Chemically strengthened glass and method for manufacturing chemically strengthened glass

1. A chemically tempered glass, comprising:

a compressive stress layer having a compressive stress of 20 MPa or more continuously from a main surface in a depth direction thereof;

a tensile stress layer that is arranged on an inner side with respect to the compressive stress layer in a sheet thickness direction and has a tensile stress of 20 MPa or more continuously in a depth direction thereof; and

a stress-neutral layer arranged between the compressive stress layer and the tensile stress layer,

wherein the stress-neutral layer

has a compressive stress of less than 20 MPa and/or a tensile stress of less than 20 MPa continuously in the sheet thickness direction, and

has a thickness of 5.3% or more of a sheet thickness, and

wherein the chemically tempered glass is prepared by a process comprising:

(1) immersing a glass to be tempered containing an alkali metal as a composition in a first molten salt to subject the glass to a first ion exchange treatment; and

(2) immersing the glass in a second molten salt to subject the glass to a second ion exchange treatment to provide the chemically tempered glass,

wherein a treatment time of the first ion exchange treatment is 3 or more times as long as a treatment time of the second ion exchange treatment, and the treatment time of the first ion exchange treatment is 112 hours or more.

2. The chemically tempered glass according to claim 1 , wherein the stress-neutral layer further comprises a neutral core layer that continuously has a compressive stress 0.01 or less times as large as a maximum compressive stress value of the compressive stress layer and/or a tensile stress 0.1 or less times as large as a maximum tensile stress value of the tensile stress layer, and that has a thickness of 2.5% or more of the sheet thickness.

3. The chemically tempered glass according to claim 1 , wherein the thickness of the stress-neutral layer is 25% or less of the sheet thickness.

4. The chemically tempered glass according to claim 1 , wherein the stress-neutral layer extends from a position equal to or shallower than 8% of the sheet thickness toward a center of the sheet thickness.

5. The chemically tempered glass according to claim 1 , wherein the tensile stress layer extends from a position equal to or deeper than 19% of the sheet thickness with respect to the main surface toward a region including a central portion of the sheet thickness.

6. The chemically tempered glass according to claim 1 ,

wherein the chemically tempered glass has a thickness of 1.0 mm or less,

wherein the compressive stress layer has a maximum compressive stress of 600 MPa or more, and

wherein the tensile stress layer has a maximum tensile stress of from 20 MPa to 72 MPa.

7. The chemically tempered glass according to claim 1 , wherein the stress-neutral layer has a compressive stress of from 0 MPa to 20 MPa continuously over a thickness of 3% or more of a sheet thickness T.

8. The chemically tempered glass according to claim 1 ,

wherein the stress-neutral layer comprises

a low-compressive stress layer having a compressive stress, and

a low-tensile stress layer having a tensile stress, and

wherein the low-compressive stress layer and the low-tensile stress layer are alternately formed.

9. The chemically tempered glass according to claim 1 , wherein, when a compressive stress change amount per unit depth in a case where a stress change in the compressive stress layer in the depth direction is linearly approximated by using a least-squares method is represented by A1 (Mpa/μm), and a compressive stress change amount per unit depth in a case where a stress change in the stress-neutral layer in a depth direction thereof is linearly approximated by using the least-squares method is represented by A2 (Mpa/μm), a relationship of A1/A2>210 is satisfied.

10. The chemically tempered glass according to claim 1 , wherein, when a position in the depth direction between the compressive stress layer and the tensile stress layer at which a stress becomes zero is represented by DOL_zero, and a stress change amount per unit depth in a case where a tensile stress change from the position DOL_zero to a depth of DOL_zero+15 (μm) is linearly approximated by using a least-squares method is represented by B, a relationship of B≥−0.90 is satisfied.

11. The chemically tempered glass according to claim 1 , wherein the chemically tempered glass comprises the compressive stress layer and the stress-neutral layer on each side of front and back main surfaces thereof.

12. The chemically tempered glass according to claim 1 , wherein the chemically tempered glass comprises as a glass composition, in terms of mol %, 50% to 80% of SiO 2 , 5% to 30% of Al 2 O 3 , 5% to 25% of Na 2 O, 0% to 10% of Li 2 O, and 0% to 30% of MgO.

13. The chemically tempered glass according to claim 12 , wherein the chemically tempered glass further comprises as a glass composition, in terms of mol %, 0.01% to 10% of P 2 O 5 and 0.01% to 3% of SnO 2 .

14. A method of producing the chemically tempered glass according to claim 1 , comprising:

immersing a glass to be tempered containing an alkali metal as a composition in a first molten salt to subject the glass to first ion exchange treatment; and

then immersing the glass in a second molten salt to subject the glass to second ion exchange treatment to provide the chemically tempered glass,

wherein the first molten salt comprises a molten salt containing 18,500 ppm or more of a desorbed ion desorbed from the glass in the first ion exchange treatment,

wherein a concentration of the desorbed ion in the second molten salt is less than a concentration of the desorbed ion in the first molten salt, and

wherein a treatment time of the first ion exchange treatment is 3 or more times as long as a treatment time of the second ion exchange treatment.

15. The method of producing the chemically tempered glass according to claim 14 ,

wherein the desorbed ion comprises a sodium ion, and

wherein the concentration of the desorbed ion in the second molten salt is less than 1,000 ppm.