Modified NiCr20Ti PVD-coating of Liquid Rocket Engines Combustion Chambers

doi:10.21203/rs.3.rs-8250378/v1

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Modified NiCr20Ti PVD-coating of Liquid Rocket Engines Combustion Chambers

https://doi.org/10.21203/rs.3.rs-8250378/v1

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The equipment and technology for applying heat-resistant coatings to the working surface of the liquid-propellant rocket engine (LPRE) combustion chamber are presented. The application of experimental coatings was carried out by the vacuum-arc method in a deep vacuum and in a vacuum in a nitrogen atmosphere for coating modification. The coatings were applied to the inner surface of a copper simulator of the LPRE combustion chamber. The coating material was NiCr20Ti alloy. The structure, phase composition, atomic-crystalline structure and hardness of the obtained coatings were investigated. The coating consists of two main structural components: a condensed matrix and a droplet phase of the cathode material. Nitriding helps to reduce the content of the droplet component. The coating is a Ni-Cr solid solution, which has a face-centered cubic lattice. Nitriding leads to the appearance of additional phases CrN, Cr₂N in an amount of up to 10% and the formation of a finer crystalline structure. The coherent scattering regions (CSR) for coatings obtained without nitriding are symmetrical in crystallographic directions and have dimensions of ~ 250 Å. Nitriding leads to a slight decrease in the CSR size to 200–210 Å and their asymmetry. The coatings contain residual tensile stresses ranging from 100–250 MPa with relative strain values ε = - (0.06–0.14) %. Nitriding increased the hardness of the coatings from 383–414 HV to 544–615 HV.

The created protective coatings will ensure an increase in the operational characteristics and service life of engines, which is important with the transition of space technology to reusable systems.

PVD coating

Deposition

Nitriding

Rocket engine combustion chamber

One of the main elements of liquid-propellant rocket engines (LPRE) is the combustion chamber, in which fuel and oxidizer are mixed and burned. The combustion process in a liquid rocket engine occurs at high temperatures and high pressure. Both of these factors contribute to an increase in heat fluxes to the chamber walls. Heated gases move along the combustion chamber and nozzle at a very high speed. As a result, the coefficient of convective heat transfer from hot combustion products to the engine chamber walls increases sharply [1].

The presence of strong heat fluxes leads to the fact that the temperature of the combustion chamber and nozzle walls reaches the melting point of their material and above [2].

Given the rapidity of the processes occurring in the LPRE combustion chamber, only a small part (fractions of a percent) of all the heat generated in the chamber is transferred to the structure. However, given the high combustion temperature and the significant amount of heat released, even a small part of it is enough for thermal destruction of the chamber, so its cooling is used.

Most of the LPRE chambers have external cooling, in which the coolant flows through the path formed by the inner and outer shells or walls of the combustion chamber and the nozzle [3–5].

The working inner surface of the combustion chamber of the LPRE rocket launchers is made of copper alloys, such as CuCrZr, which are characterized by high thermal conductivity, which is necessary to increase the efficiency of rocket engines. However, the relatively low heat resistance of copper alloys limits the service life of the combustion chambers [6].

Traditionally, the working surfaces of combustion chambers made of copper alloys are protected by heat-resistant steel, which is connected to the walls of the combustion chambers by soldering [7]. The steel grade is selected from the conditions of ensuring the necessary operational properties of the combustion chambers, high-quality soldering, manufacturability of processing, and acceptable cost. Sometimes it is difficult to simultaneously fulfill these conditions. At the same time, modern trends in the development of rocket and space technology – increasing the use of reusable stages and the need to reduce the cost of launches – require an increase in the resource of the combustion chambers of rocket engines and a decrease in the cost of their manufacture, which allows significantly reducing the cost of launching cargo into orbit [8].

Reducing the impact of heat flows and increasing the resource of combustion chambers made of copper alloys can be achieved by applying protective coatings to their working surfaces, which increase the resource and specific impulse of the engine due to an increase in heat resistance and, accordingly, the operating temperature in the chamber.

An important characteristic of heat-resistant metal alloys that can be used as protective coatings is the presence of phase and structural transformations in them at the temperatures of the thermal heating/cooling cycle. Phase recrystallization during heating/cooling negatively affects the properties of the alloy due to the dissolution and separation of secondary phases and the resulting internal stresses. Therefore, the use of Ni-Cr alloys, which are a single-phase solid solution in the entire temperature range, is attractive [9].

Requirements for vacuum-arc coatings from Ni-Cr materials and their deposition modes for LPRE combustion chambers should be related to the operating conditions of the combustion chambers during the operation of rocket engines and in terms of the complex of properties should not be lower than the requirements for their traditional analogues, in particular heat-resistant steels.

Combustion chamber coatings must be operable under conditions of high pressure (from 1 to 30 MPa), gas temperatures in the chamber up to 3800°C and the action of oxidant and fuel combustion products at temperatures of 600–800°C [10,11]. The duration of each operating cycle is 170–200 seconds [12,13].

The importance of developing new heat-resistant materials and technologies to protect the working surfaces of LPRE combustion chambers is determined by the above-mentioned factors.

The coating material was a nickel-based alloy NiCr20Ti with a chromium content of 19 − 22% and titanium of 0.15–0.35%.

The coating was applied by vacuum-arc deposition on a special installation created at the Yuzhnoye State Design Office. The inner surface of a copper imitator of a combustion chamber of LPRE was coated. Specimens for research were located in the walls of the imitator. Eight specimens with a coating thickness of > 50 µm were studied. Coating were applied to the flat surface of the cylindrical specimens of copper alloy with a diameter of 20 mm and a height of 10 mm.

Two series of coating deposition were performed. In the first series (specimens #1–4), the coating was deposited in a deep vacuum, in the second series (specimens #5–8), the coating was deposited in a vacuum in a nitrogen atmosphere for its modification.

Before coating, the combustion chamber imitator was heated by a glow discharge in a nitrogen atmosphere (see Table 1) to a temperature of ~ 400°C. Combustion of the glow discharge took place from the outer surface of the imitator.

Table 1

Heating modes of the combustion chamber imitator in the glow discharge
Nitrogen pressure in vacuum chamber (Pa)	Glow discharge current (A)	Electrical potential difference (V)	Process time (min)
5.3–6.7	5.5–7.0	800–900	20–30

After heating, the inner surface of the combustion chamber imitator was ion cleaned according to the modes in Table 2.

Table 2

Ion cleaning modes of the internal surface of the combustion chamber imitator
Pressure in vacuum chamber (Pa)	Arc discharge current on cathode (A)	Bias voltage on combustion chamber (V)	Process time (min)
0.004–0.008	up to 3	800–1000	5–10

During the ion-cleaning process and subsequent deposition process, the combustion chamber imitator's temperature ranged from 350–400°C. Vacuum-arc deposition was carried out in accordance with the modes in Table 3.

Table 3

Modes of vacuum arc deposition
Specimen number	Pressure in vacuum chamber (Pa)	Nitrogen, N₂	Arc discharge current on cathode (A)	Arc burning voltage on cathode (V)	Process time (min)
1–4	0.0004–0.0008	–	150–190	24–27	240–260
5–8	0.008–0.012	+	150–190	24–27	240–260

Surface and cross-section studies were carried out using a stereomicroscope "Konus Crystal Pro 7x-45x Stereo" and a metallographic light microscope MIM-10. The coating structure was revealed by electrolytic etching of the cross-section of the coatings on an electrolytic polishing machine "Electro-P" (Devco S.r.l) in a 10% oxalic acid solution.

X-ray studies were carried out on an Ultima IV "Rigaku" diffractometer in CuKα (λ_СuKα1 = 0.154187 nm) radiation. X-ray photography was carried out from the horizontal surface of the coating in the reflection angle range of 20–130 degrees. The calculation of the parameters of the crystal lattice, microdeformations and defect density was carried out using the Williamson-Hall method. The dimensions of the coherent scattering regions (CSR) were determined by the Selyakov-Scherrer formula.

The hardness of the coatings was determined by measuring microhardness on a PMT-3 hardness tester and calculated according to ISO 6507-1. The load was 30–50 g.

3.1 Coating Equipment

To conduct experimental work, an imitator of the combustion chamber of LPRE was designed, which has dimensions as close as possible to the combustion chamber (Fig. 1), but is simpler to manufacture. The specimens were placed in holes made at four points of the combustion chamber simulator.

To apply coatings to the working surface of the combustion chambers of the LPRE, a specialized vacuum installation has been created at the Yuzhnoye State Design Office (Fig. 2). The vacuum chamber is made in the form of a cylinder with a total volume of 0.49 m³. The combustion chamber imitator is installed inside the vacuum chamber (Fig. 3). The evaporator, fixed on the chamber cover, passes through the cavity of the imitator coaxially to it. The evaporator is an anode (three tungsten rods) and a cathode (a metal tube made of NiCr20Ti alloy with a diameter of 38.8 mm, a length of 398 mm, and a wall thickness of 6 mm).

3.2 Research of the Coatings Structure

The calculated coating thickness was 44–122 µm and depends on the location of the specimens in the combustion chamber imitator. The smallest coating thickness corresponds to the specimens with the longest distance from the evaporator – on the imitator bell, the largest – at the narrowest point between the two bells, which simulates the critical section of the combustion chamber.

The surface of the coatings (Fig. 4) of all specimens has a similar character: there is a slight roughness (for specimens #5–8 it is slightly larger than for specimens #1–4), which is primarily due to solidified droplets of the sprayed cathode. The droplets are evenly distributed over the surface, there are no microcracks. Specimens #5–8 have a darker color, which is typical of nitrogen-containing coatings.

The coating in cross-section (Fig. 5) has two structural components – the main matrix of the plasma-condensed coating (region 1) and the droplet component in the form of flat flattened droplets parallel to the substrate surface (region 2), and close to spherical volumetric droplets (region 3). The formation of volumetric structural components leads to the formation of regions that are not filled with the structural component of the matrix during the coating growth process and form small pores. The largest amount of the volumetric droplet phase, and accordingly, pores, was formed in specimens #2 and #3, which is probably due to the minimum distance from the substrate surface to the cathode.

Figure 8 presents images of the cross-sectional microstructure of the coatings of specimens #5 to 8 that were obtained in a nitrogen environment.

When nitrogen is added to the vacuum chamber, it dissolves in the main condensed matrix of the coating without significantly changing its appearance on the cuts. The number of pores in the coatings decreases. This is due to a decrease in the number of bulk droplets in plasma with higher atomic densities.

Specimens #4 and #8 have shown a deviation in the orientation of the macroregions growth boundaries from the normal direction (Fig. 6), which corresponds to the angle of inclination of the conical surface of the nozzle 0.5×86° relative to its central axis of symmetry (Fig. 1).

3.3 Research of Phase Composition and Atomic-Crystalline Structure of the Coatings

Specimens #2, #3, #6 and #7 were selected for X-ray examination. Figure 7 shows the X-ray images of the studied specimens.

Analysis of the X-ray diffraction patterns shows that the coatings of specimens #2 and #3, which were deposited in a nitrogen-free environment, are a Ni-Cr solid solution with a face-centered cubic lattice. The X-ray diffraction patterns of specimens #6 and #7 show traces of additional phases, which are identified as chromium nitrides and oxides (CrN; Cr₂N; Cr₂O₃), which are technically difficult to separate due to the low intensity of their diffraction maxima. In addition, a feature of the X-ray diffraction patterns of coatings deposited in a nitrogen environment is the increase of the background under the NiCr traces, which corresponds to the formation of a finely crystalline structure on the border with the amorphous state. This crystalline structure is most likely associated with additional phases and, according to calculations by the “Rigaku” diffractometer program, constitutes ~ 10% of the composition of the deposited material.

Table 4 presents data on the shift of the imprints for different crystallographic planes and the calculation of the relative deformation of the coatings for the studied specimens.

Table 4

Change in interplanar distance and relative deformation of NiCr20Ti coatings
	2θ	θ	sin θ	d, Å _exp.	d, Å _cal.	Δd _{exp.– cal}.	ε, %
specimen #2
100					3.558293
111	44.04302	22.02151	0.374954	2.054381	2.054381	0	0
200	51.36462	25.68231	0.433380	1.77742	1.77915	-0.00173	-0.097
220	75.6403	37.82015	0.613184	1.256227	1.258047	-0.00182	-0.145
specimen #3
100					3.554742
111	44.08933	22.04466	0.375329	2.052331	2.052331	-0.00156	-0.076
200	51.40836	25.70418	0.433724	1.776010	1.777371	-0.00145	-0.082
220	75.6733	37.83665	0.613412	1.255762	1.256791	-0.00103	-0.079
specimen #6
100					3.557328
111	44.0556	22.0278	0.375056	2.053824	2.053824	0	0
200	51.36368	25.68184	0.433373	1.777446	1.778664	-0.001218	-0.0685
220	75.6575	37.82875	0.613303	1.255985	1.257705	-0.00122	-0.0688
specimen #7
100					3.553639
111	44.10373	22.05186	0.375445	2.051694	2.051694	0	0
200	51.42839	25.71419	0.433882	1.775366	1.776819	-0.00145	-0.082
220	75.72047	37.86023	0.613737	1.255097	1.256401	-0.0013	-0.104

For the entire series of specimens under study, the shift of the imprints corresponds to a decrease in the interplanar distances compared to the calculated values for all crystallographic directions, i.e. there are residual tensile stresses, with a relative strain value in the range ε = - (0.06–0.14) %. At a value of the elastic modulus for polycrystalline nichrome of 180 GPa, these stresses fluctuate within 100–250 MPa. Table 5 presents data on the broadening of diffraction maxima (βcosθ) and their asymmetry (S) for the directions (111), (200), (220) of the studied coatings for calculating the dimensions of the CSR.

Table 5

Broadening of diffraction maxima and their asymmetry in NiCr20Ti coatings
	θ	β, deg	βcosθ, rad	4sinθ	S, deg	D _cal., Å	D _approx., Å
specimen #2
111	22.02151	0.333	0.0054	1.499	0.141	285	250
200	25.68231	0.566	0.0089	1.734	0.240	173
220	37.82015	0.679	0.0094	2.453	0.288	163
specimen #3
111	22.04466	0.3320	0.0054	1.501	0.141	285	250
200	25.70418	0.5654	0.0089	1.735	0.240	173
220	37.83665	0.6984	0.0096	2.454	0.296	160
specimen #6
111	22.0278	0.4034	0.0065	1.500	0.1713	237	200
200	25.68184	0.6762	0.0107	1.733	0.6762	143
220	37.82875	0.8166	0.0113	2.453	0.3468	136
specimen #7
111	22.05186	0.3352	0.0054	1.501	0.1423	285	210
200	25.71419	0.7059	0.0111	1.735	0.2998	139
220	37.86023	0.5946	0.0082	2.454	0.2525	188

The broadening of the (111), (200) and (220) imprints for the coatings of specimens #2 and #3 deposited in a nitrogen-free environment is almost the same. This indicates the symmetry of the CSR in all directions. For coatings obtained in a nitrogen-free environment, the average OCR size is 250 Å. Adding nitrogen to the vacuum chamber leads to a slight decrease in the CSR size to 200–210 Å. For specimen #7, a slight asymmetry of the CSR in the directions is observed. The CSR size of the additional phases CrN, Cr₂N in coatings obtained in a nitrogen environment reaches amorphous values of ~ 20 Å.

Analysis of the intensity of diffraction maxima in the X-ray diffraction patterns of the studied coatings indicates their different textural state. Table 6 shows the ratio of the intensity of the maxima (111), (200) and the average value of their asymmetry coefficient.

Table 6

Textural state and anisotropy of diffraction maxima of NiCr20Ti coatings
Specimen number	Intensity (111/200)	S (deg.)	Nitrogen, N
2	3.0	0.223	–
3	3.0	0.223	–
6	2.85	0.38	+
7	2.56	0.232	+

In coatings deposited in a nitrogen-free environment, the highest intensity is observed for (111) reflections with the same ratio (І₁₁₁/І₂₀₀) and the same values of asymmetry S, which reflects the homogeneity of their structural state. Adding nitrogen to the vacuum chamber leads to a redistribution of the intensity of diffraction reflections, which indicates a change in texture. For specimens #6 and #7, an increase in the (200) reflection is observed, which may indicate a tangential (layer-by-layer) nature of the coating growth. An increase in the asymmetry of the reflections may indicate the formation of a multilayer structure with different nitrogen contents in the Ni-Cr structure and is determined by the formation of an “extended” lattice with anisotropy of the CSR sizes.

3.4 Research of the Coatings Hardness

The results of measuring the hardness on the surface, in the cross-section, the absolute (∆HV) and the relative (ε) errors are shown in Table 7.

Table 7

Surface and cross-sectional hardness of coatings
Specimen number	Surface hardness (HV)	∆HV	ε (%)	Hardness in cross-section (HV)	∆HV	ε (%)
1	407	38	9	414	23	6
2	396	39	10	377	36	9
3	338	34	10	347	29	8
4	325	23	7	383	30	8
5	587	93	16	601	84	14
6	523	71	14	615	86	14
7	543	50	9	544	64	12
8	592	99	17	598	55	9

The hardness of the specimens on the surface of the coating and in the cross-section have close values and are within the absolute measurement error. The error of measuring the hardness of nitrided specimens is increased compared to non-nitrogen specimens. This is because when measuring hardness, the indenter pyramid sometimes falls into the region of the bulk droplet phase, the hardness of which is much lower compared to the hardness of the nitrogen-modified matrix.

Since the discrepancy in the hardness values for specimens of the same series (located in different places of the imitator) is within the absolute measurement error, it is impossible to draw conclusions about the dependence of the microhardness of the coating on the position of the specimen in the combustion chamber imitator. It is advisable to take the average value for a series of specimens obtained under the same deposition conditions. Thus, the average hardness of the non-nitrided NiCr20Ti coating is 380 HV, the nitrided one is increased by 50% and reaches 570 HV.

Equipment has been developed and the technology of vacuum-arc deposition of heat-resistant coatings on combustion chambers of liquid rocket engines made of copper alloys has been worked out.

Vacuum-arc deposition of heat-resistant NiCr20Ti coating on a combustion chamber simulator made of copper alloy was performed. For additional strengthening of the coating, it was nitrided during the deposition process.

The coating consists of two main structural components: a condensed matrix and a droplet phase of the cathode material. Nitriding helps to reduce the content of the droplet component.

The coating is a solid solution of Ni-Cr, which has a face-centered cubic lattice. Adding nitrogen to the chamber leads to the appearance of additional phases CrN, Cr₂N in a small amount of up to 10% and the formation of a more finely crystalline structure.

The CSR for coatings obtained in the plasma of the cathode material without nitrogen are symmetrical in crystallographic directions and have dimensions of ∼250 Å. Adding nitrogen to the chamber leads to a slight decrease in the size of the CSR to 200–210 Å and their asymmetry.

The coatings contain residual tensile stresses ranging from 100–250 MPa with relative deformation values of ε = - (0.06–0.14) %.

The hardness of non-nitrided coatings is 383–414 HV; nitriding increased the hardness by ∼50% to 544–615 HV.

The created protective coatings are promising for the manufacture of rocket engine combustion chambers with an increased service life, including for multiple use.

Author Contribution

Conceptualization, supervision and project administration: Volodymyr Nadtoka.Methodology: Volodymyr Nadtoka, Dmytro Bondar, Maksym KraievFormal analysis and investigation: Dmytro Bondar, Maksym Kraiev, Violeta Kraievа.Visualization: Dmytro Bondar, Violeta Kraievа.Writing – original draft preparation: Dmytro Bondar.Writing – review and editing: Dmytro Bondar, Maksym Kraiev, Volodymyr Nadtoka.All authors read and approved the final manuscript.

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No competing interests reported.

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You are reading this latest preprint version

Modified NiCr20Ti PVD-coating of Liquid Rocket Engines Combustion Chambers

Status:

Version 1

Abstract

Figures

1 Introduction

2 Material and Methods

3 Results and Discussion

3.1 Coating Equipment

3.2 Research of the Coatings Structure

3.3 Research of Phase Composition and Atomic-Crystalline Structure of the Coatings

3.4 Research of the Coatings Hardness

4 Conclusions

Declarations

Author Contribution

References

Additional Declarations

Status:

Version 1

Specimen number	Surface hardness (HV)	∆HV	ε (%)	Hardness in cross-section (HV)	∆HV	ε (%)
1	407	38	9	414	23	6
2	396	39	10	377	36	9
3	338	34	10	347	29	8
4	325	23	7	383	30	8
5	587	93	16	601	84	14
6	523	71	14	615	86	14
7	543	50	9	544	64	12
8	592	99	17	598	55	9

Specimen number	Surface hardness (HV)	∆HV	ε (%)	Hardness in cross-section (HV)	∆HV	ε (%)
1	407	38	9	414	23	6
2	396	39	10	377	36	9
3	338	34	10	347	29	8
4	325	23	7	383	30	8
5	587	93	16	601	84	14
6	523	71	14	615	86	14
7	543	50	9	544	64	12
8	592	99	17	598	55	9

Specimen number	Surface hardness (HV)	∆HV	ε (%)	Hardness in cross-section (HV)	∆HV	ε (%)
1	407	38	9	414	23	6
2	396	39	10	377	36	9
3	338	34	10	347	29	8
4	325	23	7	383	30	8
5	587	93	16	601	84	14
6	523	71	14	615	86	14
7	543	50	9	544	64	12
8	592	99	17	598	55	9