Glass-ceramic substrate with matte coating and compressive stress zone

Abstract

Glass-ceramic product with a glass-ceramic substrate and a matte coating, wherein • the glass-ceramic substrate has a surface, • the matte coating is applied to the surface of the glass-ceramic substrate, and • the matt coating creates a compressive stress zone near the surface in the glass-ceramic substrate.

Description

[0001] The invention relates to a glass-ceramic product with a matte coating, in particular a glass-ceramic product with a matte coating that creates a near-surface compressive stress zone in the glass-ceramic substrate. State of the art

[0002] Glass ceramic cooktops have become increasingly popular in recent decades, replacing traditional cooktops in many households. They offer a smooth, easy-to-clean surface and a modern appearance. Glass ceramic cooktops are typically manufactured with a transparent or translucent surface, allowing the heating elements or markings underneath to be seen.

[0003] Recently, there has been a trend toward matte finishes on glass-ceramic cooktops. Matte finishes can offer aesthetic advantages and better integrate into modern kitchen designs. However, producing matte finishes on glass-ceramic substrates presents manufacturers with technical challenges.

[0004] One way to create matte surfaces is to apply a coating to the glass-ceramic substrate. However, this can lead to problems such as insufficient coating adhesion or reduced thermal shock resistance. Particularly at the high temperatures that can occur during operation of a cooktop, there is a risk of the coating detaching from the substrate. These problems become more pronounced the thicker the applied coating.

[0005] EP 1492737 B1 discloses a method for producing a glass-ceramic plate with an enamel coating. The enamel coating is applied to a green glass and fired during the ceramization of the green glass. However, this process can result in stresses between the enamel coating and the glass-ceramic substrate, which can lead to a reduction in strength.

[0006] It was recognized that a glass-ceramic product was needed that had a matte surface while offering improved thermal shock resistance and coating adhesion. Disclosure of the invention

[0007] The present disclosure relates to a glass-ceramic product comprising a glass-ceramic substrate and a matte coating, wherein the glass-ceramic substrate has a surface, the matte coating is applied to the surface of the glass-ceramic substrate, and the matte coating creates a near-surface compressive stress zone in the glass-ceramic substrate.

[0008] This configuration enables improved adhesion of the matte coating to the glass-ceramic substrate and increases the product's thermal shock resistance. The compressive stress zone compensates for thermal stresses that can occur during use, especially in cooktops.

[0009] Preferably, the matte coating comprises a glass flux having the following composition in percent by weight on an oxide basis: SiO2: 75 - 85%, B2O3: 10 - 15%, Al2O3: 0.1 - 5%, Na2O: 1 - 5%, K2O: 0.1 - 1.5%.

[0010] This specific composition of the glass flux enables optimal formation of the compressive stress zone while ensuring good adhesion and durability of the matte coating.

[0011] A glass flux of this composition is particularly suitable for firing the coating during the ceramization of the substrate, which offers significant advantages for the production of the glass or glass-ceramic product according to the invention. The glass flux used can also consist of a mixture of different types of glass. By mixing different types of glass, it is possible, for example, to adapt the glass flux's physical properties to the requirements for producing the coating.

[0012] Preferably, the glass flux has a grain size distribution with a D 10 ) of 1 to 2 µm, a D 90 from 15 to 20 µm and a D 99 from 22 to 26 µm.

[0013] This grain size distribution contributes to the formation of a uniform and durable matte surface and positively influences the formation of the compressive stress zone.

[0014] The matt coating preferably has a thickness of 5 to 10 µm.

[0015] This layer thickness offers a good compromise between sufficient matting and the creation of the desired compressive stress zone without compromising the thermal properties of the glass-ceramic substrate.

[0016] Preferably, the compressive stress zone extends to a depth of about 2 mm and has a maximum compressive stress between a depth of 0.5 mm and 1 mm.

[0017] This distribution of the compressive stress zone ensures optimal compensation of thermal stresses and improves the overall stability of the glass-ceramic product.

[0018] Preferably, the compressive stress zone has compressive stresses of up to 1 MPa.

[0019] These compressive stress values ​​provide effective compensation for occurring tensile stresses and increase the fracture strength of the glass-ceramic product.

[0020] Preferably, at least one decorative layer is located between the matt coating and the surface of the glass-ceramic substrate.

[0021] The integration of a decorative layer enables aesthetic design options without compromising the advantageous properties of the matte coating and the compressive stress zone.

[0022] Preferably, the decorative layer has a thickness of less than 5 µm.

[0023] This low thickness of the decorative layer ensures that the formation of the compressive stress zone is not significantly affected and the adhesion of the matt coating is maintained.

[0024] The glass ceramic product is preferably designed as a hob.

[0025] The use as a hob makes optimal use of the improved thermal shock resistance and the aesthetic advantages of the matte surface.

[0026] Preferably, the matt coating has a core height (Sk) of less than 10 µm.

[0027] This low core height contributes to a uniform and pleasant matte surface that is easy to clean while maintaining the desired optical properties.

[0028] The core height describes the height of the area within the area material portion of the coating's surface that accounts for 100% of the equivalence line (also known as the equivalent line) of the area material portion. This area is also referred to as the core surface. The area material portion (also known as the "areal material portion") describes the height range of the coating's surface over which the portion of the coating material in the area of ​​the surface in question is allocated. Effectively, each height within the coating's surface is assigned the portion of the surface material that lies above this height. In a graphical representation of this curve, the ordinate represents the height within the coating's surface, while the abscissa represents the material portion in % that falls within the area above this height.

[0029] The equivalence degree is the secant of the surface material content curve with the lowest detectable slope (or gradient), whose intersection points with the surface material content curve are 40% apart with respect to their abscissa. By extrapolating the equivalence degrees to abscissa values ​​of 0% and 100%, the corresponding ordinate values ​​can be determined, with the core height being the distance between these ordinate values. Accordingly, a low core height means a very compact and therefore resistant coating, since a large portion of the surface material is concentrated in a narrow area. Short description of the characters

[0030] The invention will be described in more detail below with reference to the figures, without limitation. Like reference numerals designate like or similar elements.

[0031] They show: Fig. 1 is a graphical representation of mechanical stresses in a coated glass-ceramic, in accordance with one aspect of the present disclosure; Fig. 2 is another graphical representation of mechanical stresses in a coated glass-ceramic, in accordance with one aspect of the present disclosure; Fig. 3 is a graphical representation of the mechanical stresses in a coated glass-ceramic with a different decoration, in accordance with an aspect of the present disclosure; Fig. 4 is a graphical representation of stress measurements in a coated glass-ceramic with a decorative matte coating, in accordance with one aspect of the present disclosure; Fig. 5 is another graphical representation of stress measurements in a coated glass-ceramic with a decorative matte coating, in accordance with one aspect of the present disclosure; and Fig. 6 is a cross-sectional view of a glass-ceramic product with a matte coating and decorative layer, in accordance with one aspect of the present disclosure. Detailed description

[0032] In the following, similar or identical features are identified by the same reference symbols.

[0033] The present invention relates to a glass-ceramic product 100. As in Fig. 1, the glass-ceramic product 100 includes a glass-ceramic substrate 102 and a matte coating 104 applied to a surface of the glass-ceramic substrate 102.

[0034] The matte coating 104 serves to impart a matte appearance to the glass-ceramic product 100 while ensuring improved mechanical stability. This is achieved by creating a near-surface compressive stress zone in the glass-ceramic substrate 102, which is created by the special composition and processing of the matte coating 104.

[0035] The glass-ceramic product 100 can be used, for example, as a cooktop or as a decorative glass element. The matte coating 104 enables an aesthetically pleasing surface while simultaneously improving durability and resistance to thermal stress.

[0036] Fig. Figure 1 shows a cross-section of a glass-ceramic product 100 in three different embodiments. The glass-ceramic product 100 comprises a glass-ceramic substrate 102 with an upper substrate surface 106 and a lower substrate surface 116. The glass-ceramic substrate 102 has a substrate thickness D, which can be, for example, 4 mm.

[0037] A matte coating 104 is applied to a portion of the upper substrate surface 106, forming a coating surface 114. The matte coating 104 has a thickness of 5-10 µm. The matte coating 104 consists of a glass flux with a composition of 75-85% SiO2, 10-15% B2O3, 0.1-5% Al2O3, 1-5% Na2O, and 0.1-1.5% K2O. The coating surface 114 of the matte coating 104 has a reduced core height (Sk) of less than 10 µm.

[0038] In the Fig. In the embodiment shown in Figure 1 b), a decorative layer 108 is located between the glass-ceramic substrate 102 and the matte coating 104. This layer can be applied selectively so that it does not extend over the entire upper substrate surface 106. Rather, the decorative layer 108 can be used to create, for example, a cooking zone marking.

[0039] The Fig. The embodiment shown in Figure 1 c) has an uncoated area 110 in which the matte coating 104 and the decorative layer 108 are not applied, thereby exposing the upper substrate surface 106. Adjacent to this uncoated area 110, a display element 112 is integrated into the glass-ceramic product 100. The uncoated area 110 allows an unobstructed view of the display element 112.

[0040] The compressive stress layer d1 extends to a depth of approximately 2 mm into the glass-ceramic substrate 102, with the maximum stress occurring between 0.5 mm and 1 mm. This compressive stress layer contributes to the stability of the glass-ceramic product 100.

[0041] The glass-ceramic product 100 can withstand temperatures exceeding 500°C, particularly up to 650°C, while maintaining the advantageous compressive stress zone. This property makes the glass-ceramic product 100 particularly suitable for applications exposed to high temperatures, such as cooktops.

[0042] The Fig. 2 and Fig. Figure 3 shows graphs depicting the mechanical stresses in a 4 mm thick glass-ceramic coated on its surface. In these graphs, the stress in the glass-ceramic is plotted in MPa versus the depth z in the glass-ceramic.

[0043] In both figures two graphs are shown that show the measured voltage σ in MPa in the glass-ceramic substrate over the depth z in mm in the glass-ceramic substrate. The depth is measured orthogonally to the coated surface of the substrate. The upper graph shows the stress profile of the coating according to the invention, while the lower graph represents the stress profile of a conventional decoration according to the state of the art. The glass-ceramic substrate begins at z=0, while the coating would be found in the range z<0. However, measuring the stress ratios in the coating is not feasible with the methodology used.

[0044] The measurements were performed at three different measurement positions, which are represented by the different lines in the graphs. To measure the stress profiles, a 1 mm wide strip was cut from a suitably prepared glass-ceramic substrate with a corresponding coating, the cut surfaces of which were then polished. Subsequently, the stress birefringence between the surfaces was measured in the z-direction, perpendicular to the cut surfaces, using a suitable measuring device. The stress birefringence thus determined was then converted into stresses. A stress-optical constant of 3.2 TPa was used. -1 which is typical for the material of the glass-ceramic substrate. The side of the substrate facing away from the coating was provided with nubs, and the measuring section was selected so that the end of the measuring section lies between the nubs.

[0045] When looking at the upper graphs in Fig. 2 and Fig. 3, it is noticeable that a tensile stress initially develops beneath the inventive decoration (z > 0). This tensile stress reaches values ​​of approximately 5 to 10 MPa near the surface. With increasing depth, this tensile stress transforms into compressive stress. The compressive stress reaches its maximum at a depth of approximately 0.5 to 1 mm and has values ​​of approximately -0.5 to -1 MPa. With further increasing depth, the compressive stress gradually decreases and approaches zero again.

[0046] In contrast, the lower graphs in Fig. 2 and Fig. 3, which represent the conventional decor, exhibit a significantly different stress profile. No pronounced compressive stress zone is observed here. The stresses remain close to zero or slightly in the tensile stress range throughout the entire depth of the glass-ceramic.

[0047] The comparison between the coating according to the invention and the conventional decoration clearly shows that the coating according to the invention creates a characteristic compressive stress zone in the glass-ceramic. This compressive stress zone extends from approximately 0.5 mm to 2 mm deep and can contribute to improving the stability and durability of the glass-ceramic.

[0048] It should be noted that in the graphs, positive stress values ​​describe tensile stress, while negative values ​​represent compressive stress. The observed stress profiles are consistent across different measurement positions and samples, indicating a reproducible and reliable generation of the compressive stress zone by the coating according to the invention.

[0049] The Fig. 4, Fig. 5 and Fig. 6 show stress profiles in a glass ceramic with a matt coating and different decorative layers.

[0050] Fig. Figure 4 shows a comparison between the matte coating according to the invention and another known decoration. The graphs show the mechanical stresses in a 4 mm thick glass ceramic coated on its surface. The stress in the glass ceramic is plotted in MPa against the depth z in the glass ceramic. The upper graph shows the invention, in which a tensile stress develops below the inventive decoration (z>0), which is not observed in this way in a decoration according to the prior art (lower graph). The measurement was carried out at three different measuring positions. Positive stress values ​​​​describe a tensile stress, while negative values ​​​​describe a compressive stress.

[0051] Fig. Figure 5 shows a stress profile for a glass-ceramic with a matte coating and a decorative layer between the coating and the substrate surface. The graph contains three overlapping lines representing different positions (pos. 1, 2, and 3) on the sample. All three lines follow a similar pattern: they start with a sharp peak of high tensile stress (about 10 MPa) near the surface (z = 0 mm), then rapidly decrease to negative values ​​(about -0.5 MPa) of compressive stress within the first 0.5 mm. The stress then gradually increases back towards 0 MPa as the depth increases to about 3 mm. At about 3.5 mm depth, there is a sudden drop in stress for all three positions.

[0052] Fig.Figure 6 also shows a stress profile for a glass-ceramic with a matte coating and a decorative layer between the coating and the substrate surface. The graph contains three overlapping curves in different colors (black, red, and blue), representing positions 1, 2, and 3. All three curves follow a similar pattern, starting with a sharp peak of high tensile stress (about 7-8 MPa) near the surface (z = 0 mm), followed by a rapid decrease into a compressive stress range (negative values) between about 0.5 and 2 mm depth. The compressive stress reaches a maximum of about -0.5 MPa at about 1 mm depth. Beyond 2 mm, the stress gradually returns to values ​​close to zero.

[0053] These graphs illustrate that the compressive stress zone effect occurs even when a decorative layer is present between the matte coating and the substrate surface. This shows that the matte coating can be applied over various types of decorative inks, while the compressive stress zone effect can still be observed.

[0054] The glass-ceramic product can be manufactured by a multi-step process that involves applying a matte coating to an unceramized glass substrate and then ceramizing it.

[0055] For the matte coating, a glass flux is used that has a specific particle size distribution.

[0056] The glass flux preferably has a D 10 -value of 1 to 2 µm, a D 90 -value of 15 to 20 µm and a D 99 -value of 22 to 26 µm

[0057] This particle size distribution contributes to the formation of the desired surface structure and stress conditions in the finished coating.

[0058] The composition of the glass flux includes in weight percent on an oxide basis: SiO2: 75 - 85% Al2O3: 0.1 - 5% B2O3: 10 - 15% Na2O: 1 - 5% K2O: 0.1 - 1.5%

[0059] A particularly preferred composition of the glass flux is: SiO2: 81% B2O3: 13% Al2O3: 2% Na2O: 3.5% K2O: 0.5%

[0060] The ground glass flux is mixed with a printing medium and applied to the unceramized glass substrate using screen printing. The screen printing process allows for precise control of the layer thickness and distribution.

[0061] After the coating is applied, the ceramization process begins. This can be carried out according to various temperature profiles. An example temperature profile includes the following steps: 1. Heating from room temperature to 680°C within 23 minutes 2. Temperature increase from 680°C to 800°C within 19 minutes 3. Temperature increase from 800°C to 918°C (maximum temperature) within 24 minutes 4. Maintain the maximum temperature for 10 minutes 5. Cooling to 800°C within 20 minutes 6. Rapid cooling to room temperature within less than 150 minutes. An alternative temperature profile can look like this: 1. Rapid heating from room temperature to 740°C in 20 to 26 minutes, preferably 24 minutes 2. Temperature increase from 740°C to 825°C in 12 to 18 minutes, preferably 14 minutes 3. Temperature increase from 825°C to 930°C (maximum temperature) in 4 to 8 minutes, preferably 6 minutes 4. Maintain the maximum temperature for 4 to 8 minutes, preferably 6 minutes 5. Cooling to 800°C within 8 to 16 minutes, preferably within 10 minutes 6. Rapid cooling to room temperature

[0062] During the ceramization process, the glass flux partially melts, forming a layer with a rough surface. At the same time, a near-surface compressive stress zone develops in the glass-ceramic substrate.

[0063] Variations in the manufacturing process can include various aspects: 1. Composition of the glass flux: The proportions of the individual oxides can be varied within the specified ranges to optimize the properties of the coating. 2. Particle size distribution: Adjusting the particle size distribution can influence the surface roughness and melting behavior of the glass flow. 3. Application method: In addition to screen printing, alternative application methods such as spray coating or roller coating can be considered for special applications. Furthermore, application of the coating using an inkjet printing process may also be considered. 4. Ceramicization profile: The temperature control during ceramicization can be adjusted to optimize the formation of the compressive stress zone. 5. Multi-layer coatings: The application of multiple layers with different compositions can lead to more complex stress profiles or surface properties.

[0064] These variations allow the manufacturing process to be adapted to specific requirements and can contribute to optimizing product properties.

[0065] Although the present invention has been described using preferred embodiments, it is not limited thereto but can be modified in many ways. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] EP 1492737 B1

[0005]

Claims

[1] Glass-ceramic product comprising a glass-ceramic substrate and a matte coating, wherein • the glass-ceramic substrate has a surface, • the matte coating is applied to the surface of the glass-ceramic substrate, and • the matt coating creates a compressive stress zone near the surface in the glass-ceramic substrate. [2] A glass-ceramic product according to claim 1, wherein the matte coating comprises a glass flux having the following composition in weight percent on an oxide basis: SiO2: 75 - 85% B2O3: 10 - 15% Al2O3: 0.1 - 5% Na2O: 1 - 5% K2O: 0.1 - 1.5% [3] Glass-ceramic product according to claim 1 or 2, wherein the glass flux has a grain size distribution with a D 10 from 1 to 2 µm, a D 90 from 15 to 20 µm and a D 99 from 22 to 26 µm. [4] A glass-ceramic product according to any one of the preceding claims, wherein the matte coating has a thickness of 5 to 10 µm. [5] A glass-ceramic product according to any one of the preceding claims, wherein the compressive stress zone extends to a depth of about 2 mm and has a maximum compressive stress between a depth of 0.5 mm and 1 mm. [6] A glass-ceramic product according to any one of the preceding claims, wherein the compressive stress zone has compressive stresses of up to 1 MPa. [7] Glass-ceramic product according to one of the preceding claims, wherein at least one decorative layer is located between the matt coating and the surface of the glass-ceramic substrate. [8] Glass-ceramic product according to claim 7, wherein the decorative layer has a thickness of less than 5 µm. [9] Glass ceramic product according to one of the preceding claims, wherein the glass ceramic product is designed as a cooking hob. [10] Glass-ceramic product according to one of the preceding claims, wherein the matt coating has a core height (Sk) of less than 10 µm.

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