U.S. patent application number 13/211661 was filed with the patent office on 2012-03-01 for two-step method for strengthening glass.
Invention is credited to Sinue Gomez, Lisa Ann Lamberson, Robert Michael Morena.
Application Number | 20120052271 13/211661 |
Document ID | / |
Family ID | 44645795 |
Filed Date | 2012-03-01 |
United States Patent
Application |
20120052271 |
Kind Code |
A1 |
Gomez; Sinue ; et
al. |
March 1, 2012 |
TWO-STEP METHOD FOR STRENGTHENING GLASS
Abstract
A method of strengthening an alkali aluminoborosilicate glass. A
compressive layer extending from a surface of the glass to a depth
of layer is formed by exchanging larger metal cations for smaller
metal cations present in the glass. In a second step, metal cations
in the glass are exchanged for larger metal cations to a second
depth in the glass that is less than the depth of layer and
increase the compressive stress of the compressive layer. Formation
of the compressive layer and replacement of cations with larger
cations can be achieved by a two-step ion exchange process. An
alkali aluminoborosilicate glass having a compressive layer and a
crack indentation threshold of at least 3000 gf is also
provided.
Inventors: |
Gomez; Sinue; (Corning,
NY) ; Lamberson; Lisa Ann; (Painted Post, NY)
; Morena; Robert Michael; (Lindley, NY) |
Family ID: |
44645795 |
Appl. No.: |
13/211661 |
Filed: |
August 17, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61377136 |
Aug 26, 2010 |
|
|
|
Current U.S.
Class: |
428/213 ;
65/30.14 |
Current CPC
Class: |
C03C 21/002 20130101;
C03C 3/091 20130101; C03C 4/18 20130101; C03C 3/097 20130101; Y10T
428/2495 20150115; C03C 3/095 20130101; Y10T 428/315 20150115; C03C
2204/00 20130101 |
Class at
Publication: |
428/213 ;
65/30.14 |
International
Class: |
C03C 21/00 20060101
C03C021/00; B32B 33/00 20060101 B32B033/00; B32B 7/02 20060101
B32B007/02 |
Claims
1. A method of strengthening an alkali aluminoborosilicate glass,
the method comprising the steps of: a. providing the alkali
aluminoborosilicate glass, the glass comprising alkali metal
cations; b. forming a compressive layer extending from a surface of
the glass to a depth of layer, wherein the compressive layer is
under a compressive stress; and c. replacing at least a portion of
the alkali metal cations in the compressive layer with a larger
alkali metal cation to a second depth that is less than the depth
of layer, wherein replacing the alkali metal cations in the
compressive layer with the larger alkali metal cation increases the
compressive stress.
2. The method of claim 1, wherein the step of forming the
compressive layer comprises replacing lithium cations with sodium
cations to the depth of layer of the glass.
3. The method of claim 2, wherein the step of forming the
compressive layer comprises immersing the glass in a first ion
exchange bath comprising a sodium salt.
4. The method of claim 3, wherein the first ion exchange bath
further comprises a potassium salt.
5. The method of claim 1, wherein the step of replacing the alkali
metal cations in the compressive layer with the larger alkali metal
cation comprises replacing at least one of sodium cations and
lithium cations with potassium cations within the second depth.
6. The method of claim 5, wherein the step of replacing the alkali
metal cations in the compressive layer with the larger alkali metal
cation comprises immersing the glass in a second ion exchange bath
comprising a potassium salt.
7. The method of claim 1, wherein the step of providing the alkali
aluminoborosilicate glass comprises providing an alkali
aluminoborosilicate glass comprising: 50-70 mol % SiO.sub.2; 5-15
mol % Al.sub.2O.sub.3; 5-20 mol % B.sub.2O.sub.3; 2-15 mol %
Li.sub.2O; 0-20 mol % Na.sub.2O; and 0-10 mol % K.sub.2O.
8. The method of claim 7, wherein the alkali aluminoborosilicate
glass further comprises at least one of: 0-10 mol % P.sub.2O.sub.5;
0-5 mol % MgO; 0-1 mol % CeO.sub.2; and 0-1 mol % SnO.sub.2.
9. The method of claim 1, wherein the step of providing the alkali
aluminoborosilicate glass comprises providing an alkali
aluminoborosilicate glass having a crack initiation threshold of at
least 1000 gf upon indentation with a Vickers indenter.
10. The method of claim 1, wherein, after replacing the alkali
metal cations in the compressive layer with the larger alkali metal
cation, the surface of the glass has a crack initiation layer
threshold of at least 3000 gf upon indentation with a Vickers
indenter.
11. The method of claim 1, wherein, after replacing alkali metal
cations in the compressive layer with the larger alkali metal
cation, the compressive stress is at least 500 MPa and the depth of
layer is at least 50 .mu.m.
12. The method of claim 1, wherein the depth of layer is in a range
from 70 .mu.m up to 290 .mu.m.
13. The method of claim 1, wherein the second depth is in a range
from 5 .mu.m up to 20 .mu.m.
14. The method of claim 1, wherein the step of providing the alkali
aluminoborosilicate glass comprises providing an alkali
aluminoborosilicate glass sheet having a thickness of less than 2
mm.
15. A method of strengthening an alkali aluminoborosilicate glass,
the method comprising the steps of: a. providing the alkali
aluminoborosilicate glass, the glass comprising lithium cations and
sodium cations; b. replacing lithium cations with sodium cations to
form a compressive layer extending from a surface of the glass to a
depth of layer, wherein the compressive layer is under a
compressive stress; and c. replacing sodium cations and lithium
cations with potassium cations to a second depth that is less than
the depth of layer, and wherein replacing the lithium cations and
the sodium cations in the compressive layer with the potassium
cations increases the compressive stress.
16. The method of claim 15, wherein the step of replacing the
lithium cations with the sodium cations comprises immersing the
glass in a first ion exchange bath, the first ion exchange bath
comprising a sodium salt.
17. The method of claim 16, wherein the first ion exchange bath
further comprises a potassium salt.
18. The method of claim 15, wherein the step of replacing sodium
cations and optionally lithium cations with potassium cations
comprises immersing the glass in a second ion exchange bath, the
second ion exchange bath comprising a potassium salt.
19. The method of claim 15, wherein the step of providing the
alkali aluminoborosilicate glass comprises providing an alkali
aluminoborosilicate glass comprising: 50-70 mol % SiO.sub.2; 5-15
mol % Al.sub.2O.sub.3; 5-20 mol % B.sub.2O.sub.3; 2-15 mol %
Li.sub.2O; 0-20 mol % Na.sub.2O; and 0-10 mol % K.sub.2O.
20. The method of claim 19, wherein the alkali aluminoborosilicate
glass further comprises at least one of: 0-10 mol % P.sub.2O.sub.5;
0-5 mol % MgO; 0-1 mol % CeO.sub.2; and 0-1 mol % SnO.sub.2.
21. The method of claim 15, wherein the step of providing the
alkali aluminoborosilicate glass comprises providing an alkali
aluminoborosilicate glass having a crack initiation threshold of at
least 1000 gf upon indentation with a Vickers indenter.
22. The method of claim 15, wherein, after replacing sodium cations
and optionally lithium cations with potassium cations, the surface
of the glass has a crack initiation layer threshold of at least
3000 gf upon indentation with a Vickers indenter.
23. The method of claim 15, wherein, after replacing sodium cations
and optionally lithium cations with potassium cations, the
compressive stress is at least 500 MPa and the depth of layer is at
least 50 .mu.m.
24. The method of claim 15, wherein the depth of layer is in a
range from 70 .mu.m up to 290 .mu.m.
25. The method of claim 15, wherein the second depth is in a range
from 5 .mu.m up to 20 .mu.m.
26. The method of claim 15, wherein the step of providing the
alkali aluminoborosilicate glass comprises providing an alkali
aluminoborosilicate glass sheet having a thickness of less than 2
mm.
27. An alkali aluminoborosilicate glass comprising lithium cations,
sodium cations, and potassium cations, wherein the glass has a
surface having a compressive layer extending from the surface to a
depth of layer and is enriched in potassium cations to a second
depth that is less than the depth of layer, and wherein the surface
of the glass has a crack initiation layer threshold of at least
3000 gf upon indentation with a Vickers indenter.
28. The glass of claim 27, wherein the surface has a compressive
stress of at least 500 MPa, and wherein the depth of layer is at
least 50 .mu.m.
29. The glass of claim 27, wherein the depth of layer is in a range
from 70 .mu.m up to 290 .mu.m.
30. The glass of claim 27, wherein the second depth is in a range
from 5 .mu.m up to 20 .mu.m.
31. The glass of claim 27, wherein the alkali aluminoborosilicate
glass comprises: 50-70 mol % SiO.sub.2; 5-15 mol % Al.sub.2O.sub.3;
5-20 mol % B.sub.2O.sub.3; 2-15 mol % Li.sub.2O; 0-20 mol %
Na.sub.2O; and 0-10 mol % K.sub.2O.
32. The glass of claim 31, wherein the alkali aluminoborosilicate
glass further comprises at least one of: 0-10 mol % P.sub.2O.sub.5;
0-5 mol % MgO; 0-1 mol % CeO.sub.2; and 0-1 mol % SnO.sub.2.
33. The glass of claim 27, wherein the glass is a glass sheet
having a thickness of up to 2 mm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn.119 of U.S. Provisional Application Ser. No.
61/377,136 filed on Aug. 26, 2010, the content of which is relied
upon and incorporated herein by reference in its entirety.
BACKGROUND
[0002] The disclosure relates to strengthened alkali
aluminoborosilicate glasses. More particularly, the disclosure
relates to a method of strengthening such glasses. Even more
particularly, the disclosure relates to strengthening such glasses
by ion exchange.
[0003] The ion exchange process can be used to strengthen
alkali-containing glasses by creating compressive stress layers in
the surface region of the glass. In general, lithium-containing
aluminosilicate glasses are ion exchanged more readily than
sodium-containing glasses and greater depths of compression can be
obtained in lithium-containing aluminosilicate glasses at lower
temperatures and shorter times. However, such lithium-containing
aluminosilicate glasses tend to have lower strain and anneal
points, and lower temperatures are required for treatment to avoid
structural relaxation. In addition, the exchange of sodium for
lithium in the glass, results in lower surface compression--which
translates into lower surface strength--when compared to the
surface compression achieved with the exchange of potassium for
sodium in the glass.
SUMMARY
[0004] A method of strengthening an alkali aluminoborosilicate
glass is provided. A compressive layer extending from a surface of
the glass to a depth of layer is formed by exchanging larger metal
cations for smaller metal cations present in the glass. In a second
step, metal cations in the glass are exchanged for larger metal
cations to a second depth that is less than the depth of layer. The
second step increases the compressive stress of the compressive
layer. For example, sodium cations are exchanged in the first step
for lithium cations that are present in the glass to the depth of
layer, and potassium cations are then exchanged in the second step
for sodium cations and lithium cations in the glass to the second
depth. The exchange of the potassium cations for sodium and lithium
cations increases the compressive stress of the layer. Formation of
the compressive layer and replacement of cations with larger
cations can be achieved by a two-step ion exchange process. An
alkali aluminoborosilicate glass having a compressive layer and a
crack indentation threshold of at least 3000 gf is also
provided.
[0005] Accordingly, one aspect of the disclosure is to provide a
method of strengthening an alkali aluminoborosilicate glass. The
method comprises the steps of: providing an alkali
aluminoborosilicate glass comprising alkali metal cations; forming
a compressive layer extending from a surface of the glass to a
depth of layer, wherein the compressive layer is under a
compressive stress; and replacing at least a portion of the alkali
metal cations with a larger alkali metal cation to a second depth
that is less than the depth of layer, and wherein replacing the
alkali metal cations with the larger alkali metal cation increases
the compressive stress.
[0006] A second aspect of the disclosure is to provide a method of
strengthening an alkali aluminoborosilicate glass. The method
comprises the steps of: providing the alkali aluminoborosilicate
glass comprising lithium cations and sodium cations; replacing at
least a portion of the lithium cations with sodium cations to form
a compressive layer that extends from a surface of the glass to a
depth of layer and is under compressive stress; and replacing at
least a portion of the sodium cations and the lithium cations with
potassium cations to a second depth that is less than the depth of
layer, wherein the compressive layer is enriched in potassium
cations to the second depth, and wherein replacing sodium cations
and lithium cations with potassium cations increases the
compressive stress of the compressive layer.
[0007] A third aspect of the disclosure is to provide an alkali
aluminoborosilicate glass. The glass comprises lithium cations,
sodium cations, and potassium cations. The glass has a surface
having a compressive layer extending from the surface to a depth of
layer and is enriched in potassium cations to a second depth that
is less than the depth of layer. The surface of the glass has a
crack initiation threshold of at least 3000 gf upon indentation
with a Vickers indenter.
[0008] 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
[0009] FIG. 1 is a schematic cross-sectional view of a glass sheet,
described herein, having strengthened surfaces;
[0010] FIG. 2 is a plot of crack initiation loads obtained for
alkali aluminoborosilicate glasses measured before strengthening
and following strengthening by ion exchange processes;
[0011] FIG. 3a is plot of Na.sub.2O concentration profile following
the first step of a two-step ion exchange process; and
[0012] FIG. 3b is plot of K.sub.2O concentration profile following
the second step of a two-step ion exchange process.
DETAILED DESCRIPTION
[0013] 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 used herein, the
indefinite articles "a," "an," and the corresponding definite
article "the" means "at least one" or "one or more," unless
specified otherwise.
[0014] Referring to the drawings in general and to FIG. 1 in
particular, 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.
[0015] As used herein, the term "enriched," unless otherwise
specified, means that the concentration of a specified element or
ionic specie is greater than the average concentration of that
element or ionic specie within the bulk of the glass. As used
herein, the term "glass" refers to alkali aluminoborosilicate
glasses, unless otherwise specified.
[0016] Methods of strengthening alkali aluminosilicate glass are
provided. in one embodiment, the method comprises the steps of:
providing the alkali aluminoborosilicate glass; initially forming a
compressive layer extending from a surface of the glass to a depth
of layer; and replacing at least a portion of alkali metal cations
with a larger alkali metal cation to a second depth that is less
than the depth of layer. Replacing the alkali metal cations with
the larger alkali metal cation increases the compressive stress in
the compressive layer and increases the damage resistance of the
surface of the glass. The compressive layer inhibits the
introduction of flaws at the surface and prevents crack initiation
and propagation through the depth of the layer. In some
embodiments, the method is carried out through the use of a
two-step ion exchange process.
[0017] In the first step of the method, an alkali
aluminoborosilicate glass is provided. In some embodiments, the
glass is provided in the form of a sheet having a thickness of
about 2 mm or less. Such sheets can be formed by down-draw methods
known in the art such as slot-draw or fusion-draw processes, or by
other methods known in the art. The glass, in some embodiments,
comprises monovalent lithium cations, and sodium cations. The glass
can additionally include monovalent potassium cations. The presence
of such alkali metal cations in the glass is typically represented
by the oxide species Li.sub.2O, Na.sub.2O, and K.sub.2O. In some
embodiments, the alkali aluminoborosilicate glass comprises,
consists essentially of, or consists of: 50-70 mol % SiO.sub.2;
5-15 mol % Al.sub.2O.sub.3; 5-20 mol % B.sub.2O.sub.3; 2-15 mol %
Li.sub.2O; 0-20 mol % Na.sub.2O; and 0-10 mol % K.sub.2O. In some
embodiments, the glass can further comprise at least one of: 0-10
mol % P.sub.2O.sub.5; 0-5 mol % MgO; 0-1 mol % CeO.sub.2; and 0-1
mol % SnO.sub.2.
[0018] Non-limiting compositions and physical properties of
representative glasses are listed in Table 1. Crack initiation
thresholds, which were determined by indentation with a Vickers
indenter, are also listed for the compositions in Table 1. Crack
initiation thresholds, expressed in Kgf, were measured: 1) prior to
ion exchange of the glass ("pre-IX" in Table 1); 2) following
single-step ion exchange (IX) of the glass for 10 hours in a
390.degree. C. molten salt bath containing 60% KNO.sub.3 and 40%
NaNO.sub.3 by weight; 3) following single step ion exchange of the
glass for 10 hours in a 390.degree. C. molten NaNO.sub.3 salt bath;
and 4) following a two-step ion exchange process comprising ion
exchange of the glass for 10 hours in a 390.degree. C. molten
NaNO.sub.3 salt bath, followed by ion exchange for 30 minutes in a
390.degree. C. molten KNO.sub.3 salt bath. depths of layer (DOL) of
the compressive layers formed by the ion exchange processes,
expressed in microns (.mu.m) are also listed in Table 1.
TABLE-US-00001 TABLE 1 Compositions and physical properties of
representative alkali aluminoborosilicate glasses. (Mol % ) 1 2 3 4
5 6 7 SiO.sub.2 65.7 65.7 65.7 65.7 65.7 65.7 65.2 Al.sub.2O.sub.3
12.3 12.3 12.3 12.3 10.3 10.3 11.1 B.sub.2O.sub.3 9.1 9.1 9.1 7.1
11.1 9.1 6.2 Li.sub.2O 5 7 5 7 5 7 2.3 Na.sub.2O 6.6 4.6 4.6 6.6
6.6 6.6 9.8 K.sub.2O 1.3 1.3 3.3 1.3 1.3 1.3 2.7 MgO 0 0 0 0 0 0
2.2 CaO 0 0 0 0 0 0 0.32 P.sub.2O.sub.5 0 0 0 0 0 0 0 SnO.sub.2 0 0
0 0 0 0 0.1 CeO.sub.2 0 0 0 0 0 0 0.05 Strain pt. 492 491 492 480
468 468 491 Anneal pt. 540 538 542 521 510 507 534 Softening pt.
785 786 799 731 709 691 743 CTE 66 62 67.2 69.9 65.3 68.9 79
Density 2.334 2.327 2.331 2.261 2.335 2.369 2.413 Crack initiation
1000 1000 1000 1000 1000 500 1000 threshold pre-IX, Kgf DOL of IX
294 270 221 245 245 240 123 60KNO.sub.3/40NaNO.sub.3 @ 390.degree.
C., 10 h Crack initiation 5000 7000 4000 6000 4000 6000 4000
threshold, Kgf DOL of IX NaNO.sub.3 294 270 196 245 245 265 108 @
390.degree. C., 10 h Crack initiation 3000 3000 3000 3000 3000 3000
3000 threshold, Kgf DOL of IX 294 270 196 245 245 265 1)NaNO.sub.3
@ 390.degree. C. 10 h 2) KNO.sub.3 390.degree. C., 30 min Crack
initiation 5000 6000 6000 10000 5000 6000 6000 threshold, Kgf (Mol
%) 8 9 10 11 12 13 14 SiO.sub.2 65.2 65.2 65.2 65.2 65.2 65.2 65.2
Al.sub.2O.sub.3 11.1 11.1 11.1 11.1 11.1 9.1 11.1 B.sub.2O.sub.3
6.2 6.2 6.2 6.2 6.2 8.2 6.2 Li.sub.2O 4.3 6.3 8.3 12.1 8.3 2.3 5
Na.sub.2O 7.8 5.8 3.8 0 5.8 9.8 9.8 K.sub.2O 2.7 2.7 2.7 2.7 0.7
2.7 0 MgO 2.2 2.2 2.2 2.2 2.2 2.2 2.2 CaO 0.32 0.32 0.32 0.32 0.32
0.32 0.32 P.sub.2O.sub.5 0 0 0 0 0 0 0 SnO.sub.2 0.1 0.1 0.1 0.1
0.1 0.1 0.1 CeO.sub.2 0.05 0.05 0.05 0.05 0.05 0.05 0.05 Strain pt.
480 479 476 483 479 481 487 Anneal pt. 522 520 517 523 519 522 529
Softening pt. 723 720 715 721 717 710 740 CTE 75 70 66 60 66 77.7
71.7 Density 2.407 2.398 2.39 2.372 2.396 2.415 2.405 Crack
initiation 500 1000 1000 500 1000 500 500 threshold pre-IX, Kgf DOL
of IX 172 157 147 123 221 98 235 60KNO.sub.3/40NaNO.sub.3 @
390.degree. C., 10 h Crack initiation 5000 8000 8000 >9000
>10000 4000 7000 threshold, Kgf DOL of IX NaNO.sub.3 157 186 74
137 167 132 147 @ 390.degree. C., 10 h Crack initiation 3000 3000
3000 3000 3000 3000 3000 threshold, Kgf DOL of IX 1)NaNO.sub.3 @
390.degree. C. 10 h 2) KNO.sub.3 390.degree. C., 30 min Crack
initiation 9000 8000 >9000 >10000 10000 7000 9000 threshold,
Kgf (Mol % ) 15 16 17 18 19 20 21 SiO.sub.2 65.2 64.7 63.7 65.2
64.7 63.7 64.7 Al.sub.2O.sub.3 12.3 12.3 12.3 12.3 12.3 12.3 11.1
B.sub.2O.sub.3 9.1 9.1 9.1 9.1 9.1 9.1 6.2 Li.sub.2O 5 5 5 7 7 7
6.3 Na.sub.2O 6.6 6.6 6.6 4.6 4.6 4.6 5.8 K.sub.2O 1.3 1.3 1.3 1.3
1.3 1.3 2.7 MgO 0 0 0 0 0 0 2.2 CaO 0 0 0 0 0 0 0.32 P.sub.2O.sub.5
0.5 1 2 0.5 1 2 0.5 SnO.sub.2 0.05 0.05 0.05 0.05 0.05 0.05 0.1
CeO.sub.2 0 0 0 0 0 0 0 Strain pt. 488.6 487 491 491 482 486 478
Anneal pt. 537.7 536 540 538 528 535 520 Softening pt. 795.3 788.7
789 806 774 774 729 CTE 65.9 65.1 66.2 61.5 63.1 63.4 Density 2.332
2.332 2.327 2.327 2.325 2.321 2.392 Crack initiation threshold
pre-IX, Kgf DOL of IX 244 195 195 244 224 215 146
60KNO.sub.3/40NaNO.sub.3 @ 390.degree. C., 10 h Crack initiation
threshold, Kgf DOL of IX NaNO.sub.3 @ 390.degree. C., 10 h Crack
initiation threshold, Kgf DOL of IX 1)NaNO.sub.3 @ 390.degree. C.
10 h 2) KNO.sub.3 390.degree. C., 30 min Crack initiation
threshold, Kgf (Mol % ) 22 23 SiO.sub.2 64.2 63.2 Al.sub.2O.sub.3
11.1 11.1 B.sub.2O.sub.3 6.2 6.2 Li.sub.2O 6.3 6.3 Na.sub.2O 5.8
5.8 K.sub.2O 2.7 2.7 MgO 2.2 2.2 CaO 0.32 0.32 P.sub.2O.sub.5 1 2
SnO.sub.2 0.1 0.1 CeO.sub.2 0 0 Strain pt. 483 500 Anneal pt. 527
549 Softening pt. 748 796 CTE Density 2.387 2.376 Crack initiation
threshold pre-IX, Kgf DOL of IX 166 195 60KNO.sub.3/40NaNO.sub.3 @
390.degree. C., 10 h Crack initiation threshold, Kgf DOL of IX
NaNO.sub.3 @ 390.degree. C., 10 h Crack initiation threshold, Kgf
DOL of IX 1)NaNO.sub.3 @ 390.degree. C. 10 h 2) KNO.sub.3
390.degree. C., 30 min Crack initiation threshold, Kgf Strain,
anneal, and softening points are expressed in degrees Celsius;
coefficients of thermal expansion (CTE) are expressed as 10.sup.7
K.sup.-1; and densities are expressed in g/cm.sup.3.
[0019] The alkali aluminoborosilicate glass as provided has
intrinsically high damage resistance; i.e., the glass has high
damage resistance prior to--or without--any chemical or thermal
strengthening or tempering. Such damage resistance is measured or
characterized by the resistance of the glass to crack formation
and/or crack propagation upon indentation with a Vickers indenter.
In some embodiments, the glass has a crack initiation threshold
(i.e., the Vickers indenter load at which cracks are first
observed) of at least about 1000 gf before strengthening and, in
particular embodiments, in a range from about 1000 gf up to about
2000 gf, prior to strengthening. Examples of glass compositions
that intrinsically have crack initiation thresholds in this range
are listed in Table 2. In comparison, soda-lime glasses have low
damage tolerance, and form cracks when indented at loads as low as
100 gf. Even when ion-exchanged, soda-lime glass typically has a
damage tolerance of less than 1000 gf.
TABLE-US-00002 TABLE 2 Crack initiation thresholds for mixed alkali
aluminoborosilicate glasses before strengthening. Composition (mol
%) a b c d e f g h SiO.sub.2 65.7 65.7 65.2. 65.7 65.7 64.7 65.7
63.7 Al.sub.2O.sub.3 12.3 12.3 11.1 11.3 10.8 11.3 10.3 11.3
B.sub.2O.sub.3 9.1 0.1 6.2 11.1 11.6 12.1 12.1 13.1 Li.sub.2O 5 7
8.3 4.6 4.6 4.6 4.6 4.6 Na.sub.2O 6.6 4.6 3.8 6.2 6.2 6.2 6.2 6.2
K.sub.2O 1.3 1.3 2.7 1.1 1.1 1.1 1.1 1.1 MgO 0 0 2.2 0 0 0 0 0 CaO
0 0 0.32 0 0 0 0 0 SnO.sub.2 0 0 0.1 0 0 0 0 0 Crack 1000 1000 1000
2000 1000 1000 1000 1000 Initiation Load (gf)
[0020] The formation of the compressive layer and increase damage
resistance described herein can, in some embodiments, be achieved
by a two-step ion exchange process. 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 as the
ions present in the glass. The exchange of metal cations is
typically carried out in a molten salt bath, with larger cations
from the bath typically replacing smaller cations within the glass.
Ion exchange is limited to a region extending from the surface of
the glass article to a depth (depth of layer, or "DOL") below the
surface. By way of example, ion exchange of alkali metal-containing
glasses can be achieved by immersing the glass in at least one
molten salt bath containing a salt such as, but not limited to,
nitrates, sulfates, and chlorides of at least one alkali metal ion.
The temperature of such molten salt baths is typically in a range
from about 380.degree. C. up to about 450.degree. C., with
immersion times ranging up to about 16 hours. However, temperatures
and immersion times that are different from those described herein
can also be used. The replacement or exchange of smaller cations
within the glass with larger cations from the bath creates a
compressive stress in the region near the surface of the glass to
the depth of layer. The compressive stress near the surface gives
rise to a central tension in an inner or central region of the
glass so as to balance forces within the glass.
[0021] The step of initially forming the compressive layer provides
a compressive layer having an unusually deep depth of layer. In
some embodiments, the step of forming the compressive layer
comprises replacing smaller alkali metal cations with larger alkali
metal cations. In a particular embodiment, this step comprises
replacing lithium cations in the glass with sodium cations from,
for example, a molten salt bath, by ion exchange to the depth of
layer below the surface of the glass. The exchange of Na.sup.+ ions
for Li.sup.+ ions achieves an advantageously deep depth of layer
(e.g., d.sub.1, d.sub.2, in FIG. 1). In such embodiments, the depth
of layer is at least 50 .mu.m, and can, in some embodiments, extend
from the surface to a depth in a range from about 70 .mu.m up to
about 290 .mu.m.
[0022] The exchange of Na.sup.+ ions for Li.sup.+ ions can be
achieved by immersing the glass in a first ion exchange bath
comprising at least one molten sodium salt. In some embodiments,
the sodium salt is sodium nitrate (NaNO.sub.3). In some
embodiments, the ion exchange bath contains only sodium salt; i.e.,
no other metal salts are intentionally added to the bath. In other
embodiments, however, the first ion exchange bath further includes
salts of other alkali metals such as, but not limited to, potassium
nitrate (KNO.sub.3). In one non-limiting example, the first ion
exchange bath comprises 40% NaNO.sub.3 and 60% KNO.sub.3 by weight.
In another non-limiting example, the first ion exchange bath
comprises 20% NaNO.sub.3 and 80% KNO.sub.3 by weight.
[0023] Following formation of the compressive layer, at least a
portion of the small alkali metal cations (e.g., Li.sup.+,
Na.sup.+) in the compressive layer are replaced by a single, larger
alkali metal cation specie to a second depth (e.g., d.sub.1',
d.sub.2', in FIG. 1) that is less than the depth of layer. In some
embodiments, this is achieved by exchanging potassium ions for at
least one of sodium ions and lithium ions in the glass. The
exchange of K.sup.+ ions for Na.sup.+ ions and Li.sup.+ to the
second depth increases the surface compressive stress of the glass.
In some embodiments, the second depth extends from the surface to a
depth in a range from about 5 .mu.m up to about 20 .mu.m. Such ion
exchange is achieved by immersing the glass in a second ion
exchange bath comprising a potassium sodium salt. In some
embodiments, the potassium salt is potassium nitrate
(KNO.sub.3).
[0024] After the glass has been strengthened by forming the deep
compressive layer and replacing smaller ions with larger ions to a
lesser depth using the two-step ion exchange process described
hereinabove, the glass has a crack initiation threshold of at least
3000 gf when indented with a Vickers indenter, a compressive stress
of at least 500 MPa, and a depth of layer of at least 50 .mu.m.
[0025] An alkali aluminoborosilicate glass comprising lithium
cations, sodium cations, and potassium cations is also provided.
The glass has a compressive stress layer extending from a surface
of the glass to a depth of layer. The compressive layer of the
glass is enriched in potassium cations to a second depth that is
less than the depth of layer. The surface also has a crack
initiation threshold of at least 3000 gf when indented with a
Vickers indenter.
[0026] A cross-sectional view of an alkali aluminoborosilicate
glass sheet strengthened by the methods described herein is
schematically shown in FIG. 1. In the non-limiting example shown in
FIG. 1, strengthened glass sheet 100 has a thickness t, a first
surface 110 and second surface 120 that are substantially parallel
to each other, central portion 115. Compressive layers 112, 122
extend from first surface 110 and second surface 120, respectively,
to depths of layer d.sub.1, d.sub.2, below each surface.
Compressive layers 112, 122 are under a compressive stress, while
central portion 115 is under a tensile stress, or in tension. The
tensile stress in central portion 115 balances the compressive
stresses in compressive layers 112, 122, thus maintaining
equilibrium within strengthened glass sheet 100. Each of
compressive layers 112, 122 of the glass is enriched in potassium
cations to second depths d.sub.1', d.sub.2', respectively, wherein
second depths d.sub.1', d.sub.2' are less than depths of layer
d.sub.1, d.sub.2. Whereas a glass sheet having compressive layers
112, 122 extending from opposite surfaces 120, 120 is shown in FIG.
1, the glass described herein can have a strengthened single
surface, rather than multiple strengthened surfaces. This can be
achieved, for example, by masking one of surfaces 110, 120 during
the two-step ion exchange process, which is described herein and is
used to strengthen glass sheet 100.
[0027] In some embodiments, the first depth of layer extends from
the surface of the glass to a depth in a range from about 70 .mu.m
up to about 290 .mu.m. In some embodiments, the second depths
d.sub.1', d.sub.2' extend from surfaces 112, 122 to a depth in a
range from about 5 .mu.m up to about 20 .mu.m.
[0028] The compressive layer is, in some embodiments, formed by the
methods described hereinabove, such as, for example, the two-step
ion exchange process described above. As previously described
herein, the method comprises the steps of: providing the alkali
aluminoborosilicate glass; forming a compressive layer extending
from a surface of the glass to a depth of layer (d.sub.1, d.sub.2
in FIG. 1); wherein the compressive layer is under a compressive
stress; and replacing alkali metal cations in the compressive layer
with a larger alkali metal cation to a second depth (d.sub.1',
d.sub.2' in FIG. 1) that is less the depth of layer. The
replacement of the alkali metal cations with a larger alkali metal
cationic specie increases the compressive stress in the compressive
layer.
[0029] In some embodiments, the glass is in the form of a sheet
having a thickness of about 2 mm or less. Such sheets can be formed
by down-draw methods known in the art such as slot-draw or
fusion-draw processes, or by other methods known in the art. In
some embodiments, the alkali aluminoborosilicate glass comprises,
consists essentially of, or consists of: 50-70 mol % SiO.sub.2;
5-15 mol % Al.sub.2O.sub.3; 5-20 mol % B.sub.2O.sub.3; 2-15 mol %
Li.sub.2O; 0-20 mol % Na.sub.2O; and 0-10 mol % K.sub.2O. In some
embodiments, the glass can further comprise at least one of: 0-10
mol % P.sub.2O.sub.5; 0-5 mol % MgO; 0-1 mol % CeO.sub.2; and 0-1
mol % SnO.sub.2. Compositions and physical properties, and damage
resistance of representative glasses are listed in Table 1.
[0030] The glasses described herein intrinsically (i.e., prior to
thermal or chemical strengthening (e.g., ion exchange)) possess
high levels of damage resistance. Such damage resistance is
measured or characterized by the resistance of the glass to crack
formation and/or crack propagation upon indentation with a Vickers
indenter. In some embodiments, the glass has a crack initiation
threshold (i.e., the Vickers indenter load at which cracks are
first observed) of at least about 1000 gf before strengthening and,
in particular embodiments, in a range from about 1000 gf up to
about 2000 gf, prior to strengthening. Examples of glass
compositions that intrinsically have crack initiation thresholds in
this range are listed in Table 2. In comparison, soda-lime glasses
have low damage tolerance, and form cracks when indented at loads
as low as 100 gf. Even when ion-exchanged, soda-lime glass
typically has a damage tolerance of less than 1000 gf.
[0031] The tolerance to damage and strength of the alkali
aluminoborosilicate glasses having intrinsic damage resistance can
be greatly enhanced by the use of a 2 step ion-exchange process.
The two-step strengthening/ion exchange processes described
hereinabove provide new opportunities for the use of such glasses
in those consumer electronics applications where high strength and
scratch resistance are desirable. Such applications include, but
are not limited to, cover plates, display windows, display screens,
touch screens, and the like for portable or hand-held electronic
communication and entertainment devices.
EXAMPLES
[0032] The following examples illustrate the features and
advantages of the methods and glasses described herein and are in
no way intended to limit the disclosure or appended claims
thereto.
Example 1
[0033] Crack initiation thresholds were measured with a Vickers
indenter for representative alkali aluminoborosilicate glass
compositions before ion exchange and following ion exchange under
different conditions.
[0034] Glass samples having selected compositions were subjected to
single-step ion exchange by immersion in a single molten salt bath.
Different glass samples having these same compositions were
subjected to two-step ion exchange in multiple salt baths, in
accordance with the methods described hereinabove.
[0035] The single-step ion exchange baths used were: a) a
390.degree. C. molten NaNO.sub.3 salt bath; and b) a 390.degree. C.
molten salt bath containing 60% KNO.sub.3 and 40% NaNO.sub.3 by
weight. The glass samples that were ion exchanged in the pure
NaNO.sub.3 bath were immersed in the bath for 5 hours. Glass
samples that were ion exchanged in the KNO.sub.3/NaNO.sub.3 bath
were immersed in the bath for 10 hours.
[0036] Two sets of two-step, multiple ion exchange baths were used.
The first set of multiple baths consisted of a first bath of molten
NaNO.sub.3 salt at 390.degree. C., followed by a second bath of
molten KNO.sub.3 salt at 390.degree. C. Glass samples were immersed
in the first (NaNO.sub.3) bath for 10 hours, and then immersed in
the second (KNO.sub.3) bath for 30 minutes. The second set of
multiple baths consisted of a first bath of molten NaNO.sub.3 salt
at 410.degree. C., followed by a second bath of molten KNO.sub.3
salt at 410.degree. C. Glass samples were immersed in the first
(NaNO.sub.3) bath for 10 hours, and then immersed in the second
(KNO.sub.3) bath for 10 minutes.
[0037] For comparison, crack initiation thresholds for soda lime
glass were measured before ion exchange and after ion exchange in a
410.degree. C. KNO.sub.3 bath for 8 hours.
[0038] Crack initiation thresholds were measured for different
compositions before and after ion exchange and are plotted in FIG.
2. For comparison, crack initiation thresholds for soda lime glass
(SLS in FIG. 2) were measured before ion exchange and following ion
exchange in a 410.degree. C. KNO.sub.3 bath for 8 hours.
Single-step ion exchange of the glasses provided approximately a
three-fold improvement in damage resistance. The two-step ion
exchange processes, as described hereinabove, allowed the use of
loads as high as 10,000 gf before crack formation is observed.
Example 2
[0039] The effect of the second ion exchange step is illustrated by
the data shown in Table 3, which lists depths of layer (DOL) and
compressive stresses (CS) measured for 1 mm alkali
aluminoborosilicate glass samples following different ion exchange
(IX) procedures. The compositions of the samples listed in Table 3
are: i) 65.7 mol % SiO.sub.2; 12.3 mol % Al.sub.2O.sub.3; 9.1 mol %
B.sub.2O.sub.3; 5 mol % Li.sub.2O; 6.6 mol % Na.sub.2O; 1.3 mol %
K.sub.2O; 0.1 mol % SnO.sub.2; and 0.15 CeO.sub.2; ii) 65.7 mol %
SiO.sub.2; 10.3 mol % Al.sub.2O.sub.3; 12.1 mol % B.sub.2O.sub.3;
4.6 mol % Li.sub.2O; 6.2 mol % Na.sub.2O; 1.1 mol % K.sub.2O; 0.1
mol % SnO.sub.2; and 0.15 CeO.sub.2; and iii) 57.9 mol % SiO.sub.2;
12.1 mol % Al.sub.2O.sub.3; 18.1 mol % B.sub.2O.sub.3; 4.6 mol %
Li.sub.2O; 6.2 mol % Na.sub.2O; 1.1 mol % K.sub.2O; 0.1 mol %
SnO.sub.2; and 0.15 CeO.sub.2.
[0040] One set of samples of each composition was ion exchanged in
a single-step process by immersion in a 390.degree. C. molten salt
bath containing 80 wt % KNO.sub.3 and 20 wt % NaNO.sub.3 for 5
hours. Two-step ion exchange consisted of immersion a 390.degree.
C. molten salt bath containing 80 wt % KNO.sub.3 and 20 wt %
NaNO.sub.3 for 5 hours followed by immersion in a 410.degree. C.
KNO.sub.3 molten salt bath for 1 hour. Immersion in the second ion
exchange bath in accordance with the methods described hereinabove
increased the compressive stress of all samples.
[0041] A second set of samples of each composition was ion
exchanged in a single-step process by immersion in a 390.degree. C.
molten salt bath containing 60 wt % KNO.sub.3 and 40 wt %
NaNO.sub.3 for 5 hours. Two-step ion exchange consisted of
immersion a 390.degree. C. molten salt bath containing 60 wt %
KNO.sub.3 and 40 wt % NaNO.sub.3 for 5 hours followed by immersion
in a 410.degree. C. KNO.sub.3 molten salt bath for 1 hour.
Immersion in the second ion exchange bath in accordance with the
methods described hereinabove increased the compressive stress of
all samples.
TABLE-US-00003 TABLE 3 Depth of layer (DOL) and compressive stress
(CS) of alkali aluminoborosilicate using different ion exchange
(IX) conditions. Sample (i) Sample (ii) Sample (iii) DOL CS DOL CS
DOL CS IX procedure (.mu.m) (MPa) (.mu.m) (MPa) (.mu.m) (MPa)
Single-step 195 577 195 540 107 562 80 wt % KNO.sub.3/20 wt %
NaNO.sub.3; 5 hrs at 390.degree. C. Two-step 791 711 726 1) 80 wt %
KNO.sub.3/20 wt % NaNO.sub.3; 5 hrs at 390.degree. C.; 2)
KNO.sub.3; 1 hr at 410.degree. C. Single-step 195 516 146 494 88
490 60 wt % KNO.sub.3/40 wt % NaNO.sub.3; 5 hrs at 390.degree. C.
Two-step 800 761 707 1) 60 wt % KNO.sub.3/40 wt % NaNO.sub.3; 5 hrs
at 390.degree. C.; 2) KNO.sub.3; 1 hr at 410.degree. C.
[0042] The strengths of alkali aluminoborosilicate glass samples
before ion exchange and after single-step and two-step ion exchange
processes were also measured using ring-on-ring measurements
performed on polished surfaces of 1 mm thick glass samples. All
samples comprised 65.7 mol % SiO.sub.2; 10.3 mol % Al.sub.2O.sub.3;
12.1 mol % B.sub.2O.sub.3; 4.6 mol % Li.sub.2O; 6.2 mol %
Na.sub.2O; and 1.1 mol % K.sub.2O. The measured ring-on-ring
strength of the sample before ion exchange was 131.+-.45 MPa.
Single-step ion exchange in a 390.degree. C. molten salt bath
containing 60 wt % KNO.sub.3 and 40 wt % NaNO.sub.3 for 6 hours
yielded a ring-on-ring strength of 491.+-.108 MPa. Two-step ion
exchange consisted of immersion in a 390.degree. C. molten salt
bath containing 60 wt % KNO.sub.3 and 40 wt % NaNO.sub.3 for 6
hours followed by immersion in a 410.degree. C. KNO.sub.3 molten
salt bath for 1 hour yielded a ring-on-ring strength of 647.+-.215
MPa. The two-step ion exchange process, in accordance with the
methods described hereinabove, thus resulted in an increase in
ring-on-ring strength of about 30% over the single-step
process.
Example 3
[0043] Electron microprobe analysis was used to investigate
Na.sub.2O and K.sub.2O concentrations in alkali aluminoborosilicate
samples following a two-step ion exchange process carried out in
accordance with the methods described hereinabove. FIG. 3a is a
plot of the Na.sub.2O concentration profile determined by electron
microprobe analysis following the first ion exchange step
(immersion in a 390.degree. C. molten NaNO.sub.3 salt bath for 5
hours). FIG. 3b is a plot of the K.sub.2O concentration profile
determined by electron microprobe analysis following the second ion
exchange step (immersion in a 410.degree. C. molten KNO.sub.3 salt
bath for: a) 0 minutes (i.e., corresponding to single-step ion
exchange); b) 10 minutes; c) 20 minutes; and d) 60 minutes. The
concentrations of Na.sup.+ and K.sup.+ ions correspond to the
Na.sub.2O and K.sub.2O concentrations, respectively, shown in FIGS.
3a and 3b. The K.sup.+ and Na.sup.+ concentrations on the glass
surface relate to compressive stress, while the distance these ions
diffuse into the glass relates to the depth of layer of the
compressive layer. As previously described hereinabove, the first
step of the two-step process (FIG. 3a) develops the deep depth of
layer, while the second step (FIG. 3b) adds a shallow layer that
provides the glass with a high compressive stress. The time for
which the second step is allowed to proceed dictates the K.sup.+
surface concentration and thus the compressive stress level that is
ultimately achieved. For example, a surface compressive stress of
895 MPa was observed for sample d (FIG. 3b), which was ion
exchanged in the second KNO.sub.3 bath for 60 minutes
[0044] 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. 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.
* * * * *