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Mechanical properties of B–C–N films synthesized by an electron beam excited plasma-CVD method
Surface and Coatings Technology 169 – 170 (2003) 270–273
Mechanical properties of B–C–N films synthesized by an electron beam excited plasma-CVD method Takeshi Hasegawaa,*,1, Kazuhiro Yamamotob, Yozo Kakudateb, Masahito Bana,c a FCT Project, Japan Fine Ceramics Center, FCT Laboratory, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan c Technical Institute, Kawasaki Heavy Industries, Ltd., 118 Futatsuzuka, Noda, Chiba 278-8585, Japan
b
Abstract We have synthesized B–C–N films by an electron beam excited plasma-CVD method and characterized structures and properties of the obtained films. In this paper, tribological properties of the films under dry-lubrication were investigated with a ball-on-disc type friction tester. The hardness of the films increases monotonically with increasing boron content and both the friction coefficient and the wear rate increase with increasing the hardness. From these results the low friction coefficient can be attributed to the formation of a sp2-bonded B–C–N structure. And it is suggested that the cause of the wear for the B–C–N films may be a microscopic peeling, because the film should become more brittle and the internal stress of the film should become larger for the harder film. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Plasma-CVD; B–C–N films; Hardness; Friction coefficient; Wear rate
1. Introduction In recent years, materials in the B–C–N system have been investigated very well. Although diamond is the hardest material, its application to machining tools is limited because of the oxidation at high temperature and the poor abrasive resistance for iron-based alloys. On the other hand, while cubic phase BN is next to diamond in the hardness, it is chemically more stable than diamond. Cubic B–C–N phases are expected to be superhard materials and to have high corrosion resistance at high temperature. Therefore it is of great interest and of technological importance to obtain the cubic B–C– N phases and to control their prominent properties by varying their compositions. The cubic B–C–N phases have been synthesized so far only by high pressure-high temperature treatments w1–3x. For the industrial applications, however, it is important to form hard B–C–N films on other materials as mechanical hard coating. Although many attempts to *Corresponding author. Tel.: q81-298-61-9982; fax: q81-298-614796. E-mail address: [email protected] (T. Hasegawa). 1 Under temporary transfer from Kawasaki Heavy Industries, Ltd.
produce the B–C–N films by using various CVD or PVD methods have been reported w4–8x, few studies have dealt the relationship among their composition, structure and mechanical properties. In a previous paper w9x, we reported the synthesis of B–C–N films by an electron beam excited plasma (EBEP)-CVD method and results of analysis of their compositions. In the present study, the mechanical properties of B–C–N films in terms of the hardness, the friction coefficient and the wear rate with varying deposition conditions are investigated. 2. Experimental procedure B–C–N films are synthesized with the EBEP-CVD system, a schematic of which is shown in Fig. 1. An argon d.c. glow-discharge is generated by using thermal electrons from a LaB6 filament as a trigger. By applying a voltage between a discharge and acceleration electrodes, an electron beam is extracted from the discharge electrode and directed into a process chamber to produce a plasma of process gases. A substrate is heated by radiation from a graphite heater. A negative bias is induced to the substrate with a 2 MHz r.f. generator.
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 0 3 9 - 2
T. Hasegawa et al. / Surface and Coatings Technology 169 – 170 (2003) 270–273
271
Table 1 Fraction of process gases and composition of the films No.
Flow rate
Atomic concentration in the film (%)
(sccm)
1 2 3 4 5 6 7 8
Fig. 1. Schematic diagram of the EBEP-CVD system.
B–C–N films were deposited from mixtures of 10% B2H6 in He, CH4, N2 and H2 on polished p-type Si (1 0 0) substrates which were in situ pre etched by an argon plasma for 10 min at 0.13 Pa. A temperature of the substrate is maintained at 950 K, the total gas pressure is 1.33 Pa, the acceleration voltage of the electron beam is 80 V, and the bias voltage to the substrate is y300 V in all deposition experiments. The hardness of the films was estimated with a nanoindentation tester operated in a continuous stiffness mode in which fused silica was used as a control material. Tribological properties, the friction coefficient and the wear rate, of the films under dry-lubrication were investigated with a ball-on-disc type friction tester. The samples were placed on a table whose temperature regulated at 373 K. The applied load was 1 N and the sliding velocity was 0.1 mys. A counterpart material was a mirror-polished ball with a diameter of 10 mm made of SUJ2 bearing steel. After the sliding distance reached 80 m, a shape of a wear tracks was measured by interferometry with white light, then the wear rate was estimated from a depth and a width of the wear tracks.
B2H6
CH4
N2
He
H2
B
C
N
1 2 3 4 5 6 7 8
8 8 8 8 8 8 8 8
10 10 10 10 10 10 10 10
79 78 77 76 75 74 73 72
32 32 32 32 32 32 32 32
10.6 20.7 28.2 32.6 39.3 46.6 50.3 55.4
75.1 59.3 47.2 41.7 34.7 30.7 28.1 25.2
11.7 17.3 21.1 22.5 21.9 18.8 16.6 14.5
BxCyN
B0.9C6.4N B1.2C3.4N B1.3C2.2N B1.5C1.9N B1.8C1.6N B2.5C1.6N B3.0C1.7N B3.8C1.7N
the films was approximately 200 nm. As shown in a composition triangle for the ternary system B–C–N (Fig. 2), there seems to be a tendency that the composition of the films approaches that of B4C in the case of higher concentration of boron in the process gases. Hardness of the films increases with increasing boron concentration and reaches 30 GPa, as shown in Fig. 3. Friction coefficients obtained from the ball-on-disc tests are in the range of 0.13–0.80 and a dependence of the friction coefficient on the hardness is shown in Fig. 4. B0.9C6.4N film (sample 1) that has the lowest hardness exhibits the lowest friction coefficient and the friction coefficient increases with increasing the hardness of the film. This tendency can be attributed to a formation of a phase with a layered structure, because in a previous paper w9x we found from results of XPS, FT-IR and XRD measurements that the films with the low hardness consist of an sp2-bonded B–C–N structure, while an
3. Results and discussion B–C–N films were deposited under the condition that a B2H6 flow rate was varied keeping CH4 and N2 flow rates constant. The flow rates of process gases and atomic concentrations of B, C and N in deposited films measured with X-ray photoelectron spectroscopy (XPS) are summarized in Table 1 where compositions expressed as BxCyN are also indicated. The thickness of
Fig. 2. Ternary boron, carbon, and nitrogen composition triangle of the films synthesized with varying the B2H6 flow rate, as listed in Table 1.
272
T. Hasegawa et al. / Surface and Coatings Technology 169 – 170 (2003) 270–273
Fig. 3. Hardness of the B–C–N films synthesized under different flow rates of B2H6, as listed in Table 1.
Fig. 5. Dependence of the wear rate on the hardness of the obtained B–C–N films.
sp3-bonded structure appears in the films with high hardness. The similar tendency between the friction coefficient and the hardness was reported for B–C–N films deposited by r.f. magnetron sputtering w10x. A wear rate estimated from a depth and a width of a wear track is in the range of 3.5=10y7 –8.1=10y6 mm3 yN m and is plotted as a function of the hardness in Fig. 5. The wear rate increases with increasing the hardness, and the hardest films (samples 7 and 8) were worn out during the friction test. After the test, partial peelings of the films from the substrate were observed for samples 5 and 6. The relationship between the wear rate and the hardness of the obtained films is opposite of that observed for tetrahedral amorphous carbon (taC) films w11x. It is considered that the cause of the wear for ta-C films is the adhesion between the film and the counterpart material. On the other hand, the wear mechanism for the present B–C–N films may be a microscopic peeling, because the film should become more brittle and the internal stress of the film should become larger for the harder film. This mechanism is also
suggested by the partial peelings observed in the samples 5 and 6, and an irregular fluctuation of friction coefficient–time curves recorded during the friction coefficient measurement. 4. Conclusions Tribological properties, the friction coefficient and the wear rate, have been investigated in relation to the hardness for B–C–N thin films synthesized by an EBEP-CVD method. The hardness of the films increases monotonically with increasing boron content and both the friction coefficient and the wear rate increase with increasing the hardness. From these results the low friction coefficient can be attributed to the formation of a sp2-bonded B–C–N structure. And it is suggested that the cause of the wear for the B–C–N films may be a microscopic peeling, because the film should become more brittle and the internal stress of the film should become larger for the harder film. Acknowledgments This work was supported by the Frontier Carbon Technology (FCT) project, which was consigned to JFCC by NEDO. References
Fig. 4. Dependence of the friction coefficient on the hardness of the obtained B–C–N films.
w1x Y. Kakudate, M. Yoshida, S. Usuba, H. Yokoi, S. Fujiwara, Trans. Mater. Res. Soc. Jpn. Part B 14 (1994) 1447. w2x E. Knittle, R.B. Kaner, R. Jeanloz, M.L. Cohen, Phys. Rev. B 51 (1995) 12149. w3x V.L. Solozhenko, D. Andrault, G. Fiquet, M. Mezouar, D.C. Rubie, Appl. Phys. Lett. 78 (2001) 1385. w4x S. Ulrich, H. Ehrhadt, T. Theel, et al., Diamond Relat. Mater. 7 (1998) 839. w5x Y. Wada, Y.K. Yap, M. Yoshimura, Y. Mori, T. Sasaki, Diamond Relat. Mater. 9 (2000) 620.
T. Hasegawa et al. / Surface and Coatings Technology 169 – 170 (2003) 270–273 w6x A. Lousa, J. Esteve, S. Muhl, E. Martinez, Diamond Relat. Mater. 9 (2000) 502. w7x M.C. Polo, E. Martinez, J. Esteve, J.L. Andujar, ´ Diamond Relat. Mater. 8 (1999) 423. w8x E.H.A. Dekempeneer, J. Meneve, S. Kuypers, J. Smeets, Thin solid films 281 (1996) 331.
273
w9x T. Hasegawa, K. Yamamoto, Y. Kakudate, Diamond Relat. Mater. 11 (2002) 1290. w10x E. Martinez, A. Lousa, J. Esteve, Diamond Relat. Mater. 10 (2001) 1892. w11x E. Martinez, J.L. Andujar, ´ M.C. Polo, J. Esteve, J. Robertson, W.I. Milne, Diamond Relat. Mater. 10 (2001) 145.
Mechanical properties of B–C–N films synthesized by an electron beam excited plasma-CVD method Takeshi Hasegawaa,*,1, Kazuhiro Yamamotob, Yozo Kakudateb, Masahito Bana,c a FCT Project, Japan Fine Ceramics Center, FCT Laboratory, Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan c Technical Institute, Kawasaki Heavy Industries, Ltd., 118 Futatsuzuka, Noda, Chiba 278-8585, Japan
b
Abstract We have synthesized B–C–N films by an electron beam excited plasma-CVD method and characterized structures and properties of the obtained films. In this paper, tribological properties of the films under dry-lubrication were investigated with a ball-on-disc type friction tester. The hardness of the films increases monotonically with increasing boron content and both the friction coefficient and the wear rate increase with increasing the hardness. From these results the low friction coefficient can be attributed to the formation of a sp2-bonded B–C–N structure. And it is suggested that the cause of the wear for the B–C–N films may be a microscopic peeling, because the film should become more brittle and the internal stress of the film should become larger for the harder film. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Plasma-CVD; B–C–N films; Hardness; Friction coefficient; Wear rate
1. Introduction In recent years, materials in the B–C–N system have been investigated very well. Although diamond is the hardest material, its application to machining tools is limited because of the oxidation at high temperature and the poor abrasive resistance for iron-based alloys. On the other hand, while cubic phase BN is next to diamond in the hardness, it is chemically more stable than diamond. Cubic B–C–N phases are expected to be superhard materials and to have high corrosion resistance at high temperature. Therefore it is of great interest and of technological importance to obtain the cubic B–C– N phases and to control their prominent properties by varying their compositions. The cubic B–C–N phases have been synthesized so far only by high pressure-high temperature treatments w1–3x. For the industrial applications, however, it is important to form hard B–C–N films on other materials as mechanical hard coating. Although many attempts to *Corresponding author. Tel.: q81-298-61-9982; fax: q81-298-614796. E-mail address: [email protected] (T. Hasegawa). 1 Under temporary transfer from Kawasaki Heavy Industries, Ltd.
produce the B–C–N films by using various CVD or PVD methods have been reported w4–8x, few studies have dealt the relationship among their composition, structure and mechanical properties. In a previous paper w9x, we reported the synthesis of B–C–N films by an electron beam excited plasma (EBEP)-CVD method and results of analysis of their compositions. In the present study, the mechanical properties of B–C–N films in terms of the hardness, the friction coefficient and the wear rate with varying deposition conditions are investigated. 2. Experimental procedure B–C–N films are synthesized with the EBEP-CVD system, a schematic of which is shown in Fig. 1. An argon d.c. glow-discharge is generated by using thermal electrons from a LaB6 filament as a trigger. By applying a voltage between a discharge and acceleration electrodes, an electron beam is extracted from the discharge electrode and directed into a process chamber to produce a plasma of process gases. A substrate is heated by radiation from a graphite heater. A negative bias is induced to the substrate with a 2 MHz r.f. generator.
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 0 3 9 - 2
T. Hasegawa et al. / Surface and Coatings Technology 169 – 170 (2003) 270–273
271
Table 1 Fraction of process gases and composition of the films No.
Flow rate
Atomic concentration in the film (%)
(sccm)
1 2 3 4 5 6 7 8
Fig. 1. Schematic diagram of the EBEP-CVD system.
B–C–N films were deposited from mixtures of 10% B2H6 in He, CH4, N2 and H2 on polished p-type Si (1 0 0) substrates which were in situ pre etched by an argon plasma for 10 min at 0.13 Pa. A temperature of the substrate is maintained at 950 K, the total gas pressure is 1.33 Pa, the acceleration voltage of the electron beam is 80 V, and the bias voltage to the substrate is y300 V in all deposition experiments. The hardness of the films was estimated with a nanoindentation tester operated in a continuous stiffness mode in which fused silica was used as a control material. Tribological properties, the friction coefficient and the wear rate, of the films under dry-lubrication were investigated with a ball-on-disc type friction tester. The samples were placed on a table whose temperature regulated at 373 K. The applied load was 1 N and the sliding velocity was 0.1 mys. A counterpart material was a mirror-polished ball with a diameter of 10 mm made of SUJ2 bearing steel. After the sliding distance reached 80 m, a shape of a wear tracks was measured by interferometry with white light, then the wear rate was estimated from a depth and a width of the wear tracks.
B2H6
CH4
N2
He
H2
B
C
N
1 2 3 4 5 6 7 8
8 8 8 8 8 8 8 8
10 10 10 10 10 10 10 10
79 78 77 76 75 74 73 72
32 32 32 32 32 32 32 32
10.6 20.7 28.2 32.6 39.3 46.6 50.3 55.4
75.1 59.3 47.2 41.7 34.7 30.7 28.1 25.2
11.7 17.3 21.1 22.5 21.9 18.8 16.6 14.5
BxCyN
B0.9C6.4N B1.2C3.4N B1.3C2.2N B1.5C1.9N B1.8C1.6N B2.5C1.6N B3.0C1.7N B3.8C1.7N
the films was approximately 200 nm. As shown in a composition triangle for the ternary system B–C–N (Fig. 2), there seems to be a tendency that the composition of the films approaches that of B4C in the case of higher concentration of boron in the process gases. Hardness of the films increases with increasing boron concentration and reaches 30 GPa, as shown in Fig. 3. Friction coefficients obtained from the ball-on-disc tests are in the range of 0.13–0.80 and a dependence of the friction coefficient on the hardness is shown in Fig. 4. B0.9C6.4N film (sample 1) that has the lowest hardness exhibits the lowest friction coefficient and the friction coefficient increases with increasing the hardness of the film. This tendency can be attributed to a formation of a phase with a layered structure, because in a previous paper w9x we found from results of XPS, FT-IR and XRD measurements that the films with the low hardness consist of an sp2-bonded B–C–N structure, while an
3. Results and discussion B–C–N films were deposited under the condition that a B2H6 flow rate was varied keeping CH4 and N2 flow rates constant. The flow rates of process gases and atomic concentrations of B, C and N in deposited films measured with X-ray photoelectron spectroscopy (XPS) are summarized in Table 1 where compositions expressed as BxCyN are also indicated. The thickness of
Fig. 2. Ternary boron, carbon, and nitrogen composition triangle of the films synthesized with varying the B2H6 flow rate, as listed in Table 1.
272
T. Hasegawa et al. / Surface and Coatings Technology 169 – 170 (2003) 270–273
Fig. 3. Hardness of the B–C–N films synthesized under different flow rates of B2H6, as listed in Table 1.
Fig. 5. Dependence of the wear rate on the hardness of the obtained B–C–N films.
sp3-bonded structure appears in the films with high hardness. The similar tendency between the friction coefficient and the hardness was reported for B–C–N films deposited by r.f. magnetron sputtering w10x. A wear rate estimated from a depth and a width of a wear track is in the range of 3.5=10y7 –8.1=10y6 mm3 yN m and is plotted as a function of the hardness in Fig. 5. The wear rate increases with increasing the hardness, and the hardest films (samples 7 and 8) were worn out during the friction test. After the test, partial peelings of the films from the substrate were observed for samples 5 and 6. The relationship between the wear rate and the hardness of the obtained films is opposite of that observed for tetrahedral amorphous carbon (taC) films w11x. It is considered that the cause of the wear for ta-C films is the adhesion between the film and the counterpart material. On the other hand, the wear mechanism for the present B–C–N films may be a microscopic peeling, because the film should become more brittle and the internal stress of the film should become larger for the harder film. This mechanism is also
suggested by the partial peelings observed in the samples 5 and 6, and an irregular fluctuation of friction coefficient–time curves recorded during the friction coefficient measurement. 4. Conclusions Tribological properties, the friction coefficient and the wear rate, have been investigated in relation to the hardness for B–C–N thin films synthesized by an EBEP-CVD method. The hardness of the films increases monotonically with increasing boron content and both the friction coefficient and the wear rate increase with increasing the hardness. From these results the low friction coefficient can be attributed to the formation of a sp2-bonded B–C–N structure. And it is suggested that the cause of the wear for the B–C–N films may be a microscopic peeling, because the film should become more brittle and the internal stress of the film should become larger for the harder film. Acknowledgments This work was supported by the Frontier Carbon Technology (FCT) project, which was consigned to JFCC by NEDO. References
Fig. 4. Dependence of the friction coefficient on the hardness of the obtained B–C–N films.
w1x Y. Kakudate, M. Yoshida, S. Usuba, H. Yokoi, S. Fujiwara, Trans. Mater. Res. Soc. Jpn. Part B 14 (1994) 1447. w2x E. Knittle, R.B. Kaner, R. Jeanloz, M.L. Cohen, Phys. Rev. B 51 (1995) 12149. w3x V.L. Solozhenko, D. Andrault, G. Fiquet, M. Mezouar, D.C. Rubie, Appl. Phys. Lett. 78 (2001) 1385. w4x S. Ulrich, H. Ehrhadt, T. Theel, et al., Diamond Relat. Mater. 7 (1998) 839. w5x Y. Wada, Y.K. Yap, M. Yoshimura, Y. Mori, T. Sasaki, Diamond Relat. Mater. 9 (2000) 620.
T. Hasegawa et al. / Surface and Coatings Technology 169 – 170 (2003) 270–273 w6x A. Lousa, J. Esteve, S. Muhl, E. Martinez, Diamond Relat. Mater. 9 (2000) 502. w7x M.C. Polo, E. Martinez, J. Esteve, J.L. Andujar, ´ Diamond Relat. Mater. 8 (1999) 423. w8x E.H.A. Dekempeneer, J. Meneve, S. Kuypers, J. Smeets, Thin solid films 281 (1996) 331.
273
w9x T. Hasegawa, K. Yamamoto, Y. Kakudate, Diamond Relat. Mater. 11 (2002) 1290. w10x E. Martinez, A. Lousa, J. Esteve, Diamond Relat. Mater. 10 (2001) 1892. w11x E. Martinez, J.L. Andujar, ´ M.C. Polo, J. Esteve, J. Robertson, W.I. Milne, Diamond Relat. Mater. 10 (2001) 145.