U.S. patent application number 13/693282 was filed with the patent office on 2013-04-18 for damage resistant, chemically toughened protective cover glass.
The applicant listed for this patent is Gregory Scott Glaesemann, James Joseph Price, Robert Sabia, Nagaraja Shashidhar. Invention is credited to Gregory Scott Glaesemann, James Joseph Price, Robert Sabia, Nagaraja Shashidhar.
Application Number | 20130095310 13/693282 |
Document ID | / |
Family ID | 40939130 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130095310 |
Kind Code |
A1 |
Glaesemann; Gregory Scott ;
et al. |
April 18, 2013 |
DAMAGE RESISTANT, CHEMICALLY TOUGHENED PROTECTIVE COVER GLASS
Abstract
The invention is directed to a high strength, chemically
toughened protective glass article, the glass article having a high
damage tolerance threshold of at least 1500 g as measured by the
lack of radial cracks when the load is applied to the glass using a
Vickers indenter; preferably greater than 2000 g s measured by the
lack of initiation of radial cracks when the load is applied to the
glass using a Vickers indenter
Inventors: |
Glaesemann; Gregory Scott;
(Corning, NY) ; Price; James Joseph; (Corning,
NY) ; Sabia; Robert; (Corning, NY) ;
Shashidhar; Nagaraja; (Painted Post, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Glaesemann; Gregory Scott
Price; James Joseph
Sabia; Robert
Shashidhar; Nagaraja |
Corning
Corning
Corning
Painted Post |
NY
NY
NY
NY |
US
US
US
US |
|
|
Family ID: |
40939130 |
Appl. No.: |
13/693282 |
Filed: |
December 4, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12367185 |
Feb 6, 2009 |
8367208 |
|
|
13693282 |
|
|
|
|
61065167 |
Feb 8, 2008 |
|
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Current U.S.
Class: |
428/220 |
Current CPC
Class: |
C03C 3/091 20130101;
Y10T 428/315 20150115; C03C 21/002 20130101; Y10T 428/31
20150115 |
Class at
Publication: |
428/220 |
International
Class: |
C03C 3/091 20060101
C03C003/091 |
Claims
1. A high strength, chemically toughened protective glass article,
said protective glass article having a thickness less than or equal
to 2.0 millimeters, a high damage tolerance threshold of at least
6000 g as measured by the lack of the presence of radial cracks
when the load is applied to the glass using a Vickers indenter, and
a compressive stress greater than 600 MPa.
2. The glass article according to claim 1, wherein said the glass
of said cover glass is selected from the group consisting of alkali
containing aluminosilicate glasses, alkali containing
aluminoborosilicate glasses, alkali containing borosilicate glasses
and alkali containing glass-ceramics.
3. The glass article according to claim 1, wherein the composition
of the glass of said article comprising, before any ion exchange to
chemically strengthen, 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 %, 0-0.5%
(As.sub.2O.sub.3,SnO.sub.2); 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 %.
4. The glass article according to claim 1, wherein the composition
of the glass of said article comprising, before any ion exchange to
chemically strengthen, 64-68% SiO.sub.2, 10-12% Al.sub.2O.sub.3,
0-2% B.sub.2O.sub.3, 12-15% Na.sub.2O, 2-4% K.sub.2O, 5-7% MgO,
>0-1% CaO, 0-0.5% (As.sub.2O.sub.3, SnO.sub.2), 0-1%
(Sb.sub.2O.sub.3, SnO.sub.2), and >0-1% TiO.sub.2.
5. The article according to claim 1, wherein said article has a
strength of 200 MPa or greater after blasting the surface of said
article with SiC grit according to ASTM Method C158.
6. The article according to claim 1, wherein said article has a
strength of 300 MPa or greater after blasting the surface of said
article with SiC grit according to ASTM Method C158.
7. The article according to claim 1, wherein the surface of said
article has been chemically toughed to a depth of at least 40
.mu.m.
Description
PRIORITY
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/367,185 filed on Feb. 6, 2009, the content
of which is relied upon and incorporated herein by reference in its
entirety, and the benefit of priority under 35 U.S.C. .sctn.120 is
hereby claimed, which claims the benefit of priority under 35
U.S.C. .sctn.119 of U.S. Provisional Application Ser. No.
61/065,167 filed on Feb. 8, 2008 the content of which is relied
upon and incorporated herein by reference in its entirety.
FIELD
[0002] The invention is directed to protective glass covers, and in
particular to chemically toughened, damage resistant glass covers
suitable for use in electronic devices.
BACKGROUND
[0003] The use of mobile devices having larger displays is becoming
more ubiquitous in devices such as cell phones, hand held games,
MP3 players, watches, laptop computers, mobile GPS and other
display screens in cars, touch panel screens, and other electronic
devices without limitation. At least a portion of the cover plate
is transparent, so as to allow the user to view a display. For some
applications, the cover plate is sensitive to the user's touch. As
the use of such devices increases the probability of the cover
glass breaking or suffering damage due to accidents, cleaning
negligent use and ordinary use also increases. The presently
available cover glasses are not designed or selected to survive the
high levels of abuse or some or the ordinary accident that can
occur such as sharp contact or impact with another object. Due to
frequent contact, such cover plates must have high strength and be
scratch resistant.
[0004] The "selection criteria" for the existing glasses, though
not always a requirement, has typically been limited to the
following: [0005] 1. A minimum height that the glass will have to
survive when a 135 g ball is dropped on the glass that is supported
in a prescribed manner; [0006] 2. A minimum strength as measured by
a four point bend test; and [0007] 3. A hardness which although
measured is typically not a requirement. These "criteria" for
existing protective glass used in display devices are not well
understood. Moreover, the primary test method for accepting cover
glass as fit-for-use is a ball drop test, a test known to the
inventors for its inability to accurately assess damage resistance
of glass because it is sensitive to existing surface flaws and not
to the introduction of new flaws. Strength testing, for example,
immediately after ion exchange, has also been used as a predictor
of the protective capability of cover glass. These tests will
naturally lead one to value high surface compressive stress over a
deep ion exchange layer. The inventors have found that this is
incorrect and the opposite is actually true. Consequently, the
present thin cover glass in these devices has not been optimized
for glass and ion exchange properties that directly relate to
abrasion resistance and visual appearance in these devices. For
example, current soda lime silicate ("SLS") glass used in mobile
devices is mechanically hindered by inherent limitations in its ion
exchange capability.
[0008] The criteria described above has been applied to select
glasses that are primarily in the soda-lime silicates family,
including version in which the alumina content is
elevated--referred to as aluminosilicates or
modified-aluminosilicates. [United States Patent Application
Publication 2008-0286548 A1 mentioned above discloses some glass
compositions that are an improvement over cover glass formulations
of the prior art]. We have found that these criteria do not
describe actual failure modes observed in the field for these
devices. The requirements defined by the prior art does not predict
how much load the glass can withstand when the mobile device is
dropped on a sharp object such a small stone. It also does not
predict how the glass will survive after the mobile device has
experienced in-service use with accumulated damage on the surface.
The prior art requirements can result in parts with unacceptably
poor strength and scratches. The present invention overcomes the
difficulties present in glasses currently used as protective covers
and/or touch screens in electronic devices.
SUMMARY
[0009] The invention is directed to a high strength, chemically
toughened protective and/or interactive (e.g., a touch screen)
glass article, the glass article having a high damage tolerance
threshold of at least 1500 g as measured by the lack of the
presence of radial cracks when the load is applied to the glass
using a Vickers indenter. In one embodiment the high damage
tolerance threshold of at least 2000 g. In one embodiment the high
damage tolerance threshold of at least 4000 g. In another
embodiment the high damage tolerance threshold of at least 6000
g.
[0010] In a further embodiment the high strength, chemically
toughened protective glass article is transparent.
[0011] In an additional embodiment the high strength, chemically
toughened protective glass article is opaque and/or
non-transparent.
[0012] In one embodiment the invention is directed to a protective
glass made of a soda lime glass, an alkali containing
aluminosilicate glass, an alkali containing aluminoborosilicate
glass, an alkali containing borosilicate glass or an alkali
containing glass-ceramic that has been ion exchanged to have a high
damage tolerance threshold of at least 2000 g as measured by the
lack of the presence of radial cracks when the load is applied to
the glass using a Vickers indenter. In one embodiment the high
damage tolerance threshold of at least 4000 g. In another
embodiment the high damage tolerance threshold of at least 6000
g.
[0013] The invention is also directed to a method of designing the
ion-exchange parameters in thin glass articles for use as
protective cover sheets, the method having the steps of:
[0014] choosing the depth of the compression layer required to
achieve the desired level of damage resistance as measured by a
Vickers indenter test and/or a scratch resistant test using a Knoop
diamond indenter;
[0015] selecting the compressive stress that will allow a designed
maximum tensile stress to develop in the center of the glass
article; and
[0016] diluting an ion-exchange bath containing alkali metal ions
having a diameter larger than that of sodium ions with sodium ions
to achieve the desired compressive stress.
[0017] The invention is also directed to a method of making a
chemically strengthened glass article suitable for use as a
protective cover glass the method having the steps of:
[0018] providing a glass sheet, the glass sheet being made of a
glass selected from the group consisting of soda lime glass, an
alkali containing aluminosilicate glass, an alkali containing
aluminoborosilicate glass, an alkali containing borosilicate glass
and alkali containing glass-ceramics;
[0019] chemically strengthening the glass sheet by ion-exchanging
Na and/or Li ions in the surface of the glass for larger alkali
ions (or other larger exchangeable ions), the chemical exchange
being to a depth of at least 40 .mu.m from the surface of the
sheet; and
[0020] finishing the sheet by cutting and polishing as required
(including and edge cutting, grinding and polishing) to make the
glass article;
[0021] wherein when finished, the glass article has a damage
tolerance threshold of at least 2000 g as measured by the lack of
the presence of radial cracks when the load is applied to the glass
using a Vickers indenter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 illustrates a commercially used glass material and
the scratches present on the glass from use.
[0023] FIG. 2 illustrates a commercially used glass material and
the damage that can occurred to the glass upon contact sharp
contact or impact with an object.
[0024] FIG. 3 is a schematic diagram illustrating a glass with an
ion-exchanged layer of depth DOL, surface compressive stress
C.sub.S and central tension C.sub.T.
[0025] FIG. 4 is a graph showing the strength of selected glass
materials before and after ion exchange.
[0026] FIG. 5 is a graph illustrating the onset of strength
limiting radial cracks as measured by the Vickers indentation
method.
[0027] FIG. 6 is a graph illustrating the strength of selected
ion-exchanged glasses after abrasion by air blasting using SiC
particles.
[0028] FIG. 7 is a graph illustrating lateral crack initiation
threshold (visual defects) as measured using a Vickers indent
[0029] FIG. 8 illustrates the scratch damage induced by sliding a
Knoop diamond indenter across the surface of a commercially
available cover glass
[0030] FIG. 9 illustrates the scratch damage induced by sliding a
Knoop diamond indenter across the surface of a cover glass
chemically strengthened according to the invention.
[0031] FIG. 10 is an enlargement of the area indicated by arrow 60A
and illustrates the chipping that occurs in the commercial
glass.
[0032] FIG. 11 is an enlargement of the area indicated by arrow 50A
and illustrates the lateral cracks that occur in the commercial
glass.
[0033] FIG. 12 is a graph illustrating the relationship between DOL
and CS for glasses of varying thickness (generally) and for glass
of the invention.
[0034] FIG. 13 is a graph illustrating the Critical Load vs. Depth
of layer (DOL) for chemically strengthened soda-lime glass
presently used as a protective glass and glasses of the
invention.
DETAILED DESCRIPTION
[0035] As used herein, the terms "chemical strengthening,"
"chemical toughening" and "ion-exchanging", and similar terms, mean
exchanging alkali ions in a glass composition with larger diameter
alkali ions. All glass compositions given herein are for glass
before any ion exchange. Also herein, it is to be understood that
the glass article being claimed can be both protective and/or
interactive, for example, a touch screen. As used in FIGS. 8-11,
the arrow 200 indicates the scratch direction. As may be used
herein regarding glass compositions, the term "consisting
essentially of" means that the composition contains the recited
materials and amounts, and excludes contaminants that may be
present in the glass
[0036] In general what is disclosed is a thin protective cover
glass that has been chemically strengthened to have a high damage
threshold of at least 2000 g as measured by the lack of the
presence of radial cracks when the load is applied to the glass by
a Vickers indenter. While the invention can be used to make cover
glasses of any thickness (for example, 30 mm), cover glasses for
use in electronic devices, and particularly hand-held devices, have
to be thin for weight reasons and generally have a thickness of
less than or equal to 5.0 mm; preferably less than or equal to 2.0
mm; in some embodiments less than 1.7 mm; and in additional
embodiments less than 1.2 mm. The difficulty with the thin cover
glasses is that while being thin, the glass must be capable of
withstanding an abrasive in-service environment and also be
resistant to cracking, peeling and other types of damage. As mobile
display manufacturers transition existing and future products from
plastic display covers to glass coves, glass will be exposed to
ever increasing levels of abuse. Examples of scratch and impact
damage on a commercial ion-exchanged sodium aluminosilicate glass
used in presently available cell phones are shown in FIGS. 1 and 2.
FIG. 1 illustrates the scratches on the protective glass cover that
occur through ordinary use. FIG. 2 illustrates the damage that can
occur to the same glass upon contact sharp contact or impact with
an object. The same type of glass is used in other electronic
devices.
[0037] The invention, in one aspect, is directed to a thin,
ion-exchanged (chemically toughened) cover glass optimized so that
it is resistant to damage and failure when used in mobile (or non
mobile) display devices. The performance of this glass is described
in terms of tests (pre-existing or developed) specifically designed
to quantify damage tolerance and failure resistance. The
compression layer of the glass of the invention has been optimized
to be at least 40 .mu.m in depth, which is deeper than other
ion-exchanged cover glass used in these devices, and has a
compressive stress of at least 600 MPa; preferably a compressive
stress of at least 700 MPa. It is this combination of depth of
layer (DOL) and compressive stress (CS) that provides the superior
resistance to crack initiation and failure.
[0038] When the limit of maximum tensile strength is imposed on
thin glass articles, the CS and DOL will have to be limited. This
limit can be reached by achieving the maximum CS while controlling
the depth of the compression layer or it can be controlled by
achieving a desired DOL while limiting the maximum CS. The DOL can
be limited by controlling the time while the CS can be limited by
controlling the concentration of sodium ions on the ion exchange
bath. The limit of maximum tensile stresses for glass articles of
various thicknesses (500 to 1000 .mu.m) is shown in FIG. 12 and the
lines drawn for each thickness are for when CT (tension in the
center) is 54 MPa. In FIG. 12 the "+" data points indicate the
DOL/CS relationship for chemically strengthened sodium
aluminosilicate glass ("C" in FIG. 12 legend) according to the
invention. The CS/DOL values to the left of the + data points can
be achieved by dilution of the ion-exchange bath.
[0039] If a DOL of 60 .mu.m is desired, the maximum compressive
stress that can be developed in the surface of the glass article is
330, 520 and 700 MPa for 0.5, 0.7 and 0.9 mm articles,
respectively. If a specified impact resistance is required, it is
generally desirable to target a depth of layer while limiting the
surface compressive stress. The impact load is related to the dept
of layer as is illustrated in FIG. 13. In FIG. 13 the
.tangle-solidup. 90 represent results obtained with a chemically
strengthened soda lime glass as is now commercially used and the
symbol .box-solid. C represents results obtained using the
chemically strengthened sodium aluminosilicate glass according to
the invention. The net result is that the chemical strengthening
parameters for thin glass can be controlled to achieve a desired
level of damage resistance.
[0040] From a frangibility viewpoint, it has been found desirable
that the level of tensile stress in the center of a glass of
approximately 1 mm thickness should be below approximately 54 MPa
as calculated from the CS and DOL measurements with FSM-6000
surface stress meter. This MPa value will vary with the thickness
of the glass, the MPa value rising as the glass gets thinner and
falling as the glass gets thicker.
[0041] As will be shown by the data presented herein, the
chemically toughened (strengthened) glasses of the invention have a
number of improved and highly desirable characteristics. Among
these are: [0042] 1. Greater resistance to surface chipping from a
sharp object striking the glass surface than other glass that glass
presently used in these devices. [0043] 2. Greater resistance to
the initiation of strength limiting flaws, flaws proven to be
present in existing devices with a cover glass. [0044] 3. Machining
and handling flaws induced prior to ion exchange are enveloped by
the exchanged layer and placed in compression. This makes the final
glass product more tolerant to the finishing process. [0045] 4.
Reduced finishing costs as glass surface can be formed directly
during glass making using the fusion process
[0046] The invention can be practiced with glass compositions that
can be chemically strengthened (that is, contains an element or
elements in the glass can be ion-exchanged). Glasses that are
particularly suited to the invention are alkali containing
aluminosilicate glasses, alkali containing borosilicate glasses,
alkali containing aluminoborosilicate glasses and alkali contain
glass-ceramics. In preferred embodiments the glasses and
glass-ceramics are transparent. The glass can be chemically
strengthened by ion exchange and the compositions can be down-drawn
into sheets. The glass has a melting temperature of less than about
1650.degree. C. and a liquidus viscosity of at least 130 kpoise
and, in one embodiment, greater than 250 kpoise. The glass can be
ion exchanged at relatively low temperatures and to a depth of at
least 30 .mu.m.
[0047] One exemplary sodium aluminosilicate glass has, before ion
exchange, a composition 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 %, 0-0.5%
(As.sub.2O.sub.3,SnO.sub.2); 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 %.
[0048] Other exemplary sodium aluminosilicate glasses have a
composition, in weight percent before ion exchange, of 64-68%
SiO.sub.2, 10-12% Al.sub.2O.sub.3, 0-2% B.sub.2O.sub.3, 12-15%
Na.sub.2O, 2-4% K.sub.2O, 5-7% MgO, >0-1% CaO, 0-0.5%
(As.sub.2O.sub.3,SnO.sub.2), 0-1% (Sb.sub.2O.sub.3,SnO.sub.2), and
0-1% TiO.sub.2. The arsenic and antimony are frequently added to
glass compositions as fining agents to aid in eliminating gaseous
inclusions in the glass. However, arsenic and antimony are
generally regarded as hazardous materials. Accordingly, in one
embodiment, the glass is substantially free of antimony and
arsenic, comprising less that about 0.05 wt % of each of the oxides
of these elements. In applications where a fining agent may be
necessary it is advantageous to use a nontoxic component such as
tin, halides, or sulfates to produce the fining effect. Tin (IV)
oxide (SnO.sub.2) and combinations of tin (IV) oxide and halides
are particularly useful as fining agents and can be used to replace
the arsenic and antimony in the foregoing compositions.
[0049] The glass composition to be used for making the chemically
toughened glass of the invention can be made into sheets using
suitable processes; for example, fusion drawing, slot draw, rolled
sheet, precision pressing and other methods known in the art. The
preferred methods are draw-down methods such as fusion drawing and
slot drawing because they result in a glass with a relatively
pristine surface. These draw-down methods are used in the
large-scale manufacture of ion-exchangeable flat glass.
[0050] The fusion draw process uses a drawing tank that has a
channel for accepting molten glass raw material. The channel has
weirs that are open at the top along the length of the channel on
both sides of the channel. When the channel fills with molten
material, the molten glass overflows the weirs. Due to gravity, the
molten glass flows down the outside surfaces of the drawing tank.
These outside surfaces extend down and inwardly so that they join
at an edge below the drawing tank. The two flowing glass surfaces
join at this edge to fuse and form a single flowing sheet. The
fusion draw method offers the advantage that, since the two glass
films flowing over the channel fuse together, neither outside
surface of the resulting glass sheet comes in contact with any part
of the apparatus. Thus, the surface properties are not affected by
such contact.
[0051] The slot draw method is distinct from the fusion draw
method. Here the molten raw material glass is provided to a drawing
tank. The bottom of the drawing tank has an open slot with a nozzle
that extends the length of the slot. The molten glass flows through
the slot/nozzle and is drawn downward as a continuous sheet
therethrough and into an annealing region. Compared to the fusion
draw process, the slot draw process provides a thinner sheet as
only a single sheet is drawn through the slot, rather than two
sheets being fused together, as in the fusion down-draw
process.
[0052] In order to be compatible with down-draw processes, the
alkali aluminosilicate glass described herein has a high liquidus
viscosity. In one embodiment, the liquidus viscosity is at least
130 kilopoise (kpoise) and, in another embodiment, the liquidus
viscosity is at least 250 kpoise.
[0053] In one embodiment, the glass is strengthened by
ion-exchange. As used herein, the term "ion-exchanged" is
understood to mean that the glass is strengthened by ion-exchange
processes that are known to those skilled in the glass fabrication
arts. Such ion exchange processes include, but are not limited to,
treating the heated alkali aluminosilicate glass (or other suitable
alkali-containing glass) with a heated solution containing ions
having a larger ionic radius than ions that are present in the
glass surface, thus replacing the smaller ions with the larger
ions. Potassium ions, for example, could replace sodium or lithium
ions in the glass. Alternatively, other alkali metal ions having
larger atomic radii, such as rubidium (Rb) or cesium (Cs) could
replace smaller alkali metal ions in the glass, including
potassium. Similarly, other alkali metal salts such as, but not
limited to, sulfates, halides, and the like may be used in the ion
exchange process. Generally the times and temperatures for
ion-exchange are 380-460.degree. C. and 3-16 hours, respectively,
for when using the compositions described herein using a 100%
potassium nitrate bath. The exact time and temperature required are
dependent of the exact glass composition being ion-exchanged. In
one embodiment, the down-drawn glass is chemically strengthened by
placing it a molten salt bath comprising KNO.sub.3 for a
predetermined time period to achieve ion exchange. In one
embodiment, the temperature of the molten salt bath is about
430.degree. C. and the predetermined time period is about eight
hours. In another embodiment the ion-exchange is first carried out
using K ion to achieve the desired depth of exchange and then
carried out using Ce or Rb ion to further strengthen the surface by
exchange with K ions relative close to the surface.
[0054] Down-draw processes produce surfaces that are relatively
pristine. Because the strength of the glass surface is controlled
by the amount and size of surface flaws, a pristine surface that
has had minimal contact has a higher initial strength. When this
high strength glass is then chemically strengthened, the resultant
strength is higher than that of a surface that has been a lapped
and polished. Chemical strengthening or tempering by ion exchange
also increases the resistance of the glass to flaw formation due to
handling. Accordingly, in one embodiment, the down-drawn alkali
aluminosilicate glass has a warpage of less than about 0.5 mm for a
300 mm.times.400 mm sheet. In another embodiment, the warpage is
less than about 0.3 mm.
[0055] Surface compressive stress refers to a stress caused by the
substitution during chemical strengthening of an alkali metal ion
contained in a glass surface layer by an alkali metal ion having a
larger ionic radius. In one embodiment potassium ions are
substituted for sodium ions in the surface layer of the glass
described herein. The glass has a surface compressive stress of at
least about 200 MPa. In one embodiment, the surface compressive
stress is at least about 600 MPa. In a further embodiment the
surface compressive strength is at least 700 MPa. The alkali
aluminosilicate glass has a compressive stress layer that has a
depth of at least 40 .mu.m.
[0056] The replacement of smaller ions by larger ions at a
temperature below that at which the glass network can relax
produces a distribution of ions across the surface of the glass
that results in a stress profile. The larger volume of the incoming
ion produces compressive stress (CS) on the surface and tension in
the center (CT) of the glass. The compressive stress is related to
the central tension by the following relationship:
CS=CT.times.(t-2DOL)/DOL
where t is the thickness of the glass and DOL is the depth of
exchange.
[0057] To exemplify the invention, a sodium aluminosilicate glass
composition which falls the composition as described above in
Paragraph [0033] was fusion drawn into sheets. Sample "C" was
comprised of the following analyzed composition, in mol %:67%
SiO.sub.2, 10% Al.sub.2O.sub.3, 0.45% B.sub.2O.sub.3, 13.6%
Na.sub.2O, 2.6% K.sub.2O, 5.8% MgO, 0.06% CaO and 0.33%
As.sub.2O.sub.3. This Sample C was evaluated along with three
non-Corning commercially available soda lime glasses and identified
below as Samples "X", "Y", and "Z" which are advertised as being
chemically strengthenable (that is, they are not chemically
strengthened as received but can be chemically strengthened by the
purchaser). All four samples were ion-exchanged under the same
conditions, exchanging K ions for Na ions. All glass samples were 1
mm thick. All glass samples were optimized for ion exchange. Table
1 shows the depth of ion-exchange for the four samples
TABLE-US-00001 TABLE 1 X Y Z C DOL .mu.m) 15 14 12 63 CS (MPa) 532
500 768 708 Temp., .degree. C. 390 430 410 410 Time (Hr) 12 7 11 12
X and Y are soda lime glasses containing 1-2 wt % alumina. Z is a
soda lime glass containing about 3 wt % alumina C is a sodium
aluminosilicate glass containing 8-10 wt % alumina.
[0058] Table 1 illustrates that using the glasses described herein,
one is able to obtain a chemically strengthen glass that has a
depth of layer DOL (which is the depth to which K ions are
exchanged for Na ions) of greater than 40 .mu.m and a surface
compressive stress CS greater than 600 MPa; preferably greater than
700 MPa.
[0059] As indicted above, all glass samples were ion-exchanged by
the same process. Consequently, the samples can be directly
compared for resistance to glass damage after ion exchange, which
comparison is illustrated in FIGS. 4-7. Generally, flaws after
finishing are better contained within the ion-exchange layer as is
evidenced by the increase in strength after ion exchange.
[0060] In FIGS. 4-7 both ion-exchanged and non-ion-exchanged
samples of each of X, Y, Z and C were evaluated. The
non-ion-exchanges samples were all abraded to the same 50 MPa
strength level and then ion-exchanged. FIG. 4 illustrates that all
samples had the same strength before ion-exchange, Sample C was
approximately 100 MPa stronger than Samples X, Y and Z after
abrasion, but before ion exchange
[0061] FIG. 5 illustrates the onset of strength limiting cracks as
measured by the Vickers indentation method all have the same
approach for each of the four samples. Samples X, Y and Z all
exhibit the onset of radial cracks at a critical load in the range
of 800-1000 g. Sample C did not exhibit radial cracking until a
critical load was greater than 6000 g. The critical load of Sample
C was thus at least 6.times. greater than the load of the other
samples.
[0062] FIG. 6 illustrates the strength of ion-exchanges Samples X,
Y, Z and C after blasting with sharp, hard SiC particles according
to ASTM Method C158. The x-axis, labeled "Contact Force Factor" or
"CFF", is a combination of grit size and blasting pressure. The
blasted SiC particles abrade the glass surface. The strength of the
glass after SiC blasting was measured using the ring-on ring
method. The results shown in FIG. 6 shown that Samples X, Y and Z
all have an initial strength (in MPa) of between 450 and 500,
whereas Sample C has an initial strength of approximately 575 MPa.
After SiC blasting at a CFF of approximately 10, Samples X, Y and X
all shown a strength in the range of 80-100 MPa whereas Sample C
showed an average strength of approximately 400 MPa.
[0063] FIG. 7 illustrates the load that was required to initiate
lateral cracking which is responsible for chipping. The lateral
crack threshold (visible defects) was measured using a Vickers
indenter. [There is no ASTM method for the Vickers indenter test,
but the method is described in articles by T. Tandon et al.,
"Stress Effects in Indentation Fracture Sequences," J. Am. Ceram
Soc. 73 [9] 2619-2627 (1990); R. Tandon et al., "Indentation
Behavior of Ion-Exchanges Glasses," J. Am. Ceram Soc. 73 [4]
970-077 (1990); and P. H. Kobrin et al., "The Effects of Thin
Compressive Films on Indentation Fracture Toughness Measurements,"
J. Mater. Sci. 24 [4] 1363-1367 (1989)]. The numbers above each bar
represent the depth of the ion-exchanged layer of each sample and
are also found in Table 1. The results illustrated in FIG. 7 shown
that the Critical Load required to initiate cracking and chipping
of Samples X, Y and Z is in the approximate range of 800-1400 g,
where for Sample C no lateral cracking, and hence no chip
formation, was observed at loads as high as 6000 g. The results
indicate that Sample C is at least 4.times. more resistant to
lateral cracking than Samples X, Y and Z.
[0064] FIGS. 8 and 9 illustrate in improves resistance of the glass
of the present invention over a commercially available glass used
as a protective cover glass. The test was conducted using ASTM
G171-03 scratch test method and the Micro-Tribometer mod.UMT-2. The
UMT is a commercial instrument (CETR Inc., Campbell, Calif.) that
permits various form of tribological testing including scratch
tests. An appropriate reference is V. Le Houerou et al., "Surface
Damage of Soda-lime-silica Glasses: Indentation Scratch Behavior,"
J. Non-Cryst Solids, 316 [1] 54-63 (2003). In this test a Knoop
indenter is dragged across the surface with an ever increasing
indentation load to a maximum load of 500 grams in approximately
100 seconds (so as to distinguish glass-to-glass differences).
[0065] FIGS. 8 and 9 illustrate the scratch induced by sliding a
Knoop diamond indenter across the surface of glass Sample Y and C,
respectively, at an ever increasing load. Numerals 30 and 40
represent the start and finish point of the scratch test for each
sample. For both Samples Y and C there was grooving and pealing of
glass from the indenter groove as might be expected. However, in
Sample Y are three stages of damage which are the scratch groove,
lateral cracking (numeral 50, lines A and B) and chipping (numeral
60, lines A and B). The lateral cracking and chipping of Sample Y
occurs at a load of less than 200 grams. Median crack vents are
also generated. There is no evidence of lateral cracking or
chipping in Sample C, which shows only the scratch groove. The
Sample C glass article does not exhibit lateral cracking until
loads exceeding 500 grams in this test. FIG. 10 is an enlargement
of the area of Sample Y denoted by arrow 60A in FIG. 8 and
illustrates the chipping that occurred at that point for this
sample. Similar chipping can be found in Sample Y in the area
denoted by arrow 60B and elsewhere along the groove. FIG. 11 is an
enlargement of the Sample Y area denoted by arrow 50A in FIG. 8 and
illustrates the lateral cracks found in Sample Y. Similar lateral
cracks can be found elsewhere in Sample Y along the groove
line.
[0066] Unlike float glass, which has been used to make cover
glasses, fusion formed and slot drawn glass does not have to be
thinned during finishing. Once the edges are prepared, the glass is
ready for product assembly. This lowers the cost of manufacturing
the cover glass, especially for devices requiring large glass
surface areas, for example, ATM touch screens, laptop computers and
other large screen devices. The beneficial surface forming surface
area forming can also impact manufacturing process step
utilization. Equipments investments and process time can be devoted
to edge grinding operations which in turn can permit more rigorous
process control and hence improved strength of the ground edge, an
area that often is the first to fail.
[0067] 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 invention.
Accordingly, various modifications, adaptations, and alternatives
may occur to one skilled in the art without departing from the
spirit and scope of the present invention.
* * * * *