Durable glass ceramic cover glass for electronic devices

Abstract

The invention relates to glass articles suitable for use as electronic device housing/cover glass which comprise a glass ceramic material. Particularly, a cover glass comprising an ion-exchanged glass ceramic exhibiting the following attributes (1) optical transparency, as defined by greater than 90% transmission at 400-750 nm; (2) a fracture toughness of greater than 0.6 MPa·m^1/2; (3) a 4-point bend strength of greater than 350 mpa; (4) a vickers hardness of at least 450 kgf/mm² and a vickers median/radial crack initiation threshold of at least 5 kgf; (5) a Young's Modulus ranging between about 50 to 100 GPa; (6) a thermal conductivity of less than 2.0 W/m° C., and (7) and at least one of the following attributes: (i) a compressive surface layer having a depth of layer (DOL) greater and a compressive stress greater than 400 mpa, or, (ii) a central tension of more than 20 mpa.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This
wherein t is the thickness of the glass ceramic article. Unless otherwise specified, central tension CT and compressive stress CS are expressed herein in megaPascals (MPa), whereas thickness t and depth of layer DOL are expressed in millimeters.

It should be noted that in addition to single step ion exchange processes, multiple ion exchange procedures can be utilized to create a designed ion exchanged profile for enhanced performance. That is, a stress profile created to a selected depth by using ion exchange baths of differing concentration of ions or by using multiple baths using different ion species having different ionic radii. Additionally one or more heat treatments can be utilized before or after ion exchange to tailor the stress profile.

This requisite fracture toughness in excess of 0.6 MPa·m^1/2, in combination with 20 μm/surface compressive stress exceeding 400 MPa combination (or CT exceeding 20 MPa), the Vickers hardness/indentation threshold requirements, and the 4-point bend strength of greater than 350 MPa, all function to result in an cover glass which is sufficiently strong and durable so as to withstand typical consumer use/applications. One measure of this durability feature which the aforementioned ion-exchanged glass ceramic article is capable of meeting is the ability of the ion exchanged glass ceramic article to withstand a standard drop testing requirement involving numerous (e.g., 5) impacts/drops from a height of one meter onto a hard surface such as concrete or granite.

Referring now particularly to the thermal conductivity attribute, it should be noted that thermal conductivities of the desired level, particularly of less than 4 W/m° C., are likely to result in a cover glass that remains cool to the touch even in high temperatures approaching as high as 100° C. Preferably, a thermal conductivity of less than 3 W/m° C., and less than 2 W/m° C. are desired. Representative thermal conductivities* (in W/m° C.) for some suitable silicate glass-ceramics (discussed in detail below) include the following:

Cordierite glass-ceramic         3.6
β-spodumene (Corningware)        2.2
β-quartz (Zerodur)               1.6
Wollastonite (Example 9 - below) 1.4
Machinable mica (Macor)          1.3

*(see A. McHale, Engineering properties of glass-ceramics, in Engineered Materials Handbook, Vol. 4, Ceramics and Glasses, ASM International 1991, hereby incorporated by reference in its entirety.)

Other glass-ceramics which exhibit the requisite thermal conductivity feature included lithium disilicate based and canasite glass ceramics both of which are expected to exhibit thermal conductivity value of less than 4.0 W/m° C. For comparison, it should be noted that a ceramic such as alumina may exhibit undesirable thermal conductivities as high as 29.

In another exemplary embodiment the article, particularly the glass ceramic cover glass exhibits radio and microwave frequency transparency, as defined by a loss tangent of less than 0.015 over the frequency range of between 500 MHz to 3.0 GHz. This radio and microwave frequency transparency feature is especially important for wireless hand held devices that include antennas internal to the device. This radio and microwave transparency allows the wireless signals to pass through the cover glass and in some cases enhances these transmissions. Furthermore, it may also be desirable to be transparent in the infrared to allow wireless optical communication between electronic devices; specifically an infra-red transparency of greater than 80% at wavelengths ranging from 750 to 2000 nm. For example IR communication can be used to download music files to a portable music player, or workout data can be uploaded from a GPS or heart rate monitor to a computer for analysis.

In certain embodiments the glass ceramic cover glass has at least one surface exhibiting a Ra roughness of less than 50 nm, preferably less than 15 nm. In order to achieve this level of surface roughness, one option is to polish the surface using standard polishing techniques so as to achieve the requisite surface roughness of less than 50 nm, preferably less than 15 nm. Alternatively, the glass ceramic article can formed using a mold having a polished or non-textured surface so as to achieve the requisite surface roughness of less than 50 nm, preferably less than 15 nm.

One specific glass ceramic is the β-quartz solid solution shown in Table 12:

TABLE 12

SiO2                  65.3 (wt %)
Al2O3                 20.1 (wt %)
B2O3                  2.0 (wt %)
Li2O                  3.6 (wt %)
Na2O                  0.3 (wt %)
K2O
MgO                   1.8 (wt %)
MgF2
CaO
CaF2
SrO
BaO
ZnO                   2.2
P2O5
TiO2                  4.4
ZrO2
SnO2                  0.3
Crystal               (1)
Strain (° C.)         792
Anneal (° C.)         876
CTE (×10−7/° C.) 25-300° C.
Density (g/cm3)       2.525
Liq. Temp             1210
Liq. Visc (Poise)     18000
RoR Strength (MPa)    350
IX RoR Strength (MPa) 700
Fract Tough (MPa      1
m1/2)
Modulus (Mpsi)        12.448
Shear Mod (Mpsi)      5.001
P Ratio               0.245

Generally, the process for forming any of the representative glass-ceramic materials detailed herein comprises melting a batch for a glass consisting essentially, in weight percent on the oxide basis as calculated from the batch, of a composition within the range set forth above. It is within the level of skill for those skilled in the glass-ceramic art to select the required raw materials necessary as to achieve the desired composition. Once the batch materials are sufficiently mixed and melted, the process involves cooling the melt at least below the transformation range thereof and shaping a glass article therefrom, and thereafter heat treating this glass article at temperatures between about 650-1,200° C. for a sufficient length of time to obtain the desired crystallization in situ. The transformation range has been defined as that range of temperatures over which a liquid melt is deemed to have been transformed into an amorphous solid, commonly being considered as being between the strain point and the annealing point of the glass.

The glass batch selected for treatment may comprise essentially any constituents, whether oxides or other compounds, which upon melting to form a glass will produce a composition within the aforementioned range. Fluorine may be incorporated into the batch using any of the well-known fluoride compounds employed for the purpose in the prior art which are compatible with the compositions herein describe

Heat treatments which are suitable for transforming the glasses of the invention into predominantly crystalline glass-ceramics generally comprise the initial step of heating the glass article to a temperature within the nucleating range of about 600-850° C. and maintaining it in that range for a time sufficient to form numerous crystal nuclei throughout the glass. This usually requires between about ¼ and 10 hours. Subsequently, the article is heated to a temperature in the crystallization range of from about 800-1,200° C. and maintained in that range for a time sufficient to obtain the desired degree of crystallization, this time usually ranging from about 1 to 100 hours. Inasmuch as nucleation and crystallization in situ are processes which are both time and temperature dependent, it will readily be understood that at temperatures approaching the hotter extreme of the crystallization and nucleation ranges, brief dwell periods only will be necessitated, whereas at temperatures in the cooler extremes of these ranges, long dwell periods will be required to obtain maximum nucleation and/or crystallization.

Additionally, the heat treatment can be optimized to produce glass ceramics with high transmission properties. Such procedures are described in U.S. Publ. No. 2007/0270299, herein incorporated by reference in its entirety.

It will be appreciated that numerous modifications in the crystallization process are possible. For example, when the original batch melt is quenched below the transformation range thereof and shaped into a glass article, this article may subsequently be cooled to room temperature to permit visual inspection of the glass prior to initiating heat treatment. It may also be annealed at temperatures between about 550-650° C. if desired. However, where speed in production and fuel economies are sought, the batch melt can simply be cooled to a glass article at some temperature just below the transformation range and the crystallization treatment begun immediately thereafter.

Glass-ceramics may also be prepared by crystallizing glass fits in what is referred to as powder processing methods. A glass is reduced to a powder state, typically mixed with a binder, formed to a desired shape, and fired and crystallized to a glass-ceramic state. In this process, the relict surfaces of the glass grains serve as nucleating sites for the crystal phases. The glass composition, particle size, and processing conditions are chosen such that the glass undergoes viscous sintering to maximum density just before the crystallization process is completed. Shape forming methods may include but are not limited to extrusion, pressing, and slip casting.

Various modifications and variations can be made to the materials, methods, and articles described herein. Other aspects of the materials, methods, and articles described herein will be apparent from consideration of the specification and practice of the materials, methods, and articles disclosed herein. It is intended that the specification and examples be considered as exemplary.

Claims

13. An article suitable as a cover glass for a portable electronic device, the article comprising a glass ceramic, the glass ceramic having a primary crystalline phase of transparent spinel or transparent mullite and exhibiting:

a. optical transparency of greater than 60%, as defined by the transmission of light over the range of from 400-750 nm through 1 mm of the glass ceramic;

b. colorlessness, as defined by having the values of L*≥90; 0.1≥a*≥−0.1; and 0.4≥b*≥−0.4 on the CIE 1976 Lab color space as measured in transmission through 1 mm of glass ceramic; and

c. at least one of the following attributes:

(i) a fracture toughness of greater than 1.0 MPa·m^1/2;

(ii) an mor of greater than 135 mpa

(iii) a knoop hardness of at least 400 kg/mm²;

#20# (iv) a thermal conductivity of less than 4 W/m° C.; and or

(v) a porosity of less than 0.1%.