Mechanical Property and Corrosion Behavior of Powder-Metallurgy-Processed 3D Graphene-Networks-Reinforced Al Matrix Composites
Round 1
Reviewer 1 Report
This manuscript deals with elaboration and characterization of graphene-aluminum composites combining powder metallurgy and SPS which is certainly a hot topic.
This work could be improved by adding more information about samples microstructure and morphology, currently there is no SEM data about composite powders nor sintered samples. Since graphene dispersion is of paramount importance and the authors report a decrease in hardness for the highest graphene content tested and they ascribe this to graphene agglomeration, SEM data on powders and bulk samples could reinforce this hypothesis.
The discussion about relative density and mechanical properties would benefit from further information regarding grain size i.e. since it is a well-known factor influencing hardness, and GN addition is likely to modify it, is the Al grain size similar in pure Al and composite bulk samples?
The reviewer has one major reservation about figure 6 which currently does not make sense (the curves shapes and values seem wrong). Tensile strength is expected around 250 MPa not 2000 MPa and max strain is expected about 10% not 90%!
Once this issue is sorted, such values should be compared to results already reported in the literature.
Here are some additional comments:
l. 68: “To date, most of research on graphene reinforced AMCs has concentrated on low content of graphene (below 5 wt%) [23-25]. But the range of graphene content is still very researchable.” This last sentence is not easy to understand.
l.77: “The main process of PM is: preparation and preparation of raw materials, obtaining raw embryos, sintering and forming [32].” Embryos is not the right word (subsequent use l. 118)
l.111: “oscillated ultrasonic » replace with “sonicated”
l.120: “The sintering was done in temperature 120 of 600 ℃ for 10 minutes under the vacuum atmosphere” please add heating rate and pressure applied.
l.130-131: what was the loading applied (and time) for Vickers microhardness testing?
l.139-140: “Characteristic XRD peaks of aluminum at their respective crystallographic phase” this sentence should be modified
Supplementary materials should be removed since they consist purely of figures from the main text.
Author Response
Responds to the reviewer’s comments
(crystals-2247875)
Dear Editor:
We appreciate the reviewer’s insightful comments. All these comments are valuable and helpful for improving our paper and providing the important guidelines to our studies. The manuscript has been revised according to the suggestions and comments of the reviewers. A point-by-point response to the reviewer’s comments is presented in the following.
The revised manuscript with the major revisions is marked for your convenience. After improvement following the reviewers' suggestions, the revised paper should become more interesting and provide more valuable information to the readers. Once again, we earnestly appreciate Editor/Reviewers’ work and hope the corrections will meet the requirement for publication in Crystals.
Thank you very much for your consideration!
Sincerely yours,
Yifang Ouyang
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To reviewer #1:
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Reviewer Comments: This manuscript deals with elaboration and characterization of graphene-aluminum composites combining powder metallurgy and SPS which is certainly a hot topic.
Comment 1: This work could be improved by adding more information about samples microstructure and morphology, currently there is no SEM data about composite powders nor sintered samples. Since graphene dispersion is of paramount importance and the authors report a decrease in hardness for the highest graphene content tested and they ascribe this to graphene agglomeration, SEM data on powders and bulk samples could reinforce this hypothesis.
Author Response: Thank you for your precious comments. We have added SEM images (Fig. 4 and Fig. 5) and microstructure analysis of the 3DGN/Al composite powders and sintered samples to demonstrate the dispersion of 3DGN in composite powders and bulk composites in the revised manuscript as following:
Lines 235-252:
Fig. 4 illustrates the microstructure of ball milled pure aluminum specimen and ball milled 3DGN/Al composite powders. It can be seen from Fig. 4(a) that, after the ball milling, pure aluminum powder changed from spherical shape to flakes, or was crushed into powder aggresomes due to aggregation. This flake-like aluminum is favorable because it can provide more sites than aggresomes for 3DGN attachment. As depicted in Fig. 4(b) and (c), 3DGNs were mainly attached on the surface of the flake-like aluminum (showed in green arrow) and kept the original wrinkled structure, which indicates the effective distribution of 3DGN in 3DGN/Al composite powders and the possibility of producing a cohesive link between 3DGN and aluminum. The two most critical factors for the preparation of 3DGN/Al composites are precisely the dispersion and the structural stability of 3DGN. Intriguingly, a piece of 3DGN was observed in Fig. 4(d) and (e), which was not found in the other 3DGN/Al composite powders. The presence of large pieces of 3DGN is the evidence for 3DGN agglomeration. That is to say, 0.075%-3DGN/Al composite powders and 0.150%-3DGN/Al composite powders achieved homogeneous distribution of 3DGN, while 0.225% and 0.300% 3DGN were achieved in 3DGN/Al composite powders with some extent of agglomeration.
Lines 257-275:
The morphologies of sintered pure Al sample and 3DGN/Al composites with different 3DGN contents and the EDS mapping analysis of 3DGN/Al composites with 0.300% 3DGN are given in Fig. 5. All sintered specimens were nearly smooth and dense. A very small number of pores (as marked in Fig. 5) appear on the surface of pure aluminum specimen in Fig. 5(a), indicating obviously that pure aluminum specimen is the densest. From Fig. 5(b) and (c), one can see that pores increase with the increasing addition of 3DGN. A more detailed observation of Fig. 5(d) and (e) reveals that the large pores were formed by several small pores connected together. The 3DGN agglomeration aforementioned above may also lead to the formation of porosity. In addition, the apparent morphologies of Fig. 5(d) and (e) differ from those of Fig. 5(a), (b) and (c), in which some gray or dark areas (already marked with red arrows) are observed. Further EDS analysis and elemental mapping of the gray or dark areas of 0.300%-3DGN/Al composites sample were performed and shown in Fig. 5(f). The distribution of carbon element (shown in Fig. 5(f1)) and aluminum element (shown in Fig. 5(f2)) demonstrate that the gray or dark areas result from 3DGN agglomeration. Thereby, it can be inferred that the gray or dark areas of 0.225%-3DGN/Al composites sample in Fig. 5(d) also result from the 3DGN agglomerations, though it is not as distinct as Fig. 5(e). This inference is also consistent with the aforementioned statements that the morphology of large 3DGN agglomerates was observed only in both of 0.225%-3DGN/Al and 0.300%-3DGN/Al composite powders.
Comment 2: The discussion about relative density and mechanical properties would benefit from further information regarding grain size i.e. since it is a well-known factor influencing hardness, and GN addition is likely to modify it, is the Al grain size similar in pure Al and composite bulk samples?
Author Response: We appreciate this comment. The Al grain size is similar in both of pure Al and composite bulk samples. The addition of 3DGN can refine the Al grain to a certain extent and lead to an increase in hardness. However, the 3DGN content in this study is very small, the effect of grain refinement is limited, and the process of SPS results in the growth of grain, which is the reason why the hardness is not significantly improved in this investigation.
Comment 3: The reviewer has one major reservation about figure 6 which currently does not make sense (the curves shapes and values seem wrong). Tensile strength is expected around 250 MPa not 2000 MPa and max strain is expected about 10% not 90%! Once this issue is sorted, such values should be compared to results already reported in the literature.
Author Response: We apologize for our mistake in labeling Fig. 6 in the original manuscript. It has now been corrected as compressive curves. The prepared 3DGN/Al composites have the same good ductility as pure aluminum. They do not fracture under compression test until the limit of machine pressure is applied, and the composites were pressed into thin sheets.
Comment 4: l. 68: “To date, most of research on graphene reinforced AMCs has concentrated on low content of graphene (below 5 wt%) [23-25]. But the range of graphene content is still very researchable.” This last sentence is not easy to understand.
Author Response: Thank you for your suggestion. We have more detailed explanation for this section in the revised manuscript to facilitate the understanding of why we are studying the effect of different 3DGN contents on 3DGN/Al composites. It was added as the lines 102-104:
But the atomic percentage is still large due to the low mass of carbon atom. Many researchers believed that the optimal value of graphene is between 0.3-1.0 wt% [16], and it seems to be an interesting topic to further investigate the effect of content for graphene.
Comment 5: l.77: “The main process of PM is: preparation and preparation of raw materials, obtaining raw embryos, sintering and forming [32].” Embryos is not the right word (subsequent use l. 118).
Author Response: We are very sorry for our incorrect writing, and the right word is “green compact”.
Comment 6: l.111: “oscillated ultrasonic » replace with “sonicated”.
Author Response: Thank you for your comment, and “oscillated ultrasonic” has been replaced with “sonicated”.
Comment 7: l.120: “The sintering was done in temperature 120 of 600 ℃ for 10 minutes under the vacuum atmosphere” please add heating rate and pressure applied.
Author Response: Thanks for this suggestion, and the heating rate of 100 ℃/min and pressure applied of 50 MPa have been added in the " Preparation of composites " section as the following:
Lines 169-171:
The sintering was done at a heating rate of 100 °C per minute up to 600 °C and hold at 600 °C for 10 minutes. The process of sintering was carried out under the vacuum atmosphere and the pressure of 50 MPa.
Comment 8: l.130-131: what was the loading applied (and time) for Vickers microhardness testing?
Author Response: We are thankful for the comment. We have added the conditions for measurement of hardness, including the loading applied and dwell time for Vickers microhardness testing, they are 9.8 N and 10 s, respectively, in the revised manuscript.
Lines 182-183:
The Vickers hardness (HV) of the developed composites was tested by a hardness tester (HWDM-3) at a test load of 9.8 N and dwell time of 10 s.
Comment 9: l.139-140: “Characteristic XRD peaks of aluminum at their respective crystallographic phase” this sentence should be modified.
Author Response: Thanks. We agree with your comment and have modified this sentence in the revised manuscript as “The characteristic XRD peaks of aluminum (space group is Fm-3m (225), ……” (lines 197-198 in the revised manuscript).
Comment 10: Supplementary materials should be removed since they consist purely of figures from the main text.
Author Response: Thank you for underlining this deficiency. Supplementary materials have been removed.
Reviewer 2 Report
This work deals with the mechanical properties and corrosion resistance of Al reinforced with 3D graphene networks. The Authors found that the corrosion resistance of the composites is lower than that of unreinforced Al. The gain in hardness and strength was very small with the addition of graphene. It appears that it may be difficult to attract the reader's attention to these results. To merit publication, the work should be strengthened by more detailed microstructural studies and a more detailed discussion of mechanisms responsible for the observed behaviors.
Compressive true stress-true strain curves should be shown.
The title of the paper should be revised: "Mechanical property and corrosion behavior of powder metallurgy-processed 3D graphene network-reinforced Al matrix composites".
Author Response
Responds to the reviewer’s comments
(crystals-2247875)
Dear Editor:
We appreciate the reviewer’s insightful comments. All these comments are valuable and helpful for improving our paper and providing the important guidelines to our studies. The manuscript has been revised according to the suggestions and comments of the reviewers. A point-by-point response to the reviewer’s comments is presented in the following.
The revised manuscript with the major revisions is marked for your convenience. After improvement following the reviewers' suggestions, the revised paper should become more interesting and provide more valuable information to the readers. Once again, we earnestly appreciate Editor/Reviewers’ work and hope the corrections will meet the requirement for publication in Crystals.
Thank you very much for your consideration!
Sincerely yours,
Yifang Ouyang
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To reviewer #2:
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Comment 1: This work deals with the mechanical properties and corrosion resistance of Al reinforced with 3D graphene networks. The Authors found that the corrosion resistance of the composites is lower than that of unreinforced Al. The gain in hardness and strength was very small with the addition of graphene. It appears that it may be difficult to attract the reader's attention to these results. To merit publication, the work should be strengthened by more detailed microstructural studies and a more detailed discussion of mechanisms responsible for the observed behaviors.
Author Response: Thanks for your instructive suggestion. We have added more detailed microstructural studies in the " Microstructure " section and a detailed elaboration of the mechanism based on the observed results as following in the revised manuscript:
Lines 235-252:
Fig. 4 illustrates the microstructure of ball milled pure aluminum specimen and ball milled 3DGN/Al composite powders. It can be seen from Fig. 4(a) that, after the ball milling, pure aluminum powder changed from spherical shape to flakes, or was crushed into powder aggresomes due to aggregation. This flake-like aluminum is favorable because it can provide more sites than aggresomes for 3DGN attachment. As depicted in Fig. 4(b) and (c), 3DGNs were mainly attached on the surface of the flake-like aluminum (showed in green arrow) and kept the original wrinkled structure, which indicates the effective distribution of 3DGN in 3DGN/Al composite powders and the possibility of producing a cohesive link between 3DGN and aluminum. The two most critical factors for the preparation of 3DGN/Al composites are precisely the dispersion and the structural stability of 3DGN. Intriguingly, a piece of 3DGN was observed in Fig. 4(d) and (e), which was not found in the other 3DGN/Al composite powders. The presence of large pieces of 3DGN is the evidence for 3DGN agglomeration. That is to say, 0.075%-3DGN/Al composite powders and 0.150%-3DGN/Al composite powders achieved homogeneous distribution of 3DGN, while 0.225% and 0.300% 3DGN were achieved in 3DGN/Al composite powders with some extent of agglomeration.
Lines 257-275:
The morphologies of sintered pure Al sample and 3DGN/Al composites with different 3DGN contents and the EDS mapping analysis of 3DGN/Al composites with 0.300% 3DGN are given in Fig. 5. All sintered specimens were nearly smooth and dense. A very small number of pores (as marked in Fig. 5) appear on the surface of pure aluminum specimen in Fig. 5(a), indicating obviously that pure aluminum specimen is the densest. From Fig. 5(b) and (c), one can see that pores increase with the increasing addition of 3DGN. A more detailed observation of Fig. 5(d) and (e) reveals that the large pores were formed by several small pores connected together. The 3DGN agglomeration aforementioned above may also lead to the formation of porosity. In addition, the apparent morphologies of Fig. 5(d) and (e) differ from those of Fig. 5(a), (b) and (c), in which some gray or dark areas (already marked with red arrows) are observed. Further EDS analysis and elemental mapping of the gray or dark areas of 0.300%-3DGN/Al composites sample were performed and shown in Fig. 5(f). The distribution of carbon element (shown in Fig. 5(f1)) and aluminum element (shown in Fig. 5(f2)) demonstrate that the gray or dark areas result from 3DGN agglomeration. Thereby, it can be inferred that the gray or dark areas of 0.225%-3DGN/Al composites sample in Fig. 5(d) also result from the 3DGN agglomerations, though it is not as distinct as Fig. 5(e). This inference is also consistent with the aforementioned statements that the morphology of large 3DGN agglomerates was observed only in both of 0.225%-3DGN/Al and 0.300%-3DGN/Al composite powders.
Comment 2: Compressive true stress-true strain curves should be shown.
Author Response: Thank you for your comment. The Fig. 6 in the original manuscript has been corrected, as well as it is compressive true stress-true strain curves, and which has been noted as Fig. 8 in the revised manuscript.
Comment 3: The title of the paper should be revised: "Mechanical property and corrosion behavior of powder metallurgy-processed 3D graphene network-reinforced Al matrix composites".
Author Response: Thank you for your suggestion. We think it is an excellent suggestion, and the title has been revised to "Mechanical property and corrosion behavior of powder metallurgy-processed 3D graphene network-reinforced Al matrix composites" in the revised manuscript.
Reviewer 3 Report
This manuscript reports an experimental investigation on mechanical and corrosion properties of a series of 3D graphene network-reinforced Al matrix composites. Results and Discussion are appropriate, conclusions are quite sound. English style is sufficient. Therefore, the manuscript can be accepted just after some minor corrections:
1. line 30 :"Its' " without apostrophe.
line 32 and 33: "superelectrical properties and tribological properties..", remove the first "properties".
line 90: 3DGN: the first time that an acronym is used it must be explicitated.
lines 134 and 135: please indicate the scan rate used for the polarization tests and give some details on how the impedance measurements were done ( i.e., at Ecorr?, frequency range ?, etc, etc.)
par. 3.1 : Microstructure: characterization is not complete; is it possible to add some SEM images of the sintered samples? This could important , for instance, to have a clue about any porosity or microstructural defects.
line 210: Legend of Fig. 6: "Compressive" and not "comprehensive"
Fig. 7(b) polarization curves: although passive current density changes with composition, I note that the breakdown onset of the passive films (i.e., start of the pitting zone) are being not much altered by the 3DGN additions. This seems in contrast with statement of the authors, when say that too much 3DGN destroys the stability of the Al passive film. If this were the case, I suppose that one should find the onset of passive film breakdown at a distinct lower potential than pure Al. Please provide some more detailed comments about the reasons for this behavior and in general about pitting susceptibility of these composites.
Author Response
Responds to the reviewer’s comments
(crystals-2247875)
Dear Editor:
We appreciate the reviewer’s insightful comments. All these comments are valuable and helpful for improving our paper and providing the important guidelines to our studies. The manuscript has been revised according to the suggestions and comments of the reviewers. A point-by-point response to the reviewer’s comments is presented in the following.
The revised manuscript with the major revisions is marked for your convenience. After improvement following the reviewers' suggestions, the revised paper should become more interesting and provide more valuable information to the readers. Once again, we earnestly appreciate Editor/Reviewers’ work and hope the corrections will meet the requirement for publication in Crystals.
Thank you very much for your consideration!
Sincerely yours,
Yifang Ouyang
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To reviewer #3:
--------------------------------------------------------------------
Reviewer Comments: This manuscript reports an experimental investigation on mechanical and corrosion properties of a series of 3D graphene network-reinforced Al matrix composites. Results and Discussion are appropriate, conclusions are quite sound. English style is sufficient. Therefore, the manuscript can be accepted just after some minor corrections:
Comment 1: line 30 :"Its' " without apostrophe..
Author Response: Thank you for your comment. The apostrophe has been removed in the revised manuscript.
Comment 2: line 32 and 33: "superelectrical properties and tribological properties.", remove the first "properties".
Author Response: We appreciate this comment. The first "properties" has been removed.
Comment 3: line 90: 3DGN: the first time that an acronym is used it must be explicitated.
Author Response: We are thankful for the comment. “3DGN” has now been explicitated as “3D graphene networks (3DGN)” in the line 134 of the revised manuscript.
Comment 4: lines 134 and 135: please indicate the scan rate used for the polarization tests and give some details on how the impedance measurements were done ( i.e., at Ecorr?, frequency range ?, etc, etc.).
Author Response: Thank you for your suggestion. We have added more details for this section in the revised manuscript as the lines 188-194:
A three-electrode cell was used with a saturated calomel electrode (SCE) as a reference, a platinum mesh as the counter electrode (CE). The corrosion cell of 1 cm2 was used as working surface. The impedance measurements were performed with an AC signal amplitude of 5 mV rms, 10 points/decade and a frequency range of 0.01 Hz-100000 Hz. Tafel curve was recorded from corrosion potential (Ecorr) -0.2 V to Ecorr + 0.2 V, with scan rate 1 mV/s. The Ecorr and corrosion current density (Icorr) were obtained by fitting the experimental data to Tafel curve.
Comment 5: par. 3.1 : Microstructure: characterization is not complete; is it possible to add some SEM images of the sintered samples? This could important, for instance, to have a clue about any porosity or microstructural defects.
Author Response: Thanks for this suggestion, and we have added SEM image (Fig. 5) of the sintered samples and more detailed microstructural studies in the " Microstructure " section as following in the revised manuscript:
Lines 257-275:
The morphologies of sintered pure Al sample and 3DGN/Al composites with different 3DGN contents and the EDS mapping analysis of 3DGN/Al composites with 0.300% 3DGN are given in Fig. 5. All sintered specimens were nearly smooth and dense. A very small number of pores (as marked in Fig. 5) appear on the surface of pure aluminum specimen in Fig. 5(a), indicating obviously that pure aluminum specimen is the densest. From Fig. 5(b) and (c), one can see that pores increase with the increasing addition of 3DGN. A more detailed observation of Fig. 5(d) and (e) reveals that the large pores were formed by several small pores connected together. The 3DGN agglomeration aforementioned above may also lead to the formation of porosity. In addition, the apparent morphologies of Fig. 5(d) and (e) differ from those of Fig. 5(a), (b) and (c), in which some gray or dark areas (already marked with red arrows) are observed. Further EDS analysis and elemental mapping of the gray or dark areas of 0.300%-3DGN/Al composites sample were performed and shown in Fig. 5(f). The distribution of carbon element (shown in Fig. 5(f1)) and aluminum element (shown in Fig. 5(f2)) demonstrate that the gray or dark areas result from 3DGN agglomeration. Thereby, it can be inferred that the gray or dark areas of 0.225%-3DGN/Al composites sample in Fig. 5(d) also result from the 3DGN agglomerations, though it is not as distinct as Fig. 5(e). This inference is also consistent with the aforementioned statements that the morphology of large 3DGN agglomerates was observed only in both of 0.225%-3DGN/Al and 0.300%-3DGN/Al composite powders.
Comment 6: line 210: Legend of Fig. 6: "Compressive" and not "comprehensive".
Author Response: Thank you for your comment, we apologize for our mistake. The “comprehensive” has been replaced with “compressive”.
Comment 7: Fig. 7(b) polarization curves: although passive current density changes with composition, I note that the breakdown onset of the passive films (i.e., start of the pitting zone) are being not much altered by the 3DGN additions. This seems in contrast with statement of the authors, when say that too much 3DGN destroys the stability of the Al passive film. If this were the case, I suppose that one should find the onset of passive film breakdown at a distinct lower potential than pure Al. Please provide some more detailed comments about the reasons for this behavior and in general about pitting susceptibility of these composites.
Author Response: Thanks for this suggestion and we have added more details about the reason and the pitting susceptibility of the composites in the revised manuscript. From the polarization curve we can see that the breakdown onset of the passive films of the 3DGN/Al composites is lower than that of pure aluminum, but the breakdown onset of the passive film may not be significantly altered, as you have mentioned. Nevertheless, the slight change in the breakdown onset of passive film indicates to some extent the instability of the passive film on the aluminum surface, which is also, as we have explained, the small addition of 3DGN only lead to the local destruction of the continuity of the passive film, not completely destroy the integrity of the Al passive film, and thus we can only see slight changes in the breakdown onset of the passive films. As for the pitting susceptibility of 3DGN/Al composites, it is strongly related to the integrity of the passive film formed on its surface. The higher the pitting potential (the potential of the breakdown onset of the passive films), the higher pitting resistance of the composites and the higher pitting susceptibility. As mentioned above, the breakdown onset of the passive films of the 3DGN/Al composites is lower than that of pure aluminum, implying that the pitting resistance and the pitting susceptibility of pure aluminum is the highest, while the pitting resistance and the pitting susceptibility of the 3DGN/Al composites become inferior.
Author Response File: 
Author Response.docx
Reviewer 4 Report
In the manuscript "Mechanical property, corrosion behavior of powder metallurgy processed 3D graphene networks reinforced Al matrix composites" synthesized and investigated pure aluminum samples and graphene/Al composites with weight percentages of 0.075, 0.150, 0.225, and 0.300 using a powder metallurgy technique with BM and SPS.
The comments are as follows:
1. In line 140, replace the "crystalline phase" with a "phase". A space group/structural type can be specified for aluminium.
2. Based on the XRD data, it is difficult to speak about the phase composition, however, an important characteristic that may indicate the absence of interaction between the matrix and graphene can be the lattice spacing. It follows from the presented data to determine the lattice spacing of the aluminum and reasonably conclude that there is no interaction.
3. In Figure 4, above the bars of the histogram, indicate the values of the achieved densities as a percentage of the theoretical.
4. In the caption to figure 6, it is necessary to indicate that the curves are not сomprehensive, but compression test curves.
5. The supplementary materials duplicates the drawings of the manuscript. what's the point of listing them twice?
6. In the introduction, it is necessary to describe in more detail the novelty of this work and the differences from a large number of works already published on this topic.
7. In the work, it is necessary to provide SEM photographs of the microstructure of sintered samples to prove the 3d distribution of reinforced particles.
The manuscript may be published after corrections.
Author Response
Responds to the reviewer’s comments
(crystals-2247875)
Dear Editor:
We appreciate the reviewer’s insightful comments. All these comments are valuable and helpful for improving our paper and providing the important guidelines to our studies. The manuscript has been revised according to the suggestions and comments of the reviewers. A point-by-point response to the reviewer’s comments is presented in the following.
The revised manuscript with the major revisions is marked for your convenience. After improvement following the reviewers' suggestions, the revised paper should become more interesting and provide more valuable information to the readers. Once again, we earnestly appreciate Editor/Reviewers’ work and hope the corrections will meet the requirement for publication in Crystals.
Thank you very much for your consideration!
Sincerely yours,
Yifang Ouyang
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To reviewer #4:
--------------------------------------------------------------------
Reviewer Comments: In the manuscript "Mechanical property, corrosion behavior of powder metallurgy processed 3D graphene networks reinforced Al matrix composites" synthesized and investigated pure aluminum samples and graphene/Al composites with weight percentages of 0.075, 0.150, 0.225, and 0.300 using a powder metallurgy technique with BM and SPS. The comments are as follows:
Comment 1: In line 140, replace the "crystalline phase" with a "phase". A space group/structural type can be specified for aluminium.
Author Response: Thank you for your comment. A space group of aluminum (Fm-3m (225)) has been given and the sentence with "crystalline phase" which you point out has been revised to “The characteristic XRD peaks of aluminum (space group is Fm-3m (225), ……” in lines 197-198 of the revised manuscript.
Comment 2: Based on the XRD data, it is difficult to speak about the phase composition, however, an important characteristic that may indicate the absence of interaction between the matrix and graphene can be the lattice spacing. It follows from the presented data to determine the lattice spacing of the aluminum and reasonably conclude that there is no interaction.
Author Response: We appreciate this comment. As shown by XRD, the diffraction peaks of aluminum demonstrate no any apparent shift, which means that there is no obvious solution of carbon in Al, namely, there is no obvious interaction between Al and 3DGN. It is likely that there are too few of such interactions and therefore failed to be detected. However, as can be seen in Fig. 4 and Fig. 5 provided in our revised manuscript, 3DGN is just tightly adhered to the surface of aluminum, and there is a high possibility of adhesion between them. As similar as Palei et al.[1] reported, the improve hardness may attribute to the attachment between Al and graphene. We can probably infer from this that a small amount of adhesion is also occurring in our 3DGN/Al composites. The only slight increase in hardness in the present experiments may be indicative of this conjecture. This adhesion, together with other mechanisms such as grain refinement, is the result of a slight increase in hardness.
Comment 3: In Figure 4, above the bars of the histogram, indicate the values of the achieved densities as a percentage of the theoretical.
Author Response: Thanks for your suggestion. We have added a sentence of “The experimental density as a percentage of the theoretical density is the relative density, which can be used to characterize the densification. (lines 180-182 of the revised manuscript)” to understand the relative density on the bars of the histogram.
Comment 4: In the caption to figure 6, it is necessary to indicate that the curves are not сomprehensive, but compression test curves.
Author Response: We are really sorry for our careless mistake. Thank you for your reminder. We have corrected the “сomprehensive” into “compressive”.
Comment 5: The supplementary materials duplicates the drawings of the manuscript. what's the point of listing them twice?
Author Response: We are extremely grateful to you for pointing out this problem and we have removed the supplementary materials.
Comment 6: In the introduction, it is necessary to describe in more detail the novelty of this work and the differences from a large number of works already published on this topic.
Author Response: We are thankful for the comment and have added more detail of the novelty of this work and the differences from others in the introduction as following:
Lines 62-84:
The type of graphene materials incorporated into AMCs is of importance. In the past decade, the most widely used reinforcements of graphene materials reinforcements in AMCs are 1D carbon nanotubes and 2D graphene nanosheets. While 3D graphene material is rising and expected to be a potential graphene material due to its easy preparation compared to single-layer graphene, unique wrinkled structure and economical cost. It is considered as a new type of reinforcements, which is capable of structural stability and hydrophobicity with keeping the large specific surface area and good electrical conductivity of graphene [20]. There are a lot of researches on the use of 1D and 2D graphene materials in AMCs aforementioned above. However, there is quite a few reports on the application of 3D graphene materials in other MMCs [21]. For example, Algul et al. [22] prepared 3D graphene-nickel matrix composites with higher wear resistance and lower friction coefficient than the nickel matrix, attributed to the solid lubrication effect of graphene. The 3D graphene-copper matrix composites reported by Chen et al. [23] has a yield strength and tensile strength of 290 MPa and 308 MPa, respectively, and the structure of 3D graphene is well preserved in the bulk composites. The successful application and enhancing effect of 3D graphene materials on MMCs encouraged us to investigate the effect on 3D graphene in 3D graphene materials reinforced AMCs. Bastwros et al. [24] have successfully prepared graphene nanoflakes (few layers of graphene thickness) reinforced AMCs and found a 47% increase in flexural strength due to the addition of graphene. Moreover, the unique property advantages of 3D graphene materials for the mechanical properties and corrosion resistance of AMCs deserve further study. In order to better understand the effect of 3D graphene materials as a reinforcement in AMCs, it is promising to study the effect of 3D graphene materials on the properties of AMCs.
Comment 7: In the work, it is necessary to provide SEM photographs of the microstructure of sintered samples to prove the 3d distribution of reinforced particles.
Author Response: Thank you for your suggestion. We have added more SEM photographs (Fig. 5 in the revised manuscript) and investigations of sintered 3DGN/Al composites samples to prove the distribution of 3DGN in the " Microstructure " section as following:
Lines 257-275:
The morphologies of sintered pure Al sample and 3DGN/Al composites with different 3DGN contents and the EDS mapping analysis of 3DGN/Al composites with 0.300% 3DGN are given in Fig. 5. All sintered specimens were nearly smooth and dense. A very small number of pores (as marked in Fig. 5) appear on the surface of pure aluminum specimen in Fig. 5(a), indicating obviously that pure aluminum specimen is the densest. From Fig. 5(b) and (c), one can see that pores increase with the increasing addition of 3DGN. A more detailed observation of Fig. 5(d) and (e) reveals that the large pores were formed by several small pores connected together. The 3DGN agglomeration aforementioned above may also lead to the formation of porosity. In addition, the apparent morphologies of Fig. 5(d) and (e) differ from those of Fig. 5(a), (b) and (c), in which some gray or dark areas (already marked with red arrows) are observed. Further EDS analysis and elemental mapping of the gray or dark areas of 0.300%-3DGN/Al composites sample were performed and shown in Fig. 5(f). The distribution of carbon element (shown in Fig. 5(f1)) and aluminum element (shown in Fig. 5(f2)) demonstrate that the gray or dark areas result from 3DGN agglomeration. Thereby, it can be inferred that the gray or dark areas of 0.225%-3DGN/Al composites sample in Fig. 5(d) also result from the 3DGN agglomerations, though it is not as distinct as Fig. 5(e). This inference is also consistent with the aforementioned statements that the morphology of large 3DGN agglomerates was observed only in both of 0.225%-3DGN/Al and 0.300%-3DGN/Al composite powders.
References
- Palei, B. B.; Dash, T.; Biswal, S. K. Graphene reinforced aluminum nanocomposites: synthesis, characterization and properties. J. Mater. Sci. 2022, 57 (18), 8544-8556.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
In answer to comment 1 the authors have added new data and a new paragraph lines 235-252 but the word “aggresome” (used twice) is definitely not appropriate here.
The addition of powders SEM in Figure 4 is most welcome, although one should not read too much in pictures at such low magnifications (for graphene-metal composites).
The addition of bulk samples SEM in Figure 5 is also welcome, it is unfortunate that sample etching was not performed before SEM observation as it should have revealed more in depth information about sample microstructure.
In answer to comment 2, the authors claim that “The Al grain size is similar in both of pure Al and composite bulk samples.” which would be an acceptable answer if it was backed by experimental data.
Author Response
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To reviewer #1:
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Comment 1: In answer to comment 1 the authors have added new data and a new paragraph lines 235-252 but the word “aggresome” (used twice) is definitely not appropriate here.
Author Response: Thank you for your precious comments. We have replaced “aggresome” with “agglomeration”.
Comment 2: The addition of powders SEM in Figure 4 is most welcome, although one should not read too much in pictures at such low magnifications (for graphene-metal composites).
Author Response: Thanks. We appreciate this comment. Even though the magnification of the powders SEM is low, we can still get some useful information from SEM images, such as that the aluminum particles and 3DGN can be clearly distinguished from the different morphologies, such as attachment on the surface of aluminum, and agglomerations, which are useful for the next analysis.
Comment 3: The addition of bulk samples SEM in Figure 5 is also welcome, it is unfortunate that sample etching was not performed before SEM observation as it should have revealed more in depth information about sample microstructure.
Author Response: Thank you for your comment, which is very useful for our research in the future. As for the graphene reinforced aluminum matrix composites, most of published work presented the apparent morphology, porosity etc. of composites without etching [1]. However, the etching is very useful to reveal more information about sample microstructure, which will be taken into account in our next work.
Comment 4: In answer to comment 2, the authors claim that “The Al grain size is similar in both of pure Al and composite bulk samples.” which would be an acceptable answer if it was backed by experimental data.
Author Response: Thank you for your comment. In order to check the Al grain size of the pure Al and composite bulk samples, the pure Al and 0.300%-3DGN/Al composites samples were etched by HF with one minute, and the morphologies were shown in the Fig. 1R. Fig. 1R (a) showed the morphology of the pure aluminum specimen, and the average Al grain size is 18.9 μm obtained by software statistics. Fig. 1R (b) showed the morphology of the 0.300%-3DGN/Al composites specimen, and the average Al grain size is 18.3 μm obtained by software statistics. From the results, we can assume that the Al grain sizes of both pure Al and composite bulk samples are very close to each other.
Fig. 1R The morphologies of (a) pure aluminum and (b) 0.300%-3DGN/Al composites after etching by HF.
Reference
- Li, J.; Zhang, X.; Geng, L. Improving graphene distribution and mechanical properties of GNP/Al composites by cold drawing. Mater. Des. 2018, 144, 159-168.
Author Response File: Author Response.docx
Reviewer 2 Report
The Authors claimed they corrected Figure 6 (now Figure 8) but they are the same in the original and revised manuscripts. It is necessary to include the compressive true stress-true strain curves of the Al and composites. Thank you.
Author Response
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To reviewer #2:
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Comment 1: The Authors claimed they corrected Figure 6 (now Figure 8) but they are the same in the original and revised manuscripts. It is necessary to include the compressive true stress-true strain curves of the Al and composites. Thank you.
Author Response: Thanks for your comment. In our work, the prepared pure aluminum and 3DGN/Al composites samples were carried out in compression test. They do not fracture under compression test until the limit of machine pressure is applied, then the pure aluminum and composites were pressed into thin sheets, thus we cannot measure the strength limit. As a result, we obtain the stress-strain curve as shown in Figure 8 in the revised manuscript. Simultaneously, according to the below equations, the compressive true stress-true strain curve is given in the figure below and it has the same trend as the stress-strain curve. We could not get other more valuable information from the compressive true stress-true strain curve, so we only used the stress-strain curve which obtained from the compression test rather than the true stress-true strain curve.
σ’=σ*（1+ε） (1)
ε‘=ln(1+ε) (2)
where σ’ and ε‘ are true-stress and true-strain, respectively, and are stress and strain obtained from compression test.
Fig. 2R The compressive true stress-true strain of 3DGN/Al composites with different 3DGN contents.
Author Response File: Author Response.docx
Reviewer 4 Report
The comments have been corrected and the manuscript can be published.
Author Response
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To reviewer #4:
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Comment 1: The comments have been corrected and the manuscript can be published.
Author Response: Thank you. We are extremely grateful to you for your suggestions to improve our paper.
Author Response File: Author Response.docx
