JP7716038B2 - Tempered glass sheet, method for manufacturing tempered glass sheet, and glass sheet for tempering - Google Patents

Description

The present invention relates to tempered glass sheets, a method for manufacturing tempered glass sheets, and glass sheets for tempering, and in particular to tempered glass sheets, a method for manufacturing tempered glass sheets, and glass sheets for tempering that are suitable as cover glass for touch panel displays in mobile phones, digital cameras, PDAs (personal digital assistants), and the like.

Ion-exchanged tempered glass sheets are used as cover glass for touch panel displays in applications such as mobile phones, digital cameras, and PDAs (personal digital assistants) (see Patent Document 1 and Non-Patent Document 1).

Japanese Patent Application Laid-Open No. 2006-83045 Special table 2016-524581 publication Special Publication No. 2011-510903

Tetsuro Izumiya et al., "New Glass and its Physical Properties," First Edition, Management Systems Research Institute, August 20, 1984, pp. 451-498

However, if a smartphone is accidentally dropped on the road or other surface, the cover glass may break, rendering the smartphone unusable. To prevent this from happening, it is important to increase the strength of the tempered glass plate.

Increasing the stress depth is an effective way to increase the strength of tempered glass sheets. More specifically, when a smartphone is dropped and the cover glass collides with the road surface, protrusions and sand particles on the road surface penetrate the cover glass, reaching the tensile stress layer and causing breakage. Therefore, by increasing the stress depth of the compressive stress layer, it becomes more difficult for protrusions and sand particles on the road surface to reach the tensile stress layer, reducing the probability of breakage of the cover glass.

Lithium aluminosilicate glass is advantageous for achieving deep stress depth. In particular, by immersing a glass plate to be tempered made of lithium aluminosilicate glass in a molten salt containing _NaNO3 and ion-exchanging the Li ions in the glass with the Na ions in the molten salt, a tempered glass plate with deep stress depth can be obtained.

However, with conventional lithium aluminosilicate glass, there is a risk that the compressive stress value of the compressive stress layer will be too small. On the other hand, if the glass composition is designed to increase the compressive stress value of the compressive stress layer, there is a risk that chemical stability will decrease.

Furthermore, conventional lithium aluminosilicate glasses have insufficient clarity, which can lead to bubbles remaining in the glass when formed into a sheet. On the other hand, when tin oxide (SnO ₂ ) is introduced into the glass composition as a fining agent to reduce bubbles, devitrification particles of SnO ₂ are generated, which can make forming into a sheet difficult.

The present invention was developed in consideration of the above circumstances, and its technical objective is to provide a tempered glass sheet that is resistant to breakage when dropped, has excellent chemical stability and clarity, and is less likely to develop devitrification particles during forming.

As a result of various investigations, the present inventors have found that the above technical problems can be solved by restricting the glass composition to a predetermined range, and have proposed this invention. That is, the tempered glass plate of the present invention is a tempered glass plate having a compressive stress layer on a surface thereof, and contains, in mol %, 40 to 80% SiO ₂ , 6 to 25% Al ₂ O ₃ , 0 to 10% B ₂ O ₃ , 3 to 15% Li ₂ O , 1 to 21% Na ₂ O , 0 to 10% K ₂ O , 0 to 10% MgO , 0 to 10% ZnO , 0 to 15% P ₂ O ₅ , and 0.001 to 0.30% SnO ₂ , and satisfies ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ]) × ([Al ₂ O ₃ ] + [Li ₂ O] + [Na ₂ O] + [K ₂ O] + [MgO] + [CaO] + [SrO] + [BaO] + [ZnO])) ≥ 0.40. Here, [Li ₂ O] refers to the mol% content of Li ₂ O. [Na ₂ O] refers to the mol% content of Na ₂ O. [K ₂ O] refers to the mol% content of K ₂ O. [Al ₂ O ₃ ] refers to the mol% content of Al ₂ O ₃ . ([ _Li2O ] + [ _Na2O ] + [ _K2O ])/[ _{Al2O3 ]} refers to the value obtained by dividing the total amount of _Li2O , _Na2O , and _K2O by the content of _Al2O3 . [ _SiO2 ] refers to the mole percent content of _SiO2 . [ _{B2O3 ]} refers to the mole percent content of _B2O3 . _{[ P2O5} ] refers to the mole percent content of _P2O5 . _{[ SnO2} ] refers to the mole percent content of _SnO2 . [ _MgO ] refers to the mole percent content of _MgO . [CaO] refers to the mole percent content of CaO. [SrO] refers to the mole percent content of SrO. [BaO] refers to the mole percent content of BaO. [ZnO] refers to the mole percent content of ZnO. ([ _SiO2 ] + [ _B2O3 ] + [ _P2O5 ]) / ₍ (100 × [ _SnO2 ]) × ([ _Al2O3 ] + [ _Li2O ] + [ _Na2O ] + [ _K2O ] + [ _MgO ] + [CaO] + [ _SrO ] + [BaO] + [ZnO])) refers to the value obtained by dividing the _total amount of _SiO2 , _B2O3 , and _P2O5 by the product _of 100 times the _SnO2 content and the _total amount of _Al2O3 , _Li2O , _Na2O , _K2O , MgO, CaO, SrO, BaO, and ZnO.

In the tempered glass plate of the present invention, the content of B ₂ O ₃ is preferably 0.1 to 3 mol %.

In the tempered glass plate of the present invention, the content of _SnO2 is preferably 0.045 mol% or less.

Furthermore, in the tempered glass sheet of the present invention, the Cl content is preferably 0.02 to 0.3 mol%.

The tempered glass plate of the present invention has a compressive stress layer on a surface thereof, and contains, in mole percent, 40 to 80% SiO ₂ , 6 to 25% Al ₂ O ₃ , 0 to 10% B ₂ O ₃ , 3 to 15% Li ₂ O , 1 to 21% Na ₂ O , 0 to 10% K ₂ O , 0 to 10% MgO , 0 to 10% ZnO , 0 to 15% P ₂ O , _0.001 to 0.045% SnO ₂ , and 0.02 to 0.3% Cl, and satisfies ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al ₂ O ₃ ]+[Li ₂ O]+[Na ₂ O]+[K ₂ O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40.

The tempered glass plate of the present invention has a compressive stress layer on a surface thereof, and contains, in mole %, a glass composition of 40 to 80% SiO ₂ , 6 to 25% Al _{2 O 3} , 0.1 to ₃ % B ₂ O 3 , 3 to 15% Li ₂ O , 1 to 21% Na ₂ O , 0 to 10% K 2 O , 0 to 10% MgO , 0 to 10% ZnO , ₀ to 15% P ₂ O , 0.001 to 0.30% SnO ₂ , and 0.02 to 0.3% Cl, and satisfies ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al ₂ O ₃ ]+[Li ₂ O]+[Na ₂ O]+[K ₂ O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40.

In the tempered glass plate of the present invention, the content of P ₂ O ₅ is preferably 2.5 mol % or more.

In the tempered glass plate of the present invention, the content of Fe ₂ O ₃ is preferably 0.001 to 0.1 mol %.

In the tempered glass plate of the present invention, the content of TiO ₂ is preferably 0.001 to 0.1 mol%.

In addition, in the tempered glass sheet of the present invention, the compressive stress value of the outermost surface of the compressive stress layer is preferably 200 to 1200 MPa. Here, the "compressive stress value of the outermost surface" and "stress depth" refer to values measured from a phase difference distribution curve observed using, for example, a scattered light photoelastic stress meter SLP-1000 (manufactured by Orihara Manufacturing Co., Ltd.). The stress depth refers to the depth at which the stress value becomes zero. When calculating the stress characteristics, the refractive index of each measurement sample is set to 1.51, and the photoelastic constant is set to 29.0 [(nm/cm)/MPa].

Furthermore, in the tempered glass plate of the present invention, the stress depth of the compressive stress layer is preferably 50 to 200 μm.

Furthermore, it is preferable that the compressive stress value of the tempered glass sheet of the present invention at a depth of 2.5 μm be 350 MPa or more. This increases the bending strength.

Furthermore, it is preferable that the tempered glass sheet of the present invention has an average compressive stress value of 85 MPa or more at a depth of 30 to 45 μm. This increases drop strength.

In addition, in the tempered glass sheet of the present invention, the temperature at which the high-temperature viscosity is 10 ^2.5 dPa·s is preferably lower than 1650° C. Here, the "temperature at which the high-temperature viscosity is 10 ^2.5 dPa·s" can be measured by, for example, a platinum sphere pull-up method.

The tempered glass sheet of the present invention preferably has an overflow confluence surface in the center in the sheet thickness direction. Here, the "overflow downdraw method" is a method for producing a glass sheet by allowing molten glass to overflow from both sides of a refractory shaped body, and drawing the overflowing molten glass downward while allowing it to join at the lower end of the refractory shaped body.

The tempered glass plate of the present invention is also preferably used as a cover glass for a touch panel display.

Furthermore, it is preferable that the tempered glass sheet of the present invention has a stress profile in the thickness direction that has at least a first peak, a second peak, a first bottom, and a second bottom.

The method for producing a tempered glass plate of the present invention is directed to a glass having a glass composition containing, in mole percent, 40 to 80% SiO ₂ , 6 to 25% Al _{2 O 3 ,} 0 to 10% B ₂ O 3 , 3 to 15% Li ₂ O , 1 to 21% Na ₂ O , 0 to 10% K 2 O , 0 to 10% MgO, 0 to 10% ZnO, 0 to 15% P ₂ O ₅ , and 0.001 to 0.30% SnO ₂ , wherein ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al _{2 and an} ion exchange step _of performing ion exchange treatment on the glass plate to be tempered multiple times to obtain _a tempered glass plate having a compressive stress layer on the surface.

The glass plate to be tempered of the present invention is an ion-exchangeable glass plate to be tempered, and has a glass composition, in mole %, of SiO ₂ 40 to 80%, Al ₂ O ₃ 6 to 25%, B ₂ O ₃ 0 to 10%, Li ₂ O 3 to 15%, Na ₂ O 1 to 21%, K ₂ O 0 to 10%, MgO 0 to 10%, ZnO 0 to 10%, P ₂ O ₅ 0 to 15%, and SnO ₂ 0.001 to 0.30%, in which ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86 and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al ₂ O ₃ ]+[Li ₂ O]+[Na ₂ O]+[K ₂ O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40.

The glass plate to be tempered of the present invention is an ion-exchangeable glass plate to be tempered, and contains, in mole percent, 40 to 80% SiO ₂ , 6 to 25% Al ₂ O ₃ , 0 to 10% B ₂ O ₃ , 3 to 15% Li ₂ O , 1 to 21% Na ₂ O , 0 to 10% K ₂ O , 0 to 10% MgO, 0 to 10% ZnO, 0 to 15% P ₂ O , _0.001 to 0.045% SnO ₂ , and 0.02 to 0.3% Cl, and the relationship ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86 is satisfied, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al ₂ O ₃ ]+[Li ₂ O]+[Na ₂ O]+[K ₂ O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40.

The glass plate to be tempered of the present invention is an ion-exchangeable glass plate to be tempered, and has a glass composition, in mole %, of SiO ₂ 40 to 80%, Al ₂ O ₃ 6 to 25%, B ₂ O ₃ 0.1 to 3%, Li ₂ O 3 to 15%, Na ₂ O 1 to 21%, K ₂ O 0 to 10%, MgO 0 to 10%, ZnO 0 to 10%, P ₂ O ₅ 0 to 15%, SnO ₂ 0.001 to 0.30%, and Cl 0.02 to 0.3%, in which ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al ₂ O ₃ ]+[Li ₂ O]+[Na ₂ O]+[K ₂ O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40.

FIG. 1 is an explanatory diagram illustrating a stress profile having a first peak, a second peak, a first bottom, and a second bottom. 10 is a stress profile of a strengthened glass sheet according to Example 3. 10 is a stress profile of a strengthened glass sheet according to Example 3.

The tempered glass plate (glass plate to be tempered) of the present invention is a tempered glass plate having a compressive stress layer on a surface thereof, and contains, in mol %, 40 to 80% SiO ₂ , 6 to 25% Al ₂ O ₃ , 0 to 10% B ₂ O ₃ , 3 to 15% Li ₂ O , ₁ to 21% Na 2 O , 0 to 10% K ₂ O , 0 to 10% MgO, 0 to 10% ZnO, 0 to 15% P ₂ O ₅ , and 0.001 to 0.30% SnO ₂ , and satisfies ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[ _SnO2 ])×([ _Al2O3 ]+[ _Li2O ]+[ _Na2O ]+[ _K2O ]+[ _MgO ]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40. The reasons for limiting the content range of each component are as follows. In the explanation of the content range of each component, % refers to mol % unless otherwise specified.

_SiO2 is a component that forms the glass network. If the _SiO2 content is too low, vitrification becomes difficult, and the thermal expansion coefficient becomes too high, which tends to reduce thermal shock resistance. Therefore, the preferred lower limit range of _SiO2 is 40% or more, 45% or more, 50% or more, 55% or more, 57% or more, and particularly 59% or more. On the other hand, if the _SiO2 content is too high, meltability and formability tend to decrease, and the thermal expansion coefficient becomes too low, making it difficult to match the thermal expansion coefficient of surrounding materials. Therefore, the preferred upper limit range of _SiO2 is 80% or less, 70% or less, 68% or less, 66% or less, 65% or less, 64.5% or less, 64% or less, 63% or less, and particularly 62% or less.

Al ₂ O ₃ is a component that enhances ion exchange performance, and also enhances strain point, Young's modulus, fracture toughness, and Vickers hardness. Therefore, the preferred lower limit range of Al ₂ O ₃ is 6% or more, 7% or more, 8% or more, 10% or more, 12% or more, 13% or more, 14% or more, 14.4% or more, 15% or more, 15.3% or more, 15.6% or more, 16% or more, 16.5% or more, 17% or more, 17.2% or more, 17.5% or more, 17.8% or more, 18% or more, more than 18%, 18.3% or more, particularly 18.5% or more, 18.6% or more, 18.7% or more, and 18.8% or more. On the other hand, if the Al ₂ O ₃ content is too high, the high-temperature viscosity increases, and meltability and moldability tend to deteriorate. Furthermore, devitrification crystals are likely to precipitate in the glass, making it difficult to form it into a plate by the overflow downdraw method or the like. In particular, when an alumina-based refractory is used as the formed refractory and the glass is formed into a plate by the overflow downdraw method, devitrification crystals of spinel are likely to precipitate at the interface with the alumina-based refractory. Furthermore, acid resistance is also reduced, making it difficult to apply to an acid treatment process. Therefore, _the preferred upper limit range of _Al2O3 is 25% or less, 21% or less, 20.5% or less, 20% or less, 19.9% or less, 19.5% or less, 19.0% or less, particularly 18.9% or less. Setting the content of _Al2O3 , which has a significant effect on ion _exchange performance, within a preferred range makes it easier to form a profile having a first peak, a second peak, a first bottom, and a second bottom.

B ₂ O ₃ is a component that reduces the high-temperature viscosity and density, stabilizes the glass, makes it difficult for crystals to precipitate, and lowers the liquidus temperature. Furthermore, it is a component that increases the binding force of oxygen electrons by cations and reduces the basicity of the glass. If the _{B 2} O ₃ content is too low, the stress depth in the ion exchange between Li ions contained in the glass and Na ions in the molten salt becomes too deep, and as a result, the compressive stress value (CS _Na ) of the compressive stress layer tends to become small. Furthermore, there is a risk that the glass becomes unstable and the devitrification resistance decreases. Furthermore, if the basicity of the glass becomes too high, the amount of O ₂ released by the reaction of the fining agent decreases, the foaming property decreases, and bubbles may remain in the glass when formed into a plate. Therefore, the preferable _lower limit range of _B2O3 is 0% or more, 0.10% or more, 0.12% or more, 0.15% or more, 0.18% or more, 0.20% or more, 0.23% or more, 0.25% or more, 0.27% or more, 0.30% or more, 0.35% or more, particularly 0.4% or more. On the other hand, if _{the B2O3} content is too high, the stress depth may become shallow. In particular, the efficiency of ion exchange between Na ions contained in the glass and K ions in the molten salt is likely to decrease, and the stress depth of the compressive stress layer (DOL_ZERO _K ) is likely to become small. Therefore, the preferred upper limit range _{of B2O3} is 10% or less, 5% or less, 4% or less, 3.8% or less, 3.5% or less, 3.3% or less, 3.2% or less, 3.1% or less, 3% or less, 2.9% or less, 2.8% or less, 2.5% or less, 2.0% or less, 1.5% or less, 1.0% or less, less than 1.0%, 0.8% or less, and particularly 0.5% _{or less. If the B2O3 content} is within the preferred range, it becomes easier to form a profile having a first peak, a second peak, a first bottom, and a second bottom.

Alkali metal oxides are ion-exchange components that reduce high-temperature viscosity and improve melting and moldability. However, if the alkali metal oxide content ([Li ₂ O] + [Na ₂ O] + [K ₂ O]) is too high, the thermal expansion coefficient may increase. Furthermore, acid resistance may decrease. Therefore, the preferred lower limit of the alkali metal oxide content ([Li ₂ O] + [Na ₂ O] + [K ₂ O]) is 10% or more, 11% or more, 12% or more, 13% or more, 14% or more, 14.2% or more, 14.5% or more, 14.8% or more, 15% or more, 15.2% or more, 15.5% or more, 15.8% or more, particularly 16% or more. The preferred upper limit is 25% or less, 23% or less, 20% or less, 19% or less, particularly 18% or less.

Li ₂ O is an ion-exchange component, particularly an essential component for ion-exchanging Li ions contained in the glass with Na ions in the molten salt to obtain a deep stress depth. Li ₂ O also reduces high-temperature viscosity, improves meltability and formability, and increases Young's modulus. Therefore, the preferred lower limit of Li ₂ O is 3% or more, 4% or more, 5% or more, 5.5% or more, 6.5% or more, 7% or more, 7.3% or more, 7.5% or more, 7.8% or more, particularly 8% or more. Therefore, the preferred upper limit of Li ₂ O is 15% or less, 13% or less, 12% or less, 11.5% or less, 11% or less, 10.5% or less, less than 10%, 9.9% or less, 9% or less, particularly 8.9% or less.

Na ₂ O is an ion exchange component and a component that reduces high-temperature viscosity and improves meltability and formability. Na ₂ O also improves devitrification resistance, particularly suppressing devitrification caused by reaction with alumina-based refractories. Therefore, the preferred lower limit of Na ₂ O is 1% or more, 2% or more, 3% or more, 4% or more, 5% or more, 6% or more, 7% or more, 7.5% or more, 8% or more, 8.5% or more, 8.8% or more, and particularly 9% or more. On the other hand, if the Na ₂ O content is too high, the thermal expansion coefficient becomes too high, which tends to reduce thermal shock resistance. Furthermore, the component balance of the glass composition may be disrupted, which may actually reduce devitrification resistance. Therefore, the preferred upper limit of Na ₂ O is 21% or less, 20% or less, 19% or less, particularly 18% or less, 15% or less, 13% or less, 11% or less, and particularly 10% or less.

K ₂ O is a component that reduces high-temperature viscosity and improves meltability and moldability. However, if the K ₂ O content is too high, the thermal expansion coefficient becomes too high, which tends to reduce thermal shock resistance. In addition, the compressive stress value of the outermost surface tends to decrease. Therefore, the preferred upper limit range of K ₂ O is 10% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, particularly 1.5% or less. In addition, from the viewpoint of increasing the stress depth, the preferred lower limit range of K ₂ O is 0% or more, 0.1% or more, 0.3% or more, particularly 0.4% or more.

The preferred lower limit of ([ _Li2O ] + [ _Na2O ] + [ _K2O ])/ _{[ Al2O3} ] is preferably 0.86 or more, 0.87 or more, particularly 0.88 or more. If ([ _Li2O ] + [ _Na2O ] + [ _K2O ])/ _{[ Al2O3} ] is too small, the efficiency of ion exchange tends to decrease. On the other hand, if the molar ratio ([ _Li2O ] + [ _Na2O ] + [ _K2O ])/ _{[ Al2O3} ] is too large, the efficiency of ion exchange tends to decrease. Therefore, the preferred upper limit range of ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ] is preferably 2.0 or less, 1.8 or less, 1.7 or less, 1.6 or less, 1.5 or less, 1.4 or less, 1.3 or less, 1.2 or less, 1.1 or less, 1.0 or less, particularly 0.95 or less.

([ _SiO2 ] + _{[ B2O3} ] + [ _P2O5 ])/((100 × [ _SnO2 ]) × ([ _Li2O ] + [ _Na2O ] + [K2O] + [ _MgO ] + [CaO] + [ _SrO ] + [BaO] + [ZnO] + [ _Al2O3 ] ₎ ) is preferably 0.40 or more, 0.41 or more, 0.42 or more, 0.43 or more, 0.44 or more, 0.45 or more, 0.48 or more, 0.50 or more, 0.51 or more, 0.52 or more, 0.53 or more, 0.54 or more, particularly 0.55 or more. _{If the molar ratio ([SiO2] + [B2O3 ] +} [ _{P2O5 ]} )/((100 × [ _SnO2 ]) × ([ _Li2O ] + [ _Na2O ] + [ _K2O ] + [MgO] + [CaO] + [SrO] + [BaO] + [ZnO] + [ _Al2O3 ])) is too small, _SnO2 particles are likely to precipitate. Also, less oxygen is released from the _fining agent during melting and molding, making it more likely that bubbles will remain in the glass when it is molded into a plate. _The upper limit _{of ([SiO2] + [B2O3 ] +} [ _P2O5 ])/((100 × [ _SnO2 ]) × ([ _Li2O ] + [ _Na2O ] + [ _K2O ] + [MgO] + [CaO] + [SrO] + [BaO] + [ZnO] + [ _Al2O3 ] ₎ ) is not particularly limited, but in order to enhance clarity while suppressing devitrification, it is preferably 4.0 or less, 3.0 or less, 2.0 or less, 1.8 or less, 1.5 or less, 1.2 or less, 1.0 or less, 0.90 or less, 0.80 or less, and particularly 0.70 or less.

[Li ₂ O]/([Na ₂ O]+[K ₂ O]) is preferably 0.4 to 1.0, 0.5 to 0.9, particularly 0.6 to 0.8. If [Li ₂ O]/([Na ₂ O]+[K ₂ O]) is too small, there is a risk that the ion exchange performance will not be fully exhibited. In particular, the efficiency of ion exchange between Li ions contained in the glass and Na ions in the molten salt is likely to decrease. On the other hand, if the molar ratio [Li ₂ O]/([Na ₂ O]+[K ₂ O]) is too large, devitrification crystals are likely to precipitate in the glass, making it difficult to form it into a plate shape by the overflow downdraw method or the like. [Li ₂ O]/([Na ₂ O]+[K ₂ O]) refers to the value obtained by dividing the Li ₂ O content by the combined amount of Na ₂ O and K ₂ O.

MgO is a component that reduces high-temperature viscosity, improves meltability and formability, and increases strain point and Vickers hardness. Among alkaline earth metal oxides, it is a component that is particularly effective in improving ion exchange performance. However, if the MgO content is too high, devitrification resistance tends to decrease, and it becomes particularly difficult to suppress devitrification caused by reaction with alumina-based refractories. Therefore, the preferred MgO content is 0-10%, 0-7%, 0-5%, 0.1-3%, 0.2-2.5%, 0.3-2%, 0.4-1.5%, and especially 0.5-1.0%.

Compared to other components, CaO reduces high-temperature viscosity, improves meltability and formability, and increases strain point and Vickers hardness without reducing devitrification resistance. However, if the CaO content is too high, there is a risk of reduced ion exchange performance and deterioration of the ion exchange solution during ion exchange treatment. Therefore, the preferred upper limit range for CaO is 6% or less, 5% or less, 4% or less, 3.5% or less, 3% or less, 2% or less, 1% or less, less than 1%, 0.7% or less, 0.5% or less, 0.3% or less, 0.1% or less, 0.05% or less, and particularly 0.01% or less.

SrO and BaO are components that reduce high-temperature viscosity, improve meltability and formability, and increase the strain point and Young's modulus. However, if their content is too high, the ion exchange reaction is likely to be inhibited, the density and thermal expansion coefficient become unduly high, and the glass is prone to devitrification. Therefore, the preferred contents of SrO and BaO are 0-2%, 0-1.5%, 0-1%, 0-0.5%, and 0-0.1%, respectively, and especially 0-less than 0.1%.

ZnO is a component that enhances ion exchange performance, and is particularly effective in increasing the compressive stress value of the outermost surface. It also reduces high-temperature viscosity without reducing low-temperature viscosity. The preferred lower limit of ZnO is 0% or more, 0.1% or more, 0.3% or more, 0.5% or more, 0.7% or more, and particularly 1% or more. On the other hand, if the ZnO content is too high, the glass tends to undergo phase separation, reduce devitrification resistance, increase density, and reduce stress depth. Therefore, the preferred upper limit of ZnO is 10% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1.5% or less, 1.3% or less, 1.2% or less, and particularly 1.1% or less.

_P2O5 is a component that enhances ion exchange performance, particularly deepening the stress depth. It also improves acid resistance. Furthermore, it is a component that increases the binding force of oxygen electrons _by cations and reduces the basicity of the glass. If the _P2O5 content is too low, there is a risk that _the ion exchange performance will not be fully exhibited. In particular, the efficiency of ion exchange between Na ions contained in the glass and K ions in the molten salt is likely to decrease, and the stress depth of the compressive stress layer (DOL_ZERO _K ) is likely to decrease. Furthermore, there is a risk that the glass will become unstable and its devitrification resistance will decrease. Furthermore, if the basicity of the glass becomes too high, the amount of _O2 released by the reaction of the fining agent will decrease, reducing foamability and leaving bubbles in the glass when formed into a plate. Therefore, the preferred _lower limit range of _P2O5 is 0% or more, 0.1% or more, 0.4% or more, 0.7% or more, 1% or more, 1.2% or more, 1.4% or more, 1.6% or more, 2% or more, 2.3% or more, 2.5% or more, 2.6% or more, 2.7% or more, 2.8% or more, 2.9% or more, 3.0% or more, 3.2% or more, 3.5% or more, 3.8% or more, 3.9% or more, 4.0% or more, 4.1% or more, 4.2% or more, 4.3% or more, 4.4% or more, 4.5% or more, and particularly 4.6% or more. On the other hand, if _{the P2O5} content is too high, the glass is likely to undergo phase separation or the water resistance is likely to decrease. Furthermore, the stress depth due to ion exchange between Li ions contained in the glass and Na ions in the molten salt becomes too deep, resulting in a small compressive stress value (CS _Na ) of the compressive stress layer. Therefore, the preferred upper limit range of P ₂ O ₅ is 15% or less, 10% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4.9% or less, or 4.8% or less. If the _{P 2} O ₅ content is within the preferred range, it becomes easier to form a non-monotonic profile.

₍ [ _SiO2 ] + 1.2 × [ _P2O5 ]) - (3 × _{[ Al2O3} ] + 2 × [ _Li2O ] + 1.5 × [ _Na2O ] + [ _K2O ] + [ _B2O3 ]) is preferably -40 _% or more, -30% or more, -25% or more, -24% or more, -23% or more, -22% or more, -21% or more, -20% or more, -19% or more, and particularly -18% or more. _If ([ _SiO2 ] + 1.2 × [ _P2O5 ]) - ( ₃ × [ _Al2O3 ] + 2 × [ _Li2O ] + 1.5 × [ _Na2O ] + [ _K2O ] + _{[ B2O3} ]) is too small, acid resistance is likely to decrease. On the other hand, if ([ _SiO2 ] + 1.2 × _{[ P2O5} ]) - (3 × [ _Al2O3 ] + 2 × [ _Li2O ] + 1.5 × [ _Na2O ] + [ _K2O ] + _{[ B2O3} ]) is too large, there is a risk that the ion exchange performance will not be fully exhibited. Therefore, ([ _SiO2 ] + 1.2 × [ _P2O5 ]) - (3 × _{[ Al2O3 ]} + ₂ × [ _Li2O ] + 1.5 × [ _Na2O ] + [ _K2O ] + [ _B2O3 ]) is preferably 30 mol% or less, 20 mol% or less _, 15 mol% or less, 10 mol% or less, 5 mol% or less, and particularly 0 mol% or less. Note that ([SiO ₂ ] + 1.2 × [P ₂ O ₅ ]) - (3 × [Al ₂ O ₃ ] + 2 × [Li ₂ O] + 1.5 × [Na ₂ O] + [K ₂ O] + [B ₂ O ₃ ]) refers to the value obtained by subtracting the sum of ₃ times the content of Al ₂ O ₃ , 2 times the content of Li ₂ O, 1.5 times the content of Na ₂ O, the content of K ₂ O, and the content of B ₂ O ₃ from the sum of the content of SiO 2 _and 1.2 times the content of P ₂ O 5.

_SnO2 is a fining agent and a component that enhances ion exchange performance, but if its content is too high, devitrification resistance tends to decrease. Therefore, the preferred lower limit range for _SnO2 is 0.001% or more, 0.002% or more, 0.005% or more, 0.007% or more, particularly 0.010% or more, and the preferred upper limit range is 0.30% or less, 0.27% or less, 0.25% or less, 0.20% or less, 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.09% or less, 0.08% or less, 0.07% or less, 0.06% or less, 0.05% or less, 0.047% or less, 0.045% or less, 0.042% or less, 0.040% or less, 0.038% or less, 0.035% or less, 0.032% or less, particularly 0.030% or less.

In addition to the above ingredients, the following ingredients may also be added:

_ZrO2 is a component that increases the Vickers hardness and also increases the viscosity near the liquidus viscosity and the strain point, but if its content is too high, there is a risk that devitrification resistance will be significantly reduced. Therefore, the preferred content of _ZrO2 is 0 to 3%, 0 to 1.5%, 0 to 1%, and particularly 0 to 0.1%.

_TiO2 is a component that enhances ion exchange performance and reduces high-temperature viscosity, but if its content is too high, transparency and devitrification resistance tend to decrease. Therefore, the preferred _TiO2 content is 0 to 3%, 0 to 1.5%, 0 to 1%, 0 to 0.1%, and particularly 0.001 to 0.1%.

Cl is a fining agent. When used in combination with _SnO2 , in particular, it facilitates enlarging the bubble diameter in the glass, making it easier to exert a fining effect. Therefore, when _SnO2 and Cl are used in combination, the fining effect can be maintained even if the _SnO2 content is reduced. On the other hand, if the Cl content is too high, it is a component that has a negative impact on the environment and equipment. Therefore, the preferred lower limit range of Cl is 0% or more, 0.001% or more, 0.005% or more, 0.008% or more, 0.010% or more, 0.015% or more, 0.018% or more, 0.019% or more, 0.020% or more, 0.021% or more, 0.022% or more, 0.023% or more, 0.024% or more, 0.025% or more, 0.027% or more, 0.030% or more, 0.035% or more, 0.040% or more, 0.050% or more, 0.070% or more, 0.090% or more, particularly 0.100% or more, and the preferred upper limit range is 0.3% or less, 0.2% or less, 0.17% or less, 0.15% or less, particularly 0.12% or less.

In addition to the above, 0.001 to 1% of SO ₃ and CeO ₂ may be added as fining agents.

_Fe2O3 is _an impurity that is inevitably mixed in from raw materials. The preferred content of _Fe2O3 is less than 1000 _ppm (less than 0.1%), less than 800 ppm, less than 600 ppm, less than 400 ppm, particularly less than 300 ppm. If the content _{of Fe2O3} is too high, the transmittance of the cover glass is likely to decrease. On the other hand, the preferred lower limit range _{of Fe2O3} is 10 ppm or more, 20 ppm or more, 30 ppm or more, 50 ppm or more, 80 ppm or more, particularly 100 ppm or more. If the content of _Fe2O3 is too low, the raw material cost is likely to rise due to the use _of high-purity raw materials.

Rare earth oxides such as _{Nd2O3 , La2O3} , _{Y2O3 , Nb2O5 , Ta2O5} , and _Hf2O3 are components that increase Young's modulus. However, _the raw material cost is high, and adding a large amount of them tends to reduce devitrification resistance. Therefore, _the preferred content of rare earth oxides is 5% or less, 4% or less, 3 _% or less, 2% or less, 1% or less, 0.5% or less, and particularly 0.1% or less.

From an environmental perspective, the tempered glass plate (glass plate to be tempered) of the present invention preferably contains, as a glass composition, substantially no As ₂ O ₃ , Sb ₂ O ₃ , PbO, and F. From an environmental perspective, it is also preferable that the tempered glass plate substantially contains no Bi ₂ O _3. The phrase "substantially does not contain ..." means that the specified components are not actively added as glass components, but impurity-level addition is permitted, and specifically refers to a case where the content of the specified components is less than 0.05%.

The tempered glass sheet (glass sheet to be tempered) of the present invention preferably has the following properties:

The density is preferably 2.55 g/cm or ^less , 2.53 g/cm ^or less, 2.50 g/cm ^or less, 2.49 g/cm ^{or less} , or 2.45 g/cm ^{or less} , particularly 2.35 to 2.44 g/cm. The ^lower the density, the more lightweight the tempered glass sheet can be. The "density" can be measured by the well-known Archimedes method or the like.

The thermal expansion coefficient at 30 to 380° C. is preferably 150×10 ⁻⁷ /° C. or less, 100×10 ⁻⁷ /° C. or less, particularly 50×10 ⁻⁷ to 95×10 ⁻⁷ /° C. Note that the "thermal expansion coefficient at 30 to 380° C." refers to a value obtained by measuring the average thermal expansion coefficient using a dilatometer.

The softening point is preferably 950°C or less, 930°C or less, 920°C or less, 910°C or less, or 900°C or less, and particularly 880 to 900°C. If the softening point is too high, bending by heat treatment becomes difficult. Note that "softening point" refers to the value measured based on the method of ASTM C338.

The temperature at which the high-temperature viscosity is 10 ^2.5 dPa s is preferably 1660°C or lower, less than 1600°C, 1590°C or lower, 1580°C or lower, 1570°C or lower, or 1560°C or lower, particularly preferably 1400 to 1550°C. If the temperature at which the high-temperature viscosity is 10 ^2.5 dPa s is too high, the melting property and formability decrease, making it difficult to form the molten glass into a plate. Note that the "temperature at which the high-temperature viscosity is 10 ^2.5 dPa s" refers to a value measured by the platinum sphere pulling method.

The liquidus viscosity is preferably 10 ^3.74 dPa s or more, 10 ^4.5 dPa s or more, 10 ^4.8 dPa s or more, 10 ^4.9 dPa s or more, 10 ^5.0 dPa s or more, 10 ^5.1 dPa s or more, 10 ^5.2 dPa s or more, 10 ^5.3 dPa s or more, 10 ^5.4 dPa s or more, particularly 10 ^5.5 dPa s or more. The higher the liquidus viscosity, the better the resistance to devitrification, and the less likely devitrification particles are to occur during molding. Here, "liquidus viscosity" refers to the value of the viscosity at the liquidus temperature measured by the platinum sphere pulling method. The "liquidus temperature" is defined as the highest temperature at which devitrification (devitrification particles) is observed inside the glass when a glass powder that passes through a standard sieve of 30 mesh (500 μm) and remains on a 50 mesh (300 μm) sieve is placed in a platinum boat and held in a temperature gradient furnace for 24 hours, and then the platinum boat is removed.

The Young's modulus is preferably 70 GPa or more, 74 GPa or more, 75 to 100 GPa, and particularly 76 to 90 GPa. If the Young's modulus is low, the cover glass will be more likely to bend if the plate thickness is thin. The "Young's modulus" can be calculated using the well-known resonance method.

The tempered glass sheet of the present invention has a compressive stress layer on its surface. The compressive stress value of the outermost surface is preferably 200 MPa or more, 220 MPa or more, 250 MPa or more, 280 MPa or more, 300 MPa or more, 310 MPa or more, and particularly 320 MPa or more. The higher the compressive stress value of the outermost surface, the higher the Vickers hardness. On the other hand, if extremely large compressive stress is formed on the surface, the tensile stress inherent in the tempered glass will become extremely high, and there is a risk of significant dimensional change before and after the ion exchange treatment. For this reason, the compressive stress value of the outermost surface is preferably 1200 MPa or less, 1100 MPa or less, 1000 MPa or less, 900 MPa or less, 700 MPa or less, 680 MPa or less, 650 MPa or less, and particularly 600 MPa or less. Note that shortening the ion exchange time or lowering the temperature of the ion exchange solution tends to increase the compressive stress value of the outermost surface.

The stress depth is preferably 50 μm or more, 60 μm or more, 80 μm or more, 100 μm or more, 110 μm or more, 120 μm or more, 130 μm or more, and particularly 140 μm or more. The deeper the stress depth, the more difficult it is for road surface protrusions and sand particles to reach the tensile stress layer when the smartphone is dropped, reducing the probability of breakage of the cover glass. On the other hand, if the stress depth is too deep, there is a risk of significant dimensional change before and after the ion exchange treatment. Furthermore, the compressive stress value of the outermost surface tends to decrease. Therefore, the stress depth is preferably 200 μm or less, 180 μm or less, and particularly 170 μm or less. Note that the stress depth tends to deepen if the ion exchange time is extended or the temperature of the ion exchange solution is increased.

The compressive stress value at a depth of 2.5 μm is preferably 350 MPa or more, 360 MPa or more, 370 MPa or more, 380 MPa or more, 390 MPa or more, 400 MPa or more, 410 MPa or more, 420 MPa or more, 430 MPa or more, 440 MPa or more, 450 MPa or more, 460 MPa or more, 470 MPa or more, 480 MPa or more, 490 MPa or more, 500 MPa or more, 510 MPa or more, 520 MPa or more, 530 MPa or more, 540 MPa or more, 550 MPa or more, and particularly 600 MPa or more. The higher the compressive stress value at a depth of 2.5 μm, the higher the bending strength. On the other hand, if extremely large compressive stress is formed at a depth of 2.5 μm, the tensile stress inherent in the tempered glass sheet may become extremely high. Therefore, the compressive stress value at a depth of 2.5 μm is preferably 800 MPa or less, 750 MPa or less, 730 MPa or less, 700 MPa or less, 680 MPa or less, 650 MPa or less, 640 MPa or less, and particularly 630 MPa or less.

The average compressive stress value at a depth of 30 to 45 μm is preferably 85 MPa or more, 86 MPa or more, 87 MPa or more, 88 MPa or more, 89 MPa or more, 90 MPa or more, 92 MPa or more, 95 MPa or more, 98 MPa or more, and particularly 100 MPa or more. The higher the average compressive stress value at a depth of 30 to 45 μm, the less likely the smartphone will crack due to road surface protrusions or sand particles when dropped, reducing the probability of breakage of the cover glass. On the other hand, if the average compressive stress value at a depth of 30 to 45 μm becomes extremely high, the tensile stress inherent in the tempered glass sheet may become extremely high. For this reason, the average compressive stress value at a depth of 30 to 45 μm is preferably 150 MPa or less, 140 MPa or less, 130 MPa or less, 125 MPa or less, 120 MPa or less, 115 MPa or less, 110 MPa or less, and particularly 105 MPa or less.

The tempered glass sheet of the present invention preferably has a thickness of 2.0 mm or less, 1.5 mm or less, 1.3 mm or less, 1.1 mm or less, 1.0 mm or less, 0.9 mm or less, and particularly 0.8 mm or less. The smaller the thickness, the lighter the tempered glass sheet can be. On the other hand, if the thickness is too thin, it becomes difficult to obtain the desired mechanical strength. Therefore, the thickness is preferably 0.3 mm or more, 0.4 mm or more, 0.5 mm or more, 0.6 mm or more, and particularly 0.7 mm or more.

The method for manufacturing a tempered glass sheet of the present invention is characterized by comprising: a preparation step of preparing a glass sheet to be tempered that has the above-described glass composition; and an ion exchange step of subjecting the glass sheet to be tempered to multiple ion exchange treatments to obtain a tempered glass sheet having a compressive stress layer on its surface. While the method for manufacturing a tempered glass sheet of the present invention is characterized by performing multiple ion exchange treatments, the tempered glass sheet of the present invention encompasses not only glass sheets that have undergone multiple ion exchange treatments, but also glass sheets that have undergone only one ion exchange treatment.

The method for producing tempered glass according to the present invention is, for example, as follows. First, glass raw materials prepared to obtain the desired glass composition are charged into a continuous melting furnace, where they are heated and melted at 1400 to 1700°C. After clarification, the molten glass is preferably supplied to a forming device, formed into a sheet, and cooled. After forming into a sheet, the glass can be cut to the desired dimensions using well-known methods.

The overflow downdraw method is a preferred method for forming molten glass into a sheet. In the overflow downdraw method, the surface that will become the glass sheet does not come into contact with the surface of the refractory molding, and is formed into a sheet in a free surface state. This makes it possible to inexpensively produce unpolished glass sheets with good surface quality. Furthermore, in the overflow downdraw method, alumina-based refractories or zirconia-based refractories are used as the refractory molding. The tempered glass sheet (glass sheet to be tempered) of the present invention has good compatibility with alumina-based refractories and zirconia-based refractories (particularly alumina-based refractories), and therefore is less likely to react with these refractories to produce bubbles, pimples, etc.

In addition to the overflow downdraw method, various other forming methods can be used. For example, the float method, downdraw method (slot downdraw method, redraw method, etc.), rollout method, press method, etc. can be used.

When forming the molten glass, it is preferable to cool the molten glass through the temperature range between the annealing point and the strain point at a cooling rate of at least 3°C/min and less than 1000°C/min. The lower limit of the cooling rate is preferably at least 10°C/min, at least 20°C/min, at least 30°C/min, and particularly at least 50°C/min, and the upper limit is preferably less than 1000°C/min, less than 500°C/min, and particularly less than 300°C/min. If the cooling rate is too fast, the glass structure will become coarse, making it difficult to increase the Vickers hardness after ion exchange treatment. On the other hand, if the cooling rate is too slow, the production efficiency of glass sheets will decrease.

In the method for producing a tempered glass sheet of the present invention, multiple ion exchange treatments are performed. Preferably, the multiple ion exchange treatments include immersion in a molten salt containing _KNO3 , followed by immersion in a molten salt containing _NaNO3 . This allows for an increase in the compressive stress value of the outermost surface while ensuring a deep stress depth.

In particular, in the method for producing a tempered glass sheet of the present invention, it is preferable to perform an ion exchange treatment (first ion exchange step) by immersing the glass in a _NaNO3 molten salt or a mixed molten salt _{of NaNO3} and _KNO3 , followed by an ion exchange treatment (second ion exchange step) by immersing the glass in a mixed molten salt of _KNO3 and LiNO3. This makes it possible to form the non-monotonic stress profile shown in Figure 1, i.e., a stress profile having at least a first peak, a second peak, a first bottom, and a second bottom. As a result, it is possible to significantly reduce the probability of breakage of the cover glass when the smartphone is dropped.

In the first ion exchange process, Li ions contained in the glass undergo ion exchange with Na ions in the molten salt. When a mixed molten salt of _NaNO3 and _KNO3 is used, Na ions contained in the glass also undergo ion exchange with K ions in the molten salt. Here, the ion exchange between Li ions contained in the glass and Na ions in the molten salt is faster and more efficient than the ion exchange between Na ions contained in the glass and K ions in the molten salt. In the second ion exchange process, Na ions in the vicinity of the glass surface (a shallow region from the outermost surface to 20% of the plate thickness) undergo ion exchange with Li ions in the molten salt. Additionally, Na ions in the vicinity of the glass surface (a shallow region from the outermost surface to 20% of the plate thickness) also undergo ion exchange with K ions in the molten salt. That is, in the second ion exchange process, Na ions in the vicinity of the glass surface can be removed while K ions with a large ionic radius can be introduced. As a result, the compressive stress value of the outermost surface can be increased while maintaining a deep stress depth.

In the first ion exchange process, the temperature of the molten salt is preferably 360 to 400°C, and the ion exchange time is preferably 30 minutes to 6 hours. In the second ion exchange process, the temperature of the ion exchange solution is preferably 370 to 400°C, and the ion exchange time is preferably 15 minutes to 3 hours.

In order to form a non-monotonic stress profile, it is preferable that the NaNO3 concentration be higher than the _KNO3 concentration in the mixed molten salt of _NaNO3 and _KNO3 used in the first ion exchange step, and it is preferable that the _KNO3 concentration be higher than _{the LiNO3} concentration in the mixed molten salt of _KNO3 and _LiNO3 used in the second ion exchange step.

In the mixed molten salt of _NaNO3 and _KNO3 used in the first ion exchange step, the _KNO3 concentration is preferably 0 mass% or more, 0.5 mass% or more, 1 mass% or more, 5 mass% or more, 7 mass% or more, 10 mass% or more, 15 mass% or more, and particularly 20 to 90 mass%. If the _KNO3 concentration is too high, the compressive stress value formed during ion exchange between Li ions contained in the glass and Na ions in the molten salt may be too low. On the other hand, if the _KNO3 concentration is too low, stress measurement using a surface stress meter may become difficult.

In the mixed molten salt of _KNO3 and _LiNO3 used in the second ion exchange step, the _LiNO3 concentration is preferably greater than 0 to 5 mass%, greater than 0 to 3 mass%, greater than 0 to 2 mass%, and particularly 0.1 to 1 mass%. If the _LiNO3 concentration is too low, Na ions near the glass surface are less likely to be released. On the other hand, if the _LiNO3 concentration is too high, there is a risk that the compressive stress value formed by ion exchange between Na ions near the glass surface and K ions in the molten salt will be too low.

The present invention will be described below based on examples. Please note that the following examples are merely illustrative. The present invention is not limited to the following examples in any way.

Table 1 shows the glass compositions and glass properties of examples of the present invention (samples Nos. 1 to 8 and 12), and Table 2 shows the glass compositions and glass properties of comparative examples of the present invention (samples Nos. 9 to 11). In the table, "N.A." means not measured, "(Li ₂ O + Na ₂ O + K ₂ O)/Al ₂ O ₃ " means the molar ratio ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ], and "(Si + P + B)/((100Sn) × (Al + Li + Na + K + Mg + Ca + Sr + Ba + Zn))" means the molar ratio ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100 × [SnO ₂ ]) × ([Al ₂ O ₃ ] + [Li 2 O] + [Na ₂ O] + [K ₂ O _]) . O] + [MgO] + [CaO] + [SrO] + [BaO] + [ZnO]).

Each sample in the table was prepared as follows. First, glass raw materials were mixed to achieve the glass composition shown in the table, and melted in a platinum pot at 1600°C for 21 hours. The resulting molten glass was then poured onto a carbon plate and formed into a flat plate. It was then cooled at 3°C/min through the temperature range from the annealing point to the strain point to obtain a glass plate (glass plate to be tempered). The surface of the resulting glass plate was optically polished to a thickness of 1.5 mm, and various properties were evaluated.

Density (ρ) is a value measured using the well-known Archimedes method.

The thermal expansion coefficient in the range of 30 to 380° C. (α _{30-380° C.} ) is a value obtained by measuring the average thermal expansion coefficient using a dilatometer.

The temperature at which the high-temperature viscosity is 10 ^2.5 dPa·s (10 ^2.5 dPa·s) is a value measured by the platinum sphere pull-up method.

The softening point (Ts) was measured according to the ASTM C338 method.

The liquidus temperature (TL) was determined by placing glass powder that passed through a standard 30 mesh (500 μm) sieve and remained on the 50 mesh (300 μm) sieve in a platinum boat and holding it in a temperature gradient furnace for 24 hours. The platinum boat was then removed and observed under a microscope. This was the highest temperature at which devitrification (devitrification particles) was observed inside the glass. The liquidus viscosity (logη at TL) was the viscosity at the liquidus temperature measured using the platinum sphere pull-up method, and is expressed as logη by taking the logarithm.

The acid resistance test was carried out by using a glass sample having dimensions of 50 × 10 × 1.0 mm and mirror polished on both sides as a measurement sample, thoroughly washing it with a neutral detergent and pure water, and then immersing it in a 5% by mass HCl aqueous solution heated to 80°C for 24 hours, and calculating the mass loss per unit surface area (mg/cm ² ) before and after immersion.

The alkali resistance test was carried out by using a glass sample having dimensions of 50 × 10 × 1.0 mm and mirror polished on both sides as a measurement sample, thoroughly washing it with a neutral detergent and pure water, and then immersing it in a 5 mass % NaOH aqueous solution heated to 80°C for 6 hours, and calculating the mass loss per unit surface area (mg/cm ² ) before and after immersion.

Young's modulus (E) was calculated using a method in accordance with JIS R1602-1995 "Testing method for elastic modulus of fine ceramics."

Subsequently, each glass plate was immersed in a KNO ₃ molten salt at 430 ° C. for 4 hours to perform an ion exchange treatment to obtain a tempered glass plate having a compressive stress layer on the surface. After cleaning the glass surface, the compressive stress value (CS _K ) and stress depth (DOL_ZERO K ) of the compressive stress layer on the outermost surface were calculated from the number and spacing of interference fringes observed using a surface stress meter FSM- ₆₀₀₀ (manufactured by Orihara Seisakusho Co., Ltd.). Here, DOL_ZERO _K is the depth at which the compressive stress value becomes zero. In calculating the stress characteristics, the refractive index of each sample was 1.51 and the optical elastic constant was 29.0 [(nm / cm) / MPa].

In addition, each glass plate was immersed in a 380 ° C. NaNO ₃ molten salt for 1 hour to perform an ion exchange treatment to obtain a tempered glass plate. After cleaning the glass surface, the outermost surface compressive stress value (CS _Na ) and stress depth (DOL_ZERO _Na ) were calculated from the phase difference distribution curve observed using a scattered light photoelastic stress meter SLP-1000 (manufactured by Orihara Seisakusho Co., Ltd.). Here, DOL_ZERO _Na is the depth at which the stress value becomes zero. In calculating the stress characteristics, the refractive index of each sample was 1.51 and the photoelastic constant was 29.0 [(nm / cm) / MPa].

Furthermore, each glass plate was crushed and classified into sizes between 2 and 5.6 mm, and then heated to 1650°C. The molten glass was directly observed (High Temperature Observation; HTO). If no bubbles of 75 μm or larger were observed, the clarity was rated as "Good." Otherwise, the clarity was rated as "Good."

As is clear from Table 1, samples No. 1 to 8 and No. 12 had a large molar ratio ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ], and therefore when subjected to ion exchange treatment with KNO ₃ molten salt, the compressive stress value (CS _K ) of the compressive stress layer was 1090 MPa or more, and when further subjected to ion exchange treatment with NaNO ₃ molten salt, the compressive stress value (CS _Na ) of the outermost compressive stress layer was 279 MPa or more.

As is clear from Table 1, Samples No. 1 to 8 and No. 12 had a molar ratio ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100 × [SnO ₂ ]) × ([Al ₂ O ₃ ] + [Li ₂ O] + [Na ₂ O] + [K ₂ O] + [MgO] + [CaO] + [SrO] + [BaO] + [ZnO])) of 0.40 or more, and therefore were evaluated as having good clarity.

On the other hand, as is clear from Table 2, Samples No. 9 and 10 had a molar ratio ([ _Li2O ] + [ _Na2O ] + [ _K2O ])/ _{[ Al2O3} ] of less than 0.86 _, and therefore had lower compressive stress values (CS _K ) of _the compressive stress layer than the samples of the Examples. Also, Sample No. 11 had a molar ratio ([ _SiO2 ] + [ _B2O3 ] + [ _P2O5 ])/((100 × [ _SnO2 ]) × ([ _Al2O3 ] + [ _Li2O ] + [ _Na2O ] + [ _K2O ] + [ _MgO ] + [CaO] + [SrO] + [BaO] + [ZnO])) of less than 0.40, and therefore the evaluation of clarity was poor.

First, glass raw materials were mixed to obtain the glass compositions of Samples No. 1 and No. 6 in Table 1, and melted in a platinum pot at 1600°C for 21 hours. The resulting molten glass was then poured onto a carbon plate and formed into a flat plate. It was then cooled at 3°C/min through the temperature range from the annealing point to the strain point to obtain a glass plate (glass plate to be tempered). The surface of the resulting glass plate was optically polished to a thickness of 0.7 mm.

The obtained tempered glass sheet was subjected to an ion exchange treatment by immersing it in a NaNO ₃ molten salt (NaNO ₃ concentration 100% by mass) at 380 ° C for 3 hours, and then immersed in a mixed molten salt of KNO ₃ and LiNO ₃ (LiNO ₃ concentration 2.5% by mass) at 380 ° C for 75 minutes. Furthermore, after cleaning the surface of the obtained tempered glass sheet, the stress profile of the tempered glass sheet was measured using a scattered light photoelastic stress meter SLP-1000 (manufactured by Orihara Corporation) and a surface stress meter FSM-6000 (manufactured by Orihara Corporation). In both cases, a non-monotonic stress profile similar to that shown in FIG. 1, that is, a stress profile having a first peak, a second peak, a first bottom, and a second bottom, was obtained.

First, glass raw materials were mixed to obtain the glass compositions of Samples No. 6, 9, and 12 in Table 1, and melted in a platinum pot at 1600°C for 21 hours. The resulting molten glass was then poured onto a carbon plate and formed into a flat plate. It was then cooled at 3°C/min through the temperature range from the annealing point to the strain point to obtain a glass plate (glass plate to be tempered). The surface of the resulting glass plate was optically polished to a thickness of 0.8 mm.

The obtained tempered glass sheet was subjected to an ion exchange treatment by immersing it in a mixed molten salt of _KNO3 and _NaNO3 ( _NaNO3 concentration 60% by mass) at 380 ° C for 3 hours, and then ion exchange treatment (condition A) was performed by immersing it in a mixed molten salt of _KNO3 and _LiNO3 ( _LiNO3 concentration 1.0% by mass) at 380 ° C for 30 minutes. Furthermore, after cleaning the surface of the obtained tempered glass sheet, the stress profile of the tempered glass sheet was measured using a scattered light photoelastic stress meter SLP-1000 (manufactured by Orihara Corporation) and a surface stress meter FSM-6000 (manufactured by Orihara Corporation). In both cases, the non-monotonic stress profile shown in Figure 2 was obtained.

The obtained tempered glass plate was subjected to an ion exchange treatment by immersing it in a mixed molten salt of _KNO3 and _NaNO3 ( _NaNO3 concentration 60% by mass) at 380 ° C. for 3 hours, and then immersed in a mixed molten salt _of KNO3, _NaNO3 and _LiNO3 ( _NaNO3 concentration 4.0% by mass, _LiNO3 concentration 1.0% by mass) at 380 ° C. for 45 minutes to perform an ion exchange treatment (condition B). Furthermore, after cleaning the surface of the obtained tempered glass plate, the stress profile of the tempered glass plate was measured using a scattered light photoelastic stress meter SLP-1000 (manufactured by Orihara Seisakusho Co., Ltd.) and a surface stress meter FSM-6000 (manufactured by Orihara Seisakusho Co., Ltd.). In both cases, the non-monotonic stress profile shown in FIG. 3 was obtained.

Table 3 shows the compressive stress value (CS) at the outermost surface of the stress profile of each sample, the stress depth (DOC), the compressive stress value at a depth of 2.5 μm (CS _2.5 ), and the average compressive stress value at a depth of 30 to 45 μm (CS _30-45 ).

As is clear from Figures 3 and 4 and Table 3, Samples No. 6 and 12 are considered to have high bending strength and high drop strength because _CS2.5 is 350 MPa or more and _CS30-45 is 85 MPa or more in the stress profiles after ion exchange under Conditions A and B. On the other hand, Sample No. 9 is considered to have low drop strength because _CS30-45 is less than 85 MPa in the stress profiles after ion exchange under Conditions A and B.

The tempered glass sheet of the present invention is suitable as a cover glass for touch panel displays in mobile phones, digital cameras, PDAs (personal digital assistants), and the like. In addition to these applications, the tempered glass sheet of the present invention is also expected to be used in applications requiring high mechanical strength, such as window glass, magnetic disk substrates, flat panel display substrates, flexible display substrates, solar cell cover glass, solid-state imaging device cover glass, and automotive cover glass.

Claims (18)

A tempered glass plate having a compressive stress layer on a surface thereof contains, in mole percent, 40 to 80% SiO ₂ , 16 to 25% Al ₂ O ₃ , 0.1 to 10% B ₂ O ₃ , 3 to 15% Li ₂ O , 1 to 21% Na ₂ O , 0 to 10% K ₂ O , 0 to 10% MgO , 0 to 10% ZnO , ₀ to 15% P ₂ O , 0.001 to 0.30% SnO ₂ , and 0.02 to 0.3% Cl, wherein ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al ₂ O ₃ ]+[Li ₂ O]+[Na ₂ O]+[K ₂ O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40, and the stress depth of the compressive stress layer is 50 to 200 μm. 2. The tempered glass sheet according to claim 1, wherein the content of B ₂ O ₃ is 0.1 to 3 mol %. 3. The tempered glass sheet according to claim 1, wherein the _SnO2 content is 0.045 mol% or less. The tempered glass sheet according to any one of claims 1 to 3, characterized in that the Cl content is 0.100 to 0.3 mol%. A tempered glass plate having a compressive stress layer on its surface contains, as a glass composition, in mole %, 40 to 80% SiO ₂ , 16 to 25% Al ₂ O ₃ , 0.1 to 10% B ₂ O ₃ , 3 to 15% Li ₂ O , 1 to 21% Na ₂ O , 0 to 10% K ₂ O , 0 to 10% MgO , 0 to 10% ZnO , ₀ to 15% P ₂ O , 0.001 to 0.045% SnO ₂ , and 0.02 to 0.3% Cl, and the relationship between the mole fractions of the tempered glass and the compressive stress layer is expressed as ([SiO ₂ ] + [B ₂ O ₃ ] + [P _{2 O} ])/((100 × [SnO ₂ ]) × ([Al ₂ O ₃ ] + [Li ₂ O] + [Na ₂ O] + [K ₂ O] + [MgO] + [CaO] + [SrO] + [BaO] + [ZnO])) ≥ 0.40, ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ] ≥ 0.86, and the stress depth of the compressive stress layer is 50 to 200 μm. A tempered glass plate having a compressive stress layer on a surface thereof contains, in mole percent, 40 to 80% SiO ₂ , 16 to 25% Al ₂ O ₃ , 0.1 to 3% B ₂ O ₃ , 3 to 15% Li ₂ O , 1 to 21% Na ₂ O , 0 to 10% K ₂ O , 0 to 10% MgO , 0 to 10% ZnO , ₀ to 15% P ₂ O , 0.001 to 0.30% SnO ₂ , and 0.02 to 0.3% Cl, and the relationship ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al _{2 O} ] is ≧0.86, and ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al ₂ O ₃ ]+[Li ₂ O]+[Na ₂ O]+[K ₂ O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40, and the stress depth of the compressive stress layer is 50 to 200 μm. The tempered glass plate according to any one of claims 1 to 6, characterized in that the P ₂ O ₅ content is 2.5 mol % or more. The tempered glass sheet according to any one of claims 1 to 7, characterized in that the Fe ₂ O ₃ content is 0.001 to 0.1 mol%. The tempered glass plate according to any one of claims 1 to 8, characterized in that the content of _TiO2 is 0.001 to 0.1 mol%. The tempered glass sheet according to any one of claims 1 to 9, characterized in that the compressive stress value of the outermost surface of the compressive stress layer is 200 to 1200 MPa. The tempered glass sheet according to any one of claims 1 to 10, characterized in that the stress depth of the compressive stress layer is 100 to 200 μm. The tempered glass sheet according to any one of claims 1 to 11, characterized in that the compressive stress value at a depth of 2.5 μm is 350 MPa or more. The tempered glass sheet according to any one of claims 1 to 12, characterized in that the average compressive stress value at a depth of 30 to 45 μm is 85 MPa or more. The tempered glass sheet according to any one of claims 1 to 13, characterized in that the temperature at a high-temperature viscosity of 10 ^2.5 dPa·s is lower than 1650°C. The tempered glass sheet according to any one of claims 1 to 14, characterized in that it has an overflow confluence surface at the center in the sheet thickness direction. The tempered glass sheet according to any one of claims 1 to 15, characterized in that it is used as a cover glass for a touch panel display. The tempered glass sheet according to any one of claims 1 to 16, characterized in that the stress profile in the thickness direction has at least a first peak, a second peak, a first bottom, and a second bottom. The glass composition contains, in mole percent, SiO ₂ 40-80%, Al ₂ O ₃ 16-25 %, B ₂ O ₃ 0.1-10 %, Li ₂ O 3-15%, Na 2 O 1-21%, K _{2 O} 0-10%, MgO 0-10%, ZnO 0-10%, P ₂ O ₅ 0-15%, SnO ₂ 0.001-0.30%, and Cl 0.02-0.3%, and the relationship ([Li ₂ O] + [Na ₂ O] + [K ₂ O])/[Al ₂ O ₃ ]≧0.86 is satisfied, and the relationship ([SiO ₂ ] + [B ₂ O ₃ ] + [P ₂ O ₅ ])/((100×[SnO ₂ ])×([Al ₂ O ₃ ]+[Li ₂ O]+[Na ₂ O]+[K ₂ O]+[MgO]+[CaO]+[SrO]+[BaO]+[ZnO]))≧0.40; and an ion exchange process of performing ion exchange treatment on the glass plate to be tempered multiple times to obtain a tempered glass plate having a compressive stress layer on its surface with a stress depth of 50 to 200 μm.