DE102019119692A1 - Process for the production of environmentally stable aluminum mirrors on plastic - Google Patents

Abstract

The present invention relates to a method for producing a mirror on a plastic substrate. The method comprises the steps: - Providing a plastic substrate with surface - Coating the plastic substrate with an adhesive layer - Coating with an aluminum alloy - Coating with a dielectric layer system comprising at least two layers with different refractive indices. The method is characterized in that the aluminum alloy has a Chromium content of at least 0.5 at% and a maximum of 5 at%.

Description

The present invention relates to a method for producing environmentally stable aluminum mirrors by combined sputtering of adhesive layers, Al: Cr, dielectric protective layers and, preferably, vapor deposition of organic protective layers. The process is particularly preferably carried out in a coating batch.

Aluminum mirrors are the most widely used metal mirrors. Due to the high reflection over a wide spectral range, AI mirrors can be designed for the visible (VIS) as well as the near infrared (NIR) range. AI mirrors are typically deposited using physical vapor deposition (PVD) processes (primarily vapor deposition or sputtering).

The environmental stability of pure Al layers is not particularly high, since aluminum tends to corrode due to impurities in the ambient air. Furthermore, pure Al layers have a low abrasion resistance. Therefore, AI mirrors are used to increase the environmental stability with dielectric [e.g. JP4453886B2 , EP0456488A1 ] and / or organic protective layers [ US20040198898A1 , US4085248A , US6939018B2 ] (“enhanced aluminum mirrors”).

The dielectric protective layers also serve to increase the reflection of the AI mirrors in a defined spectral range. The limits of the spectral range with increased reflection are determined by the sequence and thickness of the dielectric protective layers with low (L) and high (H) refractive index.

A possible field of application for AI mirrors is in the automotive sector in the mirror component for head-up displays (HUD), where the Al mirror layers are deposited directly on plastic substrates (e.g. Topas COC). The resistance of such AI mirrors is checked in the automotive sector by various accelerated aging tests: Table 1 shows such typical accelerated aging tests for aluminum mirrors.

In order to achieve a sufficiently high level of environmental resistance on plastic substrates, various measures are necessary. An important step is the application of an adhesive layer between the polymer substrate and the Al mirror layer [ GB2281411A , US20040120060A1 ].

• · Die Haftschicht verbessert die Adhesion des Schichtstapels aufgrund einer besseren chemischen Bindung des Haftlayermaterials zur Substratoberfläche im Vergleich zu AI.
• · Die Haftschicht dient als Diffusions-Barriere, die den Transport von Wasser zur Substratoberfläche einschränkt, was ein Aufschwellen der Kunststoffoberfläche verhindert oder vermindert.
• · Die Haftschicht schützt die Al Schicht vor korrosiven Medien, die am interface zur Kunststoffoberfläche eindringen können.
• · Mit der Haftschicht kann die intrinsische Schichtspannung des gesamten Schichtstapels beeinflusst werden.

The functionality of the adhesive layer is based on various factors:

• · The adhesive layer improves the adhesion of the layer stack due to a better chemical bond between the adhesive layer material and the substrate surface compared to Al.
• · The adhesive layer serves as a diffusion barrier which restricts the transport of water to the substrate surface, which prevents or reduces swelling of the plastic surface.
• · The adhesive layer protects the Al layer from corrosive media that can penetrate at the interface to the plastic surface.
• · The intrinsic layer tension of the entire layer stack can be influenced with the adhesive layer.

Without an adhesive layer, complete or local delamination (pinholes) of the mirror layer stack can occur during the environmental tests.

As already shown above, dielectric protective layers (H / L stacks) can be deposited on the side of the Al layer remote from the substrate to further increase the environmental resistance. The reflection increases with the design wavelength as a function of the number of dielectric protective layers (L / H). If the mirror is to be applied to polymer substrates, due to the different coefficient of thermal expansion between the polymer substrates and the dielectric layers, care must be taken that the number of dielectric protective layers on the polymer substrates is not selected too high, otherwise delamination due to excessive layer tension comes.

The challenge now is to deposit inexpensive aluminum mirrors on plastic substrates that pass the environmental tests common in the automotive sector (Table 1).

According to the invention, the object is achieved by a mirror according to claim 9. This can be produced by the method according to claim 1. The subclaims relate to preferred embodiments of the present invention.

• - Aufbringen einer Haftschicht
• - Aufbringen einer Al-Cr Legierung mittels Zerstäubung (Sputtern)
• - Aufbringen dielektrischer Schutzschichten mittels physikalischer Abscheidung aus der Gasphase
• - Aufdampfen einer hydrophoben organischen Schutzschicht

According to the invention, a method for producing such a surface comprises the steps:

• - Application of an adhesive layer
• - Application of an Al-Cr alloy by means of atomization (sputtering)
• - Application of dielectric protective layers by means of physical deposition from the gas phase
• - Vapor deposition of a hydrophobic organic protective layer

The sputtering, the physical deposition from the gas phase and the vapor deposition are preferably carried out from the same coating chamber. Particularly preferably, the adhesive layer is also applied by means of physical deposition from the gas phase, ideally also using the same chamber.

The invention will now be explained in detail on the basis of specific examples and with the aid of the figures.

• 1a zeigt die Reflexion in Abhängigkeit der Wellenlänge für Al-Spiegel mit unterschiedlichen Cr-Konzentrationen (Einfallswinkel = 45°, unpolarisiertes Licht).
• 1b zeigt die über drei Beschichtungs-Chargen gemittelte Reflexion in Abhängigkeit der Cr-Konzentration bei unterschiedlichen Wellenlängen (Einfallswinkel = 45°, unpolarisiertes Licht).
• 2 zeigt den schematischen Schichtaufbau einer ersten Ausführungsform der vorliegenden Erfindung (Design 2)
• 3 zeigt die Reflexion in Abhängigkeit der Wellenlänge von Design 2 für zwei unterschiedliche Substrate (Einfallswinkel = 45°, unpolarisiertes Licht).
• 4 zeigt den schematischen Schichtaufbau einer zweiten Ausführungsform der vorliegenden Erfindung (Design 3)
• 5 zeigt die Reflexion in Abhängigkeit der Wellenlänge von Design 3 (Einfallswinkel = 45°, unpolarisiertes Licht).

Table 1 shows typical accelerated aging tests for Al mirrors
Table 2 shows the result of aging tests for 3 different designs as defined below.

• 1a shows the reflection as a function of the wavelength for Al mirrors with different Cr concentrations (angle of incidence = 45 °, unpolarized light).
• 1b shows the reflection averaged over three coating batches as a function of the Cr concentration at different wavelengths (angle of incidence = 45 °, unpolarized light).
• 2 shows the schematic layer structure of a first embodiment of the present invention (design 2)
• 3 shows the reflection as a function of the wavelength of design 2 for two different substrates (angle of incidence = 45 °, unpolarized light).
• 4th shows the schematic layer structure of a second embodiment of the present invention (design 3)
• 5 shows the reflection as a function of the wavelength of design 3 (angle of incidence = 45 °, unpolarized light).

In a first step, an adhesive layer is applied to a plastic substrate (here Topas) by means of magnetron sputtering in a drum system. The basic concept of such a coating system is for example in US6258218 described. In the present example, SiO _{2 is} sputtered with a thickness of 50 nm as the adhesive layer. Thicknesses between 10-100 nm are reasonable. No plasma treatment of the Topas surface is necessary prior to the process, presumably due to the reactive plasma process at the beginning of the process there is a short, intrinsic plasma treatment of the surface.

Alloying with small amounts of Cr (according to the invention ≤5 at.%) Increases the environmental stability of the Al layers. For example, Al: Cr can be achieved either by co-sputtering an Al and Cr target or by sputtering an alloyed Al: Cr target. The reflection decreases per alloyed at.% Cr by slightly more than 1 abs.%, Like 1a and 1b clarify.

With 1 at.% Cr, a significant improvement in environmental stability is achieved (Table 2), while the reduction in reflection is still acceptable for many applications. The composition of the alloyed target is therefore preferably around Al0.99Cr0.01.

In the present example, an 85 nm thick Al _0.99 Cr _0.01 layer was applied

A dielectric deep-index SiO2 layer with a thickness of 90 nm was then applied to the AlCr layer, likewise by means of sputtering. A dielectric Nb2O5 layer with a thickness of 57 nm was applied to this. The resulting layer system is shown schematically in 2 shown. In 3 the measured reflection is shown as a function of the wavelength. The corresponding example is called Design 2 below.

According to a second variant, 5 dielectric protective layers were applied by means of sputtering. These were 2 Nb2O5 layers as high-index material (H), which were arranged between 3 SiO2 layers as deep-index material (L). The dielectric protective layers had the following thicknesses: 90nm SiO2 - 57nm Nb2O5 - 95nm SiO2 - 50nm Nb2O5 - 40nm SiO2.

In this variant, a hydrophobic protective layer was then applied to further improve the environmental resistance in a damp environment (fixed climate, salt fog, condensation water). In the present case, this was a fluoropolymer, namely perfluoropolyether with anchor group (thickness = 1 monolayer). The hydrophobic protective layer also improves the abrasion resistance in a significant way due to the low coefficient of friction. The corresponding layer system is shown schematically in 4th shown. In 5 the measured reflection is shown as a function of the wavelength. The corresponding example is called Design 3 below.

In the present example, the adhesive layer, the AlCr layer and also the dielectric protective layers were sputtered while the hydrophobic protective layer was vapor-deposited. For this purpose, the drum coating system was equipped with additional evaporation boats. The shuttles are covered with a plate with many small opening holes. The boats are mounted near the chamber wall at a distance of approx. 13 cm from the outside of the substrate and point with the opening holes in the direction of the substrates. For a homogeneous coating of the substrates several boats are necessary, which are mounted at a distance of approx. 20 cm from one another. The boats are filled with tablets containing the hydrophobic material and the hydrophobic material is evaporated at an electrical power of approx. 300-500W. Since all layers were applied in the same drum system, the chamber only had to be occupied once and a vacuum built up. This enabled the process time to be reduced considerably.

As already described, the sputtering of alloyed AlCr improves the environmental resistance significantly compared to established solutions in which only pure Al is used. The combination of sputtered, dense dielectric protective layers and vapor-deposited organic protective layers further improve the environmental test resistance. To ensure a cost-effective solution, the metallic, dielectric and organic layers are sputtered / evaporated in the same chamber. It should be pointed out that up to now the organic layer cannot be sputtered because the molecules would decompose if a fluoropolymer were sputtered. The application of the organic protective layer (refractive index n = 1.3) leads to a slight shift in the spectral range with maximum reflection. However, this can be taken into account in the design of the dielectric layer structure.

For comparison purposes, a layer structure with a pure aluminum mirror was also produced. This means that an SiO2 adhesive layer was applied to a Topas substrate, then an aluminum layer was sputtered and then dielectric protective layers were applied. The reference created in this way is called Design 1 in the following. Design 1 differs from Design 2 and Design 3 in the reflective metal layer in that the metal layer is an aluminum layer without the addition of chromium.

Environmental tests have now been carried out on the various designs of the Al Spiegel layer stack. The corresponding results are listed in Table 2. It is considered passed if the following factors are met: Reflection loss <1 abs.% After environmental test and visual inspection ok. It was marked as passed with a tick. It was marked as failed with a cross. It can be clearly seen that Design 1 fails in the majority of the tests and Design 2 passes in the majority of the tests. In contrast, Design 3 passed all tests.

• - Bereitstellen eines Kunststoffsubstrats mit Oberfläche
• - Beschichten des Kunststoffsubstrats mit einer Haftschicht
• - Beschichten mit einer Aluminium Legierung
• - Beschichten mit einem dielektrischen Schichtsystem das zumindest zwei Schichten mit unterschiedlichem Brechungsindex umfasst.

A method for producing a mirror on a plastic substrate has been disclosed, comprising the steps of:

• - Providing a plastic substrate with a surface
• - Coating the plastic substrate with an adhesive layer
• - Coating with an aluminum alloy
• Coating with a dielectric layer system which comprises at least two layers with different refractive indices.

The method is characterized in that the aluminum alloy has a chromium content of at least 0.5 at% and a maximum of 5 at%.

The method can be carried out in such a way that the aluminum alloy, apart from impurities, does not contain any metals except aluminum and chromium.

The method can be carried out in such a way that the adhesive layer and / or the aluminum alloy and / or the dielectric layer system is applied by means of sputtering.

The method can be carried out in such a way that it comprises a step in which at least one hydrophobic layer is applied, preferably onto the dielectric layer system.

The method can be carried out in such a way that the hydrophobic layer comprises an organic layer and preferably a fluoropolymer and particularly preferably a perfluoropolyether with an anchor group.

The method can be carried out such that the hydrophobic layer is an organic layer and preferably a fluoropolymer and particularly preferably a perfluoropolyether with an anchor group.

The method can be carried out in such a way that the hydrophobic layer is applied by means of evaporation in a vacuum.

The method can be carried out in such a way that all layers are applied in the same coating chamber and accordingly the coating chamber does not have to be opened from coating the first, namely the adhesive layer, to the last, namely the hydrophobic layer, so that the vacuum does not have to be broken.

A plastic substrate was disclosed, the surface of which at least partially comprises a mirror coating, the mirror coating including an adhesive layer, an aluminum alloy and a dielectric layer system as a protective layer, and the aluminum alloy having a chromium content of at least 0.5 at% and a maximum of 5 at%.

The mirror coating of the plastic substrate can comprise a hydrophobic layer, the hydrophobic layer preferably comprising a fluoropolymer and particularly preferably a perfluoropolyether with an anchor group Table 1 test standard Tape test quickly DIN ISO 9211-4 Cotton cloth 100 strokes DIN ISO 9211-4 Salt fog 24h DIN ISO 9022-4 Storage 105 ° C / 500h Similar to DIN ISO 9022-2 Storage -40 ° C / 500h Similar to DIN ISO 9022-2 Fixed climate 85 ° C / 85rH / 500h Similar to DIN ISO 9022-2 40 ° C / 144h condensation, then dry at 85 ° C for 30min Similar to DIN EN ISO 6270-2 Temperature cycle: -40 ° C / + 85 ° C / 0.5h / 500 cycles Similar to DIN ISO 9022-2 Table 2 Environmental test Design 1 Design 2 Design 3 Tape test quickly ✔ ✔ ✔ Cotton cloth 100 strokes × × ✔ Salt fog 24h × ✔ ✔ Storage 105 ° C / 500h ✔ ✔ ✔ Storage -40 ° C / 500h ✔ ✔ ✔ Fixed climate 85 ° C / 85rH / 500h × ✔ ✔ 40 ° C / 144h condensation, then dry at 85 ° C for 30min × × ✔ Temperature cycle: -40 ° C / + 85 ° C / 0.5h / 500 cycles ✔ ✔ ✔ Sun cream test: (70 ° C / 48h) × × ✔

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• JP 4453886 B2 [0003]
• EP 0456488 A1 [0003]
• US 20040198898 A1 [0003]
• US 4085248 A [0003]
• US 6939018 B2 [0003]
• GB 2281411 A [0006]
• US 20040120060 A1 [0006]
• US 6258218 [0016]

Non-patent literature cited

• DIN ISO 9211-4 [0036]
• DIN ISO 9022-4 [0036]
• DIN ISO 9022-2 [0036]
• DIN EN ISO 6270-2 [0036]

Claims (10)

A method for producing a mirror on a plastic substrate, comprising the steps: - providing a plastic substrate with surface - coating the plastic substrate with an adhesive layer - coating with an aluminum alloy - coating with a dielectric layer system comprising at least two layers with different refractive indices. characterized in that the aluminum alloy has a chromium content of at least 0.5 at% and a maximum of 5 at%. Procedure according to Claim 1 , characterized in that the aluminum alloy, apart from impurities, does not contain any metals other than aluminum and chromium. Method according to one of the preceding claims, characterized in that the adhesive layer and / or the aluminum alloy and / or the dielectric layer system is applied by means of sputtering. Method according to one of the Claims 1 to 3 , characterized in that the method comprises a step in which at least one hydrophobic layer is applied, preferably on the dielectric layer system. Procedure according to Claim 4 , characterized in that the hydrophobic layer comprises an organic layer and preferably a fluoropolymer and particularly preferably a perfluoropolyether with an anchor group. Procedure according to Claim 5 , characterized in that the hydrophobic layer is an organic layer and preferably a fluoropolymer and particularly preferably a perfluoropolyether with an anchor group. Method according to one of the Claims 4 to 6th , characterized in that the hydrophobic layer is applied by evaporation in a vacuum. Procedure according to Claim 7 , characterized in that all layers are applied in the same coating chamber and accordingly the coating chamber does not have to be opened from coating the first, namely the adhesive layer to the last, namely the hydrophobic layer, so that the vacuum does not have to be broken. Plastic substrate whose surface at least partially comprises a mirror coating, the mirror coating including an adhesive layer, an aluminum alloy and a dielectric layer system as a protective layer, characterized in that the aluminum alloy has a chromium content of at least 0.5 at% and a maximum of 5 at%. Plastic substrate Claim 9 , characterized in that the mirror coating comprises a hydrophobic layer, the hydrophobic layer preferably comprising a fluoropolymer and particularly preferably a perfluoropolyether with an anchor group

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