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Aspects of the practical application of titanium alloys after low temperature nitriding glow discharge in hydrogen- free -gas media
Applied Surface Science 392 (2017) 356–361
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Perspective Article
Aspects of the practical application of titanium alloys after low temperature nitriding glow discharge in hydrogen- free -gas media N.S. Mashovets a,∗ , I.M. Pastukh a , S.M. Voloshko a,b a b
Khmelnickiy National University, Ukraine National Technical University of Ukraine “Kyiv Polytechnic Institute”, Ukraine
a r t i c l e
i n f o
Article history: Received 1 March 2016 Received in revised form 19 August 2016 Accepted 22 August 2016 Available online 15 September 2016 Keywords: Titanic alloy Nitriding in glow discharge Layer formation XPS AES
a b s t r a c t X-ray diffraction analysis, X-ray photoelectron spectroscopy, and Electron Auger-spectroscopy investigation of phase transformation on the surface of the VT8 titanium alloy after a low temperature hydrogen-free nitriding in a glow discharge. Operational characteristics of titanium alloys defined physical-mechanical characteristics of the surface and their phase composition, which depend on the process parameters of nitriding. Surface modification of titanium alloys were carried out by low-temperature nitriding in a glow discharge in hydrogen-free environment. The main advantage of this method lies in the absence of hydrogen embrittlement and complete environmental safety process. Application of the glow discharge can not only speed up the process by the order of the diffusion surface saturation with nitrogen, but also significantly alters the kinetics of the process and quality of the nitrided layer, in particular its physio-mechanical properties and phase composition. For research purposes, the standards from an ␣ +  alloy Ti-Al6-Cr2-Mo2,5 (VT8) were used. Research into the phase composition was performed by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Stratified analysis by AES was conducted by etching the surface of the samples’ argon ion beam with diameters of 1.5 mm with an energy of 3000 eV and a current density of 400 mA/cm2 . The above material shows the promise of the technology of low-temperature hydrogen-nitriding by glow discharge. This greatly expands the range of practical applications of titanium alloys. In addition, changing the technological mode allows you to manage a wide range of modified phase composition of the surface layer and as a result – to form the surface of titanium parts, taking into account the conditions of the subsequent operation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Traditionally, surface modification of titanium alloys by glow discharge nitriding was carried out at a relatively high temperature and in atmospheres containing hydrogen [1]. However, the surface layer becomes brittle, which greatly narrows the range of practical uses of the technology. When low-temperature hydrogenfree-nitriding in glow discharge (NGDs) is formed on the surface of a relatively thin layer of nitride structures and there is a relatively small depth of the diffusion zone [2]. However, due to the greater plasticity of the surface layer, new opportunities open up for the use
∗ Corresponding author at: Khmelnickiy State University, Faculty of Mechanical Engineering, Str. Institutskaya, 11 29016 Khmelnickiy, Ukraine. E-mail addresses: [email protected] (N.S. Mashovets), [email protected] (I.M. Pastukh). http://dx.doi.org/10.1016/j.apsusc.2016.08.180 0169-4332/© 2016 Elsevier B.V. All rights reserved.
of the modified titanium alloys in a variety of industries. This work is devoted to the results of the low-temperature NGDs to develop recommendations for further practical application.
2. Materials and methods of investigation Research into the phase composition was performed by Xray diffraction, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Filming X-ray diffraction peaks was carried out on a DRON-3 M CO K␣ radiation scheme –2 in the angular range of 30◦ –100◦ in increments of 0,05◦ and 2 s exposure time. With the use of X-ray diffraction, the coating thickness was also determined [3]. Stratified analysis by AES was conducted by etching the surface of the samples’ argon ion beam with diameters of 1.5 mm with an energy of 3000 eV and a current density of 400 mA/cm2 . This automatic system allows for the analysis of
N.S. Mashovets et al. / Applied Surface Science 392 (2017) 356–361
357
Fig. 1. Radiographs for the VT8 titanium alloy, nitrided under the №2 regime (a) and the № 1 regime (b).
layered depth no less than 0.5–1.0 nm. The studies were performed using the serial scanning electron Auger spectrometer JAMP-10S. X-ray photoelectron spectra were obtained on the “SERIES 800 XPS” Kratos Analytical electron spectrometer using nonmonochromatic X-ray emitter MgK␣ (1253.6 eV). Photoelectron core-level spectra were acquired using a hemispherical analyzer at a pass-energy of 50 eV with a 0.025 eV energy step. The overall resolution of the spectrometer in this operating mode was 0.95 eV measured as a full width at half maximum (FWHM) of the Ag 3d5/2
line. After subtraction of the Shirley-type background, the corelevel spectra were decomposed into their components with mixed Gaussian–Lorentzian lines by a non-linear least-squares curvefitting procedure, using the public software package XPSPEAK 4.1. The binding energies, FWHM and areas of the peaks’ components were determined from the fitting results. The carbon C1s line at 284.9 eV was taken as a reference for surface-charging corrections. The peak areas and standard sensitivity factors were used to evaluate the surface composition of the samples. On the surface of the
358
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Fig. 2. The spectra of Auger electrons for the nitrided titanium alloy VT 8 (mode №1). 1 – the original surface: Ti (N) – 28; About – 18; C – 54 at.%; 2 – etched with Ar + ions 45 min: Ti (N) – 7,2; O – 20; C – 8 at.%.
nitrided titanium alloy ␣ +  alloy Ti-Al6-Cr2-Mo2,5 (VT-8) were recorded spectra core levels for Ti2p, O1s and N1s. Surface modification of titanium alloys were carried out by low-temperature nitriding in a glow discharge in hydrogen-free environment. The main advantage of this method lies in the absence of hydrogen embrittlement and complete environmental safety process. Experimental set nitriding in a glow discharge in hydrogen-free environments was developed in Khmelnytsky National University. The installation includes the discharge chamber vacuum system, the preparation of the gaseous medium, and power supply and management. The latter was primarily designed to maintain the glow of electric discharge in the gas and at least short-term exclusion of its transition to the arc. Technical characteristics of the installation: -
Power 40 kW; Three-phase alternating current, 50 Hz 380 V; The maximum voltage in the cell 1500 V; The diameter of the vacuum chamber 600 mm; The height of the vacuum chamber 1000 mm; Minimum vacuum of 1 Pa; Operating temperature range 400–1100 ◦ C; Maximum weight of 500 kg cages.
There was control over the composition of the gaseous medium, the temperature, the pressure in the discharge chamber, the voltage on the electrodes and the current camera therein (respectively – the current density) during the process. As noted above, a feature of the installation is to use a hydrogen-free gas mixture. However, the absence of hydrogen, which is a reducing metal and neutralizes oxygen requires especially pure gas (99.9%) and a completely sealed gas supply system and vacuum chamber. Even a small amount of oxygen (0.1%) causes the formation of oxides on a metal surface and greatly reduces the efficiency of the process up to its total cessation. Therefore, in contrast to systems that operate with hydrogensaturating fluids (ammonia, a mixture of nitrogen and hydrogen), this unit has separators for oxygen and moisture. Application of the glow discharge can not only speed up the process by the order of the diffusion surface saturation with nitrogen, but also significantly alters the kinetics of the process and quality of the nitrided layer, in particular its physio-mechanical properties and phase composition. This allows for the optimization of the mechanical properties of the surface layer depending on the operating conditions of structural elements and also improves their wear resistance. Nitriding in glow discharge was conducted after different regimes, the technological parameters of the process were varied in such scopes: temperature (T ◦ C) within the limits of 540–700 ◦ C, pressure (P, Pa) – 80 to 400 Pa, time (min) – 20 to 240 min, maintenance of argon (Ar%) – 0 to 96%. Research of fretting-resistant titanic alloy Ti-Al6-Cr2-Mo2,5 were conducted on the special options which are described in work [4,5]. Low temperature NGDs were performed with the following parameters modes: • Mode 1 - surface temperature of 660 ◦ C and pressure of 160 Pa, nitriding duration of 75 min; • Mode 2 - surface temperature of 580 ◦ C, pressure of 160 Pa, nitriding duration 75 min; • Mode 3 - the surface temperature of 700 ◦ C, pressure of 240 Pa, nitriding duration 130 min; • Mode 4 - the surface temperature of 620 ◦ C, pressure of 240 Pa, the duration of the nitriding for 20 min. Modes 1 and 2 used gaseous medium by volume of components – 28% nitrogen, 72% argon; in modes 3 and 4–52% nitrogen, 48% argon.
Fig. 3. Layered change in the content of components in the depth of the surface layer of the nitrided titanium alloy VT8: a) Mode 1, b) Mode 2.
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Table 1 The binding energy of the most intense components of the O1s , N1s and Ti2p3/2 spectra before/after ion etching of the nitrided titanium alloy VT8, eV.
Mode 1 Mode 2 Mode 3 Mode 4
O1s
N1s
Ti2p3/2
530.5/529.9 530.6/530.0 530.2/530.1 530.9/529.8
397.4/397.4 397.5/397.5 397.1/397.1 397.3/397.3
459.1/453.2 459.0/455.8 458.7/455.0 458.9/455.7
The direct current power supply was used. During the process, the voltage and current change depending on the mode of nitriding. 3. Results of investigation and discussion X-ray diffraction studies have shown that as a result of low temperature NGDs on the surface of the titanium alloy VT8, the mononitride TiN (␦ phase) and – Ti2N phase is formed. It is possible to fix these phases, after nitriding at 660 ◦ C. At a temperature of 580 ◦ C, such phases are not detected, and as a result the X-rays pass deeper than the diffusion layer. The thickness of the nitride layer is fixed: for Mode 1–2.4 m, Mode 2–1.8 m, Mode 3–4.1 m, Mode 4–2.1 m. The predominant structures of the generated modified layer when a mode 1 is the phase of Ti2 N and to a lesser extent TiN (␦ phase) and in mode 2, – Ti-␣, whereas in the initial state, there is substantially Ti – ␣, Ti- phase. Radiographs for modes 1 and 2 are shown in Fig. 1. In addition, the phase composition of the modified surface was investigated by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The AES method studied the surface of the titanium alloy VT8, nitrided by modes №1, 2, 3, 4. In these cases, the distribution of elements was determined by depth in combination with argon ion sputtering material. Fig. 2 shows the Auger spectra for both the initial state and after ion etching the surface of the nitrided titanium alloy VT8. Analysis of changes in the content of elements in the surface layers of the nitrided titanium alloy under ion etching can be concluded that the surface is characterized by an increased content of carbon and oxygen which are surface contaminants. After ion etching their content drops sharply to a depth of about 50–100 Å, and then remains practically constant. By way of removal of carbon and oxygen in the Auger spectra, peaks increase for major components. Carrying out stratified analysis establishes a significant heterogeneity of distribution of the depth of all investigated elements. Fig. 3 shows the layered dependencies on the main components of the studied alloys produced by ion etching. Comparison of the peak intensities after nitriding according to various modes indicates that after nitriding at 580 ◦ C (mode 2), the intensity of the peaks is significantly lower than at 660 ◦ C (mode 1). The activity of the titanium alloy in interaction with oxygen indicates the following. Despite the fact that the pre-etched ion bombardment nitriding oxide structures were removed previously, after a brief period in air by way of XPS, oxygen compounds were detected again. Spectra were detected on the surface of the titanium alloy VT-8 when nitrided for modes 1, 2, 3, 4 were recorded for core levels of Ti2p, O1s and N1s. Given that the method gives information about the XPS surface composition at a depth of not more than 10 nm from the surface, one sample was etched by ion bombardment of Ar+ with the spectrum once again recorded. On the basis of this data on the composition of elements, the binding energies of the core electrons of atoms of oxygen, nitrogen and titanium were calculated (Table 1). For all the samples, the O1s line is rather broad and includes at least two intense components (Fig. 4). The dominant component at a binding energy of ∼530 eV is obviously related to the O2− ions in the lattice of TiO2, whereas the other component at a bind-
Fig. 4. X-ray photoelectron spectra of O1s – electrons: 1 – Mode №2; 2 – Mode №1 after ion etching; 3 – Mode №1; 4 – Mode №3; 5 – Mode №4.
ing energy of ∼532 eV corresponds to OH groups (mainly Ti OH bonds) and indicates the appreciable hydroxylation of the surface. The intensity of the peaks of oxygen for various modes of nitriding of the titanium alloy is significantly different. The maximum value corresponds to the regimes №1 and №2, but after cleaning, the ion intensity falls sharply. For modes №3 and №4 intensity is very low. This indicates that the oxide film is small. Its presence is caused by absorption of oxygen atoms during nitriding. After sputtering Ar+ , peak II is lost and the intensity of peak III is reduced, but after two hours of etching, peak III completely disappears, indicating that the removal of the surface oxide. This confirms the formation of titanium nitride on the surface of the lowtemperature glow discharge nitriding at a temperature of 580 ◦ C. The Ti 2p3/2 peak at 457.1 eV implies formation on the alloy surface of the TiNx Oy -type oxynitride [6,7]. This component, however, may also include contributions from a shake-up satellite, which was shown to arise in Ti 2p spectra of TiN at 1.5–2.8 eV higher binding energy with respect to the main Ti N peak [6,8], and reduced Ti3+ −O species. Finally, the Ti 2p3/2 peak at 458.6 eV can be ascribed to Ti4+ species in somewhat reduced (because of preliminary ion bombardment) TiO2 . The XPS spectra (Figs. 5 and 6) for mode 1, before purification by ion sputtering the Ti2p region correspond to the three groups of the doublet. The peak of the I corresponds to the titanium nitride TiN with a binding energy Eb = 455.4 eV and Eb = 461.1 eV. Peaks II and III at high binding energies are closely related to the Me O bond in various oxide forms, which corresponds to that of Ti O N (Eb = 457.8 eV and Eb = 463.5 eV) and of titanium Dioxide (Eb = 459.1 eV and Eb = 464.6 eV). The intense peak in emission of N1s photoelectrons which is observed for all the samples at binding energies of 397.1–397.5 eV (Fig. 6b) indicates the presence of N3− electronic state and formation of titanium nitride phase TiN in the VT8 alloy under all treatment conditions. The noticeable asymmetry of the N1 s line towards higher binding energies implies also formation on the surface of oxynitride-type bonds N Ti O (∼398.8 eV) and N (O, C) bonds (∼400 eV).
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Fig. 5. X-ray photoelectron spectra of Ti2p – electrons for regime №1, (a) – prior to the ion etching (b) – after ion etching (the peak of the I corresponds to TiN, the peak II – Ti-ON, peak III to TiO2 ).
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Fig. 6. a) X-ray photoelectron spectra of Ti2p – electrons, b) X-ray photoelectron spectra of N1s – electrons, where 1 – the mode №2; 2 – Mode №1 after ion etching; 3 – Mode №1; 4 – Mode №3; 5 – Mode №4.
4. Conclusions The above material shows the promise of the technology of lowtemperature hydrogen-nitriding by glow discharge. This greatly expands the range of practical applications of titanium alloys. In addition, changing the technological mode allows you to manage a wide range of modified phase composition of the surface layer and as a result – to form the surface of titanium parts, taking into account the conditions of the subsequent operation. References [1] B.N. Arzamasov, A.G. Bratuhin, S. Eliseev, T. Panagiotou, Ionic Chemical Heat Treatment of Alloys Moscow, Publishing House of the MSTU Bauman, 1999, pp. 400 (in Russian). [2] I.M. Pastukh, Theory and Practice of Without Hydrogen Glow Discharge Nitriding, KIPT, Kharkov, 2006.
[3] S.S. Gorelik, Y.A. Skakov, L.N. Rastorguev, X-ray and Electron-Optical Analysis, MISIS, Moscow, 1994, 328 p. (in Russian). [4] I.M. Pastukh, N.S. Mashovets, Yu.I. Shalapko, V.A. Kurskaja, Forecast the formation of nitrides on the surface of titanium alloys during nitriding in a glow discharge, J. Achiev. Mater. Manuf. Eng. 62 (2) (2014) 53–62. [5] N.S. Mashovets, Influence of parameters ionic nitriding the titan on physics-mechanical descriptions of nitriding layer, Her. Engine Build. 3 (2004) 122–125. [6] A. Glaser, S. Surnev, F.P. Netzer, N. Fateh, G.A. Fontalvo, C. Mitterer, Oxidation of vanadium nitride and titanium nitride coatings, Surf. Sci. 601 (2007) 1153–1159. [7] M.J. Jung, K.H. Nam, Y.M. Chung, J.H. Boo, J.G. Han, The physiochemical properties of TiOx Nx films with controlled oxygen x y partial pressure, Surf. Coat. Technol. 171 (2003) 71–74. [8] M.A. Vasylyev, S.P. Chenakin, L.F. Yatsenko, Nitridation of TiA6AlA4 V alloy under ultrasonic impact treatment in liquid nitrogen, Acta Mater. 60 (2012) 6223–6233.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Perspective Article
Aspects of the practical application of titanium alloys after low temperature nitriding glow discharge in hydrogen- free -gas media N.S. Mashovets a,∗ , I.M. Pastukh a , S.M. Voloshko a,b a b
Khmelnickiy National University, Ukraine National Technical University of Ukraine “Kyiv Polytechnic Institute”, Ukraine
a r t i c l e
i n f o
Article history: Received 1 March 2016 Received in revised form 19 August 2016 Accepted 22 August 2016 Available online 15 September 2016 Keywords: Titanic alloy Nitriding in glow discharge Layer formation XPS AES
a b s t r a c t X-ray diffraction analysis, X-ray photoelectron spectroscopy, and Electron Auger-spectroscopy investigation of phase transformation on the surface of the VT8 titanium alloy after a low temperature hydrogen-free nitriding in a glow discharge. Operational characteristics of titanium alloys defined physical-mechanical characteristics of the surface and their phase composition, which depend on the process parameters of nitriding. Surface modification of titanium alloys were carried out by low-temperature nitriding in a glow discharge in hydrogen-free environment. The main advantage of this method lies in the absence of hydrogen embrittlement and complete environmental safety process. Application of the glow discharge can not only speed up the process by the order of the diffusion surface saturation with nitrogen, but also significantly alters the kinetics of the process and quality of the nitrided layer, in particular its physio-mechanical properties and phase composition. For research purposes, the standards from an ␣ +  alloy Ti-Al6-Cr2-Mo2,5 (VT8) were used. Research into the phase composition was performed by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Stratified analysis by AES was conducted by etching the surface of the samples’ argon ion beam with diameters of 1.5 mm with an energy of 3000 eV and a current density of 400 mA/cm2 . The above material shows the promise of the technology of low-temperature hydrogen-nitriding by glow discharge. This greatly expands the range of practical applications of titanium alloys. In addition, changing the technological mode allows you to manage a wide range of modified phase composition of the surface layer and as a result – to form the surface of titanium parts, taking into account the conditions of the subsequent operation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Traditionally, surface modification of titanium alloys by glow discharge nitriding was carried out at a relatively high temperature and in atmospheres containing hydrogen [1]. However, the surface layer becomes brittle, which greatly narrows the range of practical uses of the technology. When low-temperature hydrogenfree-nitriding in glow discharge (NGDs) is formed on the surface of a relatively thin layer of nitride structures and there is a relatively small depth of the diffusion zone [2]. However, due to the greater plasticity of the surface layer, new opportunities open up for the use
∗ Corresponding author at: Khmelnickiy State University, Faculty of Mechanical Engineering, Str. Institutskaya, 11 29016 Khmelnickiy, Ukraine. E-mail addresses: [email protected] (N.S. Mashovets), [email protected] (I.M. Pastukh). http://dx.doi.org/10.1016/j.apsusc.2016.08.180 0169-4332/© 2016 Elsevier B.V. All rights reserved.
of the modified titanium alloys in a variety of industries. This work is devoted to the results of the low-temperature NGDs to develop recommendations for further practical application.
2. Materials and methods of investigation Research into the phase composition was performed by Xray diffraction, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Filming X-ray diffraction peaks was carried out on a DRON-3 M CO K␣ radiation scheme –2 in the angular range of 30◦ –100◦ in increments of 0,05◦ and 2 s exposure time. With the use of X-ray diffraction, the coating thickness was also determined [3]. Stratified analysis by AES was conducted by etching the surface of the samples’ argon ion beam with diameters of 1.5 mm with an energy of 3000 eV and a current density of 400 mA/cm2 . This automatic system allows for the analysis of
N.S. Mashovets et al. / Applied Surface Science 392 (2017) 356–361
357
Fig. 1. Radiographs for the VT8 titanium alloy, nitrided under the №2 regime (a) and the № 1 regime (b).
layered depth no less than 0.5–1.0 nm. The studies were performed using the serial scanning electron Auger spectrometer JAMP-10S. X-ray photoelectron spectra were obtained on the “SERIES 800 XPS” Kratos Analytical electron spectrometer using nonmonochromatic X-ray emitter MgK␣ (1253.6 eV). Photoelectron core-level spectra were acquired using a hemispherical analyzer at a pass-energy of 50 eV with a 0.025 eV energy step. The overall resolution of the spectrometer in this operating mode was 0.95 eV measured as a full width at half maximum (FWHM) of the Ag 3d5/2
line. After subtraction of the Shirley-type background, the corelevel spectra were decomposed into their components with mixed Gaussian–Lorentzian lines by a non-linear least-squares curvefitting procedure, using the public software package XPSPEAK 4.1. The binding energies, FWHM and areas of the peaks’ components were determined from the fitting results. The carbon C1s line at 284.9 eV was taken as a reference for surface-charging corrections. The peak areas and standard sensitivity factors were used to evaluate the surface composition of the samples. On the surface of the
358
N.S. Mashovets et al. / Applied Surface Science 392 (2017) 356–361
Fig. 2. The spectra of Auger electrons for the nitrided titanium alloy VT 8 (mode №1). 1 – the original surface: Ti (N) – 28; About – 18; C – 54 at.%; 2 – etched with Ar + ions 45 min: Ti (N) – 7,2; O – 20; C – 8 at.%.
nitrided titanium alloy ␣ +  alloy Ti-Al6-Cr2-Mo2,5 (VT-8) were recorded spectra core levels for Ti2p, O1s and N1s. Surface modification of titanium alloys were carried out by low-temperature nitriding in a glow discharge in hydrogen-free environment. The main advantage of this method lies in the absence of hydrogen embrittlement and complete environmental safety process. Experimental set nitriding in a glow discharge in hydrogen-free environments was developed in Khmelnytsky National University. The installation includes the discharge chamber vacuum system, the preparation of the gaseous medium, and power supply and management. The latter was primarily designed to maintain the glow of electric discharge in the gas and at least short-term exclusion of its transition to the arc. Technical characteristics of the installation: -
Power 40 kW; Three-phase alternating current, 50 Hz 380 V; The maximum voltage in the cell 1500 V; The diameter of the vacuum chamber 600 mm; The height of the vacuum chamber 1000 mm; Minimum vacuum of 1 Pa; Operating temperature range 400–1100 ◦ C; Maximum weight of 500 kg cages.
There was control over the composition of the gaseous medium, the temperature, the pressure in the discharge chamber, the voltage on the electrodes and the current camera therein (respectively – the current density) during the process. As noted above, a feature of the installation is to use a hydrogen-free gas mixture. However, the absence of hydrogen, which is a reducing metal and neutralizes oxygen requires especially pure gas (99.9%) and a completely sealed gas supply system and vacuum chamber. Even a small amount of oxygen (0.1%) causes the formation of oxides on a metal surface and greatly reduces the efficiency of the process up to its total cessation. Therefore, in contrast to systems that operate with hydrogensaturating fluids (ammonia, a mixture of nitrogen and hydrogen), this unit has separators for oxygen and moisture. Application of the glow discharge can not only speed up the process by the order of the diffusion surface saturation with nitrogen, but also significantly alters the kinetics of the process and quality of the nitrided layer, in particular its physio-mechanical properties and phase composition. This allows for the optimization of the mechanical properties of the surface layer depending on the operating conditions of structural elements and also improves their wear resistance. Nitriding in glow discharge was conducted after different regimes, the technological parameters of the process were varied in such scopes: temperature (T ◦ C) within the limits of 540–700 ◦ C, pressure (P, Pa) – 80 to 400 Pa, time (min) – 20 to 240 min, maintenance of argon (Ar%) – 0 to 96%. Research of fretting-resistant titanic alloy Ti-Al6-Cr2-Mo2,5 were conducted on the special options which are described in work [4,5]. Low temperature NGDs were performed with the following parameters modes: • Mode 1 - surface temperature of 660 ◦ C and pressure of 160 Pa, nitriding duration of 75 min; • Mode 2 - surface temperature of 580 ◦ C, pressure of 160 Pa, nitriding duration 75 min; • Mode 3 - the surface temperature of 700 ◦ C, pressure of 240 Pa, nitriding duration 130 min; • Mode 4 - the surface temperature of 620 ◦ C, pressure of 240 Pa, the duration of the nitriding for 20 min. Modes 1 and 2 used gaseous medium by volume of components – 28% nitrogen, 72% argon; in modes 3 and 4–52% nitrogen, 48% argon.
Fig. 3. Layered change in the content of components in the depth of the surface layer of the nitrided titanium alloy VT8: a) Mode 1, b) Mode 2.
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Table 1 The binding energy of the most intense components of the O1s , N1s and Ti2p3/2 spectra before/after ion etching of the nitrided titanium alloy VT8, eV.
Mode 1 Mode 2 Mode 3 Mode 4
O1s
N1s
Ti2p3/2
530.5/529.9 530.6/530.0 530.2/530.1 530.9/529.8
397.4/397.4 397.5/397.5 397.1/397.1 397.3/397.3
459.1/453.2 459.0/455.8 458.7/455.0 458.9/455.7
The direct current power supply was used. During the process, the voltage and current change depending on the mode of nitriding. 3. Results of investigation and discussion X-ray diffraction studies have shown that as a result of low temperature NGDs on the surface of the titanium alloy VT8, the mononitride TiN (␦ phase) and – Ti2N phase is formed. It is possible to fix these phases, after nitriding at 660 ◦ C. At a temperature of 580 ◦ C, such phases are not detected, and as a result the X-rays pass deeper than the diffusion layer. The thickness of the nitride layer is fixed: for Mode 1–2.4 m, Mode 2–1.8 m, Mode 3–4.1 m, Mode 4–2.1 m. The predominant structures of the generated modified layer when a mode 1 is the phase of Ti2 N and to a lesser extent TiN (␦ phase) and in mode 2, – Ti-␣, whereas in the initial state, there is substantially Ti – ␣, Ti- phase. Radiographs for modes 1 and 2 are shown in Fig. 1. In addition, the phase composition of the modified surface was investigated by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The AES method studied the surface of the titanium alloy VT8, nitrided by modes №1, 2, 3, 4. In these cases, the distribution of elements was determined by depth in combination with argon ion sputtering material. Fig. 2 shows the Auger spectra for both the initial state and after ion etching the surface of the nitrided titanium alloy VT8. Analysis of changes in the content of elements in the surface layers of the nitrided titanium alloy under ion etching can be concluded that the surface is characterized by an increased content of carbon and oxygen which are surface contaminants. After ion etching their content drops sharply to a depth of about 50–100 Å, and then remains practically constant. By way of removal of carbon and oxygen in the Auger spectra, peaks increase for major components. Carrying out stratified analysis establishes a significant heterogeneity of distribution of the depth of all investigated elements. Fig. 3 shows the layered dependencies on the main components of the studied alloys produced by ion etching. Comparison of the peak intensities after nitriding according to various modes indicates that after nitriding at 580 ◦ C (mode 2), the intensity of the peaks is significantly lower than at 660 ◦ C (mode 1). The activity of the titanium alloy in interaction with oxygen indicates the following. Despite the fact that the pre-etched ion bombardment nitriding oxide structures were removed previously, after a brief period in air by way of XPS, oxygen compounds were detected again. Spectra were detected on the surface of the titanium alloy VT-8 when nitrided for modes 1, 2, 3, 4 were recorded for core levels of Ti2p, O1s and N1s. Given that the method gives information about the XPS surface composition at a depth of not more than 10 nm from the surface, one sample was etched by ion bombardment of Ar+ with the spectrum once again recorded. On the basis of this data on the composition of elements, the binding energies of the core electrons of atoms of oxygen, nitrogen and titanium were calculated (Table 1). For all the samples, the O1s line is rather broad and includes at least two intense components (Fig. 4). The dominant component at a binding energy of ∼530 eV is obviously related to the O2− ions in the lattice of TiO2, whereas the other component at a bind-
Fig. 4. X-ray photoelectron spectra of O1s – electrons: 1 – Mode №2; 2 – Mode №1 after ion etching; 3 – Mode №1; 4 – Mode №3; 5 – Mode №4.
ing energy of ∼532 eV corresponds to OH groups (mainly Ti OH bonds) and indicates the appreciable hydroxylation of the surface. The intensity of the peaks of oxygen for various modes of nitriding of the titanium alloy is significantly different. The maximum value corresponds to the regimes №1 and №2, but after cleaning, the ion intensity falls sharply. For modes №3 and №4 intensity is very low. This indicates that the oxide film is small. Its presence is caused by absorption of oxygen atoms during nitriding. After sputtering Ar+ , peak II is lost and the intensity of peak III is reduced, but after two hours of etching, peak III completely disappears, indicating that the removal of the surface oxide. This confirms the formation of titanium nitride on the surface of the lowtemperature glow discharge nitriding at a temperature of 580 ◦ C. The Ti 2p3/2 peak at 457.1 eV implies formation on the alloy surface of the TiNx Oy -type oxynitride [6,7]. This component, however, may also include contributions from a shake-up satellite, which was shown to arise in Ti 2p spectra of TiN at 1.5–2.8 eV higher binding energy with respect to the main Ti N peak [6,8], and reduced Ti3+ −O species. Finally, the Ti 2p3/2 peak at 458.6 eV can be ascribed to Ti4+ species in somewhat reduced (because of preliminary ion bombardment) TiO2 . The XPS spectra (Figs. 5 and 6) for mode 1, before purification by ion sputtering the Ti2p region correspond to the three groups of the doublet. The peak of the I corresponds to the titanium nitride TiN with a binding energy Eb = 455.4 eV and Eb = 461.1 eV. Peaks II and III at high binding energies are closely related to the Me O bond in various oxide forms, which corresponds to that of Ti O N (Eb = 457.8 eV and Eb = 463.5 eV) and of titanium Dioxide (Eb = 459.1 eV and Eb = 464.6 eV). The intense peak in emission of N1s photoelectrons which is observed for all the samples at binding energies of 397.1–397.5 eV (Fig. 6b) indicates the presence of N3− electronic state and formation of titanium nitride phase TiN in the VT8 alloy under all treatment conditions. The noticeable asymmetry of the N1 s line towards higher binding energies implies also formation on the surface of oxynitride-type bonds N Ti O (∼398.8 eV) and N (O, C) bonds (∼400 eV).
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Fig. 5. X-ray photoelectron spectra of Ti2p – electrons for regime №1, (a) – prior to the ion etching (b) – after ion etching (the peak of the I corresponds to TiN, the peak II – Ti-ON, peak III to TiO2 ).
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Fig. 6. a) X-ray photoelectron spectra of Ti2p – electrons, b) X-ray photoelectron spectra of N1s – electrons, where 1 – the mode №2; 2 – Mode №1 after ion etching; 3 – Mode №1; 4 – Mode №3; 5 – Mode №4.
4. Conclusions The above material shows the promise of the technology of lowtemperature hydrogen-nitriding by glow discharge. This greatly expands the range of practical applications of titanium alloys. In addition, changing the technological mode allows you to manage a wide range of modified phase composition of the surface layer and as a result – to form the surface of titanium parts, taking into account the conditions of the subsequent operation. References [1] B.N. Arzamasov, A.G. Bratuhin, S. Eliseev, T. Panagiotou, Ionic Chemical Heat Treatment of Alloys Moscow, Publishing House of the MSTU Bauman, 1999, pp. 400 (in Russian). [2] I.M. Pastukh, Theory and Practice of Without Hydrogen Glow Discharge Nitriding, KIPT, Kharkov, 2006.
[3] S.S. Gorelik, Y.A. Skakov, L.N. Rastorguev, X-ray and Electron-Optical Analysis, MISIS, Moscow, 1994, 328 p. (in Russian). [4] I.M. Pastukh, N.S. Mashovets, Yu.I. Shalapko, V.A. Kurskaja, Forecast the formation of nitrides on the surface of titanium alloys during nitriding in a glow discharge, J. Achiev. Mater. Manuf. Eng. 62 (2) (2014) 53–62. [5] N.S. Mashovets, Influence of parameters ionic nitriding the titan on physics-mechanical descriptions of nitriding layer, Her. Engine Build. 3 (2004) 122–125. [6] A. Glaser, S. Surnev, F.P. Netzer, N. Fateh, G.A. Fontalvo, C. Mitterer, Oxidation of vanadium nitride and titanium nitride coatings, Surf. Sci. 601 (2007) 1153–1159. [7] M.J. Jung, K.H. Nam, Y.M. Chung, J.H. Boo, J.G. Han, The physiochemical properties of TiOx Nx films with controlled oxygen x y partial pressure, Surf. Coat. Technol. 171 (2003) 71–74. [8] M.A. Vasylyev, S.P. Chenakin, L.F. Yatsenko, Nitridation of TiA6AlA4 V alloy under ultrasonic impact treatment in liquid nitrogen, Acta Mater. 60 (2012) 6223–6233.