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Sintering iron using a hollow cathode discharge
Materials Science and Engineering A343 (2003) 163 /169 www.elsevier.com/locate/msea
Sintering iron using a hollow cathode discharge S.F. Brunatto a, I. Ku¨hn b, A.N. Klein b, J.L.R. Muzart b, a
b
Departamento de Engenharia Mecaˆnica, Universidade Federal do Parana´-81531-990, Curitiba, PR, Brazil LABMAT, Departamento de Engenharia Mecaˆnica, Universidade Federal de Santa Catarina-88040-900, Floriano´polis, SC, Brazil Received 20 November 2001; received in revised form 15 May 2002
Abstract Unalloyed iron samples were sintered in the presence of a hollow cathode glow discharge, generated in a gas mixture of 80% Ar/ 20% H2 at pressures ranging from 133 to 400 Pa. The sample, which worked as the inner cathode of the discharge, was heated by the bombardment of strongly accelerated ions and fast neutrals created in the cathode sheath. The outer cathode of the hollow geometry consisted of a stainless steel AISI 310 cylinder. The enhanced ionization obtained in the hollow cathode configuration provided a high current density (25 mA cm 2) and consequently a high temperature could be attained (1250 8C). The samples were sintered at 1150 8C for times ranging from 30 to 240 min. The temperature was adjusted by varying the time on/off of the pulsed power supply used to generate the discharge. Microstructural results are presented and it is shown that samples may be successfully sintered in a hollow cathode discharge. In addition, atoms of Cr and Ni sputtered from the outer stainless steel cathode were deposited onto the sample surface and diffused, during sintering, resulting in the formation of a layer approximately 20 mm thick containing these elements. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma sintering; Unalloyed iron; Hollow cathode discharge
1. Introduction Sintering of ceramics and steels is conventionally carried out in a furnace in a controlled atmosphere. Several alternative techniques have been developed since 1966, as mentioned by Johnson et al. [1], to obtain components with improved properties, resulting in new applications for powder technology. Plasmas generated in microwave cavities, in hollow cathode devices and microwave or RF-inductively coupled discharges have all been used for sintering ceramics [2 /5]. In the above mentioned papers it was reported that ceramics could be sintered to high density at high sintering rates. Heat transfer mechanism and cleaning of the particle surfaces in the plasma environment are suggested to be responsible for the enhanced sintering. However, the plasma reactivity may be an impediment to sintering as discussed by Park and McNallan [6]. They concluded that
Corresponding author. Tel.: /55-48-234-0084; fax: /55-48-2340059 E-mail address: [email protected] (J.L.R. Muzart).
sintering of SiC whisker reinforced mullite matrix composites was not successfully carried out because of chemical reactions occurring in the plasma atmosphere. High-density samples at high rate sintering may also be obtained using the plasma activated sintering or spark plasma sintering process [7 /12]. The pulsed voltage applied to the sample removes the oxide film and adsorbed gases from particle surfaces of non-oxide ceramics or metals, and activates the sintering. Following the cleaning of the particle surfaces, a direct current associated with a pulsed one flows among the powders and, thus, the contact resistance between particles generates heat by the Joule effect. This localized heating accelerates the formation of necks, thus activating sintering [13]. Recently, sintering of metallic components using an abnormal glow discharge containing hydrogen and argon has been described [14 /16]. The abnormal glow discharge is characterized by full covering of the cathode by the glow region [17], supplying a uniform treatment. The principle of heating is based on the ion and fast neutrals bombardment of the sample. A negatively biased voltage was applied to the sample, which worked as the cathode of the abnormal glow
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discharge, generating an electric field in the cathode sheath, where ions are strongly accelerated. Collisions between ions and argon atoms or hydrogen molecules of the gas discharge in the cathode sheath result in a flow of fast neutrals toward the cathode [17]. The bombardment of ions and fast neutrals heat the sample. The modified linear abnormal glow discharge, using the hollow cathode configuration, also results in the bombardment of the cathode by ions and fast neutrals, consequently causing its heating. Using the hollow cathode geometry, the ionization rate is higher than that of the linear abnormal glow discharge. The ionization rate of the former is around ten times higher than that of the latter considering an argon discharge generated at 300 V, under 100 Pa in a configuration whose distance inter-cathode is 5 mm [18]. As a result, an increased heating efficiency is obtained in the same proportion as compared to the linear discharge. Taking advantage of the high ionization rate, hollow cathode discharges of gas mixtures containing hydrogen, nitrogen and methane were also used to surface treat steels in nitriding [19] and carburizing or carbonitriding [20]. In this paper, a D.C. pulsed hollow cathode discharge in an argon/hydrogen gas mixture was used to sinter unalloyed iron. The advantage of using a mixture of Ar/H2 lies in the production of hydrogen atoms and a consequent reduction of the oxides present in the pressed pellets. The high ionization rate obtained in the hollow cathode configuration resulted in an efficient heating of the samples. The discharge was maintained at relatively low-pressures (133 /400 Pa) in a gas flow of high purity gas mixtures; thus, atmospheres of elevated purity levels could easily be obtained. Measurements of temperature as a function of the distance between the inner and outer cathodes as well as the influence of the power applied to the discharge are presented. Microstructural aspects of sintered samples are shown and it was verified that samples might be successfully sintered in a hollow cathode discharge. In addition, the effect of atoms sputtered from the outer cathode resulted in the formation of a surface layer containing Cr and Ni.
2. Experimental and materials The plasma sintering apparatus is shown in Fig. 1. The reactor consisted of a stainless steel cylinder 350 mm in diameter and 380 mm in height. Samples 10 mm in diameter and 10 mm in high were placed on a steel support that functions as the inner cathode of the discharge. The outer cathode was an AISI 310 stainless steel cylinder, located concentrically to the sample. The diameters of the outer cathode were 16, 22 and 28 mm corresponding to inter-cathode distances of 3, 6 and 9 mm, respectively. The height of the external cylinder was 25 mm and the central cathode consisted of the support,
Fig. 1. Experimental apparatus. (1) Sample, (2) Outer cathode, (3) Window, (4) Capacitance manometer, (5) Mechanical pump, (6) Gas mixture inlet. Icc, inner cathode current and It, total current.
the sample and a cylinder on the top, such that the inner cathode total height was also 25 mm, so as to generate a uniform electric field and consequently a homogeneous discharge. The inner cathode and the outer cylinder were negatively biased at the same voltage, using a square waveform pulsed power supply of 3.6 kW. The voltage was fixed to 460 V and the power transferred to the plasma was adjusted by varying the time switched on (ton) of the pulse. The ton of the pulse could be varied from 10 to 180 ms and the total on/off time was 200 ms. The sample temperature was varied by adjusting the on/ off time of the pulsed voltage and was measured using a chromel /alumel (type K) thermocouple. This thermocouple was protected with a stainless steel cover, 1.5 mm in diameter, electrically isolated with Al2O3 and inserted 8 mm into the sample holder. To ensure a uniform bombardment of ions, leading to a thermal equilibrium between the parts, the sample and holder were made of the same material. Prior to sintering, the system was pumped down by a two-stage mechanical pump until a residual pressure of less than 1.3 Pa (0.01 Torr) was reached. The gas mixture consisting of 80% argon (99.999% pure) and 20% hydrogen (99.998% pure) was adjusted using two Datametrics mass flow controllers whose full scale value was 8.3 /10 6 standard m3 s 1 (500 sccm) and 3.3 / 10 6 standard m3 s 1 (200 sccm), respectively. The total gas flow was set to 5 /10 6 standard m3 s 1 (300 sccm) in order to maintain a ‘clean’ atmosphere of the discharge. The pressure in the vacuum chamber was
S.F. Brunatto et al. / Materials Science and Engineering A343 (2003) 163 /169
adjusted by manual valves and measured using an Edward capacitance manometer of 1.33 /103 Pa (10 Torr) full scale. Samples of unalloyed iron powder were compacted at a pressure of 600 MPa by a double action pressing with moving die body. The size of the powder particles ranged between 30 and 200 mm and the mean of the particle size distribution was around 100 mm. Plasma sintering was carried out at 1150 8C and a heating rate of around 25 8C min 1. Prior to sintering, the samples were cleaned in an argon/hydrogen discharge at a pressure of 133 Pa and at a temperature of 450 8C for 30 min. The sintered samples were cut and mounted in bakelite for microstructural analysis. The exposed surfaces were then ground and polished with diamond paste to a 1-mm finish. Scanning electron microscopy (SEM) was carried out by using a Philips XL-30 equipment, and the chemical composition of the sample surface was obtained using energy dispersive X-ray analysis (EDX). Mass loss on samples was measured with a 0.1-mg precision balance.
165
Fig. 2. Variation of sample temperature as a function of the pulse power supply switched on ton for pressures of 133 and 400 Pa. The pulse voltage was 460 V and the distance between cathodes a /6 mm.
3. Results and discussion In Section 3.1, the characterization of the hollow cathode configuration is presented, particularly the evolution of temperature as a function of cathode geometry, the discharge pressure and the power transferred to the glow discharge. The characteristics of the sintered samples are presented in Section 3.2 with an emphasis on the sputtering effect produced on the sample during sintering. 3.1. Characterization of the hollow cathode discharge The variation of the sample temperature as a function of ton for pressures of 133 and 400 Pa is presented in Fig. 2. The distance a between the inner and outer cathodes was fixed at a value of 6 mm, and the pulse voltage was fixed at 460 V. As expected, temperature increased when ton increased and at a pressure of 133 Pa, even for the major ton of the pulse, a temperature of 1150 8C was not attained. At a pressure of 400 Pa, as the discharge current increased as a function of pressure, a temperature of 1200 8C for ton /60 ms was reached. Maintaining the pulse voltage at 460 V and the pressure at 400 Pa, measurements of temperature as a function of ton are presented in Fig. 3 for three values of the distance a between the inner and outer cathodes (3, 6 and 9 mm). In order to evaluate the influence of the hollow cathode configuration, the outer cathode was removed and measurements of temperature in this configuration were also presented in Fig. 3. A significant increase of temperature was observed in the hollow configuration as
Fig. 3. Variation of sample temperature as a function of the pulse power supply switched on ton for distance between cathodes a of 3, 6 and 9 mm and without external cathode. The pulse voltage was 460 V and the pressure 400 Pa.
compared to the linear abnormal glow discharge; the higher the confinement of the hollow cathode configuration, the more accentuated was the increase of
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temperature. For the distance between cathodes of 9 mm, a temperature of 1150 8C was reached when ton was 180 ms, i.e. the upper ton limit, so it may be difficult to maintain the sintering temperature for 1 h. Otherwise, when the distance a /3 mm, the discharge was confined in such a way that the ionization increase was excessively high, as discussed by Von Engel [18]. As a consequence, the current density and thus the temperature increased rapidly for a small increase of ton, which makes it very hard to achieve accurate temperature values. In others words, a variation of ton of roughly 1 ms results in a variation of about 20 8C at 1150 8C (cf. Fig. 3). For a distance a /6 mm, a temperature of 1150 8C was obtained for a ton of 48 ms and may be maintained with a precision of less than 10 8C. Theses conditions, distance inter-cathode a /6 mm, pulse voltage of 460 V, gas mixture of 80% Ar/20% H2 and pressure of 400 Pa are convenient to carry out sintering and will be addressed later on.
3.2. Characterization of unalloyed sintered samples
Fig. 4. SEM of transversely cross-sectioned samples, sintered at 1150 8C, for times of (a) 30 min and (b) 240 min.
Fig. 5. EDX analysis of the sample surface (a) lateral part and (b) base.
Micrographs of transversely cross-sectioned samples, sintered at 1150 8C for 30 and 240 min are presented in Fig. 4(a) and (b), respectively. When sintering was carried out for 30 min an efficient formation of necks was observed, which, as expected, increased with processing time. In addition, numerous small pores observed in samples sintered for 30 min disappeared when the processing time was increased to 240 min, and as a consequence, growth of the pore size could be identified. This indicated that sintering of unalloyed iron, using hollow cathode discharge technique might be successfully carried out. These results were in good agreement with those presented by Batista et al. [15]. In that paper, sintering was carried out using a linear
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abnormal glow discharge and an activation of mass transport by sintering was observed, which was attributed to back-scattering of atoms sputtered from the sample surface. In order to investigate the effect of sputtering, EDX analysis was carried out on the lateral part of the sample surface, which was exposed to deposition of atoms sputtered from the outer cathode. The results showed, Fig. 5(a), that besides the observation of iron peaks, chromium and nickel were also deposited on the surface. EDX analysis of the base of the sample in contact with the sample holder is presented in Fig. 5(b) and only iron was detected on the surface. That confirmed that chromium and nickel elements were sputtered from the outer cathode and deposited on the sample surface. At the sintering temperature, atoms of Cr and Ni deposited on the sample surface diffused into the material, forming a layer containing Cr and Ni elements, as shown in Fig. 6(a) and (b). As expected, by increasing the processing time, the depth of the layer enriched with alloying elements also increased. However, at the same time that atoms were sputtered from the outer cathode and deposited on the sample, it was also bombarded by ions. Thus, a sputtering of atoms from the sample surface may also occur. A mass loss of the sample is shown in Fig. 7, and increased for higher sintering times. The mass of the samples was typically around 5 g and the mass loss ranged from 15 to 65 mg. This mass loss
167
Fig. 7. Mass loss percent of the sample sintered at 1150 8C as a function of processing time.
showed that in spite of the atoms of Cr, Ni and certainly Fe being deposited on the sample surface coming from the outer cathode, a higher quantity was sputtered out of the sample. Such an effect may be explained in terms of the higher intensity of ion bombardment of the inner cathode as compared to the internal surface of the outer cathode. When sintering was carried out at a temperature of 1150 8C, using an inter-cathode distance of 6 mm and a pressure of 400 Pa, the current measured at the inner cathode was 160 and 300 mA at the outer. In order to evaluate the ion current density on the lateral surface of the inner cathode, the ionization rate in the hollow cathode region as well as of the base and top of the sample, where a linear abnormal discharge was generated, must be considered. By using results published by Guntherschulze in 1930 and presented by Von Engel [18], for the discharge parameters of the experiment discussed here it can be evaluated that the rate: I (hollow cathode)=I (linear discharge) :7
Fig. 6. Profiles of Cr and Ni concentration on the lateral part of the sample, sintered at a temperature of 1150 8C for times of (a) 60 min and (b) 120 min.
Subtracting the contributions of the base and top which was corrected by the factor 1/7, the current calculated on the lateral part of the sample was around 155 mA and the current density around 20 mA cm 2. Using the same procedure, but applied to the external part of the outer cathode, the current at the internal part of the outer cathode may be evaluated to give a value of 260 mA and a current density of 15 mA cm 2. Consequently, ion bombardment and sputtering were more intense on the sample than on the outer cathode. In addition, results presented by Timanyuk and Tkachenko [21] showed that the dark space at the inner cathode of an annular discharge is thinner than that observed at the outer cylinder. In this case, fewer collisions between ions and atoms or molecules occur in the cathode sheath of
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168
Table 1 Atomic percent concentration of Cr and Ni in atoms on the external and internal surfaces of the outer cathode Element
Cr Ni
Concentration in atoms (%) External surface
Internal surface
26.5 16.5
17.0 9.0
The processing time was 30 min at a pressure of 400 Pa and the sample temperature was 1150 8C.
amount of Cr and Ni deposited on the surface was observed for a time around 80 min. This behavior resulted from the decrease in alloy elements on the internal surface of the outer cathode as a function of time. Thus, the concentration of these elements in the gas phase was reduced and as the diffusion from the surface to the core remained, a decrease of Cr and Ni on the sample surface was observed for a sintering time of 240 min.
4. Conclusion the inner cathode, resulting in higher kinetic energies of ions and, consequently, in a higher sputtering yield on the sample. The higher sputtering observed on the sample surface was confirmed by the results presented in Table 1. For a sintering time of 30 min, the atom percent concentration was 26% of Cr and 16.5% of Ni on the external surface of the outer cathode. This was the same as the initial composition of the stainless steel AISI 310, according to what it is generally accepted, i.e. the stoichiometry was maintained during sputtering. On the contrary, it was observed that the concentration of Cr and Ni decreased strongly at the internal surface. This clearly indicated that a significant quantity of iron atoms was deposited on the internal surface of the outer cathode, what was in good agreement with the results presented in Fig. 7, where a higher sputtering was verified on the sample surface. The percent concentration of Cr and Ni on the sample surface as a function of sintering time is presented in Fig. 8. A maximum
An annular hollow cathode discharge may be successfully used to sinter metallic components. Atoms sputtered from the outer cathode were deposited onto the sample surface and by diffusion formed a layer containing elements such as Cr and Ni when an outer stainless steel cylinder was used. Such a layer containing alloy elements as chromium, whose affinity for nitrogen is high, certainly is suitable for surface treatment such as plasma nitriding. In addition, it was shown that the sputtering of atoms from the inner cathode was stronger than that observed on the internal surface of the outer cathode, resulting in a loss of mass of the sample. Certainly, the intense ion bombardment may produce a significant modification of the sample surface morphology. Further studies are being carried out so as to investigate this.
Acknowledgements This work was performed using funds from a Finep/ MCT (PRONEX) and CNPq research grant.
References
Fig. 8. Cr and Ni concentration as a function of processing time on the lateral surface of the sample. Sintering temperature: 1150 8C.
[1] D.L. Johnson, W.B. Sanderson, J.M. Knowlton, E.L. Kemer, M.Y. Chen, Sci. Sintering 20 (2/3) (1988) 109. [2] R.A. Page, S. Spooner, W.B. Sanderson, D.L. Johnson, J. Am. Ceram. Soc. 71 (12) (1988) 1125. [3] M. Bengisu, O.T. Inal, J. Mater. Sci. 29 (1994) 5475. [4] H. Su, D.L. Johnson, J. Am. Ceram. Soc. 79 (12) (1996) 3199. [5] N.P. Tandian, E. Pfender, Plasma Chem. Plasma Proc. 17 (3) (1997) 353. [6] Y.S. Park, M.J. McNallan, J. Mater. Sci. 32 (1997) 523. [7] J.R. Groza, S.H. Risbud, K. Yamazaki, J. Mater. Res. 7 (10) (1992) 2643. [8] R.S. Mishra, J.A. Schneider, J.F. Shackelford, A.K. Mukherjee, Nanostruct. Mater. 5 (5) (1995) 525. [9] L.L. Ye, Z.G. Liu, K. Raviprasad, M.X. Quan, M. Umemoto, Z.Q. Hu, Mater. Sci. Eng. A 241 (1998) 290. [10] T. Takeuchi, E. Be´tourne´, M. Tabuchi, H. Kageyama, Y. Kobayashi, A. Coats, F. Morrison, D.C. Sinclair, A.R. West, J. Mater. Sci. 34 (1999) 917. [11] T. Takeuchi, M. Tabuchi, H. Kageyama, J. Am. Ceram. Soc. 82 (4) (1999) 939.
S.F. Brunatto et al. / Materials Science and Engineering A343 (2003) 163 /169 [12] T. Murakami, M. Komatsu, A. Kitahara, M. Kawahara, Y. Takahashi, Y. Ono, Intermetallics 7 (1999) 731. [13] A. Matsumoto, K. Katoh, K. Andoh, Corr. Eng. 44 (1995) 751. [14] J.L.R. Muzart, V.J. Batista, C.V. Franco, A.N. Klein, Adv. Powder Metall. Particul. Mater. 3 (1997) 77. [15] V.J. Batista, R. Binder, A.N. Klein, J.L.R. Muzart, Int. J. Powder Metall. 34 (8) (1998) 55. [16] V.J. Batista, M. Mafra, J.L.R. Muzart, A.N. Klein, N. Back, Mater. Sci. Forum 299 (3) (1999) 249.
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[17] B. Chapman, Glow Discharge Processes: Sputtering and Plasma Etching, Wiley, New York, 1980, p. 100. [18] A. Von Engel, Ionized Gases, AIP Press, Woodbury, NY, 1994, p. 228. [19] M. Benda, J. Vlcek, V. Cibulka, J. Musil, J. Vac. Sci. Technol. A 15 (5) (1997) 2636. [20] K. Terakado, R. Urao, M. Ohmori, Metall. Mater. Trans. A 27A (1996) 401. [21] V.A. Timanyuk, V.M. Tkachenko, Sov. Phys. Tech. Phys. 34 (7) (1989) 832.
Sintering iron using a hollow cathode discharge S.F. Brunatto a, I. Ku¨hn b, A.N. Klein b, J.L.R. Muzart b, a
b
Departamento de Engenharia Mecaˆnica, Universidade Federal do Parana´-81531-990, Curitiba, PR, Brazil LABMAT, Departamento de Engenharia Mecaˆnica, Universidade Federal de Santa Catarina-88040-900, Floriano´polis, SC, Brazil Received 20 November 2001; received in revised form 15 May 2002
Abstract Unalloyed iron samples were sintered in the presence of a hollow cathode glow discharge, generated in a gas mixture of 80% Ar/ 20% H2 at pressures ranging from 133 to 400 Pa. The sample, which worked as the inner cathode of the discharge, was heated by the bombardment of strongly accelerated ions and fast neutrals created in the cathode sheath. The outer cathode of the hollow geometry consisted of a stainless steel AISI 310 cylinder. The enhanced ionization obtained in the hollow cathode configuration provided a high current density (25 mA cm 2) and consequently a high temperature could be attained (1250 8C). The samples were sintered at 1150 8C for times ranging from 30 to 240 min. The temperature was adjusted by varying the time on/off of the pulsed power supply used to generate the discharge. Microstructural results are presented and it is shown that samples may be successfully sintered in a hollow cathode discharge. In addition, atoms of Cr and Ni sputtered from the outer stainless steel cathode were deposited onto the sample surface and diffused, during sintering, resulting in the formation of a layer approximately 20 mm thick containing these elements. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma sintering; Unalloyed iron; Hollow cathode discharge
1. Introduction Sintering of ceramics and steels is conventionally carried out in a furnace in a controlled atmosphere. Several alternative techniques have been developed since 1966, as mentioned by Johnson et al. [1], to obtain components with improved properties, resulting in new applications for powder technology. Plasmas generated in microwave cavities, in hollow cathode devices and microwave or RF-inductively coupled discharges have all been used for sintering ceramics [2 /5]. In the above mentioned papers it was reported that ceramics could be sintered to high density at high sintering rates. Heat transfer mechanism and cleaning of the particle surfaces in the plasma environment are suggested to be responsible for the enhanced sintering. However, the plasma reactivity may be an impediment to sintering as discussed by Park and McNallan [6]. They concluded that
Corresponding author. Tel.: /55-48-234-0084; fax: /55-48-2340059 E-mail address: [email protected] (J.L.R. Muzart).
sintering of SiC whisker reinforced mullite matrix composites was not successfully carried out because of chemical reactions occurring in the plasma atmosphere. High-density samples at high rate sintering may also be obtained using the plasma activated sintering or spark plasma sintering process [7 /12]. The pulsed voltage applied to the sample removes the oxide film and adsorbed gases from particle surfaces of non-oxide ceramics or metals, and activates the sintering. Following the cleaning of the particle surfaces, a direct current associated with a pulsed one flows among the powders and, thus, the contact resistance between particles generates heat by the Joule effect. This localized heating accelerates the formation of necks, thus activating sintering [13]. Recently, sintering of metallic components using an abnormal glow discharge containing hydrogen and argon has been described [14 /16]. The abnormal glow discharge is characterized by full covering of the cathode by the glow region [17], supplying a uniform treatment. The principle of heating is based on the ion and fast neutrals bombardment of the sample. A negatively biased voltage was applied to the sample, which worked as the cathode of the abnormal glow
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 3 8 3 - 0
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discharge, generating an electric field in the cathode sheath, where ions are strongly accelerated. Collisions between ions and argon atoms or hydrogen molecules of the gas discharge in the cathode sheath result in a flow of fast neutrals toward the cathode [17]. The bombardment of ions and fast neutrals heat the sample. The modified linear abnormal glow discharge, using the hollow cathode configuration, also results in the bombardment of the cathode by ions and fast neutrals, consequently causing its heating. Using the hollow cathode geometry, the ionization rate is higher than that of the linear abnormal glow discharge. The ionization rate of the former is around ten times higher than that of the latter considering an argon discharge generated at 300 V, under 100 Pa in a configuration whose distance inter-cathode is 5 mm [18]. As a result, an increased heating efficiency is obtained in the same proportion as compared to the linear discharge. Taking advantage of the high ionization rate, hollow cathode discharges of gas mixtures containing hydrogen, nitrogen and methane were also used to surface treat steels in nitriding [19] and carburizing or carbonitriding [20]. In this paper, a D.C. pulsed hollow cathode discharge in an argon/hydrogen gas mixture was used to sinter unalloyed iron. The advantage of using a mixture of Ar/H2 lies in the production of hydrogen atoms and a consequent reduction of the oxides present in the pressed pellets. The high ionization rate obtained in the hollow cathode configuration resulted in an efficient heating of the samples. The discharge was maintained at relatively low-pressures (133 /400 Pa) in a gas flow of high purity gas mixtures; thus, atmospheres of elevated purity levels could easily be obtained. Measurements of temperature as a function of the distance between the inner and outer cathodes as well as the influence of the power applied to the discharge are presented. Microstructural aspects of sintered samples are shown and it was verified that samples might be successfully sintered in a hollow cathode discharge. In addition, the effect of atoms sputtered from the outer cathode resulted in the formation of a surface layer containing Cr and Ni.
2. Experimental and materials The plasma sintering apparatus is shown in Fig. 1. The reactor consisted of a stainless steel cylinder 350 mm in diameter and 380 mm in height. Samples 10 mm in diameter and 10 mm in high were placed on a steel support that functions as the inner cathode of the discharge. The outer cathode was an AISI 310 stainless steel cylinder, located concentrically to the sample. The diameters of the outer cathode were 16, 22 and 28 mm corresponding to inter-cathode distances of 3, 6 and 9 mm, respectively. The height of the external cylinder was 25 mm and the central cathode consisted of the support,
Fig. 1. Experimental apparatus. (1) Sample, (2) Outer cathode, (3) Window, (4) Capacitance manometer, (5) Mechanical pump, (6) Gas mixture inlet. Icc, inner cathode current and It, total current.
the sample and a cylinder on the top, such that the inner cathode total height was also 25 mm, so as to generate a uniform electric field and consequently a homogeneous discharge. The inner cathode and the outer cylinder were negatively biased at the same voltage, using a square waveform pulsed power supply of 3.6 kW. The voltage was fixed to 460 V and the power transferred to the plasma was adjusted by varying the time switched on (ton) of the pulse. The ton of the pulse could be varied from 10 to 180 ms and the total on/off time was 200 ms. The sample temperature was varied by adjusting the on/ off time of the pulsed voltage and was measured using a chromel /alumel (type K) thermocouple. This thermocouple was protected with a stainless steel cover, 1.5 mm in diameter, electrically isolated with Al2O3 and inserted 8 mm into the sample holder. To ensure a uniform bombardment of ions, leading to a thermal equilibrium between the parts, the sample and holder were made of the same material. Prior to sintering, the system was pumped down by a two-stage mechanical pump until a residual pressure of less than 1.3 Pa (0.01 Torr) was reached. The gas mixture consisting of 80% argon (99.999% pure) and 20% hydrogen (99.998% pure) was adjusted using two Datametrics mass flow controllers whose full scale value was 8.3 /10 6 standard m3 s 1 (500 sccm) and 3.3 / 10 6 standard m3 s 1 (200 sccm), respectively. The total gas flow was set to 5 /10 6 standard m3 s 1 (300 sccm) in order to maintain a ‘clean’ atmosphere of the discharge. The pressure in the vacuum chamber was
S.F. Brunatto et al. / Materials Science and Engineering A343 (2003) 163 /169
adjusted by manual valves and measured using an Edward capacitance manometer of 1.33 /103 Pa (10 Torr) full scale. Samples of unalloyed iron powder were compacted at a pressure of 600 MPa by a double action pressing with moving die body. The size of the powder particles ranged between 30 and 200 mm and the mean of the particle size distribution was around 100 mm. Plasma sintering was carried out at 1150 8C and a heating rate of around 25 8C min 1. Prior to sintering, the samples were cleaned in an argon/hydrogen discharge at a pressure of 133 Pa and at a temperature of 450 8C for 30 min. The sintered samples were cut and mounted in bakelite for microstructural analysis. The exposed surfaces were then ground and polished with diamond paste to a 1-mm finish. Scanning electron microscopy (SEM) was carried out by using a Philips XL-30 equipment, and the chemical composition of the sample surface was obtained using energy dispersive X-ray analysis (EDX). Mass loss on samples was measured with a 0.1-mg precision balance.
165
Fig. 2. Variation of sample temperature as a function of the pulse power supply switched on ton for pressures of 133 and 400 Pa. The pulse voltage was 460 V and the distance between cathodes a /6 mm.
3. Results and discussion In Section 3.1, the characterization of the hollow cathode configuration is presented, particularly the evolution of temperature as a function of cathode geometry, the discharge pressure and the power transferred to the glow discharge. The characteristics of the sintered samples are presented in Section 3.2 with an emphasis on the sputtering effect produced on the sample during sintering. 3.1. Characterization of the hollow cathode discharge The variation of the sample temperature as a function of ton for pressures of 133 and 400 Pa is presented in Fig. 2. The distance a between the inner and outer cathodes was fixed at a value of 6 mm, and the pulse voltage was fixed at 460 V. As expected, temperature increased when ton increased and at a pressure of 133 Pa, even for the major ton of the pulse, a temperature of 1150 8C was not attained. At a pressure of 400 Pa, as the discharge current increased as a function of pressure, a temperature of 1200 8C for ton /60 ms was reached. Maintaining the pulse voltage at 460 V and the pressure at 400 Pa, measurements of temperature as a function of ton are presented in Fig. 3 for three values of the distance a between the inner and outer cathodes (3, 6 and 9 mm). In order to evaluate the influence of the hollow cathode configuration, the outer cathode was removed and measurements of temperature in this configuration were also presented in Fig. 3. A significant increase of temperature was observed in the hollow configuration as
Fig. 3. Variation of sample temperature as a function of the pulse power supply switched on ton for distance between cathodes a of 3, 6 and 9 mm and without external cathode. The pulse voltage was 460 V and the pressure 400 Pa.
compared to the linear abnormal glow discharge; the higher the confinement of the hollow cathode configuration, the more accentuated was the increase of
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temperature. For the distance between cathodes of 9 mm, a temperature of 1150 8C was reached when ton was 180 ms, i.e. the upper ton limit, so it may be difficult to maintain the sintering temperature for 1 h. Otherwise, when the distance a /3 mm, the discharge was confined in such a way that the ionization increase was excessively high, as discussed by Von Engel [18]. As a consequence, the current density and thus the temperature increased rapidly for a small increase of ton, which makes it very hard to achieve accurate temperature values. In others words, a variation of ton of roughly 1 ms results in a variation of about 20 8C at 1150 8C (cf. Fig. 3). For a distance a /6 mm, a temperature of 1150 8C was obtained for a ton of 48 ms and may be maintained with a precision of less than 10 8C. Theses conditions, distance inter-cathode a /6 mm, pulse voltage of 460 V, gas mixture of 80% Ar/20% H2 and pressure of 400 Pa are convenient to carry out sintering and will be addressed later on.
3.2. Characterization of unalloyed sintered samples
Fig. 4. SEM of transversely cross-sectioned samples, sintered at 1150 8C, for times of (a) 30 min and (b) 240 min.
Fig. 5. EDX analysis of the sample surface (a) lateral part and (b) base.
Micrographs of transversely cross-sectioned samples, sintered at 1150 8C for 30 and 240 min are presented in Fig. 4(a) and (b), respectively. When sintering was carried out for 30 min an efficient formation of necks was observed, which, as expected, increased with processing time. In addition, numerous small pores observed in samples sintered for 30 min disappeared when the processing time was increased to 240 min, and as a consequence, growth of the pore size could be identified. This indicated that sintering of unalloyed iron, using hollow cathode discharge technique might be successfully carried out. These results were in good agreement with those presented by Batista et al. [15]. In that paper, sintering was carried out using a linear
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abnormal glow discharge and an activation of mass transport by sintering was observed, which was attributed to back-scattering of atoms sputtered from the sample surface. In order to investigate the effect of sputtering, EDX analysis was carried out on the lateral part of the sample surface, which was exposed to deposition of atoms sputtered from the outer cathode. The results showed, Fig. 5(a), that besides the observation of iron peaks, chromium and nickel were also deposited on the surface. EDX analysis of the base of the sample in contact with the sample holder is presented in Fig. 5(b) and only iron was detected on the surface. That confirmed that chromium and nickel elements were sputtered from the outer cathode and deposited on the sample surface. At the sintering temperature, atoms of Cr and Ni deposited on the sample surface diffused into the material, forming a layer containing Cr and Ni elements, as shown in Fig. 6(a) and (b). As expected, by increasing the processing time, the depth of the layer enriched with alloying elements also increased. However, at the same time that atoms were sputtered from the outer cathode and deposited on the sample, it was also bombarded by ions. Thus, a sputtering of atoms from the sample surface may also occur. A mass loss of the sample is shown in Fig. 7, and increased for higher sintering times. The mass of the samples was typically around 5 g and the mass loss ranged from 15 to 65 mg. This mass loss
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Fig. 7. Mass loss percent of the sample sintered at 1150 8C as a function of processing time.
showed that in spite of the atoms of Cr, Ni and certainly Fe being deposited on the sample surface coming from the outer cathode, a higher quantity was sputtered out of the sample. Such an effect may be explained in terms of the higher intensity of ion bombardment of the inner cathode as compared to the internal surface of the outer cathode. When sintering was carried out at a temperature of 1150 8C, using an inter-cathode distance of 6 mm and a pressure of 400 Pa, the current measured at the inner cathode was 160 and 300 mA at the outer. In order to evaluate the ion current density on the lateral surface of the inner cathode, the ionization rate in the hollow cathode region as well as of the base and top of the sample, where a linear abnormal discharge was generated, must be considered. By using results published by Guntherschulze in 1930 and presented by Von Engel [18], for the discharge parameters of the experiment discussed here it can be evaluated that the rate: I (hollow cathode)=I (linear discharge) :7
Fig. 6. Profiles of Cr and Ni concentration on the lateral part of the sample, sintered at a temperature of 1150 8C for times of (a) 60 min and (b) 120 min.
Subtracting the contributions of the base and top which was corrected by the factor 1/7, the current calculated on the lateral part of the sample was around 155 mA and the current density around 20 mA cm 2. Using the same procedure, but applied to the external part of the outer cathode, the current at the internal part of the outer cathode may be evaluated to give a value of 260 mA and a current density of 15 mA cm 2. Consequently, ion bombardment and sputtering were more intense on the sample than on the outer cathode. In addition, results presented by Timanyuk and Tkachenko [21] showed that the dark space at the inner cathode of an annular discharge is thinner than that observed at the outer cylinder. In this case, fewer collisions between ions and atoms or molecules occur in the cathode sheath of
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Table 1 Atomic percent concentration of Cr and Ni in atoms on the external and internal surfaces of the outer cathode Element
Cr Ni
Concentration in atoms (%) External surface
Internal surface
26.5 16.5
17.0 9.0
The processing time was 30 min at a pressure of 400 Pa and the sample temperature was 1150 8C.
amount of Cr and Ni deposited on the surface was observed for a time around 80 min. This behavior resulted from the decrease in alloy elements on the internal surface of the outer cathode as a function of time. Thus, the concentration of these elements in the gas phase was reduced and as the diffusion from the surface to the core remained, a decrease of Cr and Ni on the sample surface was observed for a sintering time of 240 min.
4. Conclusion the inner cathode, resulting in higher kinetic energies of ions and, consequently, in a higher sputtering yield on the sample. The higher sputtering observed on the sample surface was confirmed by the results presented in Table 1. For a sintering time of 30 min, the atom percent concentration was 26% of Cr and 16.5% of Ni on the external surface of the outer cathode. This was the same as the initial composition of the stainless steel AISI 310, according to what it is generally accepted, i.e. the stoichiometry was maintained during sputtering. On the contrary, it was observed that the concentration of Cr and Ni decreased strongly at the internal surface. This clearly indicated that a significant quantity of iron atoms was deposited on the internal surface of the outer cathode, what was in good agreement with the results presented in Fig. 7, where a higher sputtering was verified on the sample surface. The percent concentration of Cr and Ni on the sample surface as a function of sintering time is presented in Fig. 8. A maximum
An annular hollow cathode discharge may be successfully used to sinter metallic components. Atoms sputtered from the outer cathode were deposited onto the sample surface and by diffusion formed a layer containing elements such as Cr and Ni when an outer stainless steel cylinder was used. Such a layer containing alloy elements as chromium, whose affinity for nitrogen is high, certainly is suitable for surface treatment such as plasma nitriding. In addition, it was shown that the sputtering of atoms from the inner cathode was stronger than that observed on the internal surface of the outer cathode, resulting in a loss of mass of the sample. Certainly, the intense ion bombardment may produce a significant modification of the sample surface morphology. Further studies are being carried out so as to investigate this.
Acknowledgements This work was performed using funds from a Finep/ MCT (PRONEX) and CNPq research grant.
References
Fig. 8. Cr and Ni concentration as a function of processing time on the lateral surface of the sample. Sintering temperature: 1150 8C.
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