Patent Description:
The present invention relates to a method comprises: immersing a glass-based substrate comprising a glass ceramic in a molten salt bath to produce a glass-based article, wherein prior to the immersion of any glass-based substrate in the molten salt bath, the molten salt bath comprises: a nitrate salt, and a nitrite salt in an amount of greater than or equal to about <NUM> wt% and less than or equal to <NUM> wt%. The nitride salt comprises NaNO<NUM>.

<CIT>, <CIT>, <CIT> and <CIT> disclose methods of chemical strengthening glass substrates by immersing the same into molten salt baths of nitrate salts containing nitride salts.

In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that, unless otherwise specified, terms such as "top," "bottom," "outward," "inward," and the like are words of convenience and are not to be construed as limiting terms. In addition, whenever a group is described as comprising at least one of a group of elements and combinations thereof, it is understood that the group may comprise, consist essentially of, or consist of any number of those elements recited, either individually or in combination with each other. Similarly, whenever a group is described as consisting of at least one of a group of elements or combinations thereof, it is understood that the group may consist of any number of those elements recited, either individually or in combination with each other. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range as well as any ranges therebetween. As used herein, the indefinite articles "a," "an," and the corresponding definite article "the" mean "at least one" or "one or more," unless otherwise specified. It also is understood that the various features disclosed in the specification and the drawings can be used in any and all combinations.

Unless otherwise specified, all compositions of the glass ceramics described herein are expressed in terms of mole percent (mol%), and the constituents are provided on an oxide basis. Unless otherwise specified, all temperatures are expressed in terms of degrees Celsius (°C).

It is noted that the terms "substantially" and "about" may be utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. For example, a glass that is "substantially free of K<NUM>O" is one in which K<NUM>O is not actively added or batched into the glass, but may be present in very small amounts as a contaminant, such as in amounts of less than about <NUM> mol%.

Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying examples and drawings.

Strengthened glass-based articles may be produced by chemical strengthening. The strengthened glass-based articles exhibit improved mechanical strength and reliability. In one method of chemical strengthening, glass-based substrates are ion exchanged to produce a strengthened glass-based article. As used herein, a "glass-based article" refers to an ion exchanged glass-based article, and a "glass-based substrate" refers to the glass-based precursor that is chemically strengthened to form the glass-based article. The glass based substrates used in the method of the invention comprise glass ceramics.

The ion exchange process includes immersing a glass-based substrate in a molten alkali salt bath to exchange the alkali ions from the salt bath for alkali ions that are present in the glass-based substrate. The molten alkali salt bath is maintained at a temperature below the glass transition temperature of the glass-based substrate. Where the ions exchanged from the molten salt bath into the glass-based substrate have a larger ionic radius than the ions that they replace in the glass-based substrate, a glass-based article is produced that includes a compressive stress layer at the surface. This compressive stress layer strengthens the glass-based article, and reduces the likelihood of breakage from the introduction of surface flaws to the glass-based article. These strengthened glass-based articles are commonly employed in applications where scratch-resistance, abrasion resistance, and fracture resistance are desired.

The molten salt bath composition employed in ion exhanged processes affects the efficiency and effectiveness of the ion exchange, and thereby impacts the properties of the glass-based articles produced by ion exchange. Impurities in the molten salt bath may be detrimental to the ion exchange process. In some circumstances, impurities may form an inorganic layer on the surface of the glass-based substrate that inhibits the ion exchange, thereby reducing the mechanical strength of the resulting glass-based article. This "blocking" inorganic layer issue is particularly prevalent for glass-based substrates that include soda lime silicates. It has also been observed that an inorganic layer may be formed on the surface of glass-based substrates that contain lithium and/or sodium. The inorganic layer may contribute to a hazy appearance of the glass-based article, which is undesirable. Impurities that may be present in the molten salt bath and contribute to the formation of the inorganic layer include magnesium and calcium. To avoid the formation of an inorganic layer, a high purity molten salt bath may be utilized that includes impurities at a level below a part per million. However, high purity molten salt baths impart an added cost that is undesirable.

The methods described herein address the formation of an inorganic layer on glass-based articles due to the presence of impurities in a molten salt bath, without requiring the use of a high purity molten salt bath. Additionally, the methods described herein allow the production of a glass-based article that is substantially free or free of haze without requiring post-ion exchange polishing or removal of an inorganic layer.

The methods described herein prevent or minimize the formation of an inorganic layer on glass-based articles by increasing the presence of a nitrite salt in molten nitrate salt baths. The increase in nitrite salt concentration is achieved by adding a nitrite salt to a fresh molten nitrate salt bath. As utilized herein, a "fresh molten nitrate salt bath" is a bath in which no glass-based substrates have been submersed for the purposes of ion exchange.

Without wishing to be bound by any theory, the increased nitrite salt concentration in the molten nitrate salt bath may prevent the deposition of impurities in the molten salt bath as oxides in an inorganic layer on the surface of the glass-based article. The deposition occurs when oxide species of the impurities are present in the molten salt bath. The impurities are often originally present in the molten salt bath as nitrate salts. These nitrate salts may then decompose to form an oxide of the impurity as well as the gasous compoents nitrogen dioxide and oxygen. The decomposition reaction proceeds at elevated temperatures, such as above about <NUM> for Mg(NO<NUM>)<NUM>, producing impurity oxide constituents in the molten salt bath. The decomposition reaction is demonstrated by the following equation, where magnesium is utilized as a representative impurity:.

<NUM>(NO<NUM>)<NUM> → 2MgO + 4NO<NUM> + O<NUM>.

Accordingly, an increase in the nitrite concentration in the molten salt bath inhibits the decomposition reaction (shifts quilibrium towards the left in the above equation), preventing the formation of oxides in the molten salt bath. The impurity oxides present in the bath have been shown to attach to the surface of the glass-based article during the ion exchange process. The resulting deoposits may produce haze and an inorganic layer of the type described above which reduces the mechanical strength of the glass-based articles. Thus, reducing the impurity oxides present in the molten salt bath reduces the incidence of haze and produces glass-based articles with increased strength.

The method of increasing the nitrite (-NO<NUM>) salt content of a molten nitrate (-NO<NUM>) salt bath includes adding a nitrite salt directly to the bath. The nitrite salt is added to the bath in any amount sufficient to reduce and/or eliminate the formation of impurity oxides in the bath. The molten nitrate salt bath includes a nitrite salt in an amount of greater than or equal to about <NUM> wt% and less than or equal to <NUM> wt%, such as about <NUM> wt% to about <NUM> wt%. In some embodiments, the nitrite salt is added to the bath in an amount of about <NUM> wt%.

In some embodiments, the nitrite salt added to the molten nitrate salt bath may be a salt of the same component as the nitrate salt. The nitrite salt comprises NaNO<NUM>.

The molten nitrate salt bath used as a starting point for any of the above nitrite concentration increase methods may be any molten nitrate salt bath suitable for ion exchange. In some embodiments, the molten nitrate salt bath may include at least one of NaNO<NUM>, LiNO<NUM>, KNO<NUM>, RbNO<NUM>, and AgNO<NUM>. In some embodiments, the molten nitrate salt bath may include a mixture of nitrates, such as sodium nitrate and potassium nitrate. The composition of the molten nitrate salt bath may be selected based on the composition of the glass-based substrate and the desired stress profile characteristics.

The nitrite salt containing molten nitrate salt baths may be employed to produce a strengthened glass-based article. The glass-based articles are produced by immersing a glass-based substrate in the nitrite salt containing molten salt bath. The resulting glass-based articles have a compressive stress layer extending from the surface of the glass-based article to a depth of compression.

The glass-based articles may have a flexural strength of at least about <NUM> MPa, such as at lesat about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, or more.

The glass-based articles may be substantially free or free of haze. In some embodiments, the glass-based articles may have a surface that is at least <NUM>% free of haze. Stated differently, at least <NUM>% of the surface area of the glass-based articles may exhibit no haze. In some embodiments, the glass-based articles are free of haze.

The glass-based articles may be substantially free or free of deposits of magnesium and calcium. In some embodiments, the glass-based articles may have a surface that is at least <NUM>% free of deposits of magnesium and calcium. Stated differently, at least <NUM>% of the surface area of the glass-based articles may exhibit no deposits of magnesium and calcium. In some embodiments, the glass-based articles are free of deposits of magnesium and calcium.

The glass-based substrates utilized to form the glass-based articles described herein may be any appropriate material. In some embodiments, the glass-based substrates are a glass, such as a soda lime silicate glass, lithium silicate glass, or a sodium silicate glass. In some embodiments, the glass-based substrate may be a glass ceramic, such as a petalite crystalline phase containing glass ceramic. In some embodiments, the glass-based substrate includes at least one of Li<NUM>O, Na<NUM>O, K<NUM>O, and Rb<NUM>O.

The strengthened glass-based articles disclosed herein may be incorporated into another article such as an article with a display (or display articles) (e.g., consumer electronics, including mobile phones, tablets, computers, navigation systems, wearable devices (e.g., watches) and the like), architectural articles, transportation articles (e.g., automotive, trains, aircraft, sea craft, etc.), appliance articles, or any article that requires some transparency, scratch-resistance, abrasion resistance or a combination thereof. An exemplary article incorporating any of the strengthened articles disclosed herein is shown in <FIG>. Specifically, <FIG> show a consumer electronic device <NUM> including a housing <NUM> having front <NUM>, back <NUM>, and side surfaces <NUM>; electrical components (not shown) that are at least partially inside or entirely within the housing and including at least a controller, a memory, and a display <NUM> at or adjacent to the front surface of the housing; and a cover substrate <NUM> at or over the front surface of the housing such that it is over the display. In some embodiments, at least one of the cover substrate <NUM> or a portion of housing <NUM> may include any of the strengthened articles disclosed herein.

As a non-limiting exemplary embodiment, <NUM> wt% of NaNO<NUM> was added to a NaNO<NUM> molten salt bath. A petalite crystalline phase containing glass ceramic substrate was ion exchanged in the nitrite containing bath for <NUM> hours at a temperature of <NUM>. As a comparative example, a petalite crystalline phase containing glass ceramic substrate was ion exchanged in a nitrite-free NaNO<NUM> bath for <NUM> hours at a temperature of <NUM>. The nitrite-free bath was not aged prior to the ion exchange. As shown in <FIG>, the glass ceramic article produced with the nitrite containing bath <NUM> was haze-free, while the comparative glass ceramic article ion exchanged in the nitrite-free bath <NUM> exhibited a hazy surface. The surface of the glass ceramic article ion exchanged in the nitrite-free bath <NUM> was examined with a scanning eletron microscope (SEM), and inhomogeneous deposits <NUM> were detected as shown in <FIG>.

A portion of the surface of the glass ceramic article ion exchanged in the nitrite-free bath <NUM> containg a deposit <NUM> was subjected to an energy dispersive spectroscopy (EDS) analysis. As shown in <FIG>, the EDS analysis indicates that the deposit contains magnesium. By contrast, an area of the surface that did not contain a deposit did not contain magnesium, as shown in <FIG>. These results support the conclusion that the deposits and resulting haze are due at least in part to impurities, such as magnesium and calcium, present in the molten salt bath.

As additional reference examples, petalite containing glass ceramic substrates were ion exchanged in sodium nitrate baths that were aged at a temperature of <NUM> for <NUM> hours and <NUM> hours, under the same ion exchange conditions as the previous example. The flexural strength of the glass ceramics ion exchanged in the unaged (nitrite-free) bath, <NUM> wt% NaNO<NUM> added bath, <NUM> hour aged bath, and <NUM> hour aged bath was then measured with ring on ring testing, as described below. As shown in <FIG>, the unaged (nitrite-free) bath produced a glass ceramic article with a significantly lower flexural strength than the glass ceramic articles produced with the nitrite containing baths of the type described herein. <FIG> is a Weibull plot, where each data point represents the percentage of the samples that fail at a given flexural stress. This data further indicates that the presence of haze is correlated to a reduced flexural strength in ion exchanged articles.

According to one or more embodiments, the increase or decrease in strength on one side of a glass-based substrate can be determined using ring on ring (RoR) testing. The strength of a material is defined as the stress at which fracture occurs. The RoR test is a surface strength measurement for testing flat glass specimens, and ASTM C1499-<NUM>(<NUM>), entitled "Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature," serves as the basis for the RoR test methodology described herein.

For the RoR test, a glass-based article as shown in Figure <NUM> is placed between two concentric rings of differing size to determine equibiaxial flexural strength (i.e., the maximum stress that a material is capable of sustaining when subjected to flexure between two concentric rings). In the RoR configuration <NUM>, the glass-based article <NUM> is supported by a support ring <NUM> having a diameter D2. A force F is applied by a load cell (not shown) to the surface of the glass-based article by a loading ring <NUM> having a diameter D <NUM>.

The ratio of diameters of the loading ring and support ring D1/D2 may be in a range from <NUM> to <NUM>. In some embodiments, D1/D2 is <NUM>. Loading and support rings <NUM>, <NUM> should be aligned concentrically to within <NUM>% of support ring diameter D2. The load cell used for testing should be accurate to within ±<NUM>% at any load within a selected range. Testing is carried out at a temperature of <NUM>±<NUM> and a relative humidity of <NUM>±<NUM>%.

For fixture design, the radius r of the protruding surface of the loading ring <NUM> is in a range of h/<NUM> ≤ r ≤ <NUM>/<NUM>, where h is the thickness of glass-based article <NUM>. Loading and support rings <NUM>, <NUM> are made of hardened steel with hardness HRc > <NUM>. RoR fixtures are commercially available.

The intended failure mechanism for the RoR test is to observe fracture of the glass-based article <NUM> originating from the surface 430a within the loading ring <NUM>. Failures that occur outside of this region - i.e., between the loading ring <NUM> and support ring <NUM> - are omitted from data analysis. Due to the thinness and high strength of the glass-based article <NUM>, however, large deflections that exceed ½ of the specimen thickness h are sometimes observed. It is therefore not uncommon to observe a high percentage of failures originating from underneath the loading ring <NUM>. Stress cannot be accurately calculated without knowledge of stress development both inside and under the ring (collected via strain gauge analysis) and the origin of failure in each specimen. RoR testing therefore focuses on peak load at failure as the measured response.

Claim 1:
A method, comprising:
immersing a glass-based substrate comprising a glass ceramic in a molten salt bath to produce a glass-based article,
wherein prior to the immersion of any glass-based substrate in the molten salt bath, the molten salt bath comprises:
a nitrate salt, and
a nitrite salt in an amount of greater than or equal to <NUM> wt% and less than or equal to <NUM> wt%, wherein the nitrite salt comprises NaNO<NUM>..