Method for determining the layer thickness of an at least partially transparent surface coating

Abstract

The invention relates to a method for determining the layer thickness of an at least partially transparent surface coating on the surface of a substrate, using a measuring device for emitting focused radiation. The invention is characterized in that the distance between the surface of the substrate and the measuring device for emitting focused radiation is changed by a relative movement between the measuring device and the substrate, wherein the image of the radiation is captured in different relative positions, wherein in the case of an image with a clear outline edge, focusing is inferred at the interface between the surface coating and the substrate and the current relative position is stored as the first position, and wherein in the case of an image surrounded by scattering effects, focusing is inferred at the interface between the surface coating and the air and the current relative position is stored as the second position, after which the difference between the first and second positions is used to determine theTaking into account the refractive index of the transparent surface coating, the layer thickness of the at least partially transparent surface coating is determined.

Description

[0001] The invention relates to a method for determining the layer thickness of an at least partially transparent surface coating on the surface of a substrate, using a measuring device for emitting focused radiation.

[0002] German patent DE 196 32 763 A1 describes a measuring head for observing the development of photoresist on silicon wafers, which are used to structure integrated circuits by coating them with a thin layer of photoresist. A structural pattern is then exposed and developed onto the wafer. During development, a critical decrease in the layer thickness can occur, which can be observed and measured with the measuring head. This head comprises a converging lens and an annular detector in its focal plane, with its optical axis perpendicular to the photoresist layer. Monochromatic light is focused into the inner opening of the annular detector, projected by the converging lens as a collimated beam onto the photoresist layer, and reflected back onto itself. The reflected light falls directly back as long as the surface is unstructured.Structures that form in the resist layer during development, however, diffract the light, so that some of the light reaches the ring detector. The signal on the ring detector thus records the temporal progression of the photoresist development.

[0003] Unfortunately, the measuring head is unsuitable for measuring the thickness of a fully cured surface coating, such as a partially transparent paint layer or a clear coat.

[0004] The patent application from the USA: US 2018 / 0024048 A1 discloses a method for determining the thickness of a transparent surface coating on the surface of a substrate using a measuring device for emitting focused radiation, wherein the respective position of two interfaces is inferred from a backscattering of the radiation and a thickness of the surface coating is determined from this.

[0005] The following information on this topic was published in the journal "Applied Optics" (Dong, Y. et al.: Nondestructive analysis of automotive paints with spectral domain optical coherence tomography. In: Applied Optics, Vol. 55, No. 13 (2016), pp. 3695-3700): The use of optical coherence tomography (OCT) as an analytical tool for the non-destructive measurement and characterization of the individual paint layer thicknesses of multilayer automotive paints was demonstrated. A graph-based segmentation method was used for the automated analysis of the thickness measurements of the upper monochromatic layers. The thickness values ​​measured with OCT showed good agreement with optical microscopy and ultrasound techniques, which are the current standard in the automotive industry. Due to the high axial resolution (5.5 µm), it was shown that OCT can resolve the thickness of individual paint layers down to 11 µm.With its high lateral resolution (12.4 µm), the OCT method was also able to measure the cross-sectional area of ​​the aluminum flakes found in metallic automotive paint. The measured values ​​ranged from 300 µm to 1850 µm. In summary, the proposed OCT method is a non-contact, high-resolution measurement technique with the potential to become a standard for quality assurance in vehicle coatings.

[0006] PCT application WO 2015 / 044168 A1 discloses a method for producing a glass or glass-ceramic article with locally modified transmission, in which electromagnetic radiation is directed by a laser onto a localized area of ​​the glass or glass-ceramic article's surface. The laser's power density is selected such that it is below the ablation threshold, thereby reducing the transmission in the irradiated area. The invention further relates to a product comprising a glass or glass-ceramic element with a first area and an adjacent second area. The second area occupies a smaller proportion of the surface area of ​​the glass or glass-ceramic element than the first area and is more intensely colored.The second area has at least one first color-giving component, such that the integral light transmission in the visible spectral range is reduced in the second area compared to the first area, wherein the first color-giving element reduces the integral light transmission in the visible spectral range of the second area and the first and second areas have a common microstructure.

[0007] For example, the simultaneously filed German application number 10 2024 138 957.5, submitted by the same applicant and entitled "Method for producing a structure in the volume of an at least partially transparent material using a laser beam," describes a method for producing an optical structure in the volume of an at least partially transparent material, such as a surface coating, using a laser beam. If such structures are to be precisely positioned in all three dimensions within a surface coating, e.g., the clear coat of a vehicle, then measuring the thickness and / or topography of the surface coating would be necessary.

[0008] The object of the present invention is therefore to create an improved method for determining the layer thickness of an at least partially transparent surface coating, which allows for simple, reliable and process-safe measurement, which can in particular be integrated into a manufacturing process.

[0009] According to the invention, this problem is solved by a method with the features in claim 1, and in particular in the characterizing part of claim 1. Advantageous embodiments of the method according to the invention are set forth in the dependent claims.

[0010] The method according to the invention is thus used to determine the layer thickness of an at least partially transparent surface coating on the surface of a substrate. It utilizes a measuring device for emitting focused radiation. Unlike the prior art mentioned above, the distance between the surface of the substrate and the measuring device for emitting focused radiation is changed by a relative movement between the measuring device and the substrate. The image of the radiation is then captured in various relative positions. In the case of an image with a clear outline, e.g., an elliptical or circular image, focusing at the interface between the surface coating and the substrate is inferred, and the current relative position is stored as the first position.If, however, an image is captured that is surrounded by scattering effects, then focusing on the interface between the surface coating and the air is inferred, and the current relative position is stored as a second position. The layer thickness can then be determined from the difference between the first and second positions. Since the propagation speed of light varies in different media, the path difference by which the focusing optics have moved relative to the substrate surface does not directly correspond to the thickness of the surface coating, but must be multiplied by the refractive index of the intervening material, in this case, the refractive index of the surface coating. Therefore, if the refractive index of the surface coating is known, the desired thickness can be easily calculated from the path difference.Preferably, a well-known image processing software is used to quickly and efficiently capture and differentiate the images in the first and second positions.

[0011] A particularly advantageous embodiment of the invention further provides that the topography of the surface coating is inferred from the changing distance between the first and / or second position and the measuring device along a travel path transverse to the surface coating. In addition to the pure layer thickness, the course of the layer or its surface in space can thus also be recorded.

[0012] In a highly advantageous further development of the method, laser radiation is used as the focused beam. Using a laser, it is very simple and efficient to generate the focused beam with sufficient brightness for reliable image acquisition.

[0013] The method according to the invention can be used for any type of layer thickness measurement of surface coatings. It can be used on both very small and very large objects, such as vehicles, to determine the thickness of a clear coat layer. The measuring device can be guided, for example, by a multi-axis robot to or over the area where the layer thickness is to be measured. This can be done either while the measuring device is moving, i.e., "on-the-fly," or after the measuring device has been moved into a measuring position and remains there during the measurement.

[0014] According to a very favorable further development of the invention, the layer thickness and / or the topography of the surface coating in a predetermined area of ​​the surface can be recorded and used directly for further processing processes and / or stored for later processing processes.

[0015] In addition to simply measuring the layer thickness, e.g. for quality testing or assurance, it is now particularly interesting if, according to an advantageous further development of the method according to the invention, the measured layer thickness and / or topography of the surface coating in the specified area is used to introduce a specified structuring into the inner volume of the surface coating, given the now known position of the surface coating in space.

[0016] According to a preferred further development, the specified structuring can be realized as an optical structuring comprising modification bubbles formed in a uniform, periodic, or random arrangement in at least one plane within the volume. Such structures or structurings can modify the optical properties, such as the colors, reflections, and / or transparency of the surface coating.

[0017] Such structures are known from two unpublished older German patent applications with file numbers 10 2024 138 585.5 and 10 2024 138 590.1. These applications describe the process of providing materials, particularly surface coatings or varnishes, with optical structures that enable graphic representation and color changes. This is achieved by forming modification bubbles within the material in a uniform, periodic, or random arrangement in at least one plane within the volume. This allows, for example, the creation of iridescent colors in the case of periodic structures spaced on the order of the wavelength of visible light. The structures function in the manner of an optical grating.

[0018] Furthermore, it can be provided that the modification bubbles are introduced into the surface coating via a focused laser beam from a structuring device moving relative to the surface. They are therefore preferably introduced into the inner volume of the surface coating by means of a focused laser beam, in particular a focused ultrashort pulse laser.

[0019] It is particularly advantageous if the structuring device is also configured to emit focused radiation to determine the layer thickness and / or topography of the surface coating and to infer its thickness and / or topography. The structuring device and the measuring device can therefore be combined. This reduces the complexity and enables excellent correlation between the measured parameters and their use in processing, since the two devices are in a fixed, unchanging position relative to each other and are moved together.

[0020] The method according to the invention can preferably be used for a surface coating which is formed as a paint layer, in particular on a clear coat, on a vehicle or on a component for a vehicle.

[0021] Further advantageous embodiments will also become clear from the following exemplary embodiment, which is described with reference to the figures.

[0022] This shows: Fig. 1 a schematically represented system for carrying out the method for determining the layer thickness of an at least partially transparent surface coating and for structuring the internal volume of the surface coating; Fig. 2. An image in a first position of the measuring device to the surface coating and a schematic view of its formation; Fig. 3. An image in a second position of the measuring device relating to the surface coating and a schematic view of its formation; Fig. 4 a cross-section through a substrate with a transparent lacquer layer and a structure inside the lacquer layer; Fig. 5 a top view of the representation according to Fig. 4 in a first embodiment a) and a second embodiment b); Fig. 6. A cross-section through a substrate with a transparent lacquer layer and an alternative design of the structure inside the lacquer layer; and Fig. 7 a photographic illustration of a surface coating according to the invention on a section of a rim.

[0023] In the presentation of the Fig. Figure 1 is a system 10 for determining the layer thickness of an at least partially transparent surface coating 1 on the surface of a vehicle 11, using a measuring device 16 for emitting a focused measuring beam M. The system 10 can also be used to produce an optical structure 3 in the inner volume of the surface coating 1 of the vehicle 11, here its clear coat, as will be described in more detail later.

[0024] To determine the layer thickness and topography of the clearcoat 1, a focused visible light in the form of a laser beam is preferably used as the measuring beam M. In principle, radiation in a different wavelength range would also be conceivable, provided that a suitable device is used to capture the required image. The process engineering solution for the automated determination of the layer thickness and topography of the clearcoat 1 is implemented via an optical evaluation of the focal points / laser spots. For this purpose, the distance between the measuring device 16 and the surface coating 1 is changed. This results in different images of the clearcoat 1, which are displayed in the Fig. 2 and Fig. 3 are shown on the left.

[0025] In the presentation of the Fig. The focus F of the measuring beam M is located on the underside of the clear coat 1, i.e., at an interface 17 between the clear coat 1 and a substrate 2 supporting it, in this case, for example, the body panel or a base coating thereof. The result in the image on the left, which can be captured by a camera 18, is a single focal point F. This can have any shape, e.g., an ellipse. However, it always has a clear outline edge that is not surrounded by scattering effects. The measuring device 16 is located at a distance e from a visible surface 4 of the clear coat 1.

[0026] The measuring device 16 is now moved until the measuring beam M is focused on the visible surface 4 of the clear coat 1. This is the interface between the clear coat 1 and the surrounding air. Without changing the focusing distance of the measuring beam M itself, the following is then created: Fig. 3. Image shown on the left. The focal point F, now lying on the view-side surface 4, is still visible. However, the central focal point F is now surrounded by scattering ST, which is caused by the Fig. The reflection at the interface 17, indicated on the right, is due to the change in the distance from e to e' between the measuring device 16 and the visible surface 4. The thickness d of the clear coat 1 can then be determined by measuring the distance from e to e'.

[0027] As explained above, the speed of light propagation varies in different media. Therefore, the path difference (e'-e) does not directly correspond to the layer thickness d of clear coat 1. Rather, d = (e'-e)*n, where n is the refractive index of the clear coat. However, if the refractive index n of clear coat 1 is known, which is typically the case, the layer thickness d of clear coat 1 can be easily calculated from the path difference (e'-e).

[0028] The example of the one in Fig. One camera (indicated) captures 18 images, which can then be analyzed to determine the layer thickness and, by considering the position of the measuring device in space, also the topography of the clear coat layer. This can be achieved with automated focus position determination through the integration of image recognition software. The two relevant focus positions of the Fig. 2 and Fig. 3 for determining the layer thickness d of the clear coat 1 are the visible surface 4 and the interface 17 between the substrate 2 and the clear coat 1.

[0029] Structures 3 (see below) can now be placed in the inner volume of the clear coat 1. Fig. 4) are introduced as volume modifications. The measurement can take place either before or during the manufacturing process for the structures 3. If it takes place before the manufacturing process, it can be performed particularly easily immediately before the production of the structures 3 at the respective location, or at every nth location, depending on the tolerance of the clearcoat layer 1 in space. However, the entire clearcoat layer 1 can also be measured in space in a first step in order to then carry out the manufacturing process for the structures 3. To verify the material present in the (partially) transparent clearcoat layer 1, an inline verification analysis of the clearcoat is performed.

[0030] The fabrication of the structures 3 can be carried out using a laser beam S, preferably using a focused ultrashort pulse laser.

[0031] A structuring device 13 serves to emit at least one focused laser beam S. The structuring device 13 can include a laser scanner to move the laser beam S precisely within the clear coat 1 and a lens to focus it. This allows the predefined structure 3 to be formed from modification bubbles 5, which will be described in more detail later. In laser processing and laser welding, the term "laser scanner" typically refers to a processing optic with at least one laser that can be directed to different points and / or along different paths via movable mirrors.

[0032] The structuring device 13 is moved relative to the vehicle 11 by an industrial robot 14 with up to 6 axes. Additionally, the vehicle 11 can be moved on an assembly frame or carriage 15, for example, in a flow production line. Typically, however, it will remain stationary during the production of the structure 3, and only the industrial robot 14 moves the structuring device 13 to the various required positions relative to the vehicle 11. The desired structures can then be incorporated into the clear coat 1, preferably using an ultrashort pulse laser. In the illustration of the Fig. 1. Through the structures 3, for example, a logo 12 is realized, which is indicated here as an example circle.

[0033] The measuring beam M of the measuring device 16 and the laser beam S used to produce the structures 3 can preferably originate from the same laser source. For this purpose, another beam / light source, e.g., from the visible spectrum, can be coaxially coupled into the beam-guiding optics of the laser beam S used for processing (or, if necessary, provided by a separate focusing optic) to obtain the measuring beam M. Ideally, a measuring device 16 is used instead that can detect the wavelength generated by the laser beam S itself. Then the laser beam S can also be used for the measurement. Inaccuracies due to a misalignment of the laser beam S on the one hand and the radiation source for the measuring beam on the other can thus be avoided.

[0034] In the presentation of the Fig. Figure 4 shows a section of logo 12 in a schematic and highly magnified cross-section. The clear coat 1 is applied to a substrate designated 2, in this case, for example, the body panel or a base coating thereof. Within the thickness of the clear coat 1, the structures 3 are arranged in three planes, one behind the other, starting from a visible surface 4 of the clear coat 1. Each of these planes can contain a dot matrix.

[0035] In the two in Fig. In the top views shown in Figures 5 a) and b), these dot matrices can be seen in two possible embodiments. Fig. 5 a) can be seen that here individual lines 6, each consisting of a series of - in Fig. 6 and Fig. 7. Modification bubbles 5, as described in more detail below, are formed, resulting in a regular rectangular grating. This acts as an optical grating and can very precisely and homogeneously alter the optical properties across its extent, particularly the transmission and / or reflection within the clear coat 1.

[0036] The lines 6 are formed by a series of modification bubbles 5, which is indicated here by the dotted representation of the lines 6. With a spacing x of the lines 6 of a few tens of micrometers, e.g., approximately 30 micrometers, but in any case above the order of magnitude of visible light, black, white, and shades of gray can be produced. This allows for a high degree of design flexibility. The somewhat more complex optical gratings enable the achievement of very homogeneous colors or color gradients. If this is not absolutely necessary, alternatives can be used. Fig. 3 b) equally good results can be achieved. Instead of the periodically repeating lines 6, individual modification bubbles 5 are used, which are distributed in a random pattern. The color nuances then result solely from an average area density of the modification bubbles 5. This is less computationally intensive and less demanding in terms of planning. However, the resulting gray tone may appear somewhat less homogeneous than with the implementation according to Fig. 5 a).

[0037] It is particularly interesting when, in an optical grating, as in Fig. 5a) shows that the distance between the modification bubbles 5 of each individual line 6 is on the order of 400 to 780 nm. The distance x between the individual lines 6 is, for example, more than 30 µm. This allows, as detailed in the earlier German application with file number 10 2024 138 585.5, an iridescent color effect, i.e., a shimmering in rainbow colors, to be achieved.

[0038] With both variants, structures 3 can be subsequently introduced into an already applied layer of clear coat 1 via the structuring device 13, e.g. in the form of the brand logo 12.

[0039] If different layers are provided with different grids, different distances between the modification bubbles 5 and / or distances x between the lines 6 – and thus colors in the sense mentioned above – can be used, further increasing the design possibilities. In addition, the individual layers can now also be changed in their angle to each other and to the visible surface 4. This is shown in the representation of the Fig. 6 to recognize, which are otherwise analogous to Fig. 4 is to be understood.

[0040] The modification bubbles 5 themselves are droplet-shaped. Their diameter ranges from 0.1 to 10 µm, typically from approximately 2 to 8 µm. These modification bubbles alter the refractive index of the clear lacquer 1 used. According to current understanding, this is likely due to a reorganization / rapid solidification of the molten clear lacquer 1, as well as a local change in chemical composition caused by irradiation.

[0041] In conclusion, in Fig.Figure 7 shows a photograph of a section of a rim 7 for the vehicle 11. The rim 7 is sealed with clear coat 1. Structures 3 are incorporated into this clear coat 1 in at least one plane, particularly parallel to the visible surface 4 of the clear coat 1. These structures represent a logo designated 12, in the image the brand logo "Maybach" (corresponding to a protected trademark of one of the co-applicants). The entire optical effect that makes the brand logo 12 visible as a structure 3 within the clear coat 1 is based on modification bubbles 5, which are formed in a lattice structure.

Claims

[1] Method for determining the layer thickness (d) of an at least partially transparent surface coating (1) on the surface (17) of a substrate (2), using a measuring device (16) for emitting focused radiation (M), characterized by, that the distance between the surface (17) of the substrate (2) and the measuring device (16) for emitting focused radiation (M) is changed by a relative movement between the measuring device (16) and the substrate (2), wherein the image of the radiation is recorded in different relative positions, wherein in the case of an image with a clear outline edge, a focusing on the interface (17) between the surface coating (1) and the substrate (2) is inferred and the current relative position is stored as the first position (e), and wherein in the case of an image which is surrounded by scattering effects (ST), a focusing on the interface (4) between the surface coating (1) and air is inferred and the current relative position is stored as the second position (e'), after which the difference (e'-e) between the first and second position is used to determineTaking into account the refractive index (n) of the transparent surface coating (1), the layer thickness (d) of the at least partially transparent surface coating (1) is determined. [2] Method according to claim 1, characterized by , that the changing distance between the first and / or the second position (e, e') and the measuring device (16) along a travel path perpendicular to the surface coating (1) allows conclusions to be drawn about the topography of the surface coating (1). [3] Method according to claim 1 or 2, characterized by , that laser radiation is used as the focused radiation (M). [4] Method according to claim 1, 2 or 3, characterized by , that the layer thickness (d) of the surface coating (1) is recorded in a specified area of ​​the surface and is used directly for further processing processes and / or stored for later processing processes. [5] Method according to claim 2 or 3, characterized by , that the topography of the surface coating (1) is captured in a specified area of ​​the surface and is used directly for further processing processes and / or stored for later processing processes. [6] Method according to claim 4 or 5, characterized by , that the detected layer thickness (d) and / or topography of the surface coating (1) is used to selectively introduce a predetermined structure (3) into the internal volume of the surface coating (1). [7] Method according to claim 6, characterized by , that the specified structure (3) is realized as an optical structure (3) which includes modification bubbles (5) which are formed in a uniform periodic or random arrangement in at least one plane in the volume. [8] Method according to claim 7, characterized by, that the modification bubbles (5) are introduced via a focused laser beam (S) of a structuring device (13) moving relative to the surface of the surface coating (1). [9] Method according to claim 8, characterized by , that the structuring device (13) is further equipped to emit the focused radiation (M) to determine the layer thickness (d) and / or topography of the surface coating (1). [10] Method according to any one of claims 1 to 9 characterized by , that the measuring device (16) and / or the structuring device (13) are moved relative to the surface coating (1) by means of a multi-axis robot (14). [11] Method according to any one of claims 1 to 10 characterized by its use on a coating, in particular a clear coat (1), on or in a vehicle (11) or on a component (7) for a vehicle (11).

Images