Method for making strengthened glass having reduced induced curvature using differential chemistry

1. A method for making a strengthened glass substrate with a reduced induced curvature utilizing differential chemistry, the method comprising:

providing a glass substrate having a first surface and a second surface as two opposing surfaces opposed to each other in the glass substrate,

wherein the glass substrate is characterized by having a varied glass chemical composition including host alkali ions situated in the glass substrate and having an average host ionic radius,

wherein a first glass chemical composition of the varied glass chemical composition in a first volume located at and below the first surface is different than a second glass chemical composition of the varied glass chemical composition in a second volume located at and below the second surface,

wherein the first volume is a treatment-rich volume and the second volume is a treatment-poor volume;

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

wherein the medium composition is associated with an ion exchange rate of the invading alkali ions of the medium composition;

providing a modified exchange medium including a modified medium composition including invading alkali ions,

wherein the modified medium composition is associated with a modified ion exchange rate of the invading alkali ions of the modified medium composition, and

wherein the modified ion exchange rate is slower than the ion exchange rate;

applying the modified exchange medium to a surface of the treatment-rich volume;

applying the exchange medium to a surface of the treatment-poor volume; and

conducting ion exchange while applying both the exchange medium and the modified exchange medium to produce the strengthened glass substrate,

wherein conducting ion exchange while applying both the exchange medium and the modified exchange medium is configured to produce a reduced net bending moment about a mid-plane of the strengthened glass substrate.

2. The method of claim 1 , wherein the modified medium composition includes at least one additive associated with reducing the rate of ion exchange of the modified exchange medium.

3. The method of claim 2 , wherein the additive includes ions associated with competing with the invading alkali ions of the modified medium composition for access to the exchange-rich surface during ion exchange.

4. The method of claim 2 , wherein the additive includes at least one of monovalent ions and divalent ions.

5. The method of claim 2 , wherein the additive comprises at least one of NaNO 3 and Ca(NO 3 ) 2 .

6. The method of claim 1 , wherein the substrate comprises a variation in at least one of chemical composition and chemical structure in the substrate.

7. The method of claim 6 , wherein at least one of the chemical composition and chemical structure in the treatment-poor volume is different than in the treatment-rich volume.

8. The method of claim 1 , wherein a net bending moment about mid-plane is about zero in the strengthened glass substrate.

9. The method of claim 1 , wherein a compressive stress varies at different locations of the strengthened glass substrate.

10. The method of claim 1 , wherein conducting ion exchange is performed at a constant temperature.

11. The method of claim 1 , wherein conducting ion exchange is performed while applying about an equal temperature to the treatment-poor volume and the treatment-rich volume.

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

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

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

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

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

17. The method of claim 1 , wherein the substrate has a width of about 3.0 millimeters or less.

18. The method of claim 1 , wherein the treatment-poor volume includes latent tin impurities from a float glass process utilized in producing the substrate.

19. The method of claim 1 , wherein conducting ion exchange while applying both the exchange medium and the modified exchange medium is configured to produce a fully nullified net bending moment about the mid-plane of the strengthened glass substrate.