RU2283363C2 - Method of making erosion-resistant heat-protective coats - Google Patents

Abstract

FIELD: metallurgy; making erosion-resistant heat-protective coats by plasma spraying.

SUBSTANCE: proposed method may be used in rocketry in manufacture of combustion chambers for liquid propellant engines with cermet erosion-resistant heat-protective coats on base of ZrO₂ NiCr composition. Proposed method includes spraying of nichrome precoat followed by spraying of cermet composition from mechanical powder mixture containing 50-80 wt-% of zirconium dioxide and 50-20 wt-% of nichrome. For spraying cermet composition, use is made of mechanical mixture containing zirconium dioxide and nichrome powders at size of particles of 10-40 mcm and 40-100 mcm, respectively. Powder mixture is delivered under nozzle exit section of plasmatron in way of its motion relative to surface to be coated. Calcium oxide is used as stabilizing additive in zirconium dioxide powder in the amount of 4-6 wt-%.

EFFECT: increase of adhesion strength by 2-3 times; enhanced heat resistance of cermet coats due to phase transition from metal precoat to main coat.

1 dwg, 1 tbl, 1 ex

Description

The invention relates to the field of rocketry and can be used for thermal and erosion protection of the fire walls of the LRE combustion chambers by applying plasma spraying erosion-resistant heat-resistant coatings (ETZP).

One of the urgent tasks associated with improving the performance of plasma heat-protective coatings is the task of increasing their adhesive strength and heat resistance.

There are various ways to increase the adhesion and heat resistance of ETZP plasma spraying. One of them is the application of a metal sublayer with a thickness of 80 ÷ 120 μm from a heat-resistant alloy, for example nichrome (NiCr), under a heat-shielding coating (see, for example, OST 92-1406-68 “Coatings of erosion-resistant non-metallic”). This method allows to obtain for plasma heat-resistant coatings based on zirconium dioxide (ZrO ₂ ) tear strength σ _A ~ 3.5 ÷ 4.5 MPa and heat resistance n ~ 6 ÷ 8 cycles.

There is also known (see German patent No. 25152366) a method for applying plasma spraying coatings on products made of pure copper, which consists in the fact that the intermediate layer is first applied with a thickness of 0.1 mm from powdered phosphorus-copper solder, and then at least one a layer of an alloy containing copper as an alloying component. After applying the second layer, the coating is fused, heating the product to approximately 1000 K. The adhesion of plasma coatings to copper is increased by this method by creating an intermediate diffusion phosphorus-containing layer. Further, a decrease in the copper content in subsequent layers of the coating provides a gradual transition to the desired composition of the coating while maintaining its high adhesive properties. Thus, in the specified method, the well-known idea of increasing the adhesion strength of coatings by creating a diffusion zone is successfully used. The disadvantage of this method is that the increase in adhesion is due to the use of fusible phosphorus-copper solder, which is not applicable in coatings operating at temperatures above 1300 K.

There is also a method of obtaining ETZP with increased values of tear strength and heat resistance (see. "Powder metallurgy and sprayed coatings", edited by B.O. Mitin, M., Metallurgy, 1987, p. 560), adopted as a prototype, in wherein an increase in the service characteristics of plasma coatings is achieved by adding plastic material, for example, nichrome, to the coating and using transition layers between the substrate and the coating having a variable content of plastic additive decreasing from the substrate to the main coating. Such layers, for example, can be:

- 1st layer 95 ÷ 65% weight. NiCr + 5 ÷ 35% weight. ZrO ₂ ;

- 2nd layer 65 ÷ 35% weight. NiCr + 35 ÷ 65% weight. ZrO ₂ ;

- 3rd layer 5 ÷ 35% weight. NiCr + 95 ÷ 65% weight. ZrO ₂ .

Thus, this method also implements a solution to create a phase transition zone from the substrate to the coating.

The described method allows to increase the adhesive strength of ETZP to a value of σ _A ~ 7 ÷ 8 MPa and heat resistance to n ~ 8 ÷ 10 cycles. The disadvantage of this method is that the above characteristics do not provide operability under the conditions of exposure to high-temperature gas flows KS LRE promising samples of rocket technology. The disadvantage of this method is also significant difficulties in ensuring stability and reproducibility of applying multilayer coatings on the complex internal surfaces of the combustion chambers of rocket engines. In addition, coating in several passes is not technologically advanced and impairs the cohesive characteristics of the heat-protective coating package as a whole.

The aim of the present invention is to increase the adhesion strength and heat resistance of heat-protective coatings by creating a phase transition zone at the same time (in one technological cycle) with the formation of the main heat-protective coating.

This goal is achieved by the fact that in the known method for producing erosion-resistant heat-protective coatings, which consists in applying a nichrome sublayer by plasma spraying and then spraying a cermet composition from a mechanical powder mixture containing 50 ÷ 80% by weight. zirconium dioxide and 50 ÷ 20% weight. nichrome, cermet composition is prepared from zirconia and nichrome powders with a particle size of 10 ÷ 40 μm and 40 ÷ 100 μm, respectively, and the mixture is fed into the plasma jet under the cut of the plasma torch nozzle in the direction of its movement relative to the sprayed surface, while as a stabilizing additive for zirconia powder use calcium oxide, the content of which is 4 ÷ 6% weight.

The drawing schematically shows illustrating this method of producing coatings supply of mechanical cermet mixture under the cut of the plasma torch nozzle in the direction of its movement relative to the sprayed surface.

The proposed method for producing coatings provides the formation of a phase transition zone from the metal sublayer to the initial ETZP composition due to the relationship of the mechanical cermet mixture supply circuit (under the cut of the plasma torch nozzle in the direction of its movement) with the particle size distribution and physicomechanical characteristics of the cermet composition components.

The estimated calculations showed that the differences in the kinetic energy of the oxide and metal particles after exiting the injector, due to the difference in their density and size, lead to an uneven distribution of the components of the mechanical cermet mixture over the cross section of the plasma jet. So, in the low-enthalpy zone of the plasma jet (see the shaded area in the drawing), which, with the given scheme for moving the plasma torch and feeding the cermet composition into the plasma jet, forms a part of the ETZP adjacent to the metal sublayer, an increased content of nichrome particles is formed. The energy characteristics of this zone are insignificant and increase from the outer boundary to the central part of the plasma jet (see, for example, V.V. Kudinov, V.M. Ivanov “Plasma application of refractory coatings”, M., Mechanical Engineering, 1981). However, there is enough energy to ensure that the particles of nichrome powder of the above granulation located in this zone are melted and, therefore, participate in the formation of the coating with almost the same utilization rate as if they were in the central (highly enthalpy) region of the plasma jet. As regards refractory and low heat-conducting particles of zirconium dioxide, the picture will be different. In the zone under consideration, only the smallest particles of zirconium dioxide will melt, and the size of the melted particles and their number will increase from the outer boundary of the zone to the center due to an increase in enthalpy characteristics.

Thus, the complex of these factors, including a certain size of the powder particles of the cermet composition and the scheme of its feeding into the plasma jet, allows simultaneously with the formation of the coating to obtain the phase transition zone from the metal sublayer to the initial coating composition.

It is known that zirconium dioxide has polymorphic transformations:

Polymorphic transformations are accompanied by a change in volume, which, obviously, negatively affects the heat resistance of coatings operating under the influence of high-temperature gas flows of rocket engines, when the temperature difference on the coating and under it is ~ 1000 °. To reduce the consequences of this negative phenomenon, stabilizing additives, such as Y ₂ O ₃ , CaO, MgO, etc., are introduced into ZrO ₂ .

The proposed method for producing coatings was tested for ZrO ₂ powder stabilized with CaO. It was experimentally established that the content of the stabilizing additive with the indicated composition of the starting powders and the conditions for introducing the mixture into the plasma jet affects the thermal stability of the ETZP.

The content of the stabilizing additive in the zirconia powder was determined empirically, based on the heat resistance criterion for the resulting coatings, while maintaining their thermal properties. The properties of cermet coatings based on ZrO ₂ powders containing CaO additive in the amount of 0–3% by weight, 4–6% by weight, 7–10% by weight were studied. It was found that cermet coatings obtained using powder containing 4–6% by weight have the highest heat resistance. CaO.

Examples.

The samples made of copper alloys of the Brh08 type were applied by plasma spraying of coatings consisting of a nichrome sublayer and cermet. Kermet was prepared in two formulations: 80% by weight. ZrO ₂ + 20% weight. NiCr and 50% weight. ZrO ₂ + 25% weight. NiCr. A zirconia dioxide powder was used with a granulation of 10–40 μm and nichrome powder with a particle size of 40–100 μm. The mechanical cermet mixture was fed into the plasma jet under the cut of the plasma torch nozzle in the direction of its movement along the generatrix of rotation bodies of the sprayed surfaces.

To obtain comparative data, cermet heat-protective coatings were simultaneously applied to samples of the same copper alloy in a known manner.

The phase composition of the coatings and the distribution of the metal component over the thickness were controlled by the metallographic method.

The determination of adhesive strength and heat resistance was carried out in accordance with the requirements of the methods set forth in OS 92-1406-68 "Coatings erosion-resistant non-metallic."

The obtained physicomechanical and thermophysical properties of the coatings are summarized in table 1.

As can be seen from the table, the use of the proposed method for producing erosion-resistant heat-protective coatings in comparison with the known solutions allows to increase the adhesion strength and heat resistance of ETZP by 2-3 times due to the creation of a phase transition zone in a single technological cycle with the formation of the main coating.

Claims (1)

A method of producing erosion-resistant heat-protective coatings, including plasma spraying of a nichrome sublayer and subsequent spraying of a cermet composition from a mechanical powder mixture containing 50 ÷ 80 wt.% Zirconia and 50 ÷ 20 wt.% Nichrome, characterized in that a mechanical composition is used for spraying the cermet composition a mixture containing zirconia and nichrome dioxide powders with a particle size of 10 ÷ 40 and 40 ÷ 100 μm, respectively, and the powder mixture is supplied under the cut of the plasma torch nozzle in the direction of its movement relative to apylyaemoy surface, wherein as a stabilizing additive in the zirconia powder used calcium oxide contents of which amounts to 4 ÷ 6 wt.%.

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