TWI872506B - Strengthened lithium-free aluminoborosilicate glass - Google Patents

Abstract

The present invention describes a strengthened lithium-free aluminoborosilicate glass. The lithium-free aluminoborosilicate glass undergoes a double ion-exchange process to achieve a depth of compressive layer greater than 50 μm. The chemically strengthened aluminoborosilicate glass survives during a greater number of device drops before failure.

Description

Strengthened lithium-free aluminum borosilicate glass

The present invention describes an aluminum borosilicate glass. More specifically, the present invention focuses on a lithium-free glass composition. The present invention describes a lithium-free glass composition that exhibits higher strength through a double ion exchange strengthening process.

In recent years, electronic devices equipped with liquid crystal displays, organic light-emitting diode displays or the like have been widely used. Since glass materials have high surface hardness, they are widely used as cover glass materials for displays of such electronic devices. Since glass is a typical brittle material, such cover glass is usually subjected to strengthening treatment. There is a continuous concern about reducing the thickness and weight of electronic devices, resulting in a demand for thinner cover glass.

Chemical strengthening is an important strengthening process for glass sheets. For thin glass sheets such as cover glass for displays, chemical strengthening is often used to strengthen the cover glass. In the chemical strengthening process, glass containing monovalent alkali metal ions at the surface is immersed in a molten salt bath at high temperature. During the process, monovalent alkali metal ions with a larger radius in the salt bath are able to replace monovalent alkali metal ions with a smaller radius on the glass surface, thereby forming a compressive stress layer at the glass surface. The larger densely packed monovalent alkali metal ions at the glass surface then produce high compressive stress, which in turn provides higher strength. The compression layer further serves to suppress defects that can lead to glass failure, including low tensile strength.

In recent years, lithium aluminosilicate glass has been widely used as a cover glass for displays of electronic devices due to its superior mechanical properties over lithium-free glass. Specifically, the superior mechanical properties are achieved by the fact that the smallest lithium ion among alkali metal ions can first undergo ion exchange with the sodium ion, which is the next larger ion after the lithium ion, and then the sodium ion undergoes another ion exchange with the next larger ion, namely the potassium ion, in a double ion exchange (DIOX) process. This overall DIOX process produces an increased compression depth, made possible by the presence of lithium ions, while achieving greater surface compressive stress. However, due to its widespread use in battery packs that power smartphones, laptops, electric vehicles, and the like, the demand for lithium is enormous. The huge demand for lithium has outstripped its supply, resulting in increased procurement costs for lithium. Furthermore, it has been predicted that lithium will soon be insufficient to meet the current demand. Therefore, the present invention focuses on methods or processes for enhancing the performance of lithium-free aluminum borosilicate glass. Therefore, there is a need for lithium-free aluminum borosilicate glass that exhibits a desired surface compressive stress and layer depth when undergoing ion exchange.

Invention Goal

Some objects of the present invention are described herein. One object of the present invention is to provide an aluminum borosilicate glass composition. Another object of the present invention is to provide an aluminum borosilicate glass that is substantially free of lithium.

Another object of the present invention is to allow lithium-free aluminum borosilicate glass to undergo a double ion exchange process to achieve a compression layer depth greater than 50 μm.

Another object of the present invention is to provide a lithium-free aluminum borosilicate composition having higher strength and better service life.

Other objects and advantages of the present invention will become more apparent from the following description, which is not intended to limit the scope of the present invention.

In one embodiment of the present invention, an aluminum borosilicate glass composition is disclosed. The present invention discloses an aluminum borosilicate glass composition that is substantially free of lithium.

In one embodiment, the glass composition of the lithium-free aluminum borosilicate glass includes _SiO2 in a range of about 40 wt% to about 70 wt%, _Al2O3 in a range of about 5 wt% to about 35 wt%, and _B2O3 in a range of about 0.5 wt% to about 10 _wt %. In addition, it includes alkali metal _oxides such as _Na2O in a range of about 5 wt% to about 25 wt% and _K2O in a range of about 0 wt% to about 5 wt%, and alkali earth metal oxides such as MgO in a range of about 0 wt% to about 7 wt%. In addition, it includes _P2O5 in a range of about 0 wt% to about 10 _wt %, _ZrO2 in a range of about 0 wt% to about 6 wt%, SnO2 in a range of about 0 wt% to about 2 wt _{%, Fe2O3 in} a range of about 0 wt% to about 3 wt%, _CeO2 in a range of about 0 wt% to about ₃ wt%, and _TiO2 in a range of about 0 wt% to about 5 wt%.

In one embodiment, lithium-free aluminum borosilicate glass can exhibit higher strength when it undergoes a double ion exchange process.

In one embodiment, lithium-free aluminum borosilicate glass undergoes a double ion exchange process to achieve a compression layer depth greater than 50 μm.

In one embodiment, the lithium-free aluminum borosilicate glass has better durability in use, higher crack resistance, and higher sharp impact strength. In addition, the glass can survive a large number of device drops before failure occurs.

In one embodiment, the lithium-free aluminum borosilicate glass can be used as a substrate for a touch panel display and as a back cover for such displays, such as a liquid crystal display (LCD), a field emission display (FED), a plasma display (PD), an electroluminescent display (ELD), an organic light emitting diode (OLED) display, a micro-LED, or the like. The lithium-free aluminum borosilicate glass is used for protection of electronic devices with display screens, such as mobile phones, entertainment devices, tablet computers, laptops, digital cameras, wearable devices, or the like.

These and other aspects, advantages and salient features of the present invention will become apparent from the following detailed description.

Priority claim

This application claims priority over Indian provisional application serial number 202221022507 filed on April 15, 2022, the entire contents of which are incorporated herein by reference.

In the following description, in several views shown in the drawings, the same reference numerals represent the same or corresponding parts. It should also be understood that, unless otherwise specified, terms such as "top", "bottom", "outside", "inside" and the like are for convenient use and should not be interpreted as restrictive terms. In addition, whenever a group is described as comprising at least one of a set of elements and combinations thereof, it should be understood that the group may contain, consist essentially of or consist of any number of those elements, either individually or in combination with each other. Similarly, whenever a group is described as consisting of at least one of a set of elements or combinations thereof, it should be understood that the group may consist of any number of those elements, either individually or in combination with each other. Unless otherwise specified, when enumerating the range of values, it includes the upper and lower limits of the range and any range therebetween. As used herein, unless otherwise specified, the indefinite article "a/an" and the corresponding definite article "the" mean "at least one" or "one or more". It should also be understood that the various features disclosed in the specification and drawings can be used in any and all combinations.

As used herein, the term "glass article" is used in its broadest sense to include any article made entirely or partially of glass. Unless otherwise specified, all compositions are expressed in weight percent (wt %). Unless otherwise specified, all temperatures are expressed in degrees Celsius (° C.). Unless otherwise specified, the coefficient of thermal expansion (CTE) is expressed in 10 ^-7 /° C. and represents a value measured in a temperature range of about 50° C. to about 300° C.

It should be noted that the terms "substantially" and "approximately" may be used herein to represent the degree of inherent uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. Such 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. For example, a glass that is "substantially free of _Li2O " is one to which _Li2O has not been actively added or batch-added to the glass, but may be present in minimal amounts as a contaminant from the raw materials.

Recently, with the advancement of technology, electronic devices such as mobile phones, tablet computers, wearable devices, digital cameras or the like are widely used. These electronic devices have display screens protected by cover glasses of different compositions. The present invention describes a lithium-free aluminum borosilicate glass composition that provides a display screen with a better service life.

The present invention describes various lithium-free aluminum borosilicate glass compositions in detail. The present invention mainly describes lithium-free aluminum borosilicate glasses and compositions thereof. The glass composition includes one or more chemical components, such as _SiO2 , _Al2O3 and _B2O3 . _The glass composition further includes an alkali metal oxide selected from the group consisting of _Na2O and _K2O . In addition, the glass composition includes one or more alkaline oxides, such as MgO, CaO, _SrO and BaO. It _may also contain other chemical components, such as _ZrO2 , _Fe2O3 , _CeO2 , _P2O5 , _TiO2 or _the like. In addition, it may also contain refining agents such as SnO ₂ , chlorides, sulfates or the like. The properties of lithium-free aluminoborosilicate glass are highly affected by the amounts of the components of the glass composition.

In one embodiment, _SiO2 is a component that forms a glass network structure. In the case of too high a _SiO2 content, the glass is difficult to melt and form, or the glass has too low a thermal expansion coefficient and is difficult to have the same thermal expansion coefficient as the surrounding material. On the other hand, in the case of too low a _SiO2 content, it is difficult to vitrify. In addition, such glasses have an increased thermal expansion coefficient, which tends to reduce thermal shock resistance. Therefore, the glass composition requires an optimal weight % of _SiO2 . For example, the glass composition may include about 40 weight % to about 70 weight % of _SiO2 .

In one embodiment, Al ₂ O ₃ is a component that enhances suitability for single and/or multiple ion exchange. Al ₂ O ₃ further has the effect of enhancing the heat resistance and Young's modulus of the glass. In the case where the content of Al ₂ O ₃ is too high, devitrified crystals tend to separate in the glass, making it difficult to form glass by an overflow down-draw process or the like. In addition, such glasses have increased viscosity at high temperatures and are difficult to melt. When the content of Al ₂ O ₃ is too low, it is possible that the glass may not have sufficient suitability for single and/or multiple ion exchange. In these respects, the glass composition requires an optimal weight % of Al ₂ O _3. For example, the glass composition may include about 5 weight % to about 35 weight % of Al ₂ O ₃ .

In one embodiment, _B2O3 is _a component that has the effect of lowering the liquidus temperature, high temperature viscosity and density of the glass, and further has the effect of enhancing the suitability of the glass for single and/or multiple ion exchange. In addition, the presence _{of B2O3} causes a reduction in the depth of the compressive stress layer formed by chemical strengthening. From these aspects, the glass composition requires an optimal weight % _{of B2O3} . For example, the glass composition may include about 0.5 weight % to about 10 weight % _{of B2O3} .

In one embodiment, _Na2O is a component used in chemical strengthening treatment to increase the surface compressive stress and the depth of the surface compressive stress layer by replacing sodium ions with potassium ions. However, increasing the _Na2O content beyond an appropriate limit may result in a situation where the surface compressive stress may decrease. In view of these aspects, the glass composition requires an optimal weight % of _Na2O . For example, the glass composition may include about 5 weight % to about 25 weight % of _Na2O .

Similar to _Na2O , _K2O is a component that increases the meltability of glass. A reduced content of _K2O increases the ion exchange rate in chemical strengthening and thereby increases the depth of the surface compressive stress layer, but simultaneously decreases the liquidus temperature TL of the glass composition. Therefore, _K2O is preferably contained in a small amount. From these aspects, the glass composition requires an optimal wt% of _K2O . For example, the glass composition may include about 0 wt% to about 5 wt% of _K2O .

Furthermore, the aluminum borosilicate glass contains substantially no lithium, ie in particular Li ₂ O. Since the presence of Li ₂ O improves the Young's modulus and fracture toughness of the glass, the content of Al ₂ O ₃ is adjusted to improve the Young's modulus of the lithium-free aluminum borosilicate glass.

In one embodiment, MgO is an alkali earth metal, wherein the content of MgO may be about 0 wt % to about 7 wt %. In one embodiment, P ₂ O ₅ is a component that enhances the suitability of the glass for ion exchange and is highly effective, especially when increasing the depth of the compressive stress layer. Since a higher P ₂ O ₅ content may cause phase separation in the glass or may reduce water resistance, the glass composition may include about 0 wt % to about 10 wt % P ₂ O ₅ .

In one embodiment, the glass composition further includes one or more refining agents, such as about 0 wt % to about 2 wt % SnO ₂ and about 0 wt % to about 3 wt % Fe ₂ O ₃ . In addition, it may also include other refining agents, such as CeO ₂ , chlorides, sulfates, or the like. In one embodiment, the glass composition may include about 0 wt % to about 6 wt % ZrO ₂ and about 0 wt % to about 5 wt % TiO ₂ . In one embodiment, the thickness of the lithium-free aluminum borosilicate glass obtained from the above glass composition is in the range of 20 microns to 2 mm.

Table 1 illustrates the following non-limiting, exemplary lithium-free aluminum borosilicate glass compositions and their properties:

Table 2 illustrates the following non-limiting, exemplary aluminum borosilicate glass compositions and their properties:

In an exemplary embodiment, when the glass composition includes about 42.64 wt % SiO ₂ , about 31.27 wt % Al ₂ O ₃ , about 4.19 wt % B ₂ O ₃ , about 18.15 wt % Na ₂ O, about 0.10 wt % K ₂ O, about 1.31 wt % MgO, about 0.17 wt % SnO ₂ , and about 2.16 wt % P ₂ O ₅ , the glass transition temperature (Tg) of the lithium-free aluminum borosilicate glass obtained from the glass composition is about 657° C., and the CTE is about 94×10 ^-7 /°C, density is about 2.44 g/cc, Young's modulus is about 70.5 GPa, annealing temperature is about 667°C, Passon's ratio is about 0.24, and shear modulus is about 28.4 GPa.

Lithium-free aluminum borosilicate glass can undergo two ion exchange process steps. In one embodiment, the ion exchange process is based on the size of the monovalent alkali metal ions. The lithium-free aluminum borosilicate glass containing monovalent alkali metal ions is treated at high temperature in a molten salt bath containing an alkali metal inorganic salt. Here, the monovalent alkali metal ions at the glass surface of the glass are ion exchanged with the monovalent alkali metal ions of the alkali metal inorganic salt. A common process is to immerse the glass in a molten salt bath of an alkali metal inorganic salt or a mixture of an alkali metal inorganic salt and other inorganic salts. The immersion time is sufficient to induce this exchange only in the surface layer of the glass object. The high compressive stress achieved by the ion exchange process helps the glass survive a relatively large number of device drops before failure occurs.

When larger alkali metal ions replace smaller alkali metal ions in the surface layer of glass at an elevated temperature below the strain point of the glass, the surface layer subsequently acquires a compressive stress. The larger alkali metal ions attempt to occupy the space previously occupied by the smaller alkali metal ions, thereby creating a compressive stress at the surface layer of the glass. Because the temperature of the molten salt bath is below the strain point of the glass, the glass cannot readjust itself to relieve the stress. Lithium-free aluminoborosilicate glass undergoes a double ion exchange process. The following are the two steps of the double ion exchange process:

Ion exchange step I:

The glass composition of the present invention is rich in Na+ ions and deficient in K+ ions. In the first step I of the double ion exchange process, the K+/Na+ ratio of the alkali metal salt used in the molten salt bath is higher than the K+/Na+ ratio in the glass. Therefore, the first step I is performed to replace the smaller alkali metal ions (Na+ ions) on the surface of the glass with the larger alkali metal ions (K+ ions) in the molten salt bath. In the first step I of the double ion exchange process, an alkali metal salt (such as sodium nitrate (NaNO ₃ ) and potassium nitrate (KNO ₃ )) is used in the ion exchange bath. Here, K+ ions of the alkaline metal inorganic salt replace Na+ ions in the surface layer of the glass. Since the ion exchange time is very long, i.e., at least about 1 hour or about more than 4 hours, the Na+ ions in the glass are initially replaced by the K+ ions in the ion exchange bath to enrich the glass surface with K+ ions. Further ion exchange causes the K+ ions in the glass to be exchanged by the Na+ ions in the ion exchange bath. This continuous process of exchanging K+ ions in the glass with Na+ ions in the ion exchange bath, followed by exchanging Na+ ions in the glass with K+ ions in the ion exchange bath, continues until the compression depth becomes greater than 50 μm and the composition of the K+/Na+ ratio in the glass becomes the same as the K+/Na+ ratio in the ion exchange bath. Although the compression stress becomes lower and lower with time and the ion exchange bath contains a mixture of Na+ ions and K+ ions, the compression depth of the glass contains the same ratio of Na+ to K+ ions as in the ion exchange bath. This gives a compression layer of greater depth and a large amount of compression stress. The compressive stress layer formed on the surface of the glass provides the glass with increased scratch and drop resistance.

Ion exchange step II:

In the second step II of the ion exchange, the K+/Na+ ratio is greater than 90%, which is much higher than the K+/Na+ ratio in the first step I of the ion exchange process. In the process of the present invention, some alkaline metal salts used so far, such as potassium nitrate (KNO ₃ ) mixed with some sodium nitrate (NaNO ₃ ) (very high concentration) provide the liquid ion exchange bath of the second step of the ion exchange. Here, the K+ ions of the alkaline metal inorganic salt replace the Na+ ions in the surface layer of the glass. Due to the very high concentration of K+ ions and the very short ion exchange time (<60 minutes), this leads to the generation of very large compressive stresses in the surface.

The aluminoborosilicate glass described herein is initially formed to be substantially free of _Li2O . When the glass undergoes a double ion exchange process, a greater number of K+ ions replace Na+ ions present on the surface of the glass. The double ion exchange process of the lithium-free aluminoborosilicate glass allows for the achievement of an increased compressive stress layer on the surface of the glass compared to a single ion exchange process of the lithium-free aluminoborosilicate glass. The double ion exchanged glass has excellent mechanical properties similar to those of lithium aluminoborosilicate glass. The ion exchange conditions of the double ion exchange process are adjusted to achieve the desired compressive stress layer on the surface of the glass.

Table 3 illustrates an exemplary sample, describing two steps of the ion exchange process for glass sample 5 with a thickness of 0.7 mm and the corresponding compression depth (DOL_zero) and compression stress (CS) for each step.

In one exemplary embodiment, the glass composition undergoes two ion exchange process steps. In the first step of the ion exchange, the glass undergoes the first step of the ion exchange process in a salt bath comprising about 75 wt % KNO ₃ and about 25 wt % NaNO _3. The preferred temperature for treating the glass material is about 415° C., and the time for treating the glass material is about 130 minutes. The stress profile of the first step of the ion exchange includes a compressive stress of about 158.43 MPa and a compression depth of about 134.53 μm. In addition, in the second step of the ion exchange, the salt bath comprises about 97 wt % KNO ₃ and about 3 wt % NaNO ₃ . The preferred temperature for treating the glass material is about 380°C, and the time for treating the glass material is about 20 minutes. The stress profile of the second step of the ion exchange includes a compressive stress greater than 940.01 MPa and a compression depth greater than 11.58 μm. The lithium-free aluminum borosilicate glass undergoes a double ion exchange process to achieve a compression layer depth greater than 50 μm.

The present invention allows lithium-free aluminoborosilicate glass to undergo a double ion exchange process to achieve a surface layer strength equivalent to that of lithium aluminoborosilicate glass. The high compressive stress achieved by the double ion exchange step helps the glass survive a greater number of device drops before failure.

Aluminum borosilicate glass can be used as a substrate for touch panel displays and as a back cover for such displays, such as liquid crystal displays (LCDs), field emission displays (FEDs), plasma displays (PDs), electroluminescent displays (ELDs), organic light emitting diode (OLED) displays, micro LEDs or the like. Aluminum borosilicate glass is used for protection of electronic devices with display screens, such as mobile phones, entertainment devices, tablet computers, laptops, digital cameras, wearable devices or the like.

Although typical embodiments have been described for illustrative purposes, the foregoing description should not be considered as limiting the scope of the present invention or the scope of the attached patent applications. Therefore, those skilled in the art may make various modifications, adaptations, and substitutions without departing from the spirit and scope of the present invention.

Claims (6)

A chemically strengthened glass article, comprising: _SiO2 in a range of about 40 wt% to about 70 wt%; _Al2O3 in a range of about 5 wt% to about 35 wt%; _B2O3 in a range of about 0.5 wt% to about ₁₀ wt _% ; Na2O in a range of about 5 wt% to about 25 wt%; _K2O in a range of about 0 wt% to about 5 wt%; MgO in a range of about 0 wt% to about 7 wt%; _P2O5 in a range of about 0 wt% to about 10 wt% _{; ZrO2} in a range of about 0 wt% to about 6 wt _% ; _SnO2 in a range of about 0 wt% to about 2 wt%; _Fe2O3 in a range of about 0 wt% to about 3 wt%; _{and CeO2} in a range of about 0 wt% to about 3 wt%. and _TiO2 in a range of about 0 wt% to about 5 wt%, wherein the glass object undergoes a double ion exchange process to achieve a K+/Na+ ion ratio greater than 90%, wherein the glass object has a compression layer depth greater than 50 μm, and wherein the glass object is substantially free of lithium. A chemically strengthened glass article as claimed in claim 1, wherein the _SiO2 content is optimized to achieve a thermal expansion coefficient that is compatible with surrounding materials. A method for treating a glass object comprises the following steps: a) providing a molten salt bath to the glass object containing K+ ions and Na+ ions; b) maintaining the K+/Na+ ion ratio in the molten salt bath higher than the K+/Na+ ion ratio in the glass; c) replacing the alkali metal ion Na+ on the surface of the glass with the alkali metal ion K+ in the molten salt bath; d) enriching the surface of the glass object with K+ ions; e) adding at least one selected from _NaNO3 or KNO ₃ ; f) replacing Na+ ions in a surface layer of the glass with K+ ions; g) maintaining a K+/Na+ ion ratio in the glass equal to a K+/Na+ ratio in the molten salt bath; h) mixing an alkali metal salt KNO ₃ with NaNO ₃ to provide a liquid ion exchange bath; i) replacing Na+ ions in the surface layer of the glass with K+ ions; and j) maintaining a K+/Na+ ion ratio greater than 90%, wherein the resulting glass object has a compression depth greater than 50 μm, and wherein the glass object is substantially free of lithium. The method of claim 3, wherein the ion exchange time in step (a) is at least 1 hour. The method of claim 3, wherein the ion exchange time in step (h) is less than 60 minutes. The method of claim 3, wherein the double ion exchange process provides the glass object with increased scratch resistance.