Plasma induced crystallization and densification of amorphous coatings

Abstract

The invention relates to a method for coating a substrate and a correspondingly coated substrate. The resulting coating has a crystalline and dense structure, which is particularly suitable for applying a further coating thereon. The method uses plasma instead of thermocuring to cure the applied solution, thus being faster in speed, lower in energy consumption, and suitable for substrates that are more sensitive to temperature.

Description

Technical Field

[0001] The invention relates to a method for coating a substrate and a correspondingly coated substrate. The resulting coating has a crystalline and dense structure which is particularly suitable for applying a further layer of coating thereon. Background Art

[0002] Many substrates are provided with coatings in order to achieve some desired effect or enhancement of performance. Depending on the application, the coatings have a thickness ranging from a few nanometers to several millimeters. For example, for optical and ophthalmic lenses, several layers of coatings ranging from nanometers to micrometers are usually applied to provide UV filtering, anti-reflective and anti-scratch properties, and chemical resistance to glass or polymers.

[0003] To apply coatings of such thickness, a sol-gel process is generally used. In a first step, a solution of the coating medium or its precursor is prepared, which is then applied to the substrate by different techniques to generate a thin liquid film. Finally, the solvent is evaporated from the film to obtain the coating.

[0004] Since these coatings on the substrate are usually amorphous after this initial coating step, curing in the form of a thermal annealing step is usually applied as a further step to crystallize, densify and / or modify the coating. Thermal annealing leads to different film modifications, depending on the temperature program applied. The evaporation of the solvent in the initial coating step is achieved by heating the substrate only to moderate temperatures (in the case of full heating), while the curing step requires higher temperatures. When the coating is applied to the substrate, there is always a certain limit to the maximum temperature that can be applied in the thermal annealing step. In many cases, the temperatures required for common coating materials are too high for temperature-sensitive substrates such as polymers. In addition, the curing step is usually also quite time-consuming and energy-consuming. Conventional annealing of sol-gel coatings is usually carried out at high temperatures of several hundred degrees Celsius and lasts for several hours.

[0005] A typical example of a coating on a glass lens made of soda-lime glass is a UV protective layer consisting of ZnO. In a standard sol-gel coating process on soda-lime glass, the ZnO coating is applied by dip coating (v=30 cm / min), with a first step of drying at 145°C for 6 min and a second step of annealing at 500°C for 60 min. The second heating step is intended to transform the coating from an amorphous state into a crystalline state and to burn off the organic binder in the system. The aim is to achieve a dense, crystalline ZnO film on soda-lime glass. However, in the case of ZnO coatings on glass, the second heating step not only leads to the burning off of the binder and the crystallization of ZnO, but also to the production of a highly porous film. Such a highly porous film is not suitable or at most very limitedly suitable for applying further layers of coating. In addition, the high temperatures also cause great thermal stresses on the substrate. Due to the temperature limitations of the substrate, the crystallization of ZnO coating systems on polymer substrates using thermal annealing is not feasible.

[0006] This example of ZnO coating illustrates the problems faced with sol-gel coatings of different kinds of coating materials on different substrates. While the curing time and temperature may vary, the basic problems of high temperature, long annealing time, high energy consumption, insufficient crystallinity and insufficient density always exist.

[0007] Furthermore, the problems described for the sol-gel process and in particular for ZnO coatings also affect other coating processes which use liquid solutions and require a thermal annealing step to solidify the initially produced amorphous layer. Summary of the invention

[0008] It is therefore an object of the present invention to provide an improved coating process which does not have the problems of the prior art or at least has them to a lesser extent. In particular, the present invention provides a coating process which results in a coating which is suitable for applying further layers thereon.

[0009] The present invention achieves this object.

[0010] The invention can be used in the field of coatings on substrates (such as glass, polymers, metals or other materials), in particular sol-gel coatings, which include an annealing step in order to densify, crystallize or otherwise change the morphology of the coating, in particular with the aim of making the coating suitable as a base layer for further layers.

[0011] The method uses plasma rather than thermal curing to cure the applied solution, making it faster, less energy intensive, and suitable for more temperature-sensitive substrates.

[0012] The inventors have found that a thermal annealing step to change the morphology (crystallization / modification state and / or density and surface roughness) and the resulting film properties (such as UV absorption, hardness, chemical resistance) can be advantageously exchanged with a plasma treatment step. Surprisingly, not only can the treatment temperature and treatment time be reduced, but also the obtained and obtainable properties of the coating are improved. This is very unexpected, because it has been believed that the crystallization of amorphous coatings requires sufficiently high temperatures and sufficiently long times to form a dense grain morphology. Therefore, it was feared that the surface roughness, porosity and grain size would be deteriorated by applying plasma instead of thermal treatment.

[0013] However, contrary to these views, the advantage achieved is that high energy can be brought into the film system, leading to desired film properties that cannot be achieved by thermal annealing at the same time and / or at substrate temperatures as low as plasma processes. This enables new combinations of film-substrate variations, especially for heat-sensitive substrates.

[0014] In a first aspect, the present invention relates to a method for coating a substrate, comprising the steps of:

[0015] a) providing a substrate;

[0016] b) applying a coating solution onto the surface of the substrate;

[0017] c) thermally drying the coating solution to form an amorphous coating;

[0018] d) treating the amorphous coating by plasma treatment to form an at least partially crystalline coating.

[0019] This method enables the object of the present invention to be achieved, namely to provide a method for obtaining a coating which is suitable as a base layer for a layer of another coating in an excellent manner. The obtained coating can have fewer pores, a higher average crystallinity, a higher density and / or a lower surface roughness. The grains are small enough to achieve low porosity and only a small area with a degree of crystallinity below 100%. Even if a binder is used in the coating solution, the coating can still have a very low porosity and an improved microstructure compared to the coating processes of the prior art using thermal annealing (especially sol-gel processes). In addition, the process can also be adjusted by changing the plasma treatment time and energy to produce a certain desired coating microstructure.

[0020] In a particularly preferred embodiment, the coating solution is a sol-gel based coating solution and is applied in a sol-gel process. Here, the term "sol-gel based coating solution" refers to a coating solution that is a colloidal solution that can serve as a precursor to an integrated network or gel and is therefore suitable for use in a sol-gel coating process.

[0021] In an embodiment, the plasma treatment comprises generating a radio frequency plasma or a microwave plasma, in particular at a frequency of 10 MHz to 300 MHz or 300 MHz to 300 GHz. Preferably, the plasma is generated at a frequency of 10 MHz to 100 MHz or 1 GHz to 100 GHz. Although a plasma generator using one of the standard frequencies can be successfully used for the present invention, by applying a non-standard frequency within the claimed range, a specific fine-tuning of the coating properties can be achieved. This allows the method to be flexibly adapted to the desired combination of substrate and coating medium and the desired properties of the final coating for the intended function and the suitability of the base layer as an additional layer.

[0022] Preferably, the plasma is generated by a radio frequency plasma generator with a capacitive electrode arrangement or a pulsed magnetron microwave generator.

[0023] In an embodiment, the generator for generating the plasma is operated at a power of 0.2 kW - 10 kW, preferably 0.3 kW - 7 kW.

[0024] In a preferred embodiment, the plasma is generated in an atmosphere of oxygen, argon, nitrogen, air or hydrogen and at ambient pressure or reduced pressure, particularly under vacuum. Preferably, the plasma is generated in an atmosphere of oxygen, air, argon or nitrogen, more preferably in an atmosphere of oxygen or air. A certain proportion of hydrogen in a hydrogen atmosphere or an argon or nitrogen atmosphere can be used for doping. In many cases, oxygen plasma is particularly advantageous when a binder is used in the coating solution because it can help burn off the binder. In addition, oxygen plasma can be a preferred atmosphere when certain precursors of the coating medium are used in the coating solution.

[0025] In a preferred embodiment, the plasma treatment time is 0.1s to 120min, preferably 1s to 60min, or 30s to 30min, or 1min to 10min. The plasma treatment time can be at least 0.1s, at least 1s, at least 30s or at least 1min. The plasma treatment time can be at most 120min, at most 60min, at most 30min or at most 10min. The plasma treatment time can be selected according to the type of plasma used and its power. For example, for radio frequency plasma, a treatment time of 30min to 120min can be selected, while for microwave plasma, a treatment time of 1s to 10min can be selected.

[0026] In an embodiment, the maximum processing temperature does not exceed 400°C, preferably does not exceed 300°C, or does not exceed 150°C. The maximum processing temperature may be at least 25°C, at least 30°C, at least 35°C, at least 40°C, or at least 50°C. The maximum processing temperature in this article may refer to the highest temperature to which the substrate and its coating are exposed during the processing, i.e., in all steps. This is equivalent to the temperature of the oven in the thermal annealing step of the prior art. These temperatures are much lower than those in the prior art and allow the use of substrates that are more sensitive to temperature.

[0027] In particularly preferred embodiments, during the plasma treatment, the substrate temperature does not exceed 400° C., preferably does not exceed 300° C. or does not exceed 150° C. The substrate temperature may be at least 25° C., at least 30° C., at least 35° C., at least 40° C. or at least 50° C. This opens up a range of new substrates, in particular polymer substrates, that could not be used before due to their temperature sensitivity for coating processes.

[0028] In a particularly preferred embodiment, during the entire process, the coating is exposed to a maximum temperature of preferably at most 150°C to 400°C for a duration of at most 0.1 min to 90 min, preferably at most 0.5 min to 60 min or at most 1 min to 30 min. The maximum temperature here refers to the actual temperature reached by the substrate and its coating when exposed during the treatment process and the time span for which it has this temperature. Compared with the thermal annealing steps of the prior art, these preferred conditions can allow lower temperatures and shorter times, while still producing excellent crystalline, low porosity and dense coatings, which are suitable for multilayer structures.

[0029] In an embodiment, the amorphous coating is treated by plasma treatment according to step d) to form a crystalline coating, and the reflectivity measured at an angle of 6° in the range of 250nm-850nm according to ISO 15368:2001 is 0.05-0.3, and in particular, the reflectivity measured in the range of 250nm-380nm is 0.05-0.15 on the coating with a thickness of 100nm-120nm. The reflectivity is not only an indicator of the crystallinity of the coating, but also an important characteristic of the coating.

[0030] In a very preferred embodiment, the amorphous coating is treated by plasma treatment according to step d) to form a crystalline coating having a porosity of less than 20%, preferably less than 15%, determined from the ratio of the inter-grain area to the total examined area in a scanning electron microscope (SEM) image. To determine this ratio, a SEM image of the coating is made and the SEM image is evaluated by image processing. 2 In a predetermined inspection area, the pore area between the crystals is detected based on the contrast and brightness difference. The porosity is then calculated by dividing the void area by the inspection area. Crystalline coatings with porosity in this range are particularly suitable as a base layer for additional layers.

[0031] In an embodiment, the method may include repeating steps b) to d) one or more times to form one or more additional layers of crystalline coating on the crystalline coating formed in the previous repetition, wherein preferably, the coating solution in step b) is different from the coating solution in the previous repetition of step b). Applying additional different layers of coating can be used to provide the substrate with additional functions as described above. For example, the lens can be provided with UV protection, anti-reflection and anti-scratch properties.

[0032] Of course, if a thicker layer of a coating is required, the same coating solution can also be used multiple times. This can be advantageous because thinner layers are more likely to crystallize with higher quality and less likely to form higher porosity than thicker layers. Since the layers of the coating produced by the method according to the invention are excellent base layers for other layers, for example, two layers with half the thickness of a single layer have a higher overall quality than a single layer.

[0033] Preferably, the substrate comprises or consists of glass, polymer, metal or alloy or a combination thereof.

[0034] In a preferred embodiment, the coating solution comprises a metal or a metal oxide, in particular a transition metal or a transition metal oxide, or a combination thereof, preferably ZnO, ZrO2, TiO2, VO2, WO3, SnO, indium tin oxide, antimony tin oxide, or precursors of these components, in particular their acetates and carbonates, and an optional binder, in particular SiO2 or TiO2. With these ingredients, the substrate can be provided with a wider range of functions. During the coating process, the precursor will react with the metal or metal oxide. The main reason for using precursors is that they are more soluble than others, which facilitates the preparation of the coating solution. Therefore, acetates and carbonates are particularly preferably used as precursors. They generally have good solubility and are easily transferred to their constituent metals or metal oxides. The binder can provide structure for the crystalline coating.

[0035] Preferably, the precursor is decomposed into a metal or metal oxide and its organic components under the action of the plasma, and these organic components are transferred to the gas phase, preferably the precursor reacts with the plasma gas. The precursor is generally an organic compound of a metal or metal oxide. Therefore, the plasma preferably decomposes the precursor and helps to transfer the organic components (e.g., CO ) to the gas phase. This is particularly effectively achieved by plasma gases that can react with the precursor. In many cases, oxygen is suitable for this purpose. It can not only drive off the organic components by oxidation, but also additionally oxidize the metal to its oxide, which is why this gas is particularly preferred for coating solutions of metal oxide coatings.

[0036] In an embodiment, the coating comprises a metal or a metal oxide, in particular a transition metal or a transition metal oxide, or a combination thereof, preferably ZnO, ZrO2, TiO2, VO2, WO3, SnO, indium tin oxide or antimony tin oxide, or consists thereof. As mentioned above, these components can provide a wider range of functions for the substrate. In some cases, one or more other ingredients, such as additives or activators, may be required to achieve the intended function or structure of the coating. However, the coating is usually composed of a single metal or metal oxide.

[0037] In a particularly preferred embodiment, step d) forms a crystalline coating having an average grain size of less than 40 nm, preferably less than 20 nm, and greater than 5 nm, preferably greater than 10 nm, when measured by scanning electron microscopy on a coating having a thickness of 80 nm to 120 nm. It has been shown that an average grain size in this range is most suitable for very dense and low porosity coatings. The inventors have found that the larger the grain size, the greater the pores (and therefore the porosity). In addition, the processing time can advantageously be very short, which shortens the process time, saves energy, and obtains a very uniform particle size distribution.

[0038] In a preferred embodiment, the morphology of the amorphous coating is altered in terms of crystalline state and / or modified state and / or density and / or surface roughness.

[0039] In a particularly preferred embodiment, the UV absorption rate of the obtained crystalline coating is 15% to 90%. The method according to the invention is particularly suitable for producing high-quality UV protective coatings. Of course, other types of functional coatings can also be produced in this way.

[0040] In an embodiment, applying the coating solution in step b) comprises spin coating, printing, spraying, roller coating, air knife coating or dipping the coating solution on the surface of the substrate. These methods are particularly suitable for applying the coating solution based on the substrate, the type of coating solution and its viscosity and the desired thickness.

[0041] In a second aspect, the present invention relates to a substrate comprising an at least partially crystalline coating, which coating is preferably obtainable by a method according to the present invention, wherein the coating has a porosity of less than 20%, preferably less than 15%, the porosity being determined based on the ratio of the inter-grain area to the total examined area in a scanning electron microscope image.

[0042] In an embodiment, the average grain size of the coating on the substrate is less than 40 nm, preferably less than 20 nm, and greater than 5 nm, preferably greater than 10 nm, when measured on a coating with a thickness of 80 nm-120 nm. This range has been shown to be the optimal range for the density and porosity of the coating. In addition, if the grain size is too small, the proportion of amorphous structure will increase because amorphous grain boundaries become dominant. The grain size can be adjusted by a combination of treatment time and plasma energy. In addition, the choice of plasma treatment type (microwave or radio frequency) also has an impact on the general grain size. As the treatment time increases, the crystallinity and / or grain size will also increase.

[0043] In a third aspect, the present invention relates to a substrate, wherein:

[0044] - The substrate comprises glass and / or polymer;

[0045] - the coating comprises ZnO; and

[0046] -The average grain size of the coating is less than 30nm.

[0047] In a fourth aspect, the present invention relates to a substrate, wherein:

[0048] - The substrate comprises glass and / or polymer;

[0049] - the coating comprises ZnO; and

[0050] - The refractive index of the coating is between 1.55 and 2.10 when measured at a wavelength of 590 nm and a coating thickness of 100 nm to 120 nm.

[0051] In an embodiment, the coating of the substrate comprises more than two layers, wherein preferably the layers are different from their adjacent layers. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] FIG. 1 is a scanning electron microscope image of a ZnO coating prepared by a prior art sol-gel process with a thermal annealing step (upper image: after drying, lower image: after annealing).

[0053] Figure 2 1 is a scanning electron microscope image of a ZnO coating prepared by a sol-gel process using 90 min O2 radio frequency plasma according to the present invention (upper image: dry sample in FIG. 1 , lower image: after annealing).

[0054] Figure 3 1 is a scanning electron microscope image of a ZnO coating prepared by a sol-gel process using 300 s O 2 microwave plasma according to the present invention (upper image: dry sample in FIG. 1 , lower image: after annealing).

[0055] Figure 4 1 to 3 are scanning electron microscope images for comparison of the annealed ZnO coatings (upper image: thermal annealed sample in FIG. 1 ; middle image: Figure 2 RF plasma annealing sample in; below: Figure 3 of the microwave plasma annealed sample). DETAILED DESCRIPTION

[0056] In order to demonstrate the effect of the present invention, a ZnO coating on a soda-lime glass substrate is selected as a demonstration coating. Such a ZnO coating is usually provided as a UV protection layer.

[0057] Comparison Examples

[0058] As a comparison, a corresponding ZnO coating was prepared in a conventional sol-gel process of the prior art with a thermal annealing step. The ZnO coating was applied by dip coating (v=30 cm / min) in an ethanol solution and dried at 145°C for 6 min in a first step. The dried coating is shown in the upper figure in FIG1 . Its thickness is 200 nm–220 nm. The coating was then annealed by heating at 500°C for 60 min. The annealed coating is shown in the lower figure in FIG1 . Its thickness is 110 nm–120 nm. It can be clearly seen from the picture that the thermal annealing process produces a higher open porosity and a lower density. Although the coating after the annealing step may be suitable for the desired purpose of UV protection, it is not possible to apply additional layers of other coatings on top of this high porosity layer, for example, to also provide anti-reflection or anti-scratch effects.

[0059] In addition, to quantify the effect, the 2D porosity of the coating was measured by scanning electron microscopy and image processing as described above. The porosity of the standard sol-gel process sample after thermal annealing was 26.7%, which is too high to allow the additional coating layer to fully integrate with it.

[0060] First example

[0061] For the examples according to the invention, the first step repeated the sol-gel process of the comparative example described above. In the first example, the obtained dried coating was then subjected to a radio frequency plasma treatment in an oxygen atmosphere for 90 min. The plasma was generated by a capacitive electrode device at a power of 540 W and a standard frequency of 13.56 MHz. During the plasma treatment, the substrate temperature did not exceed 250° C. The annealed coating was then subjected to a radio frequency plasma treatment in an oxygen atmosphere for 90 min. Figure 2 The thickness is 100nm–120nm. Compared to the annealed coating of the reference sample, the examples treated with RF plasma have smaller grains, a denser structure (confirmed by reflectivity measurements), and a higher degree of crystallinity. Oven experiments confirmed that these effects are not simply the effect of lowering the substrate temperature, but are also clearly caused by the plasma action.

[0062] The 2D porosity of the coating was measured to be 7.4%. This significant improvement in porosity makes the coating suitable for coating of additional layers. Although the treatment time was increased compared to the heat treatment time in this example, the temperature of the substrate was still reduced to half of it (from 500°C to 250°C). Moreover, the porosity, density and grain size were much better than the reference. This makes RF plasma the best choice for coating solutions that would be degraded by high energy treatments. In addition, treatment times in the range of 60 minutes are also feasible because the porosity of the coating is already in a range suitable for coating of other layers.

[0063] Second example

[0064] For the second example, the plasma treatment step was performed for 300 s using a microwave plasma generated by a magnetron in an oxygen atmosphere. The magnetron was operated with an average power of 2.2 kW and a nominal frequency of 2.45 GHz. Also in this case, during the plasma treatment, the substrate temperature did not exceed 250°C. Annealing the coating as Figure 3 As shown in the figure below. The coating thickness is also 100nm-120nm. The 2D porosity of the coating was measured to be 7.1%. Compared with the annealed coating of the first example, the example treated with microwave plasma has a smaller grain size, a denser structure and a higher crystallinity. The porosity is only slightly smaller than that in the first example, and the porosity in the first example has achieved an excellent value. In terms of processing time, it is reduced by 12 times compared to the thermal annealing of the prior art (300s versus 60min). The higher energy input of microwave plasma makes it possible to use other precursors that are more difficult to decompose in the prior art and therefore require higher temperatures in the heat treatment step.

[0065] These examples show that the method according to the invention has a number of parameters associated with the plasma treatment which allow the properties of the resulting coating to be adjusted and that the plasma treatment generally produces a better quality than the thermal treatments of the prior art. In addition to this, the method has a wider range of uses due to a wider selection of coating solutions and substrates.

[0066] exist Figure 4 A direct comparison of the annealed coatings is shown in Figure 2 to give an overview of the improvement in porosity. The table below summarizes the respective 2D porosity.

[0067] Annealing step hot RF plasma Microwave plasma 2D Porosity 26.7％ 7.4％ 7.1％

Claims

1. A method for coating a substrate, comprising the steps of: a) providing a substrate; b) applying a coating solution on the surface of the substrate; c) thermally drying the coating solution to form an amorphous coating; d) treating the amorphous coating using a plasma treatment method to form an at least partially crystalline coating.

2. The method according to claim 1, wherein: The coating solution is a sol-gel based coating solution and is applied in a sol-gel process.

3. The method according to claim 1 or 2, wherein: The maximum treatment temperature does not exceed 400°C, preferably does not exceed 300°C or does not exceed 150°C.

4. A method according to any one of the preceding claims, wherein: During the entire process, the coating is exposed to a maximum temperature of at most 150° C. to 400° C. for a duration of at most 0.1 min to 90 min, preferably at most 0.5 min to 60 min or at most 1 min to 30 min.

5. A method according to any one of the preceding claims, wherein: According to step d), the amorphous coating is treated by plasma treatment to form a crystalline coating, and on a coating with a thickness of 100 nm to 120 nm, the reflectivity measured at an angle of 6° in the range of 250 nm to 850 nm according to ISO 15368:2001 is 0.05 to 0.3, and in particular, the reflectivity measured in the range of 250 nm to 380 nm is 0.05 to 0.

15.

6. A method according to any one of the preceding claims, wherein: According to step d), the amorphous coating is treated with a plasma treatment method to form a crystalline coating, the porosity of the crystalline coating is less than 20%, preferably less than 15%, and the porosity is determined based on the ratio of the inter-grain area to the total inspection area in the scanning electron microscope image.

7. The method according to any one of the preceding claims, further comprising repeating steps b) to d) one or more times to form one or more additional layers of crystalline coating on the crystalline coating formed in the previous repetition, wherein preferably, the coating solution in step b) is different from the coating solution in step b) of the previous repetition.

8. A method according to any one of the preceding claims, wherein: The substrate includes or consists of glass, polymer, metal or alloy or a combination thereof.

9. A method according to any one of the preceding claims, wherein: The coating solution comprises a metal or a metal oxide, in particular a transition metal or a transition metal oxide, or a combination thereof, preferably ZnO, ZrO2, TiO2, VO2, WO3, SnO, indium tin oxide, antimony tin oxide, or precursors of these components, in particular their acetates and carbonates, and an optional binder, in particular SiO2 or TiO2.

10. The method according to claim 9, wherein: The precursor decomposes under the action of the plasma into metal or metal oxide and its organic components, which are transferred into the gas phase. Preferably, the precursor reacts with the plasma gas.

11. A method according to any one of the preceding claims, wherein: The coating comprises or consists of a metal or a metal oxide, in particular a transition metal or a transition metal oxide, or a combination thereof, preferably ZnO, ZrO2, TiO2, VO2, WO3, SnO, indium tin oxide or antimony tin oxide.

12. A method according to any one of the preceding claims, wherein: Step d) forms a crystalline coating, wherein the average grain size of the crystalline coating is less than 40 nm, preferably less than 20 nm, and greater than 5 nm, preferably greater than 10 nm, when measured on a coating having a thickness of 80 nm to 120 nm by image processing using a scanning electron microscope.

13. A method according to any one of the preceding claims, wherein: The plasma treatment method comprises generating a radio frequency plasma or a microwave plasma, in particular at a frequency of 10 MHz to 300 MHz or 300 MHz to 300 GHz, preferably at a frequency of 10 MHz to 100 MHz or 1 GHz to 100 GHz.

14. A method according to any one of the preceding claims, wherein: The plasma is generated in an atmosphere of oxygen, argon, nitrogen, air or hydrogen and at ambient pressure or reduced pressure, in particular under vacuum.

15. A method according to any one of the preceding claims, wherein: The plasma treatment time is 0.1 s to 120 min, preferably 1 s to 60 min, or 30 s to 30 min, or 1 min to 10 min.

16. A method according to any one of the preceding claims, wherein: During the plasma treatment, the substrate temperature does not exceed 400°C, preferably does not exceed 300°C or does not exceed 150°C.

17. A method according to any one of the preceding claims, wherein: The plasma is generated by a radio frequency plasma generator with a capacitive electrode arrangement or a pulsed magnetron microwave generator.

18. A method according to any one of the preceding claims, wherein: The generator for generating the plasma is operated at a power of 0.2 kW to 10 kW, preferably 0.3 kW to 7 kW.

19. A method according to any one of the preceding claims, wherein: The morphology of an amorphous coating is altered in terms of crystalline state and / or modified state and / or density and / or surface roughness.

20. A method according to any one of the preceding claims, wherein: The UV absorption of the resulting crystalline coating ranged from 15% to 90%.

21. A method according to any one of the preceding claims, wherein: Applying the coating solution in step b) includes spin coating, printing, spray coating, roller coating, air knife coating or dip coating the coating solution on the surface of the substrate.

22. A substrate comprising an at least partially crystalline coating, preferably obtainable by a method according to any one of the preceding claims, wherein the coating has a porosity of less than 20%, preferably less than 15%, the porosity being determined from the ratio of the inter-grain area to the total examined area in a scanning electron microscope image.

23. The substrate according to claim 22, wherein The average grain size of the coating is less than 40 nm, preferably less than 20 nm, and greater than 5 nm, preferably greater than 10 nm, when measured on a coating having a thickness of 80 nm - 120 nm.

24. The substrate according to claim 22 or 23, wherein: - the substrate comprises glass and / or polymer; - the coating comprises ZnO; and -The average grain size of the coating is less than 30nm.

25. The substrate according to any one of claims 22 to 24, wherein - the substrate comprises glass and / or polymer; - the coating comprises ZnO; and - The refractive index of the coating is between 1.55 and 2.10 when measured at a wavelength of 590 nm and a coating thickness of 100 nm - 120 nm.

26. The substrate according to any one of claims 22 to 25, wherein The coating comprises two or more layers, wherein preferably the layers are different from their adjacent layers.

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