RU2620801C1 - Method of forming coloured decorative coating by anodisation - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of obtaining a decorative coating with a changing colour during visual angle variation consists in the formation of one-dimensional photonic crystal with photonic bandgap in the visible range by anodising the surface of a valve metal or its alloy containing not less than 50% of valve metal at a cyclically varying parameters: current and voltage, wherein each cycle consists of two stages: anodization is carried out at the first stage at current stabilization in the range of 0.1 to 50 mA /cm² for a time ensuring charge flow from 0.05 to 5 C/cm²; anodization is carried out at the second stage at voltage stabilization, increasing it from the voltage value at the end of the first stage to a value lying in the range from 10 to 200 V, with a decreasing voltage rise rate from 5 V/s to 0 V/s, and is held at this value for a time ensuring charge flow from 0.05 to 5 C/cm², ensuring more than 1.4 ratio ofmaximal voltage at the second stage to minimal voltage at the first stage, wherein metal surface serves as anode during production of the decorative coating, and inert material is used as cathode, wherein anodisation charge is reduced by 0.01-10% at the first and the second stages at every next anodisation cycle, which amount lies in the range from 20 to 300.

EFFECT: invention allows to obtain coloured decorative coatings of high quality using simple and reproducible method characterized by safety and environmental friendliness due to elimination of toxic substances from the technology.

9 cl, 10 dwg, 2 tbl, 5 ex

Description

FIELD OF THE INVENTION

The invention relates to the field of photonics, and more particularly to methods for producing colored decorative coatings.

The proposed method for creating a decorative coating can be used for painting metal products in various colors, changing with a change in the viewing angle. The range of objects that can be decorated using the claimed invention is extremely diverse, these include electronic device housings (telephones, tablets, laptops, etc.), vehicle housings (airplanes, ships, cars, etc.), jewelry, interior items as well as metal cladding panels for buildings and various kinds of construction objects.

In addition, a decorative coating can be used to develop and create a wide range of high-tech optoelectronic devices that require the use of elements capable of reflecting or transmitting light with a certain adjustable wavelength (for example, Notch filters, laser generation media).

State of the art

Porous oxide films obtained by anodizing valve metals are widely used in science and technology. A striking example is the porous films of anodic alumina, which have been used for almost 100 years for decorative purposes [Patent specification GB No. 223995 (Z. No. 19839/23), 11/3/1924, London], as well as corrosion protection [Patent specification GB No. 223994 (Z. No. 19838/23), 11/03/1924, London]. It is known from the literature that, ceteris paribus, the pore diameter and average distance between them linearly increase with increasing anodizing voltage [Woo Lee, Sang-Joon Park. Porous Anodic Aluminum Oxide: Anodization and Templated Synthesis of Functional Nanostructures // Chemical Reviews, 2014, v. 114 (15), p. 7487-7556; D.I. Petukhov, K.S. Napolskii, A.A. Eliseev. Permeability of anodic alumina membranes with branched channels // Nanotechnology, 2012, v. 23, pp. 335601]. This allows you to create porous films with a modulated structure along the normal to their surface by changing the anodizing conditions in the process of formation of the oxide coating. In the case of periodic modulation of the structure on a scale comparable to the wavelength of light, such a film may possess the properties of a one-dimensional photonic crystal [J.D. Joannopoulos, R. D. Meade, J.N. Winn. Photonic crystals: molding the flow of light. Princeton University Press. 1995]. Photonic crystals of porous anode oxides with a photonic band gap in the visible spectrum have a color whose color depends on the viewing angle. Due to this property, they can find wide practical application for decorative purposes.

The known method (analogue) of coloring aluminum and its alloys by creating on their surface an oxide film having the properties of a photonic crystal [CN 102181902, Method for coloring aluminum and alloy surface thereof; Zhiyuan Ling, Yisen Liu, Yi Chang, Xing Hu; September 14, 2011]. The invention discloses a method for painting the surface of aluminum and its alloys, which consists of the following steps: 1) cleaning and polishing the surface of aluminum or aluminum alloy; 2) preliminary anodic oxidation of aluminum or an aluminum alloy in a solution of sulfuric acid with a concentration of from 0.1 to 3 mol / l under constant conditions for 20 minutes to 3 hours at a voltage of 10 to 20 V using graphite as a cathode; 3) the oxidation of aluminum or aluminum alloy at a temperature of from 0 to 5 ° C by applying a voltage that varies periodically in time, in order to obtain a layered photonic crystal of aluminum oxide. Each cycle of voltage change consists of five sections: (i) an increase in the sinusoid, (ii) a stage of constant high voltage, (iii) a linear decrease in voltage, (iv, v) two sinusoidal sections of voltage decrease. In this method, the coloring of the surface of aluminum or aluminum alloy occurs due to the presence of a photonic band gap in the photonic crystal of aluminum oxide. Moreover, various coating colors from the visible spectrum can be obtained by maintaining the duration of the anodizing pulses and varying only the voltage or maintaining the voltage and changing only the duration of the pulses.

In the analogue, as well as in the claimed solution, the intense reflection of the incident light flux in narrow spectral ranges, which determines the appearance of the color of the coating, is achieved due to the presence of a porous structure with a periodically changing refractive index created by anodizing the metal under cyclically changing conditions. However, the method for forming colored coatings described in the analogue has a number of significant disadvantages:

1) anodizing is carried out under strictly periodic conditions, which does not allow to achieve the constancy of the optical period of the created structure. The product of the structure period and the effective refractive index characterizing the optical period of the structure inevitably turns out to be less for the upper layers of the oxide film compared to the lower ones, since the upper layers are longer in an acidic electrolyte solution in which the anodic oxide is subject to chemical dissolution. This feature of the structure, in turn, leads to broadening of the spectral regions in which the incident light flux is reflected and, as a result, obtaining a pure color, characterized by a narrow band in the reflection spectrum, becomes impossible;

2) anodizing is carried out during stabilization (control) of the voltage, which does not allow to quickly reduce the voltage without stopping the growth of the oxide film. In this regard, the anodization mode proposed by the authors of the invention [CN 102181902] includes several lengthy stages of a gradual decrease in voltage, which, in turn, leads to significant time costs when obtaining a colored coating and a blurred boundary between the layers of the oxide film, characterized by low and high porosity;

3) a mandatory stage of the process is the preliminary anodic oxidation of the metal under constant conditions for from 20 minutes to 3 hours. This layer does not create and does not enhance the color of the coating, and therefore, its exclusion is desirable to accelerate the process of forming a decorative coating.

There is another method for producing photonic crystals on the surface of aluminum, selected as the closest analogue (prototype), the description of which is presented in the patent [CN 100572616 C, Method for manufacturing high quality aluminum oxide photon crystal; July 16, 2008]. This invention discloses a method for producing photonic crystals from aluminum oxide, the main stages of which are: 1) cleaning and degreasing of the surface of aluminum, electrochemical polishing; 2) anodizing aluminum in the usual way one or more times; 3) dissolution of alumina from the surface of aluminum formed during the anodizing process to obtain a clean aluminum surface; 4) anodizing the surface of aluminum obtained in stage (3) with a constant voltage of 15 to 80 V until the current density is stabilized, and then transmitting a high current density of 1-20 mA / cm ² for 10 s to 10 minutes, followed by passing a low current density of 0.1 to 10 mA / cm ² for 10 s to 10 minutes, providing a ratio of high and low current density in the range of 2-10: 1. The above procedure is repeated as many times as it is necessary to form layers in a photonic crystal. In this case, the processing time at high and low current densities is reduced by 0.5-1 second for each subsequent cycle. Due to the fact that the current density changes cyclically, an aluminum oxide film with a layered structure is formed on the aluminum surface, which has the properties of a one-dimensional photonic crystal.

In the last step, the resulting alumina photonic crystal is placed in mercuric chloride HgCl ₂ to separate the oxide film from the remaining aluminum, then the oxide film is washed and dried, thereby obtaining an alumina photonic crystal.

In this solution [CN 100572616], as well as in the previous source of information [CN 102181902], the formation of a porous structure by anodizing aluminum under periodically changing conditions is decisive. Significant disadvantages of the method of forming optically active layers proposed in the prototype are the following:

1) during the formation of a layered porous oxide film, anodization is carried out with current stabilization. In this mode, with a sharp switching of a low current density to a large one, the probability of breakdown of a dielectric film is high, which makes the formation of a photonic crystal with a homogeneous structure over a large area impossible at high current densities. This negatively affects the quality of the resulting coating and the uniformity of the color scheme. In connection with the foregoing, the anodization mode proposed by the inventors [CN 100572616] can be used only at relatively low current densities, which in turn requires a long duration of electrochemical treatment to form a coating capable of efficiently reflecting light;

2) an obligatory step in the process of obtaining a photonic crystal is the preliminary anodic oxidation of the metal under constant conditions (constant voltage). The layer formed at this stage does not create and does not enhance the color of the coating, and, therefore, its exclusion is desirable to accelerate the process of forming a decorative coating;

3) the technology for producing photonic crystals uses toxic mercury-containing substances, which is highly undesirable in mass production.

In connection with the foregoing, the development of methods for the electrochemical preparation of decorative coatings based on photonic crystals that change color when the viewing angle is changed is an important task of modern science and technology.

Disclosure of invention

In the present invention, the following meanings are given to the applicable terms:

- "metal oxide" - a compound of a chemical element (metal) with oxygen in the oxidation state -2, in which oxygen is bound to the metal;

- "porosity" - the proportion of pore volume in the total volume of the porous body;

- “pore diameter” is the value found from the cross-sectional area of the pore assuming a round cross-sectional shape of the pore (D = (4⋅S / π) ^0.5 , where D is the pore diameter, S is the cross-sectional area);

- “pore cross section” - pore cross section with a plane perpendicular to the long axis of the pore;

- “layer” - a sheet, strip, etc. of relatively small thickness compared with other linear dimensions of the product;

- "layered product" includes at least two layers bonded together;

- "decorative coating" - a layer or a combination of layers located on the surface of the product and giving it the required performance characteristics (color, surface texture, hardness, etc.);

- "photonic crystal" - a material with a periodic change in dielectric constant on a scale comparable to the wavelength of light;

- “one-dimensional photonic crystal” - a material with a periodic change in the dielectric constant in one direction at scales comparable to the wavelength of light;

- “photonic band gap” - a wavelength range forbidden for the propagation of radiation in a photonic crystal and characterized by intense reflection of light incident on the surface of the photonic crystal;

- "visible range" - the range of electromagnetic radiation with wavelengths from 380 to 780 nm;

- “current stabilization mode” - the mode of operation of the DC source, in which the operator sets the current, and the output voltage depends on the set current and the resistance of the external circuit. In this case, the current can be either constant or vary according to the program set by the operator;

- “voltage stabilization mode” - the mode of operation of the DC source, in which the operator sets the voltage, and the current depends on the specified voltage and the resistance of the external circuit. In this case, the voltage can be either constant or change according to the program set by the operator.

 The present invention is to develop a reproducible, easy-to-implement method for producing high-quality color decorative coatings characterized by a color change with a change in the viewing angle. The quality of the decorative coating is determined by the uniformity of color, its saturation, as well as the intensity of reflection of the incident light, which determines the occurrence of color.

The technical result achieved by the claimed invention consists in the formation of a decorative coating characterized by pure color from a set of spectral colors, arising due to intense (at least 80%) reflection of incident light only in a narrow spectral region of visible radiation. The technical result is achieved by changing the anodizing conditions that provide a porous oxide film on a metal surface that has the properties of a one-dimensional photonic crystal, reducing both the number of anodizing steps (by eliminating the stage associated with the formation of the preliminary layer) used in the formation of coatings, and reducing the time spent on one cycle in the process of anodizing, as well as on the formation of the coating as a whole.

Another advantage of the method is its safety / environmental friendliness due to the exclusion of toxic mercury-containing substances from the technology.

The method of creating a colored coating is based on the electrochemical oxidation (anodization) of the metal. For this purpose, so-called valve metals (Al, Ti, Zr, Hf, Nb, Ta, etc.) can be used, as well as alloys based on them with a valve metal content of at least 50%, which are uniform in anodizing non-conductive oxide film on their surface.

The problem is solved in that the method of obtaining a decorative coating with a changing color when changing the viewing angle includes the formation of a one-dimensional photonic crystal with a photonic band gap in the visible range using anodic oxidation of a metal surface with cyclically changing parameters, while using current as cyclically changing parameters and voltage, and each cycle consists of two stages: at the first stage, anodizing is carried out with current stabilization in the range from 0.1 to 50 mA / cm ² during the time providing the flow of charge from 0.05 to 5 C / cm ² ; in the second stage, anodizing is carried out with voltage stabilization, increasing it from the voltage value at the end of the first stage to a value lying in the range from 10 to 200 V, with a decreasing voltage rise rate from 5 V / s to 0 V / s, and withstand value over time, providing a charge flow from 0.05 to 5 C / cm ^2, the ratio of the maximum voltage providing the second step to the minimum voltage in the first stage of more than 1.4, the metal surface during the preparation of the decorative coating is as the anode and as the cathode is an inert material, wherein the anodization charge on the first and second stages to reduce 0.01-10% in each subsequent cycle of the anodization, the number of which ranges from 20 to 300.

It is preferable to use the surface of a valve metal or an alloy based on it with a valve metal content of at least 50% as a metal surface.

The best result is achieved when aluminum, titanium, zirconium, hafnium, niobium and tantalum are used as a valve metal.

It is preferable to carry out the anodizing in metal oxide-dissolving electrolytes, such as aqueous or aqueous-organic acid solutions, including sulfuric, orthophosphoric, selenic, hydrofluoric, oxalic, malonic, succinic, acetic, hydroxyethylidene diphosphonic acids and mixtures thereof. The concentration of acid in electrolytes is from 0.1 to 5 mol / l, and anodizing is carried out at a temperature of from -20 ° C to + 40 ° C using aqueous organic electrolytes or at a temperature of from -5 ° C to + 40 ° C when using aqueous solutions of electrolytes.

It is worth noting that at various stages of anodizing with current stabilization in the range from 0.1 to 50 mA / cm ² and voltage stabilization in the range from 10 to 200 V, the current density value can lie in the range from 0.1 to 500 mA / cm ² , and the voltage is in the range from 0.1 to 200 V.

When using dilute solutions of acids as an electrolyte, as well as during the anodization process at low temperature after the formation of a one-dimensional photonic crystal from anodic metal oxide, it is preferable to increase the pore diameter by chemical etching in acid solutions or mixtures thereof.

Preferably, after the formation of a one-dimensional photonic crystal from an anodic metal oxide with a porous structure, additional layers are applied on its surface, performing auxiliary functions, including protective, and / or strengthening, and / or anti-glare, and / or absorbing, and / or antibacterial, and / or hydrophobic and / or oleophobic, and these layers can completely or partially penetrate into the channels of the porous structure of the photonic crystal.

It is preferable, after obtaining a one-dimensional photonic crystal from an anodic metal oxide with a porous structure, to additionally form a layer under and / or in the lower part of the photonic crystal that absorbs the light transmitted through the photonic crystal.

The best result is achieved when electrochemical deposition of nanostructures is carried out in the channels of the porous structure of the photonic crystal and / or applying dark paint to one of the surfaces of the photonic crystal and / or removing the metal on the surface of which the photonic crystal is formed to form the absorbing layer.

The resulting coating is characterized by the efficiency of reflection of incident light in the region of the photonic band gap. Due to the clear boundary between adjacent layers in the structure of the formed photonic crystals, the proposed method for producing a decorative coating allows to reduce the number of layers of a porous oxide film while maintaining the color intensity at the same level, which reduces the time of formation of a decorative coating.

It is important to note that in the claimed invention, the metal anodization during stabilization (control) of the voltage at the stage of formation of the layer with a large pore diameter increases the stability of the anodization process, minimizing the probability of breakdown of the oxide film and reducing the amount of marriage. On the contrary, at the stage of formation of a layer with a small pore diameter, the anodization process is carried out during stabilization (control) of the current. This allows you to quickly reduce the voltage, forming a clear boundary between the layers, without stopping the growth of the oxide film, which in turn accelerates the process of formation of a photonic crystal.

It is worth noting that dyes are not used in the technology to add color.

Brief Description of Drawings

The invention is illustrated by graphs and figures.

In FIG. 1 is a micrograph of a transverse cleavage of a decorative coating obtained using a scanning electron microscope and showing the porous layered structure of the oxide layer.

In FIG. Figure 2 presents a graph of the dependence of the set and recorded anodizing parameters (using two cycles as an example), showing the conditions for obtaining a decorative coating. On the graph, the cycles are separated by a vertical dashed line. Each anodizing cycle consists of two stages (on the graph are divided by vertical dotted lines). At the first stage, the current is set, and the voltage is recorded, and at the second stage, the voltage is set and the current is recorded.

In FIG. Figure 3 shows the specular reflection spectrum from a decorative coating, illustrating the presence of a narrow region in the visible range, within which light is reflected from the coating surface. Reflection coefficient in this area can exceed 90%.

In FIG. Figure 4 shows the specular reflection spectra from the decorative coating, taken at various angles of incidence of light on the plane of the sample (8 °, 30 °, and 45 ° relative to the normal). The shift of the photonic band gap to the side of shorter wavelengths (indicated by the arrow) when the direction of incident light changes from normal to moving, causes the observed color change of the decorative coating.

In FIG. Figure 5 shows the specular reflection spectrum from a decorative coating obtained in a 1 M solution of sulfuric acid and consisting of 300 layers.

In FIG. 6 is a cross-sectional diagram of a decorative coating containing two additional layers. One of them performs a protective function, and the absorbing layer enhances the color.

In FIG. Figure 7 shows the specular reflection spectra from decorative coatings obtained in a 1 M solution of sulfuric acid and consisting of 50 layers. The dotted line shows the specular reflection spectrum from the decorative coating obtained by anodizing, and the solid line from the same coating, additionally containing an absorbing layer in the lower part of the oxide film. The arrows indicate a decrease in the background signal to the left and to the right of the photonic band gap in the presence of an absorbing layer, which leads to an increase in the color saturation.

In FIG. Figure 8 shows the specular reflection spectra of the decorative coating obtained in a 0.3 M solution of sulfuric acid taken at different angles of incidence of light on the plane of the sample (8 °, 15 °, and 30 ° relative to the normal). The observed shift of the reflection maximum from the region of 620 nm to the region of 580 nm leads to a change in color from red-red at normal incidence of light to yellow-green at an angle of about 30.

In FIG. Figure 9 shows the specular reflection spectra from a decorative coating obtained in a 0.3 M solution of sulfuric acid to (dashed line) and after (solid line) an additional increase in the diameter of the channels in the oxide film by chemical etching in a 1 M solution of sulfuric acid for 60 minutes at a temperature of 25 ° C. Spectra were recorded at an angle of light incidence on the sample plane of 8 ° relative to the normal.

In FIG. Figure 10 shows the specular reflection spectrum from a decorative coating obtained in a 0.3 M solution of oxalic acid. A reflection maximum is observed in the region of 650 nm, which leads to a red tint of the decorative coating when observed at a normal angle.

The implementation of the invention

The claimed invention allows to obtain high-quality decorative coatings on a metal surface, characterized by a color change with a change in the viewing angle. The technology for producing such coatings is based on the electrochemical oxidation (anodization) of a metal surface (for example, a metal foil or substrate), in which a one-dimensional photonic crystal is formed, characterized by the presence of a porous structure with many layers with alternating pore sizes (Fig. 1). Before starting the anodizing process, the surface of the metal substrate is cleaned by any methods known in the art. Then carry out its alignment and polishing also by any methods known from the prior art, providing a surface with a roughness not exceeding 100 nm. To obtain coatings with high color uniformity, the roughness should not exceed 10 nm. It is essential to use a metal substrate during the anodizing process to form a coating on it, and the cathode can be stainless steel, aluminum or any other material inert in the used electrolyte solution, for example, carbon, gold or platinum.

, где q - плотность заряда [Кл/см²], j - плотность тока [А/см²], t - время [с]. Существенным в обеспечении определенного режима анодирования на второй стадии является уменьшение скорости подъема напряжения от 5 В/с до 0 В/с. Существенным также в процессе анодирования является условие обеспечения соотношения максимального напряжения на второй стадии к минимальному напряжению на первой стадии - более 1,4. Важным для получения высококачественного покрытия является и изменение продолжительности каждого цикла процесса анодирования - заряд на первой и второй стадиях сокращают на 0,01-10% на каждом последующем цикле анодирования. Количество циклов может лежать в интервале от 20 до 300 - в зависимости от насыщенности цвета, которую необходимо получить. На цвет декоративного покрытия влияет общий заряд, затрачиваемый на анодирование на каждом цикле q=q₁+q₂, где q₁ - плотность заряда на первой стадии анодирования и q₂ - на второй. Увеличение q приводит к сдвигу фотонной запрещенной зоны в сторону больших длин волн, и как следствие изменение цвета покрытия в сторону от фиолетового к красному. Увеличение соотношения q₁/q₂ при сохранении общего заряда (q=q₁+q₂) неизменным может приводить к увеличению коэффициента отражения в области фотонной запрещенной зоны, но при этом продолжительность анодирования увеличивается. Условия формирования декоративных покрытий различных цветов включают приведенные в таблице 1 режимы анодирования, но не ограничиваются ими.A high-quality coating is formed subject to a certain anodizing regimen, which has a cyclic nature. The nature of the ongoing changes in current and voltage is significant - from low values of current density and voltage to high values within a single cycle (Fig. 2). Each cycle consists of two stages: at the first stage, anodization is carried out with stabilization of the current in the range from 0.1 to 50 mA / cm ² for a time that ensures the flow of charge from 0.05 to 5 C / cm ² ; at the second stage, anodizing is carried out with voltage stabilization, increasing it from the voltage value at the end of the first stage to a value lying in the range from 10 to 200 V, and maintaining this value for a time that ensures the flow of charge from 0.05 to 5 C / cm ² . Note that the charge flowing per unit time depends on the magnitude of the electric current [Hamburg Yu.D. Electrochemical crystallization of metals and alloys. Janus-K. 1997]:

where q is the charge density [C / cm ² ], j is the current density [A / cm ² ], t is the time [s]. It is essential to ensure a certain anodizing mode in the second stage that the voltage rise rate decreases from 5 V / s to 0 V / s. Also essential in the anodizing process is the condition for ensuring the ratio of the maximum voltage in the second stage to the minimum voltage in the first stage - more than 1.4. Important to obtain a high-quality coating is the change in the duration of each cycle of the anodizing process - the charge in the first and second stages is reduced by 0.01-10% in each subsequent anodizing cycle. The number of cycles can lie in the range from 20 to 300 - depending on the color saturation that needs to be obtained. The color of the decorative coating is affected by the total charge spent on anodizing at each cycle q = q ₁ + q ₂ , where q ₁ is the charge density at the first stage of anodization and q ₂ is at the second. An increase in q leads to a shift of the photonic band gap toward longer wavelengths, and as a result, the color of the coating changes from violet to red. An increase in the ratio q ₁ / q ₂ while maintaining the total charge (q = q ₁ + q ₂ ) unchanged can lead to an increase in the reflection coefficient in the photonic band gap, but the anodization time increases. The conditions for the formation of decorative coatings of various colors include, but are not limited to, the anodizing modes shown in Table 1.

The specular reflection spectrum at an angle of incidence of light close to normal for the yellow-green decorative coating formed by the above method is shown in FIG. 3. It is important to note that the reflection coefficient in the region of the photonic band gap exceeds 90%, which indicates the extremely high quality of the formed one-dimensional photonic crystal. A high reflection coefficient in the photonic band gap leads to a bright color of the decorative coating.

It should be noted that the color of the coating changes when the plane of the coating is rotated relative to the observer (see Table 2), which in the specular reflection spectra is expressed in the shift of the position of the photonic band gap (see Fig. 4). This feature of the decorative coating, formed by the proposed method, can be used in various design decisions in its practical use.

The color saturation of the coating increases with increasing number of layers forming a porous structure. So the coating, consisting of 300 layers (example 1, sample 10, the specular reflection spectrum is shown in Fig. 5), has a saturated blue color when observed at a normal angle passing through violet to black when the viewing angle changes from normal to the direction along the plane of the sample . In addition to increasing the number of layers in the structure of the decorative coating, an effective way to increase the color saturation is to create an absorbing layer (see example 3, Fig. 6 and 7). Moreover, to reduce the total duration of anodic oxidation of the metal during the preparation of decorative coatings, it is rational to obtain films consisting of 50-100 layers for which the reflection intensity in the region of the photonic band gap under optimally selected anodizing conditions exceeds 80-90%.

As electrolytes, aqueous or aqueous-organic solutions of acids can be used, including sulfuric, orthophosphoric, selenic, hydrofluoric, oxalic, malonic, succinic, acetic, hydroxyethylidene diphosphonic acids and mixtures thereof. The best result is achieved when an electrolyte with an acid concentration of 0.1 to 5 mol / l is used in the anodizing process, at a temperature of from -20 ° C to + 40 ° C for aqueous-organic electrolytes, or at a temperature of from -5 ° C to + 40 ° C for aqueous electrolyte solutions.

The optimum temperature range for anodizing depends on the composition of the electrolyte: the stronger the acid and its concentration, the lower the recommended temperature of the electrolyte. For strong acids, such as sulfuric or selenium, it is preferable to use low temperatures around 0 ° C in aqueous solutions of electrolytes or negative temperatures when using electrolytes based on aqueous-organic mixtures. Electrolyte freezing is not allowed. On the contrary, the temperature of low concentration solutions of weak acid electrolytes, such as oxalic, malonic and hydroxyethylidene diphosphonic, can be increased to several tens of degrees Celsius.

It is allowed to use a mixture of acids as an electrolyte with a concentration ratio of up to 1: 1. In this case, it is preferable to use an electrolyte based on one acid rather than a mixture of acids, since a simpler composition facilitates the process of adjusting the concentration of electrolyte components during its long-term use.

When using dilute solutions of acids as an electrolyte, as well as during the anodization process at low temperature after the formation of a one-dimensional photonic crystal from anodic metal oxide, pore diameter is increased by chemical etching in acid solutions or mixtures thereof at room or elevated temperature. An increase in the diameter of the channels can lead to a substantial increase in the light reflection coefficient in the region of the photonic band gap (Fig. 9) and, as a result, to an increase in the color intensity. On the other hand, an excessive increase in the diameter of the channels can degrade the mechanical characteristics of the resulting coating. The degree of increase in the diameter of the channels can reach up to 2 times, compared with the initial diameter. The final value of the increase in the diameter of the channels is determined by the optimal ratio of mechanical and optical characteristics. Depending on the initial diameter of the channels and the concentration of the etching agent, the etching time can be up to 5 hours. In this case, solutions of various acids and / or mixtures thereof, for example, sulfuric, phosphoric, oxalic, acetic and other acids with a concentration of from 0.1 to 2 mol / l, can be used. The preferred solution temperature for chemical etching is in the range of 20 to 60 ° C.

In possible embodiments of the invention, the formed coating may contain additional layers formed on the surface of the porous layer of the photonic crystal, for example, performing auxiliary functions, including protective, and / or strengthening, and / or anti-reflective, and / or absorbing, and / or antibacterial, and / or hydrophobic and / or oleophobic.

The most important of the above layers are the absorbing layer, with which you can achieve enhanced color (see example 3 and Fig. 7), and protective - prevents the penetration of liquids into the channels of the oxide film (see example 2). The formation of a protective layer is important for the following reasons.

When a one-dimensional photonic crystal is made from the anodic oxide of the metal to be treated by anodizing on the metal surface, the pores in its structure are oriented mainly perpendicular to the surface of the decorative coating and are open from the upper side.

The position of the photonic band gap, which determines the color of the decorative coating, can be estimated using the Bragg-Snell law:

where n _eff is the effective refractive index, d is the periodicity of the structure, Θ is the angle of incidence of light on the sample (measured from the normal to the film), λ _i is the wavelength corresponding to the maximum of the reflection band, m _i is the order of reflection. In turn, the effective refractive index of a photonic crystal is calculated by the formula:

where n _Ox is the refractive index of the material of the walls of the porous film of the anodic oxide, n _air ≈1 is the refractive index of air, and ƒ _Ox is the volume fraction of the space occupied by the anodic oxide.

Note that since the pores are open from the upper side after anodizing, liquid can penetrate into them, for example, in the form of raindrops when using a decorative coating for construction purposes. According to formula (2), the effective refractive index will increase, since the refractive index of the liquid is greater than the refractive index of air. In turn, according to formula (1), this leads to a shift of the photonic band gap towards longer wavelengths - a color change in the direction from violet to red. On the one hand, this effect can be used in original design solutions, but on the other, it can negatively affect the operational properties if it is necessary to obtain a constant coating color.

In order to prevent the above effect, a continuous protective layer is formed on the surface of the decorative coating from a material that is transparent in the visible spectrum and can be either polymer organic (polyvinyl chloride, polypropylene, polyester, polyethylene, fluoroplastic, etc.) and inorganic materials (Al ₂ O ₃ , SiO ₂ , etc.).

Preferably, the protective layer is formed by lamination. Note that the lamination technology is well established in the industry, in particular to protect printed products from various external influences. When laminating the decorative coating formed by the proposed method, various materials can be used, for example, polypropylene, polyvinyl chloride, polyester, etc. When coating the outer surface of the decorative coating with a protective layer, the latter can partially penetrate the channels of the oxide film (see Fig. 6).

The protective layer can be formed in other ways, including, but not limited to:

1) by vacuum deposition of a continuous transparent layer of Al ₂ O ₃ or SiO ₂ ;

2) by applying a transparent varnish by spraying it onto the surface of the workpiece. Any brand used in the automotive industry is suitable.

The best parameters of the intensity of the resulting coating are provided in the case when, after the formation of the one-dimensional photonic crystal, an absorbing layer is additionally formed located under the photonic crystal.

It should be noted that the formation of the absorbing layer can significantly enhance the color of the decorative coating by increasing the ratio of the useful signal / background. Coatings obtained during even 25 cycles with an absorbing layer have an intense color, while without it they are almost transparent.

The absorbent layer can also be formed in other ways. For example, by staining with dark paint. The best effect is achieved when using black paint. Various types of paints are suitable for coloring, for example, those used in the automotive industry. To achieve the best effect, the thickness of the paint layer should exceed 3 microns.

It is also possible to apply by lamination or stick an absorbent coating in the form of a black film with a thickness of more than 10 microns on one side of the photonic crystal. In the case of the use of glue for fixing the absorbing layer, it is necessary that it does not lose its properties under the conditions of further operation of the decorative coating.

Note that it is possible to simultaneously create several additional layers that improve the properties of the decorative coating. In particular, in FIG. Figure 6 shows an example of a decorative coating structure that simultaneously contains a protective layer to prevent liquids from entering the channels of the porous oxide film (Example 2) and an absorbing layer to increase the color intensity (Example 3).

In addition to the above two types of functional layers, depending on the application of the decorative coating, additional micro- and nanostructured polymer layers and nanocomposites based on them can be applied, which provide strengthening, antibacterial, anti-reflective, hydrophobic or oleophobic functions or combinations thereof.

The following are examples of specific embodiments of the invention, which illustrate the invention, but in no way limit its scope.

Example 1

Decorative coatings of various colors on the surface of aluminum were obtained as follows.

At the preliminary stage, aluminum plates (0.1 mm thick, aluminum content of at least 99.99%) were subjected to electrochemical polishing in a mixture containing 185 g / l CrO ₃ and 878 ml / l H ₃ PO ₄ at a temperature of 80 ° C and vigorous stirring. The Al plate, subjected to polishing, acted as the anode, and a similar aluminum plate, which exceeded the size of the anode by 2–10 times in area, acted as the cathode. The process was carried out in a pulsed galvanostatic mode at a current density of 400 mA / cm ² . The pulse length was 3 seconds, the interval between pulses was 40 seconds. The duration of electrochemical polishing was limited to 30 cycles. After polishing Al, the plates were washed with water to remove an electrolyte solution from their surface. Polishing provided Al plates with an average roughness R _{a of} not more than 10 nm.

The prepared aluminum plate was placed in an electrochemical bath. Solutions of sulfuric and oxalic acids served as the electrolyte (see table. 1). During anodization, the electrolyte was intensively mixed, and its temperature was maintained at a constant value (see table. 1). To form an oxide film with variable porosity, which subsequently acts as a photonic crystal reflecting light of certain wavelengths, the anodization method was used, cyclically changing the current density and voltage. Moreover, each cycle consisted of two stages: at the first stage, anodizing was performed in the mode of current stabilization, and at the second stage, voltage stabilization. At first (stage 1, Fig. 2), anodization was carried out in the galvanostatic mode at a current density of j ₁ until a charge q ₁ flowed (see Table 1). Then (stage 2, Fig. 2) the anodizing voltage was increased to U ₂ with a speed of ν ₂ and maintained at this value until the charge q ₂ flows (see table 1). The charges q ₁ and q _{2 were} counted from the beginning of the corresponding stage and were found by numerical integration of the current versus time q = (∫I (t) dt) / S, where q is the charge density, I is the current strength, t is time, S is the area the surface of the metal being anodized. To form coatings of different colors, the parameters q ₁ and q _{2 were} changed. Their values for the formation of decorative coatings of various colors are shown in Table 1. The charges of the corresponding stages for each subsequent anodizing cycle were reduced by 0.05-10%, depending on the composition of the electrolyte and its temperature (see Table 1). The number of anodizing cycles is shown in table 1.

After completion of the anodization process, aluminum with a porous oxide film on its surface was removed from the electrochemical bath, washed with water, and dried in air. The structure of the formed oxide films on the surface of aluminum is shown in FIG. 1. The decorative coating has a structure with variable porosity, changing quasiperiodically in the direction normal to the surface of the coating, while the pores are oriented mainly perpendicular to the surface of the decorative coating. The layered structure is formed due to different anodizing voltages at successive stages of the electrochemical oxidation process. The ratio of the average voltage in the second stage and the corresponding value in the first stage for all examples exceeded 1.5. Note that when anodizing aluminum at high current densities (more than 15 mA / cm ² ), intensive cooling of the electrolyte is required to maintain its temperature at a constant value to prevent breakdown of the oxide film.

The color of the resulting decorative coatings at a normal angle is shown in table 1. At the same time, it changed (see table 2) when the sample was rotated, which is associated with the diffractive nature of the appearance of the color.

Example 2

A colored decorative coating with a protective layer on its surface (the circuit is shown in Fig. 6) was formed as follows.

First, a one-dimensional photonic crystal was formed on the metal surface using anodization method from the anodic oxide of the metal to be treated (see Example 1). In this case, the pores formed during the anodic oxidation of the metal in metal oxide-dissolving electrolytes are oriented mainly perpendicular to the surface of the decorative coating and are open from the upper side.

Then, a transparent polyvinyl chloride film 500 microns thick was applied to the surface of the obtained crystal by lamination. After applying a polymer film to the surface of the decorative coating when it was dipped in water, the color did not change.

Example 3

A decorative coating on the surface of aluminum with enhanced color was obtained as follows.

First, a one-dimensional photonic crystal was formed from the anodic oxide of the metal to be treated on the metal surface by anodization (see Example 1). Then, to reduce the reflectivity of aluminum, the metal was deposited by cathodic reduction in the lower part of the channels of the oxide film (closer to the metal) (for the scheme, see Fig. 6). A solution containing 18 g / l SnSO ₄ and 20 g / l H ₂ SO _{4 was used} as an electrolyte for metal deposition. The deposition was carried out in a two-electrode configuration by increasing the voltage at a speed of 0.25 V / s from 0 to 6 V and then maintaining it at a constant voltage for 300 s. As a result, the channels in the lower part of the oxide film were filled with nanostructured tin, which is black due to the small particle size. In the specular reflection spectra, the presence of an absorbing layer of nanostructures in the channels of the oxide film is manifested in a significant decrease in the background intensity (see Fig. 7). This, in turn, leads to an increase in the signal / background ratio, and, as a result, to an increase in color. We note that the sample of the decorative coating after anodizing had only a slightly noticeable greenish tint, while after electrodeposition of tin a saturated green color was observed at a normal angle.

Example 4

A decorative coating on the surface of the valve metal alloy was prepared as follows.

Pre-polished according to the method described in example 1, the surface of the D16T alloy (Al content 90.9-94.7%) was anodized in a 0.3 M sulfuric acid solution. Stainless steel was used as the cathode. During the anodization, the electrolyte was intensively mixed, and its temperature was maintained in the range from 0 to 3 ° C. To form an oxide film with variable porosity, which subsequently acts as a photonic crystal that reflects light of certain wavelengths, the current density and voltage were cyclically changed during the anodization process. Moreover, each cycle consisted of two stages: at the first stage, anodizing was performed in the mode of current stabilization, and at the second stage, voltage stabilization. First (stage 1), the anodization was carried out in the galvanostatic mode at a current density of 0.35 mA / cm ² until a charge q ₁ = 225 mC / cm ² flows. Then (stage 2) the anodizing voltage was increased to 32 V at a speed of 0.4 V / s and held at that value until a charge q ₂ = 225 mC / cm ² flowed. The ratio of the average voltage in the second stage and the corresponding value in the first stage was 1.8. The charges q ₁ and q _{2 were} counted from the beginning of the corresponding stage and the numerical integration of the time dependence of the current was found. To form coatings of different colors, the parameters q ₁ and q _{2 were} changed. The charges of the corresponding stages for each subsequent anodizing cycle were reduced by 0.1%. The number of anodizing cycles was 80.

After the completion of the anodization process, the alloy with a porous oxide film on its surface was removed from the electrochemical bath, washed with water, and dried in air.

The specular reflection spectra at various angles of incidence of light on the surface of the decorative coating are shown in FIG. 8. It is easy to see that with an increase in the angle of incidence of light, the reflection maximum shifts toward shorter wavelengths, which is accompanied by a change in color from orange to green.

To increase the color intensity, an additional chemical etching of the oxide film was carried out in a 1 M solution of sulfuric acid for 60 minutes at a temperature of 25 ° C. It is clearly seen that an increase in pore diameter leads to a significant increase in the reflection intensity in the photonic band gap and a shift in its position toward shorter wavelengths (Fig. 9). In turn, this is expressed in an increase in the color saturation of the coating and its change from orange to yellow when observed at a normal angle to the surface.

Example 5

A decorative orange-red coating on the surface of aluminum was prepared as follows.

Pre-polished according to the procedure described in example 1, the aluminum surface was anodized in a 0.3 M solution of oxalic acid (H ₂ C ₂ O ₄ ). A platinum wire was used as a cathode. During the anodization process, the electrolyte was intensively mixed, and its temperature was maintained in the range from 5 to 10 ° C. To form an oxide film with variable porosity, which subsequently acts as a photonic crystal that reflects light of certain wavelengths, the current density and voltage were cyclically changed during the anodization process. Moreover, each cycle consisted of two stages: at the first stage, anodizing was performed in the mode of current stabilization, and at the second stage, voltage stabilization. First (stage 1), the anodization was carried out in the galvanostatic mode at a current density of 0.35 mA / cm ² until a charge q ₁ = 214 mC / cm ² flows. Then (stage 2) the anodizing voltage was increased to 60 V at a speed of 1 V / s and kept at this value until a charge q ₂ = 214 mC / cm ² flowed. The ratio of the average voltage in the second stage and the corresponding value in the first stage was 1.5. The charges q ₁ and q _{2 were} counted from the beginning of the corresponding stage and found by numerical integration of the current versus time. The charges of the corresponding stages for each subsequent anodizing cycle were reduced by 0.1%. The number of anodizing cycles was 80.

After the completion of the anodization process, the alloy with a porous oxide film on its surface was removed from the electrochemical bath, washed with water, and dried in air.

The spectrum of specular reflection when light is incident on the surface of the decorative coating at an angle close to normal is shown in FIG. 10. The pronounced reflection maximum at 660 nm determines the orange-red color of the coating at a normal angle.

Claims (9)

1. The method of obtaining a decorative coating with a changing color when changing the viewing angle, which consists in the formation of a one-dimensional photonic crystal with a photonic band gap in the visible range by anodizing the surface of a valve metal or an alloy based on it with a valve metal content of at least 50% with cyclically changing parameters , characterized in that the current and voltage are used as cyclically varying parameters, and each cycle consists of two stages: at the first stage of anodizing tion is carried out at constant current in the range of 0.1 to 50 mA / cm ² for a time that ensures the flow of charge from 0.05 to 5 C / cm ^2; in the second stage, anodizing is carried out with voltage stabilization, increasing it from the voltage value at the end of the first stage to a value lying in the range from 10 to 200 V, with a decreasing voltage rise rate from 5 V / s to 0 V / s, and withstand value over time, providing a charge flow from 0.05 to 5 C / cm ^2, the ratio of the maximum voltage providing the second step to the minimum voltage in the first stage of more than 1.4, the metal surface during the preparation of the decorative coating is as the anode and as the cathode is an inert material, wherein the anodization charge on the first and second stages to reduce 0.01-10% in each subsequent cycle of the anodization, the number of which ranges from 20 to 300. 2. A method of obtaining a decorative coating according to claim 1, characterized in that aluminum, titanium, zirconium, hafnium, niobium, and tantalum are used as the valve metal. 3. The method of obtaining a decorative coating according to claim 1, characterized in that the anodizing is carried out in electrolyte-dissolving electrolytes, such as aqueous or aqueous-organic solutions of acids, including sulfuric, orthophosphoric, selenic, hydrofluoric, oxalic, malonic, succinic, acetic, hydroxyethylidene diphosphonic acid and mixtures thereof. 4. A method of obtaining a decorative coating according to claim 3, characterized in that the anodizing is carried out in electrolytes with an acid concentration of from 0.1 to 5 mol / L. 5. A method of obtaining a decorative coating according to claim 3, characterized in that the anodizing is carried out at a temperature of from -20 ° C to + 40 ° C using aqueous organic electrolytes or at a temperature of from -5 ° C to + 40 ° C when using aqueous solutions of electrolytes. 6. A method of obtaining a decorative coating according to claim 1, characterized in that after the formation of a one-dimensional photonic crystal from an anodic metal oxide with a porous structure, the pore diameter is increased by chemical etching. 7. The method of obtaining a decorative coating according to claim 1, characterized in that after the formation of a one-dimensional photonic crystal from an anodic metal oxide with a porous structure, additional layers are applied to its surface, performing auxiliary functions, including protective, and / or strengthening, and / or anti-glare and / or absorbing and / or antibacterial and / or hydrophobic and / or oleophobic, and these layers can completely or partially penetrate the channels of the porous structure of the photonic crystal. 8. A method for producing a decorative coating according to claim 1, characterized in that after the formation of a one-dimensional photonic crystal from an anodic metal oxide with a porous structure, an additional layer is formed under and / or in the lower part of the photonic crystal that absorbs the light transmitted through the photonic crystal. 9. The method of obtaining a decorative coating according to claim 8, characterized in that for the formation of the absorbing layer, electrochemical deposition of nanostructures is carried out in the channels of the porous structure of the photonic crystal, and / or the deposition of a dark layer on one of the surfaces of the photonic crystal, and / or metal removal, on the surface of which a photonic crystal is formed.

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