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Formation of TiB2 films and BFe interdiffused layers by dynamic mixing
NllMlB
Nuclear Instruments and Methods in Physics Research B73 (1993) 172-177 North-Holland
Formation of TiB, films and B-Fe interdiffused by dynamic mixing
Beam Interactions with Materials&Atoms
layers
Tatsuya Yasunaga, Yasuaki Sugizaki and Hiroshi Satoh Materials Research Laboratory, Kobe Steel, Ltd., Takatsukadai, Nishi-ku, Kobe, 651-22 Japan
Received 31 August 1992
Steel specimens were treated by B ion implantation combined with simultaneous Ti evaporation (dynamic mixing), and the structure of the synthesized films and the formation of B-Fe interdiffused layer were investigated. B ion beam current density and Ti deposition rate were varied, L-130 PA/cm*, and 0.03-0.27 rim/s,, respectively. The ion energy was maintained at 40 keV. The phase of the synthesized film was found to change from cuTi to cuTi+ TiB2 and then to TiB, by decreasing the Ti deposition rate or increasing the B ion current density. The TiB phase was not observed in any condition. The formation of a B-Fe interdiffused layer beneath the synthesized film was found to be enhanced by decreasing the Ti deposition rate. The thickness of the B-Fe interdiffused layer reached up to 3 km, which was about 30 times the thickness of the 40 keV B ion implanted layer in steel as predicted by the TRIM simulation program. These thin films synthesized by the dynamic mixing showed higher adhesion compared with those deposited by the conventional rf magnetron sputtering.
1. Introduction Dynamic mixing is a new technique to produce thin films which simultaneously uses ion implantation and deposition. This technique is characterized by room temperature treatment, by high ability for controlling the structure, composition and phase formation in the growing film, and by high adhesion of the film due to the mixing effect through the film-substrate interface. This technique is particularly desirable for producing ceramic coatings such as TiN [1,2] and TIC [3] which are necessarily produced at elevated temperature by other methods. A number of research projects on fabrication of such films using this technique, without having to raise the ambient temperature to avoid substrate deterioration, have been reported. TiB, coatings are now arousing interest in the fields of protective coatings because of their high wear and corrosion resistance. They are also known to possess low electric resistance and remarkable diffusion barrier properties when interposed between metallic contacts and semiconductor substrates [4]. Several methods such as spray coating [S], chemical vapor deposition [6-91 and sputter coating [lo] have been reported for the synthesis of TiB, films. Rivikre et al. produced TiB, films by the dynamic mixing method using sputtering Correspondence to: T. Yasunaga, Materials Research Laboratory, Kobe Steel, Ltd, Takatsukadai, Nishi-ku, Kobe 651-22, Japan.
0168-583X/93/$06.00
deposition and Ar of Xe ion bombardment [ 11,121. The inert gas ion bombardment significantly enhanced the crystallization and the adhesion of the films compared with those without ion bombardment. In this study, steel substrates were treated by dynamic mixing using B ion implantation combined with Ti evaporation in order to clarify the feasibility of the synthesis of TiB, film by the reactive B ion bombardment. The structural change of the synthesized film was investigated by varying the B ion current density and Ti deposition rate to determine the condition for the synthesized TiB, single phase. In addition, the B-Fe interdiffused layer in the steel substrate was found to be formed during the synthesis, which is thought to be a specific feature of the process.
2. Experiment Substrates used for the present study were mild steel and were metallographically polished with 1 km diamond paste and then degreased in acetone. These steel specimens were treated by dynamic mixing in which B ion implantation and Ti evaporation were carried out simultaneously. B ion beam current density and Ti deposition rate were varied in the range U-130 PA/cm’, and 0.03-0.27 rim/s,, respectively. The B ion dose and thickness of the deposited Ti were dependent on the treatment time. The ion energy was maintained at 40 keV throughout all processes. The process cham-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
173
T. Yasunagaet al. / Formation of TiB, films and B-Fe layers
ber was evacuated by a cryogenic pump, and the pressure was 4 x 10-5-l x 10m4 Pa. The sample temperature during the process was monitored using a chromel-alumel thermocouple. The sample holder was cooled by circulating water on its rear surface in order to maintain the sample temperature at less than 200°C. In order to compare the adhesion of the films, another TiB, film was deposited on a separate steel specimen by conventional rf magnetron sputtering using a TiB, target. The deposition was carried out at less than 100°C in an Ar atmosphere of about 6.5 x 10-i Pa. The film thickness was about 0.5 pm. These treated surface layers were characterized by the thin film X-ray diffraction method. Composition versus depth profiles were determined by Auger electron spectroscopy with argon sputtering in which the approximate depth of each profile was estimated from its etch rate. Cross-sectional observation was also carried out using scanning electron microscopy @EM). The adhesions of the films were evaluated by a scratch test using a vibrating diamond indenter (15 pm radius) with continuously changing forces. It was automatically drawn across the sample surface at a rate of 10 pm/s which corresponded to 0.68 mN/s. The minimum force necessary to remove a film from its substrate (critical force) was determined by observing the scratch using optical microscopy and scanning electron microscopy.
28 3o”
40”
(4 aTi
I
’
30”
40”
-28
W Fe2B 1101
TiB2 op1
04 Fig. 1. Thin film X-ray diffraction patterns for dynamic mixing treated steels with a B ion beam current density of 80 PA/cm’ and Ti deposition rates of: (a) 0.20 rim/s;; (b) 0.11 rim/s;; (c) 0.05 rim/s..
3. Results
3.2. Depth profiles
3.1. X-ray diffraction patterns
Fig. 3 shows composition versus depth profiles of Fe, Ti and B for the dynamic mixing treated steels. Some profiles of other elements are eliminated from the diagrams for simplicity, although there were small amounts of 0 and C profile due to contamination in the near surface region. The B ion dose and beam current density were maintained at the same value, 5 X 1018 ions/cm2, and 80 p,A/cm*, respectively. Ti deposition rate was varied; (fig. 3a) 0.20 rim/s,, (fig. 3b) 0.11 rim/s,, (fig. 3c) 0.05 rim/s.. These conditions correspond to those shown in figs. la-lc and fig. 2 ((a)-(c)). By decreasing the Ti deposition rate from 0.20 to 0.11 rim/s,, the Ti concentration of the synthesized film was decreased from about 50 to 20 at.%, and the B concentration of the film was increased from about 50 to 75 at.%. With a further decrease in the rate to 0.05 rim/s,, however, there was no notable change in the Ti of B concentrations of the synthesized film. Other than the composition of the synthesized films, there were marked differences in the thickness of the B-Fe interdiffused layers. By decreasing the Ti deposition rate from 0.20 rim/s (a) to 0.11 rim/s (b), the thickness of the interdiffused layer was slightly increased from about 0.5 to 0.8 km. With a further decrease in the rate to 0.05 rim/s (c), however, the
Fig. 1 shows thin film X-ray diffraction patterns for the steels treated by dynamic mixing. The B ion current density was 80 pA/cm2. The Ti deposition rate was varied: (fig. la) 0.20 rim/s,, (fig. lb) 0.11 rim/s,, (fig. lc) 0.05 rim/s.. At a higher deposition rate, 0.20 rim/s (fig. la), clear peaks corresponding to aTi were observed. By decreasing the Ti deposition rate to 0.11 rim/s (fig. lb), small peaks corresponding to TiB, were superimposed on the aTi pattern. With a further decrease of the rate to 0.05 rim/s (fig. lc), the TiB, peaks were intensified, and the aTi peaks completely disappeared. Moreover, Fe,B peaks were clearly observed which means there was a considerable reaction between implanted B species and the steel substrate. Fig. 2 shows the composition of the synthesized films, determined by the X-ray diffraction patterns, as a function of the two operational parameters, B ion current density and Ti deposition rate. The composition of the synthesized films was changed from aTi single phase to aTi + TiB, mixed phase and then to TiB, single phase by decreasing the Ti deposition rate or increasing the B ion current density. The TiB phase was not observed under any condition.
T. Yasunagaet al. / Formation of TiB, films and B-Fe layers
174
s E.
layer was observed. According to the X-ray diffraction pattern (fig. lc) and the composition versus depth profile (fig. 3c), the outer layer is TiB, film synthesized on the steel surface, and the inner layer is Fe,B formed in the substrate.
0.2
Q)
H
/
aTi
s
/
‘G ._ g
3.4. Scratch test
.B
’
aTi+TiBP
0.1
% u
0
o/
0
0
/
:
i= /
0.01 ’ 0 6
’
’
’
’
;
’
50
B 0
-cc,: CC----
TiB2
.
I ’
’
’
100
’
’ I
’
ion beam current density (M/cm*)
Fig. 2. Structure of synthesized films as a function of B ion beam current density and Ti deposition rate.
thickness was drastically increased to 3.0 pm, which is about 30 times the thickness of the 40 keV B ion implanted layer in steel as predicted by using the TRIM simulation program [13].
Figs. 5a-5c show the scratches for the steels treated by dynamic mixing under the conditions shown in figs. 2a-2c. Fig. 5d shows the ones of the deposited TiB, film on steel by the rf magnetron sputtering for comparison. All of these scratches were given under the same conditions. The TiB, films deposited by sputtering (fig. 5d) was easily removed with a force of less than 13 mN, and its SEM image shows that the film was removed by brittle interfacial failure. The dynamic mixing treated steels showed higher critical forces for their film removal, 28 mN (fig. 5a) and 33 mN (fig. 5b), which mean higher adhesion due to the B-Fe interdiffused layer shown in the depth profile (figs. 3a and 3b). Although their SEM images show that the films were removed by inter-facial failure, they do no pose such brittleness as does the sputtered film. The film formed with the lowest Ti deposition rate (fig. 5~) had not been removed through the scratch test. Instead of film removal, it was smeared onto the bottom of the scratch.
3.3. Cross-sectional observation 4. Discussion Figs. 4a-4c show the cross-sectional SEM images of the steels treated by dynamic mixing under the same conditions as those shown in figs. la-lc, fig. 2 ((a)-(c)) and figs. 3a-3c, respectively. In the conditions shown in figs. 4a and 4b, single layers were observed on the steel surface. According to the X-ray diffraction patterns (figs. la and lb), the structure of the layers is (fig. la) aTi single phase and (fig. lb) aTi + TiB, mixed phase. B-Fe interdiffused layers in the steel substrates were not observed, although their composition versus depth profiles (figs. 3a and 3b) show the existence of the interdiffused layers in the substrate. In the conditions shown in fig. 4c, however, a divided modified
With varying Ti deposition rate and B ion current density, the structure of the synthesized film was found to change systematically. The phase of the films was classified in three types: aTi single phase, cwTi+ TiB, mixed phase and TiB, single phase. The TiB phase was not observed in any condition. Nakashima et al. [14] investigated Ti thin films implanted with 60 keV B ions and compared them with Ti-B compounds synthesized by conventional solid phase reaction. According to their result, the TiB phase was formed firstly at 900°C and then turned into the TiB, phase by increasing temperature during the solid phase reaction. Neverthe-
Approximate depth (pm)
(a) Fig. 3. Composition
0))
versus depth profiles of Fe, Ti and B for dynamic mixing treated steels with a B ion beam current PA/cm* and Ti deposition rates of: (a) 0.20 rim/s;; (b) 0.11 rim/s;; (c) 0.05 rim/s..
density
of 80
175
T Yasunaga et al. / Formation of TiB, films and B-Fe layers
less, only the TiB, phase was formed in the Ti films during the B ion implantation. The absence of the TiB phase in the films synthesized in this study is in agreement with the work of Nakashima et al. [14]. This phenomenon, therefore, is considered to be a specific feature of the B ion implantation or the dynamic mixing using B ion implantation. According to the Ti-B phase diagram [15], the TiB, phase is stable up to 3225°C while the TiB phase decomposes into the TiB, phase and the Ti liquid phase at 2200°C. Considering the absence of the TiB phase, the temperature of the micro-area where the B ions were stopped was thought to be high enough to decompose the TiB phase, although the temperature of the sample monitored during the process was less than 200°C. As shown in fig. 3c and fig. 4c, the thickness of the B-Fe interdiffused layer in the steel substrate was enlarged during the synthesis of the TiB, thin film. There was no difference in sample temperature, monitored during the processes, between the three dynamic mixing treatments shown in figs. 2a-2c; it was less than 200°C. However, the temperature of the near surface region might be higher during the (c) process compared with the one in (a), (b), which can enhance the B-Fe interdiffusion and the formation of the Fe,B phase. Since the B-Fe interdiffusion was enhanced under the condition where the TiB, single phase was synthesized on the substrate, the heat of the TiB, formation during the process might contribute to the
5um
(a)
(cl Fig. 4. Cross-sectional
diffusion, The cause of these phenomena has not yet been ascertained. Further research is now under way. For the scratch tests shown in fig. 5, the synthesized films by the dynamic mixing technique showed higher critical force compared with the deposited films by rf magnetron sputtering. The critical force was increased by decreasing the Ti deposition rate from (a) to (b), and there was no film removal for condition (c). According to the depth profiles shown in fig. 3, the interface between the film and the substrate was broadened by the mixing effect of the implanted B ion, and as mentioned above, the thickness of the B-Fe interdiffused layer beneath the film was enlarged followed by the formation of the Fe,B phase with a decreasing Ti deposition rate. The mixing effect between the film and the substrate contributes to the adhesion, and the formation of the Fe,B phase contributes to the hardening of the steel substrate beneath the TiB, film. The increase of the critical force (figs. 5a and 5b) and the prevention of the film removal (fig. 5c) can be explained by the fact that high adhesion and increased substrate hardness result in an increase of the critical force in the scratch test [16].
5. Conclusion
Steel substrates were treated by dynamic mixing using B ion implantation and Ti evaporation. The two
,5um,
(W
5pm
lm
SEM images of dynamic mixing treated steels with a B ion beam current density of 80 kA/cm2 deposition rates of: (a) 0.20 rim/s;; (b) 0.11 rim/s;; (c) 0.05 rim/s..
and Ti
T. Yasunaga et al. / Formation of TiB, films and B-Fe layers
176
operational parameters, B ion current density and Ti deposition rate, were varied in order to determine the condition where TiB, single phase was synthesized. The structure of the synthesized films was changed from cxTi single phase to cuTi + TiB, mixed phase and then to TiB, single phase by decreasing the Ti deposition rate or increasing the B ion current density. The
TiB phase was not observed in any condition. The absence of the TiB phase is thought to be a specific feature of the B ion implantation or the dynamic mixing using B ion implantation. The thickness of the B-Fe interdiffused layer was found to be enlarged during the synthesis of the TiB, phase, the cause of which has yet to be ascertained. These thin films
IOclm
10vm
Fig. 5. Optical
images
and SEM images of scratches
for dynamic
mixing treated
steels (B ions beam current density of 80 PA/cm’ steel by rf magnetron sputtering.
and Ti deposition rates of (a) 0.20 rim/s;; (b) 0.11 rim/s;; (c) 0.05 rim/s)) and (d) TiB, deposited
111
T. Yasunaga et al. / Formation of TiB, jZms and B-Fe layers
10pm
formed by dynamic mixing showed higher adhesion compared with those formed by the conventional rf magnetron sputtering.
Acknowledgements
This work was conducted within the scope of the program: Advanced Material-Processing and Machining System, consigned to the Advanced MaterialProcessing and Machining Technology Research Association from the New Energy and Industrial Technology Development Organization, which is carried out under the Large-Scale Project enforced by the Agency of Industrial Science and Technology, the Ministry of International Trade and Industry.
References [ll See e.g., S. Nakashima, M. Fukushima, M. Haginoya, K.
Oohata, I. Hashimoto and K. Terakado, Mater. Sci. Eng. All5 (1989) 197. Dl See e.g., T. Yano, M. Yoneda, A. Utsumi, J. Matsuda, M. Katsumura and S. Miyagawa, J. High Temp. Sot. 15 (6) (1989) 279.
[31 See e.g., S. Pimbert-Michaux and C. Chabrol, Mater. Sci. Eng. All5 (1989) 209. [41 J. Shappirio, J. Finnegan, R. Lux, D. Fox and J. Kwiatkowski, J. Vacuum Sci. Technol. A3 (6) (1985) 2255. 151 A.W. Mullendore, D.M. Mattox, J.B. Whitley and D.J. Sharp, Thin Solid Films 63 (1979) 243. [61 A.J. Becker and J.H. Blanks, Thin Solid Films 119 (1984) 241. [71 A.W. Mullendore, J.B. Whitley, H.O. Pierson and D.M. Mattox, J. Vacuum Sci. Technol. 18 (1981) 1049. PI D.M. Mattox, Thin Solid Films 63 (1979) 213. [91 H.O. Pierson and A.W. Mullendore, Thin Solid Films 95 (1982) 99. DOI J.R. Shappirio and J.J. Finnegan, Thin Solid Films 107 (1983) 81. u11 J.P. Riviere and Ph. Guesdon, Surf. Coat. Technol. 42 (1990) 81. WI J.P. Riviere, Ph. Guesdon, J. Delafond and M.F. Denanot, Surf. Coat. Technol. 45 (1991) 83. 1131J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, vol. 1, ed. J.F. Ziegler (Pergamon, New York, 1985). D41 K. Nakashima, S. Okamoto and K. Kobayashi, J. Chem. Sot. Jpn. 11 (1987) 1896. D51 J.L. Murray, P.K. Liao and K.E. Spear, Bull. Alloy Phase Diagrams 7 (6) (1986) 550. WA P.J. Burnett and D.S. Rickerby, Thin Solid Films 154 (1987) 403.
Nuclear Instruments and Methods in Physics Research B73 (1993) 172-177 North-Holland
Formation of TiB, films and B-Fe interdiffused by dynamic mixing
Beam Interactions with Materials&Atoms
layers
Tatsuya Yasunaga, Yasuaki Sugizaki and Hiroshi Satoh Materials Research Laboratory, Kobe Steel, Ltd., Takatsukadai, Nishi-ku, Kobe, 651-22 Japan
Received 31 August 1992
Steel specimens were treated by B ion implantation combined with simultaneous Ti evaporation (dynamic mixing), and the structure of the synthesized films and the formation of B-Fe interdiffused layer were investigated. B ion beam current density and Ti deposition rate were varied, L-130 PA/cm*, and 0.03-0.27 rim/s,, respectively. The ion energy was maintained at 40 keV. The phase of the synthesized film was found to change from cuTi to cuTi+ TiB2 and then to TiB, by decreasing the Ti deposition rate or increasing the B ion current density. The TiB phase was not observed in any condition. The formation of a B-Fe interdiffused layer beneath the synthesized film was found to be enhanced by decreasing the Ti deposition rate. The thickness of the B-Fe interdiffused layer reached up to 3 km, which was about 30 times the thickness of the 40 keV B ion implanted layer in steel as predicted by the TRIM simulation program. These thin films synthesized by the dynamic mixing showed higher adhesion compared with those deposited by the conventional rf magnetron sputtering.
1. Introduction Dynamic mixing is a new technique to produce thin films which simultaneously uses ion implantation and deposition. This technique is characterized by room temperature treatment, by high ability for controlling the structure, composition and phase formation in the growing film, and by high adhesion of the film due to the mixing effect through the film-substrate interface. This technique is particularly desirable for producing ceramic coatings such as TiN [1,2] and TIC [3] which are necessarily produced at elevated temperature by other methods. A number of research projects on fabrication of such films using this technique, without having to raise the ambient temperature to avoid substrate deterioration, have been reported. TiB, coatings are now arousing interest in the fields of protective coatings because of their high wear and corrosion resistance. They are also known to possess low electric resistance and remarkable diffusion barrier properties when interposed between metallic contacts and semiconductor substrates [4]. Several methods such as spray coating [S], chemical vapor deposition [6-91 and sputter coating [lo] have been reported for the synthesis of TiB, films. Rivikre et al. produced TiB, films by the dynamic mixing method using sputtering Correspondence to: T. Yasunaga, Materials Research Laboratory, Kobe Steel, Ltd, Takatsukadai, Nishi-ku, Kobe 651-22, Japan.
0168-583X/93/$06.00
deposition and Ar of Xe ion bombardment [ 11,121. The inert gas ion bombardment significantly enhanced the crystallization and the adhesion of the films compared with those without ion bombardment. In this study, steel substrates were treated by dynamic mixing using B ion implantation combined with Ti evaporation in order to clarify the feasibility of the synthesis of TiB, film by the reactive B ion bombardment. The structural change of the synthesized film was investigated by varying the B ion current density and Ti deposition rate to determine the condition for the synthesized TiB, single phase. In addition, the B-Fe interdiffused layer in the steel substrate was found to be formed during the synthesis, which is thought to be a specific feature of the process.
2. Experiment Substrates used for the present study were mild steel and were metallographically polished with 1 km diamond paste and then degreased in acetone. These steel specimens were treated by dynamic mixing in which B ion implantation and Ti evaporation were carried out simultaneously. B ion beam current density and Ti deposition rate were varied in the range U-130 PA/cm’, and 0.03-0.27 rim/s,, respectively. The B ion dose and thickness of the deposited Ti were dependent on the treatment time. The ion energy was maintained at 40 keV throughout all processes. The process cham-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
173
T. Yasunagaet al. / Formation of TiB, films and B-Fe layers
ber was evacuated by a cryogenic pump, and the pressure was 4 x 10-5-l x 10m4 Pa. The sample temperature during the process was monitored using a chromel-alumel thermocouple. The sample holder was cooled by circulating water on its rear surface in order to maintain the sample temperature at less than 200°C. In order to compare the adhesion of the films, another TiB, film was deposited on a separate steel specimen by conventional rf magnetron sputtering using a TiB, target. The deposition was carried out at less than 100°C in an Ar atmosphere of about 6.5 x 10-i Pa. The film thickness was about 0.5 pm. These treated surface layers were characterized by the thin film X-ray diffraction method. Composition versus depth profiles were determined by Auger electron spectroscopy with argon sputtering in which the approximate depth of each profile was estimated from its etch rate. Cross-sectional observation was also carried out using scanning electron microscopy @EM). The adhesions of the films were evaluated by a scratch test using a vibrating diamond indenter (15 pm radius) with continuously changing forces. It was automatically drawn across the sample surface at a rate of 10 pm/s which corresponded to 0.68 mN/s. The minimum force necessary to remove a film from its substrate (critical force) was determined by observing the scratch using optical microscopy and scanning electron microscopy.
28 3o”
40”
(4 aTi
I
’
30”
40”
-28
W Fe2B 1101
TiB2 op1
04 Fig. 1. Thin film X-ray diffraction patterns for dynamic mixing treated steels with a B ion beam current density of 80 PA/cm’ and Ti deposition rates of: (a) 0.20 rim/s;; (b) 0.11 rim/s;; (c) 0.05 rim/s..
3. Results
3.2. Depth profiles
3.1. X-ray diffraction patterns
Fig. 3 shows composition versus depth profiles of Fe, Ti and B for the dynamic mixing treated steels. Some profiles of other elements are eliminated from the diagrams for simplicity, although there were small amounts of 0 and C profile due to contamination in the near surface region. The B ion dose and beam current density were maintained at the same value, 5 X 1018 ions/cm2, and 80 p,A/cm*, respectively. Ti deposition rate was varied; (fig. 3a) 0.20 rim/s,, (fig. 3b) 0.11 rim/s,, (fig. 3c) 0.05 rim/s.. These conditions correspond to those shown in figs. la-lc and fig. 2 ((a)-(c)). By decreasing the Ti deposition rate from 0.20 to 0.11 rim/s,, the Ti concentration of the synthesized film was decreased from about 50 to 20 at.%, and the B concentration of the film was increased from about 50 to 75 at.%. With a further decrease in the rate to 0.05 rim/s,, however, there was no notable change in the Ti of B concentrations of the synthesized film. Other than the composition of the synthesized films, there were marked differences in the thickness of the B-Fe interdiffused layers. By decreasing the Ti deposition rate from 0.20 rim/s (a) to 0.11 rim/s (b), the thickness of the interdiffused layer was slightly increased from about 0.5 to 0.8 km. With a further decrease in the rate to 0.05 rim/s (c), however, the
Fig. 1 shows thin film X-ray diffraction patterns for the steels treated by dynamic mixing. The B ion current density was 80 pA/cm2. The Ti deposition rate was varied: (fig. la) 0.20 rim/s,, (fig. lb) 0.11 rim/s,, (fig. lc) 0.05 rim/s.. At a higher deposition rate, 0.20 rim/s (fig. la), clear peaks corresponding to aTi were observed. By decreasing the Ti deposition rate to 0.11 rim/s (fig. lb), small peaks corresponding to TiB, were superimposed on the aTi pattern. With a further decrease of the rate to 0.05 rim/s (fig. lc), the TiB, peaks were intensified, and the aTi peaks completely disappeared. Moreover, Fe,B peaks were clearly observed which means there was a considerable reaction between implanted B species and the steel substrate. Fig. 2 shows the composition of the synthesized films, determined by the X-ray diffraction patterns, as a function of the two operational parameters, B ion current density and Ti deposition rate. The composition of the synthesized films was changed from aTi single phase to aTi + TiB, mixed phase and then to TiB, single phase by decreasing the Ti deposition rate or increasing the B ion current density. The TiB phase was not observed under any condition.
T. Yasunagaet al. / Formation of TiB, films and B-Fe layers
174
s E.
layer was observed. According to the X-ray diffraction pattern (fig. lc) and the composition versus depth profile (fig. 3c), the outer layer is TiB, film synthesized on the steel surface, and the inner layer is Fe,B formed in the substrate.
0.2
Q)
H
/
aTi
s
/
‘G ._ g
3.4. Scratch test
.B
’
aTi+TiBP
0.1
% u
0
o/
0
0
/
:
i= /
0.01 ’ 0 6
’
’
’
’
;
’
50
B 0
-cc,: CC----
TiB2
.
I ’
’
’
100
’
’ I
’
ion beam current density (M/cm*)
Fig. 2. Structure of synthesized films as a function of B ion beam current density and Ti deposition rate.
thickness was drastically increased to 3.0 pm, which is about 30 times the thickness of the 40 keV B ion implanted layer in steel as predicted by using the TRIM simulation program [13].
Figs. 5a-5c show the scratches for the steels treated by dynamic mixing under the conditions shown in figs. 2a-2c. Fig. 5d shows the ones of the deposited TiB, film on steel by the rf magnetron sputtering for comparison. All of these scratches were given under the same conditions. The TiB, films deposited by sputtering (fig. 5d) was easily removed with a force of less than 13 mN, and its SEM image shows that the film was removed by brittle interfacial failure. The dynamic mixing treated steels showed higher critical forces for their film removal, 28 mN (fig. 5a) and 33 mN (fig. 5b), which mean higher adhesion due to the B-Fe interdiffused layer shown in the depth profile (figs. 3a and 3b). Although their SEM images show that the films were removed by inter-facial failure, they do no pose such brittleness as does the sputtered film. The film formed with the lowest Ti deposition rate (fig. 5~) had not been removed through the scratch test. Instead of film removal, it was smeared onto the bottom of the scratch.
3.3. Cross-sectional observation 4. Discussion Figs. 4a-4c show the cross-sectional SEM images of the steels treated by dynamic mixing under the same conditions as those shown in figs. la-lc, fig. 2 ((a)-(c)) and figs. 3a-3c, respectively. In the conditions shown in figs. 4a and 4b, single layers were observed on the steel surface. According to the X-ray diffraction patterns (figs. la and lb), the structure of the layers is (fig. la) aTi single phase and (fig. lb) aTi + TiB, mixed phase. B-Fe interdiffused layers in the steel substrates were not observed, although their composition versus depth profiles (figs. 3a and 3b) show the existence of the interdiffused layers in the substrate. In the conditions shown in fig. 4c, however, a divided modified
With varying Ti deposition rate and B ion current density, the structure of the synthesized film was found to change systematically. The phase of the films was classified in three types: aTi single phase, cwTi+ TiB, mixed phase and TiB, single phase. The TiB phase was not observed in any condition. Nakashima et al. [14] investigated Ti thin films implanted with 60 keV B ions and compared them with Ti-B compounds synthesized by conventional solid phase reaction. According to their result, the TiB phase was formed firstly at 900°C and then turned into the TiB, phase by increasing temperature during the solid phase reaction. Neverthe-
Approximate depth (pm)
(a) Fig. 3. Composition
0))
versus depth profiles of Fe, Ti and B for dynamic mixing treated steels with a B ion beam current PA/cm* and Ti deposition rates of: (a) 0.20 rim/s;; (b) 0.11 rim/s;; (c) 0.05 rim/s..
density
of 80
175
T Yasunaga et al. / Formation of TiB, films and B-Fe layers
less, only the TiB, phase was formed in the Ti films during the B ion implantation. The absence of the TiB phase in the films synthesized in this study is in agreement with the work of Nakashima et al. [14]. This phenomenon, therefore, is considered to be a specific feature of the B ion implantation or the dynamic mixing using B ion implantation. According to the Ti-B phase diagram [15], the TiB, phase is stable up to 3225°C while the TiB phase decomposes into the TiB, phase and the Ti liquid phase at 2200°C. Considering the absence of the TiB phase, the temperature of the micro-area where the B ions were stopped was thought to be high enough to decompose the TiB phase, although the temperature of the sample monitored during the process was less than 200°C. As shown in fig. 3c and fig. 4c, the thickness of the B-Fe interdiffused layer in the steel substrate was enlarged during the synthesis of the TiB, thin film. There was no difference in sample temperature, monitored during the processes, between the three dynamic mixing treatments shown in figs. 2a-2c; it was less than 200°C. However, the temperature of the near surface region might be higher during the (c) process compared with the one in (a), (b), which can enhance the B-Fe interdiffusion and the formation of the Fe,B phase. Since the B-Fe interdiffusion was enhanced under the condition where the TiB, single phase was synthesized on the substrate, the heat of the TiB, formation during the process might contribute to the
5um
(a)
(cl Fig. 4. Cross-sectional
diffusion, The cause of these phenomena has not yet been ascertained. Further research is now under way. For the scratch tests shown in fig. 5, the synthesized films by the dynamic mixing technique showed higher critical force compared with the deposited films by rf magnetron sputtering. The critical force was increased by decreasing the Ti deposition rate from (a) to (b), and there was no film removal for condition (c). According to the depth profiles shown in fig. 3, the interface between the film and the substrate was broadened by the mixing effect of the implanted B ion, and as mentioned above, the thickness of the B-Fe interdiffused layer beneath the film was enlarged followed by the formation of the Fe,B phase with a decreasing Ti deposition rate. The mixing effect between the film and the substrate contributes to the adhesion, and the formation of the Fe,B phase contributes to the hardening of the steel substrate beneath the TiB, film. The increase of the critical force (figs. 5a and 5b) and the prevention of the film removal (fig. 5c) can be explained by the fact that high adhesion and increased substrate hardness result in an increase of the critical force in the scratch test [16].
5. Conclusion
Steel substrates were treated by dynamic mixing using B ion implantation and Ti evaporation. The two
,5um,
(W
5pm
lm
SEM images of dynamic mixing treated steels with a B ion beam current density of 80 kA/cm2 deposition rates of: (a) 0.20 rim/s;; (b) 0.11 rim/s;; (c) 0.05 rim/s..
and Ti
T. Yasunaga et al. / Formation of TiB, films and B-Fe layers
176
operational parameters, B ion current density and Ti deposition rate, were varied in order to determine the condition where TiB, single phase was synthesized. The structure of the synthesized films was changed from cxTi single phase to cuTi + TiB, mixed phase and then to TiB, single phase by decreasing the Ti deposition rate or increasing the B ion current density. The
TiB phase was not observed in any condition. The absence of the TiB phase is thought to be a specific feature of the B ion implantation or the dynamic mixing using B ion implantation. The thickness of the B-Fe interdiffused layer was found to be enlarged during the synthesis of the TiB, phase, the cause of which has yet to be ascertained. These thin films
IOclm
10vm
Fig. 5. Optical
images
and SEM images of scratches
for dynamic
mixing treated
steels (B ions beam current density of 80 PA/cm’ steel by rf magnetron sputtering.
and Ti deposition rates of (a) 0.20 rim/s;; (b) 0.11 rim/s;; (c) 0.05 rim/s)) and (d) TiB, deposited
111
T. Yasunaga et al. / Formation of TiB, jZms and B-Fe layers
10pm
formed by dynamic mixing showed higher adhesion compared with those formed by the conventional rf magnetron sputtering.
Acknowledgements
This work was conducted within the scope of the program: Advanced Material-Processing and Machining System, consigned to the Advanced MaterialProcessing and Machining Technology Research Association from the New Energy and Industrial Technology Development Organization, which is carried out under the Large-Scale Project enforced by the Agency of Industrial Science and Technology, the Ministry of International Trade and Industry.
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