ULTRATHIN GLASS CERAMIC ARTICLE AND METHOD FOR PRODUCING AN ULTRATHIN GLASS CERAMIC ARTICLE

Abstract

An ultrathin glass-ceramic article is provided having an article thickness (t) of equal to or less than 0.3 mm and an outer surface followed towards the inside of the article by an outer layer and a central part. The glass-ceramic has a crystal phase and an amorphous phase and the outer layer includes the crystal phase. The article has a gradient structure or a layered structure.

Claims

1. An ultrathin glass-ceramic article, comprising: a crystal phase and an amorphous phase; an article thickness (t) that is equal to or less than 0.3 mm; an outer surface; a central part; and an outer layer between the outer surface and the central part; and a gradient crystalline structure in the outer layer that has a volume proportion of the crystal phase measured at or near the outer surface that is different from a volume proportion of the crystal phase in the central part.

2. The article of claim 1, wherein the volume proportion of the gradient crystalline structure increases from the outer surface towards the central part.

3. The article of claim 1, wherein the volume proportion of the gradient crystalline structure decreases from the outer surface towards the central part.

4. The article of claim 1, wherein the volume proportion of the crystal phase is in a range of 0.1 to 99 vol. %.

5. The article of claim 1, further comprising a glassy zone between the outer layer and the outer surface, the glassy zone having a thickness in a range of 1 to 400 nm.

6. The article of claim 1, further comprising a second outer surface and a second outer layer, the second outer surface being opposite to the outer surface and the second outer layer being between the second outer surface and the central part.

7. The article of claim 1, wherein the outer layer has a layer thickness (t.sub.1) is greater than 0 and ⅓ the article thickness (t).

8. The article of claim 7, further comprising a thickness ratio of the layer thickness to the article thickness (t.sub.1/t) that is greater than 0.007.

9. The article of claim 1, further comprising chemically toughening ions exchanged via an ion exchange process.

10. The article of claim 9, further comprising a compressive stress region extending from the outer surface to a depth (d), wherein the compressive stress region has a surface compressive stress at the outer surface of at least 100 MPa.

11. The article of claim 10, wherein the depth is greater than 0.01 of the article thickness (t) in microns.

12. The article of claim 9, further comprising a pen drop breakage height that is higher than t.sup.2/800 mm, wherein the thickness of the article (t) is in microns.

13. The article of claim 9, further comprising a ball drop breakage height that is higher than (t.sup.2/600)*mm, using a 20 g steel ball, wherein the thickness of the article (t) is in microns.

14. The article of claim 9, further comprising a bending radius that is less than 900*t mm, wherein the thickness of the article (t) is in millimeters.

15. The article of claim 1, wherein the article comprises a glass selected from a group consisting of a lithium-alum inate-silicate (LAS), magnesium-aluminate-silicate (MAS), zinc-aluminate-silicate (ZAS), sodium-aluminate-silicate (NAS), and lithium-disilicate (LDS) system.

16. An ultrathin glass-ceramic article, comprising: a crystal phase and an amorphous phase; an article thickness (t) that is equal to or less than 0.3 mm; an outer surface; a central part; and an outer layer between the outer surface and the central part; and a homogeneous crystal structure in the outer layer that has a volume proportion of the crystal phase measured at or near the outer surface that is homogeneous with a volume proportion of the crystal phase at the central part, and wherein the volume proportion of the crystal phase of the outer layer is different from a volume proportion of the crystal phase in the central part.

17. The article of claim 16, wherein the central part comprises a second homogeneous crystal structure.

18. The article of claim 16, wherein the central part consists of the amorphous phase.

19. A method for producing an ultrathin glass ceramic article, comprising: providing a composition of raw materials for a green glass article; melting the composition; producing the green glass article from the composition; solidifying the green glass article; heating the green glass article to a temperature (T) above a ceramization temperature and holding at the temperature (T) sufficient to convert the green glass article into the ultrathin glass ceramic article with a crystal phase and a gradient structure or a layered structure from an outer surface towards a central part; and cooling the ultrathin glass ceramic article.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1a glass-ceramic article according to the invention with decreasing gradient structure.

[0039] FIG. 1b glass-ceramic article according to the invention with increasing gradient structure.

[0040] FIG. 1c glass-ceramic article according to the invention with layered structure.

[0041] FIG. 2 a simplified schematic illustration of the pen drop test.

DETAILED DESCRIPTION

[0042] As described above, the invention provides a gradient or layered glass-ceramic article. The ultrathin article has a thickness of equal to or less than 0.3 mm, and is composed of different “layers”, “parts”, “sections” or “areas” having different proportions of crystal phase, i.e., the glass-ceramic article has a gradient structure. Because of the gradient structure, there are no real layers with distinct boundaries in the glass-ceramic article. Alternatively, the article has a layered structure with distinct boundaries within.

[0043] The crystal phase formed can be of different kinds depending on the composition of the starting green glass. The crystal phase can be α-quartz, β-quartz, cristobalite, tridymite, β-eucryptite, β-spodumene, enstatite, wollastonite, diopside, K-fluorrichterite, cordierite, nepheline, lithium disilicate, lithium metasilicate, fluorophlogopite, leucite, mullite, spinel, rutile, fluoroapatite, monazite, or mixture of the mentioned crystal phases. Especially preferred according to the invention are leucite, nepheline, β-spodumene, lithium disilicate, lithium metasilicate, magnesium dititanate.

[0044] Ceramizable glasses may be used for the invention, including lithium aluminum silicate glass ceramics (LAS), lithium silicate glass ceramics, magnesium/zinc aluminosilicate glass ceramics (MAS), magnesium silicate glass ceramics, sodium/potassium aluminosilicate glass ceramics (NaAS, KAS), phosphate glass ceramics (phosphate GC), calcium aluminosilicate glass ceramics, with compositions as below. Glass-ceramics preferably suitable for the intended applications are preferably alumina silicate based glasses such as lithium-alum inate-silicate (LAS), magnesium-aluminate-slicate(MAS), zinc-aluminate-silicate(ZAS), sodium-aluminate-silicate(NAS), lithium-disilicate(LDS), systems but not limited to. Preferably the glass-ceramic article comprises the following components in the indicated amounts (in wt.%): 40-85% SiO.sub.2, 0-15% B.sub.2O.sub.3, 0-40% Al.sub.2O.sub.3, 0-15% Li.sub.2O, 0-20% Na.sub.2O, 0-20% K.sub.2O, 0-12% ZnO, 0-15% P.sub.2O.sub.5, 0-18% TiO.sub.2, 0-5% Ag.sub.2O, 0-3% CeO.sub.2, 0-3% MnO.sub.2, 0-3% Co.sub.2O.sub.3, 0-3% Fe.sub.2O.sub.3, 0-45% MgO, 0-8% BaO, 0-18% CaO, 0-15% ZrO.sub.2, 0-3% F, 0-3% Sb.sub.2O.sub.3.

[0045] Lithium aluminum silicate glass ceramics (LAS, comprising in wt%): [0046] Al.sub.2O.sub.3: 17-30 wt % [0047] SiO.sub.2: 55-75 wt % [0048] Li.sub.2O: 2.2-5.5 wt % [0049] K.sub.2O: 0-3 wt % [0050] Na.sub.2O: 0-3 wt % [0051] Na.sub.2O+K.sub.2O: 0-4 wt % [0052] MgO: 0-3 wt % [0053] CaO: 0-2.5 wt % [0054] SrO: 0-2 wt % [0055] BaO: 0-4 wt % [0056] ZnO: 0-4 wt % [0057] B.sub.2O.sub.3: 0-2 wt % [0058] P.sub.2O.sub.5: 0-8 wt % [0059] SnO.sub.2: 0-1 wt % [0060] TiO.sub.2: 0-5.5 wt % [0061] ZrO.sub.2: 0-3.0 wt % [0062] TiO.sub.2+ZrO.sub.2: 2-6 wt %.

[0063] Lithium silicate glass ceramics (Li-disilicate, metasilicate), (comprising in wt%):

TABLE-US-00001 A1.sub.2O.sub.3:  2-25 wt % SiO.sub.2: 60-85 wt % Li.sub.2O:  5-15 wt % K.sub.2O + Na.sub.2O:  0-13 wt % Ag, Au: <0.2 wt %.

[0064] Magnesium/zinc aluminosilicate glass ceramics (MAS): (spinel, gahnite, cordierite, enstatite)Spinel/gahnite (comprising in wt%):

TABLE-US-00002 SiO.sub.2: 15-60 wt % Al.sub.2O.sub.3: 20-50 wt % MgO:  0-40 wt %, ZnO:  0-40 wt %, MgO + ZnO: 10-50 wt %.

[0065] Cordierite (comprising in wt%):

TABLE-US-00003 SiO.sub.2:  35-60 wt % Al.sub.2O.sub.3: 1 6.5-40 wt % MgO:   6-22 wt % B.sub.2O.sub.3:   0-10 wt % CaO, BaO, SrO:   0-6 wt % ZnO:   0-7.5 wt % TiO.sub.2:   1-15 wt % ZrO.sub.2:   0-10 wt % As.sub.2O.sub.3 + Sb.sub.2O.sub.3:   0-2 wt %.

[0066] Enstatite (comprising in wt%):

TABLE-US-00004 MgO: 20-35 wt % Al.sub.2O.sub.3: 2-12 wt % SiO.sub.2: 40-70 wt % Li.sub.2O: 0-2 wt % CaO: 0-4 wt % SrO: 0-12 wt % BaO: 0-17 wt % ZrO.sub.2: 0-15 wt % TiO.sub.2: 0-15 wt %.

[0067] Magnesium silicate (forsterite) (comprising in wt%):

TABLE-US-00005 SiO.sub.2: 30-60 wt % Al.sub.2O.sub.3: 10-25 wt % MgO: 13-30 wt % K.sub.2O: 0-20 wt % Na.sub.2O: 0-10 wt % TiO.sub.2: 0-15 wt % GeO.sub.2: 0-25 wt %

[0068] Sodium/potassium aluminosilicate glass ceramics (NaAS, KAS) (nepheline, kalsilite) comprising (in wt%):

TABLE-US-00006 Na.sub.2O + K.sub.2O: 5-40 wt % Al.sub.2O.sub.3: 10-40 wt % SiO.sub.2: 25-75 wt % CaO + BaO + MgO: 0-18 wt % TiO.sub.2: <10 wt % ZrO.sub.2: <10 wt %;
preferably:

TABLE-US-00007 Na.sub.2O + K.sub.2O: 10-30 wt % Al.sub.2O.sub.3: 10-35 wt % SiO.sub.2: 30-55 wt %.

[0069] Phosphate glass ceramics (comprising in wt%): (apatite, LISICON, BPO4) Apatite (comprising in wt%):

TABLE-US-00008 CaO: 5-45 wt % Al.sub.2O.sub.3: 5-38 wt % P.sub.2O.sub.5: 10-26 wt % SiO.sub.2: 10-60 wt % MgO: 0-26 wt % K.sub.2O, Na.sub.2O, Li.sub.2O: 0-10 wt % TiO.sub.2, ZrO.sub.2: 0-10 wt %

[0070] LISICON (comprising in wt%):

TABLE-US-00009 Al.sub.2O.sub.3: 0-20; preferably 4-18; more preferably 6-15.5 GeO.sub.2: 0-38; preferably < 20; more preferably < 10 Li.sub.2O: 2-12; preferably 4-8 P.sub.2O.sub.5: 30-55  TiO.sub.2: 0-35 ZrO.sub.2: 0-16 SiO.sub.2: 0-15 Cr.sub.2O.sub.3 + Fe.sub.2O.sub.3: 0-15 Ga.sub.2O.sub.3: 0-15 Ta.sub.2O.sub.5:   0-36.5 Nb.sub.2O.sub.5: 0-30 Halogenides: < 5, preferably < 3, more preferably < 0.3 M.sub.2O: < 1; preferably < 0.1; with M being an alkali cation, with the exception of Li+.

[0071] BPO4 (comprising in wt%):

TABLE-US-00010 SiO.sub.2: 10-50 wt % B.sub.2O.sub.3: 5-40 wt % P.sub.2O.sub.5: 25-75 wt % refining agents: <5 wt % M3.sub.2O.sub.3, M5.sub.2O.sub.5 and M4O.sub.2: <10 wt %.

[0072] Calcium aluminosilicate (comprising in wt%): Wollastonite (comprising in wt%):

TABLE-US-00011 SiO.sub.2: 50-65 wt % Al.sub.2O.sub.3: 0-13 wt % CaO: 15-55 wt % ZnO: 2-10 wt % K.sub.2O, Li.sub.2O, Na.sub.2O: 0-5 wt % P.sub.2O.sub.5: 0-10 wt %

[0073] Anorthite (comprising in wt%):

TABLE-US-00012 SiO.sub.2 15-54 wt % Al.sub.2O.sub.3 13-40 wt % CaO 5-22 wt % BaO, MgO 0-10 wt % TiO.sub.2 0-12 wt %

[0074] The above given compositions can further comprise refining agents such as As2O3, Sb2O3, CeO2, sulphate compounds and/or halide compounds up to a total sum of about 2 wt %.

[0075] Further glasses that are suitable for the method for producing a ceramizable green glass component are photosensitive glasses. Such a glass is sensitized, that means it is more sensitive to irradiation with ultraviolet light and can be crystallized more easily and with greater aspect ratios than a non-sensitized glass of the same composition.

[0076] According to one advantageous embodiment the size of the formed crystals is <500 nm, preferably <200 nm, preferably <150 nm, preferably <120 nm, preferably <100 nm, preferably <90 nm, preferably <80 nm, preferably <70 nm, preferably <60 nm. Some advantageous variants have a crystal size of <50 nm, preferably <40 nm. There are several advantage of controlling the grain size in a smaller level:

[0077] Firstly, gradient crystallization concentration can be formed in the glass-ceramic article. The gradient (abbreviated as G) could be define by the ratio of “crystal volume proportion in the outer layer” (measured at or near the outer surface) to “crystal volume proportion in the central part of the article”. E.g., for a 100 μm ultrathin article, in the outer layer, the crystal volume proportion can be controlled to increase with the depth to have 30% at the depth of 10 μm. In the center part of the article we have 30% crystals. Thus, the gradient would be 3% per μm. In another example having a layered structure, for a 100 μm ultrathin article, in the outer layer, the crystal volume proportion can be 30% and keep the same to have 30% at the depth of 10 μm. In the center part of the article, there is glass phase without crystals, i.e., the central part is amorphous. Thus, the gradient would be 0% per μm. It should generally be noted that all % values given for the proportion of crystal phase or amorphous phase refer to vol%.

[0078] Secondly, when the crystal grain size is smaller than 500 nm, preferably <200 nm, preferably <100 nm, preferably <50 nm the light transmission can be improved, especially when the grain size is smaller than the light wavelength, especially for the visible light (380 nm-780 nm), which makes the transparency of the glass ceramic-article possible. In a first advantageous embodiment said ultrathin glass-ceramic article has an average transmission higher than 50%, preferably >60%, preferably >75%, preferably >80%, preferably >85%, preferably >90%, preferably >95% at an article thickness of 100 μm in a spectral range 380-780 nm. An article having a high transmission can e.g., be used for cover glass or surface protection glass applications.

[0079] In a second advantageous embodiment said ultrathin glass-ceramic article has an average transmission higher than 5%, preferably >15%, preferably >25%, preferably >35%, preferably >45% in the range 380-780 nm at 100 μm thickness. An article having a quite low transmission can e.g., be used for back-cover applications, diffusor applications and other applications where high transparency is not required.

[0080] Thirdly, glass-ceramic with crystalline grains distributed in the glass matrix (amorphous phase) can significantly improve the mechanical strength of the whole body by two principles, one of which is the excellent mechanical strength of the grain itself and another one is the composite effect of grain distribution in the glass matrix. The grains can function as the secondary strengthening phase in the article. In this sense, smaller grains can increase the distribution density of the grains, which means the grains can function as crack blockers when the sample experiences external impact. Furthermore, the crystalline grain can deflect crack around its boundary, and the cracks in glass-ceramic leads to displacement of crystalline grain in the glass matrix. All of above-mentioned mechanisms require higher crack energies and result in higher crack resistance. Increased grain density result in higher resistance of cracking. Therefore, it is advantageous when the ratio of crystal size/article thickness (t) is >0.00005, preferably >0.0001, preferably >0.0005, preferably >0.001 and/or <1, preferably <0.1, preferably <0.01.

[0081] Fourthly, smaller grain size creates more residual glass phase connection between the grains. These short-distance connected residual glass phase function as the channel for ion exchange. If the grain size is too big, this big grains will isolate the residual glass phase, and the ion-exchange channel will be somehow blocked, which will lower the ion-exchange efficiency.

[0082] Fifthly, if the grain size was bigger than the DoL after chemically toughening, one big crystal would exist from the compressive stress layer to the central tension layer, which would influence the profile of the stress distribution and weaken the chemical toughening performance. In this sense, the grain size is also controlled by the ratio of grain size/Dol which is preferred less than <1, preferably <0.5, preferably <0.4, preferably <0.3, preferably <0.2, preferably <0.1, preferably <0.05.

[0083] According to an advantageous embodiment the article thickness t is lower than 300 μm, preferably less than or equal to 275 μm, preferably less than or equal to 250 pm, preferably less than or equal to 225 μm, preferably less than or equal to 200 μm, preferably less than or equal to 175 μm, preferably less than or equal to 145 μm, preferably less than or equal to 100 μm, preferably less than or equal to 70 μm, preferably less than or equal to 50 μm, preferably less than or equal to 30 μm, preferably less than or equal to 15 um and/or higher than or equal to 10 μm. Such particularly ultrathin articles are desired for various applications as described above. In particular, the thin thickness grants the article flexibility. Ultrathin glass-ceramic articles can also be used together with another thin glass or glass-ceramic element or with a thicker glass or glass-ceramic elements in order to build a laminated structure.

[0084] It is advantageous when the thickness of the outer layer (t1) of the article is in the range of >0 to t/3, wherein t is the total article thickness. Preferably the ratio of outer layer thickness/article thickness (t1/t) is >0.007, preferably >0.01, preferably >0.02.

[0085] In the following two alternative advantageous variants of gradient glass-ceramic articles will be described:

[0086] According to a first variant, the glass article has an increasing gradient structure such that the proportion of crystal phase in the outer layer is smaller than the proportion of crystal phase in the central part of the article. Here, the proportion of crystal phase in the outer layer is in the range 0.1 to 99 vol. %. In this kind of gradient glass ceramic both the outer layer and the central part of the article have a crystal phase of the desired kind. Such a glass ceramic article can for example be generated by volume crystallization caused by a special heat treatment. Such articles show great improvement of mechanical strength, i.e., scratch resistance and impact resistance (e.g., pen drop resistance). Preferably the proportion of crystal phase in the outer layer is in the range 1-40 vol. %, preferably 2-35 vol. %. In advantageous variants of the invention the proportion of crystal phase in the outer layer can be in the range 1-20 vol. %, preferably 2-15 vol. %. Further preferably the proportion of crystal phase in the central part is in the range 10-80 vol. %, preferably 15-75 vol.%. In advantageous variants of the invention the proportion of crystal phase in the central part can be in the range 20-60 vol. %, preferably 35-55 vol. %.

[0087] According to a second variant, the glass article has a decreasing gradient structure such that the proportion of crystal phase in the outer layer is higher than the proportion of crystal phase in the central part of the article. Here, the proportion of crystal phase in the outer layer is in the range 0.1 to 99.9 vol. %. This kind of glass ceramic can be generated by surface heat treatment (leading to surface crystallization), for example using a special laser, or by ion exchange applied on the green glass prior to ceramization in order to create a gradient distribution of certain ions. Such articles have improved anti-scratch properties due to high hardness of glass ceramic on the surface. Preferably the proportion of crystal phase in the outer layer is in the range 5-80 vol. %, preferably 10-75 vol. %. In advantageous variants of the invention the proportion of crystal phase in the outer layer can be in the range 5-60 vol. %, preferably 10-55 vol. %. Further preferably the proportion of crystal phase in the central part is in the range 0-70 vol. %, preferably 0-60 vol. %. I.e., this kind of gradient glass-ceramic may have a central part consisting of glass. In advantageous variants of the invention the proportion of crystal phase in the central part can be in the range 0-50 vol. %, preferably 0-40 vol. %.

[0088] In another advantageous variant of the invention the ultrathin glass ceramic article comprises a thin glassy zone on top of the outer layer, wherein the thickness of the glassy zone is in the range 1 to 400 nm, preferably in the range 3 to 300 nm, preferably in the range 5 to 200 nm. In an advantageous embodiment, the ratio of glass-zone-thickness/article thickness is less than 0.00125.

[0089] According to a further advantageous embodiment, the ultrathin glass-ceramic article according to the invention comprises a second outer surface located opposite to the first outer surface, and a second outer layer located between the second outer surface and the central part. So the thin gradient or layered glass-ceramic article has a sandwich structure: first outer surface, first outer layer, central part, second outer layer, second outer surface. In a variant of the invention, the thickness of the first outer layer is different from the thickness of the second outer layer.

[0090] According to a further development of the invention the ultrathin gradient or layered glass-ceramic article is chemically toughened preferably via an ion-exchange process which is described below. Chemically toughening improves the bending performance of the thin glass-ceramic article and other mechanical properties.

[0091] Preferably the article comprises a compressive stress region extending from the outer surface to a depth in the article (DoL), the region is defined by a compressive stress (CS) wherein a surface compressive stress (CS) at the outer surface is at least 100 MPa.

[0092] Some of the thin glass-ceramic articles can be chemically toughened in an advantageous embodiment. Thereafter, a compressive stress region extends from the outer surface to a first depth in the glass article (DoL), the region is defined by a compressive stress (CS) wherein a surface CS at the outer surface is at least 100 MPa. Such a toughened glass ceramic article has a breakage height (given in mm) of at least the figure of the thickness (t in mm) of the article multiplied by 200, wherein the breakage height is determined in a pen drop test as described above. By means of this criterion it can be decided whether a strengthened ultrathin glass-ceramic article is suitably strong enough to be used for the respective application before it becomes part of a product.

[0093] It was found that the breakage height is strongly related to crystallization ratio at certain article thickness and surface quality. Therefore, a thinner homogeneously crystallized glass-ceramic without gradient or layered structure having a high crystallization ratio is especially sensitive to breakage caused by impacts. Surprisingly it was found by the inventors that the breakage height criterion for an ultrathin gradient or layered glass-ceramic can be described by the inventive factor 200 and the thickness of the article. The inventive factor will be valid if the breakage height of the glass article is determined in the above defined pen drop test.

[0094] This test is adjusted to and is especially suitable for ultrathin glass-ceramic articles and reproduces in a quite simple manner the above mentioned problem, that is the impact contact between the glass-ceramic article (e.g., a touch display) and an external object when the article falls down or is hit.

[0095] According to an advantageous embodiment DoL is >0.01*t, preferably >0.05*t, preferably >0.1*t, t being the article thickness in the unit of micron. DoL being the depth of ion exchanged ions.

[0096] According to an advantageous embodiment the generated surface compressive stress (CS) of the article is more than 50 MPa, preferably >100 MPa, preferably higher than 150 MPa, more preferably higher than 200 MPa, more preferably higher than 300 MPa. According to preferred embodiments of the invention CS is equal to or more preferably higher than 400 MPa, more preferably higher than 500 MPa, more preferably higher than 600 MPa, further preferably higher than 700 MPa, further preferably higher than 800 MPa. However, CS should not be very high because the glass may otherwise be susceptible to self-breakage. Preferably, CS is equal to or lower than 2000 MPa, preferably equal to or lower than 1600 MPa, advantageously equal to or lower than 1500 MPa, more preferably equal to or lower than 1400 MPa. Some advantageous variants even have a CS of equal to or lower than 1300 MPa or equal to or lower than 1200 MPa.

[0097] In an advantageous variant the pen drop breakage height of the chemically toughened article is higher than t2/800 mm, preferably >t2/750 mm, preferably >t2/700 mm, preferably >t2/650 mm, wherein t is the article thickness in the unit of micron.

[0098] Further advantageous the ball drop breakage height of the chemically toughened article is higher than (t2/600)*mm, preferably >(t2/500)*mm, preferably >(t2/400)*mm, preferably >(t2/300)*mm using a 20 g steel ball, wherein t is the thickness of glass ceramic in the unit of micron.

[0099] According to a further advantageous variant the chemically toughened article has a bending radius<900*t mm, more preferable <700*t mm, even more preferable<600*t mm wherein t is the article thickness in the unit of millimeter.

[0100] In addition, a further test method is designed in order to evaluate the scratch resistance of the gradient or layered glass-ceramic article. Here, the residue 3PB strength of UTGC after scratching by a Vicker indenter with a load of 5 N is measured. In an advantageous variant of the invention the gradient or layered glass-ceramic article has a 3PB value of >60 MPa, preferably >80 MPa, preferably >100 MPa before chemical toughening. Preferably the gradient or layered glass-ceramic article has a 3PB value of >100 MPa, preferably >120 MPa, preferably >140 MPa after chemical toughening. Depending on the type of gradient or layered glass-ceramic some advantageous articles have 3PB values of >200 MPa, preferably >300 MPa, preferably even >400 MPa after chemical toughening.

[0101] In an advantageous variant the ultra-thin glass-ceramic has a high vickers hardness of >400 MPa, preferably >450 MPa, preferably >500 MPa before chemical toughening. Preferably the chemical toughened ultra-thin glass ceramic possesses a Vickers hardness of >450 MPa, preferably >500 MPa, preferably >550MPa.

[0102] In an advantageous embodiment of the ultrathin glass ceramic article the effective of CTE(A) of outer layer A is smaller than the CTE(B) of inner glass phase B. The effective CTE(A) of outer layer is preferably less than 13*10−6, is more preferably less than 11*10−6, is even more preferably less than 10*10−6 in a temperature interval of 20-300° C. The effective CTE(B) of inner layer B is preferably larger than 0*10−6 , is more preferably larger than 1*10−6, is even more preferably larger than 2*10−6 in a temperature interval of 20-300° C. Thus there is a CTE difference between the outer layer A and the central part B. Preferably the CTE difference introduces a compressive stress on the layer A and extension stress in inner part B. CTE is preferably determined according to ISO 7991:1987 (E).

[0103] According to further aspect of the invention a method for producing a thin glass-ceramic article having a gradient or layered structure is provided, comprising the following steps: [0104] a) providing a composition of raw materials for the desired green glass, [0105] b) melting the composition, [0106] c) producing a green glass article, [0107] d) allowing the green glass article to solidify, [0108] e) heating the green glass article or parts thereof to a temperature T above the ceramization temperature and holding at this temperature T for a period of time and as a result converting the green glass into a glass-ceramic having a desired crystal phase and an increasing or decreasing gradient structure or a layered structure from an outer surface towards the central part of the article, [0109] f) cooling the glass ceramic article.

[0110] First, a green glass is melted from raw materials (selected corresponding to the respective glass composition to be achieved), suitably formed and solidified to generate the precursor green glass article.

[0111] Of course, the corresponding precursor green glass ultrathin article could be produced by grinding and polishing or etching from thicker glass. These two methods are not economical and lead to a rough and maybe uneven surface quality which is quantified by Ra roughness, TTV (Total Thickness Variation) for example.

[0112] Direct hot-forming production, especially in a flat glass process like down draw, up draw, overflow fusion or float method are preferred for the mass production. Redraw method is also advantageous. These mentioned methods are economical, the glass surface quality is high (fire-polished) and the ultrathin glass with thickness from 5 μm (or even less) to 700 μm (or even higher) could be produced. For example, the down-draw/overflow fusion method could make pristine or fire-polished surface with roughness Ra less than 2 nm, preferred less than 1 nm, even preferred less than 0.5 nm, for example 0.4 or 0.3 nm. The thickness could also be precisely controlled ranging from 5 μm and 700 μm. Thin thickness grants high glass flexibility. Special float methods could also produce thin glass with pristine surface, it is economical and suitable for mass production, but the glass produced by float methods has one side as tin containing side which is different from the other side. The difference between these two sides would cause warp issue of glass after chemical toughening, and could affect printing or coating or bending or laminating process because the two sides of the article have e.g., different surface energy, morphologic deviations etc. Another variant of thin article can be produced by sawing an ultrathin green glass articles out of a thick glass ingot, bar, block etc.

[0113] The generated precursor green glass article is then crystallized to generate a gradient structure. Generally, crystallization can be formed in a heating process (laser treatment and/or heat treatment) and/or by ion exchange method after thin glass production. This can be done in batch or inline furnaces in controlled temperature and humidity environment. A roll to roll process is also feasible. By controlling the temperature/time program of the ceramization procedure (e.g., temperatures, heating rates, holding times at a defined temperature, cooling rates) the kind of crystal phase formed, the crystal size and the crystal volume proportion in the different parts of the glass-ceramic article can be adapted.

[0114] According to a first advantageous variant the ceramization step comprises heating the green glass article in a furnace up to a specific temperature for a specific time (holding time) wherein each glass-ceramic type requires its own typical ceramizing program.

[0115] Alternatively or additionally, the ceramization step comprises heating and/or irradiating the green glass article or parts thereof by a laser. Using this method it is possible to introduce heat into the article to be ceramized in a local way. For example, it is possible to only heat the surface or the bulk of the glass article in order to locally initiate crystallization in a certain depth of the article.

[0116] Alternatively or additionally, the green glass article can preferably be treated with a special laser prior to ceramization in order to influence the ceramization behavior of the green glass material.

[0117] In connection with special glass-ceramic systems (e.g., lithium silicate, and sodium silicate glass systems) is advantageous when prior to the ceramization step an ion-exchange is performed on the solidified green glass article to create a gradient ion distribution. After that, the green glass article is ceramized using a suitable method. Because of the gradient ion distribution in the green glass a glass-ceramic having a gradient structure is formed during the ceramization step.

[0118] To further improve the mechanical properties the thin glass-ceramic article can be chemically toughened/strengthened after ceramization with different known toughening processes, e.g., via an ion exchange process with salt bath or salt paste. Salts can be pure or mixed (e.g., KNO3, NaNO3, CsNO3, LiNO3, Li2SO4, K2SO4). In an advantageous further step the glass-ceramic article is chemically toughened wherein the step of chemical toughening preferably comprises an ion-exchange process.

[0119] The strengthening, as called as toughening, can be done by immersing the article into melt salt bath with cations, e.g., potassium ions, or by covering the glass e.g., by potassium ions or other alkaline metal ions contained paste and heating at high temperature at certain time. The alkaline metal ions with larger ion radius in the salt bath or the paste exchange with alkaline metal ions with smaller radius in the article, and surface compressive stress is formed due to ion exchange.

[0120] Further it is advantageous if the glass-ceramic article or a part of the glass-ceramic article is immersed in the salt bath at a temperature between 340° C. and 1200° C. for 30 seconds to 48 hours.

[0121] For some glass types it may be preferred if the chemical toughening comprises two consecutive toughening steps, wherein the first step comprises toughening with a first toughening agent and the second step comprises toughening with a second toughening agent. Preferably the first toughening agent and the second toughening agent comprise or consist of KNO3 and/or NaNO3 and/or mixtures thereof.

[0122] A chemically toughened glass-ceramic article of the invention is obtained by chemically toughening a chemically toughenable glass-ceramic article. The toughening process could be done by immersing the ultrathin article into a salt bath which contains monovalent ions to exchange with alkali ions inside glass. The monovalent ions in the salt bath has radius larger than alkali ions inside glass. A compressive stress to the glass is built up after ion-exchange due to larger ions squeezing in the glass network. After the ion-exchange, the strength and flexibility of ultrathin glass-ceramic are surprisingly and significantly improved. In addition, the CS induced by chemical toughening improves the bending properties of the toughened glass ceramic article and could increase scratch resistance of glass ceramic.

[0123] Further details of the toughening procedure and results have already been described above in connection with the described glass ceramic article.

[0124] A further aspect of the invention is the use of a thin glass ceramic article according to the invention as cover film for resistance screens, and expendable protective films for display screens, foldable/flexible phones, cameras, gaming gadget, tablet, laptops, TV, mirror, windows, aviation widows, furniture, and white goods. Preferably the inventive article can be used in the applications of industrial and consumer display substrate and cover, back-cover, fragile sensors, fingerprint sensor module substrate or cover, semiconductor package, thin film battery substrate and cover, foldable display, camera lens cover, OLEDs, PV and organic complementary metal-oxide-semiconductor (CMOS) and diffusors in various applications, e.g., lighting applications.

[0125] These and other aspects, advantages, and features will be described in more detail in the following paragraphs, drawings and appended claims, wherein the given figures are illustrative, schematic drawings.

[0126] Table 1 shows the compositions of several comparative embodiments (type 1-4) and inventive working embodiments (types 5-10) for direct hot-formed ultrathin glasses which are able to be ceramized with different kinds of crystal phases and to be chemically toughened.

TABLE-US-00013 TABLE 1 Embodiments of UTGC composition of different types Composition Type Type Type Type Type Type Type Type Type Type wt. % 1 2 3 4 5 6 7 8 9 10 SiO.sub.2 60 70 56 67 61.8 41.1 74.5 62 55 67 B.sub.2O.sub.3 1 1.2 3.0 10.3 Al.sub.2O.sub.3 20 19 26 20.25 21 26.3 4 10 22.1 19 Li.sub.2O 4 2.6 4 3 9.3 10 9.2 2 Na.sub.2O 11 0.3 0.2 0.4 7.2 14 2.6 7 K.sub.2O 1 0.1 0.5 0.5 9.7 5 4.1 4 MgO 1.9 1 1.5 3.1 2 5 BaO 0.9 1.0 CaO 1 0.05 2.5 1.3 ZrO.sub.2 2 2 1.8 1.8 0.1 3 NaF 2.5 AlF.sub.3 1.6 ZnO 1 1.5 1.9 1 P.sub.2O.sub.5 7 3 TiO.sub.2 3 2 2.7 0.2 6 4.0 4 Sb.sub.2O.sub.3 0.1 0.4 0.5 KBr Ag.sub.2O + CeO.sub.2 + 0.1 0.3 0.1 0.25 0.5 SnO.sub.2 + Fe.sub.2O.sub.3

[0127] Green glass articles 1 of the different glass types were produced either by down draw process or by producing a larger green glass body, mechanically cutting, grinding and polishing. Subsequently the green glass articles were ceramized to ultrathin glass-ceramics articles having different gradient or layered structures.

[0128] FIGS. 1a-1c show in a sketched way the principle of different embodiments of the present invention, wherein the following has to be explained:

[0129] The volume proportion of crystals rises in the direction of arrow.

[0130] “x” stands for the presence of crystals, wherein the number of “x” schematically represents the relative volume proportion of crystals, whereby it applies that “x”<“xx”<“xxx”. The term “glassy” means that there are no crystals, i.e., the central part is amorphous and consists of glass.

[0131] The glass-ceramic articles 1 shown have the following structure: first outer surface 2, first outer layer A, central part B, second outer layer A′ and second outer surface 3. The thickness of the first outer layer A is t1, the thickness of the second outer layer A′ is t2. Here, the shown articles 1 have outer layers A, A′ of the same thickness. However, of course the thicknesses t1 and t2 can be different.

[0132] FIG. 1a shows a glass-ceramic article 1 having a decreasing gradient structure, i.e., the volume proportion of crystals decreases from the outer surfaces A, A′ towards the central part B. In the outer layers A, A′, the proportion of crystals gradually or continuously changes. The central part B may have crystals with an essentially constant proportion, but in this case in a lower crystal volume proportion than that of the outer layers A, A′. Alternatively, the central part B may consists of glass, i.e., it is glassy.

[0133] FIG. 1b shows a glass-ceramic article 1 having an increasing gradient structure, i.e., the volume proportion of crystals increases from the outer surfaces A, A′ towards the central part B. In the outer layers A, A′, the proportion of crystals gradually or continuously changes. The central part B has crystals with an essentially constant proportion.

[0134] FIG. 1c shows a layered glass-ceramic article 1. In this type of embodiment the outer layers A, A′ have an essentially homogeneous crystal volume proportion from the outer surfaces 2, 3 to a certain depth (thickness t1, t2). The central part B also has an essentially homogeneous crystal volume proportion, which is higher or lower than that of the outer layers A, A′. Alternatively, the central part B can be glassy, i.e., consists of glass.

[0135] The impact resistance of chemically toughened and untoughened comparison and inventive working examples was tested with the pen drop test which was described in detail above. A simplified illustration of that test is shown in FIG. 2. As can be seen, an article 1 to be tested is placed with its second surface 3 on a 100 μm substrate 4, which consists of a 50 μm thick PE-layer 5 and a 50 μm thick PSA-layer 6. The substrate 4 with attached glass article 1 is placed on a rigid support 7. The first surface 2 of the glass article 1 is orientated upwards and impacted until breakage by a 4.5 g pen 8 with a ball-point made from tungsten carbide having a diameter of 300 μm. Step by step the drop height of the pen is increased until the glass article 1 breaks, wherein the sample is moved a little bit in order to avoid double or multiple hits. The pen drop test is performed on small samples of 20 mm×50 mm.

[0136] In addition, the impact resistance of chemically toughened and untoughened comparison and inventive working examples was tested with the ball drop test which was described in detail above. The ball drop test is performed on small samples of 50 mm×50 mm.

[0137] The breakage bending radius of chemically toughened and untoughened comparison and inventive working examples was tested with the 2 point bending method as describes above. The bending test is performed on small samples of 20 mm×70 mm.

[0138] The residue 3PB of chemically toughened and untoughened comparison and inventive working examples was tested as described above. The 3PB test is performed on small samples of 10 mm×10 mm.

Comparison Embodiment—Glass Types 1-4

[0139] Samples of different compositions and dimensions of 20 mm*50 mm, 20 mm*70 mm and 50 mm*50 mm and thicknesses of 0.07 mm, 0.1 mm, and 0.145 mm were produced as follows.

[0140] Samples of glass type 1-3 were produced in a down draw process to form ultrathin green glass articles. The samples of types 2 and 3 were then heat treated at 600-720° C. for 4-8 hours in a furnace. Because of the heat treatment β-quartz crystals (crystal phase) formed in the central part B (also called bulk) of the glass article 1 and there is still a 5 μm thick glass layer A, A′ at both surfaces 1, 3 of the article. The glass layers A, A′ have higher CTE than inner β-quartz layer (central part B), so there is stress being formed between A/B and A′/B interface.

[0141] Samples of glass type 4 were produced by an overflow process to form ultrathin green glass articles of 0.4 mm and were subsequently slimmed. Then the glass was irradiated, heated by laser (e.g., a 266 nm nano second laser) at 650-850° C. in the bulk (central part B) of the article. Because of the laser treatment β-Spodumene crystal formed in the central part B (bulk) and there was still glass layer A,A′ at both surfaces 1, 3 of the article. The glass layers A, A′ have higher CTE than the inner β-Spodumene layer (central part B), so there is stress being formed between A/B and A′/B interface.

[0142] Many samples of glass types 1-4 having the above mentioned dimensions and thicknesses were prepared, most of them were ceramized, and chemically toughened as given in table 2. After ion exchange, the toughened samples were cleaned and measured with FSM 6000 and SLP 1000.

[0143] 30 toughened samples of each thickness and each DoL were tested and evaluated in respect of impact resistance using the pen drop test and ball drop test as described above. The average breakage height was calculated as described above, and the B10 height was calculated using Weibull method. Further, for determining a breakage bending radius 30 toughened samples of each thickness and DoL were tested in the 2 point bending method described above. The average breakage bending radius was calculated.

[0144] The crystal phase was measured by XRD and the crystal size was calculated by Scherrer's formula.

TABLE-US-00014 TABLE 2 Composition types 1-4, toughening conditions and results (comparison examples) Type 1 Type 1 Type 2 Type 3 Type 4 Thickness/mm 0.07 0.1 0.1 0.1 0.145 Heat Step 1 620° C. 650° C. 655° C. treatment 1 h 1 h 1 h condition Step 2 700° C. 720° C. 730° C. 1.5 h 2 h 1.8 h Crystal phase — — β-quartz β-quartz β-quartz Crystal size/nm 40 50 30 Crystal homo- Yes Yes Yes geneous distribution in the bulk Vol. % of glass 30% 20% 45% phase in glass- ceramic Chem. Temp./ 420 420 420 420 420 Tough- ° C. ening Time/h 4 4 4 4 4 condition Salt 100% 100% 100% 100% 100% bath KNO.sub.3 KNO.sub.3 NaNO.sub.3 NaNO.sub.3 NaNO.sub.3 CS/MPa 850 856 108 132 126 DoL of K ion/μm 6.5 6.5 43 40 46 Average pen 18 31 16 17 28 drop height/mm B10 for pen 12 22 11 10 17 drop/mm Average <15 <25 <45 <45 <55 Breakage Bending radius/mm Average ball 15 23 14 18 25 drop height/cm

[0145] Type 2-4 samples are typical glass-ceramic articles which are homogeneously ceramized throughout the whole volume, i.e., they have no gradient structure or layered structure between the outer surfaces 2, 3 and the central part B. There is no outer layer having a decreasing or increasing crystal proportion, but the material is homogenously crystallized, and there might be a very thin glassy zone on top of the glass-ceramic articles. Comparing the ceramized and toughened type 2-4 results with the not ceramized but toughened type 1 results it can be seen that the impact resistance (indicated by the average pen drop height and the ball drop height) and the bending performance (indicated by the average breakage bending radius) of the ceramized types are worse than those of the only toughened glass articles. Such known glass-ceramic articles do not have better impact performance than toughened glass of comparable thickness.

Embodiment 1—Glass Type 5

[0146] Green glass of type 5 composition has been melted at 1500° C. for 4 hours and casted and annealed at 620° C. The green glass is diced and polished to the size of 20*50*0.07 mm, 20*70*0.07 mm, 50*50*0.07 mm and 10*10*0.07 mm. After that, most samples were ceramized in a furnace at a temperature of about 1000° C. for about 40 to 90 min. Thereafter, the ceramized and not ceramized samples were chemically toughened in pure KNO3. After ion exchange, the toughened samples were cleaned and measured with FSM 6000.

[0147] Toughened samples of each thickness and crystal phase proportion were tested and evaluated in respect of impact resistance using the pen drop test and the ball drop test as described above. The average breakage height was calculated as described above, and the B10 height was calculated using Weibull method. Further, for determining a breakage bending radius toughened samples of each thickness and crystal phase proportion were tested in the 2 point bending method described above. The average breakage bending radius was calculated. In addition, the residue 3PB strength was measured as described above. In each test/experiment a plurality of 30 samples of each group were tested and evaluated.

[0148] The crystal phase was measured by XRD and the crystal size was calculated by Scherrer's formula.

TABLE-US-00015 TABLE 3 Composition type 5, ceramization and toughening conditions and results Ex. 1 Ex. 2 Ex. 3 Ex. 4 Thickness/mm 0.07 0.07 0.07 0.07 Heat treatment condition Temperature/° C. 1000 1000 1000 Time/min 40 70 90 Crystal phase — Leucite Leucite Leucite Crystal size/nm 20 32 45 Crystal volume fraction at the surface 10% 17% 24% Crystal volume fraction at central part  0%  0%  0% Thickness of A + B + A/μm 5/60/5 8/54/8 12/46/12 Chemically toughening condition Temperature/° C. 420° C. 420° C. 420° C. 420° C. Time/h 4 4 4 4 Salt bath 100% KNO.sub.3 CS/MPa 345 338 326 309 DoL of K ion/μm 9.3 9.1 8.8 8.3 Average pen drop height/mm 21 28 35 41 B10 for pen drop/mm 13 18 23 28 Average breakage bending radius/mm <25 <25 <25 <25 Average ball drop height/cm 12 14 16 19 Residue 3PB strength/MPa 87 178 201 242

[0149] Inventive examples 2 to 4 show a decreasing gradient glass-ceramic type (decreasing gradient structure): The proportion (also called “fraction”) of crystal phase (here leucite) in the outer layers A, A′ is 10, 17 or 24 vol. % and thus higher than in the central part B, where it is 0 vol. %.

[0150] Comparing the not ceramized but toughened embodiment (example 1) with the ceramized and toughened inventive embodiments (examples 2-4) it can be seen that chemically toughened gradient glass-ceramic articles have better pen drop heights, ball drop heights and residue 3PB strengths and thus show improved impact performance and better scratch resistance than toughened glass of the same composition and thickness.

Embodiment 2—Glass Type 6

[0151] Green glass of type 6 composition has been melted at 1450° C. for 6 hours and casted and annealed at 580° C. The green glass was diced and polished to the size of 20*70*0.1 mm, 20*50*0.1 mm, 50*50*0.1 mm and 10*10*0.1 mm. Most small pieces were pre-treated. They were immersed in a mix salt bath (390° C. for 2 to 6 h) for a first ion exchange between Na-ions and K-ions, which could establish a decreasing gradient of K-ions from the surface to a certain depth of glass sheets. The pre-treated green glass sheets were then heat treated in furnace at a temperature of about 1100° C.-1160° C. for about 60 min., wherein a nepheline crystal phase (NaAlSiO4) was formed due to the increasing gradient of Na-ions from the surface to a certain depth. The heat treating process forms an increasing gradient of crystal phase in the outer layers A, A′.

[0152] The glass samples and glass-ceramic samples were then toughened in pure KNO3 (second ion exchange, chemically toughening). After ion-exchange, the toughened samples were cleaned and measured with FSM 6000.

[0153] Toughened samples of each thickness and crystal phase proportion were tested and evaluated in respect of impact resistance using the pen drop test and ball drop test as described above. The average breakage height was calculated as described above, and the B10 height was calculated using Weibull method. Further, for determining a breakage bending radius toughened samples of each thickness and crystal phase proportion were tested in the 2 point bending method described above. The average breakage bending radius was calculated. In addition, the residue 3PB strength was measured as described above. In each test/experiment a plurality of 30 samples of each group were tested and evaluated.

[0154] The crystal phase was measured by XRD and the crystal size was calculated by Scherrer's formula.

[0155] In these embodiments the CTE in the range 20-300° C. was determined for the outer layers A, A′ and the central part B. The CTE of the outer layers A, A′ is 7.3 ppm/K, while that of the central part B is around 8.7 ppm/K. Therefore a compressive stress is established at the interface between A/B and A′/B.

TABLE-US-00016 TABLE 4 Composition type 6, ceramization and toughening conditions and results Ex. 5 Ex. 6 Ex. 7 Ex. 8 Thickness/mm 0.1 0.1 0.1 0.1 Surface ion exchange to 50% KNO.sub.3/ 50% KNO.sub.3/ 50% KNO.sub.3/ modify the surface 50% NaNO.sub.3 50% NaNO.sub.3 50% NaNO.sub.3 composition Surface Temperature/ 390 390 390 ion ° C. exchange Time/h 2 4 4 condition K depth/μm 10 20 20 Heat Temperature/ 1100 1100 1160 treatment ° C. condition Time/h 1 1 1 Crystal phase — nepheline nepheline nepheline Crystal size/nm 60 60 80 Crystal volume fraction at  6%  6%  9% the surface Crystal volume fraction at 40% 40% 55% central part Thickness of A + B + A/μm 10/80/10 20/60/20 20/60/20 Chemically Temperature/ 430 430 430 toughening ° C. condition Time/h 2 2 2 Salt bath 100% KNO.sub.3 100% KNO.sub.3 CS/MPa 538 509 474 DoL of K ion/μm 20 18 16 Average pen drop height/mm 26 32 46 53 B10 for pen drop/mm 15 18 30 37 Average breakage bending radius/mm <20 <40 <15 <15 Average ball drop height/cm 13 16 19 22 Residue 3PB strength/MPa 93 87 459 514 Ex. 9 Ex. 10 Ex. 11 Thickness/mm 0.1 0.1 0.1 Surface ion exchange to modify the 70% KNO.sub.3/ 70% KNO.sub.3/ 70% KNO.sub.3/ surface composition 30% NaNO.sub.3 30% NaNO.sub.3 30% NaNO.sub.3 Surface ion Temperature/ 390 390 390 exchange condition ° C. Time/h 6 6 6 K depth/μm 25 25 25 Heat treatment Temperature/ 1100 1100 1160 condition ° C. Time/h 1 1 1 Crystal phase nepheline nepheline nepheline Crystal size/nm 60 60 80 Crystal volume fraction at the surface  4%  4%  7% Crystal volume fraction at central part 40% 40% 55% Thickness of A + B + A/μm 25/50/25 25/50/25 25/50/25 Chemically Temperature/ 430 430 toughening ° C. condition Time/h 2 2 Salt bath 100% KNO.sub.3 CS/MPa 410 386 DoL of K ion/μm 18 16 Average pen drop height/mm 29 42 49 B10 for pen drop/mm 16 27 31 Average breakage bending radius/mm <15 <15 Average ball drop height/cm 19 24 27 Residue 3PB strength/MPa 88 403 457

[0156] Inventive examples 6-11 show an increasing gradient glass-ceramic type. The proportion (here called “fraction”) of crystal phase (here nepheline) in the outer layers A, A′ is 4-9 vol. % and thus smaller than that of the central part B (40-55 vol. %).

[0157] Examples having a gradient glass ceramic structure (examples 6-11) show an increase of pen drop height compared with toughened but not ceramized examples 5 of the same composition and thickness. The residue 3PB strength and thus the scratch resistance is increased by chemically toughening. The improvement of pen drop height and residue 3PB strength is much better when the glass-ceramic article is additionally chemically toughened (please see example 7 in comparison to example 6 or example 10 in comparison to example 9).

Embodiment 3—Glass Type 7

[0158] Green glass of type 7 composition has been melted at 1550° C. for 4 hours and casted and annealed at 500° C. The green glass was diced and polished to the size of 20*70*0.1 mm, 20*50*0.1 mm and 50*50*0.1 mm.

[0159] Prior to ceramization the green glass articles were exposured at 355 nm ns laser inducing at both surfaces with certain depth. The frequency was 100 Hz and power was 600-800 mW. The energy was 2 J/cm2. Then the exposured samples were heat treated in a furnace in two steps as shown in table 5.

[0160] The glass samples and glass-ceramic samples were then toughened in pure KNO.sub.3. After ion-exchange, the toughened samples were cleaned and measured with FSM 6000

[0161] Toughened samples of each thickness and crystal phase proportion were tested and evaluated in respect of impact resistance using the pen drop test as described above. The average breakage height was calculated as described above, and the B10 height was calculated using Weibull method. In each test/experiment a plurality of 30 samples of each group were tested and evaluated.

[0162] The crystal phase was measured by XRD and the crystal size was calculated by Scherrer's formula.

TABLE-US-00017 TABLE 5 Composition type 7, ceramization and toughening conditions and results Ex. 12 Ex. 13 Ex. 14 Ex. 15 Thickness/mm 0.1 0.1 0.1 0.1 Heat treatment Step 1 500° C. 500° C. 500° C. condition 1 h 1 h 1 h Step 2 560° C. 560° C. 560° C. 1 h 2 h 4 h Crystal phase — Li.sub.2SiO.sub.3 Li.sub.2SiO.sub.3 Li.sub.2SiO.sub.3 Crystal size/nm 60 80 110 Crystal volume fraction 20% 27% 45% at the surface Crystal volume fraction  0%  0%  0% at central part Thickness of A + 20/60/20 28/44/28 40/20/40 B + A/μm Chemically Temper- 420 420 420 420 toughening ature/° C. condition Time/h 4 4 4 4 Salt bath 100% KNO.sub.3 CS/MPa 218 196 186 174 DoL of K ion/μm 9.3 8.7 8.6 7.5 Average pen drop 20 28 33 38 height/mm B10 for pen drop/mm 12 18 21 26 Average ball drop 13 17 20 23 height/cm

[0163] Inventive examples 13-15 show a decreasing gradient glass-ceramic type. The proportion (here called “fraction”) of crystal phase (here lithium silicate) in the outer layers A, A′ is 20-45 vol. % and thus higher than that of the central part B which is amorphous here (0%).

[0164] Comparing the not ceramized but toughened embodiment (example 12) with the ceramized and toughened inventive embodiments (examples 13-15) it can be seen that gradient glass-ceramic article have greater pen drop heights and ball drop heights and thus show improved impact performance as toughened glass of the same thickness. In this glass-ceramic type the impact performance raises with increasing crystal phase proportion, increasing thickness of outer layers, increasing crystal size.

Embodiment 4—Glass Type 8

[0165] Many samples of green glass type 8 composition having a size of 20*70*0.145 mm, 20*50*0.145 mm, 50*50*0.145 mm and 10*10*0.145 mm were prepared, ceramized and chemically toughened. Different ceramization conditions were employed.

[0166] Most small green glass samples were pre-treated. The small pieces were first immersed in a pure NaNO.sub.3 salt bath for ion exchange between Na-ions and Li-ions (390° C., 0.5-2h), which could establish a decreasing gradient of Na-ions from the surface to a certain depth of glass sheets. Then the pre-treated green glass sheets were heat treated, which formed a Lithium disilicate crystal phase (Li.sub.2Si.sub.2O.sub.5) due to the increasing gradient of Li-ions from the surface to a certain depth. The subsequent heat treating process formed an increasing gradient of crystal phase at this part. The results are displayed in Table 6 in details.

[0167] In example 17 of embodiment 4 the CTE in the range 20-300° C. was determined for the outer layers A, A′ and the central part B. The CTE at the outer surfaces is 7.7 ppm/K, while that of the central part B is around 9.8 ppm/K. Therefore, a compressive stress is established at the interface between A/B and A′/B.

[0168] The glass samples and most glass-ceramic samples were then toughened in mixture salt bath 90%KNO.sub.3/10% NaNO.sub.3. After ion-exchange, the toughened samples were cleaned and measured with FSM 6000 and SLP 1000.

[0169] Toughened samples of each thickness and crystal phase proportion were tested and evaluated in respect of impact resistance using the pen drop test as described above. The average breakage height was calculated as described above, and the B10 height was calculated using Weibull method. Further, for determining a breakage bending radius toughened samples of each thickness and crystal phase proportion were tested in the 2 point bending method described above. The average breakage bending radius was calculated. In addition, the residue 3PB strength was measured as described above. In each test/experiment a plurality of 30 samples of each group were tested and evaluated.

[0170] The crystal phase was measured by XRD and the crystal size was calculated by Scherrer's formula.

TABLE-US-00018 TABLE 6 composition type 8, ceramization and toughening conditions and results Ex. 16 Ex. 17 Ex. 18 Ex. 19 Thickness/mm 0.145 0.145 0.145 0.145 Surface ion exchange to modify 100% NaNO.sub.3 the surface composition Surface ion Temperature/° C. 390 390 390 exchange condition Time/h 0.5 1 2 Na depth/μm 15 22 30 Heat treatment Temperature/° C. 800 800 840 condition Time/h 1 1 1 Crystal phase — Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 Crystal size/nm 50 50 80 Crystal volume fraction at the surface  2%  2%  3% Crystal volume fraction at central part 43% 43% 51% Thickness of A + B + A/μm 17/111/17 25/95/25 34/77/34 Chemically toughening Temperature/° C. 380 380 380 380 condition Time/h 2 2 2 2 Salt bath 90% KNO.sub.3/ 90% KNO.sub.3/10% NaNO.sub.3 10% NaNO.sub.3 CS/MPa 838 784 757 DoL of K ion/μm 9.1 8.9 8.5 DoL of Na ion/μm 28 34 42 Average pen drop height/mm 54 45 67 73 B10 for pen drop/mm 38 32 45 56 Average breakage bending radius/mm <25 <50 <25 <25 Average ball drop height/cm 24 18 27 35 Residue 3PB strength/MPa 96 87 519 578

[0171] Inventive examples 17-19 show an increasing gradient glass-ceramic type. The proportion (here called “fraction”) of crystal phase (here lithium disilicate) in the outer layers A, A′ is 2-3 vol. % and thus smaller than that of the central part B (43-51 vol. %).

[0172] Chemically toughened inventive examples having a gradient glass ceramic structure (examples 18-19) show an increase of pen drop height, ball drop height and residue 3PB strength compared with toughened but not ceramized examples 16 of the same composition and thickness. An untoughened gradient glass ceramic article (example 17) does not seem to show improved features compared with only toughened glass (example 16) of the same composition and thickness. In this glass-ceramic type the improvement of impact resistance and residue 3PB strength is better when the glass-ceramic article is additionally chemically toughened (please see example 17 in comparison to example 18). Especially the residue 3PB strength is improved to a high extent for examples being both ceramized and toughened (examples 18, 19).

Embodiment 5 —Glass Type 9

[0173] Many samples of green glass type 9 composition having a size of 20*70*0.145 mm, 20*50*0.145 mm, 50*50*0.145 mm and 10*10*0.145 mm were prepared, ceramized, and chemically toughened.

[0174] Most small green glass samples were pre-treated. The small pieces were first immersed in a pure NaNO.sub.3 salt bath for ions exchange between Na and Li (390° C., 10-40 min), which can establish a decreasing gradient of Na-ions from the surface to certain depth of glass sheets. Then the pre-treated green glass sheets were heat treated, which can form a β-spodumene (LiAlSi.sub.2O.sub.6), Li.sub.2SiO.sub.3 crystal phase due to the increasing gradient of Li ion from the surface to certain depth. The heat treating process formed an increasing gradient of crystal phase at this part. The results are displayed in Table 7 in detail.

[0175] The glass samples and most glass-ceramic samples were then toughened in pure salt bath (NaNO.sub.3). After ion-exchange, the toughened samples were cleaned and measured with FSM 6000 and SLP 1000.

[0176] Toughened samples of each thickness and crystal phase proportion were tested and evaluated in respect of impact resistance using the pen drop test as described above. The average breakage height was calculated as described above, and the B10 height was calculated using Weibull method. Further, for determining a breakage bending radius toughened samples of each thickness and crystal phase proportion were tested in the 2 point bending method described above. The average breakage bending radius was calculated. In addition, the residue 3PB strength was measured as described above. In each test/experiment a plurality of 30 samples of each group were tested and evaluated.

[0177] The crystal phase was measured by XRD and the size is calculated by Scherrer's formula.

TABLE-US-00019 TABLE 7 composition type 9, ceramization and toughening conditions and results Ex. 20 Ex. 21 Ex. 22 Ex. 23 Thickness/mm 0.145 0.145 0.145 0.145 Surface ion exchange to modify 100% NaNO.sub.3 the surface composition Surface ion Temperature/° C. 390 390 390 exchange condition Time/h 10 20 40 Na depth/μm 10 14 20 Heat treatment Temperature/° C. 1000 1000 1000 condition Time/h 1 1 2 Crystal phase — β-spodumene (LiAlSi.sub.2O.sub.6), Li.sub.2SiO.sub.3 Crystal size/nm 60 60 80 Crystal volume fraction at the surface  4%  5%  8% Crystal volume fraction at central part 30% 30% 45% Thickness of A + B + A/μm 13/119/13 18/109/18 26/93/26 Chemically toughening Temperature/° C. 530 530 530 condition Time/h 0.5 0.5 0.5 Salt bath 100% NaNO.sub.3 100% NaNO.sub.3 CS/MPa 154 135 113 DoL of K ion/μm 20 24 29 Average pen drop height/mm 42 36 52 60 B10 for pen drop/mm 28 25 33 43 Average breakage bending radius/mm <25 <50 <25 <25 Average ball drop height/cm 18 12 21 29 Residue 3PB strength/MPa 76 67 219 278

[0178] Inventive examples 21-23 show an increasing gradient glass-ceramic type. The proportion (here called “fraction”) of crystal phase (here β-spodumene and lithium silicate) in the outer layers A, A′ is 4-8 vol. % and thus smaller than that of the central part B (30-45 vol. %).

[0179] Toughened inventive examples having a gradient glass ceramic structure (examples 22-23) show an increase of pen drop height, ball drop height and residue 3PB strength compared with toughened but not ceramized examples 20 of the same composition and thickness. An untoughened gradient glass ceramic article (example 21) does not seem to show improved features compared with only toughened glass (example 20) of the same composition and thickness. In this glass-ceramic type the improvement of impact resistance and residue 3PB strength is better when the glass-ceramic article is additionally chemically toughened (please see example 22 in comparison to example 21). Especially the residue 3PB strength is improved to a high extent for examples being both ceramized and toughened (examples 22, 23).

Embodiment 6—Glass Type 10

[0180] Green glass of type 10 composition has been melted at 1600° C. for 8 hours and casted and annealed at 630° C. The green glass is diced and polished to the size of 20*70*0.25 mm, 20*50*0.25 and 50*50*0.25 mm with many pieces.

[0181] The green glass articles were heated by laser at 1000-1300° C. by using 266 nm ns laser inducing at both surfaces with certain depth (corresponding to outer layers A and A′). The induced areas forming a layer in each case having homogeneous crystal distribution wherein the crystals are imbedded inside an amorphous glass phase. The glass samples and glass-ceramic samples were then toughened in 90% Li.sub.2SO.sub.4+10% K.sub.2SO.sub.4. The CS and DoL was not able to be measured by either FSM 6000 or SLP 1000.

[0182] Toughened samples of each thickness and crystal phase proportion were tested and evaluated in respect of impact resistance using the ball drop test as described above. The average breakage height was calculated as described above. In each test/experiment a plurality of 30 samples of each group were tested and evaluated.

TABLE-US-00020 TABLE 8 Composition type 10, ceramization and toughening conditions and results Ex. 24 Ex. 25 Ex. 26 Thickness/mm 0.25 0.25 0.25 Laser heat treatment 1200° C. 2 h 1200° C. 4 h Crystal phase MgTi.sub.2O.sub.5 MgTi.sub.2O.sub.5 Crystal size 56 74 A homogeneous crystal volume 28% 37% fraction from the surface to a depth of 30 μm Crystal volume fraction at central  0%  0% part Thickness of A + B + A/μm 30/190/30 30/190/30 Chemically Temperature/° C. 800 800 800 toughening Time/h 4 4 4 condition Salt bath 90% Li.sub.2SO.sub.4 + 10% K.sub.2SO.sub.4 Average ball drop height/cm 43 57 60

[0183] Inventive examples 25 and 26 show a layered glass-ceramic type. The proportion (here called “fraction”) of crystal phase (here MgTi.sub.2O.sub.5) in the outer layers A, A′ is 28 to 37 vol. % while the central part is amorphous, i.e., consists of glass.

[0184] Toughened inventive examples having a layered glass ceramic structure (examples 25 and 26) show an increase of ball drop height compared with toughened but not ceramized examples 24 of the same composition and thickness. Thus, a layered glass-ceramic article has improved properties.

[0185] As can be seen from the explanations and experimental data above the invention provides a new ultrathin article having improve material properties. As far as impact resistance and scratch resistance are concerned, a glass-ceramic article having a gradient structure or a layered structure has improved properties compared with both known glass articles and known glass-ceramic articles of the same thickness, especially when the ultrathin glass-ceramic article is additionally chemically toughened in addition.