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Decarburization during plasma nitriding
Surface and Coatings Technology 221 (1999) 33–38 www.elsevier.nl/locate/surfcoat
Decarburization during plasma nitriding P. Egert a,b, A.M. Maliska a, H.R.T. Silva a, C.V. Speller a, * a LABMAT/Laborato´rio de Materiais-Departamento de Engenharia Mecaˆnica/UFSC, CEP 88040-900, Floriano´polis, SC, Brazil b Departamento de Cieˆncias Exatas, Agra´rias e das Engenharias, Universidade do Sul de Santa Catarina, Campus Dudavan, 88130-000-Palhoc¸a, SC, Brazil
Abstract This work reports on the decarburization of steel surfaces nitrided in d.c. plasma discharges. Mass spectrometry was used to detect the presence of chemical species, such as C and CH , in plasma atmospheres containing hydrogen, and CO in those 3 2 containing oxygen. The active species of gaseous mixtures containing hydrogen react with superficial carbon to form CH radicals, 3 thus enabling decarburization. Conversely, although CO -forming reactions taking place in the presence of oxygen drive early 2 decarburization, at higher oxygen concentrations, an oxide coating layer is formed and acts as a reducing agent for the process. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Decarburization; Plasma nitriding; Steel surfaces
1. Introduction Plasma surface treatments have been increasingly applied to steels. As a result, numerous studies dedicated to understand the interaction of plasma species with the surface of materials have recently been carried out. As the process is developed, it has become clear that its full potential will be ultimately achieved as the fundamental species involved in the process become determined, and favourable conditions for the forming reactions to take place are established. Knowledge of the interactions existent between plasma species and material surface strongly depends on the determination of the identity of the species as well as on an evaluation of their behaviour as a function of the main plasma parameters. Although the mechanisms through which plasma species are formed play a direct role in the process, other reactions may also take place. Though indirectly related to the process, they should not be neglected. This seems to be the case for the reaction involving carbon from the steel, which may deplete the surface of the material of this element. Therefore, studies on plasma-nitriding steel should not overlook the role of carbon, since it is directly related to phase growth in the material. Such interaction may cause both the diffusion of plasma atoms towards the material and the diffusion of certain elements from the material towards * Corresponding author. E-mail address: [email protected] (C.V. Speller)
the plasma atmosphere. Studies focused on the behaviour of carbon in plasma-treated materials have shown that decarburization of the material surface takes place as a result of the interaction of the surface with the plasma [1–4]. Most of these studies directly related the process to surface reactions involving active hydrogen species and the formation of CH (x=1–3) neutral x radicals. The plasma of an electrical discharge is a nonequilibrium environment where unusual chemical species, such as neutral CH radicals can be formed x from reactions not commonly observed in conventional systems. These neutral species usually have short lifetimes, of the order of milliseconds or less. Their detection requires special plasma diagnostic techniques, such as emission spectroscopy and mass spectrometry, applied to the usually very reactive samples gathered directly from the plasma reactor. The aim of the present study is to investigate steel decarburization during surface plasma treatment as well as the decarburization mechanism of carbon steel. Mass spectrometry was used to detect the presence of neutral species, such as C and CH , or CO , when hydrogen or 3 2 oxygen are present in the gaseous atmosphere of plasma, respectively.
2. Experimental The experimental apparatus used in steel plasma nitriding is schematically shown in Fig. 1 and has been
0257-8972/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 40 7 - 7
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P. Egert et al. / Surface and Coatings Technology 221 (1999) 33–38
3. Results and discussion 3.1. Mixtures containing oxygen
Fig. 1. Schematic diagram of the experimental apparatus.
thoroughly described elsewhere [5]. A mass spectrometer was coupled to the nitriding reactor through an opening of 100 mm in diameter, allowing the passage of neutral species. These species were then introduced into an electron impact ionization chamber, in which they were ionized before entering the mass analysis chamber. In the plasma chamber, an electrical discharge is produced between the sample holder (cathode) and the walls of the chamber (anode). The entire set-up must be earthed. The system contains two set-ups that enable plasma species to be collected from two distinct regions. In one of the set-ups, the sample holder is placed approximately 2 cm from the collector orifice. In this way, the species are extracted from a region distant from the surface of the material. In the second set-up, the collecting orifice is the sample itself, so that the species can be collected directly from the region of plasma–surface interaction. This configuration was employed for the present study as it enables the detection of a very small number of neutral species formed in that region. Cylindrical AISI 1045 steel pellets were plasmatreated in Ar–O and Ar–H discharges, as well as in 2 2 gaseous mixtures containing nitrogen. The samples were later analysed by optical and scanning electron microscopy. The use of different gaseous mixtures made it possible to evaluate aspects such as the formation of nitride and oxide layers upon decarburization. The influence of other parameters such as O :H ratio, time 2 2 and temperature was also assessed. During the treatment, different neutral plasma species were simultaneously monitored in a procedure that assisted in the establishment of correlation studies between plasma diagnostics and metallographic analysis of the treated material.
In an attempt to detect neutral CO species, mass 2 spectrometry analyses of the plasma atmosphere were conducted as steel surfaces were nitrided. A more detailed study suggested that CO is formed as oxygen 2 contents ranging from 0 to 6% are introduced into the gaseous mixture [7,8]. This trend is shown in Fig. 2(a) along with a constant increase in content of carbon available from the material. The use of high-carbon materials results in an increase in the number of species formed. This is also the case when relatively high concentrations of oxygen are available in the plasma atmosphere. For oxygen contents up to 3%, the process takes place at a reasonably constant rate for each material, stabilizing after about 30 min. This can be explained as nitrogen penetrates into the iron lattice and fills its interstices, thus decreasing the supply of carbon at the surface. In addition, it is likely that the oxided surface acts as a barrier, hindering the exit of carbon at the surface. Evidence of decarburization from analyses on nitrided materials is difficult to obtain since the results are masked by the presence of the nitride layer itself. Specimens treated in Ar–O 2 atmospheres are an alternative, inasmuch as decarburization can be observed in the absence of nitrides. Monitoring CO species indicates that, under similar 2 conditions, CO formation mechanism is different in 2 both mixtures. In fact, as this may result from the interaction of plasma species with the surface of the material, the differences between curves a and b in Fig. 2 may correspond due to the different forms of interaction between the surface and either N plasma or Ar plasma 2 species. Thus, the formation of CO may also occur in 2 different ways. Decarburization of the material can be observed from samples treated in Ar–O , as can be seen in Fig. 3. After 2 a soaking period of 2 h, a continuous ferritic layer, approximately 20 mm thick, is obtained as a consequence of the dissolution of perlitic islands by plasma species. Similar results were also obtained by Nosratinia [1] and Manory [4]. The latter suggests that decarburization was due to reactions involving hydrogen at the surface. The presence of oxygen actually builds a barrier to carbon supply. Fig. 3(a) shows a relatively thick decarburized layer formed in the absence of O . This behavi2 our can be explained by the formation of an oxide layer, whose presence was determined by scanning electron microscopy, as shown in Fig. 4(a) and (b). Under mixed Ar–O atmospheres, an oxide layer develops at the 2 surface and acts as a diffusion barrier preventing carbon from exiting the material. Carbon tends to accumulate right below the oxide layer, which explains the reductive action in the rate of CO formation. 2
P. Egert et al. / Surface and Coatings Technology 221 (1999) 33–38
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Fig. 2. Variation in of CO concentration in (a) N /O and (b) Ar/O discharges. 2 2 2 2
Fig. 3. Optical micrograph of decarburized layer of AISI 1045 steel treated during 2 h at 500°C in an atmosphere of (a) pure Ar and (b) Ar–6% O. 2
Fig. 4. Electron microscopy of decarburized layer. Sample treated during 2 h at 500°C in (a) Ar–6% O mixture, showing the presence of the oxide 2 layer, and (b) pure Ar atmosphere, without the oxide layer.
The results strongly suggested that the decarburization process is a result of a non-equilibrium state forcing the system to reach an equilibrium between the reactivity of plasma species and sample atoms. The use of carbonfree gaseous mixtures induces a concentration gradient and, consequently, atoms in the material travel towards the plasma in order to keep the balance. Contrary to what is observed in conventional nitriding, in plasma
nitriding, the interaction of reactive species of the plasma with the surface of the material creates the conditions necessary for decarburization to occur. 3.2. Mixtures containing hydrogen Decarburization may also take place when hydrogen is present in the atmosphere where the plasma discharge
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P. Egert et al. / Surface and Coatings Technology 221 (1999) 33–38
Fig. 5. Variation of CH and C concentration in Ar/H discharge. 3 2 Sample heating is responsible for the negative region in the x-axis.
is obtained. When plasma diagnostics is carried out in the region close to the surface of the material, species such as C and CH are detected. In this case, the particle 3 extraction system consists of an orifice in the sample itself, which in turn acts as an interface between the discharge reactor and the analysis chamber of the spectrometer. Fig. 5 shows the formation of CH as relatively 3 high hydrogen contents are introduced into the gaseous mixture. In addition, reduction of free carbon can also be observed in the surface of the material. The results showed that reactions of active hydrogen species with carbon atoms on the surface led to the formation of CH species. Neither CH nor CH was 3 2 detected in these experiments. However, the absence of such radicals in mass spectra does not necessarily mean that they are not formed during the treatment. It is likely that these radicals are not detected due to their high reactivity and short lifetime. The process is also assessed at a microstructural level. Optical microscopy revealed that the concentration of hydrogen in the atmosphere affected the thickness of the decarburized layer ( Fig. 6). Atmospheres containing higher H concentrations induced thicker decarburized 2 layers.
In addition to being the direct result of interactions of active plasma species with surface carbon atoms, decarburization is also related to carbon diffusion in the crystal structure of iron This behaviour is initially activated by a carbon concentration gradient existing between sample and plasma atmosphere. This diffusive aspect of the process is illustrated by studies focusing on treatment parameters such as time and temperature. Samples treated for different periods show different features. Micrographs depict an increase in the thickness of decarburized layers with time as it can be seen in Fig. 7(a) and (b). In addition, decarburization is also enhanced by increasing the temperature Fig. 7(b) and (c). It has been observed that, at 300°C, there seems to be an early decarburization of the material. This result is rather significant, since it is in this temperature range (300–400°C ) that the reactor walls and samples are cleaned prior to plasma treatment. An initial H dis2 charge, which was thought to simply reduce surface oxides preparing the sample for later treatment, can also lead to surface decarburization. Surface decarburization can be either beneficial or deleterious to the process, depending on the aspect of the nitride layer. Manory [4] observed decarburization in H discharge 2 applied prior to nitriding. According to the author, reactions with hydrogen tend to remove carbon and oxygen, thus resulting in highly reactive surfaces, mostly comprising iron atoms. In fact, the author refers to decarburization as a process taking place prior to nitriding. This resulted in the formation of a ferritic surface in which the diffusion coefficients for carbon and nitrogen were improved. As a consequence, both nitriding and nitrocarburizing were enhanced. The results gathered herein were in good agreement with those reported by Nosratinia [1]. Furthermore, it can be stated that decarburization may also take place by means of reactions with O and, very likely, other 2 plasma species. Previous results [5] also indicated that
Fig. 6. Optical micrograph of decarburized layer on AISI 1045 steel treated for 2 h at 700°C in an atmosphere of (a) Ar–4% H ; and (b) Ar–25% 2 H. 2
P. Egert et al. / Surface and Coatings Technology 221 (1999) 33–38
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Fig. 7. Optical micrograph of decarburized layer on AISI 1045 steel treated in Ar–4%H during (a) 7 min at 700°C, (b) 2 h at 700°C, and (c) 2 h 2 at 300°C.
nitriding under O concentrations of up to 3% may lead 2 to the formation of reliable layers. The study shows that beyond this concentration, the presence of H becomes 2 necessary. Oxygen then reacts extensively with H to 2 form the OH radical, thus avoiding the formation of an oxide layer that would interfere in the diffusion of nitrogen atoms into the material. As a consequence, the unavoidable presence of oxygen impurities in nitriding reactors, detrimental to the process, can be bypassed. The presence of hydrogen in the atmosphere is a potential solution. Although the presence of hydrogen is favourable both for the reduction of oxides and for the removal of surface carbon, plasma nitriding can be efficiently performed in its absence. Therefore, the results presented herein made it possible to safely state that hydrogen-free atmospheres, but with oxygen impurities (below 3% for the experimental conditions referred in [5]) are also nitriding atmospheres, since CO -induced 2 surface decarburization consumes oxygen and avoids oxidation. Moreover, reactions of surface carbon with species from O impurities to form CO also play an 2 2 indirect role in the nitriding process, enhancing nitrogen diffusion. In other words, both carbon and hydrogen act as reducing agents in oxidizing atmospheres. In addition to the surface phenomenon resulting from a plasma–material interaction, carbon behaviour is also related to the formation of new nitride phases in the material. Borba [6 ] showed that the formation of a highly porous and non-uniform nitride layer allows decarburization of the material in the area located below the white layer. The phase present in this area is predominantly ferrite resulting from the diffusion of carbon through the pores of the nitride layer. When a nitride layer is uniform, decarburization is avoided, and the formation of carbon-rich phases, or precipitates in grain boundaries, is simplified. Similar results have been reported by Nosratinia [1]. The author also demonstrated that during the first stages of nitriding, part of the superficial carbon is removed through interactions with the plasma, whereas the remainder is pushed into the material, contributing to the formation of carbonrich phases.
4. Conclusions The decarburization of steel surfaces during plasma nitriding is a process that results from interactions of reactive plasma species with the surface of the material. In addition, it depends on the characteristics of the nitride layer formed, which can be either favourable or detrimental to the process. When a very uniform layer is formed, decarburization must be prevented, since the presence of such layer obstructs carbon diffusion towards the plasma. However, decarburization is enhanced through the voids of highly porous nitride layers. Decarburization taking place in the surface of the material assists plasma nitriding. The removal of surface carbon enhances nitrogen diffusion into the material. Nevertheless, as it reaches regions immediately below the nitride layer, it may become highly detrimental, if the goal is to form carbon-rich phases. In this case, the process must be avoided. The chemical reactivity of plasma species is observed to be an important factor in decarburization. Thus, the composition of the plasma atmosphere may also induce decarburization. Such is the case of mixtures containing hydrogen. Active hydrogen species tend to react with surface carbon to form CH radicals. The presence of 3 oxygen initially facilitates the decarburization through CO -forming reactions. However, for higher O concen2 2 trations (above 3%), a surface layer of oxides is formed which prevents decarburization both by oxygen and by hydrogen. Decarburization in plasma treatments is a diffusive phenomenon, as inferred by studying process parameters such as time and temperature. Decarburized layers become thicker as either time or temperature increases.
Acknowledgement This work was partially supported by Pronex, Capes/Cofecub no. 233/98 and CNPq (Brazil ). P. Egert was supported by a grant from Capes.
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References [1] M.A. Nosratinia, in: P. Vincenzin ( Ed.), Proc. High Performance Ceramic Films and Coating, Elsevier, New York, 1991, pp. 426–441. [2] L. Shi, R.R. Manory, Surf. Coat. Technol. 71 (1995) 108–111. [3] L. Shi, R.R. Manory, Surf. Coat. Technol. 71 (1995) 112–120. [4] L. Shi, R.R. Manory, Metallurg. Mater. Trans. A 27 (1996) 135–146.
[5] A.M. Maliska, P. Egert, A.R. Souza, C.V. Speller, A.N Klein, J. Mater. Sci. 32 (23) (1997) 6375–6382. [6 ] E.C. Borba, A.M. Maliska, A.N. Klein, J.L. Muzart, A.N. Souza, Le Vide, Suppl. 287, (1997) 121–124. [7] P. Egert, A. Seeber, C.V. Speller, A.M. Maliska, Le Vide, Suppl. 287 (1997) 125–128. [8] C.V. Speller, P. Egert, A.M. Maliska, in: 3rd Int. Conf. Reactive Plasmas & 14th Symp. Plasma Processing. Nara, Japan 21–24 January (1997) 313–314.
Decarburization during plasma nitriding P. Egert a,b, A.M. Maliska a, H.R.T. Silva a, C.V. Speller a, * a LABMAT/Laborato´rio de Materiais-Departamento de Engenharia Mecaˆnica/UFSC, CEP 88040-900, Floriano´polis, SC, Brazil b Departamento de Cieˆncias Exatas, Agra´rias e das Engenharias, Universidade do Sul de Santa Catarina, Campus Dudavan, 88130-000-Palhoc¸a, SC, Brazil
Abstract This work reports on the decarburization of steel surfaces nitrided in d.c. plasma discharges. Mass spectrometry was used to detect the presence of chemical species, such as C and CH , in plasma atmospheres containing hydrogen, and CO in those 3 2 containing oxygen. The active species of gaseous mixtures containing hydrogen react with superficial carbon to form CH radicals, 3 thus enabling decarburization. Conversely, although CO -forming reactions taking place in the presence of oxygen drive early 2 decarburization, at higher oxygen concentrations, an oxide coating layer is formed and acts as a reducing agent for the process. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Decarburization; Plasma nitriding; Steel surfaces
1. Introduction Plasma surface treatments have been increasingly applied to steels. As a result, numerous studies dedicated to understand the interaction of plasma species with the surface of materials have recently been carried out. As the process is developed, it has become clear that its full potential will be ultimately achieved as the fundamental species involved in the process become determined, and favourable conditions for the forming reactions to take place are established. Knowledge of the interactions existent between plasma species and material surface strongly depends on the determination of the identity of the species as well as on an evaluation of their behaviour as a function of the main plasma parameters. Although the mechanisms through which plasma species are formed play a direct role in the process, other reactions may also take place. Though indirectly related to the process, they should not be neglected. This seems to be the case for the reaction involving carbon from the steel, which may deplete the surface of the material of this element. Therefore, studies on plasma-nitriding steel should not overlook the role of carbon, since it is directly related to phase growth in the material. Such interaction may cause both the diffusion of plasma atoms towards the material and the diffusion of certain elements from the material towards * Corresponding author. E-mail address: [email protected] (C.V. Speller)
the plasma atmosphere. Studies focused on the behaviour of carbon in plasma-treated materials have shown that decarburization of the material surface takes place as a result of the interaction of the surface with the plasma [1–4]. Most of these studies directly related the process to surface reactions involving active hydrogen species and the formation of CH (x=1–3) neutral x radicals. The plasma of an electrical discharge is a nonequilibrium environment where unusual chemical species, such as neutral CH radicals can be formed x from reactions not commonly observed in conventional systems. These neutral species usually have short lifetimes, of the order of milliseconds or less. Their detection requires special plasma diagnostic techniques, such as emission spectroscopy and mass spectrometry, applied to the usually very reactive samples gathered directly from the plasma reactor. The aim of the present study is to investigate steel decarburization during surface plasma treatment as well as the decarburization mechanism of carbon steel. Mass spectrometry was used to detect the presence of neutral species, such as C and CH , or CO , when hydrogen or 3 2 oxygen are present in the gaseous atmosphere of plasma, respectively.
2. Experimental The experimental apparatus used in steel plasma nitriding is schematically shown in Fig. 1 and has been
0257-8972/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 40 7 - 7
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P. Egert et al. / Surface and Coatings Technology 221 (1999) 33–38
3. Results and discussion 3.1. Mixtures containing oxygen
Fig. 1. Schematic diagram of the experimental apparatus.
thoroughly described elsewhere [5]. A mass spectrometer was coupled to the nitriding reactor through an opening of 100 mm in diameter, allowing the passage of neutral species. These species were then introduced into an electron impact ionization chamber, in which they were ionized before entering the mass analysis chamber. In the plasma chamber, an electrical discharge is produced between the sample holder (cathode) and the walls of the chamber (anode). The entire set-up must be earthed. The system contains two set-ups that enable plasma species to be collected from two distinct regions. In one of the set-ups, the sample holder is placed approximately 2 cm from the collector orifice. In this way, the species are extracted from a region distant from the surface of the material. In the second set-up, the collecting orifice is the sample itself, so that the species can be collected directly from the region of plasma–surface interaction. This configuration was employed for the present study as it enables the detection of a very small number of neutral species formed in that region. Cylindrical AISI 1045 steel pellets were plasmatreated in Ar–O and Ar–H discharges, as well as in 2 2 gaseous mixtures containing nitrogen. The samples were later analysed by optical and scanning electron microscopy. The use of different gaseous mixtures made it possible to evaluate aspects such as the formation of nitride and oxide layers upon decarburization. The influence of other parameters such as O :H ratio, time 2 2 and temperature was also assessed. During the treatment, different neutral plasma species were simultaneously monitored in a procedure that assisted in the establishment of correlation studies between plasma diagnostics and metallographic analysis of the treated material.
In an attempt to detect neutral CO species, mass 2 spectrometry analyses of the plasma atmosphere were conducted as steel surfaces were nitrided. A more detailed study suggested that CO is formed as oxygen 2 contents ranging from 0 to 6% are introduced into the gaseous mixture [7,8]. This trend is shown in Fig. 2(a) along with a constant increase in content of carbon available from the material. The use of high-carbon materials results in an increase in the number of species formed. This is also the case when relatively high concentrations of oxygen are available in the plasma atmosphere. For oxygen contents up to 3%, the process takes place at a reasonably constant rate for each material, stabilizing after about 30 min. This can be explained as nitrogen penetrates into the iron lattice and fills its interstices, thus decreasing the supply of carbon at the surface. In addition, it is likely that the oxided surface acts as a barrier, hindering the exit of carbon at the surface. Evidence of decarburization from analyses on nitrided materials is difficult to obtain since the results are masked by the presence of the nitride layer itself. Specimens treated in Ar–O 2 atmospheres are an alternative, inasmuch as decarburization can be observed in the absence of nitrides. Monitoring CO species indicates that, under similar 2 conditions, CO formation mechanism is different in 2 both mixtures. In fact, as this may result from the interaction of plasma species with the surface of the material, the differences between curves a and b in Fig. 2 may correspond due to the different forms of interaction between the surface and either N plasma or Ar plasma 2 species. Thus, the formation of CO may also occur in 2 different ways. Decarburization of the material can be observed from samples treated in Ar–O , as can be seen in Fig. 3. After 2 a soaking period of 2 h, a continuous ferritic layer, approximately 20 mm thick, is obtained as a consequence of the dissolution of perlitic islands by plasma species. Similar results were also obtained by Nosratinia [1] and Manory [4]. The latter suggests that decarburization was due to reactions involving hydrogen at the surface. The presence of oxygen actually builds a barrier to carbon supply. Fig. 3(a) shows a relatively thick decarburized layer formed in the absence of O . This behavi2 our can be explained by the formation of an oxide layer, whose presence was determined by scanning electron microscopy, as shown in Fig. 4(a) and (b). Under mixed Ar–O atmospheres, an oxide layer develops at the 2 surface and acts as a diffusion barrier preventing carbon from exiting the material. Carbon tends to accumulate right below the oxide layer, which explains the reductive action in the rate of CO formation. 2
P. Egert et al. / Surface and Coatings Technology 221 (1999) 33–38
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Fig. 2. Variation in of CO concentration in (a) N /O and (b) Ar/O discharges. 2 2 2 2
Fig. 3. Optical micrograph of decarburized layer of AISI 1045 steel treated during 2 h at 500°C in an atmosphere of (a) pure Ar and (b) Ar–6% O. 2
Fig. 4. Electron microscopy of decarburized layer. Sample treated during 2 h at 500°C in (a) Ar–6% O mixture, showing the presence of the oxide 2 layer, and (b) pure Ar atmosphere, without the oxide layer.
The results strongly suggested that the decarburization process is a result of a non-equilibrium state forcing the system to reach an equilibrium between the reactivity of plasma species and sample atoms. The use of carbonfree gaseous mixtures induces a concentration gradient and, consequently, atoms in the material travel towards the plasma in order to keep the balance. Contrary to what is observed in conventional nitriding, in plasma
nitriding, the interaction of reactive species of the plasma with the surface of the material creates the conditions necessary for decarburization to occur. 3.2. Mixtures containing hydrogen Decarburization may also take place when hydrogen is present in the atmosphere where the plasma discharge
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P. Egert et al. / Surface and Coatings Technology 221 (1999) 33–38
Fig. 5. Variation of CH and C concentration in Ar/H discharge. 3 2 Sample heating is responsible for the negative region in the x-axis.
is obtained. When plasma diagnostics is carried out in the region close to the surface of the material, species such as C and CH are detected. In this case, the particle 3 extraction system consists of an orifice in the sample itself, which in turn acts as an interface between the discharge reactor and the analysis chamber of the spectrometer. Fig. 5 shows the formation of CH as relatively 3 high hydrogen contents are introduced into the gaseous mixture. In addition, reduction of free carbon can also be observed in the surface of the material. The results showed that reactions of active hydrogen species with carbon atoms on the surface led to the formation of CH species. Neither CH nor CH was 3 2 detected in these experiments. However, the absence of such radicals in mass spectra does not necessarily mean that they are not formed during the treatment. It is likely that these radicals are not detected due to their high reactivity and short lifetime. The process is also assessed at a microstructural level. Optical microscopy revealed that the concentration of hydrogen in the atmosphere affected the thickness of the decarburized layer ( Fig. 6). Atmospheres containing higher H concentrations induced thicker decarburized 2 layers.
In addition to being the direct result of interactions of active plasma species with surface carbon atoms, decarburization is also related to carbon diffusion in the crystal structure of iron This behaviour is initially activated by a carbon concentration gradient existing between sample and plasma atmosphere. This diffusive aspect of the process is illustrated by studies focusing on treatment parameters such as time and temperature. Samples treated for different periods show different features. Micrographs depict an increase in the thickness of decarburized layers with time as it can be seen in Fig. 7(a) and (b). In addition, decarburization is also enhanced by increasing the temperature Fig. 7(b) and (c). It has been observed that, at 300°C, there seems to be an early decarburization of the material. This result is rather significant, since it is in this temperature range (300–400°C ) that the reactor walls and samples are cleaned prior to plasma treatment. An initial H dis2 charge, which was thought to simply reduce surface oxides preparing the sample for later treatment, can also lead to surface decarburization. Surface decarburization can be either beneficial or deleterious to the process, depending on the aspect of the nitride layer. Manory [4] observed decarburization in H discharge 2 applied prior to nitriding. According to the author, reactions with hydrogen tend to remove carbon and oxygen, thus resulting in highly reactive surfaces, mostly comprising iron atoms. In fact, the author refers to decarburization as a process taking place prior to nitriding. This resulted in the formation of a ferritic surface in which the diffusion coefficients for carbon and nitrogen were improved. As a consequence, both nitriding and nitrocarburizing were enhanced. The results gathered herein were in good agreement with those reported by Nosratinia [1]. Furthermore, it can be stated that decarburization may also take place by means of reactions with O and, very likely, other 2 plasma species. Previous results [5] also indicated that
Fig. 6. Optical micrograph of decarburized layer on AISI 1045 steel treated for 2 h at 700°C in an atmosphere of (a) Ar–4% H ; and (b) Ar–25% 2 H. 2
P. Egert et al. / Surface and Coatings Technology 221 (1999) 33–38
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Fig. 7. Optical micrograph of decarburized layer on AISI 1045 steel treated in Ar–4%H during (a) 7 min at 700°C, (b) 2 h at 700°C, and (c) 2 h 2 at 300°C.
nitriding under O concentrations of up to 3% may lead 2 to the formation of reliable layers. The study shows that beyond this concentration, the presence of H becomes 2 necessary. Oxygen then reacts extensively with H to 2 form the OH radical, thus avoiding the formation of an oxide layer that would interfere in the diffusion of nitrogen atoms into the material. As a consequence, the unavoidable presence of oxygen impurities in nitriding reactors, detrimental to the process, can be bypassed. The presence of hydrogen in the atmosphere is a potential solution. Although the presence of hydrogen is favourable both for the reduction of oxides and for the removal of surface carbon, plasma nitriding can be efficiently performed in its absence. Therefore, the results presented herein made it possible to safely state that hydrogen-free atmospheres, but with oxygen impurities (below 3% for the experimental conditions referred in [5]) are also nitriding atmospheres, since CO -induced 2 surface decarburization consumes oxygen and avoids oxidation. Moreover, reactions of surface carbon with species from O impurities to form CO also play an 2 2 indirect role in the nitriding process, enhancing nitrogen diffusion. In other words, both carbon and hydrogen act as reducing agents in oxidizing atmospheres. In addition to the surface phenomenon resulting from a plasma–material interaction, carbon behaviour is also related to the formation of new nitride phases in the material. Borba [6 ] showed that the formation of a highly porous and non-uniform nitride layer allows decarburization of the material in the area located below the white layer. The phase present in this area is predominantly ferrite resulting from the diffusion of carbon through the pores of the nitride layer. When a nitride layer is uniform, decarburization is avoided, and the formation of carbon-rich phases, or precipitates in grain boundaries, is simplified. Similar results have been reported by Nosratinia [1]. The author also demonstrated that during the first stages of nitriding, part of the superficial carbon is removed through interactions with the plasma, whereas the remainder is pushed into the material, contributing to the formation of carbonrich phases.
4. Conclusions The decarburization of steel surfaces during plasma nitriding is a process that results from interactions of reactive plasma species with the surface of the material. In addition, it depends on the characteristics of the nitride layer formed, which can be either favourable or detrimental to the process. When a very uniform layer is formed, decarburization must be prevented, since the presence of such layer obstructs carbon diffusion towards the plasma. However, decarburization is enhanced through the voids of highly porous nitride layers. Decarburization taking place in the surface of the material assists plasma nitriding. The removal of surface carbon enhances nitrogen diffusion into the material. Nevertheless, as it reaches regions immediately below the nitride layer, it may become highly detrimental, if the goal is to form carbon-rich phases. In this case, the process must be avoided. The chemical reactivity of plasma species is observed to be an important factor in decarburization. Thus, the composition of the plasma atmosphere may also induce decarburization. Such is the case of mixtures containing hydrogen. Active hydrogen species tend to react with surface carbon to form CH radicals. The presence of 3 oxygen initially facilitates the decarburization through CO -forming reactions. However, for higher O concen2 2 trations (above 3%), a surface layer of oxides is formed which prevents decarburization both by oxygen and by hydrogen. Decarburization in plasma treatments is a diffusive phenomenon, as inferred by studying process parameters such as time and temperature. Decarburized layers become thicker as either time or temperature increases.
Acknowledgement This work was partially supported by Pronex, Capes/Cofecub no. 233/98 and CNPq (Brazil ). P. Egert was supported by a grant from Capes.
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