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High-dose doping of steels with nitrogen using intense pulsed ion beams
Mate~als~tte~ Nosh-Holland
14 (1992) 131-134
High-dose doping of steels with nitrogen using intense pulsed ion beams J. Piekoszewski, S&an
J. Langner, L. Nowicki, A. Turos
Institutefor Nuclear Studies, 05-400 Otwock-~wierk, Poland
L. WaliS and A. Ciurapihski Institute ofNuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland
Received 30 March 1992
The feasibility of introducing high doses of nitrogen atoms (of the order of 10” atoms/cm2) into steel by irradiation of its surface with hip-intensity pulsed ion beams is reported. The originality of this process is related to the follo~ng features: (I ) The introduction of the foreign atomic species occurs when the near-surface region is molten whereas the bulk material remains at practically unchanged temperature. (2) The whole process is a genuine single-step process occurring on the microsecond time
1. Introduction If the surface of a solid is subjected to irradiation by high-intensity pulsed ion or plasma beams, it can be modified through one of the following types of effects or combination of these: (i) purely thermal effects [ 11, (ii) thermally induced shock-wave generation [ 2 1, (iii) thermal effects in conjunction with mass deposition [ 3 1. Here, we will be concerned mostly with the last of the abovementioned cases. It is obvious, that the amount of foreign atoms necessary to alter substantially the properties of the surface layer depends on the kind of the material subjected to i~adiation. For instance, in a conventional implantation process used for doping semiconductors, the doses of dopant are in the range 10’3-10’6 atoms/cm2, whereas the modification of the surface structure of metals and ceramics requires doses in the range 1016- 1O’*atoms/ cm’. In our previous works, e.g. refs. [3,4], it was shown that it is possible to dope silicon and thus to form p+-n and n+-p junctions by using the so-called pulse impl~tation doping (PID) technique. In PID, a high-intensity, short-duration pulse, containing the dopant ions provides the dose necessary to dope 0167-577x/92/$
semiconductor and simultaneously brings an amount of energy sufficient to melt its near-surface region. While this regions is molten, rapid diffusion of the dopant into the liquid occurs, leading to the formation of the p-n junction when the epitaxial regrowth of the doped layer is terminated. The upper limit of the energy density in the pulse is set by the requirement that the amount of material removed from the surface is acceptably small. In the present paper, we demonstrate experimentally for the first time the feasibility of the introduction of high doses (up to 1O1’atoms/cm2) of nitrogen into steel by irradiation with high-intensity pulsed ion beams (IIIPIBs).
2. Experimental The irradiation experiments were performed using the rod plasma injector (RPI) type of generator, IBIS, which was described elsewhere [ 4,5 1. This device produces HIPIBs in which the power versus time transient consists of two distinct phases. In the first one, there is a sharp peak of about 1pm duration and the mean energy of ions of several keV. In the second phase, there is a long tail lasting up to several tens
05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
131
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of microseconds with ions having an energy of the order of tens of electronvolts. The total incident energy density can be set by the discharge preconditions and by varying the distance from the sample to the discharge electrodes. The RPI-type generators are capable of producing ions of practically any element which can be introduced into the electrode region in gaseous or vapour state. For the purpose of the present work, hydrogen and nitrogen ions were used. The samples for the experiment were prepared of two kinds of steel: shallow-hardening tool steel (N9) composed of 0.9 wt% C, 0.3 wt% Mn, 0.35 wt% Si, 0.2 wt% Cr, 0.25 wt% Ni, 0.25 wt% Cu and low-carbon alloy steel ( 18H2N4W) composed of 0.18 wt% 6, 0.4 wt% Mn, 0.4 wt% Si, 1.5 wt% Cr, 4.2 wt% Ni, 1.0 wt% W. The samples were prepared in the form of highly polished discs of 18 mm diameter and 2 mm thick. Each of the samples was irradiated separately with five pulses of hydrogen or nitrogen ions. Energy density of the pulse was ranging between 5 and 10 J/cm2. In order to get insight into the changes induced by irradiation in the surface structure, the back-scattering geometry of conversion X-ray Miissbauer spectroscopy (CXMS of 57Fe nuclei) was applied. Also the nuclear reaction (NRA) ‘*N (d, a, ) “C was used to determine the concentration of the retained nitrogen.
LETTERS
July I992
gen ions, and only for energy densities higher than 7 J/cm2. A typical set of CXMS spectra taken on both N9 and 18H2N4W is shown in fig. 1. For comparison, also the spectra of non-irradiated samples are displayed. The NRA measurements performed on nitrogen irradiated samples have revealed that the con~entration of the retained nitrogen ranges from 4~ lOi to 5.6~ X0” atoms/cm* depending on the energy density of the pulse. The samples of N9 and 18H2N4W, the CXMS spectra of which are shown in figs. Id and lf, were irradiated with nitrogen ions at mean energy density over five pulses equal to about 9.5 J/cm2 and the detected nitrogen concentration was about 5.6 x 1017atoms/cm2. Such high doses of the introduced nitrogen were at first surprising. It was because until recently we used to attribute the predominant role in a surface modi~cation only to the ions occurring during the sharp peak of the pulse, mistakenly disregarding the role of the long tail part of the pulse. Indeed, if only the “peak” ions were to act, then with their mean energy of the order of 10 keV, to deliver the dose of 10” atoms/cm*, the total
3. Results and discussion The inspection of CXMS spectra of 14 samples of N9 steei and 14 samples of 18H2N4W steel irradiated with either hydrogen or nitrogen ion pulses reveals the following regularities: (i ) Regardless of the kind of ion species in the pulse, the irradiation of N9 steel causes the formation of paramagnetic phase, represented in CXMS by a central peak. This phase was earlier [ 6 ] identified as an austenite occurring in the form of thin (about 0.3 pm thick) layer on top of the irradiated surface. The austenite formation starts from a relatively low energy density of the pulse, i.e. from about 4 J/cm’. (ii) In the case of 1gH2N4W steel, the substantiaf rise of the initial central peak occurs only for nitro132
Fig. 1. CXMS spectra of high-carbon tool steel N9 (a,b,c) and low-carbon alloy steel 18W2N4W (d, e, ff. (a), (d) - unirradiated; (b ), (e f - after hydrogen pulse irradiation; (c), f f f after nitrogen pulse irradiation.
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energy density in the pulse should exceed 100 J/cm’ instead of being 5-10 J/cm2 as measured in our experiments. In addition, such high energy densities would cause the evaporation of a substantial amount of the irradiated samples. The situation is quite different in the presence of the tail which actually occurs in the pulses generated by the RPI-type device. In such a case, the pulse induced process probably develops in the following way. The sharp peak heats and thus melts the near-surface layer of the sample. Then, the liquid state is sustained for a long period of time (of the order of several tens of microseconds) by the power being delivered by the low-energy ions reaching the surface during the subsequent phase of the pulse. These ions diffuse rapidly in the molten layer. A substantial amount of them remains frozen when the solidification stage is terminated. In order to check the reality of this reasoning, the onedimensional heat-flow equation was solved using the computer-based nume~cal procedure MELT VERSION 1. I [ 71. The solutions were obtained under the following assumptions: - The whole heat delivered by the pulse is deposited on the surface of the metal. - The temperature dependence of thermal conductivity and the latent heat of the solid-liquid transition are included. -The power versus time transient consists of two parts, i.e. Gaussian-shaped peak (G), followed by a constant component (C). - The total energy density, E,, in the pulse is shared equally between both parts G and C. - The material parameters such as the thermal conductivity, latent heat, specific heat, density and the melting temperature are taken from the literature [ 8 J for pure iron. The calculations showed that, for example, for the Gaussian part of 0.9 us (fwhm ) and a maximum power of P,,,,= 2.1 X lo6 W/cm*, at a total energy density of the pulse of 7.2 J/cm*, the liquid state lasts for about 11 us and the power density of part C, necessary to sustain the liquid state, equals about O.l8P,,, of part G. These results although obtained for the simplified model of the pulse shape seem to confirm at least semi-quantitatively the picture of the thermal evolution of the sample proposed above. The difference in the outcomes of pulsed ion beam irradiation of two kinds of steel, i.e. N9 and 18H2N4W
LETTERS
July 1992
mentioned in (i) and (ii) becomes obvious if we consider the differences in the elemental composition of these materials, especially in regard to the carbon content. N9 belongs to the group of high-carbon steels (0.9Oio C) and in its initial state consists almost entirely of pearlite phase. This is reflected by the Zeeman pattern in the CXMS spectrum shown in lig. la. During the pulse action the rapid heating and thus melting of the top layer of material occurs. In the freezing stage, due to a high cooling rate and a high carbon content, the austenite phase is formed. With such a high carbon content the formation of martensite starts at about 230°C and finishes well below 0°C. Therefore it is not surprising that a significant amount of austenite is observed in the irradiated samples. It is worthwhile to notice that the pulse induced process has primarily a thermal character, i.e. it is not so important what kinds of ion species were used to provide heat to the surface of the sample. In figs. 1b and lc, the relative intensity of the central peak, representing austenite phase is the same for both hydrogen and nitrogen pulses. The situation is quite different for low-carbon (0.18% C) alloy steel 18H2N4W. In the isothermal ITT (time, temperature, transformation ) diagram of this steel [ 91, the region of the pearlite transformation does not exist and bainite and martensite transformation regions overlap each other to a large extent. Therefore, cooling from the austenitizing temperature during the steel-making process leads to the formation of lowcarbon martensite and retained austenite (up to 4%). The presence of these phases is reflected in the CXMS spectrum of the initial sample as shown in fig. Id, where apart from the broad sextet there is a small central peak corresponding to the retained austenite. The quenching of the top layer is not expected to induce any essential changes in its phase composition, and therefore it is not surprising that the CXMS spectrum of the specimen irradiated with hydrogen pulses looks the same as the one for the initial material (see figs. Id and 1e). The situation changes radically when a sufficient amount of nitrogen is introduced into the liquid phase. This causes a lowering of the austenite transformation temperature and thus formation of a substantial amount of nitrogen austenite during the cooling-down stage of the pulse induced process. The presence of nitrogen austenite 133
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in irradiated sample of 18H2N4W steel is manifested by the rise of the central peak in fig. If as compared to that in fig. Id. More detailed studies of nitrogen introduction into steel by using HIPIB irradiation are in progress. The CEMS (conversion electron Mossbatter spectroscopy) instead of the CXMS technique is being applied as more suitable for the surface analysis [ 101.
Acknowledgement The authors would like to thank J.ISrolik for his continuing technical assistance and to J. Biaioskorski for computer calculations.
References [ 1 ] R. Fastow, J. Gyulai, M. Nastasi and P.G. Zielibski, J. Mater. Sci. Letters 3 ( 1984) 109.
134
July 1992
[2] A.D. Pogrebniak, G.E. Remenev, LB. Kurakin and A.E. Ligachev, Nucl. Instr. Methods B 36 (1989) 286. [3] J. Piekoszewski, M. Gryzinski, J. Langner and Z. Werner, Phys. Stat. Sol. 67a ( 198 1) K163. [4] J. Piekoszewski and J. Langner, Nucl. Instr. Methods B 53 (1991) 148. [5] J. Langner, K. Czaus, E. G&ski, M. Gryzinski, A. Horodenski, J. Piekoszewski, C. Pochrybniak, Z. Werner and W. Wojtowicz, Phys. Res. 8 (1988) 167. [6] A. Ciurapinski and L. WaliS, Inter. Rept. ICHTJ, No. 66/ I/90. [ 71 R. Baranowski, Programme for Numerical Solution of the Heat Flow Equation under Phase Change Conditions, Phys. Stat. Sol., to be published. [ 81 Y.S. Touloukian, ed., Thermophysical properties of high temperature solid materials, Vol. 1. Elements (Macmillan, London, 1967). [ 91 A.A. Popov, Probliemy metallovidienia i termitheskoi obrabotki (Mashgiz, Moscow, 1956) p. 179. [IO] D.L. Williamson, L. Wang, R. Wei and P.J. Wilbur, Mater. Letters 9 ( 1990) 302.
14 (1992) 131-134
High-dose doping of steels with nitrogen using intense pulsed ion beams J. Piekoszewski, S&an
J. Langner, L. Nowicki, A. Turos
Institutefor Nuclear Studies, 05-400 Otwock-~wierk, Poland
L. WaliS and A. Ciurapihski Institute ofNuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland
Received 30 March 1992
The feasibility of introducing high doses of nitrogen atoms (of the order of 10” atoms/cm2) into steel by irradiation of its surface with hip-intensity pulsed ion beams is reported. The originality of this process is related to the follo~ng features: (I ) The introduction of the foreign atomic species occurs when the near-surface region is molten whereas the bulk material remains at practically unchanged temperature. (2) The whole process is a genuine single-step process occurring on the microsecond time
1. Introduction If the surface of a solid is subjected to irradiation by high-intensity pulsed ion or plasma beams, it can be modified through one of the following types of effects or combination of these: (i) purely thermal effects [ 11, (ii) thermally induced shock-wave generation [ 2 1, (iii) thermal effects in conjunction with mass deposition [ 3 1. Here, we will be concerned mostly with the last of the abovementioned cases. It is obvious, that the amount of foreign atoms necessary to alter substantially the properties of the surface layer depends on the kind of the material subjected to i~adiation. For instance, in a conventional implantation process used for doping semiconductors, the doses of dopant are in the range 10’3-10’6 atoms/cm2, whereas the modification of the surface structure of metals and ceramics requires doses in the range 1016- 1O’*atoms/ cm’. In our previous works, e.g. refs. [3,4], it was shown that it is possible to dope silicon and thus to form p+-n and n+-p junctions by using the so-called pulse impl~tation doping (PID) technique. In PID, a high-intensity, short-duration pulse, containing the dopant ions provides the dose necessary to dope 0167-577x/92/$
semiconductor and simultaneously brings an amount of energy sufficient to melt its near-surface region. While this regions is molten, rapid diffusion of the dopant into the liquid occurs, leading to the formation of the p-n junction when the epitaxial regrowth of the doped layer is terminated. The upper limit of the energy density in the pulse is set by the requirement that the amount of material removed from the surface is acceptably small. In the present paper, we demonstrate experimentally for the first time the feasibility of the introduction of high doses (up to 1O1’atoms/cm2) of nitrogen into steel by irradiation with high-intensity pulsed ion beams (IIIPIBs).
2. Experimental The irradiation experiments were performed using the rod plasma injector (RPI) type of generator, IBIS, which was described elsewhere [ 4,5 1. This device produces HIPIBs in which the power versus time transient consists of two distinct phases. In the first one, there is a sharp peak of about 1pm duration and the mean energy of ions of several keV. In the second phase, there is a long tail lasting up to several tens
05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
131
Volume 14,number 2,3
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of microseconds with ions having an energy of the order of tens of electronvolts. The total incident energy density can be set by the discharge preconditions and by varying the distance from the sample to the discharge electrodes. The RPI-type generators are capable of producing ions of practically any element which can be introduced into the electrode region in gaseous or vapour state. For the purpose of the present work, hydrogen and nitrogen ions were used. The samples for the experiment were prepared of two kinds of steel: shallow-hardening tool steel (N9) composed of 0.9 wt% C, 0.3 wt% Mn, 0.35 wt% Si, 0.2 wt% Cr, 0.25 wt% Ni, 0.25 wt% Cu and low-carbon alloy steel ( 18H2N4W) composed of 0.18 wt% 6, 0.4 wt% Mn, 0.4 wt% Si, 1.5 wt% Cr, 4.2 wt% Ni, 1.0 wt% W. The samples were prepared in the form of highly polished discs of 18 mm diameter and 2 mm thick. Each of the samples was irradiated separately with five pulses of hydrogen or nitrogen ions. Energy density of the pulse was ranging between 5 and 10 J/cm2. In order to get insight into the changes induced by irradiation in the surface structure, the back-scattering geometry of conversion X-ray Miissbauer spectroscopy (CXMS of 57Fe nuclei) was applied. Also the nuclear reaction (NRA) ‘*N (d, a, ) “C was used to determine the concentration of the retained nitrogen.
LETTERS
July I992
gen ions, and only for energy densities higher than 7 J/cm2. A typical set of CXMS spectra taken on both N9 and 18H2N4W is shown in fig. 1. For comparison, also the spectra of non-irradiated samples are displayed. The NRA measurements performed on nitrogen irradiated samples have revealed that the con~entration of the retained nitrogen ranges from 4~ lOi to 5.6~ X0” atoms/cm* depending on the energy density of the pulse. The samples of N9 and 18H2N4W, the CXMS spectra of which are shown in figs. Id and lf, were irradiated with nitrogen ions at mean energy density over five pulses equal to about 9.5 J/cm2 and the detected nitrogen concentration was about 5.6 x 1017atoms/cm2. Such high doses of the introduced nitrogen were at first surprising. It was because until recently we used to attribute the predominant role in a surface modi~cation only to the ions occurring during the sharp peak of the pulse, mistakenly disregarding the role of the long tail part of the pulse. Indeed, if only the “peak” ions were to act, then with their mean energy of the order of 10 keV, to deliver the dose of 10” atoms/cm*, the total
3. Results and discussion The inspection of CXMS spectra of 14 samples of N9 steei and 14 samples of 18H2N4W steel irradiated with either hydrogen or nitrogen ion pulses reveals the following regularities: (i ) Regardless of the kind of ion species in the pulse, the irradiation of N9 steel causes the formation of paramagnetic phase, represented in CXMS by a central peak. This phase was earlier [ 6 ] identified as an austenite occurring in the form of thin (about 0.3 pm thick) layer on top of the irradiated surface. The austenite formation starts from a relatively low energy density of the pulse, i.e. from about 4 J/cm’. (ii) In the case of 1gH2N4W steel, the substantiaf rise of the initial central peak occurs only for nitro132
Fig. 1. CXMS spectra of high-carbon tool steel N9 (a,b,c) and low-carbon alloy steel 18W2N4W (d, e, ff. (a), (d) - unirradiated; (b ), (e f - after hydrogen pulse irradiation; (c), f f f after nitrogen pulse irradiation.
Volume
14, number
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energy density in the pulse should exceed 100 J/cm’ instead of being 5-10 J/cm2 as measured in our experiments. In addition, such high energy densities would cause the evaporation of a substantial amount of the irradiated samples. The situation is quite different in the presence of the tail which actually occurs in the pulses generated by the RPI-type device. In such a case, the pulse induced process probably develops in the following way. The sharp peak heats and thus melts the near-surface layer of the sample. Then, the liquid state is sustained for a long period of time (of the order of several tens of microseconds) by the power being delivered by the low-energy ions reaching the surface during the subsequent phase of the pulse. These ions diffuse rapidly in the molten layer. A substantial amount of them remains frozen when the solidification stage is terminated. In order to check the reality of this reasoning, the onedimensional heat-flow equation was solved using the computer-based nume~cal procedure MELT VERSION 1. I [ 71. The solutions were obtained under the following assumptions: - The whole heat delivered by the pulse is deposited on the surface of the metal. - The temperature dependence of thermal conductivity and the latent heat of the solid-liquid transition are included. -The power versus time transient consists of two parts, i.e. Gaussian-shaped peak (G), followed by a constant component (C). - The total energy density, E,, in the pulse is shared equally between both parts G and C. - The material parameters such as the thermal conductivity, latent heat, specific heat, density and the melting temperature are taken from the literature [ 8 J for pure iron. The calculations showed that, for example, for the Gaussian part of 0.9 us (fwhm ) and a maximum power of P,,,,= 2.1 X lo6 W/cm*, at a total energy density of the pulse of 7.2 J/cm*, the liquid state lasts for about 11 us and the power density of part C, necessary to sustain the liquid state, equals about O.l8P,,, of part G. These results although obtained for the simplified model of the pulse shape seem to confirm at least semi-quantitatively the picture of the thermal evolution of the sample proposed above. The difference in the outcomes of pulsed ion beam irradiation of two kinds of steel, i.e. N9 and 18H2N4W
LETTERS
July 1992
mentioned in (i) and (ii) becomes obvious if we consider the differences in the elemental composition of these materials, especially in regard to the carbon content. N9 belongs to the group of high-carbon steels (0.9Oio C) and in its initial state consists almost entirely of pearlite phase. This is reflected by the Zeeman pattern in the CXMS spectrum shown in lig. la. During the pulse action the rapid heating and thus melting of the top layer of material occurs. In the freezing stage, due to a high cooling rate and a high carbon content, the austenite phase is formed. With such a high carbon content the formation of martensite starts at about 230°C and finishes well below 0°C. Therefore it is not surprising that a significant amount of austenite is observed in the irradiated samples. It is worthwhile to notice that the pulse induced process has primarily a thermal character, i.e. it is not so important what kinds of ion species were used to provide heat to the surface of the sample. In figs. 1b and lc, the relative intensity of the central peak, representing austenite phase is the same for both hydrogen and nitrogen pulses. The situation is quite different for low-carbon (0.18% C) alloy steel 18H2N4W. In the isothermal ITT (time, temperature, transformation ) diagram of this steel [ 91, the region of the pearlite transformation does not exist and bainite and martensite transformation regions overlap each other to a large extent. Therefore, cooling from the austenitizing temperature during the steel-making process leads to the formation of lowcarbon martensite and retained austenite (up to 4%). The presence of these phases is reflected in the CXMS spectrum of the initial sample as shown in fig. Id, where apart from the broad sextet there is a small central peak corresponding to the retained austenite. The quenching of the top layer is not expected to induce any essential changes in its phase composition, and therefore it is not surprising that the CXMS spectrum of the specimen irradiated with hydrogen pulses looks the same as the one for the initial material (see figs. Id and 1e). The situation changes radically when a sufficient amount of nitrogen is introduced into the liquid phase. This causes a lowering of the austenite transformation temperature and thus formation of a substantial amount of nitrogen austenite during the cooling-down stage of the pulse induced process. The presence of nitrogen austenite 133
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in irradiated sample of 18H2N4W steel is manifested by the rise of the central peak in fig. If as compared to that in fig. Id. More detailed studies of nitrogen introduction into steel by using HIPIB irradiation are in progress. The CEMS (conversion electron Mossbatter spectroscopy) instead of the CXMS technique is being applied as more suitable for the surface analysis [ 101.
Acknowledgement The authors would like to thank J.ISrolik for his continuing technical assistance and to J. Biaioskorski for computer calculations.
References [ 1 ] R. Fastow, J. Gyulai, M. Nastasi and P.G. Zielibski, J. Mater. Sci. Letters 3 ( 1984) 109.
134
July 1992
[2] A.D. Pogrebniak, G.E. Remenev, LB. Kurakin and A.E. Ligachev, Nucl. Instr. Methods B 36 (1989) 286. [3] J. Piekoszewski, M. Gryzinski, J. Langner and Z. Werner, Phys. Stat. Sol. 67a ( 198 1) K163. [4] J. Piekoszewski and J. Langner, Nucl. Instr. Methods B 53 (1991) 148. [5] J. Langner, K. Czaus, E. G&ski, M. Gryzinski, A. Horodenski, J. Piekoszewski, C. Pochrybniak, Z. Werner and W. Wojtowicz, Phys. Res. 8 (1988) 167. [6] A. Ciurapinski and L. WaliS, Inter. Rept. ICHTJ, No. 66/ I/90. [ 71 R. Baranowski, Programme for Numerical Solution of the Heat Flow Equation under Phase Change Conditions, Phys. Stat. Sol., to be published. [ 81 Y.S. Touloukian, ed., Thermophysical properties of high temperature solid materials, Vol. 1. Elements (Macmillan, London, 1967). [ 91 A.A. Popov, Probliemy metallovidienia i termitheskoi obrabotki (Mashgiz, Moscow, 1956) p. 179. [IO] D.L. Williamson, L. Wang, R. Wei and P.J. Wilbur, Mater. Letters 9 ( 1990) 302.