Glass or glass-ceramic product, process for its production and ink

Abstract

The invention relates to a glass or glass-ceramic product comprising a substrate made of a glass or a glass-ceramic, wherein the substrate is provided on at least one side, on at least part of its surface, with a substantially pigment-free coating, wherein the coating comprises an at least partially molten glass flux, wherein the surface of the coating has a mean square height Sq of at least 0.1 µm and at most 2.5 µm, and wherein the coating has a core height Sk of at least 1.0 µm to at most 10 µm. The invention further relates to a method and an ink for producing the glass or glass-ceramic product.

Description

The invention relates to a glass or glass ceramic product with a coating, and to a method for producing such a glass ceramic product and to an ink for producing the glass or glass ceramic product.Prior ArtFrom the prior art, various applications for glass and glass ceramic products are known. Particularly in the case of disc-shaped glass ceramic products, the most prominent example of the application is the use as cooktop.Glass ceramic cooking surfaces are often equipped with functional coatings on the upper side that fulfil different tasks. Coatings are known in the prior art, for example from EP 2 964 854 B1, which protect such a glass ceramic cooktop from scratches by means of a layer of AlSiN deposited on the substrate. Furthermore, coatings are also known which are intended to avoid soiling of cooktops, in particular by fingerprints. Here, WO 2023 / 099 833 A1 would be mentioned by way of example.Recently, there have also been approaches which are intended to prevent both the avoidance of scratches on the surface of a substrate and the formation of fingerprints or, in general, the soiling of the cooktop. Here again, EP 4 077 231 A1 would be mentioned by way of example. Disclosed therein is a glass ceramic article obtained by a method comprising a heat treatment for ceramizing a glass suitable for forming a glass ceramic and a chemical treatment of a surface of the glass before and / or after the heat treatment for ceramization, wherein the chemical surface treatment is carried out such that after the heat treatment the arithmetic mean roughness of the surface is between 2 μm and 7 μm. With regard to the chemical treatment of the surface, the use of an acid solution based on hydrofluoric acid is described in particular.The use of hydrofluoric acid is in fact already known for etching glass and glass ceramics, but entails a number of disadvantages. On the one hand, the time required for etching a glass or glass ceramic until the aforementioned roughness has been established is comparatively high. On the other hand, hydrofluoric acid is a substance which is very problematic in terms of handling and for which a large number of safety measures have to be taken into account in order to ensure both the protection of the environment and the protection of those persons who work with the hydrofluoric acid.Furthermore, DE 10 2014 220 457 A1 discloses a method for producing a coated substrate, a disk-shaped substrate comprising at least two layers applied by means of heating and the use of the coated substrate, wherein the layer-forming material comprises at least one glass powder and particles, wherein the particles cause elevations on the layer.Furthermore, DE 10 2011 115 379 A1 describes glass or glass ceramic articles, in particular an article of this type which is provided with a haptically tactile surface, and a glass frit for producing thermally resistant layers which can be applied to the surfaces of substrates, wherein the layers can have specific haptic properties.Against this background, there is a need in the prior art for a solution by means of which it is possible to provide a glass or glass ceramic article with the least possible outlay, both in terms of time and with respect to operating safety, the surface of which article is not sensitive to scratches and largely avoids the formation of fingerprints and other soiling.Disclosure of the InventionThe object formulated above is achieved with respect to a glass or glass ceramic product having the features of claim 1. Solutions with respect to a corresponding production method and an ink to be used in this case are the subject matter of claims 14 and 17.In a first aspect, the invention relates to a glass or glass ceramic product having a substrate made of a glass or a glass ceramic, wherein the substrate is provided on at least one side on at least part of its surface with a substantially pigment-free coating, wherein the coating has an at least partially melted, preferably boron-containing glass flow, wherein the surface of the coating has an average square height Sqof at least 0.1 μm and at most 2.5 μm, and wherein the coating has a core height Skof at least 1.0 μm to at most 10 μm. According to the invention, it is provided that the glass flow has the following composition in wt % on an oxide basis:SiO 275 - 85Al 2 O 30,1 - 5B 2 O 310 - 15Na 2 O1-5K 2 O0,1 - 1,5.A glass flow of this composition is suitable in particular for firing the coating during ceramization of the substrate, which brings with it distinct advantages with regard to the production of the glass or glass ceramic product according to the invention.The term "substantially pigment-free" is understood here to mean that the coating contains less than 1% by weight of pigment, i.e. consists to an extent of at least 99% by weight of the glass flow. A "pigment" is understood here to mean particles which alter the transmission properties of the coating compared to a coating which consists exclusively of glass flow. In particular, such pigment particles can cause a coloriness of the coating or a reduced transmission of the coating.An "at least partially melted" glass flow is understood to mean a glass flow, i.e. a quantity of glass particles with a defined size distribution, in which at least some of the particles are melted and resolidified and therefore no longer exist as particles in the shape and size originally present in the glass flow. As a result of this melted portion of the particles, the particles of the glass flow are firmly connected to form a layer. At the same time, the melted part of the glass particles also serves to produce a bond to the substrate. The unmelted part of the glass particles leads to the surface not being completely planar, which would be expected in the case of a completely molten glass flow. Rather, the partial melting of the glass particles results in a defined surface roughness of the coating.The roughness of the surface of a glass or glass ceramic product according to the invention, expressed by the mean square height Sq of the surface, is at least 0.1 μm and at most 2.5 μm. The mean square height of the surface of the coating is calculated here according to the following specification: wherein A is the surface of the coating under consideration and z is the deviation of a measurement point at position (x,y) from the mean height of the coating. Particularly preferably, the mean square height Sq of the coating is greater than 0.2 μm, greater than 0.3 μm, greater than 0.4 μm or particularly preferably greater than 0.5 μm.Another parameter which describes the nature of the coating according to the invention is the core height Sk of the surface of the coating. The core height describes the height of that region within the area material fraction of the surface of the coating on which 100% of the equivalence straight line (also referred to as equivalent straight line) of the area material fraction is omitted. This region is also referred to as the core surface. The surface material fraction (also referred to as "surface material fraction") describes the height range of the surface of the coating over which fraction of the material of the coating in the region of the surface under consideration is dispensed with. Effectively, each height within the surface of the coating is thus assigned that material portion of the surface which lies above this height. In a graphical representation of this curve, the height within the surface of the coating is accordingly indicated as the ordinate, while the abscissa indicates the material fraction in %, which is dispensed with in the region above this height.The degrees of equivalence are those edges of the curve of the surface material portion with the smallest detectable gradient (or the smallest gradient), the points of intersection of which with the curve of the surface material portion with respect to their abscissa have a distance of 40%. By extrapolation of the equivalence degrees to abscissa values of 0% and 100%, the corresponding ordinate values can thus be determined, wherein the core height is the distance of these ordinate values. Accordingly, a low core height means a very compact and thus resistant coating, since a large part of the surface material is concentrated on a narrow area. The core height Sk of the coating can also be, in particular, 1.5 μm to 9 μm, 1.5 μm to 8 μm, 1.5 μm to 7 μm, 2 μm to 6 μm or particularly preferably 2 μm to 5 μm.From the overview of the values of the coating according to the invention with respect to the mean square height and the core height, it becomes clear that the coating according to the invention combines on the one hand a comparatively rough or matt surface, but at the same time also a very compact nature of the coating. Consequently, the coating according to the invention is distinguished by good properties with respect to avoiding fingerprints and other soilings, with simultaneously high resistance to scratches and wear of the coating. The coating contains only components which are harmless to health and can be produced in a simple and cost-effective manner.The glass particles of the glass flow of the coating can be in particular particles of boron-containing glass. The use of boron-containing glass flux has advantages in this case inter alia with respect to the resistance of the coating to changes in temperature and additionally improves the adhesion of the coating in particular when the substrate is a glass ceramic. In addition, when using a boron-containing glass flow, it is possible to burn in the coating during ceramization of the substrate, i.e. the conversion of a glass substrate into a glass ceramic substrate. In this way, a step in the production of the product, namely a separate stoving of the coating (so-called secondary stoving), can be avoided.The matt appearance of the coating described above is also evident according to one embodiment in that the coating has a gloss value of at most 25, measured at an angle of 60°. In this way, too, the generation of visible contaminants of the coating, in particular in the form of fingerprints, can be avoided. Thus, for example, the glossy the surface is, the more fingerprints are visible on a surface.Two important and unique parameters of the coating according to the invention have already been discussed above. However, the coating according to the invention also differs critically from coatings of the type known in the prior art with regard to further surface parameters.Thus, according to one embodiment, it is further provided that the coating has a skewness Sskof >0. The extent of the skewness (also referred to as skewness) of a surface provides information about whether the surface tends to be described more as a surface with grooves or valleys, or more as a surface with peaks. In the case of a surface with a skewness>0, i.e. a surface with grooves, the frequency of regions which project beyond the mean height value is by definition smaller than the proportion of regions which lie below the mean height value in the height distribution of the surface. Since this case can only occur if the regions lying below the mean height value are less frequently, but rather are significantly more pronounced in their height (or depth) than the regions above the mean height value, this means that the surface can be described by valleys and grooves rather than by protruding peaks.The skewness can be determined mathematically by summing the cube of all height values and dividing it by the cube of the mean square height Sq of the surface of the coating:A skewness>0, i.e. a surface with grooves instead of peaks, has the advantage that grooves are significantly more resistant than protruding peaks. Thus, to reduce the depth of a groove in the coating, significantly more material has to be removed than to reduce a tip. Consequently, a surface skewness of >0 is associated with a higher robustness of the surface finish, since the structure of the surface is less strongly attacked, for example even in abrasive cleaning processes.According to a further embodiment, it is further provided that the surface of the coating has a kurtosis Skuof >3, particularly preferably >3 and <8. The kurtosis of a surface describes the sharpness of a surface profile and is calculated as followsHere, a value of coursetosis of Sku>3 describes a surface that tends to have spikes rather than rounded. A surface which, of the type of surface structures, has serrations rather than rounded portions has the advantage that, for example, when touch-sensitive operating elements are actuated, only a smaller contact surface is formed between the surface of the coating and the finger of the operator. The visibility of fingerprints or the like can thus be effectively avoided, since these are arranged only on the very narrow tips of the surface structures.According to a further embodiment, the coating has a thickness of 2 to 10 μm. The thickness of the coating is selected in such a way that adequate stability of the coating is ensured, but at the same time the transmission properties of the substrate are influenced as little as possible by the coating. Furthermore, structures arranged below the coating are also easily recognizable.The thickness of the coating is preferably at least 3 μm, particularly preferably at least 4 μm. Furthermore, the thickness of the coating is preferably at most 9 μm, particularly preferably at most 8 μm, very particularly preferably at most 7 μm.According to a further embodiment, it is further provided that a pigmented decorative layer is arranged at least in sections between the substrate and the coating. The pigmented decorative layer is preferably applied directly to the substrate and can be produced in particular by inkjet printing methods or screen printing. When the glass or glass ceramic product is used as a cooktop, the decoration can be, for example, a cooking zone marking. By arranging the decoration below the coating, the decoration is protected from abrasion, for example as a result of cleaning of the glass or glass ceramic product. At the same time, however, the visibility of the decor and in particular its edge sharpness is only slightly impaired due to the small thickness of the coating and the absence of pigment in the coating. In this case, the decoration can in principle also be burnt in together with the coating, which simplifies the production of the glass or glass ceramic product.Alternatively, however, the decoration can also be applied to the coating, which can simplify the production of the glass or glass ceramic product. For example, the matt coating can be applied over a large surface area by means of a screen printing method and the glass or glass ceramic product thus prepared can then be provided with a decoration in a further step.According to a further embodiment, the combination of decorative layer and coating has a thickness of at most 15 μm, preferably at most 12 μm, particularly preferably at most 10 μm.It has already been stated above that the coating is applied to at least part of the surface of the substrate. This means that the coating can certainly also have recesses, for example in order to create a region in which a display can be arranged. A cutout made from the coating, i.e. a region of the surface on which no coating is applied, thus makes it possible to improve a display of the display compared to a full-surface coating. According to a further embodiment, however, it is provided that the coating is applied over the entire surface on one side of the substrate, which can simplify the production of the glass or glass ceramic product.In this case, according to a further embodiment, it is further provided that the substrate is disk-shaped and has a thickness of between 2 mm and 6 mm, preferably between 3 mm and 5 mm, particularly preferably a thickness of 4 mm. A "disc-shaped" substrate is understood to mean a substrate whose length and width are greater than its thickness by at least one order of magnitude. In particular, the glass or glass ceramic product can be a hob.Different materials can be used as substrate.Thus, according to one embodiment, it is provided that the substrate is transparent with a transmission τvisof greater than 80%, preferably greater than 85%, and has a color c* of less than 10, in particular less than 8. The color c* is calculated as effective according to the definition of the CIELab color system, i.e., a transparent and largely color-neutral substrate.The composition of such a substrate in wt % on an oxide basis can be selected, for example, as follows:SiO 264 - 68Al 2 O 319-23Li 2 O3,2 - 4,2MgO0,2 - 1,0Na 2 O+K 2 O0,1 - 1,5BaO0 - 1,5CaO + SrO0 - 1,5ZnO1 - 2,5TiO 21,6 - 2,5ZrO 21,2 - 2,0SnO 20-0,5Nd 2 O 30,005 - 0,15Fe 2 O 30,001 - 0,03Alternatively, according to a further embodiment, it is provided that the substrate is volume-colored and has a transmission τvisof less than 10%. By "bulk-colored" is meant a substrate which does not receive its coloriness by a coating, but which contains elements within the material which contribute to a coloriness of the material itself. Such volume coloration can be brought about, for example, by elements such as chromium, vanadium or molybdenum which are mixed with the glass composition. By way of example, such a substrate may have the following composition in wt % on an oxide basis:Li 2 O3,0 - 4,2Na 2 O+K 2 O0,2 - 1,5MgO0- 1,5CaO+SrO+BaO0-4ZnO0-2B 2 O 30-2Al 2 O 319 - 23SiO 260 - 69TiO 22,5-4ZrO 20,5 - 2P 2 O 50-3SnO 20,1 - <0,6TiO 2+ ZrO 2+ SnO 23,8-6V 2 O 50,01 - 0,06Fe 2 O 30,03 - 0,2In a further alternative embodiment, it is further provided that the substrate is translucent with a transmission τvisof 2% to 25% or opaque with a transmission τvisof 0.1% to 2%. In particular, this may be a glass ceramic having a composition as has been described above with reference to a transparent substrate, wherein the substrate may have a high volume fraction of keatite in the crystal phase. In the case of an opaque substrate, a coloring of L* 85-97, a* of -1.5-0.5 and b*-6-0.5 can furthermore be exhibited in the CIELAB color space. Alternatively, in the case of a translucent substrate, the chromaticity of the substrate can be configured as follows: L*=72-93, a*=-5.5-0, b*=-7-0.5 with a transmission of 2% to 10%, or alternatively: L*=60-82, a*=-7.5-2, b*=-19-4.5 with a transmission of 10% to 25%.In particular when using a glass or glass ceramic product according to the invention as a cooktop, it is provided according to a further embodiment that the substrate consists of a LAS glass ceramic, i.e. a glass ceramic of a lithium aluminum silicate. Such substrates have a very low coefficient of thermal expansion and are very resistant to temperature changes, so that they are excellently suitable in particular for the thermal requirements of a cooktop.In this case, according to a further embodiment, it is further provided that the substrate has at least one recess. In particular, such a recess can be an opening in the substrate, through which an exhaust hood (also referred to as a downdraft) can be integrated into the cooktop when the substrate is used as a cooktop. In this case, the coating on the substrate preferably extends as far as the edges of the recess, so that a seamless transition of the coated region into the recess is achieved. Such a recess in a substrate can be produced by a multiplicity of methods, in particular by drilling, milling, water jet cutting or the like.In a further aspect, the invention relates to a method for producing a glass ceramic product as described above, wherein the method has the following steps: a. providing a substrate made of glass, b. applying a layer of an ink comprising a glass flow and a printing medium to at least one partial region of a surface of the substrate, wherein the application is preferably carried out by means of screen printing, c. ceramizing the coated substrateIn the method, accordingly, in a common method step c., both the substrate is ceramized, i.e. converted from glass into glass ceramic, and the coating is fired. Consequently, a separate step for firing the coating can be omitted, which significantly simplifies the production of the glass or glass ceramic product.For ceramization of the coated substrate, the following can be carried out, for example: a) heating from room temperature to 680° C. within 23 minutes, b) temperature increase within 19 minutes from 680° C., to 800° C., c) temperature increase from 800° C. to 918° C. (maximum temperature) within 24 minutes, d) holding the maximum temperature for 10 minutes, e) cooling to 800° C. within 20 minutes, f) rapid cooling to room temperature within less than 150 minutes.Alternatively, ceramization can be carried out as follows: a) Rapid heating from room temperature to 740° C. in 20 to 26 minutes, in particular 24 minutes, b) Temperature increase from 740° C. to 825° C. in 12 to 18 minutes, in particular 14 minutes, c) Temperature increase from 825° C. to 930° C. (maximum temperature) in 4 to 8 minutes, in particular 6 minutes, d) Holding the maximum temperature for 4 to 8 minutes, in particular 6 minutes, e) Cooling to 800° C. in 8 to 16 minutes, in particular in 10 minutes, f) Rapid cooling to room temperature.According to a further embodiment, the coating is applied by means of screen printing, wherein a screen having a screen thickness of 140-31 to 54-64 is used for screen printing. In particular, a screen of screen thickness 77-55 has proven advantageous. In this case, it can also be provided that the screen used has regions in which no glass flow is transmitted to the substrate, so that recesses can be realized in the coating in a simple manner. In particular, a screen can also be used which produces a fine structure in the coating, as a result of which the degree of surface area occupancy of the coating, i.e. the ratio of coated to uncoated surface in the coated region of the substrate, becomes less than 100%. In this case, patterns can be produced in the coating, for example, which further reduce the visibility of scratches or contaminants on the surface. In this case, the area occupancy level preferably does not fall below a value of 80%.According to a further embodiment, it is further provided that a decorative layer is applied to the substrate before the coating is applied to the substrate, wherein the coating is subsequently applied at least in sections to the decorative layer. In this case, the decorative layer can be applied to the substrate in particular by means of inkjet printing methods. Alternatively, however, it can also be provided that the decorative layer is applied after method step b., i.e. is arranged on the coating.It has already been stated above that the substrate can also have a recess, for example for integrating an exhaust hood. Such a recess is preferably introduced into the substrate before the substrate is coated. In this case, the coating can subsequently be applied to the substrate as far as the edge of the recess, so that the coating is not impaired by the production process of the recess. However, it is also possible to introduce the recess into the already coated substrate after ceramization.In a further aspect, the invention relates to an ink for producing a glass or glass ceramic product as described above, wherein the ink has a boron-containing glass flow and a printing medium, wherein the glass flow has a grain size of D 10 greater than 1 μm and D 90 less than 20 μm, preferably D 90 less than 15 μm. The pressure medium is in particular a medium which contains dipropylene glycol monomethyl ether as solvent, which is particularly advantageous in particular with regard to its biocompatibility. Alternatively, a naphtha-based printing medium may be used as the solvent.The glass flow is preferably particles of a borosilicate glass, wherein the glass preferably has the following composition in wt % on an oxide basis:SiO 275 - 85Al 2 O 30,1 - 5B 2 O 310 - 15Na 2 O1 - 5K 2 O0,1 - 1,5According to a further embodiment, it is provided that the ratio of glass flow to printing medium in the ink is between 10:15 and 10:5, which has a particularly advantageous effect on the printing properties of the ink.The invention will be described in more detail below with reference to the figures and without limitation thereto. Identical reference numerals designate identical or similar elements.The following are shown: FIG. 1 : schematic representations of different embodiments of an exemplary glass or glass ceramic product, FIG. 2 : shows a schematic illustration of an exemplary method for producing an exemplary glass ceramic product FIG. 3 : shows a representation of a surface profile of a matt surface as a comparative example, FIG. 4 : a histogram of an exemplary grain size distribution of a glass flow for producing a coating, FIG. 5 : a perspective illustration of the surface profile of a coating produced with the glass flow from FIG. 4, FIG. 6 is a diagram of the surface profile along a line within the surface shown in FIG. 5 FIG. 7 shows a histogram of a further exemplary grain size distribution of a glass flow for producing a coating, FIG. 8 : a perspective illustration of the surface profile of a coating produced with the glass flow from FIG. 7, and FIG. 9 is a diagram of the surface profile along a line within the surface shown in FIG. 8.FIG. 1 shows schematic representations of various embodiments of an exemplary glass or glass ceramic product 100. FIG. 1 a) shows the simplest case in which a coating 104 is applied directly to a disk-shaped substrate 102 made of glass or glass ceramic on a surface 106. The coating has a rough surface 114 with a mean square height Sq in the range from 0.1 μm to 2.5 μm and a core height Sk of the coating 104 of 1.5 μm to 10 μm, whereby the coating 104 has a matt appearance. In this case, there is initially no further coating, in particular in the form of a decoration, provided between the coating 104 and the substrate 102. The substrate 102 is preferably a lithium aluminum silicate glass ceramic (LAS glass ceramic), which is very well suited in particular for applications as a cooktop due to its generally low coefficient of thermal expansion. The substrate 102 can be a transparent, translucent or opaque material. Further, the substrate 102 may be substantially colorless or bulk-colored.In the embodiment shown, the coating 104 is applied on the upper side of the substrate 102, which would face the user when the product 100 is used as a cooktop and on which cookware, for example, would be arranged. Even if no further coating is shown on the underside 116 of the substrate 102 in all configurations in FIG. 1, such an additional underside coating is not fundamentally excluded. Rather, in particular in the case of a transparent substrate 102, the use of a bottom side coating that is as opaque as possible is advantageous in addition to the top side coating 104, for example in order to conceal electronic components arranged below the substrate 102.The dimensions of the elements shown in FIG. 1 are greatly exaggerated for reasons of displayability and are not realistic in their relation in particular. Thus, a preferred thickness for the substrate 102 would be, for example, 4 mm, while the coating 104 preferably only has a thickness in the range from 2 to 10 μm. Likewise, the longitudinal extent of the substrate 102 is also illustrated merely by way of example and the thickness of the substrate 102 would normally be smaller than its length by at least one order of magnitude.FIG. 1 b) shows a further embodiment of the glass or glass ceramic product 100, in which a further coating 108 in the form of a decoration is additionally applied to the surface 106 of the substrate 102. In the embodiment shown, the decoration 108 is completely covered by the coating 104 and is thus protected against external influences. By directly applying the decoration 108 to the generally very smooth surface 106 of the substrate 102, a high edge sharpness of the decoration 108 can be ensured, while at the same time the rough surface 114 of the coating 104 gives the glass or glass ceramic product 100 overall a matt appearance. The decoration 108 can be, for example, cooking zone markings, manufacturer logos or other markings. In addition to the configuration illustrated here, in which the decor 108 is completely covered by the coating 104, it would also be possible within the scope of the invention for the decor 108 to be only partially covered by the coating 104, that is to say there are regions of the decor 108 which are exposed.Although in FIGS. 1 a) and 1 b) the coating 104 is in each case illustrated as a full-surface coating of a surface 106 of the substrate 102, it is also conceivable in principle for the coating 104 to extend only over part of the surface 106 of the substrate 102 and therefore for there to be regions in which the coating 104 has recesses. This case is shown in FIG. 1 c). In the embodiment shown here, which builds on the embodiment of FIG. 1 b), the coating 104 has a recess 110 in the middle of the illustration, in which the surface 106 of the substrate 102 is exposed. Such a cutout 110 can be taken into account in particular already in the production of the coating 104 by virtue of the corresponding region being cutout during the printing of the coating 104 onto the surface 106 of the substrate 102, for example by means of a corresponding masking of the substrate 102 or by means of a corresponding design of a screen for applying the coating 104 by means of a screen printing method.Such a cutout 110 can be advantageous in particular if display devices 112, for example in the form of seven-segment displays or full-color displays, are arranged below the substrate 102. An image reproduction of such display devices 112 would be distorted on transmission through the coating 104 due to its rough surface. However, this can be avoided by a corresponding cutout in the coating 104.FIG. 2 shows a schematic illustration of an exemplary method for producing an exemplary glass ceramic product 100. In a first method step 200, a substrate 102 made of glass is provided which can already be cut to the dimensions desired for the end product. Furthermore, a pretreatment of the surface 106 of the substrate 102 to be coated can also take place here, for example by polishing the surface 106 in order to set a planar surface 106 that is as smooth as possible.In a second method step 202, an ink comprising a glass flow and a print medium is then applied to at least one partial region of the surface 106 of the substrate 102. The glass flow preferably has a grain size of D 10 greater than 1 μm and D 90 less than 20 μm, preferably D 90 less than 15 μm. In order to apply the ink to the surface 106 of the substrate 102, it is possible in principle to use any desired printing methods which are suitable for processing a glass flow having the stated grain size. However, it is particularly preferred here to apply the ink by means of a screen printing method. A screen printing method has the advantage that, on the one hand, large-area coatings can be produced with little effort, while regions can be intentionally omitted in the coating 104 by a corresponding design of the screen used. In this way, structures as described above with reference to FIG. 1 c), for example, can be produced.The screen used for screen printing is to be selected in principle adapted to the printing medium used and to the glass flow. However, it has proven particularly advantageous here to use a sieve of thickness 140-31 when using a printing medium based on naphtha as solvent.In a third method step 204, the printed substrate 102 is then ceramized. For this purpose, for example, the following ceramization program can be used, which is, however, only specified by way of example and should not be understood as a restriction.g) heating from room temperature to 680° C. within 23 minutes,h) temperature increase within 19 minutes from 680° C., to 800° C.,i) Temperature increase from 800° C. to 918° C. (maximum temperature) within 24 minutesHold the maximum temperature for 10 minutesk) cooling to 800° C. within 20 minutes,Cool rapidly to room temperature within less than 150 minutes.Alternatively, ceramization may be performed as follows:g) Rapid heating from room temperature to 740° C. in 20 to 26 minutes, in particular 24 minutes,h) Temperature increase from 740° C. to 825° C. in 12 to 18 minutes, in particular 14 minutes,i) Temperature increase from 825° C. to 930° C. (maximum temperature) in 4 to 8 minutes, in particular 6 minutes,Maintaining the maximum temperature for 4 to 8 minutes, in particular 6 minutesk) cooling to 800° C. within 8 to 16 minutes, within 10 minutes,Cool rapidly to room temperature.During ceramization of the substrate 102, after a nucleation phase during which crystallisation nuclei are formed within the substrate 102, a controlled crystal growth is excited by a further increase in the temperature, whereby the glass substrate 102 is converted into a glass ceramic with defined mechanical and optical properties. Furthermore, due to the high temperatures during ceramization, the glass flow located on the surface 106 of the substrate 102 also partially melts, while the printing medium of the ink evaporates essentially without residue. In this case, the melted glass flow connects to the glass substrate 102, as a result of which a very robust coating 104 which is largely insensitive to mechanical loading is produced on the surface 106 of the substrate 102. The unmelted part of the glass flow forms an uneven structure on the surface 114 of the coating 104, by means of which the surface parameters according to the invention are brought about.FIG. 3 a) shows a schematic representation of the height profile of a surface for explaining the surface parameters according to the invention. In this case, along a measurement path within the surface for the different points along the measurement path (x-axis), the height z(x) of the surface is in each case shown above a zero line in μm. The zero line is arranged at a height for which the deviations of the local elevations of the surface from the zero line along the measurement section sum to zero. The two-dimensional case illustrated here is chosen merely for explaining the surface parameters already mentioned and described above. The surface parameters used to describe the object according to the invention are actually, however, not determined along a single line within the surface, but rather from a view of the entire surface, i.e. a three-dimensional representation of the surface.Starting from the zero line shown in FIG. 3 a), all local deviations z(x) from the zero line are squared, summed up and divided by the length of the measurement section in order to determine the mean square height Sq of the surface. From this result, the square root is subsequently drawn. In effect, a statement is thus obtained as to how much the surface deviates on average from the zero line, that is to say how rough the surface is. The larger this value, the rougher the surface.To determine the core height Sk, it is first determined for each height y on the axis of ordinates which proportion of the surface is higher than the considered y-value. This proportion is then entered as the area material proportion M on the abscissa axis. For example, in the surface of FIG. 3 a), a value of 0% for the surface material fraction would be determined at a height of 8 μm, since the surface does not deviate from the zero line over a height of 8 μm. For a height of 6 μm, a lower-setting portion of the surface would already be determined, which deviates from the zero line by more than +6 μm. By definition, in the illustration shown, a proportion of 50% would have to be assigned to the ordinate value 0 μm, since the zero line divides exactly equal proportions of the surface. In principle, however, a different zero line can also be selected as the reference point here. At a value of -10 μm, a value of more than 95% would already be determined for the area material fraction M, while from an ordinate value of approximately -13 μm 100% of the surface lies above this value.An exemplary distribution 300 of this type is shown in FIG. 3 b). In order to determine the core height Sk from this distribution, the degrees of equivalence of the distribution 300 of the area material fraction is determined in a first step. For this purpose, a range of the area material fraction M, to which an area material fraction ΔM of 40% is absent, is shifted along the axis of ordinates until that range has been found within which the baseline of the end points of the range ΔM with the curve 300 of the distribution of the area material fraction has the smallest possible slope. This is shown in FIG. 3 b) by way of example for three different regions with a respective width ΔM of 40%. For the first range ΔM 1 the corresponding points s 11 and s 12 are drawn in, which correspond to the range ΔM 1 on the curve 300 of the surface material fraction. These intersection points s 11 and s 12 result in the saw 301 of the curve 300 of the surface material fraction for the first region ΔM 1. Analogously to this, the saw edge 302 for the second region ΔM 2 has also been determined from the points of intersection s 21 and s 22, and the saw edge 303 for the third region ΔM 3 has been determined from the points of intersection s 31 and s 32 respectively.Of the curves 301, 302 and 303 shown, the middle curve 302 has the lowest detectable gradient. This curve 302 of the lowest gradient, which was determined by the method outlined above, is also referred to as equivalence degrees. To determine the core height Sk, the equivalence degrees are extrapolated to area fractions of 0% (intersection point with the axis of abscissa) and 100% (intersection point S 100). From the points of intersection of the equivalence degrees with the ordinal values for the area fractions 0% and 100%, the core height Sk is then determined as the distance of these points of intersection along the axis of abscissa. A low core height is synonymous with a high material density of the surface, which in turn contributes to a high robustness of the surface against mechanical stress. By way of example, the core height Sk of the surface shown in FIG. 3 a) is 12.7.mu.m.Another relevant parameter for describing the surface of the coating is the skewness Ssk (engl. Skewness) of the surface. As previously stated, skewness describes whether the surface can be described as having peaks (positive skewness) or having grooves (negative skewness). This is synonymous with the question as to whether the excursions of the surface of zero line tend to occur more frequently into positive regions or into negative regions. If there are therefore more positions along the measurement path for which the ordinate value is positive, the skewness Ssk is also positive, there are more positions along the measurement path for which the ordinate value is negative, the skewness Ssk is also negative. In this case, the value of the skewness can be entirely different for two surfaces with identical values for the mean square height Sq, since the excursions above or below the mean value of the surface height, on the basis of the calculation from the squared heights of the surface for determining the value Sq, does not take into account the question of the direction of the excursions. By way of example, the surface of FIG. 3 a) has a skewness Ssk of -0.33, i.e. tends to be referred to as a surface with grooves.A comparable, statistical consideration of the surface condition also allows the further surface parameter of the kurtosis Sku considered here. The kurtosis is calculated from the sum over the fourth powers of the local height of the surface z(x), normalized over the measurement path and further divided by the mean quadratic height Sq in the fourth power. In this case, it is effectively considered how frequently a specific value z occurs along the measurement path, regardless of its sign. For a value Sku=3, the various values for the height of the surface are normally distributed around the zero line. With a value for Skuof<3, the surface tends to have a rather roundish structure, while a value Sku>3 tends to refer to rather sharp, tooth-like structures of the surface. The surface shown in FIG. 3 a) has a kurtosis Skuof 2.77, i.e. has a rather rounded surface structure.In the following, it is described with reference to two exemplary embodiments how a coating having the previously described and claimed properties can be produced.For this purpose, FIG. 4 discloses a histogram of an exemplary grain size distribution of a glass flow for producing a coating. The particles of the glass flow have the following composition in % by weight on an oxide basis:SiO 281B 2 O 313Al 2 O 32Na 2 O3,5K 2 O0,5To produce the glass flow, a glass melted according to the above composition is ground, in which case wet grinding was carried out with water. The glass flow produced in this way has a grain size distribution D 10 of 1.37 μm, D 90 of 17.02 μm and D 99 of 24.10 μm.The glass flow produced in this way was then mixed with a screen printing medium based on dipropylene glycol monomethyl ether (DPM) as solvent at a paste ratio of 10:6 (glass flow to screen printing medium). The ink thus produced was then screen printed on a ceramizable glass substrate with a screen of 140-31 gauge and fired according to the ceramization program described above.FIG. 5 shows a local section over an area of approximately 1 mm 2 from the surface profile of the coating obtained in this way in a perspective view. The mean square height Sq of the surface thus obtained is 1.21 μm, the core height Sk is 3.1 μm, the skewness Ssk is 0.01, and the kurtosis Sku is 3.05. Further, the surface has an average arithmetic height Sa of 0.96 μm.The mean arithmetic height Sa is calculated from the surface A of the coating and the height z of the coating according to the following specification:By way of example, FIG. 6 also shows a representation of the surface profile along a line within the surface shown in FIG. 5.FIG. 7 shows a further histogram of an exemplary grain size distribution of a glass flow for producing a coating for an exemplary glass ceramic product. The glass flow is made of the same material as the glass flow described above with reference to FIG. 4. In the case of FIG. 7, too, the glass was ground by means of wet grinding, resulting in the grain size distribution shown. Here, D is 401,15 μm, D is 9014,72 μm, and D is 9919,89 μm. The glass flow thus obtained was mixed with a Naptha-based screen printing medium in a paste ratio of 10:12 and applied to the surface of a substrate by means of a screen of thickness 77-55 analogously to the exemplary embodiment of FIG. 4 and fired with the ceramization of the substrate.FIG. 8 shows a local section over an area of about 1 mm 2 from the surface profile of the coating obtained in this way in a perspective view. The root mean square height Sq of the surface thus obtained is 1.6 μm, the core height Sk is 3.9 μm, the skewness Ssk is 0.045, and the kurtosis Sku is 3.46.By way of example, FIG. 9 also shows a representation of the surface profile along a line within the surface shown in FIG. 8.The surface parameters Sq, Sk, Ssk, Sku and Sa used and described herein are also described, by way of example, in DIN EN ISO 25178-2:2023-09.Although the present invention has been described on the basis of preferred exemplary embodiments, it is not restricted thereto, but can be modified in a variety of ways.

Claims

Glass or glass ceramic product with a substrate made of a glass or a glass ceramic, wherein the substrate is provided on at least one side on at least part of its surface with a substantially pigment-free coating, wherein the coating has an at least partially melted glass flow, wherein the surface of the coating has a mean square height Sq of at least 0.1 μm and at most 2.5 μm, and wherein the coating has a core height Sk of at least 1.0 μm to at most 10 μm, characterized in that the glass flow has the following composition in % by weight on an oxide basis:SiO 275 - 85Al 2 O 30,1 - 5B 2 O 310 - 15Na 2 O1-5K 2 O0,1 - 1,5.Glass or glass ceramic product according to claim 1, wherein the coating has a gloss value of at most 25 at a viewing angle of 60°.Glass or glass ceramic product according to claim 1 or 2, wherein the coating has a skewness Ssk of >0.Glass or glass ceramic product according to one of the preceding claims, wherein the coating has a kurtosis Sku of > 3.Glass or glass ceramic product according to any of the preceding claims, wherein the coating has a thickness of 2 to 10 μm.Glass or glass ceramic product according to one of the preceding claims, wherein a pigmented decorative layer is arranged at least in sections between the substrate and the coating.Glass or glass ceramic product according to claim 6, wherein the combination of decorative layer and coating has a thickness of at most 15 μm.Glass or glass ceramic product according to one of the preceding claims, wherein the coating is applied over the whole surface on one side of the substrate.Glass or glass ceramic product according to one of the preceding claims, characterized in that the substrate is disc-shaped and has a thickness of between 2 mm and 6 mm, preferably between 3 mm and 5 mm, particularly preferably 4 mm.Glass or glass ceramic product according to one of the preceding claims, characterized in that the substrate is transparent with a transmission τvisof greater than 80% and has a chromaticity c* of less than 10, in particular less than 8.Glass or glass ceramic product according to one of Claims 1 to 9, characterized in that the substrate is bulk-coloured and has a transmission τvisof 2% to 10%.Glass or glass ceramic product according to one of Claims 1 to 9, characterized in that the substrate is translucent with a transmission τvisof 2% to 25% or opaque with a transmission τvisof 0.1% to 2%.Glass or glass ceramic product according to one of the preceding claims, characterized in that the substrate consists of a LAS glass ceramic.Method for producing a glass ceramic product according to one of the preceding claims, wherein the method has the following steps: a. providing a substrate made of glass, b. applying a layer of an ink comprising a glass flow and a printing medium to at least one partial region of a surface of the substrate, wherein the application is preferably carried out by means of screen printing, c. ceramizing the coated substrate.Method according to claim 14, wherein the application of the coating is effected by means of screen printing, wherein a screen having a screen thickness of 140-31 to 54-64 is used for the screen printing.Method according to claim 14 or 15, wherein a decorative layer is applied to the substrate before the application of the coating to the substrate, wherein the coating is subsequently applied at least in sections to the decorative layer.Ink for producing a glass or glass ceramic product according to one of Claims 1 to 13, wherein the ink has a boron-containing glass flow and a printing medium, wherein the glass flow has a grain size of D 10 greater than 1 μm and D 90 less than 20 μm, preferably D 90 less than 15 μm.The ink of claim 17, wherein the ratio of glass flow to print medium in the ink is between 10:15 and 10:5.

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