forming of fullerene-dispersed aluminum...
TRANSCRIPT
FORMING OF FULLERENE-DISPERSED ALUMINUM COMPOSITE
BY THE COMPRESSION SHEARING METHOD
Noboru NAKAYAMA
Akita Prefectural University,
84-4 Tsuchiya-Ebinokuti, Yurihonjyo, Akita/ 015-0055, JAPAN
Hiroyuku TAKEISHI
Chiba Institute of Technology
2-17-1 Tsudanuma, Narashino, Chiba/ 275-8588, 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 0 vol.% fullerene sample), the 1 vol.% fullerene sample, and the 15 vol.% fullerene sample were found to be 1.00, 0.94 and 0.33, 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 (C60 or C70, 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
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.
Square thread
Torque
Strain gauge
Load
Axial force
Upper plate
Shaft
Lower plate
Al Powder
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 500 MPa for the compacted powder with a compacted area of 400 mm
2. The moving distances L of the lower steel plate ranged from 0 to 10 mm. The moving speed of the lower
plate was maintained at around 0.1 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 200 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.
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=ρg/ρ0
where ρg is the density of the compacted powder and ρ0 is the density of pure aluminum (2.69 g/cm3). Regardless of the moving distance L of the lower steel plate in the compression shearing method, the relative density was high.
0 1 2 3 4 5 6 7 8 9 100.85
0.90
0.95
1.00
Re
lati
ve
de
ns
ityρ
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 2201 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.
1 µm 200 nm
40
7.2
8
R15
4
strain gauge
Moving direction
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=10 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 10 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.
1 2 3 4
100
200
300
0
Strain (%)
Str
es
s (M
Pa
)
L=2mmL=7mm
L=10mm
L=5mm
Figure 5 Stress-Strain curve of the compacted powder (pure aluminum)
(a) L=2 mm
(b) L=5 mm
(c) L=7 mm
(d) L=10 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 500 MPa for the compacted powder with a compacted area of 400 mm
2 (20 mm × 20 mm). The moving distance L of the lower steel plate was 5 mm. The moving speed of the lower
plate was maintained at around 0.1 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 5-µm thickness was naturally generated. The amounts of fullerene were a 0 - 30% volume fraction. The entire procedure was carried out in an Ar atmosphere using a glove box. The enclosed powders were mechanically mixed at 500 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 (SUS304) and had a spherical head surface of 4 mm in diameter. Friction tests were conducted using a test load of 0.2 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-Si-Cu-Mg is 150 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 10%. 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.
10 µm
Figure 8 shows TEM (Transmission Electron Microscope) images of the (a) 0 vol.% fullerene sample and (b) 30 vol.% fullerene sample. The pores cannot be observed in the 0 vol.% fullerene sample. However, the pores can be visibly observed in the
microstructure of the 30 vol.% fullerene sample. The Vickers hardness decreases due to the existence of these pores.
0 5 10 15 20 25 30
0
50
100
150
200
250V
ick
ers
h
ard
ne
ss
(H
v)
Volume fractions of fullerene (vol.%)
Figure 7 Relationship between volume fraction and Vickers hardness of Al-Si-Cu-Mg/ fullerene
(a) 0 vol.% fullerene sample (b) 30 vol.% fullerene sample
Figure 8 TEM images of the (a) 0 vol.% fullerene sample and (b) 30 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 0 vol.% fullerene sample, the friction coefficient is about 0.5-0.6. However, the friction coefficient of the 0 vol.% fullerene sample increases with the sliding distance, and repeats the increase and decrease. The friction coefficients of the 1.0-12.5 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 10 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 0 vol.% fullerene sample, the 1 vol.% fullerene sample, and the 15 vol.% fullerene sample were found to be 1.00, 0.94 and 0.33, respectively. Compared with the 0 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.
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)
Friction coefficient
(a) 0 vol.% fullerene sample
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)Friction coefficient
(b) 1.0 vol.% fullerene sample
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)
Friction coefficient
(c) 5.0 vol.% fullerene sample
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)
Friction coefficient
(d) 10.0 vol.% fullerene sample
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)
Friction coefficient
(e) 12.5 vol.% fullerene sample
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)
Friction coefficient
(f) 15.0 vol.% fullerene sample
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)
Friction coefficient
(g) 20.0 vol.% fullerene sample
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)
Friction coefficient
0 10000 20000 300000
0.2
0.4
0.6
0.8
1.0
1.2
Sliding distance (mm)
Friction coefficient
(h) 25.0 vol.% fullerene sample (i) 30.0 vol.% fullerene sample
Figure 9 Relationship between the friction coefficients of Al-Si-Cu-Mg / fullerene and the sliding distance
0 5 10 15 20 25 300
0.5
1.0
1.5
Fri
cti
on
c
oe
ffic
ien
t
Volume fractions of fullerene(%)
Figure 10 Effect of volume fraction of fullerene on friction properties
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
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