Tensile Strength of Surface Nanostructured Copper
Hoi-Lam Chan, Jian Lu *
Department of Mechanical Engineering, The Hong Kong Polytechnic University,
Kowloon, Hong Kong China
*Corresponding Author: [email protected]
ABSTRACT
The tensile strength of electrodeposited Cu subjected to surface mechanical attrition treatment (SMAT) was studied. Cu after
electrodeposition had been first studied. Electrodeposited Cu had the well-known improvement on the strength compared with
the bulk hard copper. After SMAT, although the ductility had dropped, the yield strength is further enhanced up to 50% (350MPa)
to obtain a stronger material. Similar results had been obtained in the hardness test. The hardness of the electrodeposited Cu
had further improved up to 35% after SMAT. Since the parameters of the electrodeposition affects the properties of the materials
obtained, rooms for improvement could be expected. The present study demonstrates the potential process for developing a
new kind of surface nanostructured materials for engineering applications.
1.
Introduction
Various research topics arises from Nanostructured (NS) metals and their usage are increasing gain moments in the field of
materials research regardless of structure, properties or the development of the process for bulk forms for engineering
applications. The reduction of grains size could have quite significant implications on the properties of the bulk material.
Increasing the strength and hardness were some of the well known enhanced properties that can be obtained for metals with
grains of nano-scale. A lot of literature has shown that there are numerous techniques that can be used to obtain NS materials,
mostly commonly used one employ inert gas condensation or chemical vapour condensation, pulse electrodeposition, plasma
synthesis, crystallization of amorphous solids, severe plastic deformation, consolidation of mechanically alloyed or cryomilled
powders, and Surface Mechanical Attrition Treatment (SMAT). However, there are only a few techniques, such as
electrodeposition and SMAT could produce nanostructured materials with sufficient thermal stability to allow the fabrication in
bulk form [1].
Having the advantages of achieving high rate of deposition, high resolution, high shape fidelity, available for providing artificial
materials structuring such as multilayer and being a low cost process, electrodeposition has been widely used in various
aspects of materials engineering. It has become a mature technology for materials deposition. Enabling processing under room
conditions (temperature and air), this reduces problems associated with thermal stresses; and with no vacuum required, the
equipment can be kept at low cost. Electrodeposition has long maintained the cost advantage over traditional methods or
vacuum techniques such as sputtering or evaporation [2]. Many researches has also shown that the grain size obtained from
this process could be able to reach to about 10 - 40 nm, by far a superior and more robust form is achieved.
SMAT is an approach to NS processing that has the advantages of producing 3-D bulk material on the surface of the complex
shape samples without porosity, contamination and other defects, and low cost. It is made good use of spheres excited by
frequency displacement generator to hit the surface of the samples to obtain NS surface. It is clear that the processing history
had a significant effect on mechanical properties of NS materials and SMAT is well known for providing the surface layer with
high strength, in the case of stainless steel, 3 times higher than the original materials by means of the hitting action of the
spheres. Thus, after the SMAT process, the surface mechanical, tribological, chemical and corrosion properties of the bulk
materials could subsequently be enhanced [3].
Combining these two methods to obtain NS materials in bulk form with desirable mechanical properties appears to be a low cost,
efficient and workable potential solution which is worthy to be investigated. The preliminary study on this synthesis method was
first put on Cu and aimed at studying the possible mechanical properties (tensile strength and hardness) obtained. It is
well-known that electrodeposited Cu has better strength than the conventional coarse-grained one. The SMAT process, as
mentioned above, can effectively improve the tensile strength of the samples without altering the chemical composition. Hence,
further enhancement on strength may be expected for the samples that have been treated with both of the processes. A
question thus arises: What degree can it improve? The present work aims to answer the question by a series of experiments
with trials of different parameters.
2.
2.1
Experimental
Electrodeposition
Pure copper was used for studying the proposed method. In order to ensure the purity of the deposited metal, an acidic copper
forming bath with composition of no other impurities added was used in the electrodeposition process. The substrate used was
3
Kerr Injection wax molded in a rectangular block (30 x 30 x 120 mm ). It was used instead of other metallic substrate or resin, in
order to avoid the problem of removing substrates at high temperature, which could cause undesirable change of structure of
the materials after the process of electrodeposition.
Experimental evidence has shown that pulsed electrodeposition increases the density of nucleation sites because of a high
current density during the on time, while interrupting growth and favoring renucleation during the off time, which facilitates the
formation of multiple nano-twins [4]. High purity Cu rectangular block with nano-scale growth twin lamellae are then synthesized
by means of the pulsed electrodeposition technique from an electrolyte of CuSO4. The pulsed electrodeposition was carried out
using cathodic square wave pulses by turning off the current periodically, with an on-time of 0.02 s and an off-time of 2 s. The
detailed settings and composition of the electrolyte bath will be listed in Table 1 [4].
Table 1
The settings and composition of the electrolyte bath [4].
Metal
Electrolyte
T (°C)
Power
20 ± 1
Peak = 0.5 A/cm
Remarks
composition
CuSO4 ．xH2O
Cu
(X mol/l)
2
Purity of the Cu sheet as anode = 99.99%
-double pulsed power:
Ratio of surface area of anode: that of cathode: 30:1
-on time = 0.02s
Cathode substrate: pure Fe sheet coated by a Ni-P
-off time = 2s
amorphous thin film, exposed surface area: 20 x 10
2
-Cathodic square wave
mm
pulse
Electrolyte is needed to stirred mechanically
pH = 1
The electrodeposition process had been carried out at the ambient temperature, 24°C. There were a number of parameters that
had been adjusted in the electrodeposition process to control the characteristics and the thickness of the deposited copper layer.
The deposition rate in electroplating and the surface texture of the deposited layer were controlled by the current density. The
surface texture was further regulated by the power used, e.g. the surface quality in the case of double- pulsed power used was
different from that of direct current power used, but double-pulsed power was mainly used in this work. Also, the thickness was
directly related to the duration of the electrodeposition process: the longer the duration, the thicker the deposited layer, and also
the position and direction of the rectangular block placed in the bath: the larger the surface area keeping away from each other
the thicker the deposited layer it became.
After the electrodeposition process, the electrodeposited Cu was separated from the wax substrate by de-waxing process and
further be cut into the dog-bone shape according to the ASTM E8M-04 standard, Standard Test Methods for Tension Testing of
Metallic Materials [Metric], and ready for use in the mechanical property tests or further be treated with SMAT.
2.2
SMAT
The detailed set-up and processing of the SMAT have been described previously [4]. Vibrating by the vibration generator, the
stainless steel balls were resonated to impact the sample surface. The whole surface has been impacted with a large number
shots in the short period of time as the result of the high frequency vibration. In this work, the samples were treated at room
temperature with stainless steel balls (2 mm diameter) at the frequency of 50 Hz for 30 seconds (hereafter noted as sample
SMAT-0.5), 1 min (sample SMAT-1), 2 min (sample SMAT-2), and 4 min (sample SMAT-3) respectively.
Some of the samples after electrodeposition were treated with SMAT with the time slots differences as mentioned above, while
some remained untreated for comparison.
2.3
Tensile Test
The tensile tests were carried out under an ambient condition, at 22°C and 45% humidity. The samples prepared were
mounted on the MTS RT-50 (50kN) tensile testing machine. The settings such as grip separation, extensometer gauge
length (which is always 25mm), crosshead speed (= 3 mm/ min), dimensions of sample were input for later calculations.
The extensometer (MTS 632 24F-50) was used to collect the data of the extension of the sample during the test.
The size of the sample prepared for this test is quite large, which is different from those normally used in other studies for
nano-scale materials. It is shown in fig. 1. The number of samples prepared for this test is 7 samples for each parameter,
its purpose is to obtain a validate result.
Fig.1
The standard size and shape of the deposited- Cu plate according to ASTM E8M-04 standard for tension testing of
metallic materials.
Dimensions, mm
Normal Width
6 mm
G- Gauge Length
25.0 ± 0.1
W- Width
6.0 ± 0.1
T- Thickness
Maximum thickness = 6 mm
R- Radius of fillet, min
6
L- Overall length
100
A- Length of reduced section, min
32
B- Length of grip section
30
C- Width of grip section, approximate
10
2.4
Micro -hardness Test
The hardness tests were carried out under an ambient condition, at 22 ° C and 45% humidity. The hardness of the
electrodeposited and SMATed electrodeposited samples were measured using Future-tech FM series micro-hardness tester.
The unit and magnitude of the hardness are defined by Vickers hardness (Hv). It was used to measure the hardness of the
surface of the samples.
At least 7 points were measured from each sample. Every indentation mark was observed through the optical microscope to
guaranteed its symmetric characteristic.
3.
Results and Discussion
There are various literatures have reported on the experimental results on the tensile strength of Cu. The comparison of the
mechanical properties of the nanostructured- Cu with different treating methods and that of coarse-grained copper was shown
in Table 2 [6-10].
In this work, the mechanical strength of the electrodeposited Cu obtained from different parameters, mainly the power settings
were observed. Comparing the results with the conventional Copper which had yield strength of 69 MPa, strain of 45%, it could
be easily found that the yield strength had enhanced 3 times although the strain had reduced to half. Different Power settings
including, pulsed, double-pulsed or without pulsed with different on- time or off- time combination were used, and the optimized
result among all the trials was listed in Table 3; and further comparing this work with that from the literatures, it is not hard to
discover that the combination of the yield strength and the ductility obtained from this work has been improved and the data
falling onto the separated zone from the general trend which was shown in Fig. 2.
Table 2
Data of the tensile properties of pure Cu. The data are for Copper of conventional, ultrafine and nanocrystalline
grain size, and after cold rolling to various degrees of Cold work [6 - 10].
Treatments on Cu
Yield Strength (MPa)
Uniform Elongation (%)
Ref
Annealed, coarse-grained
69
45
[6]
Room temp. rolling to 95% CW
390
4
[6]
Inert gas condensation
300 – 360
8–2
[7]
Bimodal grain size
760
3
[8]
SPD
400
12
[9]
Electrodeposition
119
22
[10]
Fig.2
The comparison of the experimental results under optimum conditions with the other literatures.
The comparison of the mechanical properties of the nanostructured- Cu with different treating methods and that of
coarse-grained copper
50
45
Uniform Elongation (%)
40
35
Annealed, coarse-grained [6]
Room temp. rolling to 95% CW [6]
30
Inert gas condensation [7]
25
Inert gas condensation [7]
Bimodal grain size [8]
20
Electrodeposition [9]
15
10
5
0
0
100
200
300
400
500
600
700
800
Yield Strength (MPa)
The experimental results obtained in this study
The optimal result obtained in this study
Table 3
Experimental tensile test results of the electrodeposited Copper samples with different power settings.
Settings
Power with 43% double- pulse
Width
Thickness
Tensile Strength
Yield Strength
Strain At Break
mm
mm
(MPa)
(MPa)
(%)
5.943
0.565
322.3
252.571
26.1
Following with the electrodeposition process, the Cu specimens were further treated with SMAT for different time periods: 30
seconds (sample SMAT-0.5), 1 min (sample SMAT-1), 2 min (sample SMAT-2), and 4 min (sample SMAT-3) respectively with the
same conditions: same circular horn (70 mm in diameter), same power excitation, stainless steel balls with same size (2 mm in
diameter), same composition and amount in this work.
Experiments showed that the longer the time the specimens put under treatment, the more severe the deformation they became.
The longer the treating time, the better the mechanical properties of the surface nanostructured Cu can be obtained until the
deformation is too serious for testing. It could be understood that Copper is a relatively soft materials, although it was
electrodeposited and the properties had been improved, the duration for the treatment could not be too long. Also, the thickness
of the specimens had taken a very important role: the thicker the specimen, more capable of withstanding the deformation due
to the SMAT process with improved mechanical properties.
The optimized result of the mechanical properties of the SMATed Electrodeposition Cu in this stage had been shown in Table 4.
Comparing the results with the electrodeposited ones, there was a significant increment in the strength while decrement in
strain. It had been fallen back onto the general trend (Fig. 3). However, the optimized conditions for electrodeposition and that for
SMAT for Copper had not yet been found in this stage. There were still rooms for improvements.
Table 4
Optimized experimental tensile test results of the SMATed electrodeposited Cu samples.
Duration for the
Width
Thickness
Tensile Strength
Yield Strength
Strain At Break
Treatment
mm
Mm
(MPa)
(MPa)
(%)
1 min
6.510
0.570
388.5
355.999
5
Fig. 3
The comparison of the mechanical properties of the samples from this work with that of the nano- Cu in other
papers.
The comparison of the mechanical properties of the nanostructured- Cu with different treating methods and that of
coarse-grained copper
50
45
Uniform Elongation (%)
40
35
Annealed, coarse-grained [6]
Room temp. rolling to 95% CW [6]
30
Inert gas condensation [7]
Inert gas condensation [7]
25
Bimodal grain size [8]
Electrodeposition [9]
20
SPD [10]
15
10
5
0
0
100
200
300
400
500
Yield Strength (MPa)
Ed- Cu
SMATed- Ed Cu
600
700
800
Besides the tensile test, the hardness test of the Electrodeposited Cu and the SMATed Ed-Cu were carried out. The hardness of
each specimen was measured five times at random locations respectively. The average value of tests was recorded as the
hardness value. 300 gram force and 10 seconds of dwell time were used in the indentation. All the specimens were polished
before indentation. The hardness values of both the untreated (only electrodeposited) and SMA treated samples were shown in
the Table 5. The result of the hardness test was consistent with the result of tensile test. The hardness of the untreated Ed-Cu
samples was greatly improved after SMAT. The improved result could be up to 34.5 %. The fact of the increment of hardness
higher than that of tensile modulus may be interpreted by the compression modulus is higher than the tensile modulus of Ed- Cu
as the hardness test is under a compression load. In addition, it is also believed that this increase in hardness is due to grain
refinement at the surface after SMAT to the nm scale [11].
Table 5
The hardness values of both the SMA untreated (only electrodeposited) and SMA treated samples.
Improvement when comparing with SMA
Vicker hardness (Hv)
treated Ed-Cu
SMA untreated Ed- Cu
94.82
--
SMA treated Ed-Cu
127.52
34.5%
4.
Conclusions
The mechanical properties, such as the tensile strength and the hardness, of the Nanostructured Cu under the treatment of
both electrodeposition and SMAT had enhanced at an optimistic degree compared with the nano-Cu processed by other
methods. Although the mechanical properties that it can obtain at this stage is not yet the best among the others, the potential of
developing a new kind of nanostructured material by the combination of these two methods could still be seen and further
improvement could be expected after further development.
Acknowledgement
This project was supported by The Hong Kong Polytechnic University Niche Areas Grant (BB90).
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