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Friction behavior of amorphous-BN—cubic-BN dual-layered film
8#IIBIJE
COATIN#S
ELSEVIER
##l#OLO? Surface and Coatings Technology 76-77 (1995) 600-603
Friction behavior of amorphous-BN-cubic-BN dual-layered film S. Watanabe, S. Miyake, M. Murakawa Nippon Institute of Technology, 4-I Gakuendai, Miyashiro, Saitama 345, Japan
Abstract
In a tribological comparison in which two kinds of BN films, i.e., cubic BN (c-BN) and amorphous BN (a-BN), were chosen as the test films, cubic BN film was found to have excellent tribological properties such as low friction coefficient and good wear resistance in air. Compared with c-BN film, the amorphous BN film was found to have a lower wear resistance, although it showed a lower friction coefficient. Therefore, it is highly desirable to combine these two excellent properties of c-BN and a-BN films, i.e., the good wear resistance of c-BN and the low friction property of a-BN. In order to achieve such a combination of properties, we conceived the idea of fabricating a dual-layered BN film having amorphous-cubic structure. More specifically, a first layer of c-BN is deposited onto a Si substrate by means of ion plating, and then a second layer of a-BN is deposited onto the c-BN layer with a gradient interlayer formed between the two BN layers. The tribological property of this film has been examined using a reciprocating tribometer under a wide load range of 0.1-4.9 N. The result shows that this combined c- and a-BN film has a superior low friction property and good wear resistance.
Keywords: BN films; Tribological properties; Friction and wear behaviour
1. Introduction Cubic boron nitride films (c-BN) can be formed by the magnetically enhanced plasma ion plating (MEP-IP) [' 1 ] developed by us. It is a reactive ion plating method using a hot-cathode plasma discharge in a parallel magnetic field. Because the hardness of c-BN is second to that of diamond, these c-BN films should be highly resistant to wear. They will be especially useful for coating surfaces that come into contact with iron-based materials where diamond cannot be used because of its high chemical affinity for iron. Accordingly, extensive research and development on the preparation of c-BN film by various physical and chemical vapor deposition processes have been carried out. However, although it is important that the tribological properties of c-BN films are investigated in detail, only three reports dealing with this topic have been published ['2-4], owing probably to the difficulty of synthesizing c-BN film itself. In these previous reports, the friction coefficients and specific wear rates of c-BN films were compared with those of various metals, ceramics and TiN films. These results show that frictional properties of the previous c-BN films are good. Accordingly, we wanted to know whether our c-BN film made by the MEP-IP method could also show the same good frictional properties as those shown by the 0257-8972/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved
previous c-BN films. As a first step of the test for our c-BN film we performed a tribological test in which quenched stainless steel (JIS SUS440C, AISI 440C) was chosen as a indenter which comes into contact with the c-BN film. In this test, we also carried out similar tests on the tribologicat properties of amorphous and hexagonal BN films prepared by the MEP-IP method to compare the results with those of the c-BN film. As expected, our c-BN film was also confirmed to have high wear resistance owing to its high strength [5]. It was also found, in another frictional test in which a diamond indenter was used instead of the stainless steel indenter, that a friction system between c-BN film and diamond as the contacting material shows a very low friction coefficient, exhibiting values of around 0.04 at a normal load of 4.9 N, which leads to less wear damage [-6]. This shows that, in a frictional system in which fracture formation and defect growth by sliding are negligible, high-strength materials such as diamond or c-BN can be recommended as practical surface materials for reducing wear [7]. From the results for both stainless steel and diamond indenters, it was clear that c-BN film can show good wear resistance. However, from the results for the stainless indenter it was clear that amorphous BN film was found to have lower wear resistance compared with c-BN film, although it showed a lower friction coefficient. Therefore, we thought it would be
S. Watanabe et al./Surface and Coatings Technology 76-77 (1995) 600-603
highly desirable to combine these two excellent properties of c-BN and a-BN films, i.e., the good wear resistance of c-BN and the low friction property of a-BN. In this work, we put into practice the idea of fabricating a dual-layered BN film having amorphouscubic structure in order to achieve such a combination of properties. More specifically, a first layer of c-BN is deposited onto a Si substrate by means of ion plating, and then a second layer of a-BN is deposited onto the c-BN layer with a gradient interlayer formed between the two BN layers. The tribological property of this film against two kinds of indenters, i.e., stainless steel and diamond indenters, has been examined using a reciprocating tribometer under a wide load range of 0.1-4.9 N.
2. Experimental details 2.I. Preparation of BN filrns The dual-layered film (a-/c-BN) was deposited onto a Si wafer (111) substrate by the MEP-IP method. A schematic diagram of the equipment used is shown in Fig. 1. The details of the equipment used are described in our previous paper [11. The film consists of two layers. Namely, a first layer of c-BN is deposited onto the Si substrate by the graded composition controlling technique [8], inserting a titanium interlayer between the gradient layer and the substrate to improve the adhesion of the film. Then a second layer of a-BN is further deposited onto the c-BN layer with a thin gradient interlayer, containing B and N, formed between the two BN layers. This can be accomplished by changing the deposition condition so that the starting condition, under which stoichiometric composition (c-BN composition) is obtained, is gradually varied toward a final condition under which amorphous or boron-rich
60i
composition (a-BN composition) can be obtained. The electron beam evaporant of 99.9% pure boron was used. The final film preparation conditions for c-BN and a-BN are shown in Table 1. The thickness of each layer estimated from our deposition rate of BN films was as follows: about 550 nm for the first c-BN layer, including the Ti interlayer with a thickness of 150 nm, and 100-150 nm for the upper a-BN layer. 2.2. Friction test A reciprocal friction testing machine [97 with a specimen holder, as shown in Fig. 2, was used for friction tests. Load was applied perpendicularly to the surface of the specimen from the z-axis direction, as shown in Fig. 2, to cause friction while the specimen holder moved reciprocally along the x-axis. To obtain the friction force by measuring the amount of deflection, a plate spring with a strain gauge was mounted on the end of a ball indenter arm. The reciprocal movement of the specimen holder was provided by a hydraulic cylinder to minimize disturbance such as vibration. The ball indenter was 6.35 mm in diameter and was made of quenched stainless steel JIS SUS440C (AISI 440C, with Vickers hardness of about 4.3 GPa, and surface roughness (Rmax) of less than 0.1 micrometer). The diamond indenter, with a tip Table 1 Deposition conditions for preparing the BN films
Total gas pressure (Pa) Ar/N a gas flow ratio Electron beam evaporant Electron beam power (kW) Discharge potential (V) Discharge current (A) Substrate bias (r.f.) (W) Substrate temperature (K)
c-BN
a-BN
6.6 x 10 .2 9 Boron (99.9%) 2.0 60 14 ~300 (13.56 MHz) ~ 673
6.6 x 10 -2 24 Boron 2.0 60 10 ~220 ~ 673
Z-axis I to
~ag~et 1
,
,
,
~gnet 2
Pumping~e-System
.~-axis~
inden~er,,~,,,,, ~
~
s/%ecimen
specimen
specimen holder X-axis/-" Gas (Ar, N2) Fig. 1. Schematic diagram of the present ion-plating (MEP-IP) system.
/
Fig. 2. Schematic illustration of the specimen hoider in a reciprocal friction testing machine.
S. Watanabe et al./Swface and Coatings Technology 76-77 (1995) 600-603
602
radius of 0.5 mm, was also used in the experiment in the same manner as in a previot~s study [6]. The diamond indenter was made of natural diamond with finished surfaces ground to roughness (Rmax) of less than 0.1 lam. The friction tests were performed at room temperature ani:t under an air atmosphere with 45% relative humidity in a semiclean room; the sliding distance of one stroke, constant sliding speed and normal load were 10 ram, 1.67 m m s -* and 0.1-4.9 N, respectively.
3.: Results and discussion The film structure was-characterized b y IR spectroscopy. Fig. 3 (c) shows an IR spectrum of the a-/c-BN dual-layered film. In Fig. 3, spectra of the first c-BN film (Fig. 3(a)) and upper a-BN film (Fig: 3(b)) are also shown. These two samples were prepared individually with the deposition conditions corresponding to the first c-BN and the upper a-BN layers in*order to check the film structure. These spectra in Fig. 3 were measured using an FT-IR spectrometer (Hitachi 1-5040, resolution of wave number 2cm -~) in the transmittance mode, with a bare Si wafer substrate as a reference. In Fig. 3 (a), a,ciear characteristic absorption band of c-BN phase [ 101 is observed at a wave number of.around 1t00 cmand two bands corresponding to the h-BN phase [11] in the graded composition-controlled layer are also observed at around 1400cm-~ and 800cmFurthermore, in spectrum (b), a characteristic broad band of a-BN [12] at around 1400 cm -* and a band at 800 cm -z are observed. As can be seen, it is confirmed
that the film showing spectrum (c) is formed by stacking films showing spectra (a) and (b). Fig. 4 shows the effect of load on the friction coefficient of the dual-layered film against the SUS440C indenter. In this figure, the results obtained from the c-BN film are also shown for comparison. To eliminate the effects of unstable factors in the initial stage of testing, these friction coefficients were calculated from the ratio of frictional force to normal force at the fifth reciprocating number. The friction coefficient of the dual-layered film is lower than that of the c-BN film under a normal load of over 0.2 N. In particular, it is found that this difference is substantial under a load of over 0.4 N. The friction coefficient of the dual-layered film was nearly equal to that of a-BN film [5]. The changes in friction coefficients of the dual-layered film against the SUS440C indenter for a number of reciprocating cycles are shown in Fig. 5. The results obtained from c-BN film are also shown in tl~is figure for comparison. The figure clearly indicates that the friction coefficient of the dual-layered film is lower than that of c-BN film, and that this low friction coefficient remains stable without large change, even if the number of reciprocations is increased. In addition, observation 0.4
I
I I I }111
I
I
I
I I IIII
I
I
I
A: a-BN/c-BN dual-layered film O: o:BN film [Indenter: S~440C (radius: 3. 175 ~ ) ]
,-0.3
1 i
111
i,it
,
'3
)hb-
'~0.2 o=
~4
"~ 0.1 a)
h-BN
c-BN
h-BN 0
0.01
0.1
0.5
1.
5,0
10,0
Indentation load (N)
Fig. 4. Dependence of friction coefficients of a-BN-c-BN and e-BN films against SUS440C indenter on normal load.
08
,!a_BNIo_NIdual-layered I l filmI I I O: c-8N film [Indenter: SUS440C (radius: 3. 175 ~n), indentation load: 2.5 N]
'5 0.6" =
904 g
I
I
-
r.~ 0" 2 i-.--~ ~
2000
1800
1600
1400
1200
1000
800
650
Wave number (ca "1)
Fig. 3. IR spectra of BN films in this experiment: (a) first layer of c-BN. film, (b) upper layer of a-BN film and (c) a-BN-c-BN duallayered film.
0
0
10
,,
20
I _..~ ~_~ r----~
,___ ~ . . _ __._~ .---~ r ----'~
30
40
50
60
70
80
90
100
Number of reciprocating cycles
Fig. 5. Changes of friction coefficient of a-BN-c-BN and c-BN films against SUS440C with number of reciprocation cycles at indentation load of 2.5 N.
S. Watanabe et at /Sutface and Coatings Technology 76-77 (1995) 600-603
= 2
:llllll I l lIII]ll I I11!Ill ' ,~: a-BN/c-BN dual-layered fi Im 0.15
0
0.01
Q : c-BN film Indenter: Diamond (radius: O.
0.1
0.5
1.0
an)]
5.0 I0.0
Indentation load (S)
Fig. 6. Dependence of friction coefficients of a-BN-c-BN and c-BN films against diamond indenter on normal load.
of the friction surface after 100 reciprocations with optical microscopy and/or SEM indicates no clear wear marks and an extremely small amount of the indenter material adhering to the film. The gradual increase in the friction coefficient may be ascribed to the gradual adhesion of the indenter material onto the film surface. However, no distinct wear damage of the film was seen, as in the case of c-BN film [5% so it is confirmed that in this friction environment the dual-layered film shows good wear resistance. Fig. 6 shows the dependence of the friction coefficient on load for the dual-layered film against a diamond indenter with tip radius of 0.5 mm. The friction coefficient of c-BN film is also shown for comparison. From this figure, it is seen that the friction coefficient of the dual-layered film is lower than that of the c-BN film throughout the entire load range, and is very low, exhibiting values of about 0.037, with a normal load of 4.9 N. This tendency is similar to the case of the SUS440C indenter. When the friction test is extended up to 100 reciprocation cycles, no clear damage of the dual-layered film is detected by visual inspection, as in the case of c-BN film [6], so it is suspected that the dual-layered film also has good wear resistance to the diamond indenter.
603
4. Conclusions
This paper dealt with the idea of fabricating a duallayered BN film having amorphous--cubic structure in order to combine the two excellent tribological properties of c-BN and a-BN films, that is, the good wear resistance of c-BN and the low friction of a-BN. The results show that this a-/c-BN dual-layered film has better tribological property than a single layer of c-BN film, i.e., this combined film has a very low friction property, which leads to less wear damage. Furthermore, it was confirmed that this good friction property was observed in the two cases of dual-layered films in contact with SUS440C and diamond indenters.
References [1] M. Murakawa and S. Watanabe, Surf. Coat. Teehnnol., 43/44 (1990) 128. [2"] K. Inagawa, K. Watanabe, K. Saitoh, Y. Yuchi and A. Itoh, Surf. Coat. TeehnnoI., 39/40 (1989) 253. [3"] S. Mineta, M. Kohata, N. Yasunaga and Y. Kikuta, Thin Solid Films, 189 (i990) 125. [4"] K. Bewilogua, J. Buth, H. Hubsch and M. Grischke, Diamond Related Mater., 2 (1993) 1206. [5"] S. Watanabe, S. Miyake and M. Murakawa, Surf. Coat. TechnnoI., 49 (1991) 406. [6"] S. Watanabe, S. Miyake and M. Murakawa, Trans. ASME, J. TriboI., in press. [71 S. Miyake, R. Kaneko, Y. Kikuya and I. Sugimoto, Trans. ASME, J. Tribol., 113 (1991) 384. [81 M. Murakawa, S. Watanabe and S. Miyake, Mater. Sci. Eng. A, I40 (1991) 753. [9.] S. Miyake, .1. Jpn. Soc, TriboIogists, 28 (1983) 419 (in Japanese). [10.] P.J. Gietisse, S.S. Mitra, J.N. Plendl, R.D. Griffis, L.C. Mansur, R. Marshall and E.A. Pascoe, Phys. Rev., I55 (1967) 1039. [1i.] R. Geick, C.H. Perry and G. Rupprecht, Phys. Rev., i46 (1966)543. [12"1 M. Murakawa and S. Watanabe, Swf. Coat. Technnol.; 43/44 (1990) 145.
COATIN#S
ELSEVIER
##l#OLO? Surface and Coatings Technology 76-77 (1995) 600-603
Friction behavior of amorphous-BN-cubic-BN dual-layered film S. Watanabe, S. Miyake, M. Murakawa Nippon Institute of Technology, 4-I Gakuendai, Miyashiro, Saitama 345, Japan
Abstract
In a tribological comparison in which two kinds of BN films, i.e., cubic BN (c-BN) and amorphous BN (a-BN), were chosen as the test films, cubic BN film was found to have excellent tribological properties such as low friction coefficient and good wear resistance in air. Compared with c-BN film, the amorphous BN film was found to have a lower wear resistance, although it showed a lower friction coefficient. Therefore, it is highly desirable to combine these two excellent properties of c-BN and a-BN films, i.e., the good wear resistance of c-BN and the low friction property of a-BN. In order to achieve such a combination of properties, we conceived the idea of fabricating a dual-layered BN film having amorphous-cubic structure. More specifically, a first layer of c-BN is deposited onto a Si substrate by means of ion plating, and then a second layer of a-BN is deposited onto the c-BN layer with a gradient interlayer formed between the two BN layers. The tribological property of this film has been examined using a reciprocating tribometer under a wide load range of 0.1-4.9 N. The result shows that this combined c- and a-BN film has a superior low friction property and good wear resistance.
Keywords: BN films; Tribological properties; Friction and wear behaviour
1. Introduction Cubic boron nitride films (c-BN) can be formed by the magnetically enhanced plasma ion plating (MEP-IP) [' 1 ] developed by us. It is a reactive ion plating method using a hot-cathode plasma discharge in a parallel magnetic field. Because the hardness of c-BN is second to that of diamond, these c-BN films should be highly resistant to wear. They will be especially useful for coating surfaces that come into contact with iron-based materials where diamond cannot be used because of its high chemical affinity for iron. Accordingly, extensive research and development on the preparation of c-BN film by various physical and chemical vapor deposition processes have been carried out. However, although it is important that the tribological properties of c-BN films are investigated in detail, only three reports dealing with this topic have been published ['2-4], owing probably to the difficulty of synthesizing c-BN film itself. In these previous reports, the friction coefficients and specific wear rates of c-BN films were compared with those of various metals, ceramics and TiN films. These results show that frictional properties of the previous c-BN films are good. Accordingly, we wanted to know whether our c-BN film made by the MEP-IP method could also show the same good frictional properties as those shown by the 0257-8972/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved
previous c-BN films. As a first step of the test for our c-BN film we performed a tribological test in which quenched stainless steel (JIS SUS440C, AISI 440C) was chosen as a indenter which comes into contact with the c-BN film. In this test, we also carried out similar tests on the tribologicat properties of amorphous and hexagonal BN films prepared by the MEP-IP method to compare the results with those of the c-BN film. As expected, our c-BN film was also confirmed to have high wear resistance owing to its high strength [5]. It was also found, in another frictional test in which a diamond indenter was used instead of the stainless steel indenter, that a friction system between c-BN film and diamond as the contacting material shows a very low friction coefficient, exhibiting values of around 0.04 at a normal load of 4.9 N, which leads to less wear damage [-6]. This shows that, in a frictional system in which fracture formation and defect growth by sliding are negligible, high-strength materials such as diamond or c-BN can be recommended as practical surface materials for reducing wear [7]. From the results for both stainless steel and diamond indenters, it was clear that c-BN film can show good wear resistance. However, from the results for the stainless indenter it was clear that amorphous BN film was found to have lower wear resistance compared with c-BN film, although it showed a lower friction coefficient. Therefore, we thought it would be
S. Watanabe et al./Surface and Coatings Technology 76-77 (1995) 600-603
highly desirable to combine these two excellent properties of c-BN and a-BN films, i.e., the good wear resistance of c-BN and the low friction property of a-BN. In this work, we put into practice the idea of fabricating a dual-layered BN film having amorphouscubic structure in order to achieve such a combination of properties. More specifically, a first layer of c-BN is deposited onto a Si substrate by means of ion plating, and then a second layer of a-BN is deposited onto the c-BN layer with a gradient interlayer formed between the two BN layers. The tribological property of this film against two kinds of indenters, i.e., stainless steel and diamond indenters, has been examined using a reciprocating tribometer under a wide load range of 0.1-4.9 N.
2. Experimental details 2.I. Preparation of BN filrns The dual-layered film (a-/c-BN) was deposited onto a Si wafer (111) substrate by the MEP-IP method. A schematic diagram of the equipment used is shown in Fig. 1. The details of the equipment used are described in our previous paper [11. The film consists of two layers. Namely, a first layer of c-BN is deposited onto the Si substrate by the graded composition controlling technique [8], inserting a titanium interlayer between the gradient layer and the substrate to improve the adhesion of the film. Then a second layer of a-BN is further deposited onto the c-BN layer with a thin gradient interlayer, containing B and N, formed between the two BN layers. This can be accomplished by changing the deposition condition so that the starting condition, under which stoichiometric composition (c-BN composition) is obtained, is gradually varied toward a final condition under which amorphous or boron-rich
60i
composition (a-BN composition) can be obtained. The electron beam evaporant of 99.9% pure boron was used. The final film preparation conditions for c-BN and a-BN are shown in Table 1. The thickness of each layer estimated from our deposition rate of BN films was as follows: about 550 nm for the first c-BN layer, including the Ti interlayer with a thickness of 150 nm, and 100-150 nm for the upper a-BN layer. 2.2. Friction test A reciprocal friction testing machine [97 with a specimen holder, as shown in Fig. 2, was used for friction tests. Load was applied perpendicularly to the surface of the specimen from the z-axis direction, as shown in Fig. 2, to cause friction while the specimen holder moved reciprocally along the x-axis. To obtain the friction force by measuring the amount of deflection, a plate spring with a strain gauge was mounted on the end of a ball indenter arm. The reciprocal movement of the specimen holder was provided by a hydraulic cylinder to minimize disturbance such as vibration. The ball indenter was 6.35 mm in diameter and was made of quenched stainless steel JIS SUS440C (AISI 440C, with Vickers hardness of about 4.3 GPa, and surface roughness (Rmax) of less than 0.1 micrometer). The diamond indenter, with a tip Table 1 Deposition conditions for preparing the BN films
Total gas pressure (Pa) Ar/N a gas flow ratio Electron beam evaporant Electron beam power (kW) Discharge potential (V) Discharge current (A) Substrate bias (r.f.) (W) Substrate temperature (K)
c-BN
a-BN
6.6 x 10 .2 9 Boron (99.9%) 2.0 60 14 ~300 (13.56 MHz) ~ 673
6.6 x 10 -2 24 Boron 2.0 60 10 ~220 ~ 673
Z-axis I to
~ag~et 1
,
,
,
~gnet 2
Pumping~e-System
.~-axis~
inden~er,,~,,,,, ~
~
s/%ecimen
specimen
specimen holder X-axis/-" Gas (Ar, N2) Fig. 1. Schematic diagram of the present ion-plating (MEP-IP) system.
/
Fig. 2. Schematic illustration of the specimen hoider in a reciprocal friction testing machine.
S. Watanabe et al./Swface and Coatings Technology 76-77 (1995) 600-603
602
radius of 0.5 mm, was also used in the experiment in the same manner as in a previot~s study [6]. The diamond indenter was made of natural diamond with finished surfaces ground to roughness (Rmax) of less than 0.1 lam. The friction tests were performed at room temperature ani:t under an air atmosphere with 45% relative humidity in a semiclean room; the sliding distance of one stroke, constant sliding speed and normal load were 10 ram, 1.67 m m s -* and 0.1-4.9 N, respectively.
3.: Results and discussion The film structure was-characterized b y IR spectroscopy. Fig. 3 (c) shows an IR spectrum of the a-/c-BN dual-layered film. In Fig. 3, spectra of the first c-BN film (Fig. 3(a)) and upper a-BN film (Fig: 3(b)) are also shown. These two samples were prepared individually with the deposition conditions corresponding to the first c-BN and the upper a-BN layers in*order to check the film structure. These spectra in Fig. 3 were measured using an FT-IR spectrometer (Hitachi 1-5040, resolution of wave number 2cm -~) in the transmittance mode, with a bare Si wafer substrate as a reference. In Fig. 3 (a), a,ciear characteristic absorption band of c-BN phase [ 101 is observed at a wave number of.around 1t00 cmand two bands corresponding to the h-BN phase [11] in the graded composition-controlled layer are also observed at around 1400cm-~ and 800cmFurthermore, in spectrum (b), a characteristic broad band of a-BN [12] at around 1400 cm -* and a band at 800 cm -z are observed. As can be seen, it is confirmed
that the film showing spectrum (c) is formed by stacking films showing spectra (a) and (b). Fig. 4 shows the effect of load on the friction coefficient of the dual-layered film against the SUS440C indenter. In this figure, the results obtained from the c-BN film are also shown for comparison. To eliminate the effects of unstable factors in the initial stage of testing, these friction coefficients were calculated from the ratio of frictional force to normal force at the fifth reciprocating number. The friction coefficient of the dual-layered film is lower than that of the c-BN film under a normal load of over 0.2 N. In particular, it is found that this difference is substantial under a load of over 0.4 N. The friction coefficient of the dual-layered film was nearly equal to that of a-BN film [5]. The changes in friction coefficients of the dual-layered film against the SUS440C indenter for a number of reciprocating cycles are shown in Fig. 5. The results obtained from c-BN film are also shown in tl~is figure for comparison. The figure clearly indicates that the friction coefficient of the dual-layered film is lower than that of c-BN film, and that this low friction coefficient remains stable without large change, even if the number of reciprocations is increased. In addition, observation 0.4
I
I I I }111
I
I
I
I I IIII
I
I
I
A: a-BN/c-BN dual-layered film O: o:BN film [Indenter: S~440C (radius: 3. 175 ~ ) ]
,-0.3
1 i
111
i,it
,
'3
)hb-
'~0.2 o=
~4
"~ 0.1 a)
h-BN
c-BN
h-BN 0
0.01
0.1
0.5
1.
5,0
10,0
Indentation load (N)
Fig. 4. Dependence of friction coefficients of a-BN-c-BN and e-BN films against SUS440C indenter on normal load.
08
,!a_BNIo_NIdual-layered I l filmI I I O: c-8N film [Indenter: SUS440C (radius: 3. 175 ~n), indentation load: 2.5 N]
'5 0.6" =
904 g
I
I
-
r.~ 0" 2 i-.--~ ~
2000
1800
1600
1400
1200
1000
800
650
Wave number (ca "1)
Fig. 3. IR spectra of BN films in this experiment: (a) first layer of c-BN. film, (b) upper layer of a-BN film and (c) a-BN-c-BN duallayered film.
0
0
10
,,
20
I _..~ ~_~ r----~
,___ ~ . . _ __._~ .---~ r ----'~
30
40
50
60
70
80
90
100
Number of reciprocating cycles
Fig. 5. Changes of friction coefficient of a-BN-c-BN and c-BN films against SUS440C with number of reciprocation cycles at indentation load of 2.5 N.
S. Watanabe et at /Sutface and Coatings Technology 76-77 (1995) 600-603
= 2
:llllll I l lIII]ll I I11!Ill ' ,~: a-BN/c-BN dual-layered fi Im 0.15
0
0.01
Q : c-BN film Indenter: Diamond (radius: O.
0.1
0.5
1.0
an)]
5.0 I0.0
Indentation load (S)
Fig. 6. Dependence of friction coefficients of a-BN-c-BN and c-BN films against diamond indenter on normal load.
of the friction surface after 100 reciprocations with optical microscopy and/or SEM indicates no clear wear marks and an extremely small amount of the indenter material adhering to the film. The gradual increase in the friction coefficient may be ascribed to the gradual adhesion of the indenter material onto the film surface. However, no distinct wear damage of the film was seen, as in the case of c-BN film [5% so it is confirmed that in this friction environment the dual-layered film shows good wear resistance. Fig. 6 shows the dependence of the friction coefficient on load for the dual-layered film against a diamond indenter with tip radius of 0.5 mm. The friction coefficient of c-BN film is also shown for comparison. From this figure, it is seen that the friction coefficient of the dual-layered film is lower than that of the c-BN film throughout the entire load range, and is very low, exhibiting values of about 0.037, with a normal load of 4.9 N. This tendency is similar to the case of the SUS440C indenter. When the friction test is extended up to 100 reciprocation cycles, no clear damage of the dual-layered film is detected by visual inspection, as in the case of c-BN film [6], so it is suspected that the dual-layered film also has good wear resistance to the diamond indenter.
603
4. Conclusions
This paper dealt with the idea of fabricating a duallayered BN film having amorphous--cubic structure in order to combine the two excellent tribological properties of c-BN and a-BN films, that is, the good wear resistance of c-BN and the low friction of a-BN. The results show that this a-/c-BN dual-layered film has better tribological property than a single layer of c-BN film, i.e., this combined film has a very low friction property, which leads to less wear damage. Furthermore, it was confirmed that this good friction property was observed in the two cases of dual-layered films in contact with SUS440C and diamond indenters.
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