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The effect of ion-mixed Ti on the BFe interdiffusion during dynamic mixing
Nuclear Instruments and Methods in Physics Research B 82 (1993) 52-56 North-Holland
Beam Interactions with Materials & Atoms
The effect of ion-mixed Ti on the B-Fe interdiffusion during dynamic mixing Tatsuya Yasunaga, Yasuaki Sugizaki, Kazuhisa Kawata and Hiroshi Satoh Materials Research Laboratory, Kobe Steel, Ltd., Takatsukadai, Nishi-ku, Kobe 651-22, Japan Received 1 December 1992 and in revised form 19 February 1993
The effects of ion-mixed Ti on the B-Fe interdiffusion during a dynamic mixing treatment on mild steel were investigated. The experiment and the resulting improvements in the mechanical surface properties are described in this paper. For the experiment, mild steel specimens were implanted with 40 keV B ions to fluences of 1 × 1018-5x 1018 ions/cm2. Separate steel specimens were treated by dynamic mixing processes, in which B-ion implantation to fluences of 1 X 1018-5X 10TM ions/cm 2 and Ti evaporation at a fixed deposition rate of 0.05 nm/s were carried out simultaneously. For the B-ion implanted steel, the thickness of the B-Fe mixed layer in the substrate was limited to less than 0.6 Ixm, and an accumulation of implanted B species in the surface region took place when the fluence exceeded 3.5 x 1018 ions/cm2. The B-ion implantation had little effect in improving the wear property of the steel. For the dynamic mixing-treated steel, TiB2 film was synthesized on the surface and B-Fe interdiffusion was found to have taken place beneath the TiB2 film during the treatment. The thickness of the B-Fe interdiffused layer reached up to 3 Ixm, about five times the thickness of the B-Fe mixed layer resulting from only the B-ion implantation. This dynamic mixing treatment was far more effective in improving the wear property of the steel than the B-ion implantation.
1. Introduction There is a growing interest in the application of ion implantation as a technique for changing the mechanical properties of metal surfaces. This process, for example, offers increased wear protection, lubricity and fatigue resistance, without dimensional change, heatinduced distortion, peeling, or chipping of coatings [1-3]. The thickness of the modified layer, however, is generally of submicron order, which is not enough to cope with harsh environments such as those endured by tools, dies, punches, etc. Thickening the modified layer by a high-dose implantation is difficult, because the concentration of the implanted species in the substrate saturates around the fluence of 1018 ions/cm 2. Wei et al. carried out N-ion implantation into a stainless steel by raising the ambient temperature up to 510°C so the implanted N ions could diffuse into the substrate [4]. According to their results, the N ions were implanted to the extremely high fluence of 1 x 1019 ions/cm 2 and then formed a modified layer which was more than 10 I~m thick and which was effective in
Correspondence to: T. Yasunaga, Materials Research Laboratory, Kobe Steel, Ltd., 5-5, Takatsukadai, 1-chome, Nishi-Ku, Kobe, Hyogo 651-22, Japan.
improving the wear resistance of the steel. Although the modified layer was thickened effectively in this case, the method could not be applied to every material because of the heat deterioration it can cause. Dynamic mixing is a new film production method which simultaneously uses ion implantation and metal evaporation. A number of studies on the fabrication of TiN films using Ti evaporation and N-ion implantation have been reported [5,6]. Adhesion of the films is generally excellent due to the effect of mixing through the film-substrate interface. This report suggests that the dynamic mixing is a technique for not only fabricating adherent films but also for thickening the interdiffused layer beneath them. About a 3 p.m thick modified layer on steel substrate was found to be formed by the dynamic mixing with B-ion implantation and Ti evaporation without having to raise the temperature.
2. Experimental procedures Substrates used in this study were mild steel which were metallographically polished with 1 ~m diamond paste and then degreased in acetone. These steel specimens were implanted with 40 keV B ions to fluences of 1.0 × 101s-5.0x 10 as ions/cm 2. Separate steel specimens were treated by dynamic mixing with simultane-
0168-583X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
T. Yasunaga et aL / Dynamic mixing treatment of mild steel ous B-ion implantation to fluences of 1.0 x 1018-5.0 × 1018 ions/cm 2 and Ti evaporation at a fixed deposition rate of 0.05 nm/s. The current density of the ion beam was maintained at 80 i~A/cm 2 throughout all processes. The determination of the deposition rate and the ion current density have been discussed in detail elsewhere [7]. The process chamber was evacuated by a cryogenic pump, and the pressure was 4 × 10-5-1 × 10 -4 Pa. Sample temperatures during the process were monitored using a chromel-alumel thermocouple, and the sample holder was cooled by circulating water on its rear surface in order to keep sample temperatures below 200°C. Composition versus depth profiles for the treated steels were determined by Auger electron spectroscopy (AES) with argon sputtering in which the approximate depth of each profile was estimated from its etch rate. The modified layers were characterized by the thin-film X-ray diffraction method at a glancing angle of 1°. Cross-sectional observation was also carried out using scanning electron microscopy (SEM). In order to evaluate the mechanical surface property, microhardness was measured using the Vickers microhardness tester with a 10 gf load. A reciprocal motion wear test was also carried out without lubricant, and the change in the friction coefficient during the test was monitored. A 10 mm diam alumina ball with 300gf load was used as the indenter. The temperature and humidity during the wear test were about 25°C and 45%, respectively, although the atmosphere was not strictly controlled.
53
3. Results and discussion
3.1. Structure of the modified layers 3.1.1. B-ion implanted steels Fig. la shows composition versus depth profiles of Fe and B for the B-ion implanted steels with various B fluences ranging from 1.0 x 1018 to 5.0 x 1018 ions/cm 2, Profiles of O and C existing in the nearsurface region were eliminated for simplicity. At the lower fluences of 1.0 × 1018 ions/cm 2 and 2.0 × 1018 ions/cm 2, all of the B species were implanted into the steel substrates. The thickness of the B - F e mixed layers increased from about 0.4 to 0.6 I~m as the B fluence increased. The projected range Rp and the range struggling ARp of 40 keV B ions in steel estimated by using the TRIM simulation program [8] are 65.6 and 27.6 nm, respectively. These calculated results suggest that the thickness of the B - F e mixed layer should be about 0.1 I~m, which is far less than the observed value. From these results, the implanted B species are thought to have diffused into the steel substrate to form a 0.6 i~m-thick B - F e mixed layer during the implantation process. At higher fluences of 3.5 × 1018 ions/cm 2 and 5.0 × 1018 ions/cm 2, most of the implanted B species were accumulated in the surface region. Although the thickness of the accumulated B layer increased with increases in the B fluence, the thickness of the B - F e mixed layer beneath the B layer did not, remaining at about 0.6 Win. These results
A
~ 100[ '~60~ -~'-~Fe ~ii ' ~EIFe ~4 0 °
o
< (a)
0.0 0.5 0.0 o.s 1.0 1 2
=
80
9 S
20
< (b)
' e///F~
0.0 3
1.0
'
'
°o.o o.5 010 0.5 1.0 1.5 0.0 1 2 3
1.0
210
0.0 A
1.0 2.0 310 Approximate depth (lam)
Fe
2.0
0.0 4
1.0 2.0 3.0 Approximate depth (.m)
Fig. 1. Composition versus depth profiles of Fe, B and Ti for (a) B-ion implanted steels and (b) dynamic mixing-treated steels with various B fluence (1: 1.0x 1018 ions/cm2; 2: 2.0× 10ts ions/cm2; 3:3.5 × 1018 ions/cruZ; 4: 5.0× 10is ions/cm2).
54
T. Yasunaga et al. / Dynamic mixing treatment of mild steel
indicate that it is impossible to form a B - F e mixed layer in a steel substrate thicker than 0.6 Ixm unless the ion energy is increased or the ambient temperature is raised during the process as done by Wei et al. [4] Fig. 2a shows the thin-film X-ray diffraction pattern for the B-ion implanted steel with the highest B fluence in which no clear peak for F e - B compound or B crystal was observed. Therefore, the B - F e mixed layer and the accumulated B layer are thought to be amorphous or composed of extremely fine crystals. Fig. 3a shows a cross-sectional image of the steel treated with B-ion implantation with the highest B fluence. A single layer is observable which corresponds to the accumulated B layer, according to the composition versus depth profile shown in figs. la-4. 3.1.2. Dynamic mixing-treated steels Fig. lb shows composition versus depth profiles of Fe, B and Ti for the dynamic mixing-treated steels with various B fluences ranging from 1.0 × 10 is to 5.0 × 10 TM ions/era 2. Each of the fluences corresponds to each of those in fig. la. The Ti deposition rate was fixed at 0.05 n m / s throughout all processes. Unlike the B-ion implantation shown in fig. la, B - F e interdiffusion in the steel substrate was found to have taken place, and the thickness of the B - F e mixed layer increased as the B fluence increased. At the highest fluence, the thickness of the B - F e mixed layer reached up to 3 Ixm, about five times the thickness of that in the B-ion implantation. The B-Ti mixed layer was also synthesized on the B - F e mixed layer during this process. Since all of the implanted B species were mixed into the B - F e or the B-Ti mixed layers, there was no accumulation of the B species such as was observed in the B-ion implantation. Fig. 2b shows the X-ray diffraction patterns for the dynamic mixing-treated steel with the highest B fluence, in which clear peaks corresponding to Fe2B and TiB z were observed. According to these results, the TiB/ film is thought to have been synthesized on the substrate, and the FezB phase is thought to have been formed in the B - F e interdiffused layer. Fig. 3b shows a cross-sectional image of the dynamic mixing treated-steel with the highest B fluence. An approximately 2 ixm-thick double layer is visible. According to the composition versus depth profile, fig. lb-4, and the X-ray diffraction pattern, fig. 2b, the outer layer corresponds to the TiB 2 film synthesized on the steel substrate, and the inner layer corresponds to the Fe2B layer formed beneath the 'riB 2 film. The cause of the B - F e interdiffusion followed by the formation of the FezB phase has not yet been ascertained. The same phenomena have already been observed, however, with other ion species, other evaporation materials and other substrates. Further research is now under way.
i 30 °
I 40 °
I 30 o
I, 40 o
20
(a)
Fe2B
"
2B
(b) Fig. 2. Thin-film X-ray diffraction patterns for (a) B-ion implanted steel and (b) dynamic mixing-treated steel. (B fluence: 5.0× l0 Is ions/cm/).
3. 2. Mechanical surface properties 3.2.1. Surface hardness Fig. 4 shows the Vickers microhardness for the steels treated by the B-ion implantation and the dynamic mixing as a function of the B fluence. The indenter load was 10 gf and the indentation ranged from 0.5 to 1.8 Ixm. It should be noted that the measured hardness is influenced by the deformation of the substrate when the indentation is not substantially less than the layer thickness [9]. The hardness of the untreated steel was Hvl20 (indentation was 1.8 ~m). For the B-ion implanted steels, although the hardness increased as the B fluence increased, it was limited to less than Hv200 (indentation was 1.4 I~m). The cause of the limitation is that the intrinsic hardness of the modified layer is not hard enough and the thickness of the layer is not thick enough to contribute to the measured hardness. For the dynamic mixing-treated steels, the surface hardness increased drastically as the B fluence increased and it reached Hv1500 (indentation was 0.5 I~m) at the highest fluence. This hardening are caused by the formation of TiB 2 film and the Fe2B layer beneath the TiB2 film. Vickers hardnesses of TiB z and FezB are about Hv3400 and Hv1300, respectively [10]. Since the thickness of the TiB 2 film was less than 0.5 Ixm, its contribution to the measured hardnesses was relatively small. On the other hand, with the increased B fluence, the FezB layer thickened up to 3 I~m, thick enough to contribute to the measured hardness. By the contribution of the Fe2B layer, the indentation depth should be decreased, with the result that the contribution of the TiB 2 film to the measured
55
T. Yasunaga et al. / Dynamic mixing treatment of mild steel
hardness should also increase. Therefore, the thickening of the B-Fe interdiffused layer and the subsequent formation of the Fe2B is essential to the increase in the measured hardness. 3.2.2. Reciprocal motion wear test
Fig. 5 shows the change in friction coefficient during the reciprocal motion wear test. For untreated steel, the friction coefficient exceeded 0.3 after seven passes. This increase in the friction coefficient results from the plastic deformation of the steel surface which generates asperities in its wear track. For the steel treated by the B-ion implantation with the highest B fluence, the duration of the low friction coefficient slightly increased up to 12 passes, but eventually the coefficient exceeded 0.3 after 25 passes. This result indicates that although the modified layer prevented
the deformation of the steel surface up to 12 passes, the layer yielded after 12 passes. For the steel treated by the dynamic mixing with the highest B fluence, the duration of the low friction coefficient drastically increased; it was less than 0.26 even after 90 passes. The modified layer produced by the dynamic mixing treatment was very effective in preventing the deformation of the surface as observed in the Vickers hardness test. This property should contribute effectively to lowering the friction coefficient during the wear test.
4. Conclusion For conventional B-ion implantations on steel substrates, the thickness of the B - F e mixed layer was limited to less than 0.6 I~m, and most of the B species
_t_
(a)
,
5pm
,
I
I pm
B
,
Til Fe2B Fe
(b)
, 5pro , Fig. 3. Cross-sectional SEM images of (a) B-ion implanted steel and (b) dynamic mtxing-treated steel. (B fluence: 5.0× 10is ions/cm2).
56
7". Yasunaga et aL / Dynamic mixing treatment of mild steel
I o
I
I
I
I T
1500
/
~) Stieoe~ Implanted steels
1000
fusion beneath the film was found to be enhanced when the B-ion implantation was combined with Ti evaporation. The thickness of the B - F e mixed layer reached 3 p.m, about five times the thickness of that resulting with only B-ion implantation. Because of the thickening effect and the formation of the TiB 2 film, the dynamic mixing treatment was far more effective than the conventional B-ion implantation in improving the wear property of steel.
_
•
500-
Acknowledgement
j.
, _
120 O0
o
t 1
I 2
= 3
B fluence(
= 4
t 5
x 1018 Ions/cm 2 )
Fig. 4. Vickers microhardness as a function of B fluence for B-ion implanted steels and dynamic mixing-treated steels.
were not mixed into the substrate but accumulated in the surface region at the higher fluences of 3.5 × 10 as ions/cm z and 5.0 × 10 t8 i o n s / c m 2. F e - B compound or B crystal were not observed in the modified layer. In contrast, TiB 2 film was formed and the B - F e interdif-
~"
0.30--~
"6
0,20
--~
"~
"
.
,
,
o Steel o B ion Implanted Steel • Dynamic mixing treated Steel .
10 .
. 20 .
. 30
.
40
50
6'0
7
'o
80'
90
Number of passes
Fig. 5. Changes in friction coefficient during a reciprocal motion wear test for untreated steel, B-ion implanted steel and dynamic mixing-treated steel. (B fluence: 5.0×1018 ions/era2), indenter: 10 mm diam alumina ball without lubricant; load: 300 gf; temperature: 25°C; humidity : 45%.
This work was conducted in the program: Advanced Material-Processing and Machining System, consigned to the Advanced Material-Processing 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 [1] P. Sioshansi, Thin Solid Films 118 (1984) 61. [2] J.E. Elder, R. Thamburaj and P.C. Patnaik, International Materials Reviews 33 6 (1988) 289. [3] K. Langguth, A. Kluge and H, Ryssel, Nucl. Instr. and Meth. B59/60 (1991) 718. [4] R. Wei, P.J. Wilbur, O. Ozturk and D.L. Williamson, Nucl. Instr. and Meth. B59/60 (1991) 731. [5] S. Nakashima, M. Fukushima, M. Haginoya, K. Oohata, I. Hashimoto and K. Terakado, Mater. Sci. Eng. Al15 (1989) 197. [6] T. Yano, M. Yoneda, A. Utsumi, J. Matsuda, M. Katsumura and S. Miyagawa, J. High Temperature Soc. 15 (6) (1989) 279. [7] T. Yasunaga, Y. Sugizaki and H. Satoh, Nucl. Instr. and Meth. B73 (1993) 172. [8] J.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). [9] B. Jonsson and S. Hogmark, Thin Solid Films 114 (1984) 257. [10] T. Oki, J. Surface Finishing Soc. 41 (5) (1990) 462.
Beam Interactions with Materials & Atoms
The effect of ion-mixed Ti on the B-Fe interdiffusion during dynamic mixing Tatsuya Yasunaga, Yasuaki Sugizaki, Kazuhisa Kawata and Hiroshi Satoh Materials Research Laboratory, Kobe Steel, Ltd., Takatsukadai, Nishi-ku, Kobe 651-22, Japan Received 1 December 1992 and in revised form 19 February 1993
The effects of ion-mixed Ti on the B-Fe interdiffusion during a dynamic mixing treatment on mild steel were investigated. The experiment and the resulting improvements in the mechanical surface properties are described in this paper. For the experiment, mild steel specimens were implanted with 40 keV B ions to fluences of 1 × 1018-5x 1018 ions/cm2. Separate steel specimens were treated by dynamic mixing processes, in which B-ion implantation to fluences of 1 X 1018-5X 10TM ions/cm 2 and Ti evaporation at a fixed deposition rate of 0.05 nm/s were carried out simultaneously. For the B-ion implanted steel, the thickness of the B-Fe mixed layer in the substrate was limited to less than 0.6 Ixm, and an accumulation of implanted B species in the surface region took place when the fluence exceeded 3.5 x 1018 ions/cm2. The B-ion implantation had little effect in improving the wear property of the steel. For the dynamic mixing-treated steel, TiB2 film was synthesized on the surface and B-Fe interdiffusion was found to have taken place beneath the TiB2 film during the treatment. The thickness of the B-Fe interdiffused layer reached up to 3 Ixm, about five times the thickness of the B-Fe mixed layer resulting from only the B-ion implantation. This dynamic mixing treatment was far more effective in improving the wear property of the steel than the B-ion implantation.
1. Introduction There is a growing interest in the application of ion implantation as a technique for changing the mechanical properties of metal surfaces. This process, for example, offers increased wear protection, lubricity and fatigue resistance, without dimensional change, heatinduced distortion, peeling, or chipping of coatings [1-3]. The thickness of the modified layer, however, is generally of submicron order, which is not enough to cope with harsh environments such as those endured by tools, dies, punches, etc. Thickening the modified layer by a high-dose implantation is difficult, because the concentration of the implanted species in the substrate saturates around the fluence of 1018 ions/cm 2. Wei et al. carried out N-ion implantation into a stainless steel by raising the ambient temperature up to 510°C so the implanted N ions could diffuse into the substrate [4]. According to their results, the N ions were implanted to the extremely high fluence of 1 x 1019 ions/cm 2 and then formed a modified layer which was more than 10 I~m thick and which was effective in
Correspondence to: T. Yasunaga, Materials Research Laboratory, Kobe Steel, Ltd., 5-5, Takatsukadai, 1-chome, Nishi-Ku, Kobe, Hyogo 651-22, Japan.
improving the wear resistance of the steel. Although the modified layer was thickened effectively in this case, the method could not be applied to every material because of the heat deterioration it can cause. Dynamic mixing is a new film production method which simultaneously uses ion implantation and metal evaporation. A number of studies on the fabrication of TiN films using Ti evaporation and N-ion implantation have been reported [5,6]. Adhesion of the films is generally excellent due to the effect of mixing through the film-substrate interface. This report suggests that the dynamic mixing is a technique for not only fabricating adherent films but also for thickening the interdiffused layer beneath them. About a 3 p.m thick modified layer on steel substrate was found to be formed by the dynamic mixing with B-ion implantation and Ti evaporation without having to raise the temperature.
2. Experimental procedures Substrates used in this study were mild steel which were metallographically polished with 1 ~m diamond paste and then degreased in acetone. These steel specimens were implanted with 40 keV B ions to fluences of 1.0 × 101s-5.0x 10 as ions/cm 2. Separate steel specimens were treated by dynamic mixing with simultane-
0168-583X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
T. Yasunaga et aL / Dynamic mixing treatment of mild steel ous B-ion implantation to fluences of 1.0 x 1018-5.0 × 1018 ions/cm 2 and Ti evaporation at a fixed deposition rate of 0.05 nm/s. The current density of the ion beam was maintained at 80 i~A/cm 2 throughout all processes. The determination of the deposition rate and the ion current density have been discussed in detail elsewhere [7]. The process chamber was evacuated by a cryogenic pump, and the pressure was 4 × 10-5-1 × 10 -4 Pa. Sample temperatures during the process were monitored using a chromel-alumel thermocouple, and the sample holder was cooled by circulating water on its rear surface in order to keep sample temperatures below 200°C. Composition versus depth profiles for the treated steels were determined by Auger electron spectroscopy (AES) with argon sputtering in which the approximate depth of each profile was estimated from its etch rate. The modified layers were characterized by the thin-film X-ray diffraction method at a glancing angle of 1°. Cross-sectional observation was also carried out using scanning electron microscopy (SEM). In order to evaluate the mechanical surface property, microhardness was measured using the Vickers microhardness tester with a 10 gf load. A reciprocal motion wear test was also carried out without lubricant, and the change in the friction coefficient during the test was monitored. A 10 mm diam alumina ball with 300gf load was used as the indenter. The temperature and humidity during the wear test were about 25°C and 45%, respectively, although the atmosphere was not strictly controlled.
53
3. Results and discussion
3.1. Structure of the modified layers 3.1.1. B-ion implanted steels Fig. la shows composition versus depth profiles of Fe and B for the B-ion implanted steels with various B fluences ranging from 1.0 x 1018 to 5.0 x 1018 ions/cm 2, Profiles of O and C existing in the nearsurface region were eliminated for simplicity. At the lower fluences of 1.0 × 1018 ions/cm 2 and 2.0 × 1018 ions/cm 2, all of the B species were implanted into the steel substrates. The thickness of the B - F e mixed layers increased from about 0.4 to 0.6 I~m as the B fluence increased. The projected range Rp and the range struggling ARp of 40 keV B ions in steel estimated by using the TRIM simulation program [8] are 65.6 and 27.6 nm, respectively. These calculated results suggest that the thickness of the B - F e mixed layer should be about 0.1 I~m, which is far less than the observed value. From these results, the implanted B species are thought to have diffused into the steel substrate to form a 0.6 i~m-thick B - F e mixed layer during the implantation process. At higher fluences of 3.5 × 1018 ions/cm 2 and 5.0 × 1018 ions/cm 2, most of the implanted B species were accumulated in the surface region. Although the thickness of the accumulated B layer increased with increases in the B fluence, the thickness of the B - F e mixed layer beneath the B layer did not, remaining at about 0.6 Win. These results
A
~ 100[ '~60~ -~'-~Fe ~ii ' ~EIFe ~4 0 °
o
< (a)
0.0 0.5 0.0 o.s 1.0 1 2
=
80
9 S
20
< (b)
' e///F~
0.0 3
1.0
'
'
°o.o o.5 010 0.5 1.0 1.5 0.0 1 2 3
1.0
210
0.0 A
1.0 2.0 310 Approximate depth (lam)
Fe
2.0
0.0 4
1.0 2.0 3.0 Approximate depth (.m)
Fig. 1. Composition versus depth profiles of Fe, B and Ti for (a) B-ion implanted steels and (b) dynamic mixing-treated steels with various B fluence (1: 1.0x 1018 ions/cm2; 2: 2.0× 10ts ions/cm2; 3:3.5 × 1018 ions/cruZ; 4: 5.0× 10is ions/cm2).
54
T. Yasunaga et al. / Dynamic mixing treatment of mild steel
indicate that it is impossible to form a B - F e mixed layer in a steel substrate thicker than 0.6 Ixm unless the ion energy is increased or the ambient temperature is raised during the process as done by Wei et al. [4] Fig. 2a shows the thin-film X-ray diffraction pattern for the B-ion implanted steel with the highest B fluence in which no clear peak for F e - B compound or B crystal was observed. Therefore, the B - F e mixed layer and the accumulated B layer are thought to be amorphous or composed of extremely fine crystals. Fig. 3a shows a cross-sectional image of the steel treated with B-ion implantation with the highest B fluence. A single layer is observable which corresponds to the accumulated B layer, according to the composition versus depth profile shown in figs. la-4. 3.1.2. Dynamic mixing-treated steels Fig. lb shows composition versus depth profiles of Fe, B and Ti for the dynamic mixing-treated steels with various B fluences ranging from 1.0 × 10 is to 5.0 × 10 TM ions/era 2. Each of the fluences corresponds to each of those in fig. la. The Ti deposition rate was fixed at 0.05 n m / s throughout all processes. Unlike the B-ion implantation shown in fig. la, B - F e interdiffusion in the steel substrate was found to have taken place, and the thickness of the B - F e mixed layer increased as the B fluence increased. At the highest fluence, the thickness of the B - F e mixed layer reached up to 3 Ixm, about five times the thickness of that in the B-ion implantation. The B-Ti mixed layer was also synthesized on the B - F e mixed layer during this process. Since all of the implanted B species were mixed into the B - F e or the B-Ti mixed layers, there was no accumulation of the B species such as was observed in the B-ion implantation. Fig. 2b shows the X-ray diffraction patterns for the dynamic mixing-treated steel with the highest B fluence, in which clear peaks corresponding to Fe2B and TiB z were observed. According to these results, the TiB/ film is thought to have been synthesized on the substrate, and the FezB phase is thought to have been formed in the B - F e interdiffused layer. Fig. 3b shows a cross-sectional image of the dynamic mixing treated-steel with the highest B fluence. An approximately 2 ixm-thick double layer is visible. According to the composition versus depth profile, fig. lb-4, and the X-ray diffraction pattern, fig. 2b, the outer layer corresponds to the TiB 2 film synthesized on the steel substrate, and the inner layer corresponds to the Fe2B layer formed beneath the 'riB 2 film. The cause of the B - F e interdiffusion followed by the formation of the FezB phase has not yet been ascertained. The same phenomena have already been observed, however, with other ion species, other evaporation materials and other substrates. Further research is now under way.
i 30 °
I 40 °
I 30 o
I, 40 o
20
(a)
Fe2B
"
2B
(b) Fig. 2. Thin-film X-ray diffraction patterns for (a) B-ion implanted steel and (b) dynamic mixing-treated steel. (B fluence: 5.0× l0 Is ions/cm/).
3. 2. Mechanical surface properties 3.2.1. Surface hardness Fig. 4 shows the Vickers microhardness for the steels treated by the B-ion implantation and the dynamic mixing as a function of the B fluence. The indenter load was 10 gf and the indentation ranged from 0.5 to 1.8 Ixm. It should be noted that the measured hardness is influenced by the deformation of the substrate when the indentation is not substantially less than the layer thickness [9]. The hardness of the untreated steel was Hvl20 (indentation was 1.8 ~m). For the B-ion implanted steels, although the hardness increased as the B fluence increased, it was limited to less than Hv200 (indentation was 1.4 I~m). The cause of the limitation is that the intrinsic hardness of the modified layer is not hard enough and the thickness of the layer is not thick enough to contribute to the measured hardness. For the dynamic mixing-treated steels, the surface hardness increased drastically as the B fluence increased and it reached Hv1500 (indentation was 0.5 I~m) at the highest fluence. This hardening are caused by the formation of TiB 2 film and the Fe2B layer beneath the TiB2 film. Vickers hardnesses of TiB z and FezB are about Hv3400 and Hv1300, respectively [10]. Since the thickness of the TiB 2 film was less than 0.5 Ixm, its contribution to the measured hardnesses was relatively small. On the other hand, with the increased B fluence, the FezB layer thickened up to 3 I~m, thick enough to contribute to the measured hardness. By the contribution of the Fe2B layer, the indentation depth should be decreased, with the result that the contribution of the TiB 2 film to the measured
55
T. Yasunaga et al. / Dynamic mixing treatment of mild steel
hardness should also increase. Therefore, the thickening of the B-Fe interdiffused layer and the subsequent formation of the Fe2B is essential to the increase in the measured hardness. 3.2.2. Reciprocal motion wear test
Fig. 5 shows the change in friction coefficient during the reciprocal motion wear test. For untreated steel, the friction coefficient exceeded 0.3 after seven passes. This increase in the friction coefficient results from the plastic deformation of the steel surface which generates asperities in its wear track. For the steel treated by the B-ion implantation with the highest B fluence, the duration of the low friction coefficient slightly increased up to 12 passes, but eventually the coefficient exceeded 0.3 after 25 passes. This result indicates that although the modified layer prevented
the deformation of the steel surface up to 12 passes, the layer yielded after 12 passes. For the steel treated by the dynamic mixing with the highest B fluence, the duration of the low friction coefficient drastically increased; it was less than 0.26 even after 90 passes. The modified layer produced by the dynamic mixing treatment was very effective in preventing the deformation of the surface as observed in the Vickers hardness test. This property should contribute effectively to lowering the friction coefficient during the wear test.
4. Conclusion For conventional B-ion implantations on steel substrates, the thickness of the B - F e mixed layer was limited to less than 0.6 I~m, and most of the B species
_t_
(a)
,
5pm
,
I
I pm
B
,
Til Fe2B Fe
(b)
, 5pro , Fig. 3. Cross-sectional SEM images of (a) B-ion implanted steel and (b) dynamic mtxing-treated steel. (B fluence: 5.0× 10is ions/cm2).
56
7". Yasunaga et aL / Dynamic mixing treatment of mild steel
I o
I
I
I
I T
1500
/
~) Stieoe~ Implanted steels
1000
fusion beneath the film was found to be enhanced when the B-ion implantation was combined with Ti evaporation. The thickness of the B - F e mixed layer reached 3 p.m, about five times the thickness of that resulting with only B-ion implantation. Because of the thickening effect and the formation of the TiB 2 film, the dynamic mixing treatment was far more effective than the conventional B-ion implantation in improving the wear property of steel.
_
•
500-
Acknowledgement
j.
, _
120 O0
o
t 1
I 2
= 3
B fluence(
= 4
t 5
x 1018 Ions/cm 2 )
Fig. 4. Vickers microhardness as a function of B fluence for B-ion implanted steels and dynamic mixing-treated steels.
were not mixed into the substrate but accumulated in the surface region at the higher fluences of 3.5 × 10 as ions/cm z and 5.0 × 10 t8 i o n s / c m 2. F e - B compound or B crystal were not observed in the modified layer. In contrast, TiB 2 film was formed and the B - F e interdif-
~"
0.30--~
"6
0,20
--~
"~
"
.
,
,
o Steel o B ion Implanted Steel • Dynamic mixing treated Steel .
10 .
. 20 .
. 30
.
40
50
6'0
7
'o
80'
90
Number of passes
Fig. 5. Changes in friction coefficient during a reciprocal motion wear test for untreated steel, B-ion implanted steel and dynamic mixing-treated steel. (B fluence: 5.0×1018 ions/era2), indenter: 10 mm diam alumina ball without lubricant; load: 300 gf; temperature: 25°C; humidity : 45%.
This work was conducted in the program: Advanced Material-Processing and Machining System, consigned to the Advanced Material-Processing 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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