# Patent

## Title
Zircon compatible, ion exchangeable glass with high damage resistance

## Abstract

An ion exchangeable glass having a high degree of resistance to damage caused by abrasion, scratching, indentation, and the like. The glass comprises alumina, B2O3, and alkali metal oxides, and contains boron cations having three-fold coordination. The glass, when ion exchanged, has a Vickers crack initiation threshold of at least 10 kilogram force (kgf).

## Background
This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/653,489, filed on May 31, 2012, and U.S. Provisional Application Ser. No. 61/748,981, filed on Jan. 4, 2013, the contents of which are relied upon and incorporated herein by reference in their entirety.


BACKGROUND
The disclosure relates to glasses that are capable of chemical strengthening by ion exchange and have intrinsic or “native” damage resistance. More particularly, the disclosure relates to such glasses that are strengthened by ion exchange and possess resistance to damage by abrasion, scratching, indentation, and other forms of sharp contact.
The ion exchange process provides a substantial improvement of glasses that are capable of being strengthened by this process to resist damage by sharp impact or indentation. To date, glasses containing network modifiers such as alkali and alkaline earth cations have been used. These cations form non-bridging oxygens (oxygens bonded to only one silicon atom), which reduce the resistance of the ion exchanged glass to damage introduced by abrasion, scratching, or the like.
SUMMARY
The present disclosure provides an ion exchangeable glass having a high degree of resistance to damage caused by abrasion, scratching, indentation, and the like. The glass comprises alumina, B2O3, and alkali metal oxides, and contains boron cations having three-fold coordination. The glass, when ion exchanged, has a Vickers crack initiation threshold of at least 10 kilogram force (kgf).
Accordingly, one aspect of the disclosure is to provide an ion exchangeable glass comprising at least about 50 mol % SiO2; at least about 10 mol % R2O, wherein R2O comprises Na2O; Al2O3, wherein Al2O3(mol %)<R2O(mol %); and B2O3, and wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≧3 mol %.
A second aspect of the disclosure is to provide a glass comprising at least about 50 mol % SiO2; at least about 10 mol % R2O, wherein R2O comprises Na2O; Al2O3; and B2O3, wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≧3 mol %, and wherein the glass has a zircon breakdown temperature that is equal to the temperature at which the glass has a viscosity in a range from about 25 kPoise to about 40 kPoise.
A third aspect of the disclosure is to provide an ion exchanged glass having a Vickers crack initiation threshold of at least about 10 kgf. The glass comprises at least about 50 mol % SiO2; at least about 10 mol % R2O, wherein R2O comprises Na2O; Al2O3, wherein Al2O3(mol %)<R2O(mol %); and B2O3, wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≧3 mol %.
A fourth aspect is to provide a glass comprising: at least about 50 mol % SiO2; at least about 10 mol % R2O, wherein R2O comprises Na2O; Al2O3; and at least 2.7 mol % B2O3 containing coordinated boron cations, wherein B2O3−(R2O−Al2O3)≧3 mol %.
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
FIG. 1 is a plot of nuclear magnetic resonance (NMR) spectra for samples of the glasses described herein.



DETAILED DESCRIPTION
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that, unless otherwise specified, terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. In addition, whenever a group is described as comprising at least one of a group of elements and combinations thereof, it is understood that the group may comprise, consist essentially of, or consist of any number of those elements recited, either individually or in combination with each other. Similarly, whenever a group is described as consisting of at least one of a group of elements or combinations thereof, it is understood that the group may consist of any number of those elements recited, either individually or in combination with each other. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range as well as any ranges therebetween. As used herein, the indefinite articles “a,” “an,” and the corresponding definite article “the” mean “at least one” or “one or more,” unless otherwise specified. It also is understood that the various features disclosed in the specification and the drawings can be used in any and all combinations.
As used herein, the terms “glass” and “glasses” includes both glasses and glass ceramics. The terms “glass article” and “glass articles” are used in their broadest sense to include any object made wholly or partly of glass and/or glass ceramic.
It is noted that the terms “substantially” and “about” may be utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
Unless otherwise specified, all compositions and concentrations described herein are expressed in mole percent (mol %). It is understood that the compositional ranges described and claimed herein represent the bulk composition of the glass as determined by those means skilled in the art, and are applicable to both the unstrengthened and strengthened glasses described herein.
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.
Accordingly, a glass having intrinsic damage resistance is provided. In one aspect, the glass, which may be strengthened so as to be resistant to damage via sharp impact, comprises at least about 50 mol % SiO2; at least about 10 mol % of at least one alkali metal oxide R2O, wherein R2O includes Na2O and, optionally, other alkali metal oxides (e.g., Li2O, K2O, Ce2O, Rb2O); alumina (Al2O3), wherein the amount of Al2O3, expressed in mol %, is less than the total amount of the alkali metal oxides present in the glass (i.e., Al2O3(mol %)<R2O(mol %)); and boron oxide (B2O3), wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≧3 mol %.
In another aspect, the glass comprises at least about 50 mol % SiO2; Al2O3; B2O3; and at least about 10 mol % R2O, wherein R2O comprises Na2O; wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≧3 mol %, and wherein the glass has a zircon breakdown temperature (i.e., the temperature at which zircon breaks down to form zirconia and silica) equal to the temperature at which the viscosity of the glass is in a range from about 25 kPoise to about 40 kPoise.
In some aspects, the glasses described herein are ion exchangeable; i.e., cations—typically monovalent alkali metal cations—which are present in these glasses are replaced with larger cations—typically monovalent alkali metal cations, although other cations such as Ag+ or Ti+—having the same valence or oxidation state. The replacement of smaller cations with larger cations creates a surface layer that is under compression, or compressive stress CS. This layer extends from the surface into the interior or bulk of the glass to a depth of layer DOL. The compressive stress in the surface layers of the glass are balanced by a tensile stress, or central tension CT, in the interior or inner region of the glass. Compressive stress and depth of layer are measured using those means known in the art. Such means include, but are not limited to measurement of surface stress (FSM) using commercially available instruments such as the FSM-6000, manufactured by Luceo Co., Ltd. (Tokyo, Japan), or the like. Methods of measuring compressive stress and depth of layer are described in ASTM 1422C-99, entitled “Standard Specification for Chemically Strengthened Flat Glass,” and ASTM 1279.19779 “Standard Test Method for Non-Destructive Photoelastic Measurement of Edge and Surface Stresses in Annealed, Heat-Strengthened, and Fully-Tempered Flat Glass,” the contents of which are incorporated herein by reference in their entirety. Surface stress measurements rely upon the accurate measurement of the stress optical coefficient (SOC), which is related to the stress-induced birefringence of the glass. SOC in turn is measured by those methods that are known in the art, such as fiber and four point bend method, both of which are described in ASTM standard C770-98 (2008), entitled “Standard Test Method for Measurement of Glass Stress-Optical Coefficient,” the contents of which are incorporated herein by reference in their entirety, and a bulk cylinder method.
In other aspects, the glasses described herein are ion exchanged and have at least one surface having a layer that is under compression to a depth of layer and an inner region that is under a tensile stress, as described hereinabove. In one aspect, the ion exchanged glass comprises at least about 50 mol % SiO2; at least about 10 mol % R2O, wherein R2O comprises Na2O; Al2O3; and B2O3, and wherein B2O3−(R2O−Al2O3)≧3 mol %. The ion exchanged glass has a Vickers crack initiation threshold of at least about 10 kilogram force (kgf).
When present in boroaluminosilicate glasses, alkali metals act as charge-balancing cations for tetrahedrally coordinated aluminum and boron. Species similar to RAlO2 or RBO2, where R is an alkali metal and aluminum are Al3+ and B3+, for example, are four-fold coordinated by oxygen. The four oxygen atoms surrounding aluminum or boron are presumed to bond to silicon atoms. Each oxygen therefore obtains one electron from the silicon atoms and, in effect, 3⁄4 of an electron from the tetrahedrally coordinated Al3+ or B3+. An alkali metal donates an electron to this configuration, thus allowing all four oxygens in the tetrahedron to obtain a full electron shell. The large alkaline earth elements Ca, Sr, and Ba are known to fill this role for aluminum. Thus, in a boroaluminosilicate glass in which R2O(mol %)+CaO(mol %)+SrO+BaO(mol %)−Al2O3(mol %)−B2O3(mol %)=0(mol %) or, alternatively, (Al2O3(mol %)+B2O3(mol %))/(R2O(mol %)+CaO(mol %)+SrO(mol %)+BaO(mol %))=1, there are exactly as many electrons donated from alkalis and alkaline earths as there are tetrahedrally coordinated aluminum and boron atoms in need of charge balance. When R2O(mol %)+CaO(mol %)+SrO(mol %)+BaO(mol %)−Al2O3(mol %)−B2O3(mol %)<0(mol %), there are more aluminum and boron atoms in need of charge balance than electrons from charge-balancing alkali or alkaline earth cations. Such glasses generally have high intrinsic or “native” damage resistance, which is considerably enhanced after ion exchange.
In these glasses, aluminum is nearly always four-fold coordinated by oxygen if enough charge-balancing cations are available. Low-valence cations perform better in a charge-balancing role than small, higher valence cations. This has been attributed to the extent to which the charge-balancing cation competes with aluminum for electron density. Large, low-valence cations are ineffective in this competition and thus leave electron density largely within the AlO4 tetrahedron. Boron is mostly four-fold coordinated in fully annealed alkali borosilicate glasses in which the amount of alkali oxide present in the glass is less than or equal to than the concentration Al2O3, as the alkali modifiers are consumed or taken up in bonds with aluminum. In fully annealed alkali borosilicate glasses in which the amount of alkali oxide present in the glass is greater than the combined concentration of B2O3 and Al2O3, boron is mostly four-fold coordinated and the excess alkali modifiers are taken up in non-bridging oxygen (NBO) bonds. In fully annealed boroaluminosilicate glasses in which B2O3≧alkali modifiers>Al2O3, however, boron assumes a mix of three and four-fold coordination, even in charge-balanced compositions. When fast-cooled from above the anneal or strain point of the glass, the formation of three-fold coordinated boron is entropically preferred. Thus, the proportion of three-fold coordinated boron may be enhanced through forming processes such as down-draw (e.g., slot- or fusion-draw) processes, or by heating the glass to a temperature above its anneal or strain point, followed by fast-cooling (e.g., at rates of at least about 4° C./s). In some embodiments, the glass is fast-cooled from a temperature that is above the anneal or strain point of the glass. To a reasonable approximation, charge-balancing cations first stabilize aluminum in four-fold coordination, and the remaining cations stabilize some fraction of boron in four-fold coordination, where the fraction may be as high as 1. The glasses described herein comprise at least 2.7 mol % and, in some embodiments, from about 2.7 mol % to about 4.5 mol % B2O3, in which the boron cations are predominantly three-fold coordinated. In some embodiments, at least about 50% of the boron cations comprising the glasses described herein are three-fold coordinated. In other embodiments, at least about 80% of the boron in these glasses is three-fold coordinated and, in still other embodiments, at least 90% of the boron cations in these glasses are three-fold coordinated. The proportion of three fold boron can be determined using 11B nuclear magnetic resonance (NMR). FIG. 1 shows NMR spectra for samples of the glasses described herein having the composition 67.45 mol % SiO2, 12.69 mol % Al2O3, 3.67 mol % B2O3, 13.67 mol % Na2O, 2.36 mol % MgO, and 0.09 mol % SnO2. The amount of four-fold coordinated boron (N4 in FIG. 1) ranges from 8.9% to 9.5%. Thus, about 90% of the boron present in each of the samples shown in FIG. 1 is three-fold coordinated.
When R2O(mol %)+MO(mol %)<Al2O3(mol %), where MO represents divalent metal oxides (e.g., MgO, ZnO, CaO, BaO, SrO, etc.), there are not enough monovalent or divalent cations to stabilize aluminum in four-fold coordination, and boron is therefore almost entirely in three-fold coordination in these glasses. In some embodiments, the glasses described herein comprise about 3 mol % to about 4.5 mol % of B2O3 which contains boron cations that are surrounded by three anions; i.e., the boron cations are three-fold coordinated and, in some embodiments, B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≦4.5 mol %. Three-fold coordinated boron is desirable because it densifies more readily when compressed, thus providing intrinsic or native damage resistance to the glass. Glasses with compositions in which R2O(mol %)+MO(mol %)−Al2O3(mol %)<0(mol %), however, generally have very high melting temperatures unless they also have very high B2O3 contents, e.g., greater than about 10 mol %. Such high amounts of B2O3 have a deleterious effect on both the surface compressive stress and rate of ion exchange, as evidenced by low depths of layer obtained in an ion exchange of fixed duration/time period. Since high CS (e.g., greater than about 650 MPa) and significant depth of layer (DOL) (e.g., greater than 30 μm) are also required to obtain good damage resistance, the additional benefits of high intrinsic damage resistance may be offset by poor ion exchange characteristics. In order to achieve optimal balance of attributes after ion exchange, it is therefore desirable to keep boron concentrations sufficiently low. Accordingly, in some embodiments, the glasses described herein comprise less than 10 mol % B2O3. Such glasses are very difficult to melt to a level of quality (e.g., defect concentration) acceptable for mass manufacturing. Depending on the application for the glass article, a marginally acceptable defect level in a continuous process is about 1 inclusion (e.g., gas bubbles, particles, or “seeds”) that are greater than about 50 μm in size (e.g., diameter, for approximately spherical particles, or major axis for elongated particles) per pound (lb) of glass.
In some embodiments, the glasses described herein comprise at least about 50 mol % SiO2, from about 9 mol % to about 22 mol % Al2O3; from about 3 mol % to about 10 mol % B2O3; from about 10 mol % to about 20 mol % Na2O; from 0 mol % to about 5 mol % K2O; at least about 0.1 mol % MgO and/or ZnO, wherein 0 mol %<MgO(mol %)+ZnO(mol %)≦6 mol %; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO(mol %)+SrO(mol %)+BaO(mol %)≦2 mol %. In some embodiments, the glasses described herein comprise from about 66 mol % to about 74 mol % SiO2.
In some embodiments, the glasses described herein comprise at least about 0.1 mol % of at least one of MgO and ZnO. In certain embodiments, the glass may comprise up to about 6 mol % MgO and/or ZnO. Magnesium and zinc behave differently from the other divalent cations. While magnesium and zinc may charge-balance aluminum to some extent, they appear to have little or no role in stabilizing boron in four-fold coordination in the glasses described herein. Thus, replacing an alkali oxide with magnesium oxide in a boroaluminosilicate glass forces boron atoms out of four-fold coordination and into three-fold coordination, resulting in higher indentation crack initiation threshold load in ion-exchanged glass.
In addition to the oxides listed above, the glasses described herein may further comprise chemical fining agents including, but not limited to, halogens or halides (compounds containing F, Cl, Br and I) and at least one of the oxides As2O3, Sb2O3, CeO2, and SnO2. These fining agents, when present, are generally batched into the raw material at a level of about 0.5 mol % or less, and have minor impact on both the rate of ion exchange and the compressive stress that are ultimately obtained. In addition, other oxides may be added at low concentrations with little or no impact on the ion exchange characteristics of these glasses. Examples of such oxides include ZrO2, which is a common contaminant introduced by zirconia refractories in melters; TiO2, which is a common tramp component of natural silica sources; Fe2O3, which is an ubiquitous tramp oxide in all but the purest chemical reagents; and other transition metal oxides that may be used to introduce color. These transition metal oxides include V2O5, Cr2O3, Co3O4, Fe2O3, MnO2, NiO, CuO, Cu2O, and the like. Concentrations of these oxides, when present in the glass, are kept at levels of less than or equal to about 2 mol %. Compared to calcium, larger alkaline earths such as Sr and Ba lead to lower diffusivity and thus significantly lower depth of the compressive layer. Because Sr and Ba are costly reagents compared to most of the other oxides in the glasses described herein and inhibit or retard ion exchange. The concentrations of SrO and BaO, when present in the glass, are therefore kept at levels of no more than about 0.5 mol %. Rubidium and cesium oxides are too large to ion exchange at an appreciable rate, are costly, and contribute to high liquidus temperatures at elevated concentrations. The concentrations of these oxides are therefore kept below 0.5 mol %. Lithium oxide is to be generally avoided, as it contributes to “poisoning” of potassium nitrate salt ion exchange baths, resulting in lower compressive stress than if the same glass were ion exchanged in a lithium-free salt bath. In addition, compared to a sodium-for-potassium exchange, the presence of lithium leads to significantly reduced diffusivity when exchanging for potassium. When present in the glass, the lithium oxide concentration should therefore be kept below about 0.5 mol %, and, in some embodiments, below about 0.1 mol %. In some embodiments, the glasses described herein are substantially free of lithium. In some embodiments, the glasses described herein are substantially free of phosphorus.
As used herein, the term “zircon breakdown temperature” or “Tbreakdown” refers to the temperature at which zircon—which is commonly used as a refractory material in glass processing and manufacture—breaks down to form zirconia and silica. In isoviscous processes such as fusion, the highest temperature experienced by the glass corresponds to a particular viscosity of the glass. For example, “T35kP” refers to the temperature at which the glass has a viscosity of 35 kilopoise (kP). The difference between the breakdown temperature and the temperature corresponding to 35,000 poise viscosity is defined as the breakdown margin Tmargin, where: Tmargin=Tbreakdown−T35kp. When the breakdown margin Tmargin is negative, zircon will breakdown to form zirconia defects at some location on the fusion isopipe. When Tmargin is zero, it is still possible that temperature excursions could cause zircon breakdown to occur. It is therefore desirable not only to make the breakdown margin positive, but to maximize Tmargin as much as possible while being consistent with all the other attributes that must be maintained in the final glass product. In some embodiments, the glasses described herein have a zircon breakdown temperature that is equal to the temperature at which the viscosity of the glass is in a range from about 25 kPoise to about 40 kPoise, in some embodiments, in a range from about 30 kPoise to about 40 kPoise, and, in a particular embodiment, from 30 kPoise to about 35 kPoise. Zircon breakdown temperatures Tbreakdown measured for various samples of the glasses described herein are listed in Table 3.
As previously mentioned, zircon will breakdown to form zirconia and silica at some location on processing hardware—such as, for example, a fusion isopipe—when the breakdown margin Tmargin is negative. This often results in the inclusion of defects, bubbles, zirconia particles, or the like in the formed glass. The presence of such inclusions in the glass is typically detected by those means know in the art such as, for example, optical microscopy. The glasses described herein have positive zircon breakdown margins and, in some embodiments, have less than 0.1 inclusions per pound of glass, wherein the inclusions are at least about 50 μm in size (i.e., diameter, for approximately spherical particles, or major axis for elongated particles). In some embodiments, the glasses described herein contain less than 0.01 inclusions per pound of glass, wherein the inclusions are at least about 50 μm in size.
In those embodiments in which the glass is strengthened by ion exchange, the glass may, in some embodiments, be ion exchanged to create a surface layer that is under a compressive stress of at least about 600 megapascals (MPa) and, in other embodiments, at least about 800 MPa. The compressive surface layer has a depth of layer of at least about 30 microns (μm). The ion exchanged glasses described herein also possess a degree of native or intrinsic damage resistance (IDR), which may be characterized by a Vickers crack initiation threshold of greater than or equal to 10 kilogram force (kgf). In some embodiments, the ion exchanged glass has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf. In other embodiments, the ion exchanged glass has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kgf. The Vickers crack initiation threshold measurements described herein are performed by applying and then removing an indentation load to the glass surface at a rate of 0.2 mm/min. The maximum indentation load is held for 10 seconds. The crack initiation threshold is defined at the indentation load at which 50% of 10 indents exhibit any number of radial/median cracks emanating from the corners of the indent impression. The maximum load is increased until the threshold is met for a given glass composition. All indentation measurements are performed at room temperature in 50% relative humidity.
When ion exchanged, the glasses described herein have a compressive stress that is sufficiently high to provide outstanding damage resistance against impact of various kinds. The glasses described herein may be ion exchanged at a rate that facilitates large scale manufacturing. While it is advantageous to provide high compressive stress by annealing the glass prior to ion exchange, a high compressive stress may also be obtained by instead cooling the glass rapidly from a high temperature (e.g., above the strain point of the glass). Such rapid cooling may occur, for example, in down drawn processes such as fusion or slot draw processes. The combination of compressive stress, depth of layer, and intrinsic damage resistance of the ion exchanged glasses described herein provides excellent resistance to visible or strength-limiting damage introduced via sharp contact and scratching.
Physical properties of an exemplary sample of the glasses described herein, labeled Glass 3 and having the composition 67.45 mol % SiO2, 12.69 mol % Al2O3, 3.67 mol % B2O3, 13.67 mol % Na2O, 0.02 mol % K2O, 2.36 mol % MgO, 0.03 mol % CaO, and 0.09 mol % SnO2, are compared to those of glasses described in U.S. patent application Ser. No. 13/533,298, filed on Jul. 1, 2011, by Matthew J. Dejneka et al., entitled “Ion Exchangeable Glass With High Compressive Stress,” which claims priority from U.S. Provisional Patent Application No. 61/503,734, filed on Jul. 1, 2011, by Matthew J. Dejneka et al., and having the same title; and U.S. Provisional Patent Application No. 61/653,485, filed on May 31, 2012, by Matthew J. Dejneka et al., entitled “Ion Exchangeable Glass With High Damage Resistance;” labeled Glasses 1 and 2, respectively, in Table 1. Glass 1, having the composition 68.84 mol % SiO2, 10.63 mol % Al2O3, 0 mol % B2O3, 14.86 mol % Na2O, 0.02 mol % K2O, 5.43 mol % MgO, 0.04 mol % CaO, and 0.17 mol % SnO2, is compatible with zircon, but has a low indentation threshold, whereas Glass 2, having the composition 64.65 mol % SiO2, 13.93 mol % Al2O3, 5.11 mol % B2O3, 13.75 mol % Na2O, 0 mol % K2O, 2.38 mol % MgO, 0.14 mol % CaO, and 0.08 mol % SnO2, has a high indentation threshold but is incompatible with zircon. Glass 3 possesses a combination of high indentation threshold and compatibility with zircon, as described hereinabove.







TABLE 1







Physical properties of glasses.










Glass
1
2
3













Anneal Point (° C.)
650
629
644


Strain Point (° C.)
601
576
589


Softening Point (° C.)
891.7
900
922.4


Density (g/cm3)
2.432
2.39
2.403


Poisson's Ratio
0.205
0.216
0.213


Shear Modulus (Mpsi)
4.287
4.051
4.142


Young's Modulus (Mpsi)
10.336
9.851
10.046


Liquidus Temperature (° C.)
1020
1000
1005


Liquidus Viscosity (kPoise)
1000
1850
2210


Primary Devit Phase
Forsterite
Nepheline
Forsterite


Zircon Breakdown
1200
1183
1230


Temperature (° C.)


Zircon Breakdown Viscosity
30.4
71.4
33.4


(kPoise)


200 Poise Temperature (° C.)
1665
1679
1757


35 kPoise Temperature (° C.):
1190
1226
1227


Refractive Index
1.50030
1.49844
1.49836


SOC (nm/MPa/cm)
29.64
32.78
31.94


Vickers Indentation
7
>30
20-30


Threshold (kgf)









Additional non-limiting examples of the compositions of the glasses described herein and their properties are listed in Tables 2 and 3, respectively. The examples listed satisfy the requirement of having intrinsic damage resistance, as characterized by high indentation threshold, and compatibility with zircon, as characterized by low zircon breakdown viscosity.







TABLE 2





Examples of compositions of the glasses described herein.
















Analyzed
Example














(mol %)
1
2
3
4
5
6
7





SiO2
67.26
67.47
67.37
67.43
67.22
67.12
67.29


Al2O3
12.05
12.08
12.07
12.03
12.03
12.03
12.05


B2O3
2.58
2.56
2.54
2.61
2.61
2.64
2.64


Na2O
14.14
13.08
14.10
13.10
14.20
13.33
13.20


K2O
0.01
0.96
0.01
0.96
0.03
0.94
0.96


MgO
3.80
3.69
3.34
3.27
3.34
3.36
2.82


CaO
0.05
0.04
0.48
0.49
0.06
0.05
0.48


ZnO
0.00
0.00
0.00
0.00
0.41
0.42
0.45


SnO2
0.09
0.09
0.09
0.09
0.09
0.09
0.09


ZrO2
0.01
0.01
0.01
0.01
0.01
0.01
0.01


Fe2O3
0.01
0.01
0.01
0.01
0.01
0.01
0.01


Total
100
100
100
100
100
100
100


(mol %)











Analyzed
Example














(mol %)
8
9
10
11
12
13
14





SiO2
67.25
66.32
66.32
66.22
66.26
67.28
67.29


Al2O3
12.04
12.73
12.76
12.72
12.74
12.04
12.03


B2O3
2.63
3.53
3.64
3.62
3.63
3.41
3.44


Na2O
13.30
13.93
12.89
13.07
13.06
13.87
12.93


K2O
0.96
0.03
0.95
0.96
0.97
0.01
0.94


Li2O


MgO
2.76
3.31
2.84
2.85
2.32
2.79
2.77


CaO
0.05
0.05
0.48
0.05
0.47
0.49
0.49


ZnO
0.89
0.00
0.00
0.40
0.45
0.00
0.00


SnO2
0.09
0.09
0.09
0.09
0.09
0.09
0.09


ZrO2
0.01
0.01
0.02
0.01
0.01
0.01
0.01


Fe2O3
0.01
0.01
0.01
0.01
0.01
0.01
0.01


P2O5
0.00
0.00
0.00
0.00
0.00
0.00
0.00


Total
100
100
100
100
100
100
100


(mol %)











Analyzed
Example














(mol %)
15
16
17
18
19
20
21





SiO2
67.18
66.27
66.33
66.16
67.23
67.61
66.82


Al2O3
12.00
12.74
12.73
12.73
12.72
12.24
12.59


B2O3
3.39
3.54
3.53
3.58
3.63
3.64
3.51


Na2O
14.10
14.11
14.12
14.19
13.91
13.96
14.47


K2O
0.04
0.01
0.01
0.01
0.01
0.04
0.01


MgO
1.82
2.27
1.79
1.84
2.34
2.35
2.45


CaO
0.49
0.05
0.04
0.48
0.05
0.06
0.05


ZnO
0.88
0.90
1.33
0.91
0.00
0.00
0.00


SnO2
0.09
0.09
0.09
0.09
0.09
0.09
0.09


ZrO2
0.01
0.01
0.01
0.01
0.01
0.01
0.01


Fe2O3
0.01
0.01
0.01
0.01
0.01
0.01
0.01


Total
100
100
100
100
100
100
100


(mol %)











Analyzed
Example














(mol %)
22
23
24
25
26
27
28





SiO2
66.59
67.05
66.38
66.98
67.05
67.09
67.23


Al2O3
12.41
12.16
12.71
12.69
12.56
12.67
12.67


B2O3
3.42
2.91
3.56
3.98
3.99
3.62
4.10


Na2O
13.40
13.34
14.19
13.91
13.55
14.16
13.97


K2O
0.66
0.85
0.01
0.01
0.01
0.01
0.03


MgO
3.01
2.88
1.79
2.21
2.05
2.24
1.83


CaO
0.12
0.06
0.04
0.03
0.03
0.03
0.06


ZnO
0.28
0.64
1.19
0.06
0.65
0.06
0.00


SnO2
0.09
0.09
0.09
0.09
0.09
0.09
0.09


ZrO2
0.01
0.01
0.03
0.01
0.01
0.01
0.01


Fe2O3
0.01
0.01
0.01
0.01
0.01
0.01
0.01


Total
100
100
100
100
100
100
100


(mol %)











Analyzed
Example














(mol %)
29
30
31
32
33
34
35





SiO2
67.31
67.32
66.96
67.43
67.09
67.45
67.11


Al2O3
12.54
12.65
12.63
12.56
12.66
12.46
12.57


B2O3
4.25
3.76
3.96
3.93
4.15
4.07
4.12


Na2O
13.62
13.76
13.84
13.54
13.64
13.50
13.64


K2O
0.01
0.01
0.01
0.01
0.01
0.01
0.01


MgO
2.11
2.37
2.47
2.41
2.33
2.38
2.42


CaO
0.04
0.04
0.04
0.03
0.04
0.03
0.04


ZnO
0.00
0.00
0.00
0.00
0.00
0.00
0.00


SnO2
0.09
0.07
0.08
0.07
0.07
0.08
0.08


ZrO2
0.01
0.01
0.01
0.01
0.01
0.01
0.01


Fe2O3
0.01
0.01
0.01
0.01
0.01
0.01
0.01


Total
100
100
100
100
100
100
100


(mol %)
















TABLE 3





Properties of glasses listed in Table 2.





















Example
1
2
3
4
5
6





Anneal Point (° C.):
632
632
632
624
633
629


Strain Point (° C.):
579
580
581
573
581
577


Softening Point (° C.):
890.3
895.7
888.6
884
891.5
888.4


Density (g/cm3):
2.418
2.417
2.421
2.422
2.424
2.425


CTE (×10−7/° C.):
77.1
79.3
77.3
79.3
77.3
80.3


Poisson's Ratio:
0.216
0.202
0.211
0.21
0.202
0.214


Shear Modulus (Mpsi):
4.216
4.232
4.234
4.241
4.228
4.221


Young's Modulus (Mpsi):
10.25
10.174
10.252
10.265
10.16
10.247


72-hr Pt Liquidus Temp (° C.):
1060
1070
1010
1000
1030
1040


Liquidus Viscosity (kPoise):
247
345
739
1458
810
810


Primary Devit Phase:
Unknown
Unknown
Unknown
Unknown
Unknown
Unknown


200-Poise Temp (° C.):
1744.5
1715.2
1733.2
1724.3
1727.5
1736.6


35-kPoise Temp (° C.):
1215.2
1210.8
1196.8
1199.6
1206.5
1213.5


Fictivation Temp (° C.):
721.8
715.8
714.3
704.5
717.2
717.1


Refractive Index:
1.50000
1.49992
1.50068
1.50104
1.50033
1.50051


SOC (nm/MPa/cm):
30.88
30.75
30.86
30.74
31.22
31.01


CS at 50 μm DOL/410° C./1
989.6
954.8
979.6
938.6
968.8
941.4


mm/Annealed/Refined (MPa):


Time to 50 μm DOL at
18.5
13.8
20.4
15
18.3
13.1


410° C./Annealed/Refined (h):


Predicted Indentation
15
16
14
14
16
15


Threshold at 410° C./8


h/Fictivated (kgf):


Zircon Breakdown
1210
1204
1207
1199
1206
1193


Temperature (° C.):


Zircon Breakdown
38
39
30
35
36
48


Viscosity (kPoise):





Example
7
8
9
10
11
12





Anneal Point (° C.):
625
629
629
622
623
620


Strain Point (° C.):
573
576
577
570
571
568


Softening Point (° C.):
883.9
889.8
890.6
885.6
889.8
883.1


Density (g/cm3):
2.428
2.422
2.411
2.414
2.418
2.421


CTE (×10−7/° C.):
79.3
79.9
76.5
78.8
78.8
78.4


Poisson's Ratio:
0.221
0.209
0.213
0.209
0.212
0.215


Shear Modulus (Mpsi):
4.241
4.231
4.164
4.197
4.189
4.193


Young's Modulus (Mpsi):
10.36
10.228
10.106
10.148
10.153
10.192


72-hr Pt Liquidus Temp (° C.):
970
995
1065
1025
1040
960


Liquidus Viscosity (kPoise):
2799
1401
287
939
805
4180


Primary Devit Phase:
Albite
Unknown
Unknown
Unknown
Unknown
Unknown


200-Poise Temp (° C.):
1721.0
1729.6
1716.9
1718.2
1722.0
1714.0


35-kPoise Temp (° C.):
1205.1
1205.4
1212.1
1209.7
1213.1
1204.1


Fictivation Temp (° C.):
1.50124
1.50102
1.49967
1.50039
1.50016
1.50090


Refractive Index:
30.67
31.18
31.5
31.13
31.54
31.53


SOC (nm/MPa/cm):
957.4
945.5
968.3
942.5
927
927.6


CS at 50 μm DOL/410° C./1
14.4
13.6
18.3
15.3
13.7
15.7


mm/Annealed/Refined (MPa):


Time to 50 μm DOL at
11
15
20
18
18
17


410° C./Annealed/Refined (h):


Zircon Breakdown
1192
1192
1202
1190
1187
1182


Temperature (° C.):


Zircon Breakdown
43
43
41
48
52
49


Viscosity (kPoise):





Example
13
14
15
16
17
18





Anneal Point (° C.):
619
617
617
627
626
622


Strain Point (° C.):
568
567
567
574
573
571


Softening Point (° C.):
877.5
876.6
871.5
890.5
889.1
881.2


Density (g/cm3):
2.415
2.415
2.429
2.425
2.432
2.43


CTE (×10−7/° C.):
76.3
78.8
76.1
76.1
76.1
76.2


Poisson's Ratio:
0.223
0.203

0.216
0.216
0.205


Shear Modulus (Mpsi):
4.221
4.226

4.158
4.145
4.197


Young's Modulus (Mpsi):
10.324
10.171

10.117
10.083
10.119


72-hr Pt Liquidus Temp (° C.):
960
950
970
970
950
965


Liquidus Viscosity (kPoise):
2975
2742
3595
2933
3783
2470


Primary Devit Phase:
Albite
Albite
Albite
Unknown
Albite
Albite


200-Poise Temp (° C.):
1692.6
1658.8
1815.2
1714.7
1725.1
1717.9


35-kPoise Temp (° C.):
1179.0
1165.6
1226.2
1208.5
1207.9
1199.5


Refractive Index:
31.07
31.11
32.52
32.02
32.19
31.79


SOC (nm/MPa/cm):
933.5
908.4

969.6
976.2
963.9


CS at 50 μm DOL/410° C./1
21.8
16.4

19.7
19.5
21.3


mm/Annealed/Refined (MPa):


Time to 50 μm DOL at
13
16

20
21
19


410° C./Annealed/Refined (h):


Zircon Breakdown
1218
1208
1205
1193
1191
1184


Temperature (° C.):


Zircon Breakdown
19
18
47
45
46
44


Viscosity (kPoise):





Example
19
20
21
22
23
24





Anneal Point (° C.):
629
621
621
629
629
626


Strain Point (° C.):
576
569
570
577
576
574


Softening Point (° C.):
898.9
884.3
877.3
893.2
896.2
890.2


Density (g/cm3):
2.404
2.406
2.412
2.42
2.426
2.429


CTE (×10−7/° C.):
76.4
76.8
78.1
78.2
78.6
76.1


Poisson's Ratio:
0.208
0.209
0.215
0.212
0.222
0.219


Shear Modulus (Mpsi):
4.141
4.161
4.152
4.222
4.222
4.158


Young's Modulus (Mpsi):
10
10.058
10.09
10.232
10.315
10.138


72-hr Pt Liquidus Temp (° C.):
980
965
960
1030
1020
955


Liquidus Viscosity (kPoise):
980
950
960
1020
1020
945


Primary Devit Phase:
3592
3855
3649
904
1358
5875


200-Poise Temp (° C.):
Unknown
Albite
Albite
Unknown
Unknown
Albite


35-kPoise Temp (° C.):
1793.2
1760.7
1749.9
1715.3
1723.8
1705.4


Fictivation Temp (° C.):
1224.1
1209.5
1196.8
1206.8
1215.8
1205.7


Refractive Index:
1.49821
1.49833
1.49955
1.50044
1.50084
1.50117


SOC (nm/MPa/cm):
31.99
31.55
31.44
31.49
31.25
32


CS at 50 μm DOL/410° C./1
944.9
920.7
944.7
1018.3


mm/Annealed/Refined (MPa):


Time to 50 μm DOL at
15.4
16.3
17.6
16.3


410° C./Annealed/Refined (h):


Predicted Indentation
23
21
21
15
13
20


Threshold at 410° C./8


h/Fictivated (kgf):


Zircon Breakdown
1216
1224
1199
1205
1195
1220


Temperature (° C.):


Zircon Breakdown
39
28
34
36
49
28


Viscosity (kPoise):
















Example
25
26
27
28
29
30





Anneal Point (° C.):
628
629
630
635
629
640


Strain Point (° C.):
576
576
577
582
576
586


Softening Point (° C.):
894.5
898
896.4
905.8
898.9
904.7


Density (g/cm3):
2.405
2.412
2.409
2.405
2.402
2.402


CTE (×10−7/° C.):
76.4
74.4
76.5
75.9
75
75.3


Poisson's Ratio:
0.225
0.219
0.206
0.207
0.223
0.214


Shear Modulus (Mpsi):
4.152
4.135
4.165
4.099
4.212
4.126


Young's Modulus (Mpsi):
10.173
10.079
10.048
9.893
10.299
10.02


72-hr Pt Liquidus Temp (° C.):
980
990
965
960
970
1000


Liquidus Viscosity (kPoise):
970
970
950
945
950
995


Primary Devit Phase:
3223
2744
4960
5696
4346
2738


200-Poise Temp (° C.):
Unknown
Unknown
Albite
Albite
Forsterite
Unknown


35-kPoise Temp (° C.):
1725.2
1739.8
1729.2
1741.3
1744.1
1753.8


Fictivation Temp (° C.):
1213.2
1216.1
1215.4
1221.0
1221.4
1232.2


Refractive Index:
1.49840
1.49910
1.49860


SOC (nm/MPa/cm):
32.52
32.61
31.88
31.73
32.06
32.13


CS at 50 μm DOL/410° C./1
980.3
990.1
1006.9
1008.9
991.8
971.9


mm/Annealed/Refined (MPa):


Time to 50 μm DOL at
17.1
19.3
16.7
17.5
18.0
15.8


410° C./Annealed/Refined (h):


Predicted Indentation
19
22
21
27
14
24


Threshold at 410° C./8


h/Fictivated (kgf):


Zircon Breakdown
1220
1250
1215
1218
1231
1223


Temperature (° C.):


Zircon Breakdown
32
21
35
37
31
40


Viscosity (kPoise):















Example
31
32
33
34
35





Anneal Point (° C.):
630
632
623
634
628


Strain Point (° C.):
576
578
572
581
575


Softening Point (° C.):
903.8
909.1
888.9
909.4
912.8


Density (g/cm3):
2.402
2.4
2.402
2.398
2.399


CTE (×10−7/° C.):
74.8
75.2
76
74.6
74.2


Poisson's Ratio:
0.224
0.221
0.219
0.218
0.217


Shear Modulus (Mpsi):
4.121
4.118
4.105
4.11
4.118


Young's Modulus (Mpsi):
10.086
10.058
10.009
10.011
10.021


72-hr Pt Liquidus Temp (° C.):
990
1005
1000
1010
1005


Liquidus Viscosity (kPoise):
980
1000
990
1000
1000


Primary Devit Phase:
2569
2319
1680
1825
1896


200-Poise Temp (° C.):
Unknown
Unknown
Unknown
Unknown
Unknown


35-kPoise Temp (° C.):
1769.7
1776.1
1756.4
1752.8
1754.2


Fictivation Temp (° C.):
1222.2
1231.5
1208.8
1226.4
1218.8


Refractive Index:


SOC (nm/MPa/cm):
32.08
32.11
32.08
32
32.21


CS at 50 μm DOL/410° C./1
970.3
962.8
947.1
940.8
953.0


mm/Annealed/Refined (MPa):


Time to 50 μm DOL at
16.3
16.3
17.8
17.1
17.0


410° C./Annealed/Refined (h):


Predicted Indentation
22
23
25
24
24


Threshold at 410° C./8


h/Fictivated (kgf):


Zircon Breakdown
1219
1232
1226
1236
1227


Temperature (° C.):


Zircon Breakdown
37
35
27
31
31


Viscosity (kPoise):









The glasses described herein are resistant to both chipping and scratching, making it well suited for use in cover plates, touch screens, watch crystals, solar concentrators, windows, screens, containers, and other applications that require strong and tough glass with good scratch resistance.
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.

## Claims

1. A glass, the glass comprising:
66-74 mol % SiO2;
at least about 10 mol % R2O, wherein R2O comprises 9-20 mol % Na2O;
9-22 mol % Al2O3, wherein Al2O3(mol %)<R2O(mol %); and
B2O3, wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≧3 mol %, and wherein the glass is ion exchangeable.

2. The glass of claim 1, wherein the glass comprises at least 0.1 mol % of at least one of MgO and ZnO.

3. The glass of claim 1, wherein the glass has a zircon breakdown temperature that is equal to a temperature at which the glass has a viscosity in a range from about 30 kPoise to about 40 kPoise.

4. The glass of claim 1, wherein the glass comprises at least about 2.7 mol % B2O3.

5. The glass of claim 4, wherein the glass comprises from about 2.7 mol % to about 4.5 mol % B2O3.

6. The glass of claim 1, wherein at least 50% of the B2O3 present in the glass comprises three-fold coordinated boron cations.

7. The glass of claim 1, wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≦4.5 mol %.

8. The glass of claim 1, wherein the glass comprises: from about 3 mol % to about 10 mol % B2O3; from 0 mol % to about 5 mol % K2O; at least, 0.1 mol % MgO+ZnO, wherein 0≦MgO≦6 and 0≦ZnO≦6 mol %; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.

9. The glass of claim 1, wherein the glass is ion exchanged and has a layer under a compressive stress of at least about 600 MPa, the layer extending from a surface of the glass into the glass to a depth of layer of at least about 30 μm.

10. The glass of claim 9, wherein the compressive stress is at least about 800 MPa.

11. The glass of claim 9, wherein the glass has a Vickers crack initiation of at least about 10 kgf.

12. The glass of claim 11, wherein the glass has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf.

13. The glass of claim 11, wherein the glass has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kgf.

14. The glass of claim 1, wherein the glass contains less than about 0.1 inclusion per pound of the glass, the inclusion having a size of at least about 50 μm.

15. The glass of claim 1, wherein the glass is down-drawn.

16. The glass of claim 1, wherein the glass has an anneal point and a strain point, and wherein the glass is fast-cooled from a temperature that is above the anneal point or the strain point.

17. A glass, the glass comprising:
at least about 50 mol % SiO2;
at least about 10 mol % R2O, wherein R2O comprises 9-20 mol % Na2O and 0-0.5 mol % Li2O;
Al2O3; and
2.7-4.5 mol % B2O3, wherein B2O3−(R2O−Al2O3)≧3 mol %, and
wherein the glass has a zircon breakdown temperature that is equal to the temperature at which the glass has a viscosity in a range from about 25 kPoise to about 40 kPoise.

18. The glass of claim 17, wherein the glass comprises at least about 0.1 mol % of at least one of MgO and ZnO.

19. The glass of claim 17, wherein the glass comprises three-fold coordinated boron cations.

20. The glass of claim 17, wherein at least 50% of the B2O3 present in the glass comprises three-fold coordinated boron cations.

21. The glass of claim 17, wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≦4.5 mol %.

22. The glass of claim 17, wherein the glass comprises: at least about 50 mol % SiO2, from about 9 mol % to about 22 mol % Al2O3; 3 mol % to 4.5 mol % B2O3; from 0 mol % to about 5 mol % K2O; at least 0.1 mol % MgO+ZnO, wherein 0≦MgO≦6 and 0≦ZnO≦6; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.

23. The glass of claim 17, wherein the glass forms at least a portion of a cover plate, a touch screen, a watch crystal, a solar concentrator, a window, a screen, or a container.

24. The glass of claim 17, wherein the glass is ion exchanged and has a layer under a compressive stress of at least about 600 MPa, the layer extending from a surface of the glass into the glass to a depth of layer of at least about 30 μm.

25. The glass of claim 24, wherein the compressive stress is at least about 800 MPa.

26. The glass of claim 24, wherein the glass has a Vickers crack initiation threshold of at least about 10 kgf.

27. The glass of claim 26, wherein the glass has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf.

28. The glass of claim 26, wherein the glass has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kgf.

29. The glass of claim 17, wherein the glass contains less than about 0.1 inclusion per pound of the glass, the inclusion having a size of at least about 50 μm.

30. The glass of claim 17, wherein the glass is down-drawn.

31. The glass of claim 17, wherein the glass has an anneal point and a strain point, and wherein the glass is fast-cooled from a temperature that is above the anneal point or the strain point.

32. A glass comprising:
66-74 mol % SiO2;
at least about 10 mol % R2O, wherein R2O comprises 9-20 mol % Na2O;
Al2O3, wherein Al2O3(mol %)<R2O(mol %); and
at least 2.7 mol % B2O3, wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≦3 mol %.

33. The glass of claim 32, wherein the glass comprises at least 0.1 mol % of at least one of MgO and ZnO.

34. The glass of claim 32, wherein the glass comprises from 2.7 mol % to 4.5 mol % B2O3.

35. The glass of claim 32, wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≦4.5 mol %.

36. The glass of claim 32, wherein at least 50% of the B2O3 present in the glass comprises three-fold coordinated boron cations.

37. The glass of claim 32, wherein the glass comprises: from about 9 mol % to about 22 mol % Al2O3; from about 3 mol % to about 10 mol % B2O3; from 0 mol % to about 5 mol % K2O; at least 0.1 mol % MgO+ZnO, wherein 0≦MgO≦6 and 0≦ZnO≦6; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.

38. The glass of claim 32, wherein the glass is ion exchanged and has a layer under a compressive stress of at least about 600 MPa, the layer extending from a surface of the glass into the glass to a depth of layer of at least about 30 μm.

39. The glass of claim 38, wherein the compressive stress is at least about 800 MPa.

40. The glass of claim 32, wherein the glass is ion exchanged and has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf.

41. The glass of claim 32, wherein the glass is ion exchanged and has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kgf.

42. The glass of claim 32, wherein the glass contains less than about 0.1 inclusion per pound of the glass, the inclusion having a size of at least about 50 μm.

43. The glass of claim 32, wherein the glass is down-drawn.

44. The glass of claim 32, wherein the glass has an anneal point and a strain point, and wherein the glass is fast-cooled from a temperature that is above the anneal point or the strain point.

45. The glass of claim 32, wherein the glass is ion exchanged and forms at least a portion of a cover plate, a touch screen, a watch crystal, a solar concentrator, a window, a screen, or a container.

46. A glass comprising:
66-74 mol % SiO2;
at least about 10 mol % R2O, wherein R2O comprises Na2O;
9-22 mol % Al2O3;
at least 2.7 mol % B2O3, wherein B2O3−(R2O−Al2O3)≧3 mol %; and
at least 0.1 mol % of at least one of MgO and ZnO.

47. The glass of claim 46, wherein the glass has a zircon breakdown temperature that is equal to a temperature at which the glass has a viscosity in a range from about 30 kPoise to about 40 kPoise.

48. The glass of claim 46, wherein the glass comprises from 2.7 mol % to about 4.5 mol % B2O3.

49. The glass of claim 46, wherein at least 50% of the B2O3 present in the glass comprises three-fold coordinated boron cations.

50. The glass of claim 46, wherein B2O3(mol %)−(R2O(mol %)−Al2O3(mol %))≦4.5 mol %.

51. The glass of claim 46, wherein the glass comprises: from about 3 mol % to about 10 mol % B2O3; from about 9 mol % to about 20 mol % Na2O; from 0 mol % to about 5 mol % K2O; at least 0.1 mol % MgO+ZnO, wherein 0≦MgO≦6 and 0≦ZnO≦6 mol %; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.

52. The glass of claim 46, wherein the glass is ion exchanged and has a layer under a compressive stress of at least about 600 MPa, the layer extending from a surface of the glass into the glass to a depth of layer of at least about 30 μm.

53. The glass of claim 52, wherein the compressive stress is at least about 800 MPa.

54. The glass of claim 52, wherein the glass has a Vickers crack initiation threshold of at least about 10 kgf.

55. The glass of claim 54, wherein the glass has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf.

56. The glass of claim 54, wherein the glass has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kgf.

57. The glass of claim 46, wherein the glass contains less than about 0.1 inclusion per pound of the glass, the inclusion having a size of at least about 50 μm.

58. The glass of claim 46, wherein the glass is down-drawn.

59. The glass of claim 46, wherein the glass has an anneal point and a strain point, and wherein the glass is fast-cooled from a temperature that is above the anneal point or the strain point.