Strengthened glass and methods for making using heat treatment

1. A method for making a strengthened substrate, the method comprising:

providing a float glass substrate, having dimensions including a width ≦ about 25.0 millimeters, characterized by having a chemical composition and a glass chemical structure including host alkali ions having an average ionic radius situated in the glass chemical structure and dimensional volumes including a treatment-rich volume and a treatment-poor volume located as opposed to each other in the provided substrate;

determining a heat-treating temperature for producing reduced induced curvature in the substrate after chemical strengthening based, at least in part, on a maximum heat-treating temperature associated with the substrate and the chemical composition;

determining a heat-treating period for producing reduced induced curvature in the substrate after chemical strengthening based, at least in part, on the heat-treating temperature and the maximum heat-treating temperature;

heating the provided substrate in a heat treatment to the heat-treating temperature ≧about 250° C. and ≦ a maximum heat-treating temperature for the heat-treating period of about 0.001-50 hours, wherein if the heat-treating temperature is ≦ about 325° C., then the heat-treating period is about 2 hours, to produce a heat-treated substrate;

providing an exchange medium including invading alkali ions having an average ionic radius that is larger than the average ionic radius of the host alkali ions;

exposing the heat-treated substrate to the exchange medium; and

conducting ion exchange to chemically strengthen the substrate while exposing the heat-treated substrate to the exchange medium to produce a strengthened substrate,

wherein the strengthened substrate includes a compressive stress layer having a depth of about 1-500 μm,

wherein the heat-treating temperature and the heat-treating period for producing reduced induced curvature in the substrate after chemical strengthening are determined, at least in part, based upon satisfying the relationship:

t≅n *( m−T ) 2

 “t” being the heat treating period in hours,

 “n” being an empirical constant,

 “m” being the maximum heat-treating temperature in ° C., and

 “T” being the heat-treating temperature in ° C.,

wherein the strengthened substrate has a reduced induced curvature after chemical strengthening based on the heat treatment.

2. The method of claim 1 , wherein the provided substrate has a width of ≦ about 2.0 millimeters,

wherein a net bending moment about mid-plane is about zero in the strengthened substrate.

3. The method of claim 1 , wherein the heat-treating temperature is about 450-650° C.

4. The method of claim 1 , wherein the heat-treating temperature is about 400-750° C.

5. The method of claim 1 , wherein if the heat-treating temperature is ≦ about 400° C., then the heat-treating period is ≧ about 2 hours.

6. The method of claim 1 , wherein the strengthened substrate has a compressive stress layer having a depth of about 5-200 μm.

7. The method of claim 1 , wherein conducting ion exchange occurs during an ion-exchange period of about 0.1 to 50 hours.

8. The method of claim 1 , wherein conducting ion exchange occurs at an ion-exchange temperature of about 200° C. to 1,400° C.

9. The method of claim 1 , wherein the provided substrate is flat.

10. The method of claim 1 , wherein the heat-treating period is ≧0.5 hours and

if the heat-treating period is about 0.5-1 hours, then the heat-treating temperature is ≧ about 500° C.;

if the heat-treating period is ≦ about 1-3 hours, then the heat-treating temperature is ≧ about 475° C.;

if the heat-treating period is ≦ about 3-6 hours, then the heat-treating temperature is ≧ about 450° C.;

if the heat-treating period is ≦ about 6-12 hours, then the heat-treating temperature is ≧ about 425° C.;

if the heat-treating period is ≦ about 12-18 hours, then the heat-treating temperature is ≧ about 400° C.; and

if the heat-treating period is ≦ about 18-25 hours, then the heat-treating temperature is ≧ about 375° C.

11. The method of claim 1 , wherein the heat-treating period is ≧0.5 hours and

if the heat-treating period is about 0.5-1 hours, then the heat-treating temperature is ≧ about 575° C.;

if the heat-treating period is ≦ about 1-3 hours, then the heat-treating temperature is ≧ about 550° C.;

if the heat-treating period is ≦ about 3-6 hours, then the heat-treating temperature is ≧ about 525° C.;

if the heat-treating period is ≦ about 6-12 hours, then the heat-treating temperature is ≧ about 500° C.;

if the heat-treating period is ≦ about 12-18 hours, then the heat-treating temperature is ≧ about 475° C.; and

if the heat-treating period is ≦ about 18-25 hours, then the heat-treating temperature is ≧ about 450° C.

12. The method of claim 1 , wherein the heat-treating period is ≧0.5 hours and

if the heat-treating period is about 0.5-1 hours, then the heat-treating temperature is ≧ about 650° C.;

if the heat-treating period is ≦ about 1-3 hours, then the heat-treating temperature is ≧ about 625° C.;

if the heat-treating period is ≦ about 3-6 hours, then the heat-treating temperature is ≧ about 600° C.;

if the heat-treating period is ≦ about 6-12 hours, then the heat-treating temperature is ≧ about 575° C.;

if the heat-treating period is ≦ about 12-18 hours, then the heat-treating temperature is ≧ about 550° C.; and

if the heat-treating period is ≦ about 18-25 hours, then the heat-treating temperature is ≧ about 525° C.

13. The method of claim 1 , further comprising

cooling the heat-treated substrate to a cooled temperature ≦ about 150° C. to produce a cooled heat-treated substrate; and

heating the cooled heat-treated substrate to the heat-treating temperature ≧ about 250° C. and ≦ the maximum heat-treating temperature for a heat-treating period of about 0.01-50 hours to produce the heat-treated substrate.

14. The method of claim 1 , wherein the provided substrate comprises a variation in at least one of chemical composition and chemical structure in the substrate and at least one of the chemical composition and chemical structure in the treatment-poor volume is different than in the treatment-rich volume.

15. The method of claim 1 , wherein the exchange medium is one of a liquid, a solid, a gas and mixtures thereof.

16. The method of claim 1 , wherein the treatment-rich volume and the treatment-poor volume are located as diametrically opposed in the substrate.

17. The method of claim 1 , wherein the method is one of a continuous process and a batch process.

18. The method of claim 1 , wherein the provided substrate comprises one of alkali aluminosilicate glass and soda-lime silicate glass.