the
diffusion of the ion into the glass surface in the predetermined
pattern. Ovens can be electric or gas (or other like) and reach
temperatures from about 250-500.degree. C. In some embodiments, the
oven can be under pressure, allowing for use of higher temperatures
during the heating step (and thereby avoiding evaporated or boiled
paste).
[0123] FIG. 12A and FIG. 12B illustrate the use of high
concentration ion pastes 1200 on the front (12A) and back (12B)
surface (1202 and 1204, respectively) of a cover glass 1206. Paste
application patterns can be used to facilitate allocation of
asymmetric strengthening, and counterbalance stress added on the
front cover with stress added to the back cover. In FIG. 12A and
12B, illustrative front and back surface patterns are
presented.
[0124] In other embodiments, the already enhanced coated glass
article is coated with the high ion concentration paste, potassium
for example, and then placed into the potassium ion bath. The
coated glass article and ion bath are then placed in the oven for
heating, such that the paste directly deposits potassium to the
glass surface, while the potassium ion bath allows for ionic
diffusion to the non-coated or exposed surfaces of the glass
article.
[0125] Altering the ion concentration in the paste, the pattern of
paste application on the glass surface, the heating parameters of
the paste, the coating thickness of the paste, provide various
design options for creating an asymmetrically strengthened glass
article.
[0126] As can be imagined, paste with high ion concentrations can
also be combined with masking, ion-barrier coatings and glass
density to further optimize the necessary chemical strengthening
for a target glass article. Also, as can be imagined, paste with
multiple ions can be used as well as coating a glass article
surface with one or more, two or more, three or more, etc.
different pastes, each having a different ion or ions
concentration.
Electric Field Assisted Asymmetric Chemical Strengthening
[0127] As shown above, embodiments herein include asymmetric glass
strengthening during the chemical strengthening process. In this
embodiment, ion transport in the ion bath is preferentially
increased toward a target surface of the glass article, thereby
increasing the diffusion of the ions at the target surface.
Increased concentration of the ion at a surface allows for an
increase in the amount of ion incorporated into the glass surface,
up to the glass article's ion saturation point, as compared to the
remainder of the article's surface not in-line with increased ion
concentration.
[0128] Aspects of this embodiment are maximized by utilizing an ion
concentration, in the ion bath, that provides for chemical
strengthening, but below the glass articles' ion saturation point.
In this aspect, the electric field would significantly increase the
ion concentration at surfaces in-line with the preferential
transport of ions across the electric field.
[0129] In an illustrative embodiment, an electric field is
established in an appropriate ion bath to preferentially diffuse
the ion across the target surface of the immersed glass article. As
shown in FIG. 13, a glass article 1304 in need of asymmetric
chemical strengthening is positioned in the ion bath 1300 between a
positive 1306 and negative electrode 1308. Electron flow through
the external circuit 1310 allows the bath ions, potassium for
example, to flow toward the negative electrode and thereby into the
front surface 1302 of the positioned glass article (shown as arrow
1312). The increase in ion concentration at the front surface of
the glass article provides for an asymmetric strengthening of the
front surface, as the front surface 1302 will have an increase in
ion diffusion, as compared to the back surface 1314 of the
glass.
[0130] Alternative embodiments for the electric filed gradient
include performing the preferential ion diffusion in combination
with coil, laser, microwave or other thermal heating (shown as
arrow 1316). In this embodiment, a glass article 1304 is exposed to
localized microwave radiation 1316, for example, where increased
chemical strengthening is required. The microwave radiation
facilitates stress relaxation at the target surface 1302. A glass
article surface receiving preferential ion diffusion in the ion
bath due to the established electric field, may have additional ion
diffusion into the surface where the microwave radiation
facilitates stress relation (provides more space for ions to enter
the glass surface). As can be imagined, a glass article 1304 so
treated could have several different asymmetrically strengthened
zones, the zone that was heated 1318 and in-line with the ions in
the electric field, zones not heated but in-line with the ions in
the electric field 1320, zones heated but not in-line with the ions
in the electric field (not shown), and zones that are neither
heated or in-line with ions in the electric field (1322).
Asymmetric Strengthening via an Introduced Pre-Bend
[0131] Asymmetric strengthening can be introduced into the surface
of a glass article by pre-stressing the glass prior to, and during,
the enhancing and strengthening process. In one embodiment, the
glass article is formed (molded, drawn, etc.) to have a pre-desired
curvature. The formed glass article is placed under the correct
force to maintain the form and then chemically strengthened using
the embodiments as described above. For example, the formed glass
article is placed in the ion exchange baths in the pre-stressed or
formed shape. Since the glass is bent while the glass is being
chemically strengthened, it is strengthened in an enhanced manner.
So, for a curved or bent glass article, the chemical strengthening
is primarily going to the outer, curved, surface (ions more easily
diffuse into the stretched glass lattice), while the compressed
inner surface undergoes limited chemical strengthening. Different
portions of the outer surface of the glass article may be
selectively chemically strengthened, or chemically strengthened
differently, and/or the glass article can be bent selectively or
differently to offset the asymmetric chemical strengthening of the
different portions. After the pre-stressed glass article is
released from its pre-bend, the outer surface will have a greater
amount of strengthening as compared to the inner, thereby showing
an asymmetric strengthening profile.
[0132] FIG. 14A-14E illustrate chemically strengthening a glass
article according to one embodiment. In FIG. 14A, the glass article
1400 is shown having a thickness T. The thickness T can be
generally as described throughout this disclosure (0.3-5 mm). The
glass article 1400 has an outer surface 1402 and an inner surface
1404.
[0133] In FIG. 14B, an ion-exchange coating (as discussed above)
1406 is coated onto the inner surface 1404 of the glass article
1400. In this way, the ion-barrier limits ion diffusion into the
inner surface of the glass article.
[0134] In FIG. 14C, the glass article has been bent such that the
bent glass article 1400' is curved inward towards the inner surface
1404. The bending of the glass article yields the glass article
with curvature C. The curvature in the glass article 1400' can be
of varying degrees, and can be imposed by force (a fixture) or by
including a heated environment (slumped over).
[0135] In FIG. 14D, the bent glass article from FIG. 14C undergoes
chemical strengthening to yield a glass article 1400''having a
strengthened region 1406. The chemically strengthened region 1406
is provided adjacent the outer surface 1402 and not adjacent the
inner surface 1404. The chemically strengthened region extends
inward from the outer surface to a depth of layer (DoL), which is
deeper into the glass than the DoL at the inner surface (which is
minimal or non-existent). Since the outer surface is chemically
strengthened substantially more than the inner surface, the
chemically strengthened glass article 1400'' can be referred to as
being asymmetrically chemically strengthened.
[0136] FIG. 14E illustrates the chemically strengthened glass
article 1400''' after completion of the chemical strengthening
process. The glass article 1400''' is depicted as planar, or at
least substantially planar, following completion of the process.
The completed glass article 1400''' has an outer surface 1402 with
increased compression and an inner surface 1404 that was both bent
inward and coated by an ion-exchange coating to limit or eliminate
chemical strengthening. In this profile design, the chemically
strengthened glass article 1400''' tends to wrap inward from the
outer surface--meaning the outer surface compresses and expands. In
such case, the warpage due to the chemical strengthening of the
outer surface but not the inner surface causes the curvature C to
be countered. Consequently, the chemically strengthened glass
article 1400''' no longer has a curvature as it had prior to the
beginning of the chemical strengthening.
Asymmetric Strengthening Different Clad Layers
[0137] FIG. 15 illustrates another Embodiment herein which includes
forming asymmetrically strengthened glass articles 1500 through
immersion of glass article clad layers 1502 in the chemical
strengthening bath 1504, where each glass article in the clad layer
has a different starting ion concentration and composition. A clad
layer having a first and second glass article is then strengthened
using the chemical strengthening processes described herein to
provide two glass articles with asymmetric strengthening.
[0138] In one aspect, since the starting compositions of the two
glass articles are different, the exposed surface and edges of each
glass article will incorporate available ions differently. The end
result of the chemical processing step will be two glass articles
with a protected surface (internal to the clad layering) and a
chemically modified exposed surface and edges. Modification of the
exposed surfaces can be made by masking or coating, or other
embodiments herein, as described previously. Any number of articles
can be strengthened in this way, for example, in FIG. 15, three
glass articles are being strengthened at the same time.
Chemical Strengthened Glass Article Bundles
[0139] In other aspects, asymmetrically strengthened glass articles
having substantially the same stress profiles can be bundled
together for common treatment to alleviate or modify the stress in
the bundled glass. Here, the glass articles can be bundled as
multiple plates to one another and treated together to maximize
efficiency. Glass articles can be bundled as non-planar parts,
treated, and then bonded to display a bonding stress or could be
pre-bent and then bonded to display the bonding stress.
Asymmetric Strengthening Glass Articles Having a Concentration
Gradient
[0140] In another embodiment, two glass articles of differing
composition can be fused together prior to the chemical
strengthening process. Here, the fused glass article will have a
top surface chemically strengthened based on its starting glass ion
concentration and composition (top glass), and a bottom surface
chemically strengthened based on its starting glass ion
concentration and composition (bottom glass).
[0141] In addition, using the same premise, one glass piece having
a concentration gradient (composition or ion) can also be
chemically strengthened to provide asymmetrically strengthened
glass. As above, the glass article has differing ions, at differing
locations of the glass article, to be exchanged in the ion baths,
such that the resultant surface will be asymmetrically
strengthened.
[0142] Design of the starting glass, including its starting ion
concentrations and locations, can therefore by used to calibrate
the ion-diffused and asymmetrically strengthened glass.
Mechanical and/or Chemical Modifications to Tune a Stress
Profile
[0143] Embodiments herein include the use of post-chemical
strengthening, mechanical and/or chemical processes, to fine tune a
glass article's stress. Where a glass article has been prepared
according to any of the embodiments described herein, fine tuning
of the compressive stress layer, for example, or tuning of the
relationship between the tensile and compressive forces, in the
glass may be required. Removal of material, either mechanically
(grinding, polishing, cutting, etc.) or chemically (application of
HF or other like acid), can be used to locally modify the stress
profile for the glass article.
[0144] For example, where it is determined that the extent of the
compressive surface stress layer is too large, or deep, removal of
some amount of the layer will relieve stress and re-calibrate the
stress profile for the glass article. These post-chemical
strengthening embodiments are particularly useful where the stress
modification need only be minor, for example removal of 10 .mu.m
from a limited region of the cover glass.
Asymmetric Chemical Strengthening During Glass Article
Production
[0145] Embodiments herein include the stepwise modification of a
glass articles stress profile based on the use of one or more of
the chemical strengthening embodiments described herein. For
example, where production of a glass article results in a
non-conforming or unsatisfactory result, the asymmetric chemical
strengthening embodiments described herein can be used to reform
the stress so as to bring the glass article into compliance. This
may entail localized asymmetric chemical strengthening, or
conversely, removal of material, with the object of adding or
removing stress where necessary to correct any defects in the glass
article.
[0146] FIG. 16 illustrates one flow diagram including the process
for asymmetrical chemical strengthening during glass article
production 1600. A glass article having already been assigned a
particular calibrated stress pattern 1602 is appropriately treated
using any of the embodiments herein described 1604. The reliability
and safety of the glass is tested by determining whether the glass
cover exhibits the correct strengthening parameters 1606. Where the
glass article conforms to the asymmetric chemical strengthening,
the glass article is submitted for its use 1608. Where a glass
article fails to exhibit its appropriate chemical strengthening, it
is passed through the processes and embodiments described herein to
reapply the appropriate chemical strengthening and tested 1610.
This process can be repeated as many times as necessary to obtain a
glass article that conforms to the standards of its use.
[0147] As such, embodiments herein include monitoring and
correction of a glass articles predetermined stress profile.
Correction can include a number of stress modifying iterations
until the desired glass article stress profile is obtained.
Asymmetric Chemical Strengthening to Manage a Fracture Pattern
[0148] Embodiments herein include asymmetrically strengthening a
glass article to exhibit or manage a predetermined fracture
pattern. FIG. 17A and 17B show illustrative chemical strengthening
1706/1708 applied to a cover sheet 1704 to minimize fracture
propagation (17A) or minimize corner damage 1710 (17B).
[0149] FIG. 18 shows a surface stress (CS) to distance graph
illustrating that points of tension 1800 can be developed along the
surface of a glass article where a fracture would be more likely to
occur than at points of high surface stress 1802.
[0150] Using any of the embodiments described herein, an optimal
fracture pattern for the particular glass article use can be
developed. Embodiments include positioning the amount of surface
compression stress, the depth of compression stress, the top
surface to bottom surface tensile to compressive stress, and the
planar tensile to compressive stress, in an optimized pattern. A
glass article can be calibrated to control the fracture pattern
upon damage or excessive wear and tear by identifying and then
incorporating the necessary compressive surface stress, depth of
stress and tensile stress, so as to facilitate a fracture in some
regions, should one occur, as compared to other regions. In this
way, a crack could be encouraged along a perimeter as compared to
the center of the cover glass, for example. In one example, more
significant tensile stress is positioned in a desired fracture
location 1706 or 1710 as compared to locations of less preference.
Crack development and propagation can be managed by the irregular
use and positioning of stress 1706, for example.
Designing a Cover Glass to Reduce Damage, or the Propagation of
Damage, Caused by an Impact
[0151] Embodiments herein result in the production of
asymmetrically strengthened cover glass for a portable electronic
device. As previously disclosed, the combination of stresses on the
cover glass are budgeted to avoid failure and maintain safety,
i.e., with a limited volume of glass, only so much ionic material
may be added to the volume before the glass will crack or fail,
simply due to the tensile stress becoming overly voluminous and
exerting sufficient pressure to crack the glass.
[0152] In embodiments herein, asymmetrically strengthened cover
glass has a stress budget optimized to resist damage caused by
impact from dropping, fumbling, hitting, and the like of the
device, e.g., a mobile phone dropping from the users hand and
falling to the floor. In this light, most portable devices, when
impacted, tend to initially impact at a corner of the device, or to
a lesser extent, impact at a perimeter straight edge of the device.
The impact is therefore aligned with the corners of the cover glass
and, to a lesser extent, the perimeter or edge of the cover glass.
It is less likely, and more infrequent, that a dropped device will
initially impact at the front side or back side of the device,
i.e., land flat on its face or flat on its back. As such,
embodiments herein are optimized to limit or reduce damage (or the
propagation of damage) in a cover glass by designing the cover
glass with the expectation that impact will result at a corner of
the cover glass, or at the very least, a perimeter straight edge of
the cover glass.
[0153] As discussed previously, asymmetric chemical strengthening
can be used to provide modified surface compression within a cover
glass. The asymmetric strengthening must conform to a stress budget
for the particular parameters of the glass. Embodiments herein
include cover glass designs where the stress budget is utilized to
provide the most impact resistance at the cover glass corners,
followed by impact resistance along the straight perimeter edges,
and to a lesser extent the substantially flat front and back
surfaces of the glass. The budgeted stress is therefore
substantially utilized at the corners, and to some extent, along
the perimeter of the cover glass. Little or no stress budget is
allocated to the center or remainder zone of the cover glass. The
imparted strengthening is adequate to enhance impact resistance
from damage. In addition, since little of the stress budget is used
in the center or remainder zone of the cover glass, that zone is
under little to no imbalance and can remain substantially flat.
[0154] FIG. 19 shows an illustrative flow diagram 1900 for
asymmetrically strengthening a glass article having multiple zones,
each zone having a different stress profile. In operation 1902, a
glass article is obtained for a desired utility based on its
dimensions, its thickness, and its inherent composition. In
operation 1904, a budget for how much stress the identified glass
can withstand is determined based on the glass's utility, and a
budget determined for enhanced resistance to impact damage caused
by a drop, for example. As described throughout, the budget must
conform to the restricted volume of the glass, as inclusion of too
much stress in the glass can cause the tensile stress to lead to
cracks or damage under normal use constraints.
[0155] In operation 1906, the glass article is then divided into
multiple zones. For example, a first zone in the glass may have the
highest amount of chemical strengthening, followed by a second
zone, followed by a third zone having the least amount of chemical
strengthening. In operation 1908, the glass article has a stress
pattern based on the three different zones, for example, a first
stress pattern having the greatest strength related to impact, a
second stress pattern having a smaller amount of strength than the
first zone, and a third stress pattern having the lowest level of
strength. In some embodiments the third zone has little or no
chemical strengthening.
[0156] FIG. 20 shows an illustrative flow diagram 2000 for
asymmetrically strengthening a cover glass for a portable
electronic device having three or more zones, each zone having a
different stress profile. In operation 2002, a cover glass is
obtained having the dimensions, thickness and composition typically
called for use in the portable electronic device of interest. In
operation 2004, a budget for how much stress the cover glass can
withstand is determined, where the budgeted stress maintains a
substantially flat cover glass with enhanced damage resistance in
the event of an impact, a drop for example. The cover glass can be
divided into three zones, a first zone corresponding to the corner
portions or areas of the cover glass, a second zone corresponding
to the straight perimeter portions (also referred to as peripheral
edge areas) of the cover glass, and a third zone corresponding to
the remainder or center area of the cover glass. In some
embodiments, the three zones refer to the top surface of the cover
glass, or to a stress profile that extends from the top surface to
the bottom surface. The first and second zones can include up to
50% of the cover glass area (leaving 50% of the cover glass area
for the third zone), up to 40% of the cover glass area (leaving 60%
of the cover glass area for the third zone), up to 30% of the cover
glass area (leaving 70% of the cover glass area for the third
zone), up to 20% of the cover glass area (leaving 80% of the cover
glass area for the third zone), up to 15% of the cover glass area
(leaving 85% of the cover glass area for the third zone), up to 10%
of the cover glass area (leaving 90% of the cover glass area for
the third zone), up to 5% of the cover glass area (leaving 95% of
the cover glass area for the third zone), up to 2.5% of the cover
glass area (leaving 97.5% of the cover glass area for the third
zone), and up to 1% of the cover glass area (leaving 99% of the
cover glass area for the third zone).
[0157] In typical embodiments herein, in operation 2006, the glass
article can be divided into a first zone that includes a first
stress pattern useful for the corner portions of the cover glass, a
second zone that includes a second stress pattern useful for the
straight perimeter portions or edge portions of the cover glass,
and a third zone that has a stress pattern useful for the remainder
of the cover glass. In operation 2008, the budgeted stress is
allocated to the three zones where the first zone is strengthened
more than the second zone, which is strengthened more than the
third zone. In some embodiments the third zone undergoes little or
no chemical strengthening, and the entirety of the stress budget is
used on the first and second zones. Using the entirety of the
stress budget on the first and second zones results in a glass
article that is under tensile stress for normal usage, but has
improved capacity to prevent or reduce damage 0caused by an impact
to the article. Also note that the first and second zones can form
a continuous perimeter around the third zone.
[0158] FIG. 21 illustrates a cover glass 2100 having three zones,
each zone having a stress pattern useful in reducing damage, or the
propagation of damage, in a cover glass. As noted above, a finite
stress budget exists for the cover glass 2100. The stress budget is
allocated to each of the three zones, where the first zone 2102
(corresponding to the corner portions or areas of the cover glass)
receives the highest amount of chemical strengthening, a second
zone 2104 (corresponding to the straight perimeter sides or
peripheral edge areas) receives the second highest amount of
chemical strengthening, and a third zone 2106 that corresponds to
the center or remainder area of the cover glass 2100 receives the
least amount of chemical strengthening. In some embodiments, the
third zone 1906 may undergo little or no chemical strengthening.
The third zone 2106 can include an external surface where a portion
thereof is typically substantially flat, rather than the entirety
of the third zone. The third zone 2106 is also surrounded by the
higher strengthened first 2102 and second 2104 zones, which form a
contiguous perimeter around the third zone. The contiguous first
and second zones forming at the periphery of the cover glass higher
strength glass that forms a protective barrier against impact to
the lower strengthened glass found in the third zone. In some
embodiments, the first zone and second zone each form an edge and
the edges can contact each other to form an oblique angle. The
stress budget is used to reduce potential impact events from
causing damage, or the propagation of damage, to the first zone
2102, and to some lesser extent, the second zone 2104, while
leaving the third zone substantially flat or unaffected by warpage.
At the least, impact is likely to be distributed to the first and
second zones of the cover glass 2100, which form a perimeter around
and surround the centrally located third zone 2106. In addition,
the first zone can be thermally heated to a temperature that allows
for increased chemical strengthening as compared to the same zone
in the absence of thermal heating. The second zone may also be
thermally heated during asymmetric strengthening to also enhance or
increase the amount of stress induced in the zone. Thermal heating
is described throughout the current specification, but can be
performed by microwave or laser heating. In some embodiments, the
temperature of the thermal heating is below the densification
temperature of the glass and in other embodiments the temperature
of the thermal heating is above the densification temperature of
the glass.
[0159] FIG. 22 shows a cross-sectional view along line 21-21' in
FIG. 21. The first zone 2102 shows an increased amount of ions 2200
to a particular depth and concentration as compared to the third
zone 2106. The change in ion concentration along the first zone
surface, and to particular depths, modifies the glass internal
stress relationship. The increased chemical strengthening to the
first zone provides additional compressive stress along the zone or
portion of the cover glass most likely to have an impact. In FIG.
22 the first zone defines a curved edge, which in this embodiment,
extends from the top surface to the bottom surface of the cover
glass. Note that this is also the zone of the cover glass most at
risk from impact, as it has a limited area to distribute force or
energy caused by the impact. The increase in the volume of ions at
the corner can thereby resist the force or energy imparted by the
impact and reduce or prevent damage to the cover glass.
Alternatively, the third zone 2106 has a much greater area to
distribute a force associated with an impact, as well as being much
less likely to be involved with the impact itself. As such, some of
the chemical strengthening not required in the third zone can be
budgeted to the first zone and still maintain a cover glass within
its budgeted amount of stress. As noted in FIG. 22, the third zone
defines an external surface that is substantially flat.
Flattening Asymmetric Stress Profiles
[0160] Embodiments herein include the process of using asymmetric
chemical strengthening, in combination with other compensating
forces, to provide useful glass articles, for example, articles
having flat surfaces.
[0161] In one embodiment, a glass article that has been
asymmetrically chemically strengthened shows a stress imbalance due
to an overall excess of compressive stress on the top surface as
compared to the bottom surface, for example. The stress imbalance
in the glass article can be counteracted by attachment to a very
stiff material, or a stiff material having a geometry that
counteracts the stress imparted by the asymmetrically strengthened
glass article. Optimal materials would counteract the glass
article's imparted asymmetric stress so as to remain flat (or
remain at the geometry required for the glass material). In typical
embodiments, the stiff material would be attached along the surface
of the glass article, typically the bottom surface. In some cases,
the stiff material would be transparent. The stiff material would
only need to be of sufficient amount and coverage to accomplish the
counteracting stress.
[0162] In another embodiment, a glass article that has been
asymmetrical chemical strengthened has its stress imbalance
counterbalanced by tailoring mechanical or chemical removal of
material. In this embodiment, polishing or other mechanical
technique can be used to optimally remove stress from the glass
article. Alternatively, aspects of the glass article's stress
imbalance can be removed by immersion of the part in chemical
removal bath, e.g., an HF bath. Glass surface not at issue in the
chemical removal bath could be sealed off from the HF or only
selective regions of the glass surface exposed to the HF. Removal
of material would be accomplished to provide a glass article with
the correct geometry or flatness (again based on counterbalancing
the overall stress in the strengthened glass article).
[0163] In still another embodiment, the required asymmetric
compressive stress (for damage control and reliability) is
counteracted by the introduction of additional, localized, chemical
strengthening. For example, use of coatings or pastes (previously
described) can be incorporated into the asymmetrically strengthened
glass article to counteract the warpage introduced by the required
asymmetric chemical strengthening. In some aspects, the coatings or
pastes can be patterned.
[0164] Embodiments herein also include not just the placement of
counteracting chemical strengthening, but include the amount of
compressive surface stress and the depth of compression of the
chemical strengthening on the glass. Here, the inclusion of a
particular compressive surface stress can act as a stiffening
barrier to prevent or counteract warpage introduced by other
asymmetric chemical strengthening. Use of a short, high spike of
potassium ions, for example, into the surface of the glass article
can act to provide a very shallow but hard sport. These hard (high
compressive surface stress layers) can have a Young's Modulus as
high as 60 to 80 and be used to prevent warpage--in a sense, act as
the stiff material discussed above.
Compensating Asymmetric Chemical Strengthening With Forming
[0165] Embodiments herein include the design and production of
glass articles that combine the advantages of asymmetric
strengthening of surfaces on a glass article, with glass
forming.
[0166] As is described throughout the present disclosure,
asymmetric chemical strengthening allows for targeted increases in
either the compressive surface stress of a glass article and/or the
depth of compression of a glass surface. In most cases, the glass
article is calibrated to have its intended utility with maximum
damage or scratch protection for the glass article. This typically
requires some combination of the processes and embodiments
described herein, for example, increased depth of compression along
the perimeter of a cover glass with normal symmetric chemical
strengthening in the center of the cover glass.
[0167] Inclusion of asymmetric chemical strengthening, however, can
introduce stress imbalance into the glass article (note the stress
profiles discussed above). When enough stress imbalance is
introduced to a glass article, the glass article will warp. Warpage
in a glass article is typically detrimental to the article's
utility and presents a limitation on how much asymmetric stress can
be introduced into a glass article.
[0168] As previously discussed, introduced warpage can be
compensated for by introduction of competing stress imbalances, for
example, introducing asymmetric chemical strengthening in a glass
article so as to both provide utility and to provide counteracting
stress. The present embodiment, however, utilizes the glass forming
process to minimize the stress imbalances introduced by asymmetric
chemical strengthening. Further, glass forming provides a stiffer
glass article which can be formed to combine with the forces
incurred through asymmetric chemical strengthening to yield a glass
article having the desired shape.
[0169] In one embodiment, a glass article is designed to counteract
the stress imbalances introduced by asymmetric chemical
strengthening with the use of glass forming. In one aspect the
asymmetric chemical strengthening is counteracted by forming the
glass article with an appropriate geometry. Appropriate glass
article geometries for a particular stress profile provide
stiffness to counteract the stress introduced by the asymmetric
chemical strengthening procedures. In an alternative embodiment,
the asymmetric chemical strengthening is combined with glass
forming to provide a desired geometry, for example, the warpage of
the strengthening is combined with glass forming curvature to yield
a desired shape.
[0170] Where a desired glass article shape entails a non-uniform
cross-sectional shape, or thickness, symmetric chemical
strengthening would actually contribute to a wider-spread potential
warpage. Asymmetric chemical strengthening allows for both
inclusion of the desired compressive stress layers and depth and
avoids the significant warpage. Glass forming combines with the
strengthening to provide an optimized glass article.
[0171] FIG. 23 is a flow chart illustrating that a glass article
can be identified and formed with the appropriate local stiffness
to counteract the proposed asymmetric chemical strengthening 2300.
The formed glass 2302 can undergo CNS and polishing 2304. The glass
article then undergoes the various steps required to introduce the
asymmetric chemical strengthening, including, for example, the use
of barrier layers, pastes, heat, etc. (2306, 2308, 2310, 2312, 2314
and 2316). The formed glass article with enhanced stiffness can be
treated multiple times to obtain a highly calibrated surface or
surface.
Optimized Glass Article Design Based on Stress Distribution
[0172] Embodiments herein include processes for calibrating the
strength of a glass article for a particular use using any one or
more of: pre-heating a glass article to a higher glass density,
modifying the edge geometries of a glass article to maximize
geometric strengthening, modified chemical strengthening using
masking, ion barrier or limiting coatings, chemical strengthening
using ion enhancing pastes and heat, thermally assisting the
chemical strengthening, directed or preferred ion diffusion using
electric fields and heat, introducing pre-stress to target
articles, and tuning the stress found in asymmetrically prepared
glass articles.
[0173] Calibration can also occur during the glass manufacturing
process, for example, through differential strengthening of glass
in clad layers, through identification of useful ion gradients and
concentrations in starting glass, and through fusing glass articles
together, and the like.
[0174] Aspects herein utilize each of the above embodiments to
calibrate a glass article, having a budgeted amount of stress, in
the vertical and horizontal axis. Budgeted and irregular stress
allows for placement of compressive stress layers of predetermined
hardness and depth on the front, back, top, sides and edges of a
glass article to both optimize the reliability of the glass article
and to make the glass article safe for its intended use. Budgeted
irregular stress in the glass article can also be offset by
counteracting stress input by other materials, or by the geometry
of the glass itself. This is particularly useful when the finished
glass article is designed to be flat or other targeted geometry. In
this manner a glass cover, for example, can be evaluated for its
intended use, i.e., how much surface compressive stress does the
article require on the top surface, bottom surface, edges, etc.,
how deep does the compressive stress need to extend at each of
these zones, how much tensile strength will result from these
compressive stress needs, how much tensile strength will result,
can the required stresses be balanced using chemical strengthening
alone, can glass forming be used, and the like. Embodiments herein
are then utilized to perform the calibration to provide a high
utility glass cover with maximized or optimized value.
[0175] The following Example is provided for illustrative purposes
only and is not intended to limit the scope of the disclosure.
EXAMPLE
Glass Forming to Compensate for Asymmetric Chemical
Strengthening
[0176] The depth of compression in ion-exchange chemical
strengthening is correlated to the ability of a glass article to
resist failure by damage induction. In this light, maximizing the
depth of compression is a significant driver in producing more
durable and reliable glass for use in portable electronic
devices.
[0177] Depth of compression in a glass article saturates once the
ions have been diffused through the thickness of the glass. This
shows that asymmetric strengthening can be used to achieve a deeper
depth of compression, and thereby facilitate a glass article's
ability to resist failure. Further, although asymmetric
strengthening introduces warpage via a stress imbalance in the
glass article, the warpage can be compensated for by using glass
forming.
[0178] The use of glass forming includes using stiffer cover glass
designs, as well as forming a cover glass geometry to compensate
for the introduced asymmetric warpage. For example, glass forming
can be used to compensate or exacerbate the asymmetric chemical
strengthening stresses to ensure that the combined procedures
results in the desired final part shape.
[0179] Depth of compression can be implemented into a cover glass
by using one or more of the asymmetric chemical strengthening
processes described herein. As shown in FIG. 24, cover glass must
generally be designed to have a balanced stress budget. As such,
cover glass 2400 shows a high compression stress (.about.750 Mpa)
but a fairly shallow depth of compression (.about.40 .mu.m), while
cover glass 2402 has a two-step strengthening, where the
compressive stress is also 750 Megapascals (Mpa) for a very limited
depth, followed by a depth of compression out to .about.100 .mu.m.
Cover glass 2404 shows a lower compressive stress (.about.450 MPa)
with a much deeper depth of compression than found in either cover
glass 2400 or 2402 (.about.150 .mu.m). Failure rates of cover glass
2400, 2402, and 2404 showed that depth of compression provided a
significant advantage over compressive stress when it comes to
failure due to impact, particularly where the drop is onto an
asphalt surface (cover glass 900=71.2% versus cover glass
904=35%).
[0180] Since asymmetric chemical strengthening is advantageous for
cover glass design, glass forming was used to correct and maintain
the stress imbalance that results from designing a material having
asymmetric strengthening.
[0181] FIGS. 25-30 illustrate one such asymmetric chemical
strengthening and glass forming procedure.
[0182] In FIG. 25, a glass cover is obtained and undergoes CNC to
fit its basic design needs. A cross section view shows the initial
cover glass geometry. FIG. 25 shows that glass forming can be used
to introduce a bend 2502 (via bending stress) at the end of the
cover glass 2500. Note that symmetric chemical strengthening of
this formed glass would result in a highly warped glass article,
and provide little value.
[0183] In FIG. 26, the cover glass 2600 can undergo further CNC and
polishing to further prepare the cover glass. Next, in FIG. 27, the
bottom flat surface 2702 of the cover glass 2700, up to the formed
bend, is coated with a layer of an ion-exchange diffusion barrier,
SiN 2704. The SiN will significantly limit ion diffusion through
the flat bottom surface of the cover glass. This will further
ensure that the covered surface remains substantially flat.
[0184] The formed and partially masked cover glass is treated under
the chemical strengthening process described herein in FIG. 28A and
28B. As can be seen from FIG. 28A, a cross section of the glass
2800 indicates that the top surface 2802 of the cover glass has a
compression layer of depth DoL formed by diffusion of potassium
2803. The bottom surface 2804, coated by the SiN has no, or very
minimal, chemical strengthening, as expected. FIG. 28B shows a
cross-sectional view of the status of the formed cover glass
2800.
[0185] FIG. 28C is the corresponding stress profile, where the top
surface 2802 of the glass cover 2800 shows a high compressive
stress and significant DoL and the bottom cover 2804, where there
was no strengthening, shows no compression and only tensile stress
(that results from balancing the stress at the top surface).
[0186] FIG. 29A and 29B illustrate that the SiN layer on the bottom
surface 2902 of the glass cover 2900 can be oxidized to SiO.sub.2
2903, which is no longer a complete barrier to chemical
strengthening. A second round of chemical strengthening is
performed on the formed glass cover to provide the cross sectional
view shown in FIG. 29A. Note that the bottom surface 2502 now
includes a shallow compressive layer 2904, while the top surface
2906 has been further enhanced with a higher surface compression
(FIG. 29B).
[0187] Finally, FIG. 30A and 30B illustrate the final cover glass
3000 that includes a cover glass geometry to complement an
asymmetric stress profile from the series of chemical strengthening
procedures. The cover glass has excellent top cover surface
compression 3002 and DoL, matched by the geometry and high
compressive stress with limited DoL of the bottom surface 3004 (see
FIG. 30B).
[0188] FIG. 30C is the corresponding stress profile, where the top
surface 3006 of the glass cover 3000 shows high surface compression
3008. The bottom surface 3010 shows some amount of surface
compression 3012, corresponding to the lower allowance of chemical
strengthening. The cover glass 3000 has a corresponding, but
budgeted amount of tensile stress 3014 to offset the top and bottom
asymmetric surface compression.
[0189] The foregoing description, for purposes of explanation, used
specific nomenclature to provide a thorough understanding of the
described embodiments. However, it will be apparent to one skilled
in the art that the specific details are not required in order to
practice the described embodiments. Thus, the foregoing
descriptions of the specific embodiments described herein are
presented for purposes of illustration and description. They are
not intended to be exhaustive or to limit the embodiments to the
precise forms disclosed. It will be apparent to one of ordinary
skill in the art that many modifications and variations are
possible in view of the above teachings.
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