Glass or glass ceramic product, method for production thereof, and ink

Abstract

The invention relates to a glass or glass ceramic product having a substrate made of glass or glass ceramic, in which the substrate is provided on at least one side of at least a portion of the surface thereof with a substantially pigment-free coating, in which the coating comprises an at least partially molten glass flux, wherein the root mean square height Sq of the coating surface is at least 0.1 [mu] m and at most 2.5 [mu] m, and wherein the center height Sk of the coating is at least 1.0 [mu] m and at most 10 [mu] m. The invention further relates to a method and to an ink for producing said glass or glass ceramic product.

Description

TECHNICAL FIELD

[0001] The present invention relates to a glass product or glass-ceramic product having a coating, as well as to a method for manufacturing said glass-ceramic product and to an ink for manufacturing said glass product or glass-ceramic product. BACKGROUND

[0002] It is known from the prior art that glass products and glass-ceramic products have a wide range of applications. In particular in the case of plate-shaped glass-ceramic products, the most prominent example of their application is as a cooking surface.

[0003] The upper side of a glass-ceramic cooking surface is usually equipped with a functional coating in order to carry out different tasks. Coatings are known from the prior art, for example from EP 2 964 854 B1, which protect such a glass-ceramic cooking surface from scratches by depositing a layer of AlSiN on the substrate. Furthermore, it is known that the coating is intended to prevent the cooking surface from becoming soiled, in particular from fingerprints. One example of this is WO 2023 / 099833 A1.

[0004] In recent years, there have been approaches that aim to prevent the scratching of the substrate surface as well as the formation of fingerprints or in general the contamination of the cooking surface. Another example of this is EP 4 077 231 A1. This document discloses a glass-ceramic product obtained by a method comprising: heat-treating a glass suitable for forming a glass-ceramic to ceramize it; and chemically treating the surface of the glass before and / or after the heat-treatment to ceramize it, wherein the chemical surface treatment is performed so that the arithmetic average roughness of the surface after the heat-treatment is between 2 pm and 7 pm. With regard to the chemical treatment of the surface, it is in particular described that an acidic solution based on hydrofluoric acid is used.

[0005] In fact, the etching of glass and glass-ceramics using hydrofluoric acid is known, but it has many drawbacks. On the one hand, the time required to etch the glass or glass-ceramic until the roughness mentioned above is reached is relatively long. On the other hand, hydrofluoric acid is a very tricky substance to handle, various safety measures must be taken into account to ensure the protection of the environment and the protection of the personnel who come into contact with it.

[0006] Against this background, there is a need in the prior art for a solution by which a glass product or glass-ceramic product can be provided with as little expenditure as possible in terms of time and occupational safety, whose surface is resistant to scratches and largely avoids the formation of fingerprints and other contaminations. SUMMARY

[0007] The above object is achieved by a glass product or glass-ceramic product having the features of claim 1. The solution relating to the respective manufacturing method and the ink used therein are the technical subject matter of claims 17 and 20, respectively. Advantageous design solutions are the technical subject matter of the dependent claims.

[0008] In a first aspect, the present invention relates to a glass product or glass-ceramic product having a substrate made of glass or glass-ceramic, wherein the substrate is provided on at least one side of at least a portion of its surface with a coating which is essentially free of pigments, wherein the coating comprises at least partially fused, preferably boron-containing, glass flux, wherein the root mean square height Sq of the coating surface is at least 0.1 pm and at most 2.5 pm, and wherein the center part height Sk of the coating is at least 1.0 pm to at most 10 pm.

[0009] The term "essentially free of pigments" means that the content of pigments in the coating is below 1 wt.-%, i.e. consists of at least 99 wt.-% of the glass flux. By "pigments" is meant particles which are able to change the transmission properties of the coating compared to a coating consisting only of the glass flux. In particular, such pigment particles would cause a discoloration or a reduction of the transmission of the coating.

[0010] The "at least partially fused" glass flux refers to a glass flux, i.e. an amount of glass particles having a defined size distribution, wherein at least a portion of the particles has been fused and solidified again, so that they are no longer present in the form of particles of the original shape and size in the glass flux. Due to the fusion of this portion of the particles, the particles of the glass flux are firmly bound together, forming a layer. At the same time, the fused portion of the glass particles also serves to bond to the substrate. The unfused portion of the glass particles means that the surface is not completely flat, which would be expected for a completely fused glass flux. In contrast, the partial fusion of the glass particles leads to a certain roughness of the surface of the coating.

[0011] The surface roughness of the glass product or glass-ceramic product according to the invention (expressed in the root mean square height Sq of the surface) is at least 0.1 pm and at most 2.5 pm. The root mean square height of the coating surface is then calculated according to the following equation: .

[0012] where A is the area of the coating to be detected and z is the deviation of the measurement point at the position (x, y) from the average height of the coating. Particularly preferably, the root mean square height Sq of the coating is greater than 0.2 pm, greater than 0.3 pm, greater than 0.4 pm or particularly preferably greater than 0.5 pm.

[0013] Another parameter describing the properties of the coating according to the application is the core height Sk of the coating surface. The core height describes the height of the area within the surface material proportion, whose isocline (also called equivalent line) falls at 100% into this height. This area is also referred to as the core surface. The surface material proportion (also referred to as "proportion of surface material") describes in which height range of the coating surface the coating material is in the proportion discussed in the surface area. In fact, each height of the coating surface is occupied by the proportion of surface material that lies above this height. In the graphical representation of the curve, the ordinate represents the height within the coating surface, while the abscissa represents the proportion of material in % that falls into the area above this height.

[0014] The equivalent degree is the secant of the surface material proportion curve with the lowest detectable slope (or smallest gradient), whose intersection with the surface material proportion curve is at a distance of 40% with respect to its abscissa. By extrapolating the equivalent degree to the abscissa values of 0% and 100%, the corresponding ordinate values can be determined, while the core height is the distance between these ordinate values. Thus, a smaller core height means a very compact and thus resistant coating, since the majority of the surface material is concentrated in a narrow area. The core height Sk of the coating can in particular also be 1.5 pm to 9 pm, 1.5 pm to 8 pm, 1.5 pm to 7 pm, 2 pm to 6 pm or particularly preferably 2 pm to 5 pm.

[0015] From the comparison of the values of the root mean square height and the core height of the coating according to the application, it is clear that the coating according to the application combines a relatively rough or matt surface and thus also very dense coating properties. Thus, the coating according to the application is characterized by good performance in terms of preventing fingerprints and other contamination, while also having very strong scratch and wear resistance. The coating contains only ingredients that are harmless to health and can be produced in a simple and economical manner.

[0016] The glass particles of the glass flux of the coating can in particular be boron-containing glass particles. The use of a boron-containing glass flux has advantages in terms of improving the thermal shock resistance of the coating and improving the adhesion of the coating, in particular when the substrate is a glass ceramic. Furthermore, when a boron-containing glass flux is used, the coating can be fired during the ceramization of the substrate, i.e. the conversion of the glass substrate into a glass ceramic substrate. In this way, one step in the production process of the product can be avoided, namely the separate firing of the coating (so-called secondary firing).

[0017] In one embodiment, the matt appearance of the coating described above is also manifested in that the coating has a gloss value of not more than 25 measured at an angle of 60°. This also helps to avoid the coating from forming visible contaminations, in particular in the form of fingerprints. The more glossy the surface is, the more pronounced the fingerprints are, for example.

[0018] Two important and unique parameters of the coating according to the present application are discussed. However, the coating according to the present application also differs significantly from the coatings known in the prior art in other surface parameters.

[0019] According to one embodiment, therefore, the coating is provided with a skewness Ssk > 0. This parameter of the skewness of the surface, also referred to as the skewness, indicates whether the surface can be described as a surface with grooves (Rillen) or valleys (Tälern) rather than a surface with peaks. For a surface with a skewness > 0, i.e. a surface with grooves, by definition the frequency of regions in the surface height profile that lie above the average height is smaller than the proportion of regions that lie below the average height. Since this only occurs when the regions below the average height occur less frequently but the regions above the average height, i.e. the height (or depth) is significantly higher than the average height, this means that this region can be described with valleys and grooves rather than with protruding peaks.

[0020] The skewness can be calculated by adding the cubes of all height values and then dividing by the cube of the root mean square height Sq of the coating surface: .

[0021] A skewness > 0, i.e. a surface with grooves rather than peaks, has the advantage that grooves are more resistant than protruding peaks. This means that, in contrast to reducing the peaks, reducing the depth of the grooves in the coating has to remove more material. The skewness > 0 of the surface is therefore associated with a higher robustness of the surface texture, since the structure of the surface is less affected by, for example, abrasive cleaning processes.

[0022] According to another embodiment, a surface is also provided which has a coating with a kurtosis Sku > 3, particularly preferably > 3 and < 8. The kurtosis of the surface describes the sharpness of the surface profile and is calculated as follows .

[0023] where a kurtosis value Sku > 3 describes a surface which tends to have jagged edges rather than rounded (rundeungen). A surface with jagged rather than rounded edges has the advantage that, for example, when operating a touch-sensitive control, the contact area between the coating surface and the operator's finger is smaller. This effectively prevents the visibility of fingerprints or the like, since they are only located on the very narrow top of the surface structure.

[0024] According to a further embodiment, the thickness of the coating layer is 2 to 10 pm. Therein, the thickness of the coating layer is chosen to ensure that the coating layer has sufficient stability, but at the same time the transmission properties of the substrate are influenced by the coating layer as little as possible. Furthermore, the structure below the coating layer remains clearly visible. Preferably, the thickness of the coating layer is at least 3 pm, particularly preferably at least 4 pm. Furthermore, the thickness of the coating layer is preferably at most 9 pm, particularly preferably at most 8 pm, most particularly preferably at most 7 pm.

[0025] According to a further embodiment, a pigmented decorative layer is also provided at least partially between the substrate and the coating layer. The pigmented decorative layer is preferably applied directly to the substrate and can in particular be produced by inkjet printing or screen printing. If the glass product or glass-ceramic product is used as a cooking surface, the decoration can be a marking of the cooking area. By arranging the decoration below the coating layer, the decoration can be protected from wear, for example due to cleaning the glass product or glass-ceramic product. At the same time, however, due to the small thickness of the coating layer and the absence of pigments in the coating layer, the visibility of the decoration, in particular its edge sharpness, is only slightly impaired. In principle, the decoration can also be fired together with the coating layer, which simplifies the production of the glass product or glass-ceramic product.

[0026] According to a preferred embodiment, when the decorative layer is viewed vertically through the coating layer from above, the root mean square roughness Rq of the contour line of the decorative layer arranged below the coating layer in the direction parallel to the surface of the substrate is at most 20 pm, preferably at most 15 pm. The contour line of the decorative layer describes the transition between the area of the substrate surface covered by the decorative layer and the area of the substrate surface which is essentially free of the decorative layer. By "essentially free" is understood to mean an area in which the thickness of the decorative layer and thus the color effect of the decoration is at least 10% but not more than 25% of the maximum color effect of the decoration. The course of the contour line is similar to the course of the surface height profile, so that known parameters describing the surface properties can be used to describe the course of the contour line. Thus, a root mean square roughness value Rq can be specified for the contour line, which essentially provides information about the root mean square value of all longitudinal coordinate values z(x) of the contour line at different points x over a measurement section of length I. The Rq value is calculated as follows: .

[0027] In practice, Rq represents the contour sharpness of the decoration profile when viewed through the structured coating layer. The smaller the Rq value, the clearer the contour of the decoration. This is particularly advantageous, since even very fine decorative elements can be perceived substantially undisturbed through the structured coating layer.

[0028] According to a further preferred embodiment, the glass product or glass-ceramic product has a decorative layer, which is covered by the coating in the first partial region and which is not covered by the coating in the second partial region. The root mean square roughness Rq of the profile line of the decorative layer in the first partial region differs from the root mean square roughness Rq of the profile line of the decorative layer in the second partial region by at most 10 %, preferably at most 5 %.

[0029] Alternatively, the decoration can also be applied to the coating, which can simplify the production of the glass product or glass-ceramic product. For example, a matt coating can be applied on a large area using a silk screen printing method, and the glass product or glass-ceramic product produced in this way can then be decorated in a further step.

[0030] According to another embodiment, the thickness of the combination of the decorative layer and the coating is at most 15 pm, preferably at most 12 pm, particularly preferably at most 10 pm.

[0031] It has already been mentioned above that the coating is applied to at least a portion of the surface of the substrate. This means that the coating can also have recesses, for example, creating an area in which a display can be arranged. The recesses in the coating, i.e. the areas of the surface which are not coated with the coating, can improve the display effect compared to a full-surface coating. According to a further embodiment, however, the coating is applied to the entire surface on one side of the substrate, which can simplify the production of the glass product or glass-ceramic product.

[0032] It has already been mentioned above that the glass flux preferably contains boron. According to a preferred embodiment, the glass flux is also provided with the following composition in % by weight on the basis of oxides: The glass flux of this composition is particularly suitable for firing the coating during the ceramization of the substrate, which has a significant advantage for the production of the glass product or glass-ceramic product according to the invention. The glass flux used can also be composed of different types of glass mixtures. By mixing different types of glass, the physical properties of the glass flux can be adapted to the requirements for producing the coating.

[0033] The mixture of such glass fluxes can also have a different composition than the above-mentioned composition. For example, such a glass flux can have the following composition in % by weight on the basis of oxides: When a particular glass frit or mixture of different glass frits is chosen to produce the coating, various material parameters of the glass frit can fundamentally influence the properties of the produced coating. For example, if the coefficient of thermal expansion of the glass frit, as well as the coefficient of thermal expansion of the coating, is only slightly different from the coefficient of thermal expansion of the substrate, this is advantageous for the thermal stability of the coating. The choice of a material with a suitable softening point also influences the surface quality of the coating. It is advantageous to choose a softening point relative to the firing temperature of the coating such that during the firing of the coating as part of the ceramization of the substrate, the particles of the glass frit only partially melt, thus achieving the desired rough surface texture.

[0034] According to another embodiment, it is also provided that the substrate is plate-shaped, with a thickness of between 2 mm and 6 mm, preferably between 3 mm and 5 mm, particularly preferably 4 mm. A "plate-shaped" substrate is understood to mean a substrate whose length and width are at least one order of magnitude greater than its thickness. In particular, the glass product or glass-ceramic product can be a cooktop.

[0035] Different materials can be used as the substrate.

[0036] Thus, according to one embodiment, the substrate is transparent, with a transmittance τ vis greater than 80%, preferably greater than 85%, and a chroma c* less than 10, in particular less than 8. The chroma c* is calculated according to the following formula according to the CIELab color system: In practice, this is a transparent and substantially color-neutral substrate.

[0037] For example, the composition of such a substrate can be chosen as follows in terms of weight% on the basis of oxides: .

[0038] Alternatively, according to another embodiment, the substrate is volumetrically colored, and the transmittance τ vis is less than 10%. "Volumetrically colored" means that the color of the substrate does not come from the coating, but from the fact that the material contains elements inside the material that contribute to the color of the material itself. Such volumetric coloring can be achieved by adding elements such as chromium, vanadium or molybdenum to the glass composition. For example, such a substrate can have the following composition in terms of weight% on the basis of oxides: .

[0039] In another alternative embodiment, it is also provided that the substrate is a translucent substrate with a transmittance τ vis of 2% to 25% or a transmissive substrate with a transmittance τ visof 0.1 to 2 %. In particular, this can be a glass-ceramic having a composition as described above for the transparent substrate, wherein the substrate can have a high volume fraction of keatite in the crystalline phase. For the opaque substrate, the CIELab color space can also have a L * 85 - 97, a * - 1.5 - 0.5 and b * - 6 - 0.5. Alternatively, for the translucent substrate, the color of the substrate can be designed as follows: when the transmittance is 2 to 10 %, L * = 72 - 93, a * = -5.5 - 0, b * = -7 - 0.5, alternatively when the transmittance is 10 to 25 %, L * = 60 - 82, a * = -7.5 - 2, b * = -19 - 4.5.

[0040] In particular, when using the glass product or glass-ceramic product according to the application as a cooking surface, another embodiment provides that the substrate consists of a LAS glass-ceramic, i.e. a glass-ceramic made of lithium aluminum silicate. The coefficient of thermal expansion of such a substrate is very low and has a strong resistance to temperature changes, and is therefore particularly suitable for the thermal requirements of a cooking surface.

[0041] According to another embodiment, it is also provided that the substrate has at least one recess. In particular, such a recess can be an opening on the substrate, through which a hood (also referred to as a downdraft hood) can be integrated into the cooking surface when the substrate is used as a cooking surface. The coating on the substrate preferably extends to the edge of the recess, thus achieving a seamless transition of the coating area to the recess. Such a recess in the substrate can be created by various methods, in particular by drilling, milling, water jet cutting, etc.

[0042] According to another embodiment, it is also provided that a halo of at most 1.2, preferably at most 1.15, is produced in the case of a substrate thickness of 4 mm, if the light source is arranged at a distance of 0.5 mm below the glass product or glass-ceramic product. More preferably, a halo of at most 1.4, preferably at most 1.3, is produced in the case of a substrate thickness of 4 mm, if the light source is arranged at a distance of 1.75 mm below the glass substrate or glass-ceramic substrate. The distances mentioned above refer to the distance between the upper side of the light source and the lower side of the substrate. The "halo" is a standard for measuring the degree of perceived distortion when the light source passes through the substrate and the coating on the substrate.

[0043] To this end, the intensity distribution of the light source recorded with the camera without the glass product or the glass-ceramic product is compared with the intensity distribution recorded with the same camera and the same light source through the glass product or the glass-ceramic product arranged at a defined distance between the light source and the bottom surface of the substrate. Preferably, the bottom surface of the substrate is smooth in order to have as low scattering as possible on the bottom surface of the substrate. In order to evaluate the scattering behavior, the half-width of the intensity distribution measured above the light source with and without the substrate is compared, wherein the value of the halo is calculated by the quotient of the half-width with the substrate divided by the half-width without the substrate. Thus, when this value is 1, the substrate arranged above the light source and provided with the coating does not influence the measured intensity distribution. However, if this value is greater than 1, the scattering behavior of the coated substrate can be observed.

[0044] In another aspect, the present application relates to a method for producing a glass-ceramic product as described above, comprising the following steps: a. providing a glass substrate, b. applying a layer of ink comprising a glass fluxing agent and a printing medium onto at least a portion of the area of the surface of the substrate, wherein the application is preferably carried out by screen printing, c. ceramming the coated substrate.

[0045] In this method, in step c, the substrate is cerammed, i.e. transformed from glass to glass-ceramic, and the coating is fired. Thus, the step of separately firing the coating can be omitted, which greatly simplifies the production of the glass product or the glass-ceramic product.

[0046] In order to ceram the coated substrate, the following steps can be employed: heating from room temperature to 680°C in 23 minutes, temperature increase from 680°C to 800°C in 19 minutes, temperature increase from 800°C to 918°C (maximum temperature) in 24 minutes, holding the maximum temperature for 10 minutes, cooling to 800°C in 20 minutes, rapid cooling to room temperature in less than 150 minutes.

[0047] Alternatively, the ceramming can be carried out as follows: rapid temperature increase from room temperature to 740°C in 20 to 26 minutes, in particular 24 minutes, temperature increase from 740°C to 825°C in 12 to 18 minutes, in particular 14 minutes, temperature increase from 825°C to 930°C (maximum temperature) in 4 to 8 minutes, in particular 6 minutes, holding the maximum temperature for 4 to 8 minutes, in particular 6 minutes, cooling to 800°C within 8 to 16 minutes, in particular within 10 minutes to 800°C, rapid cooling to room temperature.

[0048] According to another embodiment, the coating is applied by screen printing, wherein a screen having a mesh size of 140-31 to 54-64 is used for the screen printing. In particular, it has proven advantageous to use a screen having a mesh size of 77-55. It is further possible to provide that the screen used has areas in which no glass frit is transferred to the substrate, thereby creating fine recesses in the coating. In particular, it is possible to use the screen to create fine structures in the coating, thereby reducing the surface coverage of the coating, i.e. the proportion of the coated surface to the uncoated surface in the coated area of the substrate, to less than 100%. For example, it is possible to create patterns in the coating which further reduce the visibility of scratches or surface contamination. The area occupancy is preferably not less than 80%.

[0049] According to another embodiment, it is further provided to apply a decorative layer to the substrate before applying the coating to the substrate, wherein the coating is then applied at least partially to the decorative layer. The decorative layer can in particular be applied to the substrate by means of an inkjet printing method. Alternatively, however, it is also provided to apply the decorative layer after step b) of the method, i.e. to arrange it on the coating.

[0050] It has already been mentioned above that the substrate can also have recesses, for example for the integration of an extractor hood. Such recesses are preferably introduced into the substrate before the substrate is coated. In this case, the coating can be applied to the substrate up to the edge of the recess, so that the coating is not affected by the recess manufacturing process. However, it is also possible to introduce the recess into the already coated substrate after ceramization.

[0051] In another aspect, the present application relates to an ink for producing a glass product or glass-ceramic product as described above, wherein the ink comprises a glass frit containing boron and a printing medium, wherein the particle size of the glass frit is D 10 greater than 1 pm and D 90 less than 20 pm, preferably D 90 less than 15 pm. The printing medium, in particular a medium containing dipropylene glycol monomethyl ether as a solvent, is particularly advantageous in terms of biocompatibility. Alternatively, a printing medium based on naphtha can be used as a solvent.

[0052] The glass frit is preferably borosilicate glass particles, which glass can preferably have the following composition in terms of weight % on the basis of oxides: .

[0053] According to yet another embodiment, the ratio of glass flux 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. BRIEF DESCRIPTION OF DRAWINGS

[0054] The application will be described in more detail hereinafter with reference to the drawings, in which the same reference notations denote the same or similar elements. The application is not limited to the embodiments described hereinafter but can vary in many ways.

[0055] In the drawings: Figures la-lc schematic representation of different embodiments of an exemplary glass product or glass-ceramic product is shown, Figure 2 schematic representation of an exemplary method for producing an exemplary glass-ceramic product is shown, Figures 3a-3b a surface profile plot of a matte surface as a comparative example is shown, Figure 4 a histogram showing an exemplary particle size distribution of a glass flux used for preparing a coating is shown, Figure 5 a perspective view of a surface profile of a coating prepared with a glass flux according to Figure 4 is shown, Figure 6 a surface profile plot along a line within the surface shown in Figure 5 is shown, Figure 7 a histogram showing another exemplary particle size distribution of a glass flux used for preparing a coating is shown, Figure 8 a perspective view of a surface profile of a coating prepared with a glass flux according to Figure 7 is shown, Figure 9 a surface profile plot along a line within the surface shown in Figure 8 is shown, Figures 10 and 11a-11c show a diagram of a decoration located underneath a coating in a top view of an exemplary glass-ceramic product, as well as a contour line of the decoration layer resulting therefrom, and Figure 12 an exemplary measurement structure for determining a halo value is shown. DETAILED DESCRIPTION

[0056] Figures la-lc schematic representation of different embodiments of an exemplary glass product or glass-ceramic product 100 is shown. Figure laThe simplest case is shown, in which the coating 104 is applied directly on the surface 106 of a plate-shaped substrate 102 made of glass or glass-ceramic. The coating has a rough surface 114 with a root mean square height Sq in the range of 0.1 to 2.5 pm, and a center part height Sk of the coating 104 of 1.5 to 10 pm, so that the coating 104 has a matt appearance. In this case, no further coating, in particular no coating in the form of a decoration, is initially provided between the coating 104 and the substrate 102. The substrate 102 is preferably a lithium aluminosilicate glass-ceramic (LAS glass-ceramic), which is particularly suitable for use as a cooking surface due to its generally small coefficient of thermal expansion. The substrate 102 can be a transparent, translucent or opaque material. Furthermore, the substrate 102 can be essentially colorless or volumetrically colored.

[0057] In the embodiment shown, the coating 104 is applied on the upper side of the substrate 102, which faces the user when the product 100 is used as a cooking surface and a cookware, for example, is arranged thereon. Even in this case, the rough surface 114 of the coating 104 is not visible to the user, since the rough surface 114 is covered by the cookware. Figures la-lc No further coating is shown on the lower side 116 of the substrate 102 in all design variants, but fundamentally, such an additional lower side coating is not excluded. Rather, particularly in the case of a transparent substrate 102, it is advantageous to use a lower side coating that is as opaque as possible in addition to the upper side coating 104, for example, in order to hide electronic components arranged below the substrate 102.

[0058] For illustrative reasons, Figures la-lc The dimensions of the elements shown in the figures are greatly exaggerated, in particular the relationship between them is not true to scale. For example, the thickness of the substrate 102 is preferably 4 mm, while the thickness of the coating 104 is preferably only in the range of 2 to 10 pm. Likewise, the length of the substrate 102 is shown by way of example only, and in general the thickness of the substrate 102 will be at least one order of magnitude smaller than its length.

[0059] Figure lb Another embodiment of a glass or glass-ceramic product 100 is shown, in which a further coating 108 in the form of a decorative pattern is additionally applied on the surface 106 of the substrate 102. In the embodiment shown, the decoration 108 is completely covered by the coating 104, thus protected from the outside. By applying the decoration 108 directly on the usually very smooth surface 106 of the substrate 102, a high edge sharpness of the decoration 108 can be ensured, while the rough surface 114 of the coating 104 gives the glass or glass-ceramic product 100 an overall matt appearance. The decoration 108 can be, for example, a marking of a cooking area, a manufacturer's logo or other markings. In addition to the design variant shown here, in which the decoration 108 is completely covered by the coating 104, it is also possible within the scope of the present application for the decoration 108 to be only partially covered by the coating 104, i.e. for certain areas of the decoration 108 to be exposed.

[0060] Although in Figure la and 1b the coating 104 is shown as a full-surface coating of the surface 106 of the substrate 102, it is also conceivable in principle that the coating 104 extends only over a part of the surface 106 of the substrate 102, so that there is a region in which the coating 104 has a recess. This case is shown in Figure lc ). In the shown embodiment based on the Figure lb ), the coating 104 has a recess 110 in the center of the illustration, in which the surface 106 of the substrate 102 is exposed. Such a recess 110 can be taken into account, in particular during the production of the coating 104, by omitting the respective region when printing the coating 104 onto the surface 106 of the substrate 102, for example by suitably masking the substrate 102 or by suitably designing the screen used for applying the coating 104 by means of a screen printing method.

[0061] Such a recess 110 can be particularly advantageous when a display device 112, for example in the form of a seven-segment display or a full-color display, is arranged underneath the substrate 102. The image reproduction of such a display device 112 will be distorted when passing through the coating 104 due to its rough surface. However, this can be avoided by a corresponding recess in the coating 104.

[0062] Figure 2 A schematic diagram of an exemplary method for producing the exemplary glass-ceramic product 100 is shown. In a first method step 200, a substrate 102 made of glass is provided, which can already have been cut to the dimensions required for the final product. Furthermore, it is also possible here to pre-treat the surface 106 of the substrate 102 to be coated, for example to achieve a surface 106 as smooth and even as possible by polishing the surface 106.

[0063] In a second method step 202, an ink comprising a glass flux and a printing medium is applied to at least a part of the surface 106 of the substrate 102. Preferably, the particle size D 10 of the glass flux is greater than 1 pm, D 90 is less than 20 pm, preferably D 90 is less than 15 pm. In principle, any printing method suitable for processing a glass flux having a set particle size can be used to apply the ink to the surface 106 of the substrate 102. However, it is particularly preferable to use a screen printing method for applying the ink. The advantage of a screen printing method is that, on the one hand, large-area coatings can be produced with little effort, and, on the other hand, by suitably designing the screen used, it is possible to leave out specific regions in the coating 104. In this way, for example, the structures described above Figure lc can be created.

[0064] A screen with a suitable printing medium and a glass flux is chosen for the screen printing. It has proven to be particularly advantageous, however, to use a screen with a mesh size of 140-31 when using a printing medium with naphtha as solvent.

[0065] In a third method step 204, the printed substrate 102 is ceramized. For this purpose, for example, the following ceramization process can be used, which is given by way of example only, however, and is not to be understood as limiting.

[0066] from room temperature to 680°C in 23 minutes, from 680°C to 800°C in 19 minutes, from 800°C to 918°C (maximum temperature) in 24 minutes, the maximum temperature is maintained for 10 minutes, cooling to 800°C in 20 minutes, rapid cooling to room temperature in less than 150 minutes.

[0067] Alternatively, the ceramization can be carried out as follows: rapid heating from room temperature to 740°C in 20 to 26 minutes, in particular 24 minutes, from 740°C to 825°C in 12 to 18 minutes, in particular 14 minutes, from 825°C to 930°C (maximum temperature) in 4 to 8 minutes, in particular 6 minutes, the maximum temperature is maintained for 4 to 8 minutes, in particular 6 minutes, cooling to 800°C in 8 to 16 minutes, in particular 10 minutes, rapid cooling to room temperature.

[0068] During the ceramization of the substrate 102, after a nucleation phase in which crystal nuclei are formed in the interior of the substrate 102, controlled crystal growth is stimulated by further temperature increases, so that the glass substrate 102 is converted into a glass ceramic with defined mechanical and optical properties. Due to the high temperatures during the ceramization process, the glass flux located on the surface 106 of the substrate 102 can also be partially molten, while the printing medium of the ink evaporates essentially without residue. The molten glass flux combines with the glass substrate 102 to form a very robust coating 104 on the surface 106 of the substrate 102, which is essentially insensitive to mechanical stress. The non-molten parts of the glass flux form an uneven structure on the surface 114 of the coating 104, which results in the surface parameters according to the application.

[0069] Figure 3aA schematic diagram of the surface height profile is shown to explain the surface parameters according to the invention. On the measurement segment, the height z(x) of the surface above the zero line is shown in µm at various points (x-axis) located within the surface. The zero line is set at a height such that the sum of the deviations of the local surface bulges along the measurement segment from the zero line is zero. The two-dimensional case shown here is only for explaining the surface parameters that have already been named and described. However, according to the invention, the surface parameters used to describe an object are not actually determined along a single line within the surface, but rather through observation of the entire surface, i.e., a three-dimensional representation of the surface.

[0070] from Figure 3a Starting from the zero line shown, to determine the root mean square height Sq of the surface, square all local deviations z(x) from the zero line, add them together, and then divide by the length of the measurement segment. Then take the square root of the result. This effectively provides information about the average degree of deviation of the surface from the zero line, i.e., how rough the surface is. The larger the value, the rougher the surface.

[0071] To determine the center height Sk, firstly, for each height y on the vertical axis, determine the surface proportion above the considered y value. Then, input this proportion as the surface material proportion M onto the horizontal axis. For example, in... Figure 3a At a height of 8µm, the surface material percentage is 0% because the surface above 8µm does not deviate from the zero line. For a height of 6µm, the surface percentage deviating from the zero line by more than +6µm has been determined to be a low-digit percentage. By definition, in the diagram shown, the ordinate value of 0µm will be assigned a 50% percentage because the zero line precisely divides the surface into equal parts. However, in principle, another zero line can also be chosen as a control point. When the value is -10µm, the surface material percentage M exceeds 95%, and when the ordinate value is approximately -13µm, 100% of the surface is above this value.

[0072] Figure 3b An exemplary distribution 300 of this type is shown. To determine the central height Sk based on this distribution, the first step is to determine the equivalence of the surface material proportion distribution 300. For this, a region representing 40% of the surface material proportion M (ΔM) is moved along the vertical axis until a region is found where the endpoints of region ΔM have the lowest possible slope with respect to the secant line of the surface material proportion distribution curve 300. For example... Figure 3b) shows three different areas AM of respective width of 40%. For the first area AM1, the corresponding points s11 and s12 are shown, which correspond to the area AM1 on the curve 300 of the surface material proportion. The secant 301 of the curve 300 of the surface material proportion for the first area AM1 is derived by these intersection points s11 and s12. Similarly, the secant 302 for the second area AM2 is determined by the intersection points s21 and s22, and the secant 303 for the third area AM3 is determined by the intersection points s31 and s32.

[0073] In the shown curves 301, 302 and 303, the middle curve 302 has the lowest detectable slope. The curve 302 with the lowest slope determined according to the method outlined above is also referred to as the equivalent degree. In order to determine the core height Sk, the equivalent degree is extrapolated to the levels of the area proportion 0% (intersection with the abscissa axis) and 100% (intersection S 100 According to the intersection of the equivalent degree with the ordinate values of the area proportions 0% and 100%, the core height Sk is determined as the distance of these intersection points along the abscissa axis. A smaller core height means a higher density of surface material, which in turn contributes to an increased robustness of the surface against mechanical stress. For example, Figure 3a The core height Sk of the surface shown in Fig. 6 is 12.7 pm.

[0074] Another relevant parameter describing the coating surface is the skewness Ssk of the surface. As already mentioned, the skewness describes whether the surface can be described as a surface with peaks (positive skewness) or as a surface with grooves (negative skewness). This corresponds to the question of whether the deviation of the surface from the zero line tends to occur in the positive or in the negative region. If there are more positions on the measurement section where the ordinate value is positive, the skewness Ssk is also positive; if there are more positions on the measurement section where the ordinate value is negative, the skewness Ssk is also negative. For two surfaces with the same value of the root mean square height Sq, the skewness value can be different, since in the calculation of the square of the surface height for determining the Sq value, the question of the deviation in the direction is not taken into account, which leads to deviations above or below the average value of the surface height. For example, Figure 3a The surface skewness Ssk in Fig. 6 is -0.33, so it can be described as a surface with grooves.

[0075] A comparable statistical analysis of the surface texture can also be made using the kurtosis Sku, which is also considered as a surface parameter here. The kurtosis is calculated as follows: by summing the fourth powers of the local heights z(x) of the surface, the fourth powers are normalized to the measurement section, and then further divided by the root mean square height Sq of the fourth powers. In fact, this takes into account the frequency with which a certain value z occurs on the measurement section, regardless of its sign. For a value Sku = 3, the different values of the surface height are normally distributed around the zero line. When the value Sku is < 3, the surface tends to have a more rounded structure, while when the value Sku is > 3, this indicates a more pointed, more jagged structure of the surface.Figure 3a The kurtosis Sku of the surface shown in Fig. 1 is 2.77, thus having a very rounded surface structure.

[0076] The following two exemplary embodiments describe how to produce a coating having the properties described and claimed above.

[0077] Figure 4 A histogram showing an exemplary particle size distribution of a glass flux used to produce a coating is shown. The glass flux particles can have the following composition in weight % on an oxide basis: To produce the glass flux, a molten glass according to the above composition is ground, in this case wet ground using water. The particle size distribution D 10 of the glass flux produced in this way is 1.37 pm, D 90 17.02 pm, D 99 24.10 pm.

[0078] The glass flux thus produced is then mixed with a screen printing medium as solvent in a mixing ratio of 10:6 (glass flux: screen printing medium), wherein the screen printing medium is based on dipropylene glycol monomethyl ether (DPM) as solvent. The resulting ink is then screen printed onto a ceramifiable glass substrate using a screen with a mesh size of 140 - 31 and fired according to the ceramification procedure described above.

[0079] In addition to the glass flux mentioned above, another glass flux can be added to adapt the properties of the coating to the substrate used. For example, a glass flux having the following composition can be used: .

[0080] This glass flux differs from the glass flux mentioned above in its softening temperature and coefficient of thermal expansion, so that by mixing these glass fluxes, the mechanical and thermal properties of the coating produced can be adjusted.

[0081] Figure 5 A partial detail of an area of about 1 mm2of the surface profile of the coating thus obtained is shown in perspective view. The root mean square height Sq of the surface thus obtained is 1.21 pm, the core height Sk is 3.1 pm, the skewness Ssk is 0.01 and the kurtosis Sku is 3.05. In addition, the arithmetic mean height Sa of the surface is 0.96 pm.

[0082] The arithmetic mean height Sa is calculated from the coating area A and the coating height z according to the following rule: .

[0083] Figure 6 It also shows along Figure 5 Example of a surface profile with a line within the surface shown.

[0084] Figure 7 Another histogram showing an exemplary particle size distribution of a glass flux used to produce coatings for exemplary glass-ceramic products is illustrated. The glass flux is derived from the above reference... Figure 4 The described glass flux has the same material composition. Figure 7 In this case, the glass is also ground by wet grinding, resulting in the particle size distribution shown. D 10 It is 1.15µm, D 90 It is 14.72µm, D 99 The thickness was 19.89 µm. The resulting glass flux was mixed with a naphtha-based screen printing medium at a ratio of 10:12, and then processed using a method similar to... Figure 4 In the embodiment shown, a screen with a mesh size of 77-55 is applied to the substrate surface and fired together with the ceramicization of the substrate.

[0085] Figure 8 A perspective view shows a local detail of an area of ​​approximately 1 mm² on the resulting coated surface profile. The resulting surface has a root mean square height Sq of 1.6 μm, a center height Sk of 3.9 μm, a skewness Ssk of 0.045, and a kurtosis Sku of 3.46. Furthermore, the arithmetic mean height Sa of this surface is 1.25 μm.

[0086] Figure 9 It also shows along Figure 8 Example of a surface profile with a line within the surface shown.

[0087] Figure 10a A black-and-white image of an exemplary glass-ceramic product having a colored decorative layer located beneath the coating is shown. Such an image can be created, for example, using an optical microscope. The coating appears faded or worn due to its granular structure (a substantially uniform decorative layer) and, in particular, the typically very sharp decorative outline 404 (which represents the transition between the decorated area 402 and the undecorated substrate 400).

[0088] To describe the properties of contour line 404, we can first use a simple method from Figure 11a The specific route of the outline 404 can be extracted from the illustration. To do this, a grayscale value between 0 and 100 can be set as a threshold to represent the decorative color effect between 0% and 100%, for example, a value of 20. Figure 11aAll measurement points with a grey value above or below the set threshold are assigned a grey value of 0, while all measurement points with a grey value of 20 are assigned a value of 100. Instead of a discrete threshold, it is also possible to set a threshold range, for example all grey values between 15 and 20, which should be accepted as contour line.

[0089] Figure 11b The result of this operation is schematically shown in the upper half of the detail shown in Figure 11a . As can be seen in Figure 11b , applying the threshold of the grey values of the measurement points, spots within the decoration, also caused by optical distortions of the structured coating, are sometimes interpreted as contour lines. These points can be manually removed from the illustration or a region can be defined within which points are to be considered as contour lines, so that points outside this region are no longer considered.

[0090] In this way a discrete contour line can be determined, for example as shown in Figure 11a . Due to the optical distortions caused by the structured coating arranged on the decoration layer, when looking along the contour line in the x-z diagram, it can occur that there are two different points on the z-axis for which a point on the x-axis can be assigned on the contour line. For example, in Figure 11a , this is the case for the point x1, since there is not a unique value z to which this point can be assigned on the contour line, the straight line through the point x1 parallel to the z-axis intersects the contour line in two points.

[0091] However, in order to determine the root mean square value of all ordinate values of the contour line, a curve is required which assigns only one value z(x) to each value x, so that in the next step the curve has to be converted into a suitably fitted curve. This can be achieved by assigning the highest value z(x) on the contour line to each point x for which multiple ordinate values z(x) can be assigned. This is exemplarily shown in Figure 11b . Conversely, the lowest ordinate value z(x) on the contour line can be assigned to each point x, as shown in Figure 11c .

[0092] The contour line obtained in this way makes it possible to determine the root mean square value, which corresponds to the root mean square roughness value Rq. It is also possible to determine the Rq value for both contour lines of Figure 11b and 11c and to assume the average of these two values as the actual value of the contour line. In this way, it is possible to know the degree of distortion of the clear contour line inherent to the decoration by the structured coating deposited on the decoration.

[0093] In addition to the value Rq described above, further parameters can be determined to characterize the profile line, which can be derived from the analysis of the surface texture. For example, the two-dimensional parameters Sa, Ssk and Sku described above with reference to the properties of the surface 114 of the coating 104 of the exemplary glass product or glass-ceramic product 100 can also be determined in one dimension, i.e. Ra, Rsk and Rku of the profile line. The arithmetic mean roughness Ra of the profile line is preferably at most 15 pm, while the skewness Rsk is preferably less than zero. More preferably, the kurtosis Rku of the profile line is greater than 3.

[0094] Figure 12 An exemplary measurement structure for determining the halo value of the exemplary glass product or glass-ceramic product 100 is shown. In a dark room 500, a stand 504 is arranged on a vibration-damping base 502, on which a light source 506 with a light-emitting device, for example a white seven-segment display, is arranged. The workpiece 508 to be tested, in particular a plate-shaped substrate, can be placed in turn on the light source, so that a defined distance can be established between the light-emitting device and the underside of the workpiece 508 by means of a spacer arranged between the workpiece 508 and the light source 506. Above the workpiece 508, a camera 512 with a lens 514 is arranged vertically above the light source 506 or above the light-emitting device used, the lens being focused onto the surface of the light-emitting device. For example, the camera 512 can be arranged at a distance of about 30-35 cm above the light source 506.

[0095] For the determination of the halo value, a zero measurement is carried out without the workpiece 508 and a measurement with the workpiece 508, in each case in the form of a gray scale, the image of the light-emitting device being recorded. From the images obtained in this way, in particular from the intensity distribution curves obtained by recording cross sections of the images, the halo value can be determined as described above.

[0096] The surface parameters Sq, Sk, Ssk, Sku and Sa used and described herein are also described as examples in DIN EN ISO 25178-2:2023-09.

[0097] Although the application has been described using preferred embodiments, the application is not limited thereto, but can be modified in various ways.

[0098] List of reference signs 100 glass product or glass-ceramic product 102 substrate 104 coating 106 substrate surface 108 decoration 110 recess 112 display device 114 coating surface 116 substrate underside 300 surface material proportioning 301 cut line 302 equivalent degree 303 cut line

Claims

1. A glass product or glass-ceramic product having a substrate made of glass or glass-ceramic, wherein the substrate is provided on at least one side of at least a portion of its surface with a coating which is essentially free of pigments, wherein the coating comprises at least partially molten glass flux, wherein the root mean square height Sq of the coating surface is at least 0.1 pm and at most 2.5 pm, and wherein the core height Sk of the coating is at least 1.0 pm to at most 10 pm.

2. The glass product or glass-ceramic product according to claim 1, wherein the coating has a gloss value at 60° viewing angle of at most 25.

3. The glass product or glass-ceramic product according to claim 1 or 2, wherein the skewness Ssk of the coating is > 0.

4. The glass product or glass-ceramic product according to any one of the preceding claims, wherein the kurtosis Sku of the coating is > 3.

5. The glass product or glass-ceramic product according to any one of the preceding claims, wherein the coating thickness is 2 to 10 pm.

6. The glass product or glass-ceramic product according to any one of the preceding claims, wherein a colored decorative layer is at least partially provided between the substrate and the coating; preferably, when looking vertically through the decorative layer of the coating from above, the profile line of the decorative layer provided below the coating has a root mean square roughness Rq in the direction parallel to the surface of the substrate of at most 20 pm.

7. The glass product or glass-ceramic product according to claim 6, wherein the thickness of the combination of the decorative layer and the coating is at most 15 pm.

8. The glass product or glass-ceramic product according to any one of the preceding claims, wherein the coating is applied on the entire surface of one side of the substrate.

9. A glass product or glass-ceramic product, wherein the glass flux can have the following composition in weight % on the basis of oxides: 。 10. Glass product or glass-ceramic product according to any one of the preceding claims, characterized in that, the substrate is plate-shaped with a thickness of between 2 mm and 6 mm, preferably between 3 mm and 5 mm, particularly preferably 4 mm.

11. Glass product or glass-ceramic product according to any one of the preceding claims, characterized in that, The substrate is transparent, with a transmittance τ vis greater than 80%, and a chromaticity c * less than 10, in particular less than 8.

12. The glass product or glass-ceramic product according to any one of claims 1 to 10, characterized in that, The substrate is volumetrically colored and has a transmittance τ vis is 2% to 10%.

13. The glass product or glass-ceramic product according to any one of claims 1 to 10, characterized in that, The substrate is translucent, transmittance τ vis from 2% to 25%, or opaque, transmittance τ vis from 0.1% to 2%.

14. Glass product or glass-ceramic product according to any one of the preceding claims, characterized in that, the substrate consists of LAS glass-ceramic.

15. Glass product or glass-ceramic product according to any one of the preceding claims, characterized in that, a halo of at most 1.2 is produced in the case of a substrate thickness of 4 mm if a light source is arranged at a distance of 0.5 mm below the glass product or glass-ceramic product.

16. A method for producing a glass-ceramic product according to any one of the preceding claims, wherein the method comprises the following steps: providing a substrate made of glass, applying a layer of ink comprising a glass flux and a printing medium to at least a portion of the surface of the substrate, wherein the application is preferably carried out by screen printing, ceramming the coated substrate.

17. The method according to claim 16, wherein the coating is applied by screen printing, wherein the screen printing is carried out using a screen with a mesh size of 140-31 to 54-64.

18. The method according to claim 16 or 17, wherein a decorative layer is applied to the substrate before the coating is applied to the substrate, wherein the coating is then at least partially applied to the decorative layer.

19. An ink for producing the glass product or glass-ceramic product according to any one of claims 1 to 15, wherein the ink comprises a boron-containing glass flux and a printing medium. wherein the particle size D 10 greater than 1 pm, D 90 less than 20 pm, preferably D 90 less than 15 pm.

20. The ink according to claim 19, wherein the ratio of glass flux to printing medium in the ink is between 10:15 and 10:5.

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