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Ion-bombarded-assisted intermetallic compound formation in Ti-Sn bilayers
Nuclear Instruments and Methods in Physics Research
294
B14 (1986) 294-296
North-Holland.
Amsterdam
Letter to the Editor ION-BOMBARDED-ASSISTED BILAYERS E.MA,
INTERMETALLIC
COMPOUND
FORMATION
IN Ti-Sn
B.X. LIU and H.D. LI,
Department of Engineering Physics, Tsinghua University, Beijing, The People’s Republic of China Received
19 July 1985 and in revised form 7 November
1985
The formation of metal silicides by ion beam mixing, frequently using bilayers, has been studied to a great extent in recent years. It is established that, in many cases, ion mixing results in the formation of the same metal silicide as that formed by conventional thermal annealing, but at lower temperature, for example, at RT (room temperature) [l]. Comparatively speaking, the synthesis of intermetallic compounds under ion impact in metal-metal binary systems is much less investigated, except in a few Al-based systems [2]. In this letter, we report the results of a preliminary study of ion mixing behavior in Ti-Sn bilayers, emphasizing the compound formation during post-annealing. It was reported by Dearnaley [3] as an example of the BDC (bombardment-diffused-coating) technique that the tribological property of Ti was tremendously improved when it was coated with a thin layer of Sn ( - 700 A) and bombarded by 100 keV NC to a dose of 2 X 10” ions/cm* at elevated temperature (350°C). Mossbatter spectrometry study provided evidence that Ti-Sn compounds might have formed, probably Ti,Sn, or Ti,Sn,, but no definite result of phase identification has been reported. In this study, we adopted an alternative way of studying ion mixing in Ti-Sn bilayers. Samples were prepared by evaporating - 500 A Sn onto bulk Ti substrates. The ion mixing was conducted at RT using 100 keV Arf ions. The doses employed ranged from 1 X 10” to 5 x 10’” ions/cm2. RBS (Rutherford backscattering spectrometry) spectra were taken to obtain information of the mixing effect. The samples were then subjected to thermal annealing at several temperatures (200-450°C) under a vacuum of 10e5 Torr. Phase identification was done by X-ray diffraction analysis using a Dmax/IIA diffractometer before and after each annealing step. Fig. 1 shows the RBS spectra for virgin and high dose (5 x 10’6/cm2) mixed samples. A considerable amount of intermixing is clearly seen for the irradiated sample in the spectra. But for lower doses (< 1 x 10”/cm2 in our study), mixing was barely observed,
0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
probably due to the relatively low mixing efficiency of Ar+ and/or the oxide layer that might pre-exist on the bulk Ti surface. As revealed by fig. 1, the total amount of Sn decreased after bombardment to high doses, which is probably a result of the sputtering effect. Although mixing was achieved in the high dose irradiated Ti-Sn bilayers, the X-ray diffraction spectra taken afterwards showed only the diffraction lines of the Ti and Sn structures without indications of the formation of any of the four equilibrium Ti-Sn compounds. The failure to form these compounds is not surprising, for all of them have rather complicated structure and narrow composition range. According to some of the models concerning the phase formation by ion mixing [4,5] this kind of compound cannot appear due to the unfavourable growth condition available in the ion mixing process at low temperature. At present, we are not sure whether or not equilibrium or metastable solid solutions and/or the amorphous phase were formed in some parts of the mixed layer, since we failed to collect sufficient data from the X-ray diffraction spectra, especially as the bulk Ti was present as the substrate. Further work is needed to study the phase formation after RT mixing. The samples, both irradiated and unirradiated, were then put into a self-designed vacuum furnace for annealing. The annealing was performed in three steps and X-ray diffraction spectra were taken after each step. Two diffraction spectra are shown in fig. 2. These clearly exhibit the difference between the irradiated and unirradiated samples after the first annealing step. All the phase identification results after annealing are summarized in table 1. For low dose irradiated ‘samples (< 5 X 10’5/cm2), mixing was poor and the phases identified were the same as for virgin ones. Several points can be drawn from table 1: i) A medium temperature thermal annealing (350°C, 1 h) is needed for a Ti-Sn compound to form in virgin Ti-Sn bilayers, and the first phase to appear was Ti,Sn,. ii) Ion bombardment induced the compound formation at lower annealing temperature and within shorter annealing time. The first phase formed was also Ti,Sn,,
E. Ma et al. / Inkwnetahc
compound formatwn in Ti - Sn hilayers
295
Sn
Ti
I
1.54MeV
Sn I 7
- - -
-
unirradiated
5Xlo16A,+/cm2 irradiated
6 .
CHANNEL
Fig. 1. RBS spectra
He+
NIJP?BER
for virgin sample and the 5 X lOI Ar+/cm’
and the annealing treatment required was 200°C 0.5 h. iii) Sn moved into bulk Ti more deeply along with the bombardment dose and the annealing time. The
mixed sample.
lowered Sn composition in turn favoured the formation of low-Sn compounds. Thermal annealing enables the metal atoms to have
45
40
35
30
25
20
DIFFRACTION ANGLE (20)
Fig. 2. X-ray (Cu Ka 5 x 1016 Art/cm2.
radiation)
diffraction
spectra
of samples
annealed
at 200°C
for 0.5 h (a) unirradiated.
(b) irradiated
with
E. Ma et al. / Intermetallic
296 Table 1 Phases appearing
in the Ti-Sn
bilayers
after thermal
compound formation
annealing
at various
in Ti - Sn bilayers
temperatures
for 0.5 or 1 h ‘)
Irradiation dose Ar +/cm2
RT
2o0°c 0.5 h
350°c lh
450°C lh
Virgin 1 x 10lh 5x101h
Sn+Ti Sn+Ti Sn+Ti
Sn+Ti Sn+Ti+Ti,Sn, Sn *+Ti+Ti,Sns
Sn+Ti+Ti,Sn, _
Sn *+Ti+Ti,Sn, +Ti,Sn+Ti,Sn, Sn*+Ti+Ti,Sn,+Ti$n Sn*+Ti+Ti,Sn,+Ti$n
” Note:
* means that only very weak diffraction lines of the phase were observed by X-ray diffraction analysis.
higher thermal mobility, and a certain extent of diffusion, especially for Sn (melting point 232°C). is expected. However, our results show that the low temperature annealing (2OO”C, 0.5 h) itself failed to provide sufficient activation energy for the formation of compounds with new and complicated structures, and thus further annealing at higher temperature is demanded. After the ion bombardment, however, the distinct Ti-Sn interface no longer existed and the interaction between Ti and Sn could proceed during annealing in a much broader region where the atomic ratio is more suitable for the intermetallic compound to form. At the same time, the ion mixing lowered the barrier for thermal reaction and radiation enhanced diffusion might have played an important role in assisting the growth of the new phases. Consequently, a moderate thermal annealing was enough to give rise to the compound formation. The ion mixing performed in this study did not change the first appearing compound by thermal annealing. We believe that the formation of identical compound through different procedures of planar reaction may also be found in other metal-metal systems so long as the reaction is diffusion-controlled, irrespective of whether ion mixing results in the compound formation directly or just assists the reaction during post-annealing. Our observation is also consistent with the prediction of Ronay [6], which states for thermal reactions in metal-silicon systems that the first nucleating phase would be the compound neighbouring the central eutectic in the phase diagram which is closer in composition to the diffusing species. In our case, Ti,Sn, is adjacent to the central and deepest eutectic and closer
to Sn. The results obtained by Dearnaley [3] is also understandable, because their BDC experiment was executed at higher temperature (350°C). All the Sn atoms originally at the Ti surface diffused deep into Ti and distributed in a depth as thick as two microns, resulting in the formation of compounds with a low Sn concentration. In summary, a certain amount of intermixing was achieved in Ti-Sn bilayers by high dose Ar+ irradiation, but no compound was detectable after RT mixing. However, ion irradiation enhanced the thermal synthesis of Ti-Sn compound by lowering the annealing temperature and reducing the required annealing time. This provides another example of ion-bombarded-assisted reaction [7].
References [I] S.S. Lau, B.X. Liu and M-A. Nicolet, Nucl. Instr. and Meth. 209/210 (1983) 97. [2] L.S. Hung and J.W. Mayer, Nucl. Instr. and Meth. B7/8 (1985) 676. [3] G. Dearnaley and P.D. Goode, Nucl. Instr. and Meth. 189 (1981) 117. [4] L.S. Hung, M. Nastasi, J. Gyulai, and J.W. Mayer, Appl. Phys. Lett. 42 (1983) 672. [5] B.X. Liu, Nucl. Instr. and Meth. B7/8 (1985) 547. [6] M. Ronay, Appl. Phys. Lett. 42 (7) (1983) 577. [7] T. Inada, H. Kakinuma, A. Shirota, J. Matsumoto, M. Ishikirigama and Y. Furaki, Nucl. Instr. and Meth. B7/8 (1985) 576.
294
B14 (1986) 294-296
North-Holland.
Amsterdam
Letter to the Editor ION-BOMBARDED-ASSISTED BILAYERS E.MA,
INTERMETALLIC
COMPOUND
FORMATION
IN Ti-Sn
B.X. LIU and H.D. LI,
Department of Engineering Physics, Tsinghua University, Beijing, The People’s Republic of China Received
19 July 1985 and in revised form 7 November
1985
The formation of metal silicides by ion beam mixing, frequently using bilayers, has been studied to a great extent in recent years. It is established that, in many cases, ion mixing results in the formation of the same metal silicide as that formed by conventional thermal annealing, but at lower temperature, for example, at RT (room temperature) [l]. Comparatively speaking, the synthesis of intermetallic compounds under ion impact in metal-metal binary systems is much less investigated, except in a few Al-based systems [2]. In this letter, we report the results of a preliminary study of ion mixing behavior in Ti-Sn bilayers, emphasizing the compound formation during post-annealing. It was reported by Dearnaley [3] as an example of the BDC (bombardment-diffused-coating) technique that the tribological property of Ti was tremendously improved when it was coated with a thin layer of Sn ( - 700 A) and bombarded by 100 keV NC to a dose of 2 X 10” ions/cm* at elevated temperature (350°C). Mossbatter spectrometry study provided evidence that Ti-Sn compounds might have formed, probably Ti,Sn, or Ti,Sn,, but no definite result of phase identification has been reported. In this study, we adopted an alternative way of studying ion mixing in Ti-Sn bilayers. Samples were prepared by evaporating - 500 A Sn onto bulk Ti substrates. The ion mixing was conducted at RT using 100 keV Arf ions. The doses employed ranged from 1 X 10” to 5 x 10’” ions/cm2. RBS (Rutherford backscattering spectrometry) spectra were taken to obtain information of the mixing effect. The samples were then subjected to thermal annealing at several temperatures (200-450°C) under a vacuum of 10e5 Torr. Phase identification was done by X-ray diffraction analysis using a Dmax/IIA diffractometer before and after each annealing step. Fig. 1 shows the RBS spectra for virgin and high dose (5 x 10’6/cm2) mixed samples. A considerable amount of intermixing is clearly seen for the irradiated sample in the spectra. But for lower doses (< 1 x 10”/cm2 in our study), mixing was barely observed,
0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
probably due to the relatively low mixing efficiency of Ar+ and/or the oxide layer that might pre-exist on the bulk Ti surface. As revealed by fig. 1, the total amount of Sn decreased after bombardment to high doses, which is probably a result of the sputtering effect. Although mixing was achieved in the high dose irradiated Ti-Sn bilayers, the X-ray diffraction spectra taken afterwards showed only the diffraction lines of the Ti and Sn structures without indications of the formation of any of the four equilibrium Ti-Sn compounds. The failure to form these compounds is not surprising, for all of them have rather complicated structure and narrow composition range. According to some of the models concerning the phase formation by ion mixing [4,5] this kind of compound cannot appear due to the unfavourable growth condition available in the ion mixing process at low temperature. At present, we are not sure whether or not equilibrium or metastable solid solutions and/or the amorphous phase were formed in some parts of the mixed layer, since we failed to collect sufficient data from the X-ray diffraction spectra, especially as the bulk Ti was present as the substrate. Further work is needed to study the phase formation after RT mixing. The samples, both irradiated and unirradiated, were then put into a self-designed vacuum furnace for annealing. The annealing was performed in three steps and X-ray diffraction spectra were taken after each step. Two diffraction spectra are shown in fig. 2. These clearly exhibit the difference between the irradiated and unirradiated samples after the first annealing step. All the phase identification results after annealing are summarized in table 1. For low dose irradiated ‘samples (< 5 X 10’5/cm2), mixing was poor and the phases identified were the same as for virgin ones. Several points can be drawn from table 1: i) A medium temperature thermal annealing (350°C, 1 h) is needed for a Ti-Sn compound to form in virgin Ti-Sn bilayers, and the first phase to appear was Ti,Sn,. ii) Ion bombardment induced the compound formation at lower annealing temperature and within shorter annealing time. The first phase formed was also Ti,Sn,,
E. Ma et al. / Inkwnetahc
compound formatwn in Ti - Sn hilayers
295
Sn
Ti
I
1.54MeV
Sn I 7
- - -
-
unirradiated
5Xlo16A,+/cm2 irradiated
6 .
CHANNEL
Fig. 1. RBS spectra
He+
NIJP?BER
for virgin sample and the 5 X lOI Ar+/cm’
and the annealing treatment required was 200°C 0.5 h. iii) Sn moved into bulk Ti more deeply along with the bombardment dose and the annealing time. The
mixed sample.
lowered Sn composition in turn favoured the formation of low-Sn compounds. Thermal annealing enables the metal atoms to have
45
40
35
30
25
20
DIFFRACTION ANGLE (20)
Fig. 2. X-ray (Cu Ka 5 x 1016 Art/cm2.
radiation)
diffraction
spectra
of samples
annealed
at 200°C
for 0.5 h (a) unirradiated.
(b) irradiated
with
E. Ma et al. / Intermetallic
296 Table 1 Phases appearing
in the Ti-Sn
bilayers
after thermal
compound formation
annealing
at various
in Ti - Sn bilayers
temperatures
for 0.5 or 1 h ‘)
Irradiation dose Ar +/cm2
RT
2o0°c 0.5 h
350°c lh
450°C lh
Virgin 1 x 10lh 5x101h
Sn+Ti Sn+Ti Sn+Ti
Sn+Ti Sn+Ti+Ti,Sn, Sn *+Ti+Ti,Sns
Sn+Ti+Ti,Sn, _
Sn *+Ti+Ti,Sn, +Ti,Sn+Ti,Sn, Sn*+Ti+Ti,Sn,+Ti$n Sn*+Ti+Ti,Sn,+Ti$n
” Note:
* means that only very weak diffraction lines of the phase were observed by X-ray diffraction analysis.
higher thermal mobility, and a certain extent of diffusion, especially for Sn (melting point 232°C). is expected. However, our results show that the low temperature annealing (2OO”C, 0.5 h) itself failed to provide sufficient activation energy for the formation of compounds with new and complicated structures, and thus further annealing at higher temperature is demanded. After the ion bombardment, however, the distinct Ti-Sn interface no longer existed and the interaction between Ti and Sn could proceed during annealing in a much broader region where the atomic ratio is more suitable for the intermetallic compound to form. At the same time, the ion mixing lowered the barrier for thermal reaction and radiation enhanced diffusion might have played an important role in assisting the growth of the new phases. Consequently, a moderate thermal annealing was enough to give rise to the compound formation. The ion mixing performed in this study did not change the first appearing compound by thermal annealing. We believe that the formation of identical compound through different procedures of planar reaction may also be found in other metal-metal systems so long as the reaction is diffusion-controlled, irrespective of whether ion mixing results in the compound formation directly or just assists the reaction during post-annealing. Our observation is also consistent with the prediction of Ronay [6], which states for thermal reactions in metal-silicon systems that the first nucleating phase would be the compound neighbouring the central eutectic in the phase diagram which is closer in composition to the diffusing species. In our case, Ti,Sn, is adjacent to the central and deepest eutectic and closer
to Sn. The results obtained by Dearnaley [3] is also understandable, because their BDC experiment was executed at higher temperature (350°C). All the Sn atoms originally at the Ti surface diffused deep into Ti and distributed in a depth as thick as two microns, resulting in the formation of compounds with a low Sn concentration. In summary, a certain amount of intermixing was achieved in Ti-Sn bilayers by high dose Ar+ irradiation, but no compound was detectable after RT mixing. However, ion irradiation enhanced the thermal synthesis of Ti-Sn compound by lowering the annealing temperature and reducing the required annealing time. This provides another example of ion-bombarded-assisted reaction [7].
References [I] S.S. Lau, B.X. Liu and M-A. Nicolet, Nucl. Instr. and Meth. 209/210 (1983) 97. [2] L.S. Hung and J.W. Mayer, Nucl. Instr. and Meth. B7/8 (1985) 676. [3] G. Dearnaley and P.D. Goode, Nucl. Instr. and Meth. 189 (1981) 117. [4] L.S. Hung, M. Nastasi, J. Gyulai, and J.W. Mayer, Appl. Phys. Lett. 42 (1983) 672. [5] B.X. Liu, Nucl. Instr. and Meth. B7/8 (1985) 547. [6] M. Ronay, Appl. Phys. Lett. 42 (7) (1983) 577. [7] T. Inada, H. Kakinuma, A. Shirota, J. Matsumoto, M. Ishikirigama and Y. Furaki, Nucl. Instr. and Meth. B7/8 (1985) 576.