RU2683177C1 - Method for plasma deposition of nanostructured heat shield - Google Patents

Abstract

FIELD: nanotechnologies.SUBSTANCE: invention relates to a method for the plasma deposition of a nanostructured heat shield. Conical nozzle is previously installed on the cut of the supersonic plasma torch nozzle, the inner surface of which forms with the inner surface of the nozzle fracture, which allows, after a break, to establish the plasma pressure with the sprayed substance in the wall part of the nozzle equal to the pressure in the vacuum chamber. Plasma torch and the substrate is installed in the chamber with reduced pressure. Dynamic vacuum in the chamber is maintained, the plasma gas and the powder of the sprayed substance is fed into the plasma torch and the substance is sprayed by a supersonic plasma flow with the formation of molten particles of micron level and vapor phase of the sprayed substance. Then, nanoparticles formed in the wall part of the nozzle and particles of micron level of the sprayed substance are deposited onto the substrate. Substrate is moved in such a way that the layers of nanoparticles and particles of micron level of the sprayed substance overlap each other.EFFECT: technical result consists in simultaneously increasing the adhesive and cohesive strength of the shield and increasing heat resistance thereof.1 cl, 5 dwg

Description

The invention relates to coating methods and can be used in plasmochemistry, in plasma metallurgy, can also find use in the engineering industry for the protection of heat-stressed components and structural elements.

A known method of applying heat-protective coatings (TZP) to protect heat-stressed components and structural components of propulsion systems from heat and erosion damage in the combustion stream of fuels containing the condensed phase of the combustion products of mixed solid fuel (1).

In the invention according to patent (2), it is proposed to spray a nichrome sublayer and a layer of a cermet composition containing a mixture of hafnium dioxide and nickel plated tungsten into the combustion chamber of liquid propellant rocket engines (LRE), then an additional layer of yttrium stabilized hafnium dioxide is sprayed.

The disadvantage of this method is the presence of W _Ni in one of the TZ layers, which at operating temperatures and the composition of the gaseous medium in the liquid propellant rocket engine can lead to melting of W _Ni and intense oxidation, and cracking of the layer.

Patent (3) protects a method for the production of TZP, which can be used in rocketry in the manufacture of CS LRE based on the composition ZrO ₂ + NiCr. The method consists in plasma spraying in the atmosphere, first a nichrome sublayer, and then spraying a cermet composition from a mechanical powder mixture containing zirconia and nichrome. The powder mixture is supplied under the nozzle section of the plasma torch in the direction of its movement relative to the sprayed surface. As a stabilizing additive in zirconia powder, calcium oxide is used.

The disadvantage of this method is that the presence of nichrome in the heat-insulating layer of the thermal protection layer significantly increases the coefficient of thermal conductivity λ, in comparison with the thermal protection layer, consisting of one ZrO ₂ , and reduces the heat resistance, which does not allow to protect the wall of the CS LRE from increased heat fluxes.

Currently, when creating promising liquid-propellant rocket engines with improved operating characteristics (pressure and temperature), the KS considers the possibilities of more effective protection of the KS fire walls from increased relative to existing heat fluxes coming from high-temperature combustion products to the KS fire walls.

The prototype of the present invention is a method for coating nanoparticles (4), in which, under dynamic vacuum conditions, a plasma with a sprayed powder flows around the wall in the form of an angle deviated from the axis of the plasma torch, and a fan of rarefaction waves is formed at the corner point with condensation of the nanoparticles from the vapor phase of the sprayed material in a plasma-forming gas and their deposition on a substrate with the formation of a coating consisting of nanoparticles.

The disadvantages of this method include the fact that the molten particles of the micron-deposited material due to the large mass do not participate in the creation of the coating, but fly past the substrate without experiencing rotation towards it in a fan of rarefaction waves.

The technical result achieved by the claimed method consists in simultaneously increasing the adhesive and cohesive strength of the coating, increasing its heat resistance, using all the sprayed material deposited on the substrate in the form of micron level particles and in the form of nanoparticles.

To ensure a technical result, the following coating method is proposed.

In the method of plasma deposition of a nanostructured heat-protective coating on a substrate, including installing a plasma torch with a supersonic nozzle and a substrate in a reduced pressure chamber, maintaining a dynamic vacuum in the chamber, supplying a plasma-forming gas and powder of the sprayed substance into the ultrasonic nozzle of the plasma torch, and spraying the substance with a supersonic plasma stream in the chamber to form of molten particles of micron level and vapor phase of the sprayed substance, at the section of the supersonic nozzle of the plasma torch there are conical nozzles, the inner surface of which forms a kink with the inner surface of the nozzle, the angle of which is selected from the condition that the nozzle in the wall part after turning the plasma to the pressure angle mentioned is equal to the pressure in the chamber of reduced pressure and ensuring the formation of nanoparticles in the wall of the nozzle from the vapor phase the sprayed substance and particles of micron level from the sprayed substance, while in the process of spraying the substrate is moved relative to the nozzle with the provision of overlapping each different layers of nanoparticles and micron level particles.

Nanoparticles fall onto the substrate opposite the nozzle in the form of a nanostructured layer. The molten particles of the micron-level deposited material form a classical thermal spray coating on the substrate opposite the plasma torch nozzle.

In contrast to the prototype, in which the coating is sprayed only at the expense of nanoparticles, the proposed method uses all the sprayed material. Due to the increased activity of surface atoms of nanoparticles, they contribute to an increase in cohesion between layers of micron particles, but are able to withstand operating temperatures that coincide with the working temperatures of layers of micron particles, since they are formed from the same sprayed substance.

The invention is illustrated by drawings.

The figure 1 shows a plasmatron with a mounted nozzle.

The figure 2 shows the scheme of coating.

The figure 3 shows the installation diagram of the conical nozzle on the supersonic nozzle of the plasma torch.

The figure 4 shows the change in the gas-dynamic parameters of the plasma flow at point A.

The figure 5 shows an enlarged image from a scanning electron microscope of the surface layer of TZP (layer of hafnium oxide).

 The figure 1 shows a plasmatron for coating. On the supersonic nozzle B in its output part, nozzles C are installed, the inner conical surface of which forms a kink with the nozzle surface — point A. At kink A, a fan of Prandtl-Meyer rarefaction waves is realized, in which the vapor phase of the sprayed substance condenses to form nanoparticles, the presence of which in the coating significantly increases its adhesion and cohesion, as well as heat resistance. We draw attention to the fact that a rarefaction flow in the plasma torch nozzle is formed only when the plasma jet expands into a dynamic vacuum, and not into the atmosphere.

The coating scheme using such a nozzle is shown in Figure 2. The diagram shows that in the middle part under the nozzle a traditional gas-thermal coating is applied to the substrate with 1 micron level particles that pass through the main plasma torch nozzle and, due to their relatively large size, do not unfold in the nozzle in the fan rarefaction waves, and fly further to the substrate. A layer of nanoparticles 2 formed from the vapor phase in the near-wall part of the nozzle is applied at the periphery of the coating. Moving the flat substrate, as indicated by the arrows (alternately in one direction or the other), we obtain a coating in which layers of nanoparticles and layers of particles of micron level overlap each other. In the case of a cylindrical substrate, the substrate is rotated in one direction to achieve the same effect.

The figure 3 shows the installation diagram of the conical nozzle on the supersonic nozzle of the plasma torch.

Consider the change in the gas-dynamic parameters of the plasma flow at point A, which, after a kink, is designated as A '. The following notation is introduced in FIG. 4: M _A is the Mach number to point A; M _{A '} is the Mach number after the corner point A'.

As an example, take the nozzle used in the plasma torch for spraying and having the following parameters:

- the diameter of the critical section d _cr = 4 mm;

- the diameter of the output section d _c = 11 mm;

- Mach number in the output section M _A = 3.32;

- pressure in the plasmatron P ₀ = 0.15 MPa;

- the temperature in the plasmatron T ₀ = 4200 K;

- pressure in the vacuum chamber P _k = 1.5⋅10 ² Pa.

At point A, due to the nozzle, there is a kink in the flow part of the nozzle, and the flow parameters at point A ', which is located immediately after point A after the kink of the generatrix, are determined by the laws of the Prandtl-Meyer flow. The Prandtl-Mayer angle (ν) determines the relationship between the Mach number (M) of the flow in question and the angle of rotation in the fan of rarefaction waves from the number M = 1 to the number M according to the following expression:

We choose the angle of kink of the forming nozzle on the basis of the condition that the pressure in the chamber of reduced pressure P _{k is} equal to the static pressure during the passage of a reversal in the fan of rarefaction waves and assume that it coincides with the static pressure at the nozzle exit with nozzle P _{A '} , in other words

P _{A '} = P ₀ π (M _A' ) = 1.5⋅10 ² Pa,

where π (M _{A '} ) is the gas-dynamic function equal to the ratio of the static pressure P _A' to the total pressure P ₀ . From this expression, π (M _{A '} ) = 0.001, which for nitrogen corresponds to the number M _A' = 5.56.

Следовательно, угол

Let us determine the angle Θ at which the plasma flow should unfold at point A (see Fig. 4) from M _A = 3.32 to M _{A '} = 5.56, so that the static plasma pressure after a turn is equal to the pressure P _k . From the gas-dynamic tables of the Prandtl-Mayer flow it follows that

Hence the angle

From the same tables it follows that when passing through a fan of rarefaction waves from point A to point A ', the static temperature drops from 1310 K to 584.6 K, and the static pressure drops from 25.5⋅10 ² Pa to 1.5⋅10 ² Pa

Note that the rapid cooling and drop in static pressure that occurs at point A will lead to a sharp condensation of the vapor of the sprayed substance with the formation of nanoparticles.

Now we determine the diameter of the outlet section of the nozzle with the nozzle, based on the second assumption that the pressure P _k coincides with the static pressure in the outlet section of the nozzle with the nozzle.

To do this, we determine the ratio of the area of the exit section of the nozzle “a” with the nozzle (diameter d _a , see Fig. 3) to the critical area of the nozzle (diameter d _cr ) of the plasma torch at M _a = 5.56, where M _a is the Mach number in the output section of the nozzle with the nozzle, matching the number M _{A '} . From the gas dynamic tables, this ratio is equal to 38.57 for nitrogen. It follows that d _a = 24.8 mm

Consider an example of the implementation of the proposed method of spraying. Prepared samples of substrates for applying TZP are placed in a chamber with reduced pressure.

Then, in a dynamic vacuum using a plasma torch with the proposed nozzle, first a nickel sublayer is sprayed, then a layer of zirconia stabilized with 7% Y ₂ O ₃ and the upper layer is stabilized with hafnium oxide stabilized with 7% Y ₂ O ₃ .

Spraying modes:

- plasma forming gas - nitrogen.

- arc current I, A - 100 ± 20.

- voltage on the arc U, V - 90 ± 10.

- the consumption of plasma-forming (also transporting) gas is 60 l / min.

The thickness of the nickel sublayer is 25-30 microns; layer thickness (ZrO ₂ + 7% Y ₂ O ₃ ) - 65-70 microns; layer thickness (HfO ₂ + 7% Y ₂ O ₃ ) - 10-15 microns.

An enlarged image from a scanning electron microscope of the surface layer of TZP (hafnium oxide layer) is shown in Figure 5. It can be seen from this figure that hafnium nanoparticles predominate on the surface of TZP.

Fire tests of the samples were carried out in a test setup, while during the test the sample placed in the setup was cooled by water, the heat flux was created by a plasma jet flowing out of the plasma torch with the above parameters. Each test lasted 30 seconds.

Coated samples obtained without the use of a nozzle were held up to cracks 7–9 tests. The coating samples obtained using the nozzle, withstood 25 tests without cracking.

In the proposed method, nanoparticles are formed immediately before coating deposition and do not have traditional disadvantages when working with them: they do not stick together (aggregation) and do not exhibit chemical activity with their environment. Also, the formation of nanoparticles for deposition occurs in an environmentally friendly way from micron level powders, because the whole process from the formation of nanoparticles to coating occurs in the enclosed space of the chamber.

Sources

1. OST 92-1406-68 "Coatings erosion-resistant non-metallic."

2. V.V. Saigin, A.V. Safronov et al. A method for producing erosion-resistant heat-protective coatings. RF patent No. 2499078, 2012.

3. V.V. Saigin, V.P. Voevodin and others. A method of obtaining erosion-resistant heat-resistant coatings. RF patent No. 2283363, 2003.

4. T.A. Evdokimova, M.N. Polyansky et al. Method of coating. RF patent No. 2436862, 2010.

Claims (1)

The method of plasma deposition of a nanostructured heat-protective coating on a substrate, which includes installing a plasma torch with a supersonic nozzle and a substrate in a reduced pressure chamber, maintaining a dynamic vacuum in the chamber, supplying a plasma-forming gas and powder of the sprayed substance into the supersonic plasma torch nozzle and spraying the substance with a supersonic plasma stream in the chamber with the formation of molten particles of micron level and vapor phase of the sprayed substance, characterized in that at the section of the supersonic nozzle n A conical nozzle is installed in azmotron, the inner surface of which forms a kink with the inner surface of the nozzle, the angle of which is selected from the condition that the nozzle in the near-wall part of the nozzle after the plasma is turned by the pressure angle equal to the pressure in the chamber of reduced pressure and the nanoparticles are formed in the near-wall layer of the nozzle from the vapor phase of the sprayed substance and particles of micron level from the sprayed substance, while during the spraying process the substrate is moved relative to the nozzle to ensure we overlap each other layers of nanoparticles and particles of the micron level.

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