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Low temperature nitriding of iron by alternating current pretreatment
Surface & Coatings Technology 200 (2006) 6666 – 6670 www.elsevier.com/locate/surfcoat
Low temperature nitriding of iron by alternating current pretreatment Jisen Wang ⁎, Guosong Zhang, Jinquan Sun, Ying Bao, Lichen Zhuang, Hongtao Ning School of Mechanical and Electronic Engineering, Shandong University of Science and Technology, 579 Qianwangang Road, Qingdao 266510, PR China Received 4 July 2005; accepted in revised form 29 September 2005 Available online 16 November 2005
Abstract Iron by pretreatment with the surface alternating current nanocrystalline treatment was successfully nitrided at 573 K, which has been decreased to 200 K compared with the conventional gaseous nitriding temperature. The surface alternating current nanocrystalline treatment induces atom of upper surface to recombine leading to form nanograins layer, which supplies extra driving force for the nitride formation process. It is an important significance to treat the geometric complex and the precision workpieces. The samples are characterized by microhardness tester, metallographic testing and X-ray diffraction. © 2005 Elsevier B.V. All rights reserved. PACS: 77.84.Bw; 75.50.Bb; 81.07 Keywords: Nitriding; SACNT; Iron; Nanostructures
1. Introduction Nitriding to obtain a nitrided layer is a very common process of surface heat-chemical treatment, which improves the wear resistance of iron and steel [1]. Over the past few years, the nitriding processes including gas nitriding, salt bath nitriding, plasma nitriding, pulsed laser deposition, reactive magnetron sputtering and nitrogen implantation, and laser nitriding [2–11] were gradually developed. The gas nitriding qua the foremost nitriding process was widely applied in the industry because of the simple techniques, however, it is always carried out at high temperature (N 773 K) for long duration (about 20 to 80 h), and may induce serious deterioration of the substrate in many families of materials [12]. Ammonia provides the necessary nitrogen in the nitriding process, and the build-up of the nitrided layer during gaseous nitriding starts in some energetically privileged places (the so-called active centers) [13], such as grain boundaries, surface defects and inclusions [14,15]. In these areas the concentration of nitrogen is the highest, and therefore the nitride nucleation begins here after a very short time. The number of active centers has a large influence on the
⁎ Corresponding author. Tel.: +86 532 6054512; fax: +86 532 6057987. E-mail address: [email protected] (J. Wang). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.09.035
kinetics of nitrided layer formation. Lu et al. triumphantly achieved nitriding iron at lower temperatures by surface pretreatment using a recently developed surface mechanical attrition treatment (SMAT) [16–19]. However, this technique mentioned by Lu is limited to be only used for treating the pellet parts. Now we developed a new nitriding method at low temperatures (about 573 K) to enhance the chemical reaction kinetics by the surface alternating current nanocrystalline
Fig. 1. Schematic illustrations of SACNT.
J. Wang et al. / Surface & Coatings Technology 200 (2006) 6666–6670
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Table 1 The conditions of the SCANT Intensity Frequency Temperature Time
15 mA/cm2 50 Hz 50 °C 10–60 min
treatment (SACNT) differing from the surface pretreatments that had been applied [14,20–23]. 2. Experimental 2.1. The SACNT process The experiments were carried out on the square (10 mm by 10 mm) ingot samples, 2 mm thick (C ≤ 0.003%, P ≤ 0.01%, S ≤ 0.006%, Si ≤ 0.01%, Mn ≤ 0.10%). Before being treated, the specimens were annealed at 1033 K for 2 h in order to eliminate the machining stress and then sanded in SiC papers, polished with colloidal silica, and cleaned ultrasonically in acetone for 15 min. The samples were treated via SACNT as follows: two electrodes (one is inert metal, the other is specimen) were put into the electrolyte (20 wt.% FeCl2) and an alternating current was switched on (220 V/50 Hz) for a period of time (as Fig. 1). The parameters of the treatment were presented in Table 1.
Fig. 3. X-ray diffraction pattern obtained from the surface of the original and pretreated samples.
2.2. The nitriding process The specimens after the SACNT were cleaned ultrasonically in acetone for 1 min and rinsed with distilled water. Both the untreated and treated samples were immediately put into the reaction zone of the tubular resistance furnace with a highly pure ammonia gas (NH3 N 99.9%) qua nitriding gas source. The
Fig. 2. SEM images of upper surfaces of steels after the SCANT for various samples: (a and b) 50 Hz; 15 min, (c and d) 50 Hz, 30 min; (e and f) 50 Hz, 45 min.
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J. Wang et al. / Surface & Coatings Technology 200 (2006) 6666–6670
Fig. 4. Picture of the cross-section of (a) the SACNT sample at 15 A/dm2 for 15 min, (b) the sample without pretreatment, and (c) the surface of the SACNT sample after nitriding for 9 h.
nitriding course would be persisting at 573 K for 9 h (standard atmosphere). The samples were investigated using the methods as follows. Surface of the specimens after SACNT were observed by scanning electron microscopy (SEM, JEOL JSM 6330F). The observation of surface and cross-sections of samples nitrided layer was made by SEM and optical microscopy (OM). X-ray diffraction (XRD, Bruker, D8 Discover, 60 kV), using Cu Kα radiation (λ = 1.5418 Á) in the θ–2θ geometry, was applied to investigate the nitrided layer composition. The diffusion zone thickness was estimated through microhardness measurements on cross-sections of nitrided layers. The distance from the outer surface to where the microhardness was 50 HV higher than that in matrix was adopted as a thickness of the diffusion zone. Microhardness was measured using the microhardness tester with load of 0.5 N.
which the grains were gradually refined in contrast with the sample treated for 15 min (Fig. 2b), which indicated that the time of the SACNT had an important influence on the development of the samples in upper surface microlite and the depth of the surface nanostructured layer. The influence of other parameters such as intensity and frequency would be studied further. Fig. 3 shows the X-ray diffraction pattern of original and pretreated samples, in which we find that the full wave at half maximum of three main peaks of pretreated samples are larger than that of the original samples (pretreated samples: 0.343, 0.436, 0.382; original samples: 0.245, 0.327, 0.346).
3. Results 3.1. Investigations of the upper surface of iron after the SACNT The analysis of the SEM surface micrographs showed that a sub-structure of nano-sized crystallites was present on the upper surface of the samples after the SACNT at the same parameters but different times, which could be seen in Fig. 2. As we can see from Fig. 2a, c, e, the surface of the samples is refined equably, and we can see that the surface is composed of homotaxial ridge nanostructures (showed as Fig. 2b and d). Fig. 2d showed a typical SEM image of the sample by SACNT for 30 min in
Fig. 5. X-ray diffraction pattern obtained from the surface of the SACNT (at 15 A/dm2 for 15 min) sample treated for 9 h. The phases of α-Fe, γ′ and ε can be observed, respectively.
J. Wang et al. / Surface & Coatings Technology 200 (2006) 6666–6670
According to Scherrer equation, the surface grain size of pretreated samples is about 350 nm. We can draw a conclusion that the SACNT process does not induce alloy elements and form new material, but this process only refines the grains of treated samples. 3.2. Investigations of nitrided layers Observations of the cross-section (SEM) and surface (OM) of samples after gas nitriding revealed that the quick formation of continuous nitride layer, accompanied by a thick diffusion zone, was observed even at low nitrogen potential atmospheres (Fig. 4a). However, the continuous nitrided layer was not present in the nitrided samples without pretreatment (Fig. 4b). From Fig. 4c we can see that compact vermiculate nitrides are present on the surface of the SACNT sample after nitriding for 9 h. This evidence showed that the SACNT as a pretreatment had a significant influence on the kinetics of nitriding layer formation. The nanostructured surface was generated by a larger number of grain boundaries that may accelerate the atomic diffusion, and more the nanostructures stored a largely excess energy in the grain boundaries. So the gas nitriding process could be carried out at a lower temperature through pretreatment of the SACNT. The surface microhardness measurement indicated that the surface layer of pretreated samples exhibited a much greater hardness (about 360 HV) than the substrate, but there are no significant differences in the microhardness between the surface and matrix of the un-pretreated samples. A typical microhardness profile of the Fe samples gas nitrided for 9 h is displayed in Fig. 6. The microhardness of the original pure iron is about 180 HV; however, the microhardness where the distance is about 100 μm from the outer surface is 230 HV. Integrating to Fig. 4, we can estimate that the thickness of nitrided layer is about 15 μm and the diffusion layer thickness was evaluated to be about 100 μm. The variation of hardness along depth agreed well with the structural and compositional analysis results. The X-ray diffraction spectrum of the pretreated samples after gas nitriding was shown in Fig. 5, where the presence of γ and ε peaks was observed in addition to α-Fe signal. Thus we
Fig. 6. Microhardness profile obtained for nitriding samples.
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can obtain the α-Fe (bcc) cell parameters a = 2.866 Á, and the peak widths (110): 0.39 mm, (200): 0.42 mm, (211): 0.45. The γ′ and ε nitrides cell parameters are a = 3.790 Á, a = 4.708 Á, c = 4.388 Á, respectively. This information in conjunction with the micrographs indicated that nitriding started with the formation of γ′ needles shortly after the beginning of the process and then the formation of ε occurred a little later when the γ′ became saturated. It is calculated that the surface grain size of nitriding samples is about 350 nm according to Scherrer equation, which also confirms the sample surface nanostructures are present after pretreatment (Fig. 6). 4. Discussion It is suggested that the upper surface defectiveness is a crucial responsibility for the increasing kinetics of nitride layer growth [24,25]. In the nanocrystalline Fe sample, nitrogen mostly diffuses along Fe grain boundaries because it demands a much smaller activation energy (approximately half) compared with that for the lattice diffusion. Nitrogen diffusivity along grain boundaries at 573 K is estimated to be about 3.3 * 10− 7 cm2/s (based on an activation energy of about 0.6 times that for the lattice diffusion), which are about three orders of magnitude larger than the lattice diffusion at the same temperature and even higher than the nitrogen diffusivity in the lattice at 773 K [16]. During the SACNT process, there are bonds formation and bonds breaking involved due to the ion transfer of the samples surface by the action of electric field, which induces to form the recombined nanostructured layer at the sample upper surface, and brings on crystal defect such as dislocations, positive hole and so on. The nanostructures store largely excess energy in the grain boundaries and at the same time the breaking bonds also supply a great number of energy, which afford extra driving force for the nitride formation process. The stored energy in a ball-milled nanocrystalline Fe sample with 10 nm grain sizes is estimated in about 2.3 kJ/mol [26]. With this stored excess energy, we can draw a conclusion that it is possible to form nitrides in the nanocrystalline Fe phase at 573 K according to the negative value of Gibbs free energy. Based on the Fick A2 c Second principle Ac At ¼ D Ax2 , the SACNT induces the D (diffusion coefficient) to increase and the interface diffusion rate becomes larger than the interface reaction rate. Otherwise, the sample surface can be depurated by the SACNT process and micromelting, which can increase the ion adsorption and reaction capacities assisting the gas nitriding process to be implemented. The samples without pretreatment do not form nitride on the surface because of the very high nitrogen potential threshold value at 573 K. But, as we can see from this experiment, the samples with the SACNT are nitrided successfully at the same temperature. From a kineticsppoint ffiffiffiffi of view, the SACNT process must enlarge the value of b= D and must result in the debasing of the nitrogen potential threshold value ri [27] ðri ¼ rc pffi pffiffiffi which makes it become feasible to 1−expðb2 t=DÞerfcðb t = DÞÞ form nitride on the surface of iron at lower temperature by SACNT.
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5. Conclusion In summary, the SACNT as a pretreatment is crucially important to activate the sample surface and obtain surface nanostructures, and it is not only environment friendly, convenient, inexpensive and efficient, but also fits for the treatment of the geometric complex and the precision workpieces. The process induces chemical bonds to break and reform because of the ion transgression and make the grain refined. The surface generates a larger number of grain boundaries, dislocations and positive holes, which all enhance the nitrogen diffusion. Moreover the SACNT process does not induce stress, so the application of this gas nitriding process will become more extensive lying in little transmutation, geometric complex and precision workpieces. Acknowledgement Jisen Wang and Guosong Zhang gratefully acknowledge support of this work by the program of Science and Technology Bureau of Qingdao under Grant No. 03-2-IR-18 and Shandong Provincial Education Department under Grant No. J05A05. References [1] T. Bell, Source Book on Nitriding, American Society for Metals, Metals Park, OH, 1977. [2] D. Liedtke, Nitrieren, Mcrkblatt, vol. 447, VdEh, Düsseldorf, 1974. [3] H. Kunst, D. Liedtke, Badnitrieren von Eisenwerkstoffen-Untersuchungen an Nitridschichten, Of Tribologic, Reibung-Verschleiss-Schmierung, vol. 7, 1983, p. 39. [4] H. Boyer (Ed.), Case Hardening of Steel, ASM International, Metals Park, OH, 1987.
[5] L. Abada, G. Rixecher, F. Aubertin, P. Schaaf, U. Gonser, Phys. Status Solidi, A Appl. Res. 139 (1993) 181. [6] D.L. Williamson, O. Ozturk, S. Glick, R. Wei, P.J. Wilber, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 191 (59/60) (1988) 737. [7] G. Marest, Defect Diffus. Forum 57–58 (1988) 273. [8] M. Niederdrenk, P. Schaaf, K.P. Lieb, O. Schulte, J. Alloys Compd. 237 (1996) 81. [9] L. Rissanen, M. Neubauer, K.P. Lieb, P. Schaaf, J. Alloys Compd. 274 (1/ 2) (1998) 74. [10] P. Schaaf, A. Emmel, E. Schubert, H.W. Bergmann, K.P. Lieb, Hyperfine Interact. 92 (1994) 1361. [11] P. Schaaf, A. Emmel, C. Illgner, K.P. Lieb, E. Schubert, H.W. Bergmann, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 197 (1995) (L1–0L4). [12] J. Mongis, J.P. Peyre, C. Tournier, Heat Treat. Met. 3 (1984) 71. [13] T. Burakowski, T. Wierzchon, Inzynieria Powierzchni Metali, WNT, Warszawa, 1995, Poland. [14] W.D. Jentzsch, S. Boher, Krist. Tech. 14 (5) (1979) 617. [15] H. Du, J. Agren, Z. Met.Kd. 86 (8) (1995) 522. [16] W.P. Tong, N.R. Tao, Z.B. Wang, J. Lu, K. Lu, Science (2003) 299. [17] K. Lu, J. Lu, J. Mater. Sci. Technol. 15 (1999) 193. [18] K. Lu, J. Lu, J. Mater. Sci. Eng., A 375–377 (2004) 38 (15 July). [19] N.R. Tao, M.L. Sui, K. Lu, J. Lu, Nanostruct. Mater 11 (1999) 433. [20] H. Du, N. Lange, J. Agren, Surf. Eng. 11 (4) (1995) 301. [21] F.K. Neumann, Parkt Metallogr. 5 (1968) 473. [22] Ju. Lachtin, Ja. Kogen, W.E. Kol'cov, U. Bojnazarov, MITOM 3 (1993) 31. [23] J. Baranowska, K. Szczecinski, M. Wysiecki, Vacuum 63 (2001) 517. [24] Ionic Bombardment Theory and Application, Gordon and Breach, New York, 1962. [25] H. Gnaser, Low Energy Ion Irradiation of Solid Surface, Springer, Berlin, 1999. [26] H. Fecht, in: G.C. Hadjipanayis, R.W. Siegel (Eds.), J. in Nanophase Materials: Synthesis, Properties, Applications, Kluwer Academic, Dordrecht, Netherlands, 1994. [27] D.Z. Kong, Principle of Chemical Heat Treatment, Publishing Company of Aviation, Beijing, China, 1980.
Low temperature nitriding of iron by alternating current pretreatment Jisen Wang ⁎, Guosong Zhang, Jinquan Sun, Ying Bao, Lichen Zhuang, Hongtao Ning School of Mechanical and Electronic Engineering, Shandong University of Science and Technology, 579 Qianwangang Road, Qingdao 266510, PR China Received 4 July 2005; accepted in revised form 29 September 2005 Available online 16 November 2005
Abstract Iron by pretreatment with the surface alternating current nanocrystalline treatment was successfully nitrided at 573 K, which has been decreased to 200 K compared with the conventional gaseous nitriding temperature. The surface alternating current nanocrystalline treatment induces atom of upper surface to recombine leading to form nanograins layer, which supplies extra driving force for the nitride formation process. It is an important significance to treat the geometric complex and the precision workpieces. The samples are characterized by microhardness tester, metallographic testing and X-ray diffraction. © 2005 Elsevier B.V. All rights reserved. PACS: 77.84.Bw; 75.50.Bb; 81.07 Keywords: Nitriding; SACNT; Iron; Nanostructures
1. Introduction Nitriding to obtain a nitrided layer is a very common process of surface heat-chemical treatment, which improves the wear resistance of iron and steel [1]. Over the past few years, the nitriding processes including gas nitriding, salt bath nitriding, plasma nitriding, pulsed laser deposition, reactive magnetron sputtering and nitrogen implantation, and laser nitriding [2–11] were gradually developed. The gas nitriding qua the foremost nitriding process was widely applied in the industry because of the simple techniques, however, it is always carried out at high temperature (N 773 K) for long duration (about 20 to 80 h), and may induce serious deterioration of the substrate in many families of materials [12]. Ammonia provides the necessary nitrogen in the nitriding process, and the build-up of the nitrided layer during gaseous nitriding starts in some energetically privileged places (the so-called active centers) [13], such as grain boundaries, surface defects and inclusions [14,15]. In these areas the concentration of nitrogen is the highest, and therefore the nitride nucleation begins here after a very short time. The number of active centers has a large influence on the
⁎ Corresponding author. Tel.: +86 532 6054512; fax: +86 532 6057987. E-mail address: [email protected] (J. Wang). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.09.035
kinetics of nitrided layer formation. Lu et al. triumphantly achieved nitriding iron at lower temperatures by surface pretreatment using a recently developed surface mechanical attrition treatment (SMAT) [16–19]. However, this technique mentioned by Lu is limited to be only used for treating the pellet parts. Now we developed a new nitriding method at low temperatures (about 573 K) to enhance the chemical reaction kinetics by the surface alternating current nanocrystalline
Fig. 1. Schematic illustrations of SACNT.
J. Wang et al. / Surface & Coatings Technology 200 (2006) 6666–6670
6667
Table 1 The conditions of the SCANT Intensity Frequency Temperature Time
15 mA/cm2 50 Hz 50 °C 10–60 min
treatment (SACNT) differing from the surface pretreatments that had been applied [14,20–23]. 2. Experimental 2.1. The SACNT process The experiments were carried out on the square (10 mm by 10 mm) ingot samples, 2 mm thick (C ≤ 0.003%, P ≤ 0.01%, S ≤ 0.006%, Si ≤ 0.01%, Mn ≤ 0.10%). Before being treated, the specimens were annealed at 1033 K for 2 h in order to eliminate the machining stress and then sanded in SiC papers, polished with colloidal silica, and cleaned ultrasonically in acetone for 15 min. The samples were treated via SACNT as follows: two electrodes (one is inert metal, the other is specimen) were put into the electrolyte (20 wt.% FeCl2) and an alternating current was switched on (220 V/50 Hz) for a period of time (as Fig. 1). The parameters of the treatment were presented in Table 1.
Fig. 3. X-ray diffraction pattern obtained from the surface of the original and pretreated samples.
2.2. The nitriding process The specimens after the SACNT were cleaned ultrasonically in acetone for 1 min and rinsed with distilled water. Both the untreated and treated samples were immediately put into the reaction zone of the tubular resistance furnace with a highly pure ammonia gas (NH3 N 99.9%) qua nitriding gas source. The
Fig. 2. SEM images of upper surfaces of steels after the SCANT for various samples: (a and b) 50 Hz; 15 min, (c and d) 50 Hz, 30 min; (e and f) 50 Hz, 45 min.
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J. Wang et al. / Surface & Coatings Technology 200 (2006) 6666–6670
Fig. 4. Picture of the cross-section of (a) the SACNT sample at 15 A/dm2 for 15 min, (b) the sample without pretreatment, and (c) the surface of the SACNT sample after nitriding for 9 h.
nitriding course would be persisting at 573 K for 9 h (standard atmosphere). The samples were investigated using the methods as follows. Surface of the specimens after SACNT were observed by scanning electron microscopy (SEM, JEOL JSM 6330F). The observation of surface and cross-sections of samples nitrided layer was made by SEM and optical microscopy (OM). X-ray diffraction (XRD, Bruker, D8 Discover, 60 kV), using Cu Kα radiation (λ = 1.5418 Á) in the θ–2θ geometry, was applied to investigate the nitrided layer composition. The diffusion zone thickness was estimated through microhardness measurements on cross-sections of nitrided layers. The distance from the outer surface to where the microhardness was 50 HV higher than that in matrix was adopted as a thickness of the diffusion zone. Microhardness was measured using the microhardness tester with load of 0.5 N.
which the grains were gradually refined in contrast with the sample treated for 15 min (Fig. 2b), which indicated that the time of the SACNT had an important influence on the development of the samples in upper surface microlite and the depth of the surface nanostructured layer. The influence of other parameters such as intensity and frequency would be studied further. Fig. 3 shows the X-ray diffraction pattern of original and pretreated samples, in which we find that the full wave at half maximum of three main peaks of pretreated samples are larger than that of the original samples (pretreated samples: 0.343, 0.436, 0.382; original samples: 0.245, 0.327, 0.346).
3. Results 3.1. Investigations of the upper surface of iron after the SACNT The analysis of the SEM surface micrographs showed that a sub-structure of nano-sized crystallites was present on the upper surface of the samples after the SACNT at the same parameters but different times, which could be seen in Fig. 2. As we can see from Fig. 2a, c, e, the surface of the samples is refined equably, and we can see that the surface is composed of homotaxial ridge nanostructures (showed as Fig. 2b and d). Fig. 2d showed a typical SEM image of the sample by SACNT for 30 min in
Fig. 5. X-ray diffraction pattern obtained from the surface of the SACNT (at 15 A/dm2 for 15 min) sample treated for 9 h. The phases of α-Fe, γ′ and ε can be observed, respectively.
J. Wang et al. / Surface & Coatings Technology 200 (2006) 6666–6670
According to Scherrer equation, the surface grain size of pretreated samples is about 350 nm. We can draw a conclusion that the SACNT process does not induce alloy elements and form new material, but this process only refines the grains of treated samples. 3.2. Investigations of nitrided layers Observations of the cross-section (SEM) and surface (OM) of samples after gas nitriding revealed that the quick formation of continuous nitride layer, accompanied by a thick diffusion zone, was observed even at low nitrogen potential atmospheres (Fig. 4a). However, the continuous nitrided layer was not present in the nitrided samples without pretreatment (Fig. 4b). From Fig. 4c we can see that compact vermiculate nitrides are present on the surface of the SACNT sample after nitriding for 9 h. This evidence showed that the SACNT as a pretreatment had a significant influence on the kinetics of nitriding layer formation. The nanostructured surface was generated by a larger number of grain boundaries that may accelerate the atomic diffusion, and more the nanostructures stored a largely excess energy in the grain boundaries. So the gas nitriding process could be carried out at a lower temperature through pretreatment of the SACNT. The surface microhardness measurement indicated that the surface layer of pretreated samples exhibited a much greater hardness (about 360 HV) than the substrate, but there are no significant differences in the microhardness between the surface and matrix of the un-pretreated samples. A typical microhardness profile of the Fe samples gas nitrided for 9 h is displayed in Fig. 6. The microhardness of the original pure iron is about 180 HV; however, the microhardness where the distance is about 100 μm from the outer surface is 230 HV. Integrating to Fig. 4, we can estimate that the thickness of nitrided layer is about 15 μm and the diffusion layer thickness was evaluated to be about 100 μm. The variation of hardness along depth agreed well with the structural and compositional analysis results. The X-ray diffraction spectrum of the pretreated samples after gas nitriding was shown in Fig. 5, where the presence of γ and ε peaks was observed in addition to α-Fe signal. Thus we
Fig. 6. Microhardness profile obtained for nitriding samples.
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can obtain the α-Fe (bcc) cell parameters a = 2.866 Á, and the peak widths (110): 0.39 mm, (200): 0.42 mm, (211): 0.45. The γ′ and ε nitrides cell parameters are a = 3.790 Á, a = 4.708 Á, c = 4.388 Á, respectively. This information in conjunction with the micrographs indicated that nitriding started with the formation of γ′ needles shortly after the beginning of the process and then the formation of ε occurred a little later when the γ′ became saturated. It is calculated that the surface grain size of nitriding samples is about 350 nm according to Scherrer equation, which also confirms the sample surface nanostructures are present after pretreatment (Fig. 6). 4. Discussion It is suggested that the upper surface defectiveness is a crucial responsibility for the increasing kinetics of nitride layer growth [24,25]. In the nanocrystalline Fe sample, nitrogen mostly diffuses along Fe grain boundaries because it demands a much smaller activation energy (approximately half) compared with that for the lattice diffusion. Nitrogen diffusivity along grain boundaries at 573 K is estimated to be about 3.3 * 10− 7 cm2/s (based on an activation energy of about 0.6 times that for the lattice diffusion), which are about three orders of magnitude larger than the lattice diffusion at the same temperature and even higher than the nitrogen diffusivity in the lattice at 773 K [16]. During the SACNT process, there are bonds formation and bonds breaking involved due to the ion transfer of the samples surface by the action of electric field, which induces to form the recombined nanostructured layer at the sample upper surface, and brings on crystal defect such as dislocations, positive hole and so on. The nanostructures store largely excess energy in the grain boundaries and at the same time the breaking bonds also supply a great number of energy, which afford extra driving force for the nitride formation process. The stored energy in a ball-milled nanocrystalline Fe sample with 10 nm grain sizes is estimated in about 2.3 kJ/mol [26]. With this stored excess energy, we can draw a conclusion that it is possible to form nitrides in the nanocrystalline Fe phase at 573 K according to the negative value of Gibbs free energy. Based on the Fick A2 c Second principle Ac At ¼ D Ax2 , the SACNT induces the D (diffusion coefficient) to increase and the interface diffusion rate becomes larger than the interface reaction rate. Otherwise, the sample surface can be depurated by the SACNT process and micromelting, which can increase the ion adsorption and reaction capacities assisting the gas nitriding process to be implemented. The samples without pretreatment do not form nitride on the surface because of the very high nitrogen potential threshold value at 573 K. But, as we can see from this experiment, the samples with the SACNT are nitrided successfully at the same temperature. From a kineticsppoint ffiffiffiffi of view, the SACNT process must enlarge the value of b= D and must result in the debasing of the nitrogen potential threshold value ri [27] ðri ¼ rc pffi pffiffiffi which makes it become feasible to 1−expðb2 t=DÞerfcðb t = DÞÞ form nitride on the surface of iron at lower temperature by SACNT.
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5. Conclusion In summary, the SACNT as a pretreatment is crucially important to activate the sample surface and obtain surface nanostructures, and it is not only environment friendly, convenient, inexpensive and efficient, but also fits for the treatment of the geometric complex and the precision workpieces. The process induces chemical bonds to break and reform because of the ion transgression and make the grain refined. The surface generates a larger number of grain boundaries, dislocations and positive holes, which all enhance the nitrogen diffusion. Moreover the SACNT process does not induce stress, so the application of this gas nitriding process will become more extensive lying in little transmutation, geometric complex and precision workpieces. Acknowledgement Jisen Wang and Guosong Zhang gratefully acknowledge support of this work by the program of Science and Technology Bureau of Qingdao under Grant No. 03-2-IR-18 and Shandong Provincial Education Department under Grant No. J05A05. References [1] T. Bell, Source Book on Nitriding, American Society for Metals, Metals Park, OH, 1977. [2] D. Liedtke, Nitrieren, Mcrkblatt, vol. 447, VdEh, Düsseldorf, 1974. [3] H. Kunst, D. Liedtke, Badnitrieren von Eisenwerkstoffen-Untersuchungen an Nitridschichten, Of Tribologic, Reibung-Verschleiss-Schmierung, vol. 7, 1983, p. 39. [4] H. Boyer (Ed.), Case Hardening of Steel, ASM International, Metals Park, OH, 1987.
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