3.1. Effect of MWCNT Content on the Properties of the Nanocomposites
Figure 3a–d presents the FE-SEM images showing their surface morphology of the composite materials in comparison with the unreinforced bulk iron. The surface analysis of the internal cross-section (at 10 mm from the circumference of the samples (i.e., mold surface)) of the sintered materials using FE-SEM and EDS showed all Fe-Al based materials (
Figure 3b–f) had a relatively dense area, similar to that of pure Fe compact (
Figure 3a), and another area containing Al with some porosities. The microstructure of the later area can be attributed to high temperature sintering, which is higher than the melting point of aluminum. In fact, the high temperature led to the formation of aluminum liquid, which have flowed between the solid particles of other components and wetted them causing considerable change in microstructures of the final materials. The FE-SEM images in
Figure 3c,d show the dispersion of MWCNT in composites as small MWCNT clusters and single MWCNTs. The high magnification (×50,000) of the area with MWCNT clusters showed that the MWCNT were embedded in the metal matrix.
The dispersion of each component was observed using EDS mapping and the images are shown in
Figure 3e,f. It can be seen from
Figure 3e,f that the area with a high concentration of Al had more porosities than the area with a high concentration of Fe. The carbon (C) that represents MWCNT was found all-over the surface of the Fe-Al-MWCNT composites, which suggested a good dispersion of MWCNTs in the composites. This may be explained by the fact that the mixing process of MWCNT and Al using a sieve prior to the ball milling have contributed to the reduction of the MWCNT clusters size and improved the dispersion of MWCNT within nanocomposites. The presence of carbon on all the surface of the sample may also be attributed to the promoted diffusion of carbon atoms (from MWCNT and stearic acid) within the material due to the high sintering temperature (800 °C) and long sintering time (10 min) [
37]. Another issue that was observed was the presence of a higher concentration of oxygen in the area with high aluminum content in comparison to the other area of the sample. It is believed that the polished areas of the samples were sensitive to the oxidation after being exposed to the air, especially the area of melted aluminum. This was different from the previous study on the Fe-Al-MWCNT nanocomposites that were sintered at 600 °C where the oxidation tended to occur on the surface of iron area and the solid aluminum particles were clearly and distinctively separated from iron particles and MWCNT [
16,
18]. Additionally, the existence of oxygen in the materials may be resulted from the contamination during the ball milling due to the use of stearic acid.
The phase identification in the materials with a different content of MWCNT (0–2 vol%) was studied using X-ray diffraction.
Figure 4a,b shows the XRD plots of the sintered composites in comparison to pure powder materials. The detailed analysis of XRD patterns was conducted in
Figure 4b–f to check whether some carbides or other additional phases were formed in the composites. One can notice, in
Figure 4b, that some iron oxides peaks, namely Fe
3O
4 (JCPDS Ref No: 019-0629) and α–Fe
2O
3 (JCPDS Ref No: 87-1164) were observed in XRD patterns. The formation of iron carbide (Fe
3C: JCPDS 06-0686) was observed in the composites reinforced with MWCNT as shown in
Figure 4e,f. This agrees with the previous studies [
38] that have reported that when CNTs are used as a reinforcement in the metal matrix, there is a chance for CNT to react with metals and form the carbides. In fact, the carbides were formed due to the promotion of carbon atom diffusion at high sintering temperature [
27,
37].
The disappearance of these Al peaks (Al(111) and Al(311)) in the nFe-Al based materials indicates that Al has partially reacted with iron to form Fe-Al intermetallic compounds like FeAl or Fe
3Al [
39,
40]. The others peaks of Al (200) and (220) may be hidden behind the Fe (110) and Fe (220) respectively. As shown in
Figure 4c–f, the zooming of high intensity peaks of pure Fe in the angular (2 theta) range of 40–50°, the peaks of Fe-Al compounds were observed in the composites and they can be ascribed to FeAl (110) (JCPDS Ref No: 01-1257) or Fe
3Al (220) (JCPDS Ref No: 06-0695) [
39]. It was noticed that the intensity of intermetallic compounds decreased when the MWCNT content was increased while decreasing the Al content.
The highest peaks of the composites were broadened when compared to that of pure metals. Indeed, the broadening of the peaks can be justified by the greater full-width-at-half maximum (FWHM) of the Al (200)/Fe (110) XRD-peaks for the composites than for metals (Al and Fe) as shown in
Figure 4g. The average crystallite sizes were calculated by considering the FWHM obtained for the Al (200)/Fe (110) peaks of the materials using the Scherrer equation [
35]. It can be seen, in
Figure 4g, that MWCNT reinforced composites had a smaller crystallite size ranging between 24.5 and 30 nm than that of Al (46.09 nm), Fe (48.61 nm) and 80nFe-20Al (39.68 nm). The smaller crystallite sizes in nFe-Al composites indicate that larger microstrains were induced in nFe-Al based materials when compared with the pure nFe compact [
41]. The reduction of crystallite sizes in nFe-Al based materials may be ascribed to the subgrains formation, dislocations and grain boundary sliding that occurred in the materials when the heterogeneous particles interacted and their atoms tried to diffuse during the spark plasma sintering process due to the applied heat and sintering pressure.
The chemical state of the surface of the composites with a different content of MWCNT was analyzed using XPS, as shown in
Figure 5.
Figure 5a shows the survey spectra of the composites and
Figure 5b–e compares the composites for the peaks of Fe 2p, Al 2p, O 1s and C 1s respectively. It can be seen that all composites had similar spectra of the abovementioned elements but the major peaks of the spectra detected in composites with 2 vol% of MWCNT (i.e., 80nFe-18Al-2MWCNT) were found to be sharper with higher intensities in comparison to that of other composites. The C 1s peaks were detected in all composites even in the composite without MWCNT (i.e., 80nFe-20Al). The appearance of C 1s peaks in all composites can be ascribed to the absorption of adventitious carbon on the surface of the composites due to air exposure [
42,
43].
The deconvolution of the Fe 2p, Al 2p, O 1s and C 1s peaks for 80nFe-18Al-2MWCNT composite are shown in
Figure 5f–i. Generally, the Fe 2p spectra present two major peaks Fe 2p
1/2 and Fe2p
3/2.
Figure 5f shows that the Fe 2p
3/2 XPS peak exhibited Fe
0, Fe
2+ and Fe
3+ peaks located at 706.69 eV, 709.96 eV and 714.32 eV respectively [
44,
45,
46]. The ratio of Fe
2+/Fe
3+ in Fe 2p
3/2 was about 2:1, which was greater than the ratio 1:2 reported in [
47,
48]. This suggests the coexistence of both Fe
2O
3 and Fe
3O
4 in the analyzed sample, which also confirmed the XRD analysis in
Figure 4b. The presence of Fe
2O
3 was also confirmed by the detection of the satellite peak at 719.68 eV, which is associated with Fe 2p
3/2 for Fe
2O
3 [
48]. The Fe 2p
1/2 XPS peak comprised of the Fe
2+ (723.18 eV) and Fe
3+ (729.22 eV) peaks [
44].
Figure 5g that Al
0 and Al
3+ [
44,
47] were detected at the binding energy of 72.36 eV and 74.54 eV. It can be seen in
Figure 5h that O 1s peak had three peaks after deconvolution, which are located at 529.65 eV, 530 eV and 531.81 eV, which correspond to Fe
2+, Fe
3+ and Al
3+ respectively [
44,
45,
47]. Based on the XPS analysis, it can be concluded that there was the formation of iron oxides (Fe
2O
3 and Fe
3O
4) and aluminum oxide (Al
2O
3) on the surface of the composites. This supports the EDS mapping (in
Figure 3e,f) and XRD analysis (in
Figure 4), except that the XRD did not detect the Al
2O
3 peaks probably due to the very low content of aluminum oxide in the materials. Based on XRD and XPS analysis, it can be assessed that the FeAl/Fe3Al were formed in materials.
Figure 5i shows that C 1s peaks can be divided into three peaks with binding energy of 285.14 eV, 285.68 eV and 288.74 eV, which were attributed to C–C, C–O and C=O bonding respectively [
49]. Note that as it can be seen in
Figure 5e C 1s peaks were found to be similar in the analyzed samples, except that a very slight shift of major peaks in C 1s spectra to lower binding energy was noticed in composites reinforced by MWCNT. This may be explained by the increase in species with C–C bonding on the surface of the sample when MWCNTs were used as reinforcement due to the presence of MWCNT in the form of single or clusters as was observed with SEM images in
Figure 3c,d.
The variation of magnetic properties according to the CNT content in composites was investigated. For comparison, a pure iron (nanoparticles) compact was sintered and machined under the same conditions as the composites. Moreover, those properties were compared with the commercial sendust core (Fe-Si-Al alloys, code: CS610125), which was purchased from Chang Sung Corporation (South Korea) and machined for sampling.
The hysteresis loops of the above materials were generated by VSM and are plotted in
Figure 6. As it can be seen, the composites had the hysteresis loops with slopes comparatively similar to that of pure iron compact and sendust core. The comparison among the composites showed that the incorporation of MWCNT in composites did not significantly affect the saturation magnetization. In this case, all Fe-Al based composites had a saturation magnetization close to 148.8 A.m
2/kg, which is higher than that of sendust core (122.2 A.m
2/kg) as shown in
Table 2.
Table 2 compares the typical magnetic properties and the density of composites with the commercial sendust core. It can be seen that composites had higher coercivity (1995.70–2368.63 A/m) and higher remanence (1.16–1.22 A·m
2/kg) than sintered pure iron and sendust core (Fe-Si-Al alloys). Both the coercivity and remanence were increased with an increase of MWCNT content. The increase in coercivity with respect to the MWCNT content in composites can be explained by the fact that by increasing the amount of MWCNT, which are not magnetic materials, there is a chance of increasing the number and size of MWCNT clusters, which in turn promotes the decoupling of neighboring magnetic grain domains and result in high coercivity [
50]. The high coercivity of the nFe-Al based composite was also attributed to the high coercivity of the initial iron nanoparticles and to the formation of carbides and FeAl/Fe
3Al compounds, as was shown in XRD analysis (
Figure 4). In addition, as shown in
Figure 4g, the materials exhibited small grain sizes (<50 nm), which can explain such high coercivity of the materials because the grain boundaries of those small grains would promote the magnetic domain-mall pinning effect [
51], and therefore demagnetization of the materials would require high magnetic energy leading to greater coercivity.
The sintering process of the iron nanoparticles has reduced coercivity up to 90.42% and improved the saturation magnetization as compared to the as-received iron nanopowders (see
Figure 6 and
Table 2).
As shown in
Table 2, although there was not significant change in the density of the composites, the density decreased as the content MWCNT increased and the composites had comparatively lower density than the pure iron compact. That is because MWCNT is lighter than both Al and Fe, and Al is lighter than Fe.
Figure 7a shows the electrical properties of the sintered materials. It can be seen that all composites exhibited about twofold higher resistivity than the pure iron compact. These results were in good agreement with the fact that the addition of a small amount of Al in the Fe matrix would result in material with improved electrical resistivity [
13,
16,
17,
18]. A decrease in electrical resistivity of the composites was observed as the result of increasing the volume content of MWCNT. Basically, when an magnetic materials is placed in varying magnetic field, the eddy currents are created within the materials, which in turn generate magnetic fields circulating in the opposite direction to the original field and cause the energy loss known as eddy current losses [
52]. Those losses are inversely proportional to resistivity [
53,
54]. That means that the magnetic materials with higher resistivity are needed to reduce eddy current losses in the magnetic circuit. Therefore, the composites manufactured in this work would generate lower eddy current losses than the pure iron compact.
The effect of MWCNT content on mechanical properties was also evaluated. The MWCNTs were found to increase the hardness of composites (see
Figure 7b). The hardness was improved up to 19.3% and 24% by using CNT 2 vol% in comparison with pure Fe compact and 80nFe-20Al respectively. This can be attributed to the presence of iron carbide formed during the sintering of the composites that contain MWCNTs and their small crystallite size as shown in XRD analysis,
Figure 4.
Figure 7c illustrated the transverse rupture strength (TRS) of the composites with respect to the MWCNT content and in comparison with the compact of Fe nanoparticles. It was found that the MWCNTs have reduced the strength of the composite materials and the more MWCNTs were used the lower the strength that was observed. The material without MWCNT (80nFe-20Al) had the TRS of 707.88 MPa. However, after adding MWCNT 1 vol% and 2 vol% as reinforcements, the strengths were dropped to 640.85 MPa and 585.95 MPa respectively. This contracted what was expected, which was the improvement of the mechanical strength of the Fe-Al based nanocomposites due to the MWCNT reinforcement as was reported on the strengthening of metal matrix composites with carbon nanotube materials in literature [
55].
The composites had a lower strength than the pure iron compact. This trend might be associated with the presence of pores in the area of high concentration of Al. Moreover, the network structure of MWCNTs makes them difficult to be separated one from the others and causing them to be dispersed in small cluster forms (aggregation) [
56]. Therefore, an aggregate of MWCNT in composites may be considered as the pores due to insufficient MWCNT-metal particles bonding that might cause insufficient load transfer between metallic domains and the MWCNTs and therefore led to the reduced transverse rupture strength. Moreover, the reduction of the mechanical strength of the nFe-Al- MWCNT materials was caused by the formation of brittle iron carbides.
3.3. Effect of Combining Fe Nanoparticles and Fe Microparticles
The mixture of nanoparticles and microparticles is another factor that might affect the properties of the powder metallurgical parts. Here, the use of iron particles depending on the nanoparticles (nFe) to microparticles (mFe) volume ratio was considered in the 80Fe-19Al-1MWCNT composites. The iron nanoparticles had an average size of 90–110 nm and purity of 99.9% whereas the iron microparticles had an average particle size of 40–50 μm and purity of 99.9%. The powders of composites were ball milled for 24 h and spark plasma sintered at 800 °C as described in the experimental section.
Figure 12 shows the microstructure of the composites according to the mixture ratio of iron (Fe) nanoparticles and Fe microparticles. As illustrated in
Figure 12a,b, the ball milling of nanoparticles and microparticles caused the nanoparticles to be coated on the surface of the large particles resulting in core-shell structures formation. Two kinds of core-shells were formed after ball milling, namely the (mFe-nFe) core-shells and the (Al-nFe) core-shells. Then, during the liquid phase sintering, the (Al-nFe) core-shells were destroyed due to the melting of Al causing the transformation of the Al solid in Al liquid and the Al liquid penetrated between the nearby Fe nanoparticles. That caused the rearrangement of solid particles in the liquid phase within the composites where the liquid had to fill the gap between the solid particles. Then the crystallization of the liquid–solid phase resulted in non-uniformity of the high Al concentration region. The presence of a small amount of iron in the area of high Al concentration is the iron nanoparticles that were merged in the Al liquid. On the other hand, (mFe-nFe) core-shells formed on the homogenous component by diffusion bonding among the Fe microparticles (mFe) and Fe nanoparticles (nFe) due to relatively high temperature sintering. In
Figure 12c,d, the EDS mappings of local area on the polished surface of the composites show a non-uniform surface at the region of high Al concentration and a uniform surface at the region of high Fe concentration.
The phase identification studied with XRD is shown in
Figure 13. The composite with a high content of iron microparticles had lower intensity peaks. On the other hand, the peaks of the composites with 50/50 nano-to-micro ratio (i.e., 40nFe-40mFe-19Al-1MWCNT) showed a wider base and higher peak intensity than the other composites. This implies that the microstrains were largely induced in the later composite (40nFe-40mFe-19Al-1MWCNT) when compared with other composite materials [
41]. A detailed analysis of XRD-peaks revealed the presence of iron oxides peaks in all composites, Fe-Al compounds (FeAl/Fe
3Al) and iron carbide in all composites as shown in
Figure 13b–e. These compounds resulted from the chemical reactions between the components during the sintering process.
Figure 13f shows that the composites 40nFe-40mFe-19Al-1MWCNT exhibited the biggest crystallite size of iron carbides and smallest crystallite size of Fe and Fe-Al compounds.
The magnetic hysteresis loops of the composites are shown in
Figure 14a,b and
Figure 14c,d compares the hysteresis of the as-received iron nanoparticles and iron microparticles.
Table 4 compares the magnetic properties obtained by VSM measurements and the density of the composites, which are compared with iron nanoparticles based compact, as-received Fe nanopowders and Fe micropowders. The nFe-Al-MWCNT nanocomposite consisting of iron nanoparticles only had the highest coercivity of 2175.64 A/m, which may be attributed to higher coercivity of the iron nanoparticles (15,309.71 A/m) as shown in
Figure 14c,d and
Table 4. The mixture of iron nanoparticles and iron microparticles have enhanced the saturation magnetization and reduced both the coercivity and remanence. The 40nFe-40mFe-19Al-1MWCNT exhibited the highest magnetization of 157.82 A·m
2/kg and lowest coercivity of 1083.36 A/m.
As shown in
Table 4, the density of the composites with only iron nanoparticles had the highest density (6.28 g/cm
3) among the composites but lower than the pure iron nanoparticles compact (7.47 g/cm
3). Obviously, the densification reduced as the content of large particles increased. Actually, the iron particles are harder than the aluminum particles. Therefore, the small particles of iron are easily packed with aluminum because they are harder and have a small surface of contact, which allows them to easily penetrate on the surface of aluminum particles due to particle–particles impacts during ball milling and also due to compaction. On the other hand, the larger iron particles induced large deformation of aluminum particles and among themselves during the compaction, which may not completely eliminate the space between particles and consequently resulted in lower density of the compact. Moreover, the large Fe particles (microparticles) require high compacting pressure than Al particles to eliminate the space between them and are not easily packed as for small Fe particles (nanoparticles). It can be assessed that the number of small voids between particles in compacts may have been greater in the composites with a high volume content of Fe microparticles, which reduced their densities.
As shown in
Figure 15a, the slight increase in electrical resistivity was observed as the content of iron microparticles increased. This may be associated with the presence of small pores within the composites that reduced the electrical contact inside the composites and increased the resistance of the electricity flow. The volume resistivity of each composite was more than two times than that of pure iron compact.
Figure 15b compares the hardness of composites with a different content of nanoparticles and microparticles. It was found that the incorporation of Fe microparticles in composites has improved about 16.5% in microhardness. Interestingly, the composites with nanoparticles and microparticles mixture exhibited more than 10.8% in hardness than the pure iron (nanoparticles) sintered part. This was attributed to the availability of a large and hard cross-section created by the iron microparticles that can support the load applied by the indenter of the Vickers hardness tester. In addition, from the results shown in
Figure 13f and
Figure 15b, there were no obvious dependency between Vickers microhardness and crystallite size, thus the increase in hardness with particle size may be attributed to the phase transition (soft to hard) and the decrease of interfacial motions [
59].
The results on mechanical transverse rupture strength (TRS) of composites depending on the nano-micro Fe mixture are illustrated in
Figure 15c. On the basis of 80 vol% of iron powder in the overall composite volume; 80 vol% of nanoparticle (80nFe), the combination of 40 vol% nanoparticles and 40 vol% microparticles (40nFe-40mFe), and the combination of 20 vol% nanoparticles and 60 vol% microparticles (20nFe-60mFe) were compared. The results in
Figure 15c show that the strength had reduced with an increase of Fe microparticles content. The reason for this decrease in strength might be because, firstly, the nanoparticles have large surface area to volume ratio than the microparticles. This makes nanoparticles to be easily packed with lower porosities than for microparticles and leads to a dense and stronger compact. Secondly, during milling of powders, the nanoparticles have formed a core-shell on the surface of the microparticles. Additionally, when the large volume fraction of nanoparticles was used, the excess amount of them has remained separated to microparticles. The remainders have filled the space among the microparticles during the powder packing and bonded with other neighborhood particles during compaction and sintering. Thus, the larger the volume fraction of nanoparticles with respect to the content of microparticles the smaller were the unfilled spaces (pores) among the particles. Additionally, then with enough compacting pressure while sintering, the greater the strength of materials was achieved. In fact, for composites with a high content of Fe microparticles, high compaction pressure or high sintering temperature is required to achieve full densification and high strength.