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Section 3. Materials Science
Saydakhmedov Ravshan Khalhodjaevich, Tashkent State Technical University, Professor, E-mail: ravshansaid@mail.ru Kadirbekova Kutрinisa Karimovna, Tashkent State Technical University, Associate professor, E-mail: niso_51@mail.ru Bakhadirov Kudratkhon Gayratovich, Tashkent State Technical University, Senior teacher, E-mail: bahadirov@gmail.com
INVESTIGATION OF THE STRUCTURE, DISPERSION, AND PROPERTIES OF NANOSTRUCTURAL COATINGS BASED ON CHROME MODIFIED BY MAGNETRON SPUTTERING
Abstract: In the article nanostructured coatings based on chromium, formed by magnetron sputtering with preliminary surface treatment by ion source have been investigated. The thickness, adhesion strength and of chrome coatings were determined. It was found that, depending on the formation regimes, the coatings consist of crystallites with dimensions from 45 nm to 200 nm.
Keywords: Chromium coatings, magnetron sputtering, adhesion strength, corrosion resistance, thickness coatings, nanostructures.
Introduction
Recently, interest in the study of materials with nanocrystalline structure has been increased, since the reduction of crystal size below a certain point, etc. to drive a radical change in physic-chemical properties of these materials. The most significant change of properties of nanomaterials e reaches in the range of crystallite sizes up to 100 nm. Thin films and coatings can be obtained by PVD and CVD methods. Thus, well-known coating carbide and titanium nitride on the beam are ionplasma deposition of it, which leads to the formation of nanocrystalline structure [1, 2].
Coatings based on chromium and chromium nitride used in industry as a solid thin film to protect the items and possessing excellent wear resistance, high hardness,
sufficient strength, good adhesion to the substrate, good corrosion resistance and heat resistance up to 600 °C [3-6]. In the industry galvanic coatings of chromium are widely used on steels and alloys, characterized by high chemical and mechanical resistance, however they are obtained by environmentally harmful chemical technology, requiring it to be treated with sewage treatment plants. When chromium plating and electroplating using toxic hexavalent chromium, as well as to a significant tensile stress which lead to the appearance of the grid of cracks in the coating immediately after precipitation [7, 8].
In a number of cases, it is advisable to use vacuum methods for setting chromium coatings magnetron sputtering. Due to the low deposition temperature of coatings by magnetron sputtering, it is possible to form them
on non-metallic materials, in particular on the reflecting surfaces of the headlamps and as decorative coatings on other parts of cars.
Currently, the automotive industry uses aluminum alloys to produce reflective and decorative coatings. Coatings based on aluminum do not meet the requirements for their reflective and corrosive properties. A chromium-based coating can be used successfully as a reflective coating in the automotive industry. Coatings based on chromium nitride are solid e coatings which can successfully apply abrasion I e coating to protect wear surfaces her parts and tools.
In [9] investigated multilayer based coating and chromium nitride and carbo-nitride and chromium. Multi-layer alternating layers of coating based on CrN/CrCN are formed by the PVD (CAD - cathode arc deposition). It has been experimentally established that the wear resistance of multilayer coatings based on chromium nitride and chromium carbonitride is higher than the individual coatings CrN and Cr CN. A sublayer of chromium 0.1^m thick was used as a sublayer.
Composition, microstructure and residual stresses, as well as the effects ofchromium coatings obtained by PVD (Physical Vapor Deposition) studied in ah [10, 11]. CrN coating, are obtained on a titanium substrate by physical vapor deposition (PVD) with enhanced capabilities.
Proceeding from the foregoing, it is necessary to note the relevance of the investigation of the technology of formation of coatings based on chromium and chromium nitride by the ion-plasma method.
The objective of this study is to study the structure, composition, and properties of chromium-based metal and non-metallic materials formed by magnetron sputtering. Investigation of the effects of magnetron sputtering regimes on nanoparticle structures and sizes.
Methods of research
For the deposition of chromium-based coatings, a magnetron sputtering method was used, with preliminary treatment of the surface with a source of ions. Argon ions were used to treat the surface of the products. The treatment with ions was used to improve the adhesion strength of chromium plated coatings.
After obtaining a starting vacuum degree ofthe order of 10-2 Pa, the tooling with moving samples and a control plate was processed by the ion source in the following modes:
- the control of the working gas (argon) is 2-10-1 Pa;
- discharge voltage - 4,5-5,0 kV;
- current discharge ion source - 80-100 mA;
- density ofcurrent onto the surface - up to 1 mA/cm2;
- in the time of surface treatment of samples - 3 minutes.
Directly at the time of the ion source operation, a magnetron source was turned on, then the rigging was transferred to a position opposite the source of the spray. Precipitation of chromium coatings was carried out in the following technological regimes:
- The distance from the cathode to the surface of the samples - 120-130mm;
- the control of the working gas (argon) is 2-10-1 Pa;
- Voltage cathode spray - minus 550V;
- current of discharge during spraying 2.0-2.2A;
- in the time of precipitation of the chromium coating - 5-20 minutes.
The temperature of the samples during the deposition of the coatings did not exceed 150 °C. The thickness of the vacuum-deposited chromium coatings was determined from the reference silicon wafer, which was under the same processing conditions as the steel samples.
The thickness was measured on a microinterferom-eter of the MII-4 type using the standard procedure. According to the measurements made, the maximum thickness of the chromium- based coating was about 2.4^m for a settling time of 20 minutes. Studies have shown that the thickness of chromium coatings is proportional to the deposition time in the interval of 2-20 minutes.
Adhesive strength of coatings was investigated by the method of normal detachment from the surface of coatings of glued metal rods. The coated sample was placed in a cassette with vertical 7 rods (diameter of the glued part 1 mm), which were adhered to the coating and torn off using a tearing machine. Studies have shown that the adhesive strength exceeded 70-80MPa (700 kg/cm2), since the gap occurred over the glue.
The corrosion problems observed in the coating materials are usually the result of penetration of aggressive reactants through defects and their entry onto the substrate. Coating, especially multi-layer, improves corrosion resistance durability of the material. Coatings based on Cr and CrN with a dense structure and finely dispersed crystals make them less permeable for an aggressive environment. The absence of direct diffusion channels due to non-columnar structure, as a result of
which the rate of diffusion of oxygen through the coatings is significantly reduced.
Proceeding from this, we were investigated and the chemical stability of samples with chromium-based coatings in a solution of nitric and hydrofluoric acid, which He is shown on their high corrosion resistance. For coatings less than 1^m thick, the number of corrosion points (pores in the coating) was 5-10 times greater than for coatings with a thickness of 2.3-2.5^m. As the experiment shows, corrosion resistance changes with a change in the thickness of the nanostructured coating based on chromium, which implies the existence of through channels or their absence with increasing thickness of the coatings. Another important feature Strongly nanostructured chromium coatings are structurally constituent, i.e. dimensions of nanoparticles in coatings which have a significantly impact on e corrosion system "coating-substrate".
High-temperature with the oxidation resistance is one of the most attractive properties of nanocoat-ings. This property is highly dependent on, and so on from the phase and chemical composition of the film. If the continuous path along the grain boundaries from the coating surface through the entire thickness to the substrate is interrupted, it is possible to increase the oxidation resistance. This second can be achieved if the amorphous film structure [12].
Given all these features, by e-contact microscopy was investigated, and s surface morphology and particle size in the structure of chrome coatings.
Information on the surface relief and the size of the agglomerates of chromium nanoparticles was obtained by transmission electron microscopy. To study the surface, two-step Pt / C replicas were obtained [13, 14].
Results and discussion
Electron microscopic studies have shown that structures with chromium nanoparticles are observed for the samples (Fig. 1, 2). It is shown that the formation of nanostructures from Cr with the maximum number of chromium-containing phases are obtained when 1-pro-vided spherical nanoparticles formed of chromium nano particles from 45 nm to 130 nm (Fig. 1), while the sample prepared with 2-condition observed nanoparticles chromium a larger size (66-200 nm) (Fig. 2)
Conclusions
Revealed what, varying the process conditions can be controlled dimensions and volumetric quantity of nanoparticles into the coatings ofchromium, for example, at a pressure in the vacuum chamber P = 3-10-2 Pa - nanoparticle sizes in the range from 50 to 100 nm, and at a pressure P = 3-10-3 Pa - volumetric content of the nanoparticles sizes as follows: nanoparticles s with dimensions of 70 nm in coatings are present in an amount of55%, with dimensions of 120 nm - 15%, and with dimensions of 170 nm - 30%.
Figure 1. The amount of nanoparticles: 97% - 45 nm, an number of cities account for major nanoparticles of 50 nm in size and 100 nm
It has been established, with increasing thickness, the corrosion resistance of chromium coatings varies several times. With an increase in coating thickness from to 2.3-2.5^m, the corrosion resistance increases by a factor of 5-10.
On the basis of the conducted researches it is possible to draw a conclusion about the wide possibilities of using vacuum-deposited magnetron sputtering of chromium coatings instead of galvanic coatings.
Figure 2. Number of nanoparticles: 55% - 70 nm; 15% -120 nm; 30% -170 nm
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