ИССЛЕДОВАНИЯ ПО АЗОТИРОВАНИЮ ЧЕЛНОКА ШВЕЙНОЙ МАШИНЫ (ВЫСОКОКАЧЕСТВЕННОЙ СТАЛИ У9А) ЭЛЕКТРОННЫМ ПУЧКОМ В ВАКУУМНОЙ СРЕДЕ

УДК 677.07

Abstract

This article presents the results of experimental studies focused on the electron beam nitriding process of titanium under forevacuum pressure conditions. The research mainly investigates how variations in gas pressure and displacement potential affect the formation and properties of the nitrided surface layer. During the experiments, different processing parameters were applied in order to evaluate their influence on the behavior of the electron beam plasma and on the characteristics of the treated titanium surface. Detailed measurements and analytical observations were carried out to determine changes in the mass-charge composition of the plasma generated during the nitriding process. The obtained results demonstrated that the plasma composition has a direct relationship with the structural and physical properties of the nitrided layer. In particular, changes in gas pressure and displacement potential significantly influenced the thickness, hardness, and overall quality of the modified surface. The study also revealed important correlations between plasma parameters and the efficiency of nitrogen diffusion into titanium. These findings contribute to a better understanding of electron beam surface treatment technologies and may help improve the optimization of titanium nitriding processes in industrial and scientific applications.

Аннотация

В данной статье представлены результаты экспериментальных исследований процесса электронно-лучевого азотирования титана в условиях форвакуумного давления. Основное внимание в исследовании уделено изучению влияния изменения давления газа и потенциала смещения на формирование и свойства азотированного поверхностного слоя. В ходе экспериментов применялись различные технологические параметры для оценки их воздействия на поведение плазмы электронного пучка и характеристики обработанной поверхности титана. Были проведены подробные измерения и аналитические наблюдения с целью определения изменений масс-зарядового состава плазмы, образующейся в процессе азотирования. Полученные результаты показали, что состав плазмы напрямую связан со структурными и физическими свойствами азотированного слоя. В частности, изменения давления газа и потенциала смещения существенно влияли на толщину, твердость и общее качество модифицированной поверхности. Исследование также выявило важные корреляции между параметрами плазмы и эффективностью диффузии азота в титан. Полученные выводы способствуют более глубокому пониманию технологий электронно-лучевой обработки поверхности и могут помочь в оптимизации процессов азотирования титана для промышленных и научных применений.

Keywords: vacuum, plasma cathode, electron sources, electron beams, plasma nitriding, steel, carbon, nitrogen, hardness, processing.

Ключевые слова: вакуум, плазменный катод, источники электронов, электронные пучки, плазменное азотирование, сталь, углерод, азот, твердость, обработка.

Introduction

The shuttle components of sewing machines are manufactured from specially hardened steel, with high-quality U9A steel commonly used as the material. After heat treatment, the working surfaces of the shuttle must possess a hardness of HRC 54–60. The surface finish quality of these parts must not be lower than class 10.

Nitriding of metals is a widely used method for improving the performance characteristics of structural materials. There are three main types of nitriding: gas nitriding [1], liquid nitriding in salt baths [2], and plasma nitriding [3].

Among these nitriding methods, plasma nitriding is distinguished by the short duration of the technological process, the absence of environmental pollution, and low gas and energy consumption [4]. These advantages have led to the widespread application of plasma nitriding for increasing hardness, improving corrosion resistance, and reducing surface friction [5, 6].

For the past four decades, plasma nitriding using direct current plasma nitriding (DCPN) has been widely applied in industry. This method does not possess the disadvantages inherent in gas and liquid nitriding processes. However, during treatment, the workpiece acts as a cathode under a high potential (approximately 600–700 V), which may result in surface damage caused by arc formation. In addition, overheating of sharp edges on the samples leads to non-uniform nitrogen saturation of the surface layers.

Recently, nitriding using an active screen (ASPN — Active Screen Plasma Nitriding) has been actively developed. In this method, a hollow screen electrode made of nitrided material is placed under a floating potential and serves as the cathode of the glow discharge formed between the screen and the walls of the vacuum chamber.

This approach makes it possible to eliminate the disadvantages of the DCPN method while preserving, and in some operating modes even improving, the performance characteristics of the nitrided samples.

Another alternative method is metal nitriding in low-energy electron-beam plasma (EBPN — Electron Beam Plasma Nitriding), in which an electron beam is used as a plasma generator.

The samples are positioned near the electron beam under a negative potential of approximately –350 V. The samples are externally heated, while the pressure of the nitrogen–argon gas mixture inside the vacuum chamber is maintained at 21.3 Pa. As a result, it becomes possible to increase the microhardness of U9A steel using electron-beam plasma, while the required sample temperature can be maintained by adjusting the beam parameters.

The pressure of the argon–nitrogen mixture and the accelerating voltage of the beam are typically in the ranges of 0.1–1 Pa and several hundred volts, respectively. Strengthening of the surface layers of the samples can also be achieved, primarily due to the presence of atomic nitrogen near the treated workpiece. This is associated with the technological режим of nitriding, where the electron beam voltage corresponds to the maximum dissociation of nitrogen molecules under electron impact in the energy range of 60–140 eV.

Fore-vacuum plasma-cathode electron sources operating within the pressure range of 1–100 Pa make it possible to successfully process not only conductive materials but also dielectric materials.

The use of fore-vacuum electron sources for nitrided products offers significant advantages. In particular, the ability to independently regulate the beam current, radiation energy, and gas pressure makes it possible to provide the required temperature conditions for the nitrided product and to generate dense plasma without the use of additional devices [7].

When irradiating metal samples (for example, steel or various types of unhardened steels) with an electron beam, the operating pressure and the electron beam parameters (current and voltage) can significantly influence the characteristics of the nitrided layer. In this case, an increase in gas pressure within the fore-vacuum range (up to several tens of Pascals) contributes to the intensification of the nitriding process in the electron-beam plasma.

Thus, the aim of this work was to investigate the possibility of steel nitriding in the plasma of a fore-vacuum electron source during direct irradiation of a steel sample by an electron beam, as well as to characterize the resulting modified surface layer. Studies were carried out on the mass-to-charge composition of beam-plasma ions, tribological properties, microhardness, X-ray diffraction, and elemental analysis of the nitrided surface.

Experimental Methodology and Technique. Electron Source. The electrode system of the plasma electron source used in the experiments is shown in Figure 1.

Figure 1. Electrode configuration of the electron source: 1 — cylindrical hollow cathode; 2 — anode; 3 — perforated plate (emission electrode); 4 — extractor electrode; 5, 6 — standard ceramic high-voltage insulators; 7 — electromagnetic focusing system

Structurally, the electron-beam gun is an electron-optical column that includes a cathode, anode, extractor electrode, focusing system, and cooling system (Figure 1). A hollow cathode 1 equipped with a water-cooling jacket is located in the upper part of the column. The next element is the cooled anode 2. The cathode and anode are made of stainless steel. A standard high-voltage insulator with a discharge gap of 5 mm is used to connect the cathode to the anode.

To stabilize the plasma boundary and prevent plasma penetration into the accelerating gap, a perforated plate 3 (emission electrode) made of a 1 mm thick tantalum sheet is used. The anode is connected to the extractor flange through the high-voltage insulator of the accelerating gap (the gap between the emission electrode and the extractor). The extractor 4 (accelerating electrode) is removable, made of stainless steel, and has the shape of a truncated cone with a complex geometry. To ensure the optimal operating temperature during continuous operation, the extractor is also cooled by circulating water.

An electromagnetic focusing system 7 is used for focusing the electron beam. The focusing section is made of copper wire wound on a stainless-steel frame. The main operating parameters of the source are presented in Table 1.

Table 1. Operating Parameters of the Electron Source

Experimental Parameters. The experimental setup for steel nitriding is shown in Figure 2. A prototype of the plasma source 1 was mounted on flange 8 of the vacuum chamber. The plasma electron source was powered by direct-current voltage supplies connected to the discharge and accelerating gaps, respectively.

Electrons were emitted through the perforated plate. In the anode–extractor gap, upon application of the accelerating voltage (U_{acc}), the electrons were accelerated and an initial electron beam was formed. The electron beam generated by the source entered the magnetic field of a short-focus magnetic lens, where its final formation took place.

As the electron beam propagated toward the treated sample, beam plasma was generated in the working chamber due to gas ionization. During the interaction of the electron beam with the sample surface, the sample was heated.

The working chamber was initially evacuated to a pressure of (5 \times 10^{-3}) Pa using an nEXT300D turbomolecular pump with a pumping speed of 300 L/s, after which nitrogen was introduced into the chamber up to a pressure of 8 Pa. During the nitriding process, the beam current and accelerating voltage were maintained at 100 mA and 6 kV, respectively, with a treatment duration of 75 minutes. The sample temperature was monitored using a non-contact optical pyrometer manufactured by Raytek, and the operating temperature was maintained at 900 °C.

Since the hardening of the steel surface is associated with the presence of atomic nitrogen near the treated sample, studies of the mass-to-charge composition of beam-plasma ions were carried out in order to determine the optimal parameters of the electron beam during nitriding. The investigation of the mass-to-charge composition of plasma ions was performed using a modified RGA-100 quadrupole residual gas analyzer positioned near the location intended for subsequent placement of the nitrided sample.

Figure 2. Experimental setup scheme: 1 — electron source; 2 — infrared pyrometer; 3 — vacuum sensor; 4 — electron beam; 5 — beam plasma; 6 — tantalum crucible; 7 — steel sample; 8 — cross-sectional view of the vacuum chamber walls.

Sample. As the sample, U9A steel with a thickness of 4 mm was used. A 10×10 mm specimen was cut from the material. The side of the sample exposed to the electron beam was polished using abrasive paper and subsequently cleaned with ethyl alcohol before being placed into the vacuum chamber. The sample was positioned on a tantalum crucible and maintained at ground potential [8].

Equipment and Methods for Studying the Nitrided Layer.
The microstructure, cross-sectional morphology, and elemental composition of the sample surface were investigated using a Hitachi S3400N scanning electron microscope equipped with a Bruker X’Flash 5010 energy-dispersive microanalysis system.

X-ray diffraction (XRD) analysis of the nitrided layer composition was performed using a Shimadzu XRD-6000 diffractometer (TSU TMCKP) in Bragg–Brentano geometry with monochromatic CuKα radiation. Phase composition was analyzed using the PDF-4+ database and full-profile analysis software POWDER CELL 2.4. X-ray diffraction is based on the interaction of X-rays with the crystal lattice of the studied steel.

From the X-ray diffraction data, the phase composition, coherent scattering domain (CSD) size, crystal lattice parameters, and the degree of lattice strain (Δd/d) of the formed layer were determined.

Tribological measurements at room temperature were carried out using a pin-on-disk configuration and a TRIBO tester operating in oscillating mode. The device implements the “ball-on-disk” testing method. The test sample was subjected to a 2 N load applied via a spherical tip.

For hardness measurement, a rigid holder equipped with a frictionless force sensor with a tungsten carbide tip was used. The friction coefficient was determined by measuring the deflection of the elastic arm during testing. Wear of the material was evaluated by measuring the wear track formed during the test. The wear rate was calculated using the formula:

V = 2πRA / FL,

where R is the track radius (µm), A is the cross-sectional area of the wear track (µm²), F is the applied load (N), and L is the sliding distance (m).

Nanoindentation hardness (H) was measured using a NANO hardness tester NHT-S-AX-000X (TMTSKP TPU). A diamond indenter was pressed into the coating surface with a continuously increasing load (0.8–300 mN) for 1 minute, after which the load was removed. Hardness was calculated based on the load–displacement curve and the projected contact area of the Vickers indenter imprint.

Results

Figure 3 shows the ratio of the signal intensity of nitrogen atomic ions to molecular ions as a function of experimental parameters (pressure, beam current, and accelerating voltage).

As shown in Figure 3, an almost linear increase in the fraction of nitrogen atomic ions in the beam plasma is observed with increasing pressure. An increase in the beam current from 30 to 100 mA also leads to nearly a twofold rise in the concentration of nitrogen atomic ions. With increasing accelerating voltage, a sharp decrease in the number of nitrogen atomic ions is observed in the voltage range of 500–1000 V.

A further increase in the accelerating voltage, and consequently in the irradiation energy, has almost no effect on the fraction of atomic or molecular nitrogen ions. The observed trends can be explained by the cross-sections of the most active reactions responsible for the formation of atomic and molecular nitrogen ions in the plasma under electron-beam irradiation.

Figure 3. Dependence of the ratio of atomic and molecular nitrogen ions on experimental parameters

The increase in the N⁺/N₂⁺ ratio with rising gas pressure, at constant beam current and irradiation energy, is most likely associated with an intensification of inelastic electron scattering processes in gas molecules as the pressure increases. This leads to a decrease in the mean energy of the irradiation electrons, which in turn enhances gas dissociation and increases the fraction of atomic ions. Thus, in terms of the efficiency of atomic nitrogen generation in electron-beam fore-vacuum conditions, the most favorable regime is low beam energy (500–1000 eV) at relatively high gas pressures (7–10 Pa).

However, in order to ensure a sufficient temperature for nitriding, the accelerating voltage in this experiment was increased to 6 kV, resulting in a sample temperature of 900 °C. The elemental composition of the nitrided layer was determined using scanning electron spectroscopy (Figure 4).

Figure 4. Auger spectroscopy results.

The main elements detected are iron (steel) and nitrogen; however, oxygen and a small amount of carbon are also present. The total concentration of the latter two elements does not exceed 6.2 wt.%. The presence of small amounts of oxygen and carbon in the nitrided layer is associated with their content in the near-surface region of the steel and at such levels does not lead to degradation of the nitrided layers. After 75 minutes of nitriding, the thickness of the nitrided layer was approximately 8 µm.

The presence of carbide phases in the nitrided layer was not observed in the X-ray diffraction analysis (Figure 5). At the same time, a small amount of an oxide phase corresponding to Fe₃ was detected (Figure 5 and Table 3). The low fraction of oxide phases indicates that the oxide layer has a very small thickness, occupies an insignificant volume fraction of the studied sample, and is associated with its presence on the original surface. The X-ray diffraction data fully confirm the results obtained by scanning electron microscopy.

According to Figure 5, the modified layer exhibits a polycrystalline structure with a preferred crystallographic orientation along the directions (111), (200), and (220), characteristic of the Fe₃C lattice.

Figure 5. X-ray diffraction analysis results.

The tribological investigation results (Figures 6 and 7) showed that the nitrided layer of the modified steel and the surface of the original sample exhibit different friction coefficients, with an average value of μ = 0.436 for steel (Figure 6) and μ = 0.495 for the nitrided layer (Figure 7).

Figure 6. Tribological investigation of the steel surface

Figure 7. Tribological investigation of the nitrided layer surface

The final value is typical for conventionally formed steel layers. These results demonstrate the potential for extending the service life of steel nitrided using electron beams in a medium-vacuum environment. It is noteworthy that the hardness of the nitrided layer increased by almost three times compared to the initial surface (Figure 8).

Figure 8. Hardness of the initial and nitrided samples

Conclusion

The possibility of steel nitriding in electron-beam plasma under direct irradiation using a fore-vacuum electron source has been demonstrated. The beam spectra of the nitrided layer sample indicate that its composition consists mainly of iron and nitrogen, with minor traces of carbon and oxygen. X-ray diffraction analysis revealed a polycrystalline structure of the nitrided layer.

The investigation of surface layer hardness and tribological properties showed a lower friction coefficient compared to steel and a several-fold increase in wear resistance and hardness of the nitrided layer. The obtained results demonstrate the advantages of using fore-vacuum sources for beam-plasma-based nitriding of materials.

References:

1. H Nii and A Nishimoto. Surface modification of ferritic stainless steel by active screen plasma azoting // Journal of Physics: Conference Series 379 (2012) 012052.
2. Akio Nishimoto, Kimiaki Nagatsuka, Ryota Narita, Hiroaki Nii, Katsuya Akamatsu.  Effect of the distance between screen and sample on active screen plasma azoting properties // Surface & Coatings Technology 205 (2010) S365–S368.
3. L. N. Tang, M. F. Yan Influence of Plasma Azoting on the Microstructure, Wear, and Corrosion Properties of Quenched 30CrMnSiA Steel Journal of Materials Engineering and Performance July 2013, Volume 22, Issue 7, pp 2121–2129.
4. D. Leonhardt, S.G. Walton, D.D. Blackwell, R.F. Fernsler, R.A. Meger Low-temperature azoting of stainless steel in an electron beam generated plasma // Surface & Coatings Technology 191 (2005) 255– 262.
5. N.M. Safarov, (phd) A.T. Majidov, I.M. Mirsultanov Сalculation of change of stock moisture content of the drying agent in the process of drying raw cotton in solar drying equipment. Participated in the III International Scienific Conference on Metrological Support of innovative Tehnologies (ICMSIT-III-2022) on March 3-6, / St. Petersburg-Krasnoyarsk. Russia.
6. Akbar Abrorov, Nazirjon Safarov, Fazliddin Kurbonov,  Matluba Kuvoncheva,  Khasan Saidov Mathematical model of hardening the disk-shaped saw teeth with laser beams. Participated in the II International Scienific Conference on “ASEDU-II 2021: Advances in Science, Enjineering Digital Education” on Oktober 28. 2022 / Krasnoyarsk. Russia.
7. Nazirjon Safarov,  Ilkhomjon Mirsultonov Development Of Mathematical Model Of Drying The Raw Cotton During Transportation In Pipeline By Hot Air Flow. Participated in the II International Scienific Conference on “ASEDU-II 2021: Advances in Science, Enjineering Digital Education” on Oktober 28. 2022 / Krasnoyarsk. Russia.
8. Nazirjon Safarov,  Iroda  Mukhammadjanova, Mukhammadali Kabulov Mathematical model of the process of vertical drying of raw cotton in the hot airflow. Participated in the II International Scienific Conference on “ASEDU-II 2021: Advances in Science, Enjineering Digital Education” Krasnoyarsk. Russia.
9. Nazirjon Safarov, Akbar Abrorov, and Laziz Abdullaev AAPM-2023 “Dynamik analiysis of physical and mechanical forces of acting on the needle of a shoe sewing mashine in the process of sewing leather” Journal of Phvsics: Conference Series. 2573 012036
10. Shavkat Aliev, Nazirjon Safarov  Mathematical model of heat treatment of a sewing machine rack using a laser beam universum: технические науки 2013.