FORMING OF FULLERENE-DISPERSED ALUMINUM COMPOSITE BY THE COMPRESSION SHEARING METHOD

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1 FORMING OF FULLERENE-DISPERSED ALUMINUM COMPOSITE BY THE COMPRESSION SHEARING METHOD Noboru NAKAYAMA Akita Prefectural University, 84-4 Tsuchiya-Ebinokuti, Yurihonjyo, Akita/ 15-55, JAPAN Hiroyuku TAKEISHI Chiba Institute of Technology Tsudanuma, Narashino, Chiba/ , JAPAN ABSTRACT In this paper, fullerene-dispersed aluminum composites were fabricated by the compression shearing method. The mechanical properties, friction coefficient and microstructures of the compacted powder were investigated. The addition of 1 vol.% fullerene to Al-Si-Cu-Mg improved the friction coefficient. The average friction coefficients of Al-Si-Cu-Mg (the vol.% fullerene sample), the 1 vol.% fullerene sample, and the 15 vol.% fullerene sample were found to be 1.,.94 and.33, respectively. Compared with the Al-Si-Cu-Mg results, the values for the samples with fullerene account for a reduction of 6% and 67%, respectively. This result suggests that the Al-Si-Cu-Mg/fullerene composite has excellent solid-lubricating properties. Introduction Fullerenes (C6 or C7, etc.) or cluster diamonds (CD or GCD) exhibit lubrication characteristics that cannot be matched by conventional materials. Therefore, it is likely that fullerene or the cluster diamond will be utilized as solid lubricants in a variety of applications. In recent years, composite materials containing cluster diamond (CD or GCD) uniformly dispersed in a metal matrix have been examined as ultra-high-performance solid lubricating materials with superior lubricating properties [1]-[12]. However, since the composite materials were fabricated by a powder metallurgy method (hot press or dynamic compaction method), the materials contained many pores and exhibited poor mechanical properties [9]-[12]. As a result, the composite material is not capable of producing enhanced lubricating properties. A new solidification method concept has been developed that employs compression shearing [13]. Using this method, the grain size of the fabricated material is on a nanometer scale, and the strength of the specimen is improved. When the compression shearing process is applied to powdered aluminum under room temperature and atmospheric air conditions, a thin plate specimen consisting of ultra-fine crystal grains with preferred orientation can be obtained. Conventional methods are not capable of obtaining such a specimen at room temperature. In this paper, aluminum composites were fabricated by the compression shearing method. First, pure aluminum powder was created by the compression shearing method. The mechanical properties, friction coefficient and microstructures of the compacted powder were investigated. Second, fullerene-dispersed aluminum composites were fabricated by the compression

2 shearing method. The Vickers hardness and friction coefficient of the fullerene-dispersed aluminum composites were investigated. Compression shearing method of forming under room temperature and atmospheric air conditions. Figure 1 shows a schematic drawing of the setup for the compression shearing method. The lower plate is filled with the powder, and the upper plate is loaded on the lower plate with the powder between the two surfaces. The plates are then placed inside the test equipment. Shear stress is applied to the powder by moving the lower steel plate in the direction of an axial compression presser. The compressive load P is generated by rotating an upside screw using a lever rod. The compressive load P given to the sample was determined from the value of the strain gauges. This forming procedure can be carried out under room temperature and atmospheric air without heating. Torque Square thread Shaft Strain gauge Axial force Upper plate Lower plate Al Powder Load Figure 1 Schematic diagram of compression sharing device Mechanical properties of pure aluminum formed by the compression shearing method Experiment First, pure aluminum powder was created by the compression shearing method. In this research, the powder was 99.9% pure aluminum powder of 9 µm average particle diameter produced by the gas atomizer method. On the surface of each grain, a hard and stable oxide layer of 5 µm thickness was naturally generated. The shear stress applied to the compacted powder was calculated to be 5 MPa for the compacted powder with a compacted area of 4 mm 2. The moving distances L of the lower steel plate ranged from to 1 mm. The moving speed of the lower plate was maintained at around.1 mm/s. This forming procedure was carried out under room temperature and air. Results Figure 2 shows the typical microstructure observed in the pure Al by TEM (L=2 mm). The average crystal grain size measured by the micrograph was about 2 nm. Since the powder compacted by the compression shearing method has small crystal grains (nano-size crystal grains), the mechanical properties are higher than those produced by the hot press and dynamic compaction methods.

3 1 µm 2 nm Figure 2 TEM micrograph of compression-sheared cross-section (L=2 mm) Figure 3 shows the relationship between the moving distance L of the lower steel plate and the relative density. The relative density ρ is obtained by the following equation: ρ=ρ g/ρ where ρ g is the density of the compacted powder and ρ is the density of pure aluminum (2.69 g/cm 3). Regardless of the moving distance L of the lower steel plate in the compression shearing method, the relative density was high. 1. Relative density ρ Moving distance (mm) Figure 3 Relationship between the moving distance L of the lower steel plate and relative density (for pure aluminum) Figure 4 shows the test piece used for the tensile test. The test pieces were based on a piece of JIS Z 221 No. 7. The tensile test speed was 1 mm/min. The strain of the test piece was measured by a strain gauge with a 2-mm gauge length attached to the parallel portion.

4 Moving direction R15 strain gauge Unit of measurement: mm Fig. 4 Tensile test piece. Figure 5 shows the stress-strain curve of the powder compacted by the compression shearing method. The maximum tensile strength and the elongation after fracture for the L=5 mm sample are 2 times and 13 times higher, respectively, than those for the L=2 mm sample. However, the tensile strength and elongation after fracture decreases for samples L=7 and L=1 mm. Figure 6 shows the fracture surfaces of the pure Al compacted powder. The L=2 mm sample is not long enough to obtain a large plastic deformation. However, by increasing L, the oxidation layer on the surface of the pure aluminum powder is destroyed by the shear deformation between the powder particles during compression shearing. Therefore, in the cases of L=5, 7, and 1 mm, the shape of the Al powder was not retained because the oxidation layer on the surface was broken. Furthermore, the prior particle boundaries joined together. This characteristic increased the tensile strength. From the above result, the optimum forming condition of aluminum composites by the compression shearing method was found to be L=5 mm. 3 2 Stress (MPa) 1 L=1mm L=5mm L=2mm L=7mm Strain (%) Figure 5 Stress-Strain curve of the compacted powder (pure aluminum)

5 (a) L=2 mm (b) L=5 mm 1 µm (c) L=7 mm (d) L=1 mm Figure 6 SEM micrographs of the fracture surface of compression-sheared pure aluminum Friction properties of fullerene-dispersed aluminum composite formed by the compression shearing method Experiment The shear stress applied to the compacted powder was calculated to be 5 MPa for the compacted powder with a compacted 2 area of 4 mm (2 mm 2 mm). The moving distance L of the lower steel plate was 5 mm. The moving speed of the lower plate was maintained at around.1 mm/s. This forming procedure can be carried out under room temperature and in the air without heating. The matrix consisted of a rapid-solidified Al-Si-Cu-Mg alloy powder with an average particle size of 41.4 µm. The chemical composition of the Al-Si-Cu-Mg powder is shown in Table 1. On the surface of each grain, a hard and stable oxide layer of 5µm thickness was naturally generated. The amounts of fullerene were a - 3% volume fraction. The entire procedure was carried out in an Ar atmosphere using a glove box. The enclosed powders were mechanically mixed at 5 rpm for four hours by the ball-milling method. Table 1 Chemical compositions of Al-Si-Cu-Mg (mass%) The relation between the friction characteristics and the mechanical properties (Vickers hardness, etc.) has not yet been solved completely. However, it is clear that the friction characteristics are affected by the mechanical properties. In order to investigate the influences on friction by the mechanical properties of fullerene-dispersed aluminum composites, the Vickers hardness test was performed. The load of the Vickers hardness test was 3 N and load time was 15 s. To examine the friction properties, friction measurements were carried out by the pin on disk method in air. For these measurements, the pin was made of stainless steel (SUS34) and had a spherical head surface of 4 mm in diameter. Friction tests were conducted using a test load of.2 N at a sliding speed of 1.7 mm/s. The friction test was conducted in air. Results Figure 7 shows the relationship between the Vickers hardness and volume fraction of fullerene. The Vickers hardness of Al-SiCu-Mg is 15 Hv. The Vickers hardness improves as the volume fraction of fullerene increases due to the effect of the hardness of fullerene, and the hardness number is enhanced as the volume fraction of fullerene increases up to 1%. However, the number decreases suddenly at the volume fraction of 15% and greater. Since fullerene and pores exist at the powder boundary and it is not possible to suppress the plastic deformation of the powder and destroy the oxide film of the powder surface, the binding power of the Al matrix powder decreases with the increase in fullerene.

6 Figure 8 shows TEM (Transmission Electron Microscope) images of the (a) vol.% fullerene sample and (b) 3 vol.% fullerene sample. The pores cannot be observed in the vol.% fullerene sample. However, the pores can be visibly observed in the microstructure of the 3 vol.% fullerene sample. The Vickers hardness decreases due to the existence of these pores. Vickers hardness (Hv) Volume fractions of fullerene 25 3 (vol.%) Figure 7 Relationship between volume fraction and Vickers hardness of Al-Si-Cu-Mg/ fullerene (a) vol.% fullerene sample (b) 3 vol.% fullerene sample Figure 8 TEM images of the (a) vol.% fullerene sample and (b) 3 vol.% fullerene sample Figure 9 (a)-(i) shows the relationship between the friction coefficients of Al-Si-Cu-Mg/fullerene solidified by the compression shearing method and the sliding distance. In the initial frictional stages of the vol.% fullerene sample, the friction coefficient is about However, the friction coefficient of the vol.% fullerene sample increases with the sliding distance, and repeats the increase and decrease. The friction coefficients of the vol.% fullerene sample also increase with the sliding distance, repeating the increase and decrease. However, the friction coefficients of the samples with distributed fullerene 15 vol.% or more decrease suddenly. These results suggest that fullerene has excellent solid-lubricating properties. Figure 1 shows the friction coefficients of Al-Si-Cu-Mg composites containing various volume fractions of fullerene. The addition of 1 vol.% fullerene to Al-Si-Cu-Mg improves the friction coefficient. The average friction coefficient of the vol.% fullerene sample, the 1 vol.% fullerene sample, and the 15 vol.% fullerene sample were found to be 1.,.94 and.33, respectively. Compared with the vol.% fullerene sample results, these values account for a reduction of 6% and 67%, respectively. These results suggest that the Al-Si-Cu-Mg/fullerene composite has excellent solid-lubricating properties.

7 (a) vol.% fullerene sample (b) 1. vol.% fullerene sample (c) 5. vol.% fullerene sample (d) 1. vol.% fullerene sample (e) 12.5 vol.% fullerene sample (f) 15. vol.% fullerene sample (g) 2. vol.% fullerene sample (h) 25. vol.% fullerene sample (i) 3. vol.% fullerene sample Figure 9 Relationship between the friction coefficients of Al-Si-Cu-Mg / fullerene and the sliding distance Volume fractions of fullerene(%) Figure 1 Effect of volume fraction of fullerene on friction properties

8 Conclusions Pure aluminum powder and fullerene-dispersed aluminum composites were created by the compression shearing method. The microstructures and friction coefficients of the compacted powders were investigated. The following conclusions were obtained. 1) The mechanical properties of the sample fabricated by the compression shearing method improved. 2) The friction coefficients of the samples that distributed fullerene 15% or more decreased suddenly. This result suggests that fullerene has excellent solid-lubricating properties. 3) Compared with the Al-Si-Cu-Mg results, the friction coefficient of 15% fullerene sample decreased 67%. Acknowledgments This research was sponsored by TOYO GAGE CO., LTD. The authors are grateful to encouragement and cooperation at TOYO GAGE CO., LTD References 1. H. Makita: Refinement and characterization of fine-diamond particles, New Diamond (3), T. Sano, Y. Murakoshi, et al.: Characterization of diamond dispersed Cu-matrix composite, Mater. Trans. JIM, (5), T. Xu, J. Zhao, et al.: Study on the tribological properties of ultradispersed diamond containing soot as an oil additive, Tribol. Trans., (1), T. Sasada, M. Jinbo: Role of fine diamond particles in three body wear, Rep. Chiba Institute Technol., , Q. Ouyang, K. Okada: Friction properties of aluminum-based composites containing cluster diamond, J. Vacuum Sci. Technol., A 12 (4), Q. Ouyang, K. Okada: Fundamental studies on the rolling friction of ultra-fine particles of cluster diamond, Trans. Jpn. Soc. Mech. Engineers, (585), Q. Ouyang, B. Wang, K. Okada: Atomic force microscopy investigations on the surface topographies of aluminum-based composite containing nanocluster diamond, J. Vac. Sci. Technol., B 15 (4), Q. Ouyang, K. Okada: Nano-ball bearing effect of ultra-fine particles of cluster diamond, Appl. Surf. Sci., , N. Nakayama, K. Hanada, T. Sano, S. Horikoshi, H. Takeishi: Thin Film Forming of Pure Aluminum Powders by Dynamic Compaction, Adv. Technol. Plasticity, N. Nakayama, M. Mayuzumi, K. Hanada, T. Sano, R. Tominaga, H. Takeishi: Thin-film forming of cluster diamonddispersed aluminum composite by dynamic compaction, Key Eng. Mater., , K. Hanada, N. Nakayama, M. Mayuzumi, T. Sano, H. Takeishi: Tribological properties of Al-Si-Cu-Mg alloy-based composite dispersing diamond nanocluster, Diamond and Related Materials, , K. Hanada, K. Umeda, N. Nakayama, M. Mayuzumi, H. Shikata and T. Sano: Characterization of Diamond Nanoclusters and Applications to Self-Lubricating Composites, New Diamond and Frontier Carbon Technology, (3), T. Saito, H. Takeishi and N. Nakayama: New method for the production of bulk amorphous materials of Nb-Fe-B alloys, J. Mater. Res., (3),