Influence of Plasma-Forming Gas Composition on Nitriding in Non-Selfsustained Glow Discharge with Large Hollow Cathode1

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Modification of Material Properties Influence of Plasma-Forming Gas Composition on Nitriding in Non-Selfsustained Glow Discharge with Large Hollow Cathode1

Yu.H. Akhmadeev, I.V. Lopatin, N.N. Koval, P.M. Schanin, Yu.R. Kolobov*, D.S. Vershinin*, and M.Yu. Smolyakova*

Institute of High Current Electronics SB RAS, 2/3, Akademichesky ave., Tomsk, 634055, Russia Phone: +7(3822) 49-17-13, +7(3822) 49-24-10, E-mail: [email protected] *Centre “Nanostructured materials and nanotechnologies” of Belgorod State University 2a, Koroleva st., Belgorod, 308034, Russia

Abstract – The results of influence investigation of rials and products from the oxide layer, which pre- the gas plasma composition (N2, Ar, He) on tita- vents the nitrogen diffusion and retarding the nitriding nium VT1-0 (commercially pure titanium) and process decreases. 4140 steel (0.4%-C; 1.0%-Cr) nitriding process in From this point of view, discharges with the oscil- the non-selfsustained glow discharge plasma with lation of electrons [4–6], such as non-selfsustained a large hollow cathode are presented. It is shown glow discharge with a hollow cathode are of interest. that the nitriding efficiency of 4140 steel slightly Due to the injection of charged particles from an addi- depends on the plasma-forming gas mixture com- tional plasma source, this discharge type stably ope- position. As compared to nitriding in an argon- rates at low pressure (1 Pa). The oscillation of elec- nitrogen mixture of working gases the titanium trons inside the hollow cathode results in an increase VT1-0 nitriding in a helium-nitrogen mixture re- in the generated plasma concentration. sults in a significant increase in the sample surface Preliminary experiments for small volume dis- microhardness. charge structures (600 cm3) [6] revealed this discharge type to be promising for low-temperature titanium 1. Introduction nitriding (up to 500 °C). Dependence of nitriding intensity on the type of gas added to nitrogen was de- One of the most effective methods of improving ope- tected during the experiments. rational properties of machine parts and mechanisms The aim of this work was to study the influence of is the use of materials with special properties of the the plasma-forming gas mixture in the process of tita- surface layer modified by various methods. It is more nium VT1-0 and steel 4140 nitriding in plasma of profitable than to modify the entire volume of mate- non-selfsustained glow discharge with a large hollow rial. Among the different methods to improve lifetime cathode (0.2 m3). Materials for research were selected and reliability of parts, ion-plasma surface treatment within the prospects for their widespread use in indus- in the gas discharges plasma and, in particular, nitrid- try and the existence of preliminary experimental data ing have been widely developed [1, 2] in recent years. 3 obtained by the authors and the availability of the pub- To create a nitrogen-plasma in large (0.1–1 m ) lished data on conventional nitriding in a selfsustained vacuum volumes the glow and arc discharges are mainly glow discharge. used [1, 2]. The arc discharge is interesting due to the possibility of obtaining high discharge current (50– 2. Experimental 200 A) in a continuous working mode at low voltages (20–50 V) and low working gas pressure (0.01–1 Pa) Investigation of influence of plasma-forming gas mix- [2]. That allows to generate plasma with concentration ture to a depth of the modified layer, phase composi- 9 10 –3 ne ≈ 10 –10 cm . However, the presence of erosion tion and performance characteristics of the materials of the cathode material in the arc discharge products were carried out on the experimental stand established results in narrowing the range of their technological on the basis of industrial ion-plasma deposition instal- applications. The main disadvantage of glow discharge lation NNV-6.6-I1 (Fig. 1). This installation was de- is relatively high pressure of working gas (10–100 Pa) scribed in details in [7]. and high discharge voltages (600–700 V) [3]. Under Nitriding was carried out at a working gas pressure these conditions, the ions arriving at the cathode (the (mixture of working gases) 1 Pa, the voltage of non- sample under treatment) lose most of their energy due selfsustained glow discharge was 300–400 V, the ion to multiple collisions in the cathode layer. As a result, current density on the samples was 1.2 mA/cm2. The the effectiveness of surface cleaning of treated mate- samples were fixed on a holder, which was under the

1 The work was supported by the State contract P1626 under analytical equipment of CJU “Diagnostics of structure and properties of nanomaterials” of BSU and RFBR Grant No. 08-08-92207-GFEN_a, performed at the Institute of High Current Electronics SB RAS, Tomsk. 228 Oral Session

2 Table 1. Microhardness of the samples surface layer after nitriding in various gases, GPa 3 Working gas 1 N N + Ar N + He Material 2 2 2 Gas Water 4140 steel 11.4 10.8 11.8 VT1-0 initial 2.7 3.4 4 7 6 Igl 5 U gl the near-surface layer has the maximum hardness, which thickness is 5–10 µm. The initial microhardness of 4140 steel was ≈ 2.7 GPa, and titanium VT1-0 was ≈ 2 GPa. Fig. 1. Circuit diagram of the experimental stand: 1 – glow 12 HV0.01, GPa discharge water-cooled tube anode; 2 – vacuum chamber N2 (glow discharge cathode); 3 – arc discharge grid anode; 4 – N2 + Ar arc discharge igniting electrode; 5 – magnetic coil; 6 – N + He diaphragm; 7 – samples 8 2 potential of the hollow cathode (working chamber). Nitriding process lasted for 1 h at a temperature of the 4 samples being about ∼480–500 °C. Heating and cleaning of the samples were carried out by means of ion bombardment. Ions were accelerated in the cathode layer of a non-selfsustained glow discharge. 0 100 200 300 x, μm To determine the optimal parameters of nitriding, Fig. 2. Distribution of microhardness over the depth of the a series of experiments using pure nitrogen, argon, samples 4140 steel after nitriding for 1 h at a temperature nitrogen and helium-nitrogen gas mixture were carried ≈ 500 °C out. The percentage of gases was chosen from the modes obtained previously [6] and was 40% for argon At the same time, nitriding of titanium in a gas and helium and 60% for nitrogen. mixture of helium-nitrogen results in greater increase Preliminary preparation of the samples before loa- in microhardness both on the surface of titanium ding into the vacuum chamber included the following (Table 1) and in depth (Fig. 3), compared to nitriding operations: sanding, polishing, washing with gasoline in pure nitrogen or argon-nitrogen gas mixture. and acetone in an ultrasonic bath for removing organic HV0.005, GPa contaminants; rubbing surface in alcohol. Final cleaning was carried out in argon plasma for 10 min. 4 N2 The studies of the structure and phase composition N2 + Ar of the nitrided samples were carried out by optical N2 + He metallography (optical microscope Olympus GX 71), scanning electron microscopy (Quanta 600 FEG) and 3 the X-ray diffraction (diffractometer “ARL X’TRA” for CuKα-radiation). Measurement of microhardness was carried out 2 using the automatic hardness-testing machine “DM 8B AUTO”. It was performed on the surface and in depth of the samples at loads on the indenter of 0.05 and 0 20 40 60 80 x, μm 0.1 N, with step of 10 µm from the surface into the sample. Fig. 3. Distribution of microhardness over the depth of the samples of titanium VT1-0 after nitriding in gas mixtures

3. Results and discussion Usage of helium in a mixture with nitrogen led to Microhardness of treated material surface is shown surface microhardness increasing 1.3 times, while the in Table 1. modified layer with higher hardness has a thickness of The analysis of the results presented in Table 1 ≈ 20–35 µm. demonstrates that nitriding of 4140 steel in pure nitro- The scanning electron microscopy analysis of the gen for 1 h results in the increase of microhardness microstructure of a cross section 4140 steel (Fig. 4a) both in the sample surface (Table 1) and depth of the after nitriding revealed two obviously different layers: sample (Fig. 2). For 4140 steel the addition of argon or nitride (TiN) and diffusion layer passing smoothly helium in nitrogen does not lead to significant change into the bulk material. Nitride layer has a thickness of in microhardness as compared to pure nitrogen. Harde- 4–10 µm depending on the composition of a plasma- ned layer has a thickness of about 100–150 µm, and forming gas mixture, and it is identified as Fe4N.

229 Modification of Material Properties

50 μm 5 μm

a b Fig. 4. The structure of cross sections of the samples 4140 steel (a) and titanium VT1-0 (b) after nitriding at a temperature of 480–500 °C for 1 hour in a N2 + Ar gas mixture: I – nitride layer; II – diffusion layer passing smoothly into the bulk material

As a result of titanium VT1-0 nitriding (Fig. 4b), ces [9]. To reduce the time of calculations the mathe- nitride and diffusion layers are also formed, that is matical package Maple was used. confirmed by appearance of the sample cross section. The calculations showed that the composition of Due to the fact that a layer of titanium nitride is more plasma-forming gas did not significantly impact the fragile than the diffusion layer, it could fail in the diffusion coefficient of nitrogen in the 4140 steel process of the cross section sample production. The (Table 2). thickness of the nitride layer was 2–2.3 µm for the Table 2. Estimated diffusion coefficient helium-nitrogen mixture, 1.3–1.7 µm for the argon- nitrogen mixture of working gases, and less than Gas mixture N2 Ar + N2 He + N2 0.5 µm for pure nitrogen. Diffusion 2 –14 –14 –14 Using a scanning electron microscope Quanta 600 coefficient, m /s 6.4 ⋅ 10 5.5 ⋅ 10 4.2 ⋅ 10 FEG equipped with an X-ray attachment “Trident” allowing to determine the chemical composition of Nowadays there are several theories explaining the the material the elemental chemical composition in process of nitriding in glow discharge [10]. But so far depth of the 4140 steel samples was analyzed and researchers have not reached a common opinion on the the dependences of the distribution of nitrogen in question of nitrogen saturation mechanism of mate- depth on the plasma-forming gas mixture composition rials subjected to treating in plasma of gas discharge. were obtained (Fig. 5). One approach to explain the process of saturation, the main role in the process of nitriding in a glow dis- Wt., % charge is given to atomic nitrogen, and the intensity of the process is determined by the quantity of atomic 8 N 2 nitrogen. N2 + Ar One of the ways to explain the formation of atomic 6 N2 + He nitrogen in the gas discharge, in addition to dissociation by electron impact, dissociative recombination, etc., can be ion-molecular reactions that result in the 4 dissociation of nitrogen molecules with the formation of neutral and ion of nitrogen. Ions can acquire rather high energy and be recharged in the cathode potential 2 drop, which focuses almost all of the discharge voltage. Because of the directed motion of particles in the cathode layer and subsequent recombination on the sur- 0 5 10 15 20 x, μm face the layer of adsorbed nitrogen particles is formed. Under the continuing influence of the incident flux, Fig. 5. Distribution of nitrogen in depth of the steel 4140 samples after nitriding in a mixture of gases the particles of the adsorbed layer can be sputtered or embedded into the surface creating a solid solution. Regarding the dependencies obtained, the esti- Under such conditions, the value of the nitrogen con- mates of the nitrogen diffusion rate in 4140 steel were centration is the predetermining factor for the start of carried out depending on the gas mixture composition. nitrides formation, which in its turn impairs the nitro- The calculation of the diffusion coefficient was per- gen diffusion from the surface into the treated material. formed using the second Fick’s law [8]. The solution The studies of the phase composition of nitrided of the equation was performed by finite different- samples performed by X-ray analysis using a sliding 230 Oral Session beam method showed that nitriding of 4140 steel at ness (up to 3.4 GPa) and formed by nitride phases of a temperature of 480–500 °C results in the formation titanium. Also a diffusion layer is formed (thickness of the surface layer with a complex phase composi- of 35 µm and hardness of up to 2.5 GPa) passing tion, which includes the following phases: Fe2N; smoothly into the bulk material, the hardness of which Fe3N; Fe4N; Fe3O4. The analysis of the data obtained is ≈ 2 GPa. for the nitrided samples of titanium VT1-0 in all gas The structure and microhardness of the nitrided mixtures shows that the following phases are formed: layer of technically pure titanium essentially depend

α-TiN0.3; ζ-Ti4N3–x (Fig. 6). on the composition of the working gas. The highest microhardness of the titanium surface is created by α-TiN 0.3 using a helium-nitrogen gas mixture (N2–He). I, % N2 + He 60 Nitriding of 4140 steel in the plasma of the non- selfsustained glow discharge with a hollow cathode 40 α-Ti ζ-Ti4N3–x results in the formation of a multilayer structure with 20 a hardness of 11–12 GPa and a thickness of up to 0 150 µm. The intensity of nitriding depends slightly on 34 36 38 40 42 44 the composition of the working gases mixture. 2Θ, degree Fig. 6. Part of X-ray pattern of the surface layer of titanium References VT1-0 after nitriding in the non-selfsustained glow discharge [1] T.A. Panaioti, Fiz. Khim. Obr. Mater. 4, 70–78 with a large hollow cathode (2003). Moreover, it should be noted that the phases of [2] P.M. Schanin, N.N. Koval, I.M. Goncharenko, and S.V. Grigoriev, Fiz. Khim. Obr. Mater. 3, 16–19 TiN and Ti2N were not found out, that can be ex- plained by the small thickness of the nitride layer. (2001). In this type of discharge, due to the fact that [3] Yu.M. Lakhtin, Ya.D. Kogan, G.I. Spiess, and nitiding occurs at low pressures (≈ 1 Pa), the mean Z. Bemer, Theory and Technology of Nitriding, free path of ions bombarding the surface of the treated Moscow, Metallurgiya, 1991, pp. 320. samples is comparable to the width of the cathode [4] V.V. Budyliv and R.D. Agzamov, in: Proc. of 6th sheath. Under these conditions, the ions passing the Intern. Conf. on Modif. of Mater. with Particle accelerating cathode fall (300–400 V) without collid- Beams and Plasma Flow, 2002, pp. 428–431. ing with an energy equal to the cathode fall effectively [5] A.S. Metel, S.N. Grigoriev, Yu.A. Melnik, and sputter oxides and nitrides impeding the diffusion of V.V. Panin, Plasma Phys. 35, 12, 1140–1149 nitrogen into the sample, that determines the satisfac- (2009). tory conditions of nitriding of titanium. [6] Yu.H. Akhmadeev, Yu.F. Ivanov, N.N. Koval et al., Rus. Surface, X-ray, Synchrotron and Neutron Re- 4. Conclusion search 2, 108–112 (2008). [7] I.V. Lopatin, Yu.H. Akhmadeev, N.N. Koval et al., The principal possibility of nitriding the samples of in: Proc. IX Int. Conf. on Modification of Mate- 4140 steel and titanium VT1-0 in the plasma of the rials with Particle Beams and Plasma Flows, non-selfsustained glow discharge with a large hollow 2008, pp. 312–315. cathode at low pressures (up to 1 Pa), when the treated [8] Ya.E. Geguzin, The Diffusion Zone, Moscow, samples are under the cathode potential, is shown. Nauka GRFML, 1979, pp. 342. Nitriding of titanium VT1-0 in the plasma of the [9] G. Korn and T. Korn, Handbook of Mathematics non-selfsustained glow discharge with a large hollow (for Scientists and Engineers), Moscow, Nauka, cathode at low (up to 1 Pa) pressures and rather low 1974, pp. 832. (< 500 °C) temperatures results in the formation of a [10] I.M. Pastukh, Theory and Practice Bezvodorod- multilayer structure consisting of a near-surface layer nogo Nitriding in a Glow Discharge, Kharkiv, (thickness up to 2.5 µm) possessing rather high hard- NSC KIPT, 2006, pp. 364.

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