U.S. patent application number 13/567644 was filed with the patent office on 2013-08-08 for strengthened glass articles and methods of making.
This patent application is currently assigned to CORNING INCORPORATED. The applicant listed for this patent is Kristen L. Barefoot, James Joseph Price, Jose Mario Quintal, Ronald Leroy Stewart. Invention is credited to Kristen L. Barefoot, James Joseph Price, Jose Mario Quintal, Ronald Leroy Stewart.
Application Number | 20130202868 13/567644 |
Document ID | / |
Family ID | 41268841 |
Filed Date | 2013-08-08 |
United States Patent
Application |
20130202868 |
Kind Code |
A1 |
Barefoot; Kristen L. ; et
al. |
August 8, 2013 |
STRENGTHENED GLASS ARTICLES AND METHODS OF MAKING
Abstract
A strengthened glass article having a central tension that is
below a threshold value above which the glass exhibits frangible
behavior. The central tension varies non-linearly with the
thickness of the glass. The glass article may be used as cover
plates or windows for portable or mobile electronic devices such as
cellular phones, music players, information terminal (IT) devices,
including laptop computers, and the like.
Inventors: |
Barefoot; Kristen L.;
(Corning, NY) ; Price; James Joseph; (Corning,
NY) ; Quintal; Jose Mario; (Campbell, NY) ;
Stewart; Ronald Leroy; (Elmira, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Barefoot; Kristen L.
Price; James Joseph
Quintal; Jose Mario
Stewart; Ronald Leroy |
Corning
Corning
Campbell
Elmira |
NY
NY
NY
NY |
US
US
US
US |
|
|
Assignee: |
CORNING INCORPORATED
Corning
NY
|
Family ID: |
41268841 |
Appl. No.: |
13/567644 |
Filed: |
August 6, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13451684 |
Apr 20, 2012 |
8415013 |
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13567644 |
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13289294 |
Nov 4, 2011 |
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13451684 |
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12537393 |
Aug 7, 2009 |
8075999 |
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13289294 |
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61087324 |
Aug 8, 2008 |
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Current U.S.
Class: |
428/220 |
Current CPC
Class: |
C03C 3/093 20130101;
C03C 4/18 20130101; C03C 21/002 20130101; C03C 3/078 20130101; C03C
3/083 20130101; C03C 3/087 20130101; Y10T 428/315 20150115; C03C
3/064 20130101; Y10T 428/31 20150115; C03C 3/085 20130101; C03C
3/091 20130101; C03C 3/095 20130101; C03C 2204/00 20130101 |
Class at
Publication: |
428/220 |
International
Class: |
C03C 3/078 20060101
C03C003/078 |
Claims
1. A strengthened glass article, the strengthened glass article
having a thickness t.ltoreq.0.75 mm and comprising: an outer
region; an inner region, wherein the inner region is under a
central tension CT, and wherein CT (MPa)>-15.7 (MPa/mm)t
(mm)+52.5 (MPa), wherein when the strengthened glass article is
broken by a point impact, the strengthened glass article exhibits
at least one of fragment size n.sub.1 (%.ltoreq.1 mm) of
0%.ltoreq.n.sub.1.ltoreq.5%, crack branching n.sub.3 of
0.ltoreq.n.sub.3.ltoreq.5, or combinations thereof.
2. The strengthened glass article of claim 1, wherein when the
strengthened glass is broken by a point impact, the strengthened
glass article further exhibits at least one of fragment density
n.sub.2 (fragments/cm.sup.2) of 0
fragments/cm.sup.2.ltoreq.n.sub.2.ltoreq.3 fragments/cm.sup.2,
ejection n.sub.4 (%.gtoreq.5 cm) of 0%.ltoreq..sub.4.ltoreq.2%, or
combinations thereof.
3. The strengthened glass article of claim 1, wherein the
strengthened glass article has a frangibility index of less than
3.
4. The strengthened glass article of claim 1, wherein the
strengthened glass article has a frangibility index of less than
2.
5. The strengthened glass article of claim 2, wherein the
strengthened glass article exhibits at least one of fragment size
n.sub.1 (%.ltoreq.1 mm) of 0%, fragment density n.sub.2
(fragments/cm.sup.2) of .ltoreq.1 fragments/cm.sup.2, crack
branching n.sub.3 of .ltoreq.2, ejection n.sub.4 (%.gtoreq.5 cm) of
0%, or combinations thereof.
6. The strengthened glass article of claim 5, wherein the
strengthened glass article has a frangibility index of less than
2.
7. The strengthened glass article of claim 1, wherein the
strengthened glass article is non-frangible.
8. The strengthened glass article of claim 1, wherein the
strengthened glass article comprises an alkali aluminosilicate
glass.
9. The strengthened glass article of claim 8, wherein the alkali
aluminosilicate glass comprises: 60-70 mol % SiO.sub.2; 6-14 mol %
Al.sub.2O.sub.3; 0-15 mol % B.sub.2O.sub.3; 0-15 mol % Li.sub.2O;
0-20 mol % Na.sub.2O; 0-10 mol % K.sub.2O; 0-10 mol % CaO; 0-5 mol
% ZrO.sub.2; 0-1 mol % SnO.sub.2; 0-1 mol % CeO.sub.2; less than 50
ppm As.sub.2O.sub.3; and less than 50 ppm Sb.sub.2O.sub.3; wherein
12 mol %.ltoreq.Li.sub.2O+Na.sub.2O+K.sub.2O.ltoreq.20 mol % and 0
mol %.ltoreq.MgO+CaO.ltoreq.10 mol %.
10. A strengthened glass article, the strengthened glass article
having a thickness t.ltoreq.0.75 mm and comprising: an outer
region; an inner region, wherein the inner region is under a
central tension CT, and wherein CT (MPa)>-15.7 (MPa/mm)t
(mm)+52.5 (MPa), wherein when the strengthened glass article is
broken by a point impact, the strengthened glass article exhibits
at least one of fragment size n.sub.1 (%.ltoreq.1 mm) of
0%.ltoreq.n.sub.1.ltoreq.5%, fragment density n.sub.2
(fragments/cm.sup.2) of 0
fragments/cm.sup.2.ltoreq.n.sub.2.ltoreq.3 fragments/cm.sup.2,
crack branching n.sub.3 of 0.ltoreq.n.sub.3.ltoreq.5, ejection
n.sub.4 (%.gtoreq.5 cm) of 0%.ltoreq.n.sub.4.ltoreq.2%, or
combinations thereof.
11. The strengthened glass article of claim 10, wherein the
strengthened glass article has a frangibility index of less than
3.
12. The strengthened glass article of claim 10, wherein the
strengthened glass article exhibits at least one of fragment size
n.sub.1 (%.ltoreq.1 mm) of 0%, fragment density n.sub.2
(fragments/cm.sup.2) of .ltoreq.1 fragments/cm.sup.2, crack
branching n.sub.3 of .ltoreq.2, ejection n.sub.4 (%>5 cm) of 0%,
or combinations thereof.
13. The strengthened glass article of claim 10, wherein the
strengthened glass article is non-frangible.
14. The strengthened glass article of claim 10, wherein the
strengthened glass article comprises an alkali aluminosilicate
glass.
15. The strengthened glass article of claim 14, wherein the alkali
aluminosilicate glass comprises: 60-70 mol % SiO.sub.2; 6-14 mol %
Al.sub.2O.sub.3; 0-15 mol % B.sub.2O.sub.3; 0-15 mol % Li.sub.2O;
0-20 mol % Na.sub.2O; 0-10 mol % K.sub.2O; 0-10 mol % CaO; 0-5 mol
% ZrO.sub.2; 0-1 mol % SnO.sub.2; 0-1 mol % CeO.sub.2; less than 50
ppm As.sub.2O.sub.3; and less than 50 ppm Sb.sub.2O.sub.3; wherein
12 mol %.ltoreq.Li.sub.2O+Na.sub.2O+K.sub.2O.ltoreq.20 mol % and 0
mol %.ltoreq.MgO+CaO.ltoreq.10 mol %.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/451,684 filed Apr. 20, 2012, which is a continuation of U.S.
application Ser. No. 13/289,294, filed Nov. 4, 2011, which is a
divisional of U.S. application Ser. No. 12/537,393 (now U.S. Pat.
No. 8,075,999) and claims the benefit of U.S. Provisional Patent
Application No. 61/087,324, filed Aug. 8, 2008. This application
incorporates each of the foregoing applications herein by reference
in their entireties.
BACKGROUND
[0002] Chemically strengthened glasses have been identified for use
in hand held devices, such as mobile phones, media players, and
other devices, as well as other applications requiring
transparency, high strength and abrasion resistance. However, such
glasses are potentially susceptible to frangible behavior--i.e.,
the glass energetically fragments into a large number of small
pieces when impacted with sufficient penetration force.
SUMMARY
[0003] Strengthened glasses having a central tension below a
threshold value, above which the glass exhibits frangible behavior,
are provided and described herein. The central tension varies
nonlinearly with the thickness of the glass. The glasses may be
used as cover plates or windows for portable or mobile electronic
communication and entertainment devices, such as cellular phones,
music players; and information terminal (IT) devices, such as
laptop computers and the like.
[0004] Accordingly, one aspect of the disclosure is to provide a
strengthened glass article having a thickness t.ltoreq.0.75 mm and
comprising an outer region and an inner region, wherein the inner
region is under a central tension CT, and wherein CT (MPa)>-15.7
(MPa/mm)t (mm)+52.5 (MPa), wherein when the strengthened glass
article is broken by a point impact, the strengthened glass article
exhibits at least one of fragment size n.sub.1 (%.ltoreq.1 mm) of
0%.ltoreq.n.sub.1.ltoreq.5%, crack branching n.sub.3 of
0.ltoreq.n.sub.3.ltoreq.5, or combinations thereof.
[0005] Another aspect of the disclosure is to provide a
strengthened glass article having a thickness t.ltoreq.0.75 mm and
comprising an outer region and an inner region, wherein the inner
region is under a central tension CT, and wherein CT (MPa)>-15.7
(MPa/mm)t (mm)+52.5 (MPa), wherein when the strengthened glass
article is broken by a point impact, the strengthened glass article
exhibits at least one of fragment size n.sub.1 (%.ltoreq.1 mm) of
0%.ltoreq.n.sub.1.ltoreq.5%, fragment density n.sub.2
(fragments/cm.sup.2) of 0
fragments/cm.sup.2.ltoreq.n.sub.2.ltoreq.3 fragments/cm.sup.2,
crack branching n.sub.3 of 0.ltoreq.n.sub.3.ltoreq.5, ejection
n.sub.4 (%.gtoreq.5 cm) of 0%.ltoreq.n.sub.4.ltoreq.2%, or
combinations thereof.
[0006] These and other aspects, advantages, and salient features
will become apparent from the following detailed description, the
accompanying drawings, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1a is a photograph showing strengthened glass articles
1) exhibiting frangible behavior upon fragmentation; and 2)
exhibiting non-frangible behavior upon fragmentation;
[0008] FIG. 1b is a photograph showing strengthened glass sheets
that exhibit non-frangible behavior upon fragmentation;
[0009] FIG. 2 includes plots of threshold central tension as a
function of glass thickness for strengthened alkali aluminosilicate
glass articles; and
[0010] FIG. 3 is a schematic representation of a strengthened glass
article.
DETAILED DESCRIPTION
[0011] In the following description, like reference characters
designate like or corresponding parts throughout the several views
shown in the figures. It is also understood that, unless otherwise
specified, terms such as "top," "bottom," "outward," "inward," and
the like are words of convenience and are not to be construed as
limiting terms. In addition, whenever a group is described as
comprising at least one of a group of elements and combinations
thereof, it is understood that the group may comprise, consist
essentially of, or consist of any number of those elements recited,
either individually or in combination with each other. Similarly,
whenever a group is described as consisting of at least one of a
group of elements or combinations thereof, it is understood that
the group may consist of any number of those elements recited,
either individually or in combination with each other. Unless
otherwise specified, a range of values, when recited, includes both
the upper and lower limits of the range, as well as any sub-ranges
therebetween.
[0012] Referring to the drawings in general, it will be understood
that the illustrations are for the purpose of describing particular
embodiments and are not intended to limit the disclosure or
appended claims thereto. The drawings are not necessarily to scale,
and certain features and certain views of the drawings may be shown
exaggerated in scale or in schematic in the interest of clarity and
conciseness.
[0013] Frangible behavior (also referred to herein as
"frangibility") refers to extreme fragmentation behavior of a glass
article. Frangible behavior is the result of development of
excessive internal or central tension CT within the article,
resulting in forceful or energetic fragmentation of the article
upon fracture. In thermally tempered, laminated, or chemically
strengthened (e.g., strengthened by ion exchange) glass articles,
frangible behavior can occur when the balancing of compressive
stresses (CS) in a surface or outer region of the glass article
(e.g., a plate or sheet) with tensile stress in the center of the
glass plate provides sufficient energy to cause multiple crack
branching with ejection or "tossing" of small glass pieces and/or
particles from the article. The velocity at which such ejection
occurs is a result of the excess energy within the glass article,
stored as central tension CT.
[0014] The frangibility of a glass article is a function of central
tension CT and compressive stress CS. In particular, the central
tension CT within a glass article can be calculated from the
compressive stress CS. Compressive stress CS is measured near the
surface (i.e., within 100 .mu.m), giving a maximum CS value and a
measured depth of the compressive stress layer (also referred to
herein as "depth of layer" or "DOL"). Compressive stress and depth
of layer are measured using those means known in the art. Such
means include, but are not limited to, measurement of surface
stress (FSM) using commercially available instruments such as the
FSM-6000, manufactured by Luceo Co., Ltd. (Tokyo, Japan), or the
like, and methods of measuring compressive stress and depth of
layer are described in ASTM 1422C-99, entitled "Standard
Specification for Chemically Strengthened Flat Glass," and ASTM
1279.19779 "Standard Test Method for Non-Destructive Photoelastic
Measurement of Edge and Surface Stresses in Annealed,
Heat-Strengthened, and Fully-Tempered Flat Glass," the contents of
which are incorporated herein by reference in their entirety.
Surface stress measurements rely upon the accurate measurement of
the stress optical coefficient (SOC), which is related to the
birefringence of the glass. SOC in turn is measured by those
methods that are known in the art, such as fiber and four point
bend methods, both of which are described in ASTM standard C770-98
(2008), entitled "Standard Test Method for Measurement of Glass
Stress-Optical Coefficient," the contents of which are incorporated
herein by reference in their entirety, and a bulk cylinder method.
The relationship between CS and central tension CT is given by the
expression:
CT=(CS*DOL)/(t-2DOL) (1),
wherein t is the thickness of the glass article. Unless otherwise
specified, central tension CT and compressive stress CS are
expressed herein in megaPascals (MPa), whereas thickness t and
depth of layer DOL are expressed in millimeters. The depth of the
compression layer DOL and the maximum value of compressive stress
CS that can be designed into or provided to a glass article are
limited by such frangible behavior. Consequently, frangible
behavior is one consideration to be taken into account in the
design of various glass articles.
[0015] Frangible behavior is characterized by at least one of:
breaking of the strengthened glass article (e.g., a plate or sheet)
into multiple small pieces (e.g., .ltoreq.1 mm); the number of
fragments formed per unit area of the glass article; multiple crack
branching from an initial crack in the glass article; violent
ejection of at least one fragment a specified distance (e.g., about
5 cm, or about 2 inches) from its original location; and
combinations of any of the foregoing breaking (size and density),
cracking, and ejecting behaviors. As used herein, the terms
"frangible behavior" and "frangibilty" refer to those modes of
violent or energetic fragmentation of a strengthened glass article
absent any external restraints, such as coatings, adhesive layers,
or the like. While coatings, adhesive layers, and the like may be
used in conjunction with the strengthened glass articles described
herein, such external restraints are not used in determining the
frangibility or frangible behavior of the glass articles.
[0016] FIGS. 1a and 1b illustrate examples of frangible behavior
and non-frangible behavior of strengthened glass articles upon
point impact with a sharp indenter. The point impact test that is
used to determine frangible behavior includes an apparatus that is
delivered to the surface of the glass article with a force that is
just sufficient to release the internally stored energy present
within the strengthened glass article. That is, the point impact
force is sufficient to create at least one new crack at the surface
of the strengthened glass sheet and extend the crack through the
compressive stress CS region (i.e., depth of layer) into the region
that is under central tension CT. The impact energy needed to
create or activate the crack in a strengthened glass sheet depends
upon the compressive stress CS and depth of layer DOL of the
article, and thus upon the conditions under which the sheet was
strengthened (i.e., the conditions used to strengthen a glass by
ion exchange). Otherwise, each ion exchanged glass plate shown in
FIGS. 1a and 1b was subjected to a sharp dart indenter contact
sufficient to propagate a crack into the inner region of the plate,
the inner region being under tensile stress. The force applied to
the glass plate was just sufficient to reach the beginning of the
inner region, thus allowing the energy that drives the crack to
come from the tensile stresses in the inner region rather than from
the force of the dart impact on the outer surface.
[0017] The glass sheets shown in FIGS. 1a and 1b are 50 mm.times.50
mm ion exchanged alkali aluminosilicate glass plates, each sample
having a thickness of 0.5 mm. Each of the samples had a composition
of either 66.7 mol % SiO.sub.2; 10.5 mol % Al.sub.2O.sub.3; 0.64
mol % B.sub.2O.sub.3; 13.8 mol % Na.sub.2O; 2.06 mol % K.sub.2O;
5.50 mol % MgO; 0.46 mol % CaO; 0.01 mol % ZrO.sub.2; 0.34 mol %
As.sub.2O.sub.3; and 0.007 mol % Fe.sub.2O.sub.3; or 66.4 mol %
SiO.sub.2; 10.3 mol % Al.sub.2O.sub.3; 0.60 mol % B.sub.2O.sub.3;
4.0 mol % Na.sub.2O; 2.10 mol % K.sub.2O; 5.76 mol % MgO; 0.58 mol
% CaO; 0.01 mol % ZrO.sub.2; 0.21 mol % SnO.sub.2; and 0.007 mol %
Fe.sub.2O.sub.3.
[0018] Referring to FIG. 1a, glass plate a (FIG. 1a) can be
classified as being frangible. In particular, glass plate a
fragmented into multiple small pieces that were ejected, and
exhibited a large degree of crack branching from the initial crack
to produce the small pieces. Approximately 50% of the fragments are
less than 1 mm in size, and it is estimated that about 8 to 10
cracks branched from the initial crack. Glass pieces were also
ejected about 5 cm from original glass plate a, as seen in FIG. 1a.
A glass article that exhibits any of the three criteria (i.e.,
multiple crack branching, ejection, and extreme fragmentation)
described hereinabove is classified as being frangible. For
example, if a glass exhibits excessive branching alone but does not
exhibit ejection or extreme fragmentation as described above, the
glass is still characterized as frangible.
[0019] Glass plates b, c, (FIG. 1b) and d (FIG. 1a) are classified
as not frangible. In each of these samples, the glass sheet has
broken into a small number of large pieces. Glass plate b (FIG.
2b), for example, has broken into two large pieces with no crack
branching; glass plate c (FIG. 2b) has broken into four pieces with
two cracks branching from the initial crack; and glass plate d
(FIG. 2a) has broken into four pieces with two cracks branching
from the initial crack. Based on the absence of ejected fragments
(i.e., no glass pieces forcefully ejected more than 2 inches from
their original location), no visible fragments.ltoreq.1 mm in size,
and the minimal amount of observed crack branching, samples b, c,
and d are classified as non-frangible or substantially
non-frangible.
[0020] Based on the foregoing, a frangibility index (Table 1) can
be constructed to quantify the degree of frangible or non-frangible
behavior of a glass, glass ceramic, and/or a ceramic article upon
impact with another object. Index numbers, ranging from 1 for
non-frangible behavior to 5 for highly frangible behavior, have
been assigned to describe different levels of frangibility or
non-frangibility. Using the index, frangibility can be
characterized in terms of numerous parameters: 1) the percentage of
the population of fragments having a diameter (i.e., maximum
dimension) of less than 1 mm ("Fragment size" in Table 1); 2) the
number of fragments formed per unit area (in this instance,
cm.sup.2) of the sample ("Fragment density" in Table 1); 3) the
number of cracks branching from the initial crack formed upon
impact ("Crack branching" in Table 1); and 4) the percentage of the
population of fragments that is ejected upon impact more than about
5 cm (or about 2 inches) from their original position ("Ejection"
in Table 1).
TABLE-US-00001 TABLE 1 Criteria for determining the degree of
frangibility and frangibility index. Degree Fragment of Frangi-
Fragment density frangi- bility size (fragments/ Crack Ejection
bility index (% .ltoreq.1 mm) cm.sup.2) branching (% .gtoreq.5 cm)
High 5 >20 >7 >9 >6 Medium 4 10 < n .ltoreq. 5 <
n .ltoreq. 7 7 < n .ltoreq. 9 4 < n .ltoreq. 6 20 Low 3 5
< n .ltoreq. 10 3 < n .ltoreq. 5 5 < n .ltoreq. 7 2 < n
.ltoreq. 4 None 2 0 < n .ltoreq. 5 1 < n .ltoreq. 3 2 < n
.ltoreq. 5 0 < n .ltoreq. 2 1 0 n .ltoreq. 1 n .ltoreq. 2 0
[0021] A frangibility index is assigned to a glass article if the
article meets at least one of the criteria associated with a
particular index value. Alternatively, if a glass article meets
criteria between two particular levels of frangibility, the article
may be assigned a frangibility index range (e.g., a frangibility
index of 2-3). The glass article may be assigned the highest value
of frangibility index, as determined from the individual criteria
listed in Table 1. In many instances, it is not possible to
ascertain the values of each of the criteria, such as the
fragmentation density or percentage of fragments ejected more than
5 cm from their original position, listed in Table 1. The different
criteria are thus considered individual, alternative measures of
frangible behavior and the frangibility index such that a glass
article falling within one criteria level will be assigned the
corresponding degree of frangibility and frangibility index. If the
frangibility index based on any of the four criteria listed in
Table 1 is 3 or greater, the glass article is classified as
frangible.
[0022] Applying the foregoing frangibility index to the samples
shown in FIGS. 1a and 1b, glass plate a fragmented into multiple
ejected small pieces and exhibited a large degree of crack
branching from the initial crack to produce the small pieces.
Approximately 50% of the fragments are less than 1 mm in size and
it is estimated that about 8 to 10 cracks branched from the initial
crack. Based upon the criteria listed in Table 1, glass plate a has
a frangibility index of between about 4-5, and is classified as
having a medium-high degree of frangibility.
[0023] A glass article having a frangibility index of less than 3
(low frangibility) may be considered to be non-frangible or
substantially non-frangible. Glass plates b, c, and d each lack
fragments having a diameter of less than 1 mm, multiple branching
from the initial crack formed upon impact and fragments ejected
more than 5 cm from their original position. Glass plates b, c, and
d are non-frangible and thus have a frangibility index of 1 (not
frangible).
[0024] As previously discussed, the observed differences in
behavior between glass plate a, which exhibited frangible behavior,
and glass plates b, c, and d, which exhibited non-frangible
behavior, in FIGS. 1a and 1b can be attributed to differences in
central tension CT among the samples tested. The possibility of
such frangible behavior is one consideration in designing various
glass products, such as cover plates or windows for portable or
mobile electronic devices such as cellular phones, entertainment
devices, and the like, as well as for displays for information
terminal (IT) devices, such as laptop computers. Moreover, the
depth of the compression layer DOL and the maximum value of
compressive stress CS that can be designed into or provided to a
glass article are limited by such frangible behavior.
[0025] Accordingly, in order to avoid frangibility, a glass article
should be designed to have a central tension CT at or below a
critical or threshold central tension CT for the glass article to
avoid frangibility upon impact with another object, while taking
both compressive stress CS and depth of layer DOL into account.
Based on empirical observations of the frangible behavior of glass
articles having thicknesses greater than or equal to about 2 mm,
the relationship between the "critical" or "threshold" amount of
central tension that produces unacceptable frangible behavior and
the glass thickness t was heretofore believed to be linear. An
example of the threshold central tension CT (also referred to
herein as the "threshold CT") at which the onset (also referred to
herein as the "critical" or "threshold" central tension value) of
unacceptable frangible behavior was believed to occur is plotted as
a function of thickness t in FIG. 2 (line 2).
[0026] The data represented by line 2 shown in FIG. 2 are based
upon behavior that was experimentally observed for a series of
chemically strengthened alkali aluminosilicate glass samples,
having a composition of either 66.7 mol % SiO.sub.2; 10.5 mol %
Al.sub.2O.sub.3; 0.64 mol % B.sub.2O.sub.3; 13.8 mol % Na.sub.2O;
2.06 mol % K.sub.2O; 5.50 mol % MgO; 0.46 mol % CaO; 0.01 mol %
ZrO.sub.2; 0.34 mol % As.sub.2O.sub.3; and 0.007 mol %
Fe.sub.2O.sub.3; or 66.4 mol % SiO.sub.2; 10.3 mol %
Al.sub.2O.sub.3; 0.60 mol % B.sub.2O.sub.3; 4.0 mol % Na.sub.2O;
2.10 mol % K.sub.2O; 5.76 mol % MgO; 0.58 mol % CaO; 0.01 mol %
ZrO.sub.2; 0.21 mol % SnO.sub.2; and 0.007 mol % Fe.sub.2O.sub.3,
that had been strengthened by ion exchange. Each of the samples had
a thickness of at least 2 mm. The data represented by line 2 in
FIG. 2 indicate that the relationship between the threshold central
tension CT (as determined from equation (1) and CS, DOL, and t) and
thickness t of the glass is linear (referred to hereinafter as
"linear threshold central tension CT.sub.2" or "CT.sub.2) and is
described by the equation:
CT.sub.2 (MPa)=-15.7 (MPa/mm)t (mm)+52.5 (MPa) (2).
[0027] Equation (2) is derived from experimental compressive stress
and depth of layer data that were obtained for chemically
strengthened glass samples, each having a thickness of at least 2
mm. Extrapolated to lesser thicknesses, equation (2) provides a
lower limit of CT for the strengthened glasses described herein.
Due to the relationship between central tension, compressive
stress, and depth of layer derived from data obtained for samples
in which thickness t.gtoreq.2 mm and expressed in equation (2), the
linear behavior of the threshold CT.sub.2 with respect to thickness
t limits the amount of compressive stress and depth of layer that
may be created. Consequently, design flexibility for certain
applications, particularly those in which thinner sheets of glass
are used, would be expected to be limited based upon such expected
linear behavior. For example, glass sheets would be strengthened to
achieve the CS and DOL values to achieve a central tension that is
below the threshold CT.sub.2 value predicted by equation (2) and
illustrated by line 2 in FIG. 2.
[0028] As described herein, the relationship between the critical
or threshold amount of central tension CT that produces frangible
behavior in strengthened glass articles and, in particular, glass
articles having a thickness t of less than 2 mm, has been found to
be nonlinear. Accordingly, a strengthened glass article, such as a
strengthened sheet or plate that is substantially non-frangible
(i.e., free of frangible behavior), as defined by the criteria
described herein, is provided, and schematically shown in FIG. 3.
Strengthened glass article 300 has a thickness t, an outer region
310 extending from surface 312 to a depth of layer 314, and an
inner region 320. Outer region 310 of glass article 300 is
strengthened (i.e., thermally or chemically strengthened) so as to
be under a compressive stress CS. The compressive stress CS in
outer region 310 gives rise to a central tension CT, or tensile
stress, in inner region 320, which balances the compressive stress.
The depth of the compressive stress layer DOL 314 is the depth from
the surface to the point where the measured compression stress is
reduced to zero stress at the boundary with the tensile stress zone
(inner region 320). The relationship between central tension CT and
compressive stress CS is given by equation (1), previously
presented:
CT=(CS*DOL)/(t-2DOL) (1).
[0029] Referring to FIG. 2, a threshold central tension (threshold
CT) at which the onset (also referred to herein as the critical or
threshold central tension) of unacceptable frangible behavior
actually occurs is plotted as a function of thickness t and
represented by line 1 in FIG. 2. Line 1 is based upon
experimentally observed behavior of alkali aluminosilicate glasses
having a composition of either 66.7 mol % SiO.sub.2; 10.5 mol %
Al.sub.2O.sub.3; 0.64 mol % B.sub.2O.sub.3; 13.8 mol % Na.sub.2O;
2.06 mol % K.sub.2O; 5.50 mol % MgO; 0.46 mol % CaO; 0.01 mol %
ZrO.sub.2; 0.34 mol % As.sub.2O.sub.3; and 0.007 mol %
Fe.sub.2O.sub.3; or 66.4 mol % SiO.sub.2; 10.3 mol %
Al.sub.2O.sub.3; 0.60 mol % B.sub.2O.sub.3; 4.0 mol % Na.sub.2O;
2.10 mol % K.sub.2O; 5.76 mol % MgO; 0.58 mol % CaO; 0.01 mol %
ZrO.sub.2; 0.21 mol % SnO.sub.2; and 0.007 mol % Fe.sub.2O.sub.3
that had been ion exchanged. The data represented by line 1
indicates that the relationship between central tension CT
(referred to hereinafter as "nonlinear threshold central tension
CT.sub.1" or "CT.sub.1") and thickness t of the glass is actually
nonlinear and described by the equation
CT.sub.1 (MPa).ltoreq.-38.7 (MPa/mm)ln(t) (mm)+48.2 (MPa) (3).
[0030] Equation (3) is derived from experimental measurements of
compressive stresses CS and depths of layer DOL of ion exchanged
alkali aluminosilicate glass samples, each having a thickness of
less than about 1.4 mm. It has been observed that glass articles
have a nonlinear threshold central tension CT.sub.1 that is greater
than the linear central tension CT.sub.2 defined by the previously
expected linear relationship between CT and t represented by
equation (2). An unexpected range of central tension CT.sub.1 in
which unacceptable frangible behavior is minimized or absent is
therefore described by the equation
-15.7 (MPa/mm)t (mm)+52.5 (MPa).ltoreq.CT.sub.1 (MPa).ltoreq.-38.7
(MPa/mm)ln(t) (MPa)+48.2 (MPa) (4).
[0031] The nonlinear relationship between the allowable maximum
CT.sub.1 with glass article thickness, exemplified by line 1 of
FIG. 2 and equation (3) is unexpected in light of behavior
previously observed for thicker strengthened glass samples of
similar or identical compositions. If the relationship between CT
and thickness were linear (CT.sub.2), as demonstrated by line 2 of
FIG. 2 and expressed in equation (2), the CT threshold frangibility
for part thicknesses ranging from about 0.2 up to 2 mm would be
less than that determined from equation (3), and at least one of CS
and DOL would be correspondingly limited. The depth of the
compression layer (DOL) and maximum value of compressive stress CS
at low thicknesses would also have to be reduced. Such reductions
in CS and DOL in these ranges, as dictated by the linear behavior
shown in FIG. 2, would limit design flexibility for certain
applications.
[0032] The previously expected linear behavior of the threshold CT
(CT.sub.2, expressed by line 2 of FIG. 2) provides no suggestion of
a non-linear relationship between the actual threshold CT limit
(CT.sub.1) for frangibility t (line 1 of FIG. 2) as a function of
thickness. To further illustrate this unexpected result, Table 2
lists the actual nonlinear threshold central tension CT.sub.1
calculated using equation (3) from line 1 of FIG. 2, described
herein, the linear threshold central tension CT.sub.2 calculated
using equation (2) from line 2 of FIG. 2, and the difference
between the threshold CT values (CT.sub.1-CT.sub.2) calculated
using equations (2) and (3) for glass selected thicknesses. Table
2.
TABLE-US-00002 TABLE 2 (CT.sub.1 - CT.sub.2) CT.sub.1 CT.sub.2
(-38.7 ln(t) + 48.2) - -38.7 ln(t) + 48.2 -15.7 t + 52.5 (-15.7 t +
52.5) t (mm) (MPa) (MPa) (MPa) 1.5 32.5 28.9 3.6 1.25 37.6 32.9 4.7
1.0 48.2 36.8 11.4 0.75 59.3 40.7 18.6 0.5 75.0 44.7 30.4 0.3 94.8
47.8 47.0
[0033] As can be seen from the values listed in Table 2, the
difference (CT.sub.1-CT.sub.2) between the expected threshold
CT.sub.2 predicted by the linear relationship (equation (2)) and
the actual threshold CT.sub.1 predicted by the nonlinear
relationship (equation (3)) increases with decreasing thickness t.
As CT is related to thickness t, depth of layer DOL, and
compressive stress CS (equation (1)), the greater threshold CT
values predicted by the nonlinear relationship (CT.sub.1; equation
(3)) described herein provide a greater range of CS and DOL values
that may be used to design and prepare a strengthened glass sheet
that exhibits non-frangible behavior; i.e., has a frangibility
index that is less than 3. As a result, non-frangible strengthened
glass articles can be made at certain thicknesses and strengthened
so as to have a greater threshold central tension CT than
previously believed possible.
[0034] In one embodiment, the strengthened glass article 300 is
substantially non-frangible, or free of frangible behavior, as
described hereinabove. That is, strengthened glass article 300 has
a frangibility index, as described in Table 1 herein, of less than
3. Upon impact with a force sufficient to cause fragmentation of
strengthened glass 300, the percentage n.sub.1 of the population of
the fragments having a diameter (i.e., maximum dimension) of less
than or equal to 1 mm ("Fragment size" in Table 1) is less than or
equal to 5% (i.e., 0%.ltoreq.n.sub.1.ltoreq.5%); the number of
fragments formed per unit area (in this instance, cm.sup.2) n.sub.2
of the sample ("Fragment density" in Table 1) is less than or equal
to 3 fragments/cm.sup.2; the number of cracks n.sub.3 branching
from the initial crack formed upon impact ("Crack branching" in
Table 1) is less than or equal to 5 (i.e.,
0.ltoreq.n.sub.3.ltoreq.5); and the percentage of the population of
fragments n.sub.4 that is ejected upon impact more than about 5 cm
(or about 2 inches) from their original position ("Ejection" in
Table 1) is less than or equal to 2% (i.e.,
0%.ltoreq.n.sub.4.ltoreq.2%).
[0035] The data shown in line 2 of FIG. 1 provide no suggestion of
non-linear behavior of the threshold CT limit CT.sub.1 for
frangibility as a function of thickness. As can be seen from the
values listed in Table 2, the difference between the threshold CT
predicted by equation (2) and the threshold CT predicted by the
nonlinear relationship (equation 3) increases with decreasing
thickness t.
[0036] The strengthened glass articles described herein may
comprise numerous compositions. In one embodiment, the strengthened
glass article comprises an alkali aluminosilicate glass. In some
embodiments, the alkali aluminosilicate glass comprises, consists
essentially of, or consists of: 60-70 mol % SiO.sub.2; 6-14 mol %
Al.sub.2O.sub.3; 0-15 mol % B.sub.2O.sub.3; 0-15 mol % Li.sub.2O;
0-20 mol % Na.sub.2O; 0-10 mol % K.sub.2O; 0-10 mol % CaO; 0-5 mol
% ZrO.sub.2; 0-1 mol % SnO.sub.2; 0-1 mol % CeO.sub.2; less than 50
ppm As.sub.2O.sub.3; and less than 50 ppm Sb.sub.2O.sub.3; wherein
12 mol %.ltoreq.Li.sub.2O+Na.sub.2O+K.sub.2O.ltoreq.20 mol % and 0
mol %.ltoreq.MgO+CaO.ltoreq.10 mol %. In some embodiments, the
alkali aluminosilicate glass may further comprise up 8 mol % MgO,
and/or 0 mol %.ltoreq.SnO.sub.2.ltoreq.1 mol %. In other
embodiments, the alkali aluminosilicate glass comprises, consists
essentially of, or consists of: 64 mol %.ltoreq.SiO.sub.2.ltoreq.68
mol %; 12 mol %.ltoreq.Na.sub.2O.ltoreq.16 mol %; 8 mol
%.ltoreq.Al.sub.2O.sub.3.ltoreq.12 mol %; 0 mol
%.ltoreq.B.sub.2O.sub.3.ltoreq.3 mol %; 2 mol
%.ltoreq.K.sub.2O.ltoreq.5 mol %; 4 mol %.ltoreq.MgO.ltoreq.6 mol
%; and 0 mol %.ltoreq.CaO.ltoreq.5 mol %, wherein: 66 mol
%.ltoreq.SiO.sub.2+B.sub.2O.sub.3+CaO.ltoreq.69 mol %;
Na.sub.2O+K.sub.2O+B.sub.2O.sub.3+MgO+CaO+SrO>10 mol %; 5 mol
%.ltoreq.MgO+CaO+SrO.ltoreq.8 mol %;
(Na.sub.2O+B.sub.2O.sub.3)--Al.sub.2O.sub.3.ltoreq.2 mol %; 2 mol
%.ltoreq.Na.sub.2O--Al.sub.2O.sub.3.ltoreq.6 mol %; and 4 mol
%.ltoreq.(Na.sub.2O+K.sub.2O)--Al.sub.2O.sub.3.ltoreq.10 mol %. In
a third embodiment, the alkali aluminosilicate glass comprises,
consists essentially of, or consists of: 5-50 wt % SiO.sub.2; 2-20
wt % Al.sub.2O.sub.3; 0-15 wt % B.sub.2O.sub.3; 1-20 wt %
Na.sub.2O; 0-10 wt % Li.sub.2O; 0-10 wt % K.sub.2O; and 0-5 wt %
(MgO+CaO+SrO+BaO); 0-3 wt % (SrO+BaO); and 0-5 wt %
(ZrO.sub.2+TiO.sub.2), wherein
0.ltoreq.(Li.sub.2O+K.sub.2O)/Na.sub.2O.ltoreq.0.5.
[0037] In one particular embodiment, the alkali aluminosilicate
glass has the composition: 66.7 mol % SiO.sub.2; 10.5 mol %
Al.sub.2O.sub.3; 0.64 mol % B.sub.2O.sub.3; 13.8 mol % Na.sub.2O;
2.06 mol % K.sub.2O; 5.50 mol % MgO; 0.46 mol % CaO; 0.01 mol %
ZrO.sub.2; 0.34 mol % As.sub.2O.sub.3; and 0.007 mol %
Fe.sub.2O.sub.3. In another particular embodiment, the alkali
aluminosilicate glass has the composition: 66.4 mol % SiO.sub.2;
10.3 mol % Al.sub.2O.sub.3; 0.60 mol % B.sub.2O.sub.3; 4.0 mol %
Na.sub.2O; 2.10 mol % K.sub.2O; 5.76 mol % MgO; 0.58 mol % CaO;
0.01 mol % ZrO.sub.2; 0.21 mol % SnO.sub.2; and 0.007 mol %
Fe.sub.2O.sub.3. The alkali aluminosilicate glasses described
hereinabove are, in some embodiments, substantially free of
lithium, whereas in other embodiments, the alkali aluminosilicate
glass is substantially free of at least one of arsenic, antimony,
and barium.
[0038] Non-limiting examples of such alkali aluminosilicate glasses
are described in U.S. patent application Ser. No. 11/888,213, by
Adam J. Ellison et al., entitled "Down-Drawable, Chemically
Strengthened Glass for Cover Plate," filed on Jul. 31, 2007, which
claims priority from U.S. Provisional Patent Application
60/930,808, filed on May 22, 2007, and having the same title; U.S.
patent application Ser. No. 12/277,573, by Matthew J. Dejneka et
al., entitled "Glasses Having Improved Toughness and Scratch
Resistance," filed on Nov. 25, 2008, which claims priority from
U.S. Provisional Patent Application 61/004,677, filed on Nov. 29,
2007, and having the same title; U.S. patent application Ser. No.
12/392,577, by Matthew J. Dejneka et al., entitled "Fining Agents
for Silicate Glasses," filed Feb. 25, 2009, which claims priority
from U.S. Provisional Patent Application No. 61/067,130, filed Feb.
26, 2008, and having the same title; U.S. patent application Ser.
No. 12/393,241 by Matthew J. Dejneka et al., entitled
"Ion-Exchanged, Fast Cooled Glasses," filed Feb. 26, 2009, which
claims priority from U.S. Provisional Patent Application No.
61/067,732, filed Feb. 29, 2008, and having the same title, the
contents of which are incorporated herein by reference in their
entirety.
[0039] In one embodiment, the glass articles described herein, such
as glass article 300, are chemically strengthened by ion exchange.
In this process, ions in the surface layer of the glass are
replaced by--or exchanged with--larger ions having the same valence
or oxidation state. In those embodiments in which the glass article
comprises, consists essentially of, or consists of an alkali
aluminosilicate glass, ions in the surface layer of the glass and
the larger ions are monovalent alkali metal cations, such as
Li.sup.+ (when present in the glass), Na.sup.+, K.sup.+, Rb.sup.+,
and Cs.sup.+. Alternatively, monovalent cations in the surface
layer may be replaced with monovalent cations other than alkali
metal cations, such as Ag.sup.+ or the like.
[0040] Ion exchange processes are typically carried out by
immersing a glass article in a molten salt bath containing the
larger ions to be exchanged with the smaller ions in the glass. It
will be appreciated by those skilled in the art that parameters for
the ion exchange process, including, but not limited to, bath
composition and temperature, immersion time, the number of
immersions of the glass in a salt bath (or baths), use of multiple
salt baths, additional steps such as annealing, washing, and the
like, are generally determined by the composition of the glass and
the desired depth of layer and compressive stress of the glass as a
result of the strengthening operation. By way of example, ion
exchange of alkali metal-containing glasses may be achieved by
immersion in at least one molten bath containing a salt such as,
but not limited to, nitrates, sulfates, and chlorides of the larger
alkali metal ion. The temperature of the molten salt bath typically
is in a range from about 380.degree. C. up to about 450.degree. C.,
while immersion times range from about 15 minutes up to about 16
hours. However, temperatures and immersion times different from
those described above may also be used. Such ion exchange
treatments typically result in strengthened alkali aluminosilicate
glasses having depths of layer ranging from about 10 .mu.m up to at
least 50 .mu.m with a compressive stress ranging from about 200 MPa
up to about 800 MPa, and a central tension of less than about 100
MPa.
[0041] Non-limiting examples of ion exchange processes are provided
in the U.S. patent applications and provisional patent applications
that have been previously referenced hereinabove. In addition,
non-limiting examples of ion exchange processes in which glass is
immersed in multiple ion exchange baths, with washing and/or
annealing steps between immersions, are described in U.S.
Provisional Patent Application No. 61/079,995, by Douglas C. Allan
et al., entitled "Glass with Compressive Surface for Consumer
Applications," filed Jul. 11, 2008, in which glass is strengthened
by immersion in multiple, successive, ion exchange treatments in
salt baths of different concentrations; and U.S. Provisional Patent
Application No. 61/084,398, by Christopher M. Lee et al., entitled
"Dual Stage Ion Exchange for Chemical Strengthening of Glass,"
filed Jul. 29, 2008, in which glass is strengthened by ion exchange
in a first bath is diluted with an effluent ion, followed by
immersion in a second bath having a smaller concentration of the
effluent ion than the first bath. The contents of U.S. Provisional
Patent Application Nos. 61/079,995 and No. 61/084,398 are
incorporated herein by reference in their entirety.
[0042] In one embodiment, the glass is down-drawable by processes
known in the art, such as slot-drawing, fusion drawing, re-drawing,
and the like, and has a liquidus viscosity of at least 130
kilopoise.
[0043] In some embodiments, the strengthened glass article has a
thickness of up to about 2 mm, and, in a particular embodiment, the
thickness is in a range from about 0.2 mm up to about 2 mm. In
another embodiment, the thickness of the glass article is in a
range from about 0.5 mm up to about 0.75 mm and, in another
embodiment, from about 0.9 mm up to about 2 mm. In one particular
embodiment, the strengthened glass article outer region has a depth
of layer of at least 30 m and a compressive stress of at least 600
MPa.
[0044] Methods of making a strengthened glass article that is
substantially non-frangible, or free of frangible behavior, (i.e.,
having a frangibility index, as described herein, of less than 3)
is also provided. A glass article having a thickness t is first
provided. The glass article, in one embodiment, is an alkali
aluminosilicate glass, such as those described hereinabove. A
compressive stress CS is created in an outer region of the glass
article to strengthen the glass article. The compressive stress CS
is sufficient to generate a central tension CT in a central region
of the glass article such that CT (MPa).ltoreq.-38.7 (MPa/mm)n(t)
(mm)+48.2 (MPa). In one embodiment, compressive stress CS is
sufficient to generate a central tension CT in a central region of
the glass article such that -15.7 (MPa/mm)t (mm)+52.5
(MPa).ltoreq.CT (MPa).ltoreq.-38.7 (MPa/mm)n(t) (mm)+48.2
(MPa).
[0045] In one embodiment, the compressive stress is created by
chemically strengthening the glass article, for example, by the ion
exchange processes, previously described herein, in which a
plurality of first metal ions in the outer region of the glass
article is exchanged with a plurality of second metal ions so that
the outer region comprises the plurality of the second metal ions.
Each of the first metal ions has a first ionic radius and each of
the second alkali metal ions has a second ionic radius. The second
ionic radius is greater than the first ionic radius, and the
presence of the larger second alkali metal ions in the outer region
creates the compressive stress in the outer region.
[0046] At least one of the first metal ions and second metal ions
are preferably ions of an alkali metal. The first ions may be ions
of lithium, sodium, potassium, and rubidium. The second metal ions
may be ions of one of sodium, potassium, rubidium, and cesium, with
the proviso that the second alkali metal ion has an ionic radius
greater than the ionic radius than the first alkali metal ion.
[0047] Strengthened glass articles (such as glass article 300,
shown in FIG. 3) 300 can be used as a protective cover plate (as
used herein, the term "cover plate" includes windows or the like)
for display and touch screen applications, such as, but not limited
to, portable communication and entertainment devices such as
telephones, music players, video players, or the like; and as a
display screen for information-related terminal (IT) (e.g.,
portable or laptop computers) devices; as well as in other
applications.
[0048] While typical embodiments have been set forth for the
purpose of illustration, the foregoing description should not be
deemed to be a limitation on the scope of the disclosure or
appended claims. For example, processes other than ion exchange may
be used to chemically strengthen the glass, and different means of
strengthening the glass may be used in combination with each other
to achieve compressive stress within the glass. In one alternative
embodiment, metal ions, such as silver or the like, may be used
instead of--or in combination with--alkali metal ions in the ion
exchange process. Accordingly, various modifications, adaptations,
and alternatives may occur to one skilled in the art without
departing from the spirit and scope of the present disclosure or
appended claims.
* * * * *