Polymer Composites Filled with RB Ceramics Particles as Low Friction and High Wear Resistant Filler

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1 Tribology Online, 5, 1 (21) ISSN DOI /trol.5.19 Article Polymer Composites Filled with RB Ceramics Particles as Low Friction and High Wear Resistant Filler Motoharu Akiyama 1), Takeshi Yamaguchi 2)*, Kunihiro Matsumoto 1) and Kazuo Hokkirigawa 2) 1) Minebea Co., Ltd Oaza Miyota, Miyota-machi, Kitasaku-gun, Nagano , Japan 2) Graduate School of Engineering, Tohoku University Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi , Japan *Corresponding author: yamatake@gdl.mech.tohoku.ac.jp ( Manuscript received 17 June 29; accepted 27 November 29; published 15 January 21 ) In this study, five kinds of thermoplastic resin/rb ceramics materials were developed, and their tribological properties were investigated under dry and oil lubricated conditions. RB ceramics particles with mean diameter of 15 µm were compounded with five kinds of thermoplastic resins (,,,, and ). These compounds were formed into disk geometry by injection molding. A weight fraction of RB ceramics particles were 5 wt% for, 6 wt% for and, and 7 wt% for and. Friction coefficients of the thermoplastic resin/rb ceramics s were lower than those of the neat thermoplastic resins under dry condition, and those were much lower at low sliding velocities under oil lubricated condition. A stable value of friction coefficient against sliding velocity variation was also obtained for the thermoplastic resin/rb ceramics s as compared with the neat thermoplastic resins under both dry and oil lubricated conditions. Furthermore, a remarkable reduction of specific wear rate was achieved for the newly developed s as compared with the neat resins under dry condition (67-98% reduction) and oil lubricated conditions (68-99% reduction). These results would suggest that RB ceramics particles can be applied as filler for polymer s providing both low friction and high wear resistance. Keywords: friction, wear, polymer, RB ceramics, thermoplastic resin, filler 1. Introduction Polymers are applied to tribo-elements such as cams, sliding bearings, gears, etc. usually filled with solid lubricants or reinforced with short fiber to reduce friction 1,2) and wear 1,3,4). Solid lubricants such as MoS 2, graphite and PTFE (polytetrafluoroethylene) are soft and provide low friction due to low shear strength. Fiber reinforcement with glass, carbon or aramid fiber is usually applied to polymer s to increase their load carrying capacity and wear resistance. In order to provide low friction and high wear resistance with polymers, these solid lubricants and fiber reinforcement are usually employed in combination. On the other hand, new hard porous carbon materials RB (Rice Bran) ceramics have been developed by Hokkirigawa, etc 5-8). RB ceramics are prepared by carbonizing a mixture of defatted rice bran and phenol resin in nitrogen gas environment at 9 degrees C. RB ceramics are consisted of a soft amorphous carbon corresponding to the carbonized defatted rice bran and a hard amorphous carbon corresponding to the carbonized phenol resin. It has been clarified that RB ceramics show superior tribological properties such as low friction and high wear resistance. Furthermore, friction coefficient for RB ceramics slightly increases with sliding velocity, which would provide prevention of stick-slip motion during sliding. Recently, a powder form of RB ceramics has been succeeded in development. The RB ceramics particle shows relatively high hardness (Hv = 4.4 GPa) and porous structure as shown in Fig.1 resulting in low apparent density (1.3 mg/m 3 ) equivalent to that of polymers. Thus RB ceramics particles can be dispersed uniformly in polymer resulting in good fluidity of the mixture in the cylinder of injection machine compared with other hard fillers such as glass fiber, carbon fiber, etc. Hence it would be expected that injection molding is available even if a large amount of RB ceramics particles is mixed with polymers. By using RB ceramics particles as filler for polymer s, following characteristics would be Copyright 21 Japanese Society of Tribologists 19

2 Motoharu Akiyama, Takeshi Yamaguchi, Kunihiro Matsumoto and Kazuo Hokkirigawa expected: (1) High weight fraction of RB ceramics particles with a polymer (2) Low friction (3) Prevention of stick-slip motion (4) High wear resistance On the basis of the background mentioned above, purposes of this study are to develop thermoplastic resin/rb ceramics s and to investigate their tribological properties under dry and oil lubricated conditions. Fig.1 1µm 3µm SEM images of RB ceramic particle 2. Experimental 2.1. Preparation of thermoplastic resin/rb ceramics s Figure 2 shows a schematic diagram of preparation process of thermoplastic resin/rb ceramics s. Five kinds of the thermoplastic resins such as (polyamide 66), (polyamide 11), (polyoxymethylene), (polybutylene terephthalate), and (polypropylene) were used as matrix materials. Pellets composed of each compound of RB ceramics particles and matrix resin were prepared by using a twin screw extruder. Then, by using these pellets, five kinds of thermoplastic resin/rb ceramics s were formed into disk geometry (diameter φ = 5 mm, thickness t = 3 mm) by injection molding. RB ceramics particles were compounded with each resin to the greatest weight fraction possible for the injection molding. The mean diameter of RB ceramics particles was 15 µm. The weight fractions of RB ceramics particles were 5 wt% for, 6 wt% for and, and 7 wt% for and as shown in Table 1. The resultant volume fractions of RB ceramics particles were 52 vol% for, 55 vol% for, 6 vol% for, 67 vol% for and 62 vol% for. It can be considered that difference in the possible weight fraction of RB ceramics particles by matrix resin is due to a difference of the fluidity of neat matrix resins. Neat,,,, and were also formed into the same disk geometry by injection molding. filled with glass fiber RB ceramics particles Mean diameter: d = 15µm Weight fraction: α = 5, 6 or 7wt% Thermoplastic resins ] (3wt%) (4wt%) Fig.2 (5wt%) (4wt%) (3wt%) Compounding Pellets Injection molding Five kinds of thermoplastic resin/ RB ceramics s Schematic diagram of a preparation process of the thermoplastic resin/rb ceramics s (23 wt%) was prepared as comparison. Diameter and length of the fiber were 1 µm and 3-1 µm, respectively. Mechanical properties of the neat thermoplastic resins and the s are shown in Table 1. Elastic modulus and Vickers hardness of the s filled with RB ceramics powder are higher than those of the neat resins Experimental method Friction tests were carried out using the linear reciprocating motion type friction apparatus (Fig. 3(a)) or the rotating motion type friction apparatus (Fig. 3(b)). Bearing steel (JIS SUJ2) ball with diameter of 2 mm Table 1 Mechanical properties of the neat thermoplastic resins and the thermoplastic resin/rb ceramics s /GF Weight fraction of RB ceramics particles α, wt.% Volume fraction of RB ceramics particles β, vol.% (glass fiber) 12 (glass fiber) Density ρ, Mg/m Tensile strength T, MPa Elastic modulus E, GPa Vickers hardness Hv, GPa Poisson s ratio ν Japanese Society of Tribologists ( Tribology Online, Vol. 5, No. 1 (21) / 2

3 Polymer Composites Filled with RB Ceramics Particles as Low Friction and High Wear Resistant Filler was used as a ball specimen, and the neat thermoplastic resins and the s were used as disk specimens. The linear reciprocating motion type friction apparatus was used at sliding velocities of.1,.5, and.1 m/s (the sliding velocity is defined as a steady state stage velocity not including the velocity at acceleration and deceleration periods), and the rotating motion type friction apparatus was used at sliding velocities of.1,.5, and 1. m/s. A normal load was.49 N, and number of repeat passages was 1, cycles. The tests were conducted under dry and oil lubricated conditions. Di-ester based oil (viscosity: 12.8 mm 2 /s at 313 K, 3.4 mm 2 /s at 373 K) was used as a lubricant. Wear tests were conducted using the linear reciprocating motion type friction apparatus (Fig. 3(a)) under dry and oil lubricated conditions. For the wear tests, normal load was.49 N, sliding velocity was.1 m/s. Number of repeat passages of friction were cycles under dry condition, and cycles under oil lubricated condition. The stroke of the linear reciprocating friction and wear tests was 3. mm. For the tests under oil lubricated condition, the disk specimen was fixed on the oil bath mounted on the linear motion stage. The oil was injected into the contact area with a pipette. 1. ml of the oil was injected for the reciprocating type friction and wear tests. The same amount of the oil was also injected for the rotation type friction tests for several times during the test. All the tests were conducted at room temperature. Load cell Weight Ball specimen Stage 3. Experimental results and discussion 3.1. Friction and wear properties under dry condition Figure 4 shows the variation of friction coefficient with number of repeat passages under dry condition. As shown in Fig. 4, the friction coefficient for and rapidly increased at the initial stage of friction, then took stable value around.53. The friction coefficient for the /glass fiber (GF) also increased at the initial stage of friction, then took stable value around.57. For and, the friction coefficient slightly increased at the initial stage of friction, then took stable value less than.4. The friction coefficient for gradually increased with an increase of the number of repeat passages. On the other hand for the thermoplastic resin/rb ceramics s, the friction coefficient slightly increased at the initial stage of friction, then took stable value less than.3 irrespective of the matrix resin. Figure 5 shows friction coefficient for the neat thermoplastic resins and the s at 1, cycles under dry condition. It can be seen in Fig. 5 that the friction coefficient for the thermoplastic resin/rb ceramics s was lower than that for the neat thermoplastic resin. Thus, friction of thermoplastic resin under dry condition can be reduced by mixing RB ceramics particles. Furthermore, 14% to 48% reduction of friction coefficient was obtained for the thermoplastic resin/rb ceramics s as compared with the neat thermoplastic resin. However, it can also be seen that glass fiber can not provide lower friction coefficient than that of the neat. Figure 6 shows the relationship between sliding velocity and friction coefficient for the neat, /GF and the /RB ceramics under dry condition. The friction coefficient for the /RB ceramics has less dependency on sliding velocity, while that for the neat Disk specimen (a) Linear reciprocating type.7 /RB ceramics /GF /RB ceramics /RB ceramics /RB ceramics /RB ceramics Fig.3 Weight Ball specimen Stage Disk specimen (b) Rotating type Experimental apparatuses Fig Ball: Bearing steel (R = 1mm) Normal load W =.49 N Sliding velocity v =.1 m/s Lubrication condition: Dry Number of repeat passages N, cycles Variation of the friction coefficient with number of repeat passages under dry condition Japanese Society of Tribologists ( Tribology Online, Vol. 5, No. 1 (21) / 21

4 Motoharu Akiyama, Takeshi Yamaguchi, Kunihiro Matsumoto and Kazuo Hokkirigawa Fig.5 /GF /RB ceramics /RB ceramics /RB ceramics /RB ceramics W =.49 N v =.1 m/s N = 1, cycles /RB ceramics Dry condition Friction coefficient for each neat thermoplastic resin and the thermoplastic resin/rb ceramics at 1, cycles under dry condition /RB ceramics /GF Ball: SUJ2 (R = 1 mm) Normal load :.49 N Number of repeat passages : 1, cycles Dry condition Sliding velocity v, m/s Fig.6 Relationship between sliding velocity and friction coefficient at 1, cycles under dry condition and the /GF has tendency to decrease with an increase of sliding velocity. Such stable friction coefficient over sliding velocity under dry condition was also obtained for the other polymer/rb ceramics s. These results indicate that newly developed thermoplastic resin/rb ceramics s would generate less stick-slip motion during sliding under dry condition. Figure 7 shows laser microscope images and surface profile curves of the worn surfaces of the neat, the /GF and the P66/RB ceramics after the wear test at normal load of.49 N and sliding velocity of.1 m/s under dry condition. As shown in Figs. 7(a) and (b), a plastic flow can be observed on the edge of the wear track for the neat and the /GF. Furthermore, it can be seen on the worn surface of the /GF that glass fibers are removed from the surface. On the other hand for the /RB ceramics, such a large plastic flow can not be seen on the worn surface as shown in Fig. 7(c) and the wear amount is much smaller than that of the neat and the /GF. Figure 8 shows laser microscope images and surface profile curves of the worn surfaces of ball specimens sliding against the neat, the /GF and the /RB ceramics after the wear test at normal load of.49 N and sliding velocity of.1 m/s under dry condition. As shown in Fig.8, macroscopic wear of ball specimens cannot be observed irrespective of mating disk specimens, which would be due to much higher hardness of SUJ2 ball (Hv = 7.51 GPa) than that of the neat and the s. Figure 9 shows specific wear rate of the neat thermoplastic resins and the s under dry condition. The specific wear rate of the thermoplastic resin/rb ceramics s was less than 1-8 mm 2 /N which was significantly smaller value than that of the neat thermoplastic resins. As compared with the neat thermoplastic resins, 67% to 98% reduction of specific wear rate was achieved for the thermoplastic resin/rb ceramics s under dry condition. Fig.7 Fig.8 1µm 1µm 1µm 5µm 5µm (a) (b) (c) Laser microscope images and surface profile curves of worn disk surfaces of (a), (b)/gf and (c)/rb ceramics after the wear test at normal load of.49 N and sliding velocity of.1 m/s under dry condition (a) (b) (c) 5µm 5 µm 5 µm 5 µm 1 µm Laser microscope images and surface profile curves of SUJ2 ball worn surfaces sliding against (a), (b)/gf and (c)/rb ceramics after the wear test at normal load of.49 N and sliding velocity of.1 m/s under dry condition 1 µm 1 µm Japanese Society of Tribologists ( Tribology Online, Vol. 5, No. 1 (21) / 22

5 Polymer Composites Filled with RB Ceramics Particles as Low Friction and High Wear Resistant Filler Figure 1 shows the relationship between Vickers hardness of the disk specimens and the specific wear rate of the disk specimens. It is clearly shown in Fig. 1 that the specific wear rate of the disk specimen decreases with an increase with Vickers hardness of the disks. The mechanism of such improvement of friction and wear properties of the thermoplastic resins by mixing RB ceramics particles under dry condition can be considered as follows. In the case of the neat thermoplastic resin, severe wear accompanied with the plastic flow of the surface occurred, which caused an increase of contact area resulting in high friction. On the other hand in the case of the thermoplastic resin/rb ceramics, increased hardness (shown in Table 1) would prevent the severe wear accompanied with the surface plastic flow, which resulted in prevention of significant increase of contact area. In addition, RB ceramics themselves provide low friction. Thus, the thermoplastic resin/rb ceramics showed lower friction coefficient and significantly lower specific wear rate than those of the neat thermoplastic resins. For the filled with 23 wt% glass fiber, as shown in Table 1 significant increase of hardness is not obtained compared with the neat. Thus, the surface plastic flow caused by friction was not prevented, which would result in high wear rate and high friction Friction and wear properties under oil lubricated condition Figure 11 shows the variation of the friction coefficient with number of repeat passages under oil lubricated condition. As shown in Fig. 11, the friction coefficient for the neat thermoplastic resins increased with the number of repeat passages, and that was more than.1 at 1, cycles. It can be seen that the friction coefficient for the /GF increased with an increase of the number of repeat passages, and took the value of.165 at 1, cycles which was higher than that of the neat of.131. On the other hand for the thermoplastic resin/rb ceramics s, the friction coefficient slightly decreased at the initial stage of friction, and then took low and stable value between.5 and.1. Figure 12 shows the friction coefficient for the thermoplastic resins and the s at 1, cycles under oil lubricated condition. The friction coefficient for each s filled with RB ceramics powder was lower than that for the neat thermoplastic resins. This result shows that the friction coefficient for thermoplastic resin under oil lubricated condition can also be reduced by mixing RB ceramics particles as well as the results under dry condition. Furthermore, 19% to 55% reduction of friction coefficient was obtained for the thermoplastic resin/rb ceramics s as compared with the neat thermoplastic resins under oil lubricated condition. /GF /RB ceramics /RB ceramics /RB ceramics W =.49 N v =.1 m/s /RB ceramics N = cycles Dry condition /RB ceramics Specific wear rate ws, x1-8 mm 2 /N Fig.9 Specific wear rate ws, 1-7 mm 2 /N Specific wear rate of each neat thermoplastic resin and the thermoplastic resin/rb ceramics after tested under dry condition Vickers Hardness Hv, GPa Fig.1 Relationship between Vickers hardness of the disk specimens and specific wear rate of the disks under dry condition W = 4.9 N v =.1m/s N = cycles Dry condition Ball: Bearing steel (R = 1mm) Normal load W =.49 N Sliding velocity v =.1 m/s Materials /GF /RB ceramics /RB ceramics /RB ceramics /RB ceramics /RB ceramics /RB ceramics /GF /RB ceramics /RB ceramics /RB ceramics /RB ceramics Number of repeat passages N, cycles Fig.11 Variation of the friction coefficient with number of repeat passages under oil lubricated condition Japanese Society of Tribologists ( Tribology Online, Vol. 5, No. 1 (21) / 23

6 Motoharu Akiyama, Takeshi Yamaguchi, Kunihiro Matsumoto and Kazuo Hokkirigawa Figure 16 shows specific wear rate of the neat thermoplastic resins and the s under oil lubricated condition. As shown in Fig. 16, the specific wear rate of the thermoplastic resin/rb ceramics s was less than 1-9 mm 2 /N which was.2.25 Fig.12 Friction coefficient for each neat thermoplastic resin and the thermoplastic resin/rb ceramics at 1, cycles under oil lubricated condition Figure 13 shows relationship between sliding velocity and friction coefficient for the neat, the /RB ceramics and the /GF under oil lubricated condition. The friction coefficient for the neat and the /GF under oil lubricated condition decreased with an increase of sliding velocity, and took very low value less than.5 at the sliding velocity over.1 m/s. On the other hand for the /RB ceramics s, the dependency of friction coefficient on the sliding velocity was smaller. Such a stable friction coefficient with respect to sliding velocity was also obtained for the other thermoplastic resin/rb ceramics s. On the basis of the result, it would be considered that boundary lubrication regime was kept even at higher sliding velocity for the /RB ceramics s while a lubrication mode changed from boundary to mixed lubrication regime for the neat and the /GF. Thus, the thermoplastic resin/rb ceramics would generate less stick-slip motion during sliding even under oil lubricated condition. Figure 14 shows laser microscope images and surface profile curves of the worn surfaces of the neat, the /GF and the /RB ceramics after the wear test at normal load of.49 N and sliding velocity of.1 m/s under oil lubricated condition. Wear amount of the /RB ceramics was lower than that of the neat and the /GF. Figure 15 shows laser microscope images and surface profile curves of the worn surfaces of ball specimens sliding against the neat, the /GF and the /RB ceramics after the wear test at normal load of.49 N and sliding velocity of.1 m/s under oil lubricated condition. As shown in Fig. 15, significant surface damage of ball specimens cannot be observed irrespective of mating disk specimens. Japanese Society of Tribologists ( Sliding velocity v, m/s 1 Fig.13 Relationship between sliding velocity and friction coefficient at 1, cycles under oil lubricated condition 1µm (a) 1µm 1µm 1µm.15 /RB ceramics.1 /GF.15 (b) 1µm (c) 1µm Fig.14 Laser microscope images and surface profile curves of worn disk surfaces of (a), (b) /GF and (c) /RB ceramics after the wear test at normal load of.49 N and sliding velocity of.1 m/s under oil lubricated condition 5 µm 1 µm (a) 5 µm 5 µm.5.2 W =.49N v =.1m/s N = 1 cycles Ball: Bearing steel (R = 1 mm) Normal load W =.49N /RB ceramics /RB ceramics.25 /GF /RB ceramics /RB ceramics /RB ceramics 1 µm (b) 1 µm (c) Fig.15 Laser microscope images and surface profile curves of ball worn surfaces sliding against (a), (b) /GF and (c) /RB ceramics after the wear test at normal load of.49 N and sliding velocity of.1 m/s under oil lubricated condition Tribology Online, Vol. 5, No. 1 (21) / 24

7 Polymer Composites Filled with RB Ceramics Particles as Low Friction and High Wear Resistant Filler /GF /RB ceramics /RB ceramics /RB ceramics /RB ceramics /RB ceramics W =.49 N v =.1 m/s N = cycles Specific wear rate ws, x1-8 mm 2 /N Fig.16 Specific wear rate of each neat thermoplastic resin and the thermoplastic resin/rb ceramics after tested under oil lubricated condition Specific wear rate ws, 1-8 mm 2 /N Vickers Hardness Hv, GPa Fig.17 Relationship between Vickers hardness of the disk specimens and specific wear rate of the disks under oil lubricated condition Specific wear rate ws, mm 2 /N W = 4.9 N v =.1m/s N = cycles Thermoplastic resins (Oil lubricated condition) Thermoplastic resins (Dry condition) Materials /GF /RB ceramics /RB ceramics /RB ceramics /RB ceramics /RB ceramics Materials /GF In oil Fig.18 Comprehensive tribological properties of the neat thermoplastic resins and the thermoplastic resin/rb ceramics s under dry and oil lubricated conditions Dry /RB ceramics /RB ceramics /RB ceramics /GF (Dry condition) /RB ceramics Thermoplastic resin/rb ceramics /RB ceramics s (Dry condition) /GF Ball: Bearing steel (R = 1mm) (Oil lubricated condition) Normal load W =.49 N Thermoplastic resin/rb ceramics Sliding velocity v =.1 m/s s (Oil lubricated condition) Dry or oil lubricated condition significantly smaller value than that of the neat thermoplastic resins. As compared with the neat thermoplastic resins, 68% to 99% reduction of specific wear rate was achieved for the thermoplastic resin/rb ceramics s under oil lubricated condition. As well as under dry condition, the specific wear rate of the disk specimen under oil lubricated condition has tendency to decrease with an increase of the hardness as shown in Fig. 17. Thus, it can be considered that an increased hardness by mixing RB ceramics particles provides higher wear resistance under oil lubricated condition. The contact geometry used in this study is ball and flat contact, and the ball material (SUJ2) is harder than the disk specimens. Thus, the wear of disk material produces an increase of contact area resulting in a reduction of contact pressure. Hence low wear rate of disk specimen provides small contact area resulting in a low friction coefficient at low sliding velocity under oil lubricated condition. For the neat resin and the /GF, high wear rate produced large contact area resulting in lower contact pressure. In such case, lubrication oil can easily penetrate into the contact area. Therefore, friction coefficient decreased with an increase of sliding velocity because lubrication regime can change from boundary to mixed lubrication as sliding velocity increases. Meanwhile, the polymer filled with RB ceramics powder shows higher wear resistance which maintains smaller contact area resulting in a higher contact pressure. Thus, it can be considered that the lubrication oil cannot easily penetrate into the contact area, which keeps boundary lubrication over a wide range of sliding velocity resulting in a stable low friction coefficient. Figure 18 shows comprehensive tribological properties of the neat thermoplastic resins and the thermoplastic resin/rb ceramics s under dry and oil lubricated conditions. As shown in Fig. 18, friction and wear of the thermoplastic resins used in this study can be reduced by mixing RB ceramics particles under both dry and oil lubricated conditions, which demonstrates that RB ceramics particles can be applied as the filler for polymer s providing both low friction and extremely high wear resistance. 4. Conclusions In this study, five kinds of thermoplastic resin/rb ceramics s were newly developed, and their tribological properties under dry and oil lubricated conditions were experimentally investigated. The conclusions obtained in this study are summarized as follows; (1) Friction coefficient for the thermoplastic resin/rb ceramics s was lower than that of the neat thermoplastic resins under dry condition, and much lower at low sliding velocities under oil lubricated conditions. Japanese Society of Tribologists ( Tribology Online, Vol. 5, No. 1 (21) / 25

8 Motoharu Akiyama, Takeshi Yamaguchi, Kunihiro Matsumoto and Kazuo Hokkirigawa (2) A stable friction coefficient over sliding velocity variation was also obtained for the thermoplastic resin/rb ceramics s as compared with the neat thermoplastic resins under both dry and oil lubricated conditions. (3) A remarkable reduction of specific wear rate was obtained for the newly developed s as compared with the neat resins under dry condition (67-98% reduction) or oil lubricated condition (68-99% reduction). 5. References [1] Friedrich, K., Lu, Z. and Hager A. M., Recent Advances in Polymer Composites Tribology, Wear, 19, 1995, [2] Zhang, X. R., Pei, X. Q. and Wang Q. H., Tribological Properties of MoS 2 and Carbon Fiber Reinforced Polyimide Composites, Journal of Materials Science, 43, 13, 28, [3] Hooke, C. J., Kukureka, S. N., Liao, P., Rao, M. and Chen, Y. K., Wear and Friction of Nylon-Glass Fibre Composites in Non-Conformal Contact under Combined Rolling and Sliding, Wear, 197, 1996, [4] Hanchi, J. and Eiss, N. S., Jr, Dry Sliding Friction and Wear of Short Carbon-Fiber-Reinforced Polyetheretherketone (PEEK) at Elevated Temperatures, Wear, 23-24, 1997, [5] Hokkirigawa, K., Shikano, S. and Takahashi, T., Development of Hard Porous Materials RB Ceramics Made of Rice Bran, Proc. 3rd Int. Conf. on Ecomaterials, 1997, [6] Hokkirigawa, K., Development and Application of Rice Bran Ceramics as a New Tribo-Material, Proc. Int. Trib. Conf. Nagasaki, 2, [7] Hokkirigawa, K., Chunsheng, S., Imai, K., Matsutani, A., Ando, J., Shikano, S.and Takahashi, T., Friction and Wear Properties of New Tribo-Materials RB Ceramics Made From Rice Bran, Proc. Int. Trib. Conf. Nagasaki, 2, [8] Yamaguchi, T. and Hokkirigawa, K., Development of Hard Porous Carbon Materials RB Ceramics and Their Tribological Application, Proc. 3rd Asia Int. Conf. Tribol., 26, Japanese Society of Tribologists ( Tribology Online, Vol. 5, No. 1 (21) / 26