阅读说明：本技术 具有高热膨胀系数的耐损坏的玻璃 (Damage-resistant glass with high coefficient of thermal expansion ) 是由 T·M·格罗斯 郭晓菊 于 2014-08-22 设计创作，主要内容包括：本申请涉及具有高热膨胀系数的耐损坏的玻璃。提供可离子交换的玻璃,其热膨胀系数(CTE)是至少约90×10～(-7)℃～(-1)。所述玻璃经历快速离子交换,例如于370℃-390℃的温度下在小于2小时的时间中在熔融的KNO-(3)盐浴中离子交换到大于30微米的层深度。当离子交换到30-50微米的层深度时,玻璃呈现超过30千克力(kgf)的维氏(Vickers)中间/径向裂纹引发阈值。所述玻璃是可熔合成形的,且在一些实施方式中,所述玻璃与锆石相兼容。(The present application relates to damage resistant glass having a high coefficient of thermal expansion. Providing an ion-exchangeable glass having a Coefficient of Thermal Expansion (CTE) of at least about 90x10 ‑7 ℃ ‑1 . The glass undergoes rapid ion exchange, e.g., in molten KNO at a temperature of 370 ℃ to 390 ℃ for a time of less than 2 hours 3 The ions in the salt bath are exchanged to a depth of layer greater than 30 microns. When ion exchanged to a depth of layer of 30-50 microns, the glass exhibits a Vickers (Vickers) median/radial crack initiation threshold in excess of 30 kilogram force (kgf). The glass is fusion formable and, in some embodiments, is compatible with zircon.)

1. A glass, comprising: 57 to 75 mol% SiO₂，Al₂O₃Greater than 5 mol% to 7 mol% P₂O₅And greater than 1 mol% to 5 mol% K₂O, wherein the glass does not contain B₂O₃，R₂O + R' O > 18 mol%, R₂O is at least one alkali metal oxide and R' O is at least one alkaline earth oxide, and the glass has a glass content of at least 90x10^-7℃^-1The coefficient of thermal expansion of (a).

2. The glass of claim 1, wherein the glass is ion exchanged and has a compressive layer extending from a surface of the glass to a depth of layer of at least 30 μ ι η and having a compressive stress of at least 800MPa, and wherein the ion exchanged glass has a vickers crack initiation threshold of at least 15 kgf.

3. The glass of claim 2, wherein the glass comprises KNO in a temperature range of 370 ℃ to 390 ℃₃Up to two hours of ion exchange in the ion exchange bath of (a).

4. The glass of any one of claims 1-3, wherein the glass comprises: 6 to 17 mol% Al₂O₃And 14 to 17 mol% Na₂O。

5. The glass of claim 4, further comprising up to 2 mol% MgO.

6. The glass of any one of claims 1-3, wherein the glass is MgO-free.

7. The glass of any of claims 1-3, wherein the glass has at least 95x10^-7℃^-1The coefficient of thermal expansion of (a).

8. A method of ion exchanging glass, the method comprising:

a. providing a glass comprising: 57 to 75 mol% SiO₂，Al₂O₃Greater than 5 mol% to 7 mol% P₂O₅And greater than 1 mol% to 5 mol% K₂O, wherein the glass does not contain B₂O₃，R₂O + R' O > 18 mol%, R₂O is at least one alkali metal oxide and R' O is at least one alkaline earth oxide, and the glass has a glass content of at least 90x10^-7℃^-1Coefficient of thermal expansion of (a);

b. providing an ion exchange bath, wherein the ion exchange bath comprises KNO₃And the temperature range is 370 ℃ to 390 ℃; and

c. ion exchanging the glass in the ion exchange bath for a period of time up to two hours, wherein the ion exchanged glass has a layer at a compressive stress of at least 800MPa that extends from a surface of the glass to a depth of layer of at least 30 μm, wherein the ion exchanged glass has a Vickers crack initiation threshold of at least 15 kgf.

9. The method of claim 8, wherein the depth of layer ranges from 30 μ ι η up to 50 μ ι η.

10. The method of claim 8 or 9, wherein the glass has at least 95x10^-7℃^-1The coefficient of thermal expansion of (a).

Background

The present invention relates to glass for use as a large sheet of cover glass. In particular, the invention relates to ion-exchangeable glasses for such applications. Even more particularly, the present invention relates to ion-exchangeable glasses having a coefficient of thermal expansion high enough to be used as large sheets of cover glass.

Glass is used as a protective cover for applications such as LCD displays. In some applications, such displays are supported by an external frame, which is typically made of metal, steel, or an alloy. As display sizes increase (e.g., 55 inch diagonal), it is important that the Coefficient of Thermal Expansion (CTE) of the glass match that of the frame material, otherwise the glass will be subjected to various stresses that can lead to distortion or failure. None of the commercially available glasses currently in use meets this requirement.

Summary of The Invention

Providing an ion-exchangeable glass having a Coefficient of Thermal Expansion (CTE) of at least about 90x10^-7℃^-1. The glass comprises SiO₂、Al₂O₃、P₂O₅、K₂O, and in some embodiments, MgO. The glass undergoes rapid ion exchange, e.g., in molten KNO at a temperature of 370 ℃ to 390 ℃ for a time of less than 2 hours₃The salt bath is rapidly ion exchanged to a depth of layer greater than 30 microns. When ion exchanged to a depth of layer of 30-50 microns, the glass exhibits a Vickers (Vickers) median/radial crack initiation threshold in excess of 15 kilogram force (kgf). The glass is fusion formable (i.e., has a liquidus temperature below 160 kP) and, in some embodiments, is compatible with zircon (i.e., has a zircon decomposition temperature above the 35kP temperature of the glass).

Accordingly, it is an aspect of the present invention to provide a glassGlass comprising SiO₂、Al₂O₃、P₂O₅And greater than about 1 mol% K₂O, wherein the coefficient of thermal expansion of the glass is at least about 90x10^-7℃^-1。

A second aspect of the invention is to provide an ion exchanged glass comprising SiO₂、Al₂O₃、P₂O₅And greater than about 1 mol% K₂And O. The ion exchanged glass has a coefficient of thermal expansion of at least about 90x10^-7℃^-1And a vickers crack initiation threshold of at least about 15 kgf.

A third aspect of the invention is to provide a method of ion exchanging glass. The method comprises the following steps: providing a glass comprising SiO₂、Al₂O₃、P₂O₅And greater than about 1 mol% K₂O and a coefficient of thermal expansion of at least about 90x10^-7℃^-1(ii) a Providing an ion exchange bath, wherein the ion exchange bath comprises KNO₃And a temperature of about 370 ℃ to 390 ℃; and ion exchanging the glass in the ion exchange bath for a period of time up to about 2 hours. The ion exchanged glass has a layer under a compressive stress extending from a surface of the glass to a depth of layer of at least about 30 microns.

These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, the accompanying drawings and the appended claims.

Brief description of the drawings

FIG. 1 is a schematic cross-sectional view of an ion-exchanged glass sheet; and

FIG. 2 is a schematic view of a method of ion exchanging glass.

Detailed Description

In the description below, like reference numerals designate like or corresponding parts throughout the several views shown in the figures. It should also be 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. Further, it is understood that when a group is described as comprising at least one of an element and combinations thereof, the group can comprise, consist essentially of, or consist of any number of the listed elements, 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 a combination thereof, it is understood that the group may consist of any number of those listed elements, either individually or in combination with each other. Unless otherwise indicated, a range of numerical values set forth includes both the upper and lower limits of the range, as well as any range between the upper and lower limits. As used herein, the indefinite article "a" or "an" and its corresponding definite article "the" mean "at least one", or "one or more", unless otherwise indicated. It should also be understood that the various features disclosed in the specification and drawings may be used in any and all combinations.

As used herein, the terms "glass article" and "glass articles" are used in their broadest sense to include any object made in whole or in part of glass. All units of composition are in mole percent (mol%) unless otherwise indicated. Coefficient of Thermal Expansion (CTE) in 10^-7/° c and represents a value measured over a temperature range of about 20 ℃ to about 300 ℃, unless otherwise specified.

As used herein, the term "liquidus temperature," or "T^L"refers to the temperature at which crystals first appear when the molten glass cools from the melting temperature, or the temperature at which the last point of crystals melts when the temperature is raised from room temperature. As used herein, the term "160 kP temperature" or "T^160kP"refers to the temperature at which the viscosity of the glass or glass melt is 160,000 poise (P), or 160 kilopoise (kP). As used herein, the term "35 kP temperature" or "T^35kP"refers to the temperature at which the viscosity of the glass or glass melt is 35,000 poise (P), or 35 kilopoise (kP).

It should be noted that the terms "substantially" and "about" may be used herein to represent the inherent uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent quantitative representations that may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. Thus, for example, a glass that is "substantially free of MgO" is one of the following: where no MgO is actively added or dosed into the glass, but MgO may be present in very small amounts as a contaminant.

The vickers crack initiation threshold described herein is measured by: an indentation load was applied to the glass surface and then removed at a rate of 0.2 mm/min. The maximum indentation load was held for 10 seconds. The indentation crack threshold is defined as the indentation load at which 50% of 10 indentations show any number of radial/median cracks extending from the corner of the indentation imprint. The maximum load is increased until a threshold is met for a given glass composition. All indentation measurements were performed at 50% relative humidity and room temperature.

Compressive stress and depth of layer are measured using methods known in the art. Such methods include, but are not limited to, measuring surface stress (FSM) using a commercial instrument such as FSM-6000 manufactured by Luceo corporation, tokyo, japan, or the like, methods for measuring compressive stress and depth of layer as described in ASTM1422C-99 entitled "standard specification for chemically strengthened flat glass" and ASTM1279.19779 "standard test method for nondestructive photoelastic measurement of edge and surface stresses in annealed, heat strengthened, fully tempered flat glass," the entire contents of which are incorporated herein by reference. Surface stress measurement relies on the accurate measurement of the Stress Optical Coefficient (SOC), which is related to the birefringence of the glass. The SOC is further measured by those methods known in the art, such as the fiber and four-point bending methods (all of which are described in ASTM standard C770-98(2008), entitled "standard test method for measuring stress-optical coefficient of glass", which is incorporated herein by reference in its entirety) and the block and cylinder method.

Referring to all of the drawings and in particular to FIG. 1, it is to be understood that the drawings are for purposes of describing particular embodiments of the present invention and are not to be construed as limiting the description of the invention or the appended claims. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

Described herein are ion-exchangeable glasses that have high Coefficients of Thermal Expansion (CTE) and are useful as large sheets of cover glass. The glass (also referred to herein as "high CTE glass") is also capable of ion exchange at a rate greater than that of a similar glass. After ion exchange, the glass exhibits high crack resistance as measured by vickers indentation.

The high CTE glasses described herein comprise SiO₂、Al₂O₃、P₂O₅And K₂And O. In some embodiments, the glass consists essentially of or comprises the following components: about 57 mol% to about 75 mol% SiO₂(i.e., 57 mol% SiO. ltoreq. SiO₂Less than or equal to 75 mol%); about 6 mol% to about 17 mol% Al₂O₃(i.e., 6 mol% or more and less of Al)₂O₃Less than or equal to 17 mol%); about 2 mol% to about 7 mol% P₂O₅(i.e., 2 mol% or more and P or less₂O₅Less than or equal to 7 mol%); about 14 mol% to about 17 mol% Na₂O (i.e., 14 mol% Na or more)₂O less than or equal to 17 mol%); and greater than about 1 mol% to about 5 mol% K₂O (i.e., 1 mol%<K₂O is less than or equal to 5 mol percent). In some embodiments, the glass consists essentially of or comprises the following components: about 57 mol% to about 59 mol% SiO₂(i.e., 57 mol% SiO. ltoreq. SiO₂Less than or equal to 59 mol%); about 14 mol% to about 17 mol% Al₂O₃(i.e., 14 mol% or more and less of Al)₂O₃Less than or equal to 17 mol%); about 6 mol% to about 7 mol% P₂O₅(i.e., 6 mol% or more and P or less₂O₅Less than or equal to 7 mol%); about 16 mol% to about 17 mol% Na₂O (i.e., Na is not more than 16 mol%)₂O less than or equal to 17 mol%); and greater than about 1 mol% to about 5 mol% K₂O (i.e., 1 mol%<K₂O is less than or equal to 5 mol percent). In some embodimentsThe glass also comprises up to about 2 mole percent MgO (i.e., 0 mole percent MgO. ltoreq. 2 mole percent MgO) and/or up to about 1 mole percent CaO (i.e., 0 mole percent. ltoreq. CaO. ltoreq.1 mole percent CaO). In some embodiments, the glass is substantially free of MgO. In some embodiments, the glass is substantially free of B₂O₃. The compositions, strain points, annealing points and softening points of non-limiting examples of these glasses are listed in Table 1.

Silicon oxide (SiO)₂) Is the primary network former in the glasses described herein. In some embodiments, these glasses comprise from about 57 mol% to about 75 mol% SiO₂. Higher amounts (e.g., greater than about 60 mole%) of silica tend to reduce the coefficient of thermal expansion. Thus, in some embodiments, the glass comprises from about 57 mol% to about 59 mol% SiO₂。

Alumina (Al)₂O₃) Mainly promoting ion exchange. Further, Al₂O₃Phase separation is suppressed. In some embodiments, the glasses described herein comprise from about 6 mol% to about 17 mol% Al₂O₃. In other embodiments, these glasses comprise greater than about 13 mol% Al₂O₃And, in some embodiments, from about 14 mole% to about 17 mole% Al₂O₃。

Alkali metal oxide Na₂O and K₂The presence of O increases the CTE of the glass. K₂O plays a major role in increasing CTE, followed by Na₂And O. However, K when the glass is ion exchanged₂The presence of O tends to reduce the compressive stress and reduce the zircon decomposition temperature (T) in the presence of the glass melt^{Decomposition of}). In some embodiments, the glasses described herein comprise greater than about 1 mol% K₂And O. In some embodiments, the glass comprises greater than about 1 mol% to about 5 mol% K₂And O. Na in glass₂The presence of O improves the ion exchange capacity of the glass. In some embodiments, the glass comprises from about 14 mol% to about 17 mol% Na₂O, and in other embodiments, from about 16 mol% to about 17 mol% Na₂And O. In some embodiments, the glass may also include other alkali metal oxides (Li)₂O,Rb₂O,Cs₂O), but these oxides either inhibit ion exchange and result in lower surface compressive stresses in the ion-exchanged glass or are relatively expensive. In some embodiments, the glass comprises less than about 1.5 mol% Li₂O, and in some embodiments, no or substantially no Li₂O。

The alkaline earth metal oxide MgO promotes ion exchange of the glass and increases surface compressive stress in the ion-exchanged glass, but tends to lower the thermal expansion coefficient of the glass. In some embodiments, the glasses described herein comprise up to about 2 mol% MgO. In some embodiments, the glass is free or substantially free of MgO. CaO tends to inhibit ion exchange and lower the CTE of the glass. Thus, the glass may contain up to about 1 mol% CaO.

In some embodiments, the alkali metal oxides (R) in these glasses₂O) and an alkaline earth metal oxide (R' O) in a total amount greater than about 18 mole% (i.e., R)₂O+R'O>18 mole%).

Presence of P in the glass₂O₅Ion exchange of the glass is facilitated by: adding certain cations (e.g. K)⁺) The diffusion coefficient of (c). Furthermore, P₂O₅Tends to increase the zircon decomposition temperature (T) in the presence of the glass melt^{Decomposition of}). In some embodiments, the glasses described herein comprise from about 2 mol% to about 7 mol% P₂O₅. In some embodiments, the glass comprises greater than about 5 mol% P₂O₅About 7 mol% P₂O₅(ii) a And, in some embodiments, from about 6 mol% to about 7 mol% P₂O₅。

The glasses described herein have a Coefficient of Thermal Expansion (CTE) of at least about 90x10^-7℃^-1. In other embodiments, the CTE is at least about 95x10^-7℃^-1And, in yet other embodiments, at least about 100 x10^-7℃^-1. In some embodiments, the CTE is about 90 × 10^-7℃^-1Up to about 100 x10^-7℃^-1And, in other embodiments, about 90x10^-7℃^-1Up to about 110 x10^-7℃^-1. In other embodiments, the CTE is about 95 × 10^-7℃^-1Up to about 100 x10^-7℃^-1And, in some embodiments, up to about 105 x10^-7℃^-1. The measured CTE's of the glasses shown in Table 1 are shown in tables 1 and 2. By K in glass₂The substitution of O for MgO tends to increase the CTE of the glass as shown by example 6 in tables 1 and 2. Examples 6,7, and 8 in tables 1 and 2 show the adjustment of K in the glass₂The amount of O, to "tune" or "tailor" the CTE. Since example 6 had the highest Al among the 3 glasses₂O₃And minimum K₂The O concentration, so example 6 had the highest compressive stress when ion-exchanged. Examples 9,10, and 11 show the substitution of MgO for Al₂O₃The effect on the CTE. The glasses in the series of examples 12-14 show a good balance between the inclusion of MgO and a smaller amount of K₂Conversion of the "base" glass composition of O (example 12) to contain K₂The effect on CTE of a glass containing O and substantially no MgO (example 14). Glasses in the series of examples 15-20 were shown to contain K₂O and a base glass substantially free of MgO (example 15) to a glass containing MgO and a smaller amount of K₂The effect on CTE with O glass (example 20).

The glasses described herein are in the fusion formable form; i.e. the liquidus temperature T of the glass^LAllowing them to be formed by a fusion draw process or by other down draw processes known in the art. In order to be fusion formable, the liquidus temperature of the glass should be less than the 160kP temperature T of the glass^160kP(i.e., T)^L<T^160P)。

The hardware used in the fusion draw process (e.g., isopipe) is often made of zircon. Temperature at which zircon in the isopipe decomposes to form zirconia and silica (also referred to herein as "decomposition temperature" or "T" for the sake of clarity)^{Decomposition of}") below any temperature experienced on the isopipe, the zircon will decompose to form silica and zirconia, with the result that the glass formed by the fusion process contains zirconia inclusions (also referred to as" fusion-line zirconia "). Therefore, it is desirable to feed the glass at a temperature too low to decompose zircon and form zirconiaThe lines are shaped and thus prevent the formation of zirconia defects in the glass. Alternatively, the isopipe may be made of other refractory materials (e.g., alumina), thus eliminating zircon breakdown as a factor in the fusion draw process.

Because fusion is a substantially isoviscous process, the highest temperature encountered by the glass corresponds to a particular viscosity of the glass. In standard fusion draw operations, such as those known in the art, this viscosity is about 35kP, and the temperature at which the viscosity is achieved is referred to as the 35kP temperature, or T^35kP。

In some embodiments, the high CTE glasses described herein are zircon compatible, and T^{Decomposition of}>T^35kP. For example, the composition of sample 6 (table 1) meets the CTE requirements of these glasses, but is not compatible with zircon because the 35kP temperature exceeds the zircon decomposition temperature, as shown in table 2. To make the glass compatible with zircon, the 6 composition can be varied to replace about 1 mole% Al with MgO₂O₃As indicated by the composition of sample 30. In order to make the glass compatible with zircon, the composition of sample 6 has been changed to replace the Al present in the glass with about 1 mol% MgO₂O₃As shown by the composition of sample 30 in table 1. Substitution of Al with MgO according to the zircon decomposition model₂O₃Temporary zircon decomposition temperature T^{Decomposition of}Will remain unchanged or slightly increased. As shown in tables 1 and 2, this slight change in composition changes the 35kP temperature T of the glass^35kPFrom 1244 ℃ to 1211 ℃. The glass was considered to be compatible with zircon, provided that the zircon decomposition temperature remained constant at 1215 ℃. MgO and Al₂O₃The substitutions do not significantly change the CTE, or the Compressive Stress (CS), depth of layer (DOL), and Knoop indentation threshold values of the glass when ion exchanged. For example, glass sample 28 closely approximates the composition of sample 30 and thus shows the substitution of MgO for Al₂O₃The values of CTE, CS, DOL, and indentation threshold (tables 3a and 3b) of the glass were retained. Density, T, for selected embodiments shown in Table 1^L,T^160P,T^35kPAnd T^{Decomposition of}See table 2.

In some embodiments, the methods described herein are performed using those methods known in the artThe glass of (2) is ion exchanged. In a non-limiting embodiment, the glass is immersed in a molten salt bath comprising alkali metal cations such as K⁺Which is greater than Na present in the glass⁺The cations are larger. Methods other than immersion in a molten salt bath may be used to ion exchange the glass. These methods include, but are not limited to, applying a slurry or gel containing cations to be introduced to the glass to at least one surface of the glass.

The ion exchanged glass has at least one surface layer under Compressive Stress (CS) as schematically shown in fig. 1. Glass 100 has a thickness t, a first surface 110, and a second surface 112. In some embodiments, glass 100 has a thickness t of up to about 2mm, in other embodiments up to about 1mm, in other embodiments up to about 0.7mm, and in still other embodiments up to about 0.5 mm. The glass 100 has a first layer 120 ("compressive layer") under compressive stress extending from the first surface 110 to a depth of layer d into the glass article 100₁. In the embodiment shown in FIG. 1, glass 100 also has a second compressive layer 122 under compressive stress extending from second surface 112 to a second depth of layer d₂. Glass 100 also includes a secondary glass layer₁Extend to d₂The central region 130. Central region 130 is under tensile stress or central tension, which balances or offsets the compressive stress of layers 120 and 122. Depth of layer d of the first and second compression layers 120 and 122₁And d₂Glass 100 is protected from flaw propagation by sharp impact against first and second surfaces 110 and 112 of glass 100, while compressive stresses in first and second compressive layers 120 and 122 are of a magnitude such that the flaws penetrate through a depth d of first and second compressive layers 120 and 122₁And d₂The likelihood of (c) is minimized.

In some embodiments, the ion-exchanged glasses described herein have a compressive layer that extends from the surface of the glass to a depth of layer of at least about 30 microns, and in some embodiments, the depth of layer is about 30 microns up to about 50 microns. In some embodiments, the compressive layer of glass is under a compression of at least about 700MPa when ion exchanged to a depth of layer of at least about 30 micronsUnder stress, and in other embodiments, under a compressive stress of at least about 800 MPa. Tables 3a and 3b list KNO in molten₃The compressive stress CS, depth of layer DOL, and Vickers crack indentation threshold for the glass compositions shown in Table 1 were determined after ion exchange in a salt bath at 390 ℃ and 370 ℃ respectively. Unless otherwise provided in table 2, the Stress Optical Coefficient (SOC) of the ion-exchanged glass shown in tables 3a and 3b was 30.1.

The high CTE glasses described herein also undergo rapid ion exchange. Lower CS, higher diffusion rates, and higher indentation thresholds indicate that these high CTE glasses have a more open network. For example, the glasses of the present invention may be ion-exchanged to a depth of layer greater than 30 microns in an ion-exchange bath comprising molten KNO at a temperature of about 370 ℃ to about 390 ℃ for a time of less than 2 hours₃. In specific embodiments, while in molten KNO₃Sample 6 (table 1) was ion exchanged to a compressive stress of 820MPa and a depth of layer of 50 microns when submerged at 390 ℃ for 1 hour (table 3 a).

The ion-exchanged glasses described herein have a vickers crack initiation threshold of at least about 15 kilogram force (kgf); in other embodiments, at least 20 kgf; and in yet other embodiments, at least about 30 kgf. In some embodiments, the ion exchanged glass has a vickers crack initiation threshold of at least 30kgf, in other embodiments at least 40kgf, and in still other embodiments, a vickers crack initiation threshold of at least 50 kgf. In some embodiments, the vickers crack initiation threshold is from about 30kgf up to about 50 kgf. The vickers crack indentation data for the glass compositions shown in table 1 are listed in tables 3a and 3b.

In another aspect, a method of ion exchanging glass is also provided. The steps of the method are schematically shown in fig. 2. The method 200 comprises a first step 210, wherein a glass is provided, the glass comprising SiO₂、Al₂O₃、P₂O₅And K₂O and a coefficient of thermal expansion of at least 95x10^-7℃^-1As described above. In step 220, KNO is provided₃Or mainly consisting of KNO₃Ion exchange bath of the composition. The ion exchange bath may compriseOther salts (e.g. NaNO)₃) Or may comprise KNO alone₃Or mainly consisting of KNO₃And (4) forming. Throughout the process, the ion exchange bath is maintained at a temperature of about 370 ℃ to 390 ℃. The glass is then ion exchanged in an ion exchange bath for a period of time up to about 2 hours (step 230), after which time the ion exchanged glass has a layer under compressive stress that extends from the surface of the glass to a depth of layer of at least about 30 microns, and in some embodiments, from about 30 microns to up to about 50 microns. In some embodiments, the layer of glass is under a compressive stress of at least about 700MPa, and in other embodiments, at least about 800 MPa.

In some embodiments, the ion exchanged glass has a vickers crack initiation threshold of at least about 30kgf, and in some embodiments, from about 30kgf up to about 50 kgf.

TABLE 1 compositions, strain points, annealing points, softening points and thermal expansion coefficients of the glasses.

TABLE 2 glass thermal expansion coefficients, 200 poise temperature T200,35 kpoise temperature T for the glasses listed in TABLE 1^35kP160 kpoise temperature T^160kPLiquidus temperature T^LLiquidus viscosity, zircon decomposition temperature T^{Decomposition of}Zircon breakdown viscosity, and stress optical coefficient SOC.

TABLE 3a glass in molten KNO as set forth in TABLE 1₃Compressive stress CS after ion exchange in the bath at 390 ℃, depth of layer DOL, and vickers crack indentation threshold. Unless provided in table 2, the Stress Optical Coefficient (SOC) of the ion-exchanged glass was 30.1.

TABLE 3b glass in molten KNO as set forth in TABLE 1₃Compressive stress CS after ion exchange in the bath at 370 ℃, depth of layer DOL, and vickers crack indentation threshold. Unless provided in table 2, the Stress Optical Coefficient (SOC) of the ion-exchanged glass was 30.1.

While typical embodiments have been set forth for the purpose of illustration, the foregoing descriptions should not be deemed to be a limitation on the scope of the specification or the 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 description and the appended claims.