Strengthened glass

Abstract

There is an article of manufacture. The article includes an alkali aluminosilicate glass object. The glass object is characterized by having at least one of: a surface compression of ≥about 100,000 psi with a case depth of ≥about 600 microns; and/or a compressive stress of ≥about 30,000 psi at ≥about 50 microns below a surface of the glass object and above the case depth.

Claims

1. An article of manufacture comprising: a strengthened aluminosilicate glass object characterized by having a surface compression of at least 100,000 psi at a surface with a case depth of at least 600 microns below the surface, wherein the glass object is further characterized as including at least some amounts of Li.sub.2O, Na.sub.2O and K.sub.2O.

2. The article of claim 1, wherein the glass object has a compressive stress of at least 30,000 psi at 50 microns below the surface.

3. The article of claim 1, wherein the glass object has a case depth of at least 1000 microns below the surface.

4. The article of claim 1, wherein the glass object includes a composition with varying amounts of Na.sub.2O and K.sub.2O at different depths below the surface of the glass object.

5. The article of claim 1, wherein the glass object includes 7 to 30 weight % Al.sub.2O.sub.3.

6. The article of claim 1, wherein the glass object includes less than 9 weight % Li.sub.2O.

7. The article of claim 1, wherein the glass object includes less than 3 weight % Li.sub.2O.

8. The article of claim 1, wherein the glass object includes less than 2% by weight of K.sub.2O, Na.sub.2O or a combination thereof.

9. The article of claim 1, wherein the glass object has a surface compression between 100,000 to 145,000 psi at the surface.

10. The article of claim 1, wherein the glass object has a surface compression up to 145,000 psi at the surface.

11. The article of claim 1, wherein the glass object has a surface compression greater than 145,000 psi at the surface.

12. The article of claim 1, wherein the glass object includes up to 4 weight % each of ZrO.sub.2, TiO.sub.2, MgO and ZnO, and totaling no more than 10 weight % of a combination thereof.

13. The article of claim 1, wherein the glass object is flat.

14. The article of claim 1, wherein the glass object is made from a flat glass.

15. The article of claim 1, wherein the glass object is a strengthened float glass.

16. The article of claim 1, wherein the glass object has a maximum compression at a location below the surface of the glass object.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features and advantages of the present invention become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears.

(2) In addition, it should be understood that the drawings in the figures, which highlight the aspects, methodology, functionality and advantages of the present invention, are presented for example purposes only. The present invention is sufficiently flexible, such that it may be implemented in ways other than that shown in the accompanying figures.

(3) FIG. 1 is a graph showing modulus of rupture (MOR) of a chemically strengthened glass as a function of ion exchange treatment and time. The figure shows stress relaxation at higher temperatures or longer times, in turn, suggesting that a need to optimize the ion exchange schedule exists. Taken from Nordberg et al.;

(4) FIG. 2 is a graph showing compressive stress profiles in an unbroken plate of the inventive aluminosilicate glass as a function of ion exchange treatment for various salt baths wherein circle and triangle data points correspond to use of the indicated mixed sodium and potassium salt bath used in the invention whereas the diamond and square comparative data points correspond to use of a sodium salt bath followed by a potassium salt bath;

(5) FIG. 3 is a graph showing Weibull plots of the strengths of the inventive lithium aluminosilicate silicate glass as a function of ion exchange at 475° C. for 24 hours in a sodium nitrate and potassium nitrate mixed salt bath. The inset shows the molar ratio of sodium to potassium in the mixed salt bath. Note σ.sub.f=90 MPa; 148 MPa; 403 MPa; 735 MPa; 1096 MPa at ln(σ.sub.f)=4.5; 5.0; 6.0; 6.5; and 7.0, respectively, wherein 1 MPa is about 145 psi; and cumulative probability of failure P.sub.f=98.9%, 80.8%, 63.2%, 30.8%, 12.6% and 4.8% at ln [ln {1/(1−P.sub.f)}]=1.5, 0.5, 0.0, −1.0, −2.0 and −3.0 respectively; and

(6) FIG. 4 is a graph showing compressive stress profiles in an unbroken plate of the inventive lithium aluminosilicate glass as a function of ion exchange treatment for various salt baths. The inset shows the molar ratio of sodium salt to potassium salt in the mixed salt bath.

DETAILED DESCRIPTION

(7) The present invention features chemically strengthened lithium aluminosilicate glass of a composition that provides the glass with a high strain point, which achieves unexpectedly high surface compression, deep compression case depth and high strength effective to resist fracture upon flexing from impact of high velocity projectiles. The inventive glass is characterized by a surface compression of at least 100,000 psi and a compression case depth of at least 600 microns and, in particular, of at least 1,000 microns. In particular, the glass can exhibit stress at 50 microns below a surface of the glass that is at least 30,000 psi. The compression level is 30,000 psi; hence, a tensile stress higher than 30,000 psi would be needed to cause fracture.

(8) As defined herein, the term “annealing point” is the temperature at which stresses in a glass at a viscosity of 10.sup.13 poise are relieved in minutes. The term “case depth” is defined herein as a distance measured from a surface of a glass article to a position in the interior of the article at which there is zero stress in the glass article. The surface of the glass can be any surface that is exposed to the molten salts of the ion exchange bath.

(9) The present invention uses a molten salt bath having a balanced mix such that both exchanges (sodium for lithium deep inside the glass and potassium for lithium in the surface) occur concurrently. In conventional salt baths having high sodium salt concentration, there is little potassium-lithium exchange. The inventor has recognized the advantage of using elevated temperature of the bath of 450° C., and higher, to increase the mobility of potassium ions, which require the glass to have a sufficiently high annealing point such that viscous relaxation of stresses does not occur significantly. The inventor has further recognized the usefulness for high amounts of potassium in the mixed bath to encourage potassium for lithium exchange in the surface. This results in glass having high strength adapted for use under the extreme condition of resisting fracture upon flexing from impact of high velocity projectiles.

(10) The present invention can strengthen commercially available flat glass. Glass that may be chemically strengthened according to the present invention has a composition comprising (in weight %): Li.sub.2O in an amount ranging from 3 to 9%, Na.sub.2O+K.sub.2O in an amount less than about 3.5%, and Al.sub.2O.sub.3 in an amount ranging from 7 to 30%. In particular, Na.sub.2O+K.sub.2O in the glass may ranges from 0 to 3%, and Al.sub.2O.sub.3 ranges from 18-28%. More specifically, the inventive glass has a composition consisting essentially of: Li.sub.2O in an amount ranging from 3 to 9%, Na.sub.2O+K.sub.2O in an amount less than about 3.5%, Al.sub.2O.sub.3 in an amount ranging from 7 to 30%, up to about 4% each of ZrO.sub.2, TiO.sub.2, MgO and ZnO and other similar constituents totaling no more than 10%, with the balance being SiO.sub.2 and unavoidable impurities. The inventive composition does not add B.sub.2O.sub.3 or P.sub.2O.sub.5 and these compounds are not generally present in amounts greater than 0.05 wt % or other than what is considered an impurity or tramp ingredient. Additional compositions of glasses that may be suitable for strengthening in accordance with the present invention are those disclosed in U.S. Patent Application Publication No. 2005/0090377 by Shelestak et al., incorporated herein by reference, which have an annealing point of at least 580° C.

(11) Exemplary glass that is suitable for being strengthened in the method of the present invention is a pre-cerammed glass manufactured by Nippon Electric Glass (NEG) of Japan. This glass (shown in column 2) and other suitable glasses have the compositions shown in Table 1 below, and may be treated by the salt bath temperatures and immersion times shown in Table 1. The treatments indicated in Table 1 specify the type of salt bath, and temperature, time of immersion of the glass therein. The properties reported in Table 1 specify the case depth and surface compression of the glass, respectively.

(12) TABLE-US-00001 TABLE 1 Glass Com- INVEN- INVEN- position PRIOR ART TION TION (weight %) Schott ROBAX ® NEG NEG SiO.sub.2 67.2 65.7 65.1 Al.sub.2O.sub.3 20.1 22 22.6 Li.sub.2O 3.2 4.5 4.2 MgO 1.1 0.5 0.5 CaO 0.05 BaO 0.9 ZnO 1.7 Na.sub.2O 0.4 0.5 0.6 K.sub.2O 0.23 0.3 0.3 B.sub.2O.sub.3 TiO.sub.2 2.7 2 2 ZrO.sub.2 1.7 2.5 2.9 F 0.1 Potassium Potassium Potassium Potassium & Sodium & Sodium 500° C. 500° C. 450-475° C. 450-475° C. Treatment: 8 days 2 days 8 hours 1 day Properties: 80 μm 40 μm 600 μm 1000 μm 550 MPa 850 MPa 1000 MPa 800 MPa

(13) Other glasses that may be suitable for being strengthened by the method of the present invention are ROBAX® glass by Schott Inc. having the composition shown in Table 1 (but using the inventive mixed salt bath, temperature and period of immersion) and HERCUVIT® pre-cerammed glass by PPG, Inc.

(14) Measuring surface compression and case depth herein is by the procedure prescribed by ASTM C1422-99. Measuring modulus of rupture (MOR) herein is by following ASTM standard F-394, “Biaxial Flexural Strength (MOR) of Ceramic Substrates” also known as the “ball-on-ring” procedure, which was, otherwise, prescribed for ceramic substrates. Although standard C1422-99 does not prescribe measurement of stress at 50 microns, those familiar with the chemical strengthening process recognize that superior resistance to fracture is afforded where the compression continues to be significantly large at distances representing the length of flaws typical of handling, and in extreme cases, at the base of craters produced by an impacting projectile. Handling flaws are mostly 50 microns deep, less in a commercial setting where the glass is laminated with polymer before getting scratched.

(15) Glass that is suitable for being treated to effect high strength and deep compression case depths according to the present invention has a composition that provides it with an annealing point of 580° C., or higher. This allows chemical strengthening to be carried out using a molten mixed salt bath having a temperature of 450° C. or higher and, in particular, 475° C., or higher. Although the glass can be preheated, neither the salt bath temperature nor the glass temperature should be close to the glass annealing point temperature or exceed a temperature that is about 25° C. less than the glass annealing point temperature. The lithium aluminosilicate glass has a high transition temperature (T.sub.g) of, for example, about 620-630° C.

(16) The present invention may advantageously use a high concentration of potassium salt in the dual salt bath. A ratio of moles of sodium salt to moles of potassium salt in the salt bath ranges from 1:10 to 1:2 and, in particular, is about 1:4. The amount of sodium salt in the total salt of the molten mixed salt bath is often well below 50 mol %. The composition of the salt bath may comprise sodium salt in an amount ranging from 10 mol % to 40 mol %, with the balance being potassium salt and unavoidable impurities. In particular, the salt bath may have a composition consisting essentially of sodium salt in an amount ranging from 20 mol % to 40 mol %, with the balance being potassium salt, unavoidable impurities and optional scavengers therefor. An exemplary composition of the salt bath that may advantageously be used in the present invention is about 20 mol % sodium salt, about 80 mol % potassium salt and unavoidable impurities.

(17) One way of preparing the salt bath is to weigh the salts dry in the indicated amounts, mix the salts, place the salts in the vessel that will contain the bath and turn on the electricity to the bath to melt the salt mixture and heat the bath to the specified temperature. The molten salt bath contains molten salt and no other solvents. The salt bath has no water, besides trace amounts of structure bound water. The composition of the salt bath is maintained over time, such as by periodically changing out the salt. The salts in the salt bath can be sodium nitrate and potassium nitrate. Other salts of sodium and potassium may be used instead of nitrate salts. That is, other anions such as chlorides, sulfates, phosphates and the like can be used instead of nitrates, and may be preferred when bath temperatures are high in order to reduce salt volatilization.

(18) Unavoidable impurities build up in the molten salt bath over time. As a result of the continuing exit of lithium and other ions contained in the parent glass structure from the glass, the ions build up in the bath. The amount of lithium oxide in the bath is maintained below 1 mol %, preferably below 0.1 mol %. Depending upon the chemical composition of the parent glass used, other potential impurities include aluminum, boron, calcium, magnesium, barium, zinc, titanium and zirconium. Potential impurities from the steel of the vessel that contains the bath include iron, chromium and nickel. Initially, “scavengers” or “getters” such as sodium and potassium silicate powders or flakes are used to absorb some of the impurities. After some time, the scavenging effect is reduced and the bath is dumped and replaced with uncontaminated salt. The salts have varying impurities that should be minimized as much as commercially possible. Salts of extremely high purity may be most advantageous in the present invention. Commercially obtained salt may need to be purified to a suitable level determined by one skilled in the art in view of this disclosure, such as by scavenging, before the bath is used. The scavengers are left in the bath, or the bath can first be treated by a bag containing the scavengers and the bag is subsequently removed.

(19) With the concurrent exchange of potassium and sodium ions in the bath for the lithium ions in the glass at elevated temperatures without significant viscous relaxation of the glass, high surface compression of 690 MPa to 1,000 MPa (100,000 to 145,000 psi) and a deep compression case depth on the order of up to a millimeter in as little as 1 day of immersion in the salt bath, is achieved by the invention. The resulting glass has very high strength (modulus of rupture approaching 1 GPa or 145 Kpsi).

(20) The present invention advantageously enables high speed strengthening. Compared to conventional immersion lasting often 10 days, often more, the invention can achieve very high strength in as little as 4 hours of immersion and very high strength such as for security glass (bullet-resistant applications) in as little as a day of immersion. Refer to Table 1 above for exemplary durations of immersion and resulting surface compression and case depth for the glass compositions provided.

(21) The invention may feature a chemically strengthened glass with greater than 100,000 psi (about 690 MPa) surface compression and in commonly, not less than 30,000 psi compression at 50 micron depth from the surface and a total case depth often at least 600 microns (about 24 mils).

(22) FIG. 2 shows a stress profile plot for 4 specimens of the NEG's column 2 “mother” glass (see Table 1) prior to ceramming. The square and diamond comparative data points are for sequential exchange at 450° C., first with NaNO.sub.3 and then at 475° C. with KNO.sub.3. This sequential exchange is not part of the present invention. The circle and triangle data points are for concurrent sodium and potassium salt exchange at 475° C. that may embody the invention.

(23) Shown in FIG. 3 are Weibull fracture probability plots for glass ion exchange-strengthened in mixtures of sodium nitrate (NaNO.sub.3) and potassium nitrate (KNO.sub.3) salts where the ratio of salt has been varied from 0 to 30 mol % (4.25 to 26.5 weight %) and compared with data obtained with unstrengthened and unstrengthened-plus-abraded specimens. The plots show the superiority of the 20 mol % sodium nitrate salt mixture over others. In addition, it can be seen, from steepness of the slope, that the chemically strengthened glasses have narrower strength distributions compared to the unstrengthened glass. For product specifications, one often prefers glass strength distributions to be narrow. The slope of the curves yields Weibull modulus. Whereas the Weibull modulus values for the unstrengthened glasses were around 4.5, the strengthened glasses had Weibull modulus values ranging between 9 and 18.

(24) FIG. 4 shows stress profiles for 5, 10 and 20 mol % sodium nitrate (balance potassium nitrate) baths. Again, the higher stresses achieved with ion exchange in the 20 mol % sodium nitrate bath are evident. NEG's column 2 “mother” glass shown in Table 1 was used for all specimens.

(25) On the ASTM C-1422-99 standard, the surface compression and case depth for the glass treated according to the invention (circle and triangle data points shown in FIG. 2) correspond to a glass level 5F, which is a high strength limit. Those skilled in the art will appreciate in reading this disclosure that although ASTM C-1422-99 does not prescribe any details of the profile below the surface, glass treated according to the present invention may exhibit a maximum compression value that is not exactly on the surface. While not wanting to be bound by theory, such treated glass may advantageously possess a maximum compression value that is a few microns below the surface (as disclosed in U.S. Pat. No. 6,516,634 by Green et al.). The strengthened glass of the present invention advantageously exhibits a relatively high compression value at about 50 microns below the surface compared to the Saunders patent publication.

(26) Applications for the chemically strengthened glass produced in accordance with the present invention include: glass transparencies for high-security applications, such as, bullet and blast-resistant glass, glass for armored defense vehicles, windows for government buildings and monuments, private vehicles, train and aircraft transparencies, and hurricane and earthquake-resistant windows. Lower security need applications such as bank teller windows, display cases, and ATM touch panels may also benefit from the invention.

(27) Many modifications and variations of the invention will be apparent to those of ordinary skill in the art in light of the foregoing disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than has been specifically shown and described.

(28) Although described specifically throughout the entirety of the disclosure, the representative examples have utility over a wide range of applications, and the above discussion is not intended and should not be construed to be limiting. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art recognize that many variations are possible within the spirit and scope of the principles of the invention. While the examples have been described with reference to the figures, those skilled in the art are able to make various modifications to the described examples without departing from the scope of the following claims, and their equivalents.

(29) Further, the purpose of the Abstract is to enable the U.S. Patent and Trademark Office and the public generally and especially the scientists, engineers and practitioners in the relevant art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of this technical disclosure. The Abstract is not intended to be limiting as to the scope of the present invention in any way.