Coloristic properties of decorative copper coatings applied by cold gas dynamic spraying

. The results of the coloristic properties research of art items with copper coatings applied by cold gas-dynamic spraying (CGDS), as well as the color determination technique, allowing to improve the approach to the decoration and restoration of art items, are presented. This allows us to study the quantitative properties of the color of decorative surfaces, which largely determines the quality and aesthetic properties of coatings and their subsequent decorative treatment.


Introduction
The purpose of the experimental study is to study, systematize and develop a method of evaluating the coloristic properties of decorative copper coatings applied by the CGDS method on samples of SCh15 gray cast iron.
Problems solved in the research: 1) Research of coloristic properties of decorative copper coatings, which together form complete visual images and are aimed at improving the aesthetic qualities of objects of environmental design.
2) Research of complex methods of quality evaluation of copper decorative coatings.
3) Development of technology for evaluating decorative coatings applied by CGDS method, their subsequent processing.
Scientific novelty lies in the formation of evaluation methods and the study of an alternative procedure for color measurements regarding copper coatings applied by CGDS method.

Assessment of the method effectiveness under research
Cold Gas Dynamic Spraying (CGDS) of metal coatings is a process in which a metal coating is formed by collision of cold metal particles accelerated to supersonic speed against a substrate. More detailed information on cold spraying was presented in the monographic literature, namely in the first monograph [1] and in two monographs published later [2,3].
As a result of the studies conducted by A. P. Alkhimov, V. F. Kosarev, S. V. Klinkov and others various engineering solutions were proposed to realize the optimal selection of working gas, the most suitable sizes and shapes of powder particles, the optimal spraying strategy (number of passes, speed of nozzle movement, etc.), the spray angle and preheating of the powder. The basic, most general requirements for the process of deposition of unmelted metal particles by gasdynamic spraying are presented in [4].
The cold method is called because the temperature of the particles on impact, obtained from the conversion of kinetic energy into thermal energy, and the temperature of the substrate are an order of magnitude lower than their melting temperature; on impact, plastic deformation of the particles occurs. Thus, we obtain a formed solid layer of densely packed metal particles and get rid of the side effects of high temperatures, such as oxidation of particle and substrate materials, unbalanced crystallization and high internal stresses in the machined parts.
The method has several advantages, such as: 1. The particles are transferred to the substrate in a relatively "cold" state at a speed of up to 1100 m/s. Such a small effect of temperature on the substrate does not lead to deformation of the product; 2. Particles are heated directly in contact with the substrate by converting kinetic energy into thermal energy; 3. Ability to obtain coatings that are completely identical to the composition of the sprayed material; 4. Cost-effectiveness, convenience and safety of the process. Copper, due to its reflectivity increasing towards red light, has a characteristic red color, which takes on a pink, muted, soft hue on matt surfaces. Polished copper has a brighter color and luster. This gives it unique decorative qualities and provides a wide application in the artistic processing of metals.
When considering the formation of protective and decorative coatings obtained by cold gas-dynamic spraying in the open air it is necessary to consider the effects of formation of copper oxides, which forms a certain way under pressure of the powder particles into a layer of a given thickness. During formation of the coating the particles deform, melt and crystallize. In general, the coating is a layered material consisting of particles crystallized at a very high rate. The difference between the coating properties and those of the original material is due to the complex processing of the particles during sputtering, namely their heating and successive deformation and layering upon collision with the substrate.
Today, copper is widely used in the production of artistic products mainly because of its plastic and decorative properties.
Along with deformability and corrosion resistance to environmental conditions [5], the decorativeness of copper is due to the color palette of the patina formed on its surface. A more accurate characterization of the appearance and color of the patina has become possible with the use of spectrophotometry, by which surface coloration can be measured in a more quantitative way [6][7][8][9] The chemical composition and color of the patina depends on the composition of the environment and forms a stable and durable protective layer that insulates the metal from the outside environment. Patina on copper protects the metal from further degradation. Tinting and patination of copper and copper alloys have been the subject of systematic analysis [10].
One of the important coloristic properties of a metal product are color and luster, capable of evoking visual sensations according to the wavelength of light emitted or reflected.
The direction of light reflection, together with the roughness of the surface, determines the brilliance of the metal surface [11]. Reflection is specular when the angle of incidence of the light wave is equal to the angle of reflection, and diffuse when light is reflected in different directions

Materials and methods
The basis of the research is to determine the gloss, color, and roughness of the examined surfaces. For this purpose samples of SCh15 gray cast iron, without/coating formed from powder copper C-01-01, which is supplied by Obninsk Center of Powder Spraying (TU 1793-021-40707672-00), were specially made.
Coating spraying was performed on special equipment manufactured by Obninsk Center of Powder Spraying DIMET403. Dust protection and environment protection during work with DIMET403 equipment in enclosed spaces is ensured by means of special workplace organization.
In order to expand the color range of sprayed copper coatings, 6 types of chemically active compositions used in practice in the artistic processing of metals were selected, which are presented in Table 1. The exposure time and temperature of solution heating were chosen as variable parameters.
The results of the effect of compositions on the color of copper sprayed coatings, depending on the exposure time and surface roughness are presented in Table 1.  The reflectivity of the surface is one of the characteristics of the coloristic properties, which is determined by the roughness of the coating. The evaluation of the reflectivity was carried out element by element, the individual parameters of which were determined by the styloscope method.
For this research was used profilograph-profileometer type 252, designed to measure in the laboratory conditions of roughness and sinuosity of surfaces, the cross-section of which in the measurement plane represent a straight line. The device used an inductive transducer that allows you to record the profile of the roughness in an enlarged scale as a profileogram or measure the roughness parameters in digital form on the scales of the instrument.
A Gretag Macbeth Spectroeye spectrophotometer, shown in Figure 1, was used to measure color, which allowed coloristic evaluation of laboratory samples under a constant daylight source D65.

Results and analysis
As a result of roughness measurements, the surface finish class of the samples and their roughness value were determined. The data are entered and presented in table 2. The results of determining the gloss (Table 3) allow us to state that when spraying copper with CGDS the gloss of the samples' surface averages 2%, which corresponds to the deeply matte surface class according to GOST 9.032-74.
Since the accuracy of measurements does not allow to build a clear picture of the dependence of gloss on surface roughness at the interval Ra from 3 to 5 microns using interpolation of measurement results revealed an obvious dependence of gloss on roughness (Fig. 2).  The type of histograms allows us to assume that the values of brilliance are more subject to changes against the background of changes in roughness. In this case, the selection of roughness, with a verified pattern, will allow us to predict and achieve the simulated value of gloss with high accuracy.
Based on general information on the concept and understanding of color [12][13][14], specific results were presented on color measurements of copper patina [15]. Measurements were made in RGB color models, defining color value according to three components from which as a result of mixing it is possible to achieve the result reflecting any color of visible spectrum.
According to the results of measurements, it was found that depending on the technology of obtaining the substrate, the color of chemically active solutions gives different color coordinates with a color difference from 2.18 to 12.51. This means that the effect of chemically active solutions on the color of sprayed coatings is significantly different from the effect of similar solutions on the color of cast surfaces (table 3). Table 3. Color difference between sprayed coatings and cast surfaces as a result of applying chemically active solutions at surface roughness Ra = 2-3 microns. The ColorGrab app available on PlayMarket platform was used as an alternative method of color measurements in the field. The Redmi Note 8 Pro smartphone with Android 10 operating system QP1A.190711.020 was used as the device with which the shooting and measurements were made.
Optimal conditions were developed to make measurements on all available samples. Table 4 shows the measurement results for the spectrophotometer and the smartphone, the nature of the deviations suggests that the measurement results on the smartphone lend themselves to a single correction, allowing to adapt the result with greater accuracy.
The analysis of the results showed that with the given coloristic characteristics of the copper powder coatings, the possibility to accurately design products with different decorative properties increases. The techniques used to evaluate the properties of copper powder coatings are also applicable to other metal surfaces.

Results and discussion
The main results obtained during the experiment: -to assess the decorative properties, the parameters of the coating, directly affecting its visual perception, were identified. Among these are gloss, surface color, reflectivity and roughness value. These parameters were considered and evaluated on cast iron surfaces; -the dependence of the gloss value on the roughness of powder copper coating, taking into account the chemical activity in atmospheric conditions, as well as treated with patination composition was revealed; -quantitative determination of chromaticity coordinates makes it possible to reliably determine the color when designing and restoring products with specified color parameters; -well-known recipes of patinating and coloring compositions were tested, by adjusting the soaking time and concentration of which, it is possible to achieve different effects on copper coatings obtained by cold gas-dynamic spraying. Thus, it becomes possible to combine the manufacturability of any metal and the noble shades of bronze in one artistic product.

Conclusion
As a result of this experimental research, the possibility of using an alternative method of color measurement, which involves mobile measurements in the field, having only a smartphone from the equipment, which is very convenient in today's realities. To adapt this method, it is necessary to develop measures to create standard conditions for measurements, as well as the ability to reproduce the required conditions. The primary sample of this method gives results comparable with those obtained with a spectrophotometer. At the same time, the nature of the deviations suggests that the results from the smartphone can be corrected, interpolated, and then serve as a valid alternative to the spectrophotometer measurements.