Strengthening Effects of Multi-Walled Carbon Nanotubes and Graphene …
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Table 1 Mechanical properties of pure nickel and Ni-CNT composites
Sample
Yield stress (MPa) Ultimate tensile
stress (MPa)
Elongation (%) Microhardness (HV
0.5)
Pure Ni
142.75
509.1
44
113.78 ± 2.1
Ni-Ball milled 286.91
545.73
58.5
143 ± 2.3
Ni-0.5CNT
436.8
691.38
22
180.2 ± 3.2
Ni-1CNT
403.88
479.65
6.1
193.7 ± 2.8
Ni-2CNT
288.04
293.36
1.5
165.9 ± 3.9
strength, and elongation properties of pure nickel and Ni-CNT composites are shown
in Table 1. Pure nickel exhibited a yield strength of ~143 MPa along with 44% elongation showing typical strain hardening behavior. Ni-0.5 CNT composites exhibited
a yield strength of ~461 MPa (2.5 times higher as compared to pure nickel) along
with 18% elongation. It is apparent that the tensile strength of Ni-0.5CNT and Ni1CNT nanocomposites is significantly increased due to the combining effect of CNT
reinforcement as well as nickel matrix grain refinements. It is observed that the
improvement in the tensile strength of the composites is attained at the expense of
ductility. As CNT content increases further, significant decreases in yield strength
have been observed in Ni-2CNT composites due to the presence of CNT clusters,
which act as a potential site for crack nucleation and eventually fail at lower yield
stress.
Ni-GNP Composites
X-ray Diffraction (XRD) Analysis
X-ray diffraction patterns of pure Ni and Ni-GNP composite are shown in Fig. 4.
Results illustrate that the peaks corresponding to (111), (200), (220) crystallographic
planes of FCC nickel are the only peak present. The peak corresponding to HCP
(0002) carbon is clearly visible in Ni-2GNP composite. However, this peak was not
observed in Ni-0.5GNP, and Ni-2GNP, due to the relatively low content of GNPs
in these composites. The absence of any other peaks, primarily Ni 3 C, indicates no
reaction between nickel and graphene nanoplatelets during MA and high-temperature
SPS processing.
Microstructural Analysis
SEM micrographs of pure nickel Ni-GNP nanocomposites are shown in Fig. 5. Pure
nickel exhibited a uniform microstructure without any porosity. As GNP content
increases from 0.5 to 2 wt.% in Ni-GNP composites, significant grain refinement
117
Table 1 Mechanical properties of pure nickel and Ni-CNT composites
Sample
Yield stress (MPa) Ultimate tensile
stress (MPa)
Elongation (%) Microhardness (HV
0.5)
Pure Ni
142.75
509.1
44
113.78 ± 2.1
Ni-Ball milled 286.91
545.73
58.5
143 ± 2.3
Ni-0.5CNT
436.8
691.38
22
180.2 ± 3.2
Ni-1CNT
403.88
479.65
6.1
193.7 ± 2.8
Ni-2CNT
288.04
293.36
1.5
165.9 ± 3.9
strength, and elongation properties of pure nickel and Ni-CNT composites are shown
in Table 1. Pure nickel exhibited a yield strength of ~143 MPa along with 44% elongation showing typical strain hardening behavior. Ni-0.5 CNT composites exhibited
a yield strength of ~461 MPa (2.5 times higher as compared to pure nickel) along
with 18% elongation. It is apparent that the tensile strength of Ni-0.5CNT and Ni1CNT nanocomposites is significantly increased due to the combining effect of CNT
reinforcement as well as nickel matrix grain refinements. It is observed that the
improvement in the tensile strength of the composites is attained at the expense of
ductility. As CNT content increases further, significant decreases in yield strength
have been observed in Ni-2CNT composites due to the presence of CNT clusters,
which act as a potential site for crack nucleation and eventually fail at lower yield
stress.
Ni-GNP Composites
X-ray Diffraction (XRD) Analysis
X-ray diffraction patterns of pure Ni and Ni-GNP composite are shown in Fig. 4.
Results illustrate that the peaks corresponding to (111), (200), (220) crystallographic
planes of FCC nickel are the only peak present. The peak corresponding to HCP
(0002) carbon is clearly visible in Ni-2GNP composite. However, this peak was not
observed in Ni-0.5GNP, and Ni-2GNP, due to the relatively low content of GNPs
in these composites. The absence of any other peaks, primarily Ni 3 C, indicates no
reaction between nickel and graphene nanoplatelets during MA and high-temperature
SPS processing.
Microstructural Analysis
SEM micrographs of pure nickel Ni-GNP nanocomposites are shown in Fig. 5. Pure
nickel exhibited a uniform microstructure without any porosity. As GNP content
increases from 0.5 to 2 wt.% in Ni-GNP composites, significant grain refinement