RU2763698C1 - Method for obtaining functional-gradient coatings on metal products - Google Patents

Abstract

FIELD: protective functional-gradient coating production.

SUBSTANCE: invention relates to a method for producing a protective functional-gradient coating on the surface of metal products with high wear resistance in contact media, such as friction pairs of hydraulic motors or hydraulic pumps. First, pure nickel powder with a fraction of 20-60 microns is applied to the metal surface of the products by the method of supersonic cold gas-dynamic spraying. Then, to form a strengthening sublayer, a powder composition is applied to the formed surface, consisting of a mechanical mixture of aluminum with a fraction of 20-60 microns and nickel with a fraction of 20-60 microns in a 1:1 ratio. Additionally, 10-30 wt.% coarse corundum with a fraction of 60-80 microns. After that, an aluminum coating is formed, followed by microarc oxidation (MAO) and heat treatment.

EFFECT: invention provides a functional-gradient ceramic coating on the surface of metal products in a wide range of thicknesses from 100 microns to 5 mm, with low porosity.

4 cl, 2 ex

Description

The invention relates to the field of creating protective functionally graded coatings on the surface of metal products with high wear resistance in contact media (friction pairs of hydraulic motors, hydraulic pumps). The same invention relates to the field of materials science, mechanical engineering and chemical industry.

Known methods of protection of metals [1], which consists in the fact that ceramic coatings based on nitrides and borides of titanium, zirconium, tungsten carbides, aluminum-magnesium spinel are applied to the surface of steels. Coatings are formed by plasma spraying. At the same time, it is assumed that the creation of ceramic coatings will prevent the corrosion destruction of the metal matrix during operation at elevated temperatures. The disadvantages of the methods include the formation of thin coatings that can collapse due to cyclic thermomechanical stresses during prolonged corrosion exposure, due to a significant difference in thermal expansion coefficients (TEC) at a pronounced ceramic-metal interface.

A functional coating on a steel base [2] is formed by aluminum plasma spraying, then microarc oxidation (MAO) is carried out. The thickness of the aluminum layer, which has not undergone oxidation, is 35-65 microns. In this case, the porosity of the pre-applied aluminum layer is up to 10%. The disadvantages of the method are that the plasma spraying of aluminum leads to the formation of a porous coating. An aggressive environment, upon contact with the surface, can penetrate into the steel through the through pores of the oxidized and aluminum layer. Also at temperatures of contact interaction 400 - 600 °C At the “coating-steel” interface, the processes of diffusion of aluminum into iron actively proceed, which can lead to the formation of intermetallic compounds of the “aluminum-iron” system up to the thickness of the aluminum layer that has not undergone oxidation. The result will be the embrittlement of the coating due to the deterioration of adhesion at the "intermetallic layer-ceramic" interface.

In the method [3], it is proposed to use electric arc metallization with a supersonic speed of air outflow from the spray head of the metallizer to form a preliminary layer. Electric arc metallization is carried out using filler wire AMts-3. When implementing this method, the surface of the protected material is heated, which leads to an undesirable change in the structure and properties of this material.

The method [4] includes preliminary surface preparation, increment of this surface, mechanical processing and hardening by microarc oxidation, while preliminary preparation is carried out using cubic boron nitride with a grain size of 125-150 μm at a compressed air pressure of 0.60-0.65 MPa and processing distance 80-90 mm to surface roughness R _z =100-110 μm, the surface increment is carried out by supersonic gas-dynamic spraying, where helium at a pressure of 0.40-0.45 MPa is used as a working gas, and aluminum powder with a size of particles 110-125 microns, and MAO is carried out in a silicate-alkaline electrolyte containing 2 g/l of caustic potassium and 8 g/l of liquid glass at a current density of 26-27 A/dm ² for 70-75 minutes.

However, the use of a powder with a fraction of 100–120 μm does not make it possible to obtain coatings with minimal porosity, which significantly worsens the wear resistance of the coating. It is not economically feasible to use helium for supersonic gas-dynamic spraying of aluminum, due to its higher cost (ten times) compared to air.

The closest proposed solution can be considered the formation of a functionally graded coating, according to the patent [5]. The invention relates to the field of creating protective ceramic-matrix coatings on the surface of steels with high corrosion resistance and wear resistance in aggressive environments at contact interaction temperatures of 400 - 600 ° C due to changes in the composition and structure of their surface layers.

The method consists in the fact that pure aluminum powder with a fraction of 20-60 microns is applied to the steel surface by the method of supersonic "cold" gas-dynamic spraying. Air is used as the working gas. A composite powder consisting of 20% corundum with a fraction of 60-80 μm and 80% of aluminum powder with a fraction of 20-60 μm, reinforced with over 50% nanosized particles of corundum with a fraction of up to 500 nm. Air is used as the working gas. During spraying, accumulations of nanocorundum are formed, which fill the pores of the coating. Further, the formed aluminum hardened second layer, having a porosity of not more than 5% of the volume, is subjected to microarc oxidation in a silicate-alkaline electrolyte of the composition: sodium silicate - 9 g/l, potassium hydroxide - 2 g/l, the rest is water. The duration of microarc oxidation is 1.5-2 hours, an external ceramic oxide MAO layer is formed inside a hardened aluminum second layer with corundum nanoparticles to a certain thickness with an open porosity of not more than 10%.

The resulting ceramic matrix coating has a microhardness of about 18 GPa. The open porosity of the MAO layer is not more than 7%, the porosity of the aluminum hardened layer is not more than 3% of the total volume, the adhesion of the coating to the metal base is not less than 50 MPa.

The coating given as a prototype provides good protection. The disadvantages of the prototype include the following:

- low adhesion of the coating to the metal substrate of about 50 MPa

- uncontrolled formation of an intermetallic sublayer caused by the external conditions of a particular experiment (the temperature of the liquid metal can be a variable value);

- insufficient open porosity of the outer ceramic layer of about 7% vol.;

- liquid glass contains organic substances, the composition of which varies depending on the manufacturer, as a result, the composition of liquid glass and, accordingly, the composition of the electrolyte changes.

The technical result of the invention is the creation of a ceramic functionally graded coating on metal products in a wide range of thicknesses from 100 μm to 5 mm, having a low porosity, having in its composition an adhesive nickel layer, an intermetallic reinforcing layer of the Al-Ni system and a main strong corundum layer.

The technical result is achieved by the fact that three successive layers are applied to the metal by the method of CGDN of metal powders, which, as a result of MAO and heat treatment, forms a functionally graded ceramic coating.

The presence of these layers provides a smooth change in the coefficient of thermal expansion along the thickness of the coating, as well as wear resistance and corrosion resistance under direct exposure to aggressive contact media. The formation of a ceramic functionally graded coating is carried out by three successive technological operations: cold gas-dynamic spraying (CHD), microarc oxidation (MAO) and heat treatment.

Due to the supersonic gas flow during the implementation of CGDN, the speed of the particles is about 600 m/s. As a result of severe plastic deformation upon impact, the particles are fixed on the substrate in the solid state and at a temperature well below the melting point of the sprayed material.

When applying a given first or adhesive layer, pure nickel powder with a fraction of 20-60 microns is used.

When applying a specified second hardening layer, a powder composition is applied, consisting of a mechanical mixture of aluminum with a fraction of 20-60 microns and nickel with a fraction of 20-60 microns in a ratio of 1:1. Additionally, 10-30% wt. coarse-grained corundum with a fraction of 60-80 microns.

The mechanical mixture of metal powders is subjected to pre-mixing for 3-5 hours in a drum-type mixer for homogenization.

It has been experimentally established that during the deposition of CGDN, aluminum particles smaller than 20 μm in size are carried away from the surface, since they have a small mass and do not have sufficient kinetic energy to be fixed on a steel substrate. When using a powder with a fraction of more than 60 microns, the formed coating does not have high adhesive and cohesive strength. Coarse-grained corundum particles in the composition of aluminum powder, when they hit the sprayed steel surface, fly off from it, while cleaning it from contamination, and then in the same way eliminate the oxide layer of the newly formed coating, thereby significantly increasing its cohesion and increasing thickness. Part of the coarse-grained corundum "adheres" to the coating, increasing its strength characteristics. The introduction of corundum over 30% wt. does not lead to a significant increase in the thickness of the coating, as a result of which the utilization factor of the powder does not increase during the deposition process. The ratio of metal powders 1:1 mass. is necessary to ensure the content in the layer of 60 wt%. Ni and 40% wt. Al, which is the optimal ratio for the formation of intermetallic hardening phases. The reduced aluminum content can be explained by some entrainment of less dense aluminum particles in the gas stream during CHDP.

When applying the third specified layer, aluminum powder is applied with a fraction of 20-60 μm to carry out microarc oxidation.

In accordance with the invention, air is used as a working gas in the CGDN process.

To carry out MAO, an electrolyte based on boric acid is used, containing:

- boric acid, 20-30 g/l;

- potassium hydroxide, 3-7 g/l;

- the rest is water

This electrolyte concentration is optimal to achieve the content of corundum in the coating up to 80-90 wt%. with open porosity not more than 3% vol. [6].

The duration of microarc oxidation is 1.5-2.0 hours. As a result, an external ceramic oxide MAO layer is formed inside the hardened aluminum third layer, which has a microhardness in the range of 15–20 GPa, has an open porosity of no more than 5%, and smoothly passes into the second layer from a mixture of metal components. In this case, some residual aluminum layer can be observed that has not passed into the oxide form.

After the implementation of the MAO method, the coating is subjected to heat treatment at a temperature of 650-750 °C for one hour in air. This procedure provides the necessary and optimal conditions, due to which intermetallic structures with a porosity of no more than 2% of the layer volume are formed throughout the entire volume of the second layer, including zones with residual aluminum at the boundary with the ceramic layer. Phase analysis shows the presence of Al ₃ Ni ₂ and Al ₃ Ni with inclusions of coarse-grained corundum, while residual aluminum is not detected. It is known that Al-Ni intermetallic compounds have increased strength under contact loads at high temperatures.

At the interface "nickel - metal substrate" after heat treatment, the formation of intermetallic compounds is not detected. The adhesion of the nickel first layer to the metal substrate is at least 65 MPa.

Example 1

To obtain a functionally graded coating, stainless steel samples were prepared in the form of flat plates 50x20x0.4 mm in size.

Pure nickel powder with a fraction of 20–60 µm was evenly sprayed onto the surface of the samples by the CGDN method using a robot over a thickness of 30 µm. Air was used as the working gas. A composite powder consisting of 20 wt. from corundum with a fraction of 60-80 microns, 50% from aluminum powder with a fraction of 20-60 microns and nickel powder with a fraction of 20-60 microns.

The mechanical mixture was subjected to pre-mixing for four hours in a drum mixer.

Pure aluminum powder with a fraction of 20–60 µm was evenly sprayed onto the resulting layer by the CGDN method at a thickness of 60 µm.

Further, the resulting outer layer was subjected to the MAO process in a borate electrolyte of the composition: boric acid - 23 g/l, potassium hydroxide - 4 g/l, the rest - water. The duration of the MAO process was 1.5 hours, and an oxide layer was formed inside the hardened aluminum layer to a thickness of 50 μm.

After the implementation of the MAO method, the coating was subjected to heat treatment at a temperature of 700°C for one hour in air.

The resulting functionally graded coating has a microhardness of about 18 GPa. The open porosity of the MAO layer is no more than 3%, the porosity of the intermetallic hardened layer is no more than 2% of the total volume, the adhesion of the coating to the metal base is at least 65 MPa.

Coatings were tested for wear resistance in accordance with GOST 30480 on a friction test machine 2168 UMT according to the “coated steel ring–steel ring” scheme. The pressing pressure was 110 kPa, the movement of the coated sample relative to the counterbody was 0.306 m/s, and the run was 1.1 km.

During tribological tests of this coating, wear of the counterbody occurs, which is a consequence of its high wear resistance.

Example 2

To obtain a functionally graded coating, stainless steel samples were prepared in the form of flat plates 50x20x0.4 mm in size.

Pure nickel powder with a fraction of 20–60 µm was evenly sprayed onto the surface of the samples by the CGDN method using a robot to a thickness of 40 µm. Air was used as the working gas. A composite powder consisting of 30 wt. from corundum with a fraction of 60-80 microns, 50% from aluminum powder with a fraction of 20-60 microns and nickel powder with a fraction of 20-60 microns.

The mechanical mixture was subjected to pre-mixing for five hours in a drum mixer.

Pure aluminum powder with a fraction of 20–60 µm was evenly sprayed onto the formed layer by the CGDN method at a thickness of 70 µm.

Next, the resulting outer layer was subjected to the MAO process in a borate electrolyte of the composition: boric acid - 25 g/l, potassium hydroxide 5-g/l, the rest - water. The duration of the MAO process was 2 hours; in this case, an oxide layer was formed inside the hardened aluminum layer to a thickness of 50 μm.

After the implementation of the MAO method, the coating was subjected to heat treatment at a temperature of 700°C for one hour in air.

The resulting functionally graded coating has a microhardness of about 19 GPa. The open porosity of the MAO layer is no more than 3%, the porosity of the intermetallic hardened layer is no more than 2% of the total volume, the adhesion of the coating to the metal base is at least 65 MPa.

Coatings were tested for wear resistance in accordance with GOST 30480 on a friction test machine 2168 UMT according to the “coated steel ring–steel ring” scheme. The pressing pressure was 110 kPa, the movement of the coated sample relative to the counterbody was 0.306 m/s, and the run was 1.1 km.

During tribological tests of this coating, wear of the counterbody occurs, which is a consequence of its high wear resistance.

Sources of information:

[1] Material Behavior and Physical Chemistry in Liquid Metal Systems./Ed. by H.U. Borstedt. New York: Plenum Press, 1982, p.253-264.

[2] Patent RU 90440 U1, С23С28/00, С25D11/02.

[3] Patent RU 2417146 C1, V23R 6/00.

[4] Patent RU 2486044 C1, V23R 6/00.

[5] Patent RU 2 678 045 C1.

[6] Markov, M.A. Features of the formation of ceramic coatings by microarc oxidation in an electrolyte based on boric acid / M. A. Markov, Yu. A. Kuznetsov, A. V. Krasikov, A. A. Slobodov, A. D. Bykova, S. N. Perevislov / New refractories. - 2020. - No. 5. - P.50-55.

Claims (4)

1. A method for obtaining a protective functionally graded coating on the surface of metal products, including the formation of an aluminum coating, followed by microarc oxidation (MAO) and heat treatment, characterized in that first, pure nickel powder with a fraction of 20-60 is applied to the metal surface of the products by supersonic cold gas-dynamic spraying microns, to form a reinforcing sublayer, a powder composition is applied to the formed surface, consisting of a mechanical mixture of aluminum with a fraction of 20-60 microns and nickel with a fraction of 20-60 microns in a ratio of 1:1, while additionally 10-30 wt. % coarse-grained corundum with a fraction of 60-80 microns. 2. The method according to claim 1, characterized in that MAO is carried out in an electrolyte based on boric acid containing boric acid 20-30 g/l, potassium hydroxide 3-7 g/l and water - the rest. 3. The method according to claim 1, characterized in that the heat treatment is carried out in air at 650-750 ° C for 1-2 hours. 4. The method according to claim 1, characterized in that the reinforcing sublayer is an intermetallic layer based on the aluminum-nickel system.