Mirror support for an optical mirror made of a composite material and method for its production

Description

[0001] The invention relates to a mirror support made of a composite material, in particular a mirror support for an optical mirror such as a metal mirror. The invention further relates to a method for producing the mirror support.

[0002] In optical measurement and signaling technology, there is a strong demand for mirrors with complex surfaces (e.g., freeform optics, aspheres), which have to meet high demands in terms of shaping accuracy. At the same time, the mirrors should be characterized by low weight and high rigidity. A mirror typically comprises a mirror substrate with the desired surface, on which one or more reflective layers are arranged. The at least one reflective layer is preferably applied by physical vapor deposition, for example by sputtering, vapor deposition, ion-assisted coating, or atomic layer deposition. Mirror substrates made of, for example, aluminum, copper, or brass are machined using a diamond tool, for example, until the desired surface shape is achieved. Known ultra-precision machining processes, such asDiamond turning or milling thus offers the possibility of producing metal optics with, for example, spherical, aspherical, or free-form surfaces with form deviations (PV) < 200 nm for a component diameter of 100 mm and roughness (RMS) in the range of 2 nm to 5 nm. By using machining, the achievable roughness and surface form deviation are limited to values ​​that may be sufficient for applications in the spectral infrared range, but are insufficient for shorter wavelengths.

[0003] It is known that roughness and surface shape deviations can be reduced by fine or corrective polishing. For this purpose, mirror supports are used with a mirror body, e.g., made of aluminum, with a polishing layer, e.g., made of an amorphous nickel-phosphorus alloy, which is then subjected to polishing.

[0004] The materials of the mirror body and the polishing layer are selected based on various thermal, mechanical, or manufacturing requirements. While the mirror body, especially for movable optical mirrors, should be lightweight yet dimensionally stable, the polishing layer must be polishable and inert to the ambient conditions (oxygen, moisture). For this reason, the mirror body and the polishing layer are usually made of different materials.

[0005] A disadvantage of combining different materials is that significant differences in their thermal expansion coefficients can arise. For example, the thermal expansion coefficient of the aluminum alloy Al6061, which is widely used as a substrate body (see, for example, WO 2004 / 077114 A1, US Pat. No. 6,350,176 B1), is 23.8 * 10 -6 / K, while a NiP polishing layer typically has a thermal expansion coefficient of 13.5 * 10 -6 / K.

[0006] When optical mirrors with such a composite mirror carrier are exposed to temperature changes during use, the different thermal expansion coefficients, as with a bimetallic element, lead to deformations or stresses, which can significantly impair the optical function and even lead to functional failure.

[0007] Due to the mechanical demands placed on the mirror bodies, only a few materials are currently available for the mirror supports. These include, in addition to the above-mentioned aluminum alloy Al6061, other aluminum alloys as well as beryllium and beryllium alloys. The use of beryllium can be problematic because beryllium oxide is toxic and the raw material is very expensive. In practice, in order to avoid the above-mentioned mismatch in the thermal expansion coefficients, attempts have been made to manufacture the polishing layer from a material whose thermal expansion coefficient is matched to the thermal expansion of the mirror body. The disadvantage of this concept is that experience gained from polishing the materials commonly used for polishing layers, such as NiP, cannot simply be transferred to the polishing of other materials.Furthermore, the polishing layer must be compatible with the materials of the reflective layers deposited on it.

[0008] DE 10 2005 026 418 B4 describes an aluminum composite material with silicon particles which, coated with an amorphous NiP polishing layer, is used for optical elements.

[0009] The document EP 2 065 734 A1 describes a mirror for laser processing in which a substrate made of a metal-carbon composite is provided with a reflective layer.

[0010] The publication WO 2015 / 074 677 A1 relates to a lightweight deflection mirror, for example for a galvanometer scanner, in numerically optimized lightweight construction.

[0011] The document WO 2016 / 170 043 A1 describes a method for shaping and / or correcting the shape of an optical element, in which a deviation of a shape of an optical functional surface from a target shape is determined.

[0012] One problem to be solved is to provide an improved mirror support that, in particular, exhibits improved dimensional and temperature stability, a sufficiently low mass density for moving mirror applications, and high mechanical stability. Furthermore, a suitable method for producing the mirror support is to be specified.

[0013] These objects are achieved by a mirror support and a method according to the independent patent claims. Advantageous embodiments of the invention emerge from the dependent claims.

[0014] According to at least one embodiment, the mirror support comprises a mirror body and a polishing layer. The mirror body comprises a diamond particle-reinforced aluminum composite material, wherein the aluminum composite material has a predetermined content of diamond particles. The diamond particle content is advantageously between 5% and 50% by mass, whereby the "diamond particle content" here and below refers to the mass fraction of diamond particles in the aluminum composite material. The diamond particles are diamond particles of natural or synthetic origin. In particular, the diamond particles have polycrystalline structures (PCD, Polycrystalline Diamond). This distinguishes the diamond particles, in particular, from diamond-like carbon. The diamond particles have sizes from 1 µm to 100 µm, preferably 20 µm to 50 µm. The shape of the diamond particles is polygonal, rotationally ellipsoidal, sharp-edged, or preferably spheroidal.

[0015] The diamond particles contained in the aluminum composite material achieve a high elastic modulus and thermal conductivity, which is advantageous both for lightweight applications and for achieving high dimensional accuracy. A polishing layer, such as PVD-SiC, PVD-Si, CVD-Si, PECVD-SiO 2 , PECVD-Si 3 N 4 , PVD-ZrO 2 , or preferably NiP, is applied to the mirror body.

[0016] The use of the diamond-particle-reinforced aluminum composite material as the material for the mirror body has the particular advantage of achieving high thermal conductivity and high rigidity. On the other hand, diamond-particle-reinforced aluminum composite materials have mechanical properties that do not readily permit the production of a mirror body. In particular, the comparatively high hardness and brittle properties of this material do not readily permit mechanical processing by material-removing methods such as milling or drilling. The invention makes particular use of the idea of ​​producing the mirror body from the diamond-particle-reinforced aluminum composite material by additive manufacturing.This manufacturing process advantageously enables the production of a monolithic mirror body from the diamond-particle-reinforced aluminum composite, something that would not be readily possible using conventional methods. Additive manufacturing thus makes it possible to adapt this material, which is not readily suitable for mirror bodies due to its properties that make manufacturing significantly more difficult, for this application, thus benefiting from the material's advantages, such as its hardness and thermal properties.

[0017] The diamond particle content in the diamond-reinforced aluminum composite material is selected to match the coefficient of thermal expansion of the mirror body to the coefficient of thermal expansion of the polishing layer. The term "matching" refers here to such an approximation of the coefficients of thermal expansion that the difference between the coefficients of thermal expansion of the mirror body and the polishing layer is less than 2.5 * 10 -6 / K, preferably less than 1 * 10 -6 / K. Particularly preferred embodiments of the invention are those in which the coefficients of thermal expansion of the mirror body and the polishing layer are substantially equal, so that they have a difference of, in particular, less than 0.5 * 10 -6 / K.

[0018] The adaptation of the thermal expansion coefficient of the mirror body to the thermal expansion coefficient of the polishing layer has the advantage that the mirror carrier withstands high thermal loads, especially thermal gradient loads.

[0019] According to a preferred embodiment of the invention, the mirror body consists of a diamond particle-reinforced aluminum composite material with a diamond particle content such that the thermal linear expansion coefficient in the temperature range from -180 °C to +100 °C, preferably in the range from -40 °C to +60 °C, is in the range from 10 * 10 -6< / K to 16 * 10 -6< / K. With such a mirror body, a particularly good adaptation to typically used materials of polishing layers is achieved.

[0020] The content of diamond particles in the aluminum composite material is preferably between 5% and 50% by mass, particularly preferably between 10% and 20% by mass. A diamond particle content in this range enables the thermal expansion coefficient to be adjusted to that of the polishing layer. Furthermore, the diamond particle content can improve thermal conductivity, heat capacity, and mechanical rigidity.

[0021] Varying the diamond particle content of the aluminum composite material allows adaptation to various possible materials for the polishing layer. A particularly preferred embodiment is one in which the polishing layer is formed from a nickel-phosphorus alloy, which advantageously has a coefficient of thermal expansion of 11 * 10 -6 / K to 14 * 10 -6 / K. The use of a NiP polishing layer has the particular advantage that extensive experience exists in polishing this material. The mirror substrate can thus be subjected to the same polishing procedures as those known for conventional mirror substrates.

[0022] In a preferred embodiment, the polishing layer has an RMS roughness of less than 5 nm. This achieves, in particular, high reflectivity. If the mirror support has a polishing layer with an RMS roughness of less than or equal to 1 nm, further advantages arise for the application of a mirror equipped with the mirror support for wavelengths in the visible spectral range or shorter wavelengths.

[0023] The mirror support serves in particular for the production of a mirror, in particular an optical mirror. The mirror comprises the mirror support and a reflective layer applied thereto, for example, a metal layer. The reflective layer can comprise several sublayers, for example, metal layers and / or dielectric layers.

[0024] In the method according to the invention for producing the mirror support, a mirror body comprising an aluminum composite material reinforced with diamond particles is produced by additive manufacturing and a polishing layer is subsequently applied to the mirror body, wherein the content of diamond particles in the aluminum composite material is between 5 mass% and 50 mass%, in particular between 10 mass% and 30 mass%, and is selected such that the thermal expansion coefficient of the mirror body is adapted to the thermal expansion coefficient of the polishing layer.

[0025] As already mentioned, additive manufacturing enables the production of a monolithic mirror body made of aluminum composite with a diamond particle content in the range of 5% to 50% by mass, although this material is difficult to machine due to its hardness and brittle properties. Advantageously, additive manufacturing avoids the use of material-removing processes; instead, the mirror body is created layer by layer, for example, using selective laser melting, based on a CAD model.

[0026] In a preferred embodiment, additive manufacturing is carried out by selective laser melting. In selective laser melting, the diamond-particle-reinforced aluminum composite material is applied layer by layer in powder form using a doctor blade and then completely melted locally into a solid material using laser radiation. Subsequently, another layer of powder is applied and remelted using laser radiation. This cycle is repeated until the entire mirror body is manufactured from a multitude of locally remelted powder layers.

[0027] When manufacturing the mirror body using additive manufacturing, particularly selective laser melting, lightweight structures, cooling structures, and / or support structures can be advantageously integrated into the mirror body without requiring an additional manufacturing step. Integrated cooling structures can be provided for liquid or gaseous media.

[0028] According to a preferred embodiment, the mirror support, i.e. the composite of the mirror body and the polishing layer, is subjected to a thermal treatment. In this way, dimensional stabilization of the material pairing of the mirror body and the polishing layer can advantageously be achieved. The thermal treatment is preferably carried out in a temperature range of 150 °C to 200 °C. During the thermal treatment in this temperature range, any residual stresses that may still be present can be largely eliminated, so that the mirror produced with the mirror support has a high level of stability and reproducible optical properties. The thermal treatment is preferably followed by polishing of the polishing layer in order to produce the desired shape of the mirror support with minimized roughness.

[0029] The advantageous embodiments described for the mirror carrier also apply to the method and vice versa.

[0030] Further details and advantages of the invention will become apparent from the following description of the Figures 1 and 2 visible.

[0031] They show: Figure 1A a schematic sectional view of a mirror with the mirror carrier according to a first embodiment, Figure 1B a section of the mirror with the mirror carrier according to the first embodiment, and Figure 2 a schematic view of a mirror with the mirror carrier according to a further embodiment.

[0032] The optical mirror 4 according to the embodiment of the Figures 1A and 1B has a mirror support 1 and a reflection layer 5. The mirror support 1 comprises a mirror body 2 and a polishing layer 3 comprising a NiP alloy. The optical mirror 4 of the Figure 2 has an internal cooling structure 6 and a supporting structure 7.

[0033] In the exemplary embodiment, the mirror 4 is an optical mirror with a spherical surface. The implementation of the invention is not limited to this mirror shape, but is also possible with flat or aspherically curved mirrors or free-form mirrors. The additive manufacturing of the mirror support proposed herein is particularly advantageous for producing the mirror support with a spherically or aspherically curved shape or a free form, since this is very difficult with conventional manufacturing methods (e.g. casting) due to the mechanical properties of the diamond particle-reinforced aluminum composite material. In addition, additive manufacturing makes it possible to reduce the mass of the mirror support by, for example, creating recesses or cavities with very small wall thicknesses, e.g. 0.1 mm to 1 mm, in the mirror support, which cannot be easily realized with conventional manufacturing methods.

[0034] The mirror body 2 consists of a diamond-particle-reinforced aluminum composite material with a diamond particle content, which results in a thermal expansion coefficient adapted to the NiP polishing layer. The polishing layer 3 consists of a chemically or galvanically produced amorphous nickel-phosphorus alloy with a phosphorus concentration of 10 to 15 mass% (preferably > 10.5 mass%, e.g., 12 mass%) and has a thickness of approximately 10 µm to 2000 µm.

[0035] Mirror 4, for example, is manufactured as follows. First, the mirror body 2 is produced from a diamond-particle-reinforced aluminum composite material using additive manufacturing (preferably selective laser melting). A homogeneous powder mixture consisting of Al6061 and diamond powder is used. The specific selection of the diamond particle content is based on the available material data for the composite material used.

[0036] When using a polishing layer, e.g. PVD-SiC, PVD-Si, CVD-Si, PECVD-SiO 2 , PECVD-Si 3 N 4 , PVD-ZrO 2 or preferably chemically produced NiP, with a layer thickness of < 200 µm, one surface of the blank is machined. The blank can be machined with ultra-precision using conventional hard metal tools or diamond tools (e.g. PCD tools made of polycrystalline diamond). Since the composite materials used according to the invention have a high proportion of brittle-hard inclusions compared to conventional mirror bodies and are therefore relatively brittle, shallow cutting depths and low feed rates are preferred during machining.

[0037] By using a polishing layer, e.g. galvanically produced NiP, with a layer thickness of > 200 µm, machining of the blank can be avoided.

[0038] Subsequently, a thermal treatment can be performed to reduce induced stresses. The thermal treatment is preferably carried out for a duration of 6 hours at 350 °C.

[0039] In a further step, the deposition of the polishing layer 3 takes place. The deposition of the polishing layer 3 can be carried out in particular by a galvanic or electrochemical process. The deposition preferably comprises electroless nickel plating. For this purpose, the surface of the mirror body 2 is first cleaned, activated and then subjected to deposition. After the polishing layer 3 has been applied, a further thermal treatment follows, for example for 6 hours at 150 °C, in order to reduce layer stresses in the material composite of the mirror carrier 1. A final polishing can then take place, whereby an RMS surface roughness of less than 5 nm, preferably less than 1 nm, is produced.

[0040] To produce a mirror 4 with the mirror carrier 1, a reflection layer 5 is deposited on the polishing layer 3 in a further step, preferably by physical vapor deposition. The reflection layer 5 can be a single layer or comprise multiple sublayers. The reflection layer 5 can be, for example, a metal layer. Alternatively, the reflection layer 5 can be a multilayer system, for example, a dielectric interference layer system or a combination of one or more metal layers with one or more dielectric layers.

[0041] In Figure 2A further embodiment of an optical mirror 4 is shown. The optical mirror has an internal cooling structure 6 and a support structure 7. Further configurations of the mirror 4 and the mirror support contained therein, as well as the method for its production, can correspond to the first embodiment and are therefore not explained again.

[0042] The features of the invention disclosed in the above description, the drawings and the claims may be important both individually and in combination for the realization of the invention in various embodiments.

Claims

1. A mirror support (1), comprising: - a mirror body (2) comprising a diamond particle reinforced aluminum composite material, and - a polishing layer (3), which is arranged on the mirror body (2) wherein the content of diamond particles in the aluminum composite material is between 5 % by mass and 50 % by mass and is selected such that the thermal coefficient of linear expansion of the mirror body (2) is adapted to the thermal coefficient of linear expansion of the polishing layer (3), characterized in that the entire mirror body (2) is produced by additive manufacturing from a plurality of locally remelted powder layers and a cooling structure (6) is integrated into the mirror body (2).

2. The mirror support according claim 1, wherein the content of diamond particles in the aluminum composite material is selected such that the coefficient of thermal expansion of the mirror body (2) at temperatures of -180 °C to 100 °C is in the range of 3 * 10-6 / K to 20 * 10-6 / K.

3. The mirror support according to any of the preceding claims, wherein the content of diamond particles in the aluminum composite material is between 10 % by mass and 20 % by mass.

4. The mirror support according to any of the preceding claims, in which a supporting structure (7) is integrated into the mirror body (2).

5. The mirror support according to any of the preceding claims, in which a lightweight structure is integrated into the mirror body (2).

6. The mirror support according to any of the preceding claims, in which the polishing layer (3) comprises NiP, SiC, Si, SiO2, Si3N4, or ZrO2.

7. The mirror support according to any of the preceding claims, in which the polishing layer (3) comprises a surface with an RMS roughness of at most 5 nm.

8. A mirror (4) comprising a mirror support (1) according to any of the preceding claims, wherein at least one reflection layer (5) is arranged on the polishing layer (3).

9. A method for producing a mirror support (1), comprising the steps: - producing a mirror body (2) comprising a diamond particle reinforced aluminum composite material by additive manufacturing, and - applying a polishing layer (3) on the mirror body (2), wherein the content of diamond particles in the aluminum composite material is between 5 % by mass and 50 % by mass and is selected such that the thermal coefficient of linear expansion of the mirror body (2) is adapted to the thermal coefficient of linear expansion of the polishing layer (3) and wherein the entire mirror body (2) is produced by additive manufacturing from a plurality of locally remelted powder layers and a cooling structure (6) is integrated into the mirror body (2).

10. The method according to claim 9, in which additive manufacturing is achieved by selective laser melting.

11. The method according to one of claims 9 or 10, in which a supporting structure is integrated into the mirror body (2) during additive manufacturing.

12. The method according to any of claims 9 to 11, with the further step: - thermal treatment of the composite of the mirror body (2) and the polishing layer (3) at a temperature in the range of 130 °C to 200 °C.

13. The method according to any of claims 9 to 12, with the further step: - polishing the surface of the polishing layer (3) so that it comprises an RMS roughness value of less than 5 nm.