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Tribological properties of polymer composites with diamond-like carbon flakes
Diamond & Related Materials 19 (2010) 894–898
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d i a m o n d
Tribological properties of polymer composites with diamond-like carbon flakes Tsuguyori Ohana ⁎, Takako Nakamura, Akihiro Tanaka National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Available online 22 February 2010 Keywords: Diamond-like carbon Powder Composites Tribological properties
a b s t r a c t Composites of epoxy resin with diamond-like carbon (DLC) flakes were fabricated. The DLC flakes were prepared from a DLC film deposited by chemical vapor deposition on an aluminum substrate. The tribological properties of composites were evaluated in air and water environments using a reciprocating friction tester and an AISI 440C mating ball. The friction coefficient of the epoxy composite decreased from 0.90 to 0.69 in air and from 0.71 to 0.29 in water with the addition of DLC flakes. The specific wear rate of the composite also decreased from 5 × 10− 5 to 7 × 10− 6 mm3/N m in air and from 4 × 10− 5 to 4 × 10− 6 mm3/N m in water. In contrast, the wear of the mating ball increased. Furthermore, the tribological properties of DLC flakes as an additive in water were evaluated. The suspension of powdered DLC in water reduced the friction coefficient of epoxy resin against the AISI 440C mating ball. Furthermore, the wear of the resin was negligibly small, although severe abrasive wear on the mating ball was observed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction New carbon materials such as carbon nanotubes (CNTs) [1] and carbon nanohones (CNHs) [2,3] were discovered at the end of the 20th century. These new materials have been widely investigated for possible applications, because it was thought they had high potential abilities for innovation of industry. These materials also have the potential for mechanical use [4–6]. The CNHs aggregate to form spherical particles with multiple horns and are expected to exhibit good tribological properties due to the spherical shape with submicron diameter. Previously, we investigated the tribological properties of CNTs and CNHs as additives in polymer-based composites [7]. We found that the use of these materials reduced wear. Furthermore, the friction coefficients of the composites were lower than that of the pure polymer. Therefore, we demonstrated that these new carbon nanomaterials have the potential to be used in mechanical applications. On the other hand, it is known that diamond-like carbon (DLC) films are hard with low friction properties. Therefore, DLC films have been widely applied as protective films and as a solid lubricant [8,9]. However, the application of DLC films is limited, because the processes used to apply a DLC film coating are commonly performed in a vacuum chamber. Generally, the deposition of DLC film has been carried out by a plasma process (chemical or physical vapor deposition). The size of the work material to be coated is thus limited by the size of vacuum chamber. Furthermore, the shape of the work material is also significant. It is expected that DLC films can be used to coat various sliding parts. If DLC film can be used as a solid lubricant
⁎ Corresponding author. E-mail address: [email protected] (T. Ohana). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.02.020
similar to graphite, its range of applications will be greatly expanded. Previously, we investigated the tribological properties of DLC film in a water environment [10,11]. The DLC film exhibited a low friction coefficient and high wear resistance. Interestingly, the low friction coefficient of the DLC film was retained even though partial delamination was observed in the scar track. It was considered that the partial DLC film that remained after delamination enabled the low friction coefficient to be retained. Flakes of DLC film are also expected to be used as new tribological materials as well as CNTs and CNHs. We have therefore been investigating the tribological properties of DLC powder as an additive in water and in a polymer-based composite. In this paper, the friction and wear properties of epoxy resin containing powdered DLC are reported and compared with those of composites with graphite. Furthermore, a suspension of DLC flakes in water was evaluated as an additive. The friction and wear properties were evaluated using a reciprocating friction tester. 2. Experiments 2.1. DLC flakes DLC flakes were prepared by the delamination of a DLC film deposited on an aluminum substrate. The DLC film was deposited by electron-excited plasma CVD with a DC or pulsed-bias voltage Table 1 Fabricated specimens and their hardness. Specimen
Ep
Ep-DLC
Ep-Gr0.6
EP-Gr6
Additive Vickers hardness, Hv
None 15
DC-DLC 22
Graphite (0.6 µm) 18
Graphite (0.6 µm) 18
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(DASH-400DS2P, NANOTEC). The voltage was − 3 kV for DC bias and − 3 kV with a frequency of 1 kHz and a duty of 10% for pulse bias. Details of the deposition procedure and setup are described elsewhere [10]. The substrate was removed by treatment with dilute hydrochloric acid, and the obtained powder was collected by filtration. The DLC film spontaneously disintegrated into a powder owing to its internal stress, which was − 1.3 GPa in the case of DC bias and − 1.2 GPa in the case of pulse bias (the internal stress was measured using films deposited on a Si substrate). A negative value of internal stress indicates compressive stress. The obtained DLC powder was washed with deionized water and ethanol several times. The obtained DLC powder consisted of flakes with sizes approximately 50 × 50 μm for DC bias (D-DLC) deposition and 100 × 50 μm for pulse bias deposition (P-DLC) and thicknesses of 3.4 and 1.4 μm for D-DLC and P-DLC, respectively. The DLC flakes were then ground in an agate mortar until the desired particle size was reached (approximately 6 μm). 2.2. Composites
Fig. 1. Frictional behavior of Ep and composites with various carbon powders in (a) air and (b) water.
Epoxy resin (Struers, Epofix Kit; epoxy resin: bisphenol-Aepichlorhydrine; average molecular weight MW b=700; hardener: triethylenetetramine) was used as a base material. The graphite used was commercially available graphite powder with an average particle size of 0.6 μm (T-1, Nippon Graphite Industries, Ltd.) or 6.0 μm (KS-6, Lonza). Four specimens were fabricated: (a) epoxy resin alone (Ep), (b) epoxy resin with D-DLC (Ep-DLC), (c) epoxy resin with 0.6 μm graphite particles (Ep-Gr0.6) and (d) epoxy resin with 6.0 μm graphite particles (EP-Gr6). The composite specimens (Ep-DLC, Ep-Gr0.6 and EP-Gr6) were made as follows. Epoxy resin and the hardener were mixed with 10 wt.% carbon materials (DLC flakes or graphite). Blended resins were placed as the carbon-
Fig. 2. Worn surfaces of specimens and mating balls after friction test in ambient air. Specimens: (a) Ep, (b) Ep-DLC, (c) Ep-Gr0.6 and (d) Ep-Gr6; mating balls used against (e) Ep, (f) Ep-DLC, (g) Ep-Gr0.6 and (h) Ep-Gr6.
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containing surface layer on epoxy resin in a cylindrical mold (25 mm diameter). The thickness of the carbon-containing layer was about 1 mm. The epoxy resin was hardened at room temperature overnight. The surfaces of the composites were ground by emery paper with different abrasive particle sizes. The surface average roughness Ra of the polished composites was 0.2 μm. The Vickers hardness of the composites was measured at a load of 0.25 N. Ep-DLC was harder than Ep-Gr06, Ep-Gr6 and Ep. The specimens and their hardness are summarized in Table 1. Friction and wear tests were performed using a ball-on-disktype reciprocating friction tester. The mating ball was a hardened martensite stainless-steel (AISI 440C, Hv = 830) ball of 4.76 mm diameter. The reciprocating friction stroke was 8 mm and the tests were conducted at a normal load of 1 N. The average sliding speed was 60 cycles/min and the number of cycles was 3600. The test environments were ambient air and water at room temperature. The wear volume of the composite was calculated by measuring wear scars using a noncontact-type surface profilometer. 2.3. Suspensions The powdered carbon materials (DLC flakes and graphite) were dispersed in water by an ultrasonic wave for 30 min. The concentration of the carbon additives was 1 wt.%. Friction and wear tests were performed using the same apparatus as above. The dispersed solution was added dropwise to the epoxy resin, and then the mating ball was moved at a normal load of 1 N. The suspensions of carbon materials were prepared immediately before the friction test. The reciprocating friction stroke and average sliding speed were the same as those of the friction test performed on composites above. The wear volume was also similarly measured.
composites, although the wear of mating ball was negligibly small. The wear of the mating balls against Ep-Gr6 and Ep-Gr0.6 was also small. The friction coefficient of Ep-DLC was smaller than that of Ep but larger than those of Ep-Gr6 and Ep-Gr0.6. The wear of the mating ball against Ep-DLC was greater than that against Ep, Ep-Gr6 and Ep-Gr0.6. The worn surfaces of the mating balls and the specimens after the friction test in water are shown in Fig. 4. In contrast with the worn surface of Ep in ambient air, that of Ep in water was smooth. No cracks were observed on the worn surface. Therefore, it is considered that the wear of Ep in air was more severe than that in water. The wear processes reflect the friction coefficient of Ep, which was relatively high in the friction test in air. In the case of EpDLC, the specific wear rate of the composite was 4 × 10− 6 mm3/N m in water, 10 times smaller than that of Ep. Therefore, the wear resistance was increased by the addition of DLC flakes. Asperities were more clearly observed in the worn surface of Ep-DLC after the friction test in water than after the test in air. It is considered that these asperities are DLC flakes, because the DLC flakes are much harder than the polymer. Therefore, it is concluded that the islands of DLC flakes on the scar resulted in the low friction coefficient of Ep-DLC. On the other hand, the amount of wear of the mating ball used against Ep-DLC was larger than that against Ep and the composites with graphite, although it was smaller than that in the friction test conducted in air. The specific wear rates of the mating ball against Ep-DLC were 8.2 × 10− 7 and 9.7 × 10− 7 mm3/N m in
3. Results and discussion 3.1. Frictional behavior of composites The frictional behavior of Ep and the composites with the DLC flakes and graphite is shown in Fig. 1. In ambient air, the friction coefficient of Ep decreased during the early stages of the test then reached a steady value (average friction coefficient from 1500 to 3000 cycles, μave. = 0.90). For the composites (Ep-DLC, EP-Gr0.6 and Ep-Gr6), the friction coefficients initially increased then reached steady values (μave. = 0.69, 0.58 and 0.41, respectively). The friction coefficient of Ep was the highest among the specimens. In water, the friction coefficients of Ep and Ep-Gr6 increased gradually then reached stable values (μave. = 0.72 and 0.55, respectively), whereas Ep-DLC and Ep-Gr0.6 exhibited stable and low friction coefficients throughout the friction test (μave. = 0.29 and 0.27, respectively). The reproducibility of these values of friction coefficient was invariably poor. In particular, in ambient air, they were affected by test conditions such as humidity. However, it was confirmed that inclusion of the additives caused a reduction of the friction coefficients in ambient air and water, the reduction was greater in water than in air. The friction coefficients of the Ep-DLC and EpGr0.6 were very similar. The worn surfaces of the specimens and the mating balls used in the friction test in air, as shown in Fig. 2, were observed by optical microscopy. The worn surface of Ep was considerably roughened and there were many large vertical cracks in the direction of friction. On the other hand, in the case of composites, the worn surfaces were generally smooth, although some scratched lines were seen. The wear rate of Ep-Gr0.6 was the lowest among the composites. The specific wear rates of the composites and crosssection profiles of the mating balls are shown in Fig. 3 It is considered that the generation of cracks in Ep caused the high friction coefficient. The wear of Ep was larger than those of the
Fig. 3. (a) Specific wear rates of specimens in ambient air and (b) cross-section profiles of mating balls.
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Fig. 4. Worn surfaces of specimens and mating balls after friction test in water. Specimens: (a) Ep, (b) Ep-DLC, (c) Ep-Gr0.6 and (d) Ep-Gr6; mating balls used against (e) Ep, (f) EpDLC, (g) Ep-Gr0.6 and (h) Ep-Gr6.
water and air, respectively. It is considered that the DLC powder on the scarred surface acted as an abrasive. 3.2. Frictional behavior in suspension Fig. 5 shows the frictional behavior of Ep with additives against mating ball. The friction coefficient of Ep was decreased from 0.9 to less than 0.15 upon the inclusion of additives. The friction coefficients in the suspension of DLC flakes were particularly stable and low, although those in the suspension of graphite were unstable. One of the reasons why the friction coefficients for the graphite suspensions are larger than that for the suspension of DLC flakes is a “damming” effect. It appears that the larger graphite particles could not easily penetrate the interface of the worn surface. It was reported that the addition of nanosize graphite particles resulted in a lower friction coefficient than the addition of larger graphite flakes [12], which is consistent with the results shown in Fig. 5. Furthermore, it appears
that the anisotropic properties of graphite also affect the frictional behavior. It is well known that the lubricity of graphite is related to its lamellar crystalline structure, which enables its layers to slip easily [13]. However, the orientation of particles that penetrated the interface of the worn surface was random. Therefore, it is considered that large particles strongly affect the friction coefficient. On the other hand, DLC powder has an amorphous structure. It is considered that the particles exhibit similar tribological properties in all direction. Furthermore, it was reported that flakes of DLC film promote low friction behavior in a water environment [10]. Therefore, it is considered that the DLC suspension exhibited a low and stable friction coefficient in water. In the case of the DLC suspension, the mating balls were severely worn, although no serious damage was observed for Ep. The specific wear rates of the mating ball were 1.6 × 10− 5 and 3.4 ×10− 5 mm3/N m, for D-DLC and P-DLC, respectively. In the case of 0.6 μm graphite additive, the specific wear rate of the mating ball was 5.9 × 10− 7 mm3/N m. It is considered that the DLC additive acts as a polishing material. The specific wear rate of the mating ball in DLC suspension was higher than that of the mating ball used against Ep-DLC, even though the concentration of DLC flakes in Ep-DLC was 10 times higher than that in the suspension. In the case of the friction test in the suspension, fresh DLC flakes are continuously supplied to the worn surface from the surrounding suspension. Therefore, it is considered that the amount of wear in the friction test in the suspension is larger than that in the test with the composites. It is concluded that DLC powder as an additive in a water environment can reduce the friction coefficient, although the wear of the mating ball is higher that than when a graphite additive is used. 4. Conclusion
Fig. 5. Frictional behavior of Ep with various carbon additives.
The friction and wear properties of epoxy resin containing powdered DLC film were reported and compared with those of composites with
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graphite. Furthermore, a suspension of DLC flakes in water was evaluated as an additive. Their friction and wear properties were evaluated using a reciprocating friction tester. The composite containing DLC flakes exhibited a low friction coefficient, similar to that of the composites containing graphite; the friction coefficients of the composite with DLC flakes were 0.69 and 0.29 in air and water, respectively. Furthermore, it was found that the wear resistance of epoxy resin was increased by the addition of powdered DLC, although the mating ball was severely scarred in friction test. In the case of the suspension of DLC powder, it was found that a low and stable friction coefficient was obtained compared with those obtained for suspension of graphite particles. However, the wear of the mating ball was also severe. It is considered that the DLC flakes on the scarred surface acted as a polishing material. Acknowledgements The authors thank Dr. X. Wu of AIST and Mr. W. Samoto of Tokyo Denki University for their help with the experiments.
References [1] S. Iijima, Nature 354 (7) (1991) 56. [2] S. Iijima, M. Yudasaka, R. Yamada, S. Bandow, K. Suenaga, F. Kokai, K. Takahashi, Chem. Phys. Lett. 309 (1999) 165. [3] D. Kasuya, M. Yudasaka, K. Takahashi, F. Kokai, S. Iijima, J. Phys. Chem. B 106 (2002) 4947. [4] H. Cai, F. Yan, Q. Xue, Mater. Sci. Eng. A 313 (2004) 94. [5] L. Joly-Pottuz, F. Dassenoy, B. Vacher, J.M. Martin, T. Mieno, Tribol. Int. 37 (2004) 1031. [6] Q. Wang, J. Dai, W. Li, Z. Wei, J. Jiang, Compos. Sci. Technol. 68 (2008) 1644. [7] A. Tanaka, K. Umeda, M. Yudasaka, M. Suzuki, T. Ohana, M. Yumura, S. Iijima, Tribol. Lett. 19 (2) (2005) 135. [8] C. Donnet, A. Erdemir (Eds.), Tribology of Diamond-like Carbon Films, Springer, New York, USA, 2007. [9] A. Erdemir, C. Donnet, J. Phys. D: Appl. Phys. 39 (2006) R311. [10] T. Ohana, T. Nakamura, M. Suzuki, A. Tanaka, Y. Koga, Diamond Relat. Mater. 13 (2004) 1500. [11] M. Suzuki, A. Tanaka, T. Ohana, W. Zhang, Diamond Relat. Mater. 13 (2004) 1464. [12] H.D. Houng, J.P. Tu, L.P. Gan, C.Z. Li, Wear 261 (2006) 140. [13] P.J. Bryant, P.L. Gutshall, L.H. Taylor, Wear 7 (1964) 118.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d i a m o n d
Tribological properties of polymer composites with diamond-like carbon flakes Tsuguyori Ohana ⁎, Takako Nakamura, Akihiro Tanaka National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Available online 22 February 2010 Keywords: Diamond-like carbon Powder Composites Tribological properties
a b s t r a c t Composites of epoxy resin with diamond-like carbon (DLC) flakes were fabricated. The DLC flakes were prepared from a DLC film deposited by chemical vapor deposition on an aluminum substrate. The tribological properties of composites were evaluated in air and water environments using a reciprocating friction tester and an AISI 440C mating ball. The friction coefficient of the epoxy composite decreased from 0.90 to 0.69 in air and from 0.71 to 0.29 in water with the addition of DLC flakes. The specific wear rate of the composite also decreased from 5 × 10− 5 to 7 × 10− 6 mm3/N m in air and from 4 × 10− 5 to 4 × 10− 6 mm3/N m in water. In contrast, the wear of the mating ball increased. Furthermore, the tribological properties of DLC flakes as an additive in water were evaluated. The suspension of powdered DLC in water reduced the friction coefficient of epoxy resin against the AISI 440C mating ball. Furthermore, the wear of the resin was negligibly small, although severe abrasive wear on the mating ball was observed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction New carbon materials such as carbon nanotubes (CNTs) [1] and carbon nanohones (CNHs) [2,3] were discovered at the end of the 20th century. These new materials have been widely investigated for possible applications, because it was thought they had high potential abilities for innovation of industry. These materials also have the potential for mechanical use [4–6]. The CNHs aggregate to form spherical particles with multiple horns and are expected to exhibit good tribological properties due to the spherical shape with submicron diameter. Previously, we investigated the tribological properties of CNTs and CNHs as additives in polymer-based composites [7]. We found that the use of these materials reduced wear. Furthermore, the friction coefficients of the composites were lower than that of the pure polymer. Therefore, we demonstrated that these new carbon nanomaterials have the potential to be used in mechanical applications. On the other hand, it is known that diamond-like carbon (DLC) films are hard with low friction properties. Therefore, DLC films have been widely applied as protective films and as a solid lubricant [8,9]. However, the application of DLC films is limited, because the processes used to apply a DLC film coating are commonly performed in a vacuum chamber. Generally, the deposition of DLC film has been carried out by a plasma process (chemical or physical vapor deposition). The size of the work material to be coated is thus limited by the size of vacuum chamber. Furthermore, the shape of the work material is also significant. It is expected that DLC films can be used to coat various sliding parts. If DLC film can be used as a solid lubricant
⁎ Corresponding author. E-mail address: [email protected] (T. Ohana). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.02.020
similar to graphite, its range of applications will be greatly expanded. Previously, we investigated the tribological properties of DLC film in a water environment [10,11]. The DLC film exhibited a low friction coefficient and high wear resistance. Interestingly, the low friction coefficient of the DLC film was retained even though partial delamination was observed in the scar track. It was considered that the partial DLC film that remained after delamination enabled the low friction coefficient to be retained. Flakes of DLC film are also expected to be used as new tribological materials as well as CNTs and CNHs. We have therefore been investigating the tribological properties of DLC powder as an additive in water and in a polymer-based composite. In this paper, the friction and wear properties of epoxy resin containing powdered DLC are reported and compared with those of composites with graphite. Furthermore, a suspension of DLC flakes in water was evaluated as an additive. The friction and wear properties were evaluated using a reciprocating friction tester. 2. Experiments 2.1. DLC flakes DLC flakes were prepared by the delamination of a DLC film deposited on an aluminum substrate. The DLC film was deposited by electron-excited plasma CVD with a DC or pulsed-bias voltage Table 1 Fabricated specimens and their hardness. Specimen
Ep
Ep-DLC
Ep-Gr0.6
EP-Gr6
Additive Vickers hardness, Hv
None 15
DC-DLC 22
Graphite (0.6 µm) 18
Graphite (0.6 µm) 18
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(DASH-400DS2P, NANOTEC). The voltage was − 3 kV for DC bias and − 3 kV with a frequency of 1 kHz and a duty of 10% for pulse bias. Details of the deposition procedure and setup are described elsewhere [10]. The substrate was removed by treatment with dilute hydrochloric acid, and the obtained powder was collected by filtration. The DLC film spontaneously disintegrated into a powder owing to its internal stress, which was − 1.3 GPa in the case of DC bias and − 1.2 GPa in the case of pulse bias (the internal stress was measured using films deposited on a Si substrate). A negative value of internal stress indicates compressive stress. The obtained DLC powder was washed with deionized water and ethanol several times. The obtained DLC powder consisted of flakes with sizes approximately 50 × 50 μm for DC bias (D-DLC) deposition and 100 × 50 μm for pulse bias deposition (P-DLC) and thicknesses of 3.4 and 1.4 μm for D-DLC and P-DLC, respectively. The DLC flakes were then ground in an agate mortar until the desired particle size was reached (approximately 6 μm). 2.2. Composites
Fig. 1. Frictional behavior of Ep and composites with various carbon powders in (a) air and (b) water.
Epoxy resin (Struers, Epofix Kit; epoxy resin: bisphenol-Aepichlorhydrine; average molecular weight MW b=700; hardener: triethylenetetramine) was used as a base material. The graphite used was commercially available graphite powder with an average particle size of 0.6 μm (T-1, Nippon Graphite Industries, Ltd.) or 6.0 μm (KS-6, Lonza). Four specimens were fabricated: (a) epoxy resin alone (Ep), (b) epoxy resin with D-DLC (Ep-DLC), (c) epoxy resin with 0.6 μm graphite particles (Ep-Gr0.6) and (d) epoxy resin with 6.0 μm graphite particles (EP-Gr6). The composite specimens (Ep-DLC, Ep-Gr0.6 and EP-Gr6) were made as follows. Epoxy resin and the hardener were mixed with 10 wt.% carbon materials (DLC flakes or graphite). Blended resins were placed as the carbon-
Fig. 2. Worn surfaces of specimens and mating balls after friction test in ambient air. Specimens: (a) Ep, (b) Ep-DLC, (c) Ep-Gr0.6 and (d) Ep-Gr6; mating balls used against (e) Ep, (f) Ep-DLC, (g) Ep-Gr0.6 and (h) Ep-Gr6.
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containing surface layer on epoxy resin in a cylindrical mold (25 mm diameter). The thickness of the carbon-containing layer was about 1 mm. The epoxy resin was hardened at room temperature overnight. The surfaces of the composites were ground by emery paper with different abrasive particle sizes. The surface average roughness Ra of the polished composites was 0.2 μm. The Vickers hardness of the composites was measured at a load of 0.25 N. Ep-DLC was harder than Ep-Gr06, Ep-Gr6 and Ep. The specimens and their hardness are summarized in Table 1. Friction and wear tests were performed using a ball-on-disktype reciprocating friction tester. The mating ball was a hardened martensite stainless-steel (AISI 440C, Hv = 830) ball of 4.76 mm diameter. The reciprocating friction stroke was 8 mm and the tests were conducted at a normal load of 1 N. The average sliding speed was 60 cycles/min and the number of cycles was 3600. The test environments were ambient air and water at room temperature. The wear volume of the composite was calculated by measuring wear scars using a noncontact-type surface profilometer. 2.3. Suspensions The powdered carbon materials (DLC flakes and graphite) were dispersed in water by an ultrasonic wave for 30 min. The concentration of the carbon additives was 1 wt.%. Friction and wear tests were performed using the same apparatus as above. The dispersed solution was added dropwise to the epoxy resin, and then the mating ball was moved at a normal load of 1 N. The suspensions of carbon materials were prepared immediately before the friction test. The reciprocating friction stroke and average sliding speed were the same as those of the friction test performed on composites above. The wear volume was also similarly measured.
composites, although the wear of mating ball was negligibly small. The wear of the mating balls against Ep-Gr6 and Ep-Gr0.6 was also small. The friction coefficient of Ep-DLC was smaller than that of Ep but larger than those of Ep-Gr6 and Ep-Gr0.6. The wear of the mating ball against Ep-DLC was greater than that against Ep, Ep-Gr6 and Ep-Gr0.6. The worn surfaces of the mating balls and the specimens after the friction test in water are shown in Fig. 4. In contrast with the worn surface of Ep in ambient air, that of Ep in water was smooth. No cracks were observed on the worn surface. Therefore, it is considered that the wear of Ep in air was more severe than that in water. The wear processes reflect the friction coefficient of Ep, which was relatively high in the friction test in air. In the case of EpDLC, the specific wear rate of the composite was 4 × 10− 6 mm3/N m in water, 10 times smaller than that of Ep. Therefore, the wear resistance was increased by the addition of DLC flakes. Asperities were more clearly observed in the worn surface of Ep-DLC after the friction test in water than after the test in air. It is considered that these asperities are DLC flakes, because the DLC flakes are much harder than the polymer. Therefore, it is concluded that the islands of DLC flakes on the scar resulted in the low friction coefficient of Ep-DLC. On the other hand, the amount of wear of the mating ball used against Ep-DLC was larger than that against Ep and the composites with graphite, although it was smaller than that in the friction test conducted in air. The specific wear rates of the mating ball against Ep-DLC were 8.2 × 10− 7 and 9.7 × 10− 7 mm3/N m in
3. Results and discussion 3.1. Frictional behavior of composites The frictional behavior of Ep and the composites with the DLC flakes and graphite is shown in Fig. 1. In ambient air, the friction coefficient of Ep decreased during the early stages of the test then reached a steady value (average friction coefficient from 1500 to 3000 cycles, μave. = 0.90). For the composites (Ep-DLC, EP-Gr0.6 and Ep-Gr6), the friction coefficients initially increased then reached steady values (μave. = 0.69, 0.58 and 0.41, respectively). The friction coefficient of Ep was the highest among the specimens. In water, the friction coefficients of Ep and Ep-Gr6 increased gradually then reached stable values (μave. = 0.72 and 0.55, respectively), whereas Ep-DLC and Ep-Gr0.6 exhibited stable and low friction coefficients throughout the friction test (μave. = 0.29 and 0.27, respectively). The reproducibility of these values of friction coefficient was invariably poor. In particular, in ambient air, they were affected by test conditions such as humidity. However, it was confirmed that inclusion of the additives caused a reduction of the friction coefficients in ambient air and water, the reduction was greater in water than in air. The friction coefficients of the Ep-DLC and EpGr0.6 were very similar. The worn surfaces of the specimens and the mating balls used in the friction test in air, as shown in Fig. 2, were observed by optical microscopy. The worn surface of Ep was considerably roughened and there were many large vertical cracks in the direction of friction. On the other hand, in the case of composites, the worn surfaces were generally smooth, although some scratched lines were seen. The wear rate of Ep-Gr0.6 was the lowest among the composites. The specific wear rates of the composites and crosssection profiles of the mating balls are shown in Fig. 3 It is considered that the generation of cracks in Ep caused the high friction coefficient. The wear of Ep was larger than those of the
Fig. 3. (a) Specific wear rates of specimens in ambient air and (b) cross-section profiles of mating balls.
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Fig. 4. Worn surfaces of specimens and mating balls after friction test in water. Specimens: (a) Ep, (b) Ep-DLC, (c) Ep-Gr0.6 and (d) Ep-Gr6; mating balls used against (e) Ep, (f) EpDLC, (g) Ep-Gr0.6 and (h) Ep-Gr6.
water and air, respectively. It is considered that the DLC powder on the scarred surface acted as an abrasive. 3.2. Frictional behavior in suspension Fig. 5 shows the frictional behavior of Ep with additives against mating ball. The friction coefficient of Ep was decreased from 0.9 to less than 0.15 upon the inclusion of additives. The friction coefficients in the suspension of DLC flakes were particularly stable and low, although those in the suspension of graphite were unstable. One of the reasons why the friction coefficients for the graphite suspensions are larger than that for the suspension of DLC flakes is a “damming” effect. It appears that the larger graphite particles could not easily penetrate the interface of the worn surface. It was reported that the addition of nanosize graphite particles resulted in a lower friction coefficient than the addition of larger graphite flakes [12], which is consistent with the results shown in Fig. 5. Furthermore, it appears
that the anisotropic properties of graphite also affect the frictional behavior. It is well known that the lubricity of graphite is related to its lamellar crystalline structure, which enables its layers to slip easily [13]. However, the orientation of particles that penetrated the interface of the worn surface was random. Therefore, it is considered that large particles strongly affect the friction coefficient. On the other hand, DLC powder has an amorphous structure. It is considered that the particles exhibit similar tribological properties in all direction. Furthermore, it was reported that flakes of DLC film promote low friction behavior in a water environment [10]. Therefore, it is considered that the DLC suspension exhibited a low and stable friction coefficient in water. In the case of the DLC suspension, the mating balls were severely worn, although no serious damage was observed for Ep. The specific wear rates of the mating ball were 1.6 × 10− 5 and 3.4 ×10− 5 mm3/N m, for D-DLC and P-DLC, respectively. In the case of 0.6 μm graphite additive, the specific wear rate of the mating ball was 5.9 × 10− 7 mm3/N m. It is considered that the DLC additive acts as a polishing material. The specific wear rate of the mating ball in DLC suspension was higher than that of the mating ball used against Ep-DLC, even though the concentration of DLC flakes in Ep-DLC was 10 times higher than that in the suspension. In the case of the friction test in the suspension, fresh DLC flakes are continuously supplied to the worn surface from the surrounding suspension. Therefore, it is considered that the amount of wear in the friction test in the suspension is larger than that in the test with the composites. It is concluded that DLC powder as an additive in a water environment can reduce the friction coefficient, although the wear of the mating ball is higher that than when a graphite additive is used. 4. Conclusion
Fig. 5. Frictional behavior of Ep with various carbon additives.
The friction and wear properties of epoxy resin containing powdered DLC film were reported and compared with those of composites with
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graphite. Furthermore, a suspension of DLC flakes in water was evaluated as an additive. Their friction and wear properties were evaluated using a reciprocating friction tester. The composite containing DLC flakes exhibited a low friction coefficient, similar to that of the composites containing graphite; the friction coefficients of the composite with DLC flakes were 0.69 and 0.29 in air and water, respectively. Furthermore, it was found that the wear resistance of epoxy resin was increased by the addition of powdered DLC, although the mating ball was severely scarred in friction test. In the case of the suspension of DLC powder, it was found that a low and stable friction coefficient was obtained compared with those obtained for suspension of graphite particles. However, the wear of the mating ball was also severe. It is considered that the DLC flakes on the scarred surface acted as a polishing material. Acknowledgements The authors thank Dr. X. Wu of AIST and Mr. W. Samoto of Tokyo Denki University for their help with the experiments.
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