DE102006056088B9 - Process for increasing the strength of lithium alumino-silicate glass-ceramic by surface modification and produced by this method lithium alumino-silicate glass-ceramic - Google Patents

Abstract

Process for increasing the strength of a lithium alumino-silicate glass-ceramic by surface modification, characterized by the following steps: the surface of the green glass is coated prior to ceramification with a solution or suspension of a lithium salt or an organic compound decomposable to a lithium salt during ceramification; the lithium salt has a melting point of at least 720 ° C, - the coating is dried, - coating and drying are repeated if necessary, - until the layer produced on the glass surface has a Li + ion content of 10 to 5000 mg Li + / m2, - the coated Green glass is ceramized.

Description

The invention relates to a method according to the preamble of patent claim 1.

So far, an increase in strength of lithium alumino-silicate glass-ceramic has been carried out by ion exchange in the ceramized state. For this purpose, the glass ceramic is usually immersed in a molten salt, which consists of salts with the cation to be exchanged, or coated with a salt paste or another common method for coating application is used (screen printing, spraying, brushing, etc ...). The increase in strength is achieved by the replacement of an ion with a smaller ionic radius by an ion with a larger ionic radius, such. In US 3,573,072 A or US 4,755,488 A described. During the previous ceramization, a surface layer with a residual glass phase is formed in the glass ceramic. In the residual glass phase, a pressure bias is then generated by the ion exchange. A disadvantage of the ion exchange in the salt bath is that the surface of the glass ceramic is often attacked by the melt and that the handling of the hot molten salts is expensive. An ion exchange on glass-ceramics, which takes place as a solid-state reaction, is called in DE 38 40 071 C2 the applicant described.

A disadvantage of the ion exchange processes, which are all carried out on the finished glass ceramic, is a high energy consumption, a high consumption of chemicals, a complex process control (eg salt melts at temperatures up to 850 ° C.) and an often poor surface quality of the glass ceramic after ion exchange. These disadvantages have hitherto prevented the large-scale implementation of ion exchange processes for increasing the strength and surface modification of glass ceramics.

The object of the invention is to find a method for increasing the strength and surface modification of lithium alumino-silicate glass-ceramic, which with minimal energy and chemical consumption and simple process control, the production of a glass ceramic with increased strength and, if necessary, modified surface (eg in terms of aesthetics or surface finish, roughness, etc ...).

This object is described by the method described in the patent claims or the glass ceramic.

In complete departure from the ion exchange processes known hitherto, it is possible with the method according to the invention to produce a surface modification during the ceramization of the green glass to the glass ceramic in the surface of the glass ceramic forming, with which the desired increase in strength and / or surface modification is achieved. Instead of the usual glassy surface layer, a crystalline surface layer of Keatitmischkristall (KMK) is now produced. When using the method according to the invention, it is immaterial whether the glass ceramic contains HQMK or KMK as the main crystal phase.

First, the surface of the green glass is coated with a solution of a lithium salt or the salt is applied in another common layer application method (screen printing, spraying, brushing, etc ...). The solution is dried and the green glass is passed as usual through the Keramisierungsofen and ceramized. The glass ceramic produced in this way has the strength-increasing surface layer of Keatitmischkristall (KMK) and / or other surface modifications.

Keatite mixed crystal has long been a common term in the art. By this is meant a main crystal phase in the system of lithium aluminosilicate glass-ceramics. Above the composition range for KMK, the metastable high quartz mixed crystal HQMK converts to KMK at temperatures above about 900 ° C. The solidus temperature of the phases is between 1350 ° C and 1450 ° C. Keatite mixed crystals, and with them as the most important representative β-spodumene (LiAlSi ₂ O ₆ ), like HQMK, have a "stuffed structure". They are isotypic with the tetragonal SiO ₂ modification keatite (space group P4 ₃ 2 ₁ 2 or P4 ₁ 2 ₁ 2). Si ⁴⁺ ions and Al ³⁺ ions are statistically distributed in tetrahedral oxygen coordination. Li is placed in channels like HQMK and is tetrahedrally coordinated by O. The gaps available for Li occur in pairs, but at most one of these gaps is occupied by Li. This results in a maximum proportion of 4 Li atoms per unit cell. In the pure Li ₂ O.Al ₂ O ₃ .nSiO ₂ system, the metastable HQMK converts into the stable phase β-spodumene (n = 4), or KMK, at around 3.5 ° C starting at approximately 950 ° C. It is not possible to crystallize KMK directly from the glass phase, since the formation of the KMK always proceeds via the metastable intermediate phase of the HQMK. Considering a formula unit Li _n Al _n Si _3-n O ₆ , it is true that KMK from n ≤ 1.1 forms. As the Si content increases, the transformation temperature of HQMK in KMK decreases; Similarly, the lattice constants of the KMK decrease.

The irreversible transformation of HQMK into KMK is indeed reconstructive, but nevertheless coupled only with very small atomic motions in the structure and therefore only with a very low negative reaction enthalpy. In this exothermic transformation, the 6-membered rings of the (Si, Al) tetrahedra in the HQMK structure rearrange into 5-membered and 7-membered rings. The driving force for the transformation is the increase of the Si / Al-Li distance and thus the reduction of internal stresses. Overall, there is a very close structural connection between the keatite structure and the hexagonal high quartz structure, which, when set up with another unit cell, can be considered pseudo-tetragonal.

After drying the solution, the amount of lithium compound present in the coating layer, calculated as Li ⁺ ions, should be about 10 to 2000 mg Li ⁺ / m ² . Lesser amounts of lithium ions have only a small effect, larger amounts of Li ⁺ ions can lead to optical defects, such as to a craquel formation or secondary phase precipitation on the surface (eg, Willmentit crystals at Zn-containing material). Preference is given to an amount of from 50 to 1000 mg, in particular from 100 to 500 mg of lithium ions per square meter of glass surface.

If, after the first application and drying of the lithium salt solution, sufficient lithium ion is not yet available on the glass surface, the application and drying can be repeated until the desired amount of Li ⁺ ion is present in the surface layer. The drying should preferably be rapid, so that a uniform Li-salt layer results and thus no formation of larger Li salt crystals can occur, which can lead to worse results in individual cases.

Suitable lithium salt are all lithium salts which have a melting point of more than 720 ° C, z. Li ₂ SO ₄ , Li ₂ CO ₃ and the like. It is also possible to use decomposable organic lithium compounds in the ceramizing furnace if the salts formed during the decomposition in the ceramizing furnace have a higher melting point than 720 ° C. An example of such an organic compound is z. B. lithium oxalate whose decomposition product is lithium oxide. Lithium acetate is also usable. Other examples are the lithium salts of alkane, alkene and hydroxyalkene sulfonic acids. In particular, the short-chain sulfonic acid salts having 1 to 5 carbon atoms in the organic radical are suitable. However, care must be taken that decomposition of the organic compounds does not produce byproducts adversely affecting the surface of the glass-ceramic plate. So z. For example, a fluorochemical compound is generally undesirable because of the known attack of HF on silicon.

If the lithium salt has a melting point of less than 720 ° C., the danger of surface defects forming in the surface of the glass ceramic produced becomes ever greater. So z. B. the use of melting at about 600 ° C lithium chloride often cracking in the surface cracking, whereby the strength is significantly reduced. However, this effect may deliberately be used as an appearance variation to produce aesthetic effects, provided that strength enhancement is not the primary purpose and used in applications that do not require high strength.

To achieve a good and uniform wetting and distribution of the solution or the suspension, it may be advantageous that a small amount of wetting agent is added, if the lithium compounds used or the solvent / suspending agent is not already on its own from a sufficient wetting effect feature. All known wetting agents are suitable, provided that they do not attack the surface of the part to be ceramized during their decomposition in the ceramizing furnace. The required concentration of wetting agent in the solution is different from wetting agent to wetting agent and can be determined by a person skilled in the art with few attempts.

The application of the solution or the suspension to the green glass can be done by all common methods, eg. As dipping, flooding, brushing, screen printing, spin coating, curtain coating, spraying, etc., wherein the spraying is preferred.

The layer of the salt or the organic lithium compound applied to the green glass is often very sensitive to abrasion, so that it is advantageous to add a binder to the solution. Suitable binders are all soluble in the solution of organic binders, especially cellulose ethers, gelatin, polyvinyl alcohols, polyvinylpyrrolidone, rubber resins, such as. Gum arabic, dextran, etc., and the like. After drying, the binder gives a film which protects the applied lithium salt layer from abrasion. Preference is given to cellulose ethers, polyvinylpyrrolidone and polyvinyl alcohols.

The liquid components used for the lithium salts and compounds are primarily polar solvents such as water, alcohols and ketones. However, in order to accelerate the drying of the solution applied to the glass-ceramic, it is advantageous if the solution has a high proportion of water-miscible (polar), easily evaporable, organic solvents. Particularly suitable are the lower alcohols having 1 to 5 carbon atoms, in particular methanol, ethanol and propanol.

The addition of alcohol has the additional advantage that it also acts as a wetting agent, so that can be dispensed with the particular addition of wetting agents. If a particularly fast drying is desired, the alcohol content in the solution is chosen as high as possible. However, the amount of added alcohol should not become so large that the dissolved lithium salts or compounds are precipitated or phase separation occurs. The maximum permitted amount of alcohol depends on the nature of the particular alcohol and can easily be determined by a person skilled in the art.

Good experiences are made with mixtures containing up to 60% by weight of alcohol, especially ethanol. Likewise, good experiences were made when applying the layer with alcoholic Li salt suspensions (particle size ≤ 100 microns).

The amount of the lithium salt or the lithium compound dissolved in the solvent is low, in general concentrations of 2.5 to 5 g.l ^-1 , calculated as Li ₂ O, from lithium salt or compound. The suspensions preferably have a concentration of 5-25 g of lithium salt per liter (calculated as Li ₂ O), but can also be applied more dilute.

After passing through the coated green glass plate through the Keramisierungsofen to obtain a glass ceramic having a surface layer of keatite mixed crystal (KMK) with a thickness of 1 to 3 microns. This surface layer is decisive for the increased strength. The otherwise existing glassy surface layer is practically no longer available. As a result, tensile stresses that are normally induced by the glassy layer due to their higher thermal expansion coefficient, reduced or eliminated. In addition, a harmful conversion of HQMK relics in the near-surface layer in deep quartz structure is prevented.

The layer is particularly noticeable when the green glass is converted into a high-quartz solid solution glass-ceramic. If the green glass is ceramized to a keatite mixed crystal glass ceramic, then the glass ceramic produced consists entirely of KMK glass ceramic. In contrast, a glass-ceramic which has been converted from green glass into a KMK glass-ceramic according to the prior art has a vitreous surface layer responsible for the lower strength.

The invention is suitable for the production of strength-enhanced glass ceramics from all lithium alumino-silicate green glasses. Regardless of whether the glass ceramic has a main crystal phase of KMK or HQMK, in particular it is suitable for the production of strength-enhanced plate-like structures, for. As hobs or oven plates.

Lithium alumino-silicate glass-ceramic or the corresponding green glass usually contains (in% by weight based on oxide): 55-75 SiO ₂ , 15-30 Al ₂ O ₃ , 2-6 Li ₂ O, 0-5 Na ₂ O, 0-5 K ₂ O, 0-2.2 MgO, 0-2 CaO, 0-2 SrO, 0-2.5 BaO, 0-6 ZnO ₂ , 2-9 TiO ₂ + ZrO ₂ + P ₂ O ₅ , 0-1 SnO ₂ , 0-1 F, 0-2 B ₂ O ₃ and, if necessary, fining and coloring agents.

example 1

There were glass plates with the dimensions (L × W × D) 100 mm × 100 mm × 4 mm with a composition (in wt .-% based on oxide) of Al ₂ O ₃ 19.90%, SiO ₂ 67.4 %, TiO ₂ 2.58%, Li ₂ O 3.67%, MgO 1.06%, ZnO 1.54%, ZrO ₂ 1.70%, and customary amounts of lautering and dyeing additives:
The plates were sprayed by means of a spray gun with a solution consisting of 1 l of a mixture of 440 ml H ₂ O and 560 ml ethanol in which 25 g polyvinylpyrrolidone (MW = 360000) and 13 g Li ₂ SO _{4 were} dissolved. After drying at 100 ° C, the solid surface layer formed had a thickness of 2.2 μm and contained 150 mg of Li ⁺ ions per square meter.

Subsequently, the green glass plates were subjected to a suitable ceramization program for conversion to a KMK solid solution glass ceramic. For this purpose, the green glass plates were heated in a Keramisierungsofen to the nucleation temperature of 790 ° C, held for 30 minutes at this temperature, then heated to the crystallization temperature of 1100 ° C and there for 5 min, 15 min or 30 min on this Temperature kept, then cooled to 700 ° C and then cooled outside the furnace to room temperature. The thickness of the KMK layer formed in the surface of the glass ceramic plates by the ion exchange, recognizable by their deviating structure) was determined to be about 2 μm by means of thin-layer X-ray diffraction. A glassy surface layer was virtually nonexistent.

The strength determination was carried out by means of ball drop tests. The falling-ball test is carried out according to the standard UL858 for cooking surfaces (for details see also: K. Thoma, et al., "Measurements of mechanical properties at high strain rates", MP Materialprüf, 45, 362-370, (2003)). In a falling ball test, the dynamic bending strength of a sample is tested. For this purpose, a 200 g steel ball with 36.5 mm diameter from different heights is dropped centrally on a 100 × 100 × 4 mm ³ glass ceramic. If the disk does not break, the ball is dropped in discrete steps (usually 2 cm or 5 cm steps) continuously from higher heights. The measure of strength is the drop height at which the plate breaks. The ball is guided by three bars during the trial, so that the repeatability of the central place of impact is ensured. The test is carried out on several (usually about 30) equivalent samples and the strength results are then applied as a distribution and evaluated. The strength distribution of brittle materials is approximated by a Weibull distribution. The fracture exit is due to the tensile stresses at the bottom of the test sample just below the point of impact.

For the glass ceramic sample according to the invention, the values of the ball impact strength (in [cm]) shown in Table 1 were as follows: TABLE 1 ceramization comparison According to the invention Mean value [cm] 5% fractiles Mean value [cm] 5% fractiles 1100 ° C, 5 min 82 50 75 32 1100 ° C, 15 min 51 26 70 36 1100 ° C, 30 min 15 13 57 37

It can clearly be seen that the samples exchanged according to the invention for Li-ions virtually do not show a fall in the ball-fall resistance, in particular in the 5% fractiles, which indicates a low scattering, while in the uncoated comparative samples the ball-fall resistance successively averages 82 cm drops to 15 cm on average.

Example 2

There were glass plates having the dimensions (L × W × D) 250 mm × 250 mm × 4 mm with a composition (in wt .-% based on oxide) of Al ₂ O ₃ 19.90%, SiO ₂ 67.4 %, TiO ₂ 2.58%, Li ₂ O 3.67%, MgO 1.06%, ZnO 1.54%, ZrO ₂ 1.70%, and customary amounts of lautering and dyeing additives:
The plates were sprayed by means of a spray gun with a solution consisting of 1 l of a mixture of 440 ml H ₂ O and 560 ml ethanol in which 25 g polyvinylpyrrolidone (MW = 360000) and 13 g Li ₂ SO _{4 were} dissolved. After drying at 100 ° C, the solid surface layer formed had a thickness of 2.2 μm and contained 150 mg of Li ⁺ ions per square meter.

Subsequently, the green glass plates were subjected to a suitable ceramization program for conversion to a KMK solid solution glass ceramic. For this purpose, the green glass plates were heated in a Keramisierungsofen to the nucleation temperature of 790 ° C, held for 30 minutes at this temperature, then heated to the crystallization temperature of 1075 ° C and held there for 10 min at this temperature, cooled to 300 ° C and then outside the Kiln cooled to room temperature. The thickness of the KMK layer formed in the surface of the glass ceramic plates by ion exchange was determined to be about 1.5 μm by means of thin-layer X-ray diffraction. A glassy surface layer was virtually nonexistent.

The strength determination was carried out by means of temperature difference test (TUF test). A square sample with an edge length of 250 mm and a thickness of 4 mm is locally heated with a radiant heater. The time to maximum temperature is usually about 5 minutes. This has the consequence in the sample that among other tangential tensile stresses arise that reach their maximum in the cold edge region and can lead to breakage.

In addition, on the center of the edge portion, which is the shortest distance to the heater, an injury is applied with SiC emery paper 220 grit under a surface load of 17 KPa. Thus, the breakout exit is forced there. The temperature curve until breakage is measured by a pyrometer and recorded. The break temperature is followed directly by the TUF (ΔT _max ) to:

v is the Poisson number, σ describes the fracture strength of the material. f ₁ represents a correction factor caused by the plate geometry and by the existing temperature distribution. In the case of samples prepared according to the method, the comparative batch (10 pieces, without Li ion exchange in the surface) on average withstood a temperature of 760 ° C., but with had a large scatter of the values, ie that the maximum service temperature must be well below this value. The Li-ion exchanged charge of the invention (10 hours) withstand an average temperature of 813 ° C, but showed a much lower scattering.

Example 3

There were glass plates having the dimensions (L × W × D) 250 mm × 250 mm × 4 mm with a composition (in wt .-% based on oxide) of Al ₂ O ₃ 19.07%, SiO ₂ 68.47 %, TiO ₂ 2.59%, Li ₂ O 3.59%, MgO 1.16%, ZnO 1.54%, ZrO ₂ 1.71%, and customary amounts of lautering and dyeing additives:
The plates were sprayed by means of a spray gun with a solution consisting of 1 l of a mixture of 440 ml H ₂ O and 560 ml ethanol in which 25 g polyvinylpyrrolidone (MW = 360000) and 13 g Li ₂ SO _{4 were} dissolved. After drying at 100 ° C, the solid surface layer formed had a thickness of 2.2 μm and contained 150 mg of Li ⁺ ions per square meter.

Subsequently, the green glass plates were subjected to a suitable ceramization program for conversion to a KMK solid solution glass ceramic. For this purpose, the green glass plates were heated in a Keramisierungsofen to the nucleation temperature of 790 ° C, held for 30 minutes at this temperature, then heated to the crystallization temperature of 1075 ° C and held there for 10 min at this temperature, cooled to 300 ° C and then outside the Kiln cooled to room temperature. The thickness of the KMK layer formed in the surface of the glass ceramic plates by ion exchange was determined to be about 1.5 μm by means of thin-layer X-ray diffraction. A glassy surface layer was virtually nonexistent.

Subsequently, the samples were examined for their strength analogously to Example 2. In the case of samples prepared according to the method, it can be seen that the reference batch (10 hours) withstood a temperature of 712 ± 73 ° C. The Li-ion exchanged batch (10 hours) withstood a temperature of 762 ± 83 ° C.

Example 4

Green glass plates having the dimensions (L × W × D) of 30 mm × 50 mm × 4 mm having a composition of (in% by weight based on oxide) of Al ₂ O _{3 was} 19.90%, SiO ₂ 67.4%, TiO ₂ 2.58%, Li ₂ O 3.67%, MgO 1.06%, ZnO 1.54%, ZrO ₂ 1.70%, and customary amounts of lautering and dyeing additives:
The plates were dissolved by means of a spray gun with a solution consisting of 1 liter of a mixture of 440 ml H ₂ O and 560 ml ethanol, in the 25 g polyvinylpyrrolidone (MW = 360000) and 13 g Li ₂ SO ₄ and 10 g LiCl, respectively were sprayed. After drying at 100 ° C, the solid surface layer formed had a thickness of 2.2 μm mm and contained 150 mg of Li ⁺ ions per square meter.

Subsequently, the green glass plates were subjected to a ceramization program for conversion to a high-quartz solid solution glass-ceramic. For this purpose, the green glass plates were heated in a Keramisierungsofen to the nucleation temperature of 790 ° C, held at this temperature for 30 minutes, then heated to the crystallization temperature of 900 ° C, held for 5 minutes at this temperature, cooled to 700 ° C and then outside the furnace cooled to room temperature. The thickness of the KMK layer formed in the surface of the glass ceramic plates was determined to be about 2 μm by means of thin-layer X-ray diffraction. A glassy surface layer was virtually nonexistent.

A Vickers indentation test with a Vickers diamond at 5N maximum load was performed to determine strength. The sample surface was flooded with nitrogen during the exposure and then the crack lengths of the impressions were determined within 5 minutes on a Zeiss Axioplan microscope equipped with the Zeiss Axiocam digital microscope camera. The average crack lengths are shown in Table 2 below. It can be seen that the Li-ion exchanged samples have significantly shorter crack lengths and thus higher surface strengths than the identically prepared comparative samples without Li-ion-exchanged surface. Table 2 Vickers impressions Crack length [μm] Standard deviation [μm] Comparative Example 1 94 3 Comparative Example 2 92 5 According to the invention, exchanged with LiCl 60 7 According to the invention, exchanged with Li ₂ SO ₄ 56 3

Claims (8)

Process for increasing the strength of a lithium alumino-silicate glass-ceramic by surface modification, characterized by the following steps: the surface of the green glass is coated prior to ceramification with a solution or suspension of a lithium salt or an organic compound decomposable to a lithium salt during ceramization; the lithium salt has a melting point of at least 720 ° C., - the coating is dried, - coating and drying are repeated if necessary, - until the layer produced on the glass surface has an Li ⁺ ion content of 10 to 5000 mg Li ⁺ / m ² , - The coated green glass is ceramized. A method according to claim 1, characterized in that the solution or suspension contains an organic binder. A method according to claim 1, characterized in that the solution or suspension contains as solvent and / or suspending agent, a polar organic solvent or a mixture of water and a water-miscible polar organic solvent. A method according to claim 3, characterized in that an alcohol having 1 to 5 carbon atoms is used as the organic solvent. A method according to claim 2, characterized in that polyvinylpyrrolidone, polyvinyl alcohol or a cellulose ether is used as the binder. A method according to claim 1, characterized in that lithium sulfate, lithium carbonate, lithium acetate, lithium chloride or lithium oxalate is used as the lithium salt. A method according to claim 1, characterized in that the solution contains 2.5 to 100 g · l ^-1 of lithium salt, calculated as Li ₂ O. Lithium alumino-silicate glass-ceramic produced by a method according to one of claims 1 to 7.