This application claims the benefit of priority under 35 U.S.C. Â§119 of U.S. Provisional Application Ser. No. 61/871,471 filed on Aug. 29, 2013, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates to an ion exchangeable glass with high rates of ion exchange. More particularly, the disclosure relates to an ion exchanged glass in which a surface compressive stress is achievable with reduced ion exchange times. Even more particularly, the disclosure relates to such glasses that, when ion exchanged, are resistant to surface damage.

Glasses are used in as protective covers for electronic displays and the like. As such, glasses that are used in these applications must be resistant to damage caused by impact.

SUMMARY

Ion exchangeable glasses containing SiO₂, Al₂O₃, Na₂O, MgO, B₂O₃, and P₂O₅are provided. The compressive stresses of these ion exchanged glasses are greater than 900 megapascals (MPa) at a depth of 45 or 50 microns (Î¼m) with some glasses exhibiting a compressive stress of at least 1 gigaPascals (GPa). The ion exchange rates of these glasses are much faster than other alkali aluminosilicate glasses. The ion exchanged glasses are resistant to damage caused by impact. A method of ion exchanging the glass is also provided.

Accordingly, one aspect of the disclosure is to provide a glass comprising SiO₂, Al₂O₃, Na₂O, at least one divalent metal oxide, B₂O₃, _andP₂O₅, wherein Al₂O₃(mol %)â‰§B₂O₃(mol %)+P₂O₅(mol %).

A second aspect of the disclosure is to provide an ion exchanged glass comprising SiO₂, Al₂O₃, Na₂O, at least one divalent metal oxide, B₂O₃, _andP₂O₅, wherein Al₂O₃(mol %)â‰§B₂O₃(mol %)+P₂O₅(mol %). The ion exchanged glass has a compressive layer having a compressive stress of at least about 900 MPa. The compressive layer extends from a surface of the glass to a depth of layer of at least about 45 Î¼m.

A third aspect is to provide a method of ion exchanging a glass. The glass comprises SiO₂, Al₂O₃, Na₂O, at least one divalent metal oxide, B₂O₃, and P₂O₅, wherein Al₂O₃(mol %)â‰§B₂O₃(mol %)+P₂O₅(mol %). The method includes providing an ion exchange bath comprising KNO₃. The ion exchange bath is at a temperature in a range from about 410Â° C. to 470Â° C. The method also includes ion exchanging a glass in the ion exchange bath for a time period of up to about four hours. The ion exchanged glass has a layer under a compressive stress of at least 900 MPa. The layer extends from a surface of the glass to a depth of layer of at least about 45 Î¼m.

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 schematic cross-sectional view of an ion exchanged glass;

FIG. 2 is a plot of compressive stress and depth of layer for the ion exchanged glasses listed in Table 1;

FIG. 3 is a plot of the time needed to ion exchange the glasses listed in Table 1 to a depth of layer of 50 Î¼m;

FIG. 4 is a plot Vickers crack indentation thresholds measured for samples 1-11 in Table 1; and

FIG. 5 is a plot of Vickers crack indentation thresholds measured for samples 54-60 in Table 2.

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 â€œtop,â€ â€œbottom,â€ â€œoutward,â€ â€œinward,â€ 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 â€œa,â€ â€œan,â€ and the corresponding definite article â€œtheâ€ mean â€œat least oneâ€ or â€œone or more,â€ 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 â€œglass articleâ€ and â€œglass articlesâ€ are used in their broadest sense to include any object made wholly or partly of glass. Unless otherwise specified, all compositions are expressed in terms of mole percent (mol %). Coefficients of thermal expansion (CTE) are expressed in terms of 10^âˆ’7/Â° C. and represent a value measured over a temperature range from about 20Â° C. to about 300Â° C., unless otherwise specified.

It is noted that the terms â€œsubstantiallyâ€ and â€œaboutâ€ 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. Thus, a glass that is â€œsubstantially free of MgOâ€ is one in which MgO is not actively added or batched into the glass, but may be present in very small amounts as a contaminant.

Vickers crack initiation thresholds described herein are determined 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 indentation cracking 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.

Compressive stress and depth of layer are measured using those means known in the art. Such means include, but are not limited to, measurement of surface stress (FSM) using commercially available instruments such as the FSM-6000, manufactured by Luceo Co., Ltd. (Tokyo, Japan), or the like, and methods of measuring compressive stress and depth of layer are described in ASTM 1422C-99, entitled â€œStandard Specification for Chemically Strengthened Flat Glass,â€ and ASTM 1279.19779 â€œStandard Test Method for Non-Destructive Photoelastic Measurement of Edge and Surface Stresses in Annealed, Heat-Strengthened, and Fully-Tempered Flat Glass,â€ the contents of which are incorporated herein by reference in their entirety. Surface stress measurements rely upon the accurate measurement of the stress optical coefficient (SOC), which is related to the birefringence of the glass. SOC in turn is measured by those methods that are known in the art, such as fiber and four point bend methods, both of which are described in ASTM standard C770-98 (2008), entitled â€œStandard Test Method for Measurement of Glass Stress-Optical Coefficient,â€ the contents of which are incorporated herein by reference in their entirety, and a bulk cylinder method.

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.

Described herein are glasses containing SiO₂, Al₂O₃, Na₂O, MgO, B₂O₃, and P₂O₅. All of these glasses are ion exchangeable. The compressive stresses of these ion exchanged glasses are greater than 900 megapascals (MPa) at a depth of 45 or 50 microns (Î¼m) with some glasses exhibiting a compressive stress of at least 1 gigaPascals (GPa). The ion exchange rates of these glasses are much faster than other alkali aluminosilicate glasses. In addition, the diffusivities of some these glasses approximates the diffusivities of alkali aluminosilicate glasses that contain phosphorus.

The glasses described herein comprise SiO₂, Al₂O₃, Na₂O, at least one divalent oxide, B₂O₃, and P₂O₅, wherein the amount of alumina (Al₂O₃) present in the glass is greater than or equal to total amount of B₂O₃, and P₂O₅; i.e., Al₂O₃(mol %)â‰§B₂O₃(mol %)+P₂O₅(mol %). In some embodiments, the glass consists essentially of or comprises: from about 54 mol % to about 71 mol % SiO₂(i.e., 54 mol %â‰¦SiO₂â‰¦72 mol %); from about 9 mol % to about 18 mol % Al₂O₃(i.e., 9 mol %â‰¦Al₂O₃â‰¦18 mol %); from about 9 mol % to about 18 mol % Na₂O (i.e., 9 mol %â‰¦Na₂Oâ‰¦18 mol %); at least one of MgO, CaO, ZnO, BaO, and SrO, wherein 0.5 mol %â‰¦MgO(mol %)+CaO(mol %)+ZnO(mol %)+BaO(mol %)+SrO(mol %)â‰¦4 mol %; from about 0.5 mol % to about 11 mol % B₂O₃(i.e., 4 mol %â‰¦B₂O₃â‰¦11 mol %); and from about 0.5 mol % to about 11 mol % P₂O₅(i.e., 0.5 mol %â‰¦P₂O₅â‰¦11 mol %), wherein 4 mol %â‰¦B₂O₃+P₂O₅â‰¦15 mol %.

In other embodiments, the glass consists essentially of or comprises from about 58 mol % to about 68 mol % SiO₂(i.e., 58 mol %â‰¦SiO₂â‰¦68 mol %); from about 9 mol % to about 16 mol % Al₂O₃(i.e., 9 mol %â‰¦Al₂O₃â‰¦16 mol %); from about 12 mol % to about 16 mol % Na₂O (i.e., 12 mol %â‰¦Na₂Oâ‰¦16 mol %); B₂O₃; and P₂O₅, wherein 4 mol %â‰¦B₂O₃(mol %)+P₂O₅â‰¦(mol %)â‰¦15 mol %.

Table 1 lists compositions and physical properties (densities, coefficients of thermal expansion (CTE), strain points, anneal points, softening points, Young's Modulus, molar volume, shear modulus, Poisson's ratio, stress optical coefficient (SOC), 35 kiloPoise temperature (T^{35 kP}), and liquidus temperature (T^L) of glasses described herein that were made by the fusion draw process. Compositions were analyzed using x-ray fluorescence. Anneal, strain, and softening points were determined by fiber elongation. Density was determined by the buoyancy method, CTE is the average value between room temperature and 300Â° C., SOC was determined using the diametral compression method, and TL was determined by a 72 hour hold in a gradient boat.

TABLE 1Compositions and physical properties of glasses made by thefusion draw process.Composition(mol %)Glass 1Glass 2Glass 3Glass 4Glass 5Glass 6SiO₂64.7964.3463.7663.3463.2962.53Al₂O₃13.9514.0414.1614.3214.4214.68Na₂O13.7513.9214.0814.2314.2114.49P₂O₅0.370.851.482.022.363.44B₂O₃4.474.143.753.272.992.08MgO2.512.552.612.662.552.60Anneal Pt. (Â° C.):628628629633633635Strain Pt. (Â° C.):574574576579580582Softening Pt. (Â° C.):910907.6910919920.7925.9Density (g/cm³):2.392.3942.3962.3972.3982.401CTE (Ã—10^âˆ’7/Â° C.):75.976.176.877.678.179Molar Volume (cm³/mol)27.827.9428.1428.3228.4428.78Young's modulus (Mpsi)9.8899.8079.8899.7259.6869.658Shear modulus (Mpsi)4.0584.0264.0093.9973.9973.996Poisson's ratio0.2180.2180.2330.2160.2120.209SOC32.932.8832.6332.632.4232.01T^35KP(Â° C.)121212171220122612221225SOC (nm/cm/MPa)32.9332.8832.6332.632.4232.01Liquidus T (Â° C.)920945965CompositionGlassGlass(mol %)Glass 7Glass 8Glass 91011SiO₂61.9961.5061.0460.7860.08Al₂O₃14.8214.9515.0415.1715.39Na₂O14.5914.7714.8714.9715.22P₂O₅4.034.655.135.616.24B₂O₃1.721.271.000.510.08MgO2.672.682.752.762.81Anneal Pt. (Â° C.):636636637637636Strain Pt. (Â° C.):584584584585584Softening Pt. (Â° C.):926.3927.4926.9927.1924.9Density (g/cm³):2.4022.4032.4042.4062.407CTE (Ã—10^âˆ’7/Â° C.):79.780.280.981.182Molar Volume (cm³/mol)28.9829.1829.3329.4829.7Young's modulus (Mpsi)9.6759.6739.594Shear modulus (Mpsi)3.963.9643.952Poisson's ratio0.2220.220.214SOCâˆ’3.94âˆ’3.816âˆ’3.633âˆ’3.516T^35KP(Â° C.)1028199249413.89128.6SOC (nm/cm/MPa)16.543.58098.1Liquidus T (Â° C.)31.6331.131.231.231.2

Additional examples of glasses and selected physical properties prepared in a research melter are listed in Tables 2 and 2a. The compositions reported in Table 2 are as-batched compositions. Anneal, strain and softening points were determined by fiber elongation. Density was determined by Buoyancy Method, CTE is the average value between room temperature and 300Â° C., and the stress optic coefficient was determined using the diametral compression method.

TABLE 2Compositions and physical properties of glasses made in aresearch melter.CompositionGlassGlassGlassGlassGlassGlass(mol %)121314151617SiO₂6062.66569.964.464.8Al₂O₃13.812.91210.412.913B₂O₃6.76.35.854.85Li₂ONa₂O14.513.512.610.914.313.4K₂O000000MgO10.90.90.80.70.8P₂O₅43.73.532.93SnO₂0.10.10.10.10.10.1Anneal Pt. (Â° C.):573579585601567580Strain Pt. (Â° C.):523526531541516526Softening Pt. (Â° C.):836850866913837877Density (g/cm³):2.3712.3632.3542.3332.372.353CTE (Ã—10^âˆ’7/Â° C.):76.574.170.1658073.5CompositionGlassGlassGlassGlassGlassGlass(mol %)181920212223SiO₂606060606060Al₂O₃13.813.813.813.813.813.8B₂O₃8.77.75.7420Li₂O000Na₂O14.514.514.514.514.514.5K₂O000000MgO1.01.01.01.01.01.0P₂O₅2356.78.710.7SnO₂0.10.10.10.10.10.1Anneal Pt. (Â° C.):571571574573573583Strain Pt. (Â° C.):522521523521521530Softening Pt. (Â° C.):823829847852867887Density (g/cm³):2.3722.3732.3712.3722.3712.372CTE (Ã—10^âˆ’7/Â° C.):787777767776CompositionGlassGlassGlassGlassGlassGlass(mol %)242526272829SiO₂6070.170.170.169.167.7Al₂O₃13.810.010.010.010.010.7B₂O₃6.78.36.34.37.36.9Li₂O00.00.00.00.00Na₂O14.511.211.211.210.29.6K₂O00.250.250.250.252.2MgO1.00.00.00.01.001P₂O₅402422SnO₂0.10.050.050.050.050.05Anneal Pt. (Â° C.):568612584586583576Strain Pt. (Â° C.):518578532530528523Softening Pt. (Â° C.):852827862883881873Density (g/cm³):2.3662.3422.3412.3372.3272.345CTE (Ã—10^âˆ’7/Â° C.):746766.465.663.970.1CompositionGlassGlassGlassGlassGlassGlass(mol %)303132333435SiO₂70.165.765.765.765.765.7Al₂O₃10.010.710.710.710.710.7B₂O₃6.310.77.75.3530Li₂O0.000000Na₂O9.211.611.611.611.611.6K₂O2.21.31.31.31.31.3MgO0.000000P₂O₅2035.357.710.7SnO₂0.050.10.10.10.10.1Anneal Pt. (Â° C.):584561555555551Strain Pt. (Â° C.):530514506504499Softening Pt. (Â° C.):863782811836851Density (g/cm³):2.3412.3562.3542.3492.3452.344CTE (Ã—10^âˆ’7/Â° C.):69.1CompositionGlassGlassGlassGlassGlassGlass(mol %)363738394041SiO₂65.764.565.765.765.765.7Al₂O₃10.710.710.710.711.211.2B₂O₃4.355.55.75.26.76.7Li₂O000000Na₂O11.611.611.611.611.611.1K₂O1.31.31.31.30.81.3MgO21.511.500P₂O₅4.3544444SnO₂0.10.10.10.10.10.1Anneal Pt. (Â° C.):581567566577571569Strain Pt. (Â° C.):259517517526522524Softening Pt. (Â° C.):886827824841821791Density (g/cm³):2.3672.3712.3792.3732.3812.402CTE (Ã—10^âˆ’7/Â° C.):7978778185SOC (nm/cm/MPa)33.632.833.634.133CompositionGlassGlassGlassGlassGlassGlass(mol %)424344454647SiO₂606060606060Al₂O₃13.813.813.813.812.812.3B₂O₃9.78.78.77.78.78.2Li₂O0.50.50000Na₂O14.514.514.514.514.514.5K₂O0.50.50000MgO121222P₂O₅002323SnO₂0.10.10.10.10.10.1Anneal Pt. (Â° C.):559566571570564560Strain Pt. (Â° C.):513521522520518515Softening Pt. (Â° C.):784793825824810806Density (g/cm³):2.392.4032.3722.3732.3722.381CTE (Ã—10^âˆ’7/Â° C.):818077777878CompositionGlassGlassGlassGlassGlassGlass(mol %)484950515253SiO₂606060606060Al₂O₃13.813.813.813.814.214.7B₂O₃7.27.26.26.79.78.7Li₂O0.500.500.50.5Na₂O14.514.514.514.514.915.4K₂O00.5000.50.5MgO001100P₂O₅444400SnO₂0.10.10.10.10.10.1Anneal Pt. (Â° C.):556562561568561Strain Pt. (Â° C.):506512511518508Softening Pt. (Â° C.):830839839852837850Density (g/cm³):2.3572.3632.362.3662.3432.344CTE (Ã—10^âˆ’7/Â° C.):747374747172SOC (nm/cm/MPa)33.4234.433.4933.534.6734.54CompositionGlassGlassGlassGlassGlassGlassGlass(mol %)54555657585960SiO₂57.3658.4158.5758.6457.1556.9557.14Al₂O₃16.4015.9415.9415.9117.1417.9117.19B₂O₃0.350.280.500.750.280.500.77Li₂ONa₂O16.2215.7915.7115.7216.9717.0316.92K₂OMgO3.192.942.722.471.711.481.23P₂O₅6.316.386.306.266.496.586.50SnO₂0.050.050.050.050.050.050.05Anneal Pt. (Â° C.):605596591581598593588Strain Pt. (Â° C.):658649645636653649644Softening Pt. (Â° C.):928.1930.1926.7924.6932.4932926.7Density (g/cm³):2.4232.4162.4132.412.4192.4162.413CTE (Ã—10^âˆ’7/Â° C.):84.382.681.982.285.78685.8SOC (nm/cm/MPa)30.530.930.731.630.730.431.0

TABLE 2aCompositions and physical properties of glasses made in aresearch melter.Composition(mol %)616263646566SiO₂58.1058.9060.0861.3359.3459.06Al₂O₃16.3915.8515.3815.0115.9615.89B₂O₃3.603.893.803.702.591.76Na₂O16.5816.0815.4714.7415.8916.02K₂O0.030.030.030.030.040.05MgO2.532.522.512.512.512.51CaO0.040.040.040.040.040.04P₂O₅2.632.582.582.523.534.56SnO20.100.100.100.100.100.10Anneal Pt.621627619613622630(Â° C.):Strain Pt.571573567562569577(Â° C.):Softening Pt.879885.7889.4891.4891904(Â° C.):Density2.4212.4162.4122.4062.4152.416(g/cm³):CTE84.283.180.780.182.483(Ã—10^âˆ’7/Â° C.):Molar Volume28.6528.6128.5728.5528.9129.20(cm³/mol)Young's9.9989.84510.0179.8529.6419.65modulus(Mpsi)Shear modulus3.9193.8783.8963.8883.9023.903(Mpsi)Poisson's ratio0.2760.2910.2640.2670.2360.236Composition(mol %)676869707172SiO₂63.2562.4864.1062.3564.2863.25Al₂O₃14.6114.6514.0714.6314.6414.62B₂O₃3.102.913.253.063.282.23Na₂O13.7413.7113.3113.7313.4813.73K₂O0.030.030.030.030.020.03MgO2.553.042.543.032.103.06CaO0.050.050.050.050.040.05P₂O₅2.573.022.553.002.062.93SnO20.100.100.100.090.100.10Anneal Pt.647637635630654655(Â° C.):Strain Pt.590582579577587599(Â° C.):Softening Pt.928.8918928.5914.3934.7939(Â° C.):Density2.3972.4012.3932.4012.3952.405(g/cm³):CTE74.775.773.575.774.975.6(Ã—10^âˆ’7/Â° C.):Molar Volume28.5628.6328.5128.6228.4628.51(cm³/mol)Young's9.5639.3529.4659.519.6989.6modulus(Mpsi)Shear modulus3.9913.9613.9813.973.9944.026(Mpsi)Poisson's ratio0.1980.1810.1890.1980.2140.192

Silica (SiO₂) serves as the primary glass-forming oxide in the glass compositions described herein. The concentration of SiO₂should be sufficiently high in order to provide the glass with sufficiently high chemical durability suitable for touch screen applications. However, the melting temperature (i.e., the 200 poise temperature) of pure SiO₂or high-SiO₂glasses is too high, since defects such as fining bubbles may appear. Furthermore, compared to most oxides, SiO₂decreases the compressive stress created by ion exchange. SiO₂also adds free volume to the network structure of the glass, thereby increasing the amount of point contact deformation required to form strength limiting crack systems. In some embodiments, the glass comprises from about 54 mol % to about 71 mol % SiO₂. In other embodiments, glass comprises from about 58 mol % to about 68 mol % SiO₂and, in still other embodiments, from about 60 mol % to about 70 mol % SiO₂.

Alumina (Al₂O₃) may also serve as a glass former in these glasses. Like SiO₂, alumina generally increases the viscosity of the melt. An increase in Al₂O₃relative to the alkalis (i.e., alkali metals and their oxides) or alkaline earths (i.e., alkaline earth metals and their oxides) generally results in improved durability of the glass. The structural role of the aluminum ions depends on the glass composition. When the concentration of alkali metal oxides (R₂O) is equal to or greater than the concentration of alumina (R₂O Al₂O₃), all aluminum is tetrahedrally coordinated. Alkali ions charge compensate Al³⁺ ions, so the aluminum ions act as Al⁴⁺ions, which favor tetrahedral coordination. This is the case for some of the example glasses listed in Table 1. Alkali ions that are in excess of aluminum ions tend to form non-bridging oxygens. In the other example glasses listed in Table 1, the concentration of alkali metal oxides is less than the concentration of aluminum ions (R₂Oâ‰¦Al₂O₃). Here, divalent cation oxides (Râ€²O) may also charge balance tetrahedral aluminum. While the behavior of calcium, strontium, and barium ions is equivalent to that of two alkali ions, magnesium and zinc ions do not fully charge balance aluminum in tetrahedral coordination due to their high field strength. This may result in the formation of five- and six-fold coordinated aluminum.

Generally, Al₂O₃plays a role in ion-exchangeable glasses since it enables a strong network backbone (i.e., high strain point) while allowing for the relatively fast diffusivity of alkali ions. High Al₂O₃concentrations, however, lower the liquidus viscosity of the glass. Thus, the Al₂O₃concentration in the glass should be maintained within in a reasonable range. In some embodiments, the glass comprises from about 9 mol % to about 18 mol % Al₂O₃and, in other embodiments, from about 9 mol % to about 16 mol % Al₂O₃. In still other embodiments, the glass comprises from about 10 mol % to about 16 mol % alumina.

In addition to Na₂O, the glass, in some embodiments, further comprises at least one additional alkali metal oxide (i.e., Li₂O, K₂O, Rb₂O, and/or Cs₂O). Alkali metal oxides aid in achieving low melting temperatures and low liquidus temperatures. However, the addition of alkali metal oxides dramatically increases the coefficient of thermal expansion (CTE) and lowers the chemical durability of the glass. In order to perform ion exchange, the presence of a small alkali metal oxide, such as Li₂O and Na₂O, in the glass is required to exchange with larger alkali ions (e.g., K⁺) in an ion exchange media such as, for example, a molten salt bath. Three types of ion exchange can generally be carried out: Na⁺-for-Li⁺exchange; K⁺-for-Li⁺ exchange; and K⁺-for-Na⁺exchange. Na⁺-for-Li⁺ exchange results in a deep depth of the surface compressive layer (DOL) but a low compressive stress (CS). K⁺-for-Li⁺ exchange results in a small depth of layer but a relatively large compressive stress, and K⁺-for-Na⁺ exchange results in an intermediate depth of layer and intermediate compressive stress. A sufficiently high concentration of the small alkali metal oxide in the glass is necessary to produce a large compressive stress, since compressive stress is proportional to the number of alkali ions that are exchanged out of the glass. Accordingly, in some embodiments, the glass comprises from about 9 mol % to about 18 mol % Na₂O and, in other embodiments, from about 12 mol % to about 16 mol % Na₂O. The glass may, in some embodiments, further comprise up to about 2 mol % of the other alkali metal oxides. The presence of lithium oxide (Li₂O) in the glass tends to inhibit K⁺-for-Na⁺ ion exchange and manufacturability of the glass by methods such as fusion-draw or slot-draw. Accordingly, the glasses described herein, in some embodiments, comprise less than about 1 mol % Li₂O. In other embodiments, the glass is free or substantially free of Li₂O. Similarly, the presence of K₂O in the glass tends to inhibit K⁺-for-Na⁺ ion exchange, and the amount of this alkali oxide in the glass should also be limited. In some embodiments, the glass contains less than about 2 mol % K₂O and, in other embodiments, less than about 1 mol % K₂O.

Divalent cation oxides, such as the alkaline earth oxides and ZnO, improve the melting behavior of the glass. With respect to ion exchange performance, however, the presence of divalent cations tends to decrease alkali metal ion mobility. The negative effect on ion exchange performance is especially pronounced with the larger divalent cations, such as Ba²⁺and Sr²⁺. Furthermore, the smaller divalent cation oxides (e.g., Mg²⁺, Zn²⁺, Ca²⁺) generally promote compressive stress more than the larger divalent cations. Thus, MgO, ZnO, and, in some aspects, CaO offer several advantages with respect to improved stress relaxation while minimizing the adverse effects on alkali ion diffusivity. However, when the MgO and ZnO contents are too high, they are prone to form forsterite (Mg₂SiO₄) and gahnite (ZnAl₂O₄), or willemite (Zn₂SiO₄), thus causing the liquidus temperature to rise very steeply when the MgO and ZnO concentrations exceed a certain level. In some embodiments, the glasses described herein comprise at least one of MgO, Zn, O, CaO, BaO, and SrO, where 0.5 mol %â‰¦MgO(mol %)+CaO(mol %)+ZnO(mol %)+BaO(mol %)+SrO(mol %)â‰¦4 mol %. In some embodiments, the glasses described herein comprise from 0 mol % to about 4 mol % MgO and, in other embodiments, from about 0.5 mol % to about 4 mol % MgO. Calcium and zinc oxide may be substituted for MgO. In certain embodiments, the glasses comprise up to about 4 mol % ZnO and, in other embodiments, from about 0.5 mol % to about 4 mol % ZnO. In some embodiments, the glass comprises MgO and ZnO, wherein 0.5 mol %â‰¦MgO(mol %)+ZnO(mol %)â‰¦4 mol %. Similarly, the glass may, in some embodiments, may comprise up to about 4 mol % CaO, in other embodiments, from about 0.5 mol % to about 4 mol % CaO, and in still other embodiments, the glass comprises MgO and CaO, wherein 0.5 mol %â‰¦MgO(mol %)+CaO(mol %)â‰¦4 mol %. In certain embodiments, the glasses are substantially free or free of at least one of CaO, BaO, and SrO.

The addition of B₂O₃and P₂O₅improves the damage resistance of these glasses. Boron is trigonally coordinated and thus opens up the structure when not charge balanced by alkali oxides or divalent cation oxides. The network around trigonally coordinated boron is not as rigid as that surrounding tetrahedrally coordinated boron; the bonds in trigonally coordinated boron are â€œfloppyâ€ and therefore allow the glass to tolerate some deformation before crack formation. Compared to higher coordination states, trigonal boron coordination also results in a greater amount of open space in the glass network. Furthermore, both boron and phosphorus decrease the melting viscosity and effectively help suppress zircon breakdown viscosity.

Unlike B₂O₃, P₂O₅improves diffusivity and decreases ion exchange times for the glass. However, some compressive stress capability is sacrificed by the structure formed by trigonally coordinated boron and phosphorus where the effect from P₂O₅is also pronounced. FIG. 3, which is a plot of the time needed to ion exchange the glasses listed in Table 1 to a depth of layer of 50 Î¼m, shows the effect of increased amounts of P₂O₅on the rate of ion exchange.

In some embodiments, the glass comprises from about 0.5 mol % to about 11 mol % B₂O₃and, in other embodiments, from about 2 mol % to about 10 mol % B₂O₃. In some embodiments, the amount of Al₂O₃present in the glass is greater than, or greater than or equal to, the amount of B₂O₃in the glass (i.e., Al₂O₃(mol %)â‰§B₂O₃(mol %)), and Al₂O₃(mol %)â‰§B₂O₃(mol %)+P₂O₅(mol %). In some embodiments, the glass comprises from greater than 0 mol % to about 11 mol % P₂O₅and, in other embodiments, from greater than 0 mol % to about 7 mol % P₂O₅, and, in still other embodiments, from greater than 0 mol % to about 4 mol % P₂O₅. In some embodiments, 4 mol %â‰¦B₂O₃(mol %)+P₂O₅(mol %)â‰¦15 mol %.

In some embodiments, the glasses described herein are ion exchanged using those means known in the art. In one non-limiting example, the glass is immersed in a molten salt bath containing an alkali metal cation such as, for example, K⁺, which is larger than the Na⁺cation present in the glass. Means other than immersion in a molten salt bath may be used to ion exchange of the glass. Such means include, but are not limited to, the application of a paste or gel containing the cation to be introduced into the glass to at least one surface of the glass.

The ion exchanged glass has at least one surface layer that is under a compressive stress (CS), as schematically shown in FIG. 1. Glass 100 has a thickness t, first surface 110, and second surface 112. Glass 100, in some embodiments, has a thickness t of up to about 2 mm, in other embodiments, to about 1 mm, in other embodiments, up to 0.7 mm, in still other embodiments, up to about 0.5 mm. Glass 100 has a first layer 120 under a compressive stress (â€œcompressive layerâ€ ) extending from first surface 110 to a depth of layer d₁into the bulk of the glass article 100. 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 has a central region 130 that extends from d₁to d₂. Central region 130 is under a tensile stress or central tension, which balances or counteracts the compressive stresses of layers 120 and 122. The depths of layer d₁, d₂of first and second compressive layers 120, 122 protect the glass 100 from the propagation of flaws introduced by sharp impact to first and second surfaces 110, 112 of glass 100, while the magnitude of the compressive stress in first and second compressive layers 120, 122 minimizes the likelihood of a flaw penetrating through the depth d₁, d₂of first and second compressive layers 120, 122.

In some embodiments, the ion exchanged glass described herein has a compressive layer extending from a surface of the glass to a depth of layer of at least about 45 Î¼m and, in certain embodiments, the depth of layer is at least about 50 microns (Î¼m). The compressive layer(s) of the glass, in some embodiments, are under a compressive stress of at least about 900 MPa, and, in other embodiments, at least about 1 GPa when ion exchanged to a depth of layer of at least about 45 Î¼m.

Table 3 lists ion exchange properties (compressive stress, depth of layer) determined from FSM measurements of the ion exchanged glasses listed in Table 1. FIG. 2 is a plot of compressive stress and depth of layer for the ion exchanged glasses listed in Table 1. Ion exchange was carried out for the fusion drawn samples, each having a thickness of 1.3 mm, in a refined grade KNO₃bath at 410Â° C. for 2 hours, 3 hours, 4 hours, 6 hours, 8 hours, and 12 hours. The CS and DOL values are average values, which have been corrected by assuming that SOC is 31 and the refractive index (RI) is 1.5. The compressive stresses of these glasses are generally greater than 900 MPa and, in some embodiments, greater than 1 GPa, as shown in FIG. 2. The rates of ion exchange of these glasses are significantly greater than those of alkali aluminosilicate glasses that do not contain phosphorus. In some embodiments, the glass is ion exchanged to achieve a depth of layer of at least 45 Î¼m and a compressive stress of at least about 900 MPa in an ion exchange bath comprising or consisting essentially of KNO₃at temperatures from about 410Â° C. to about 470Â° C. for up to about 5 hours, in some embodiments, for up to about 4 hours , and, in still other embodiments, for up to about 2 hours. FIG. 3 is a plot of the ion exchange time required, expressed in hours, to achieve a depth of layer of 50 Î¼m for the glasses listed in Table 1. As can be seen from FIG. 3 and Table 1, the time required to ion exchange these glasses to a DOL of 50 Î¼m decreases as the amount of P₂O₅in the glass increases.

TABLE 3Compressive stresses (CS) and depths of layer (DOL) for theglasses listed in Table 1 that were ion exchanged in a refined grade KNO₃molten salt bath at 410Â° C. for times ranging from 2 to 8 hours.Glass123456CS (MPa)101310051001991979969DOL (Î¼m)222424272932IX time (h)222222CS (MPa)1008994985974972954DOL (Î¼m)272831333539IX time (h)333333CS (MPa)972960970955950DOL (Î¼m)3739404344IX time (h)44444CS (MPa)955938936929926905DOL (Î¼m)424549525561.5IX time (h)888888Glass7891011CS (MPa)972960940926914DOL (Î¼m)3434363738IX time (h)22222CS (MPa)952946935911906DOL (Î¼m)4043434545IX time (h)33333CS (MPa)950944930910901DOL (Î¼m)4649495052IX time (h)44444CS (MPa)902900887864855DOL (Î¼m)6466687071IX time (h)88888

Table 4 lists ion exchange properties for samples 12-53 listed in Table 2. Compressive stress and depth of layer were determined from FSM measurements. Annealed samples, each having a thickness of 1 mm, were ion exchanged at 470Â° C. for 2 hours in a molten salt bath containing refined grade KNO₃. The CS and DOL values in Table 4 are average values, which have been corrected by assuming that SOC is 31.8 and RI is 1.5.

TABLE 4Ion exchange properties for samples 12-53 listed in Table 2.The samples were annealed and ion exchanged at 470Â° C. for2 hours in a molten salt bath containing refined grade KNO₃.Glass121314151617DOL (Î¼m)504948496060CS (MPa)526508505476480454Glass181920212223DOL (Î¼m)404353658676CS (MPa)585540509462390415Glass242526272829DOL (Î¼m)473042543950CS (MPa)434500433373411420Glass303132333334DOL (Î¼m)532944585872CS (MPa)385491397334334275Glass353637383940DOL (Î¼m)51494142CS (MPa)429477554568Glass414243444546DOL (Î¼m)2726353833CS (MPa)665703605547562Glass47484950515253DOL (Î¼m)36454548474647CS (MPa)513431441442457412404

Ion exchange properties for samples 54-60 in Table 2 are listed in Table 5. Compressive stress and depth of layer were determined from FSM measurements. Samples, each having a thickness of 1 mm, were ion exchanged at 420Â° C. for 2.5 hours in a molten salt bath containing refined grade KNO₃. The CS and DOL values in Table 5 are average values that have been corrected by assuming that SOC is 31.8 and RI is 1.5.

TABLE 5Ion exchange properties for samples 54-60 in Table 2. Samples,each having a thickness of 1 mm, were ion exchanged at 420Â° C.for 2.5 hours in a molten salt bath containing refined grade KNO₃.Glass54555657585960CS (MPa)942913894852930926916DOL (Î¼m)51505250545554

When ion exchanged as described herein above, the glasses described herein exhibit damage resistance that is comparable to or better than most alkali aluminosilicate glasses. This quality is at least partly attributable to the open structure resulting from the presence of B₂O₃and P₂O₅in the glass. These network formersâ€”particularly B₂O₃â€”are trigonally coordinated and open up the glass structure. This may manifest itself in higher molar volume and lower density of the glass. The trigonally coordinated network is not as rigid as a tetrahedrally coordinated network, and is therefore more capable of tolerating more deformation before crack formation occurs. In some embodiments, the ion exchanged glass has a Vickers crack initiation threshold of at least about 10 kgf. In other embodiments the Vickers crack initiation threshold is at least about 15 kgf and, in other embodiments, at least about 20 kgf. Vickers crack indentation thresholds measured for samples 1-11 in Table 1 are plotted in FIG. 4. The samples were ion exchanged at 410Â° in a molten salt bath containing refined grade KNO₃for times ranging from 2 hours to 8 hours. Sample 1 was ion exchanged for 8 hours, samples 2-5 were ion exchanged for 4 hours, samples 6-10 were ion exchanged for 3 hours, and sample 11 was ion exchanged for 2 hours. Compressive stresses and depths of layer that were obtained for these samples are listed in Table 3. Vickers crack indentation thresholds measured for samples 54-60 in Table 2 are plotted in FIG. 5. The samples were ion exchanged at 420Â° in a molten salt bath containing refined grade KNO₃for 2.5 hours. Compressive stresses and depths of layer that were obtained for these samples are listed in Table 5.

In another aspect, a method of ion exchanging a glass is also provided. Method 600 includes a first step in which an ion exchange bath comprising or consisting essentially of KNO₃is provided. The ion exchange bath may contain other salts such as, for example, NaNO₃, or may contain only or consist essentially of KNO₃. The ion exchange bathe is maintained at a temperature in a range from about 410Â° C. to 470Â° C. throughout the process. In a second step, a glass is then ion exchanged in the ion exchange bath for a time period of up to about four hours, after which time the ion exchanged glass has a layer under a compressive stress of at least about 900 MPa and, in some embodiments, at least 1 GPa. The layer extends from a surface of the glass to a depth of layer of at least about 45 Î¼m and, in some embodiments, the depth of layer is at least about 50 Î¼m. The glass that is ion exchanged in the method is a glass described hereinabove, comprising SiO₂, Al₂O₃, Na₂O, at least one divalent metal oxide, B₂O₃, and P₂O₅, wherein Al₂O₃(mol %)â‰§B₂O₃(mol %)+P₂O₅(mol %).

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.