Enhancing Graphene Retention and Electrical Conductivity of Plasma-Sprayed Alumina/Graphene Nanoplatelets Coating by Powder Heat Treatment
Round 1
Reviewer 1 Report
Manuscript ID: coatings-1232635
In the manuscript by X. Wu et al., the experimental results of plasma spraying of alumina/graphene nanoplatelets coating were presented. It was suggested that via this approach, it is possible to enhance graphene survival and thus to improve the performance of plasma-sprayed alumina/graphene nanoplatelets (Al2O3/GNPs) coatings. The effect of powder heat treatment on the microstructure, GNPs survival, and electrical conductivity of Al2O3/GNPs coatings were systematically investigated. I find that this could be a useful approach to synthesize Al2O3/GNPs coatings. I have several comments and questions that need to be properly addressed.
1.plasma spraying is a well-known technique in the coating. It is also called arc plasma spraying or arc plasma deposition. There have been a number of applications of plasma spraying, including fabrication of catalysts or corrosion-resistant coating. I think this approach can be mentioned in the introduction [For example, H. Randhawa and P. C. Johnson Surf Coat Technol 31, 303 (1987); S. H. Kim et al. Topics in Catalysis 60, 812 (2017). ;S. Hinokuma et al. Catal Sci Technol 5, 4249 (2015)]
Thank you for your kind suggestion. There may be some misunderstanding about plasma spraying. Technically speaking, plasma spraying and arc plasma deposition you mentioned are two different technologies. For plasma spraying (as shown in the following schematic a), an arc was firstly formed between cathode (tungsten electrode) and anode (copper nozzle) under a high frequency power supply. Then, working gas (Ar, H2, and so on) was ionized in the arc region and ejected from the nozzle to form plasma jet. Raw material powders (with a size of tens of micrometers) are then injected into plasma jet to undergo heating and thus to obtain molten or semi-molten droplets. Subsequently, these droplets impact onto substrate in a high speed (100-300 m/s), followed by spreading, rapid cooling, and solidification. Consequently, successive impact of droplets form lamellar-structured coatings with a thickness from tens of micrometers to a few microns.
On the other hand, for the arc plasma deposition, which is categorized as physical vapor deposition (as shown in the following schematic b), it enables the deposition of nano-films or metal nanoparticles from bulk metals or other electronic conducting materials. During arc plasma deposition process, pulsed arc discharge was triggered between cathode (target materials) and anode. Then, the metal on the cathode surface is immediately vaporized and ionized, resulting in the ejection of plasma. After the metal ions collide with the support powders, they diffuse on the surface and ultimately deposited as nanoparticles.
Therefore, the plasma spraying technology and arc plasma deposition are different in all the cases of equipment, plasma generation mode, deposition process, size of the resultant film/coating, and so on. In this way, we have not mentioned the arc plasma deposition approach in the introduction.
2. It is interesting to see that the conductivity increases as increasing heat treatment temperature [Figure 12]. Because the high conductivity is one of the desired material properties of Al2O3/GNPs coatings, there could be a possibility to obtain more conductive Al2O3/GNPs coatings if the heating temperature is higher than 1280 oC. I am wondering if the authors tested this aspect to obtain coatings with higher conductivity.
Yes, you are right, we totally agree with you that there could be a possibility to obtain more conductive Al2O3/GNPs coatings if the heat treatment temperature for powder is higher than 1280 oC. At present, we have not fabricated coatings by Al2O3/GNPs powders with heat treatment temperature higher than 1280 oC. Inspired by your comments, we will do more experiments to verify whether higher conductivity could be obtained by using Al2O3/GNPs powders with heat treatment temperature higher than 1280 oC. Moreover, we have added this explanation at the last of section 3.4 in the revised manuscript as follow:
Furthermore, heat treatment at a temperature higher than 1280 oC would be applied to the Al2O3/GNPs powders to investigate whether an Al2O3/GNPs coating with more GNPs survival and higher electrical conductivity could be achieved.
3. There have been similar approaches to obtain graphene powder or three-dimensional graphene with high conductivity. Lee et al. reported 10^6 S/m on the three-dimensional graphene layers [Lee et al. Scientific Reports 7, 11460 (2017) & Kim et al. Nature 353, 131-135 (2016)]. The obtained conductivity of the current study can be compared to those of other graphene-like materials.
Thanks for your kind and valuable suggestion. Our current work was focused on the graphene reinforced alumina coatings, the works you mentioned above were about nano-scale graphene-like materials. Since they had different materials system, scale, and microstructure, so we consider they are not comparable and we have not compared the electrical conductivity values of the current Al2O3/GNPs coating and the graphene-like materials. Inspired by your comment, we have added one sentence and cited the references you mentioned about further enhancing the electrical conductivity of ceramics by using other graphene-like materials, as highlighted at the last of section 3.4 in the revised manuscript as follow:
Noteworthy, incorporation of other types of graphene with higher electrical conductivity could also be used to further enhance the electrical conductivity of ceramic coatings. For instance, Lee et al [37, 38] reported a microporous 3D graphene-like carbons with remarkably high electrical conductivity of ~106 S m-1, which was higher than the 105 S m-1 of conventional graphene nanoplatelets fabricated by microwave explosion method [20]…
The new references added in the reference section are as follow:
37. Lee, H.; Kim, K.; Kang, S.H.; Kwon, Y.; Kim. J.H.; Kwon, Y.K.; Ryoo, R.; Park, J.Y. Extremely high electrical conductance of microporous 3D graphene-like zeolite-templated carbon framework. Rep. 2017, 7, 11460-11468, doi: 10.1038/s41598-017-11602-5.
38. Kim, K; Lee. T.; Kwon, Y.; Seo, Y.; Song, J.; Park, J.K.; Lee, H.; Park, J.Y.; Ihee, H.; Cho, S.J.; Ryoo, R. Lanthanum-catalysed synthesis of microporous 3D graphene-like carbons in a zeolite template. Nature 2016, 535, 131-135, doi: 10.1038/nature18284.
Reviewer 2 Report
In the current manuscript “Enhancing graphene survival and electrical conductivity of plasma-sprayed alumina/graphene nanoplatelets coating by powder heat treatment” the authors studied the influence of preliminary heat treatment of Al2O3-GNPs at 850 and 1280 oC on structural and electrical properties of Al2O3-GNPs films prepared by plasma spray coating method from the corresponding powders.
It was concluded that the films made on powder heat treated at higher temperature are denser and more conductive in contrast to films that used untreated Al2O3-GNPs powder.
Author concluded that as the powder heat treatment temperature increased, enhanced GNPs survival increased, as well as decreased structural defects of GNPs were achieved for the AG, AG850, and AG1280 coatings. Moreover, the electrical conductivity was reported as increased in 7 orders in comparison to Al2O3.
However, based on available data it can be possible to say that main part of added 1 wt% of GNPs disappeared during heating of Al2O3-GNPs powder up to/higher 850 oC. Moreover, after such heat treatment only insignificant part of graphene nanoplatelets can be present there. In this case, it is not clear why authors obtained such improving of electrical conductivity.
In addition, there are more comments:
1.Lines 80-82 “For instance, Garcia et al. [23] reported that ~60% of GNPs burned and only around 40% of GNPs survived the high temperature during flame spraying of GNPs/YAS (Y2O3- Al2O3-SiO2) coating. The same authors [22] also reported that only 16% of the initial GNPs retained after flame spraying by using higher melting point YAS.” At what temperature it was? Mention it.
Thank you very much for your kind comment. As you suggested, we have added the detailed temperature value of flame spraying, as highlighted in line 80 in the revised manuscript as follow:
…Garcia et al. [23] reported that ~60% of GNPs burned and only around 40% of GNPs survived the high temperature (~3000 oC) during flame spraying of GNPs/YAS (Y2O3-Al2O3-SiO2) coating…
2. Line 114 “Prior to spraying drying, nano-sized Al2O3 powders (the ratio (in wt. %) of 60 nm Al2O3 to 400 nm Al2O3 was 3:2)” Why do you use two sizes of Al2O3? Why not only one big or only small? Why this ratio? Explain it.
The reason why we chose a mixture of Al2O3 with two sizes is: Using two sizes of Al2O3 is in favor of the uniform dispersion of the nano-sized Al2O3 and the adsorption of graphene nanoplatelets on the Al2O3 particles within slurry that used for spray drying. As you suggested, we have added explanation about this issue, as highlighted from line 118 to line 120 (the updated line numbers in the revised manuscript) in the revised manuscript as follow:
…The reason for using mixture of Al2O3 powders with two sizes is to facilitate the uniform dispersion of nano-sized Al2O3 and the adsorption of GNPs on Al2O3 particles within slurry…
The ratio of 3:2 is an optimized ratio to obtain uniform Al2O3/GNPs powders. As you suggested, we have also added explanation about this issue, as highlighted in line 116 in the revised manuscript as follow:
…Prior to spray drying, nano-sized Al2O3 powders (the ratio (in wt. %) of 60 nm Al2O3 to 400 nm Al2O3 was optimized to 3:2)…
3. Why you did not measure only “nano-sized Al2O3 powders (the ratio (in wt. %) of 60 nm Al2O3 to 400 nm Al2O3 was 3:2)” for comparison and to check the effect of GNPs?
The reason why we also tested the Al2O3 coating by using commercial fuse-crushed Al2O3 powder is to compare the electrical conductivities between the Al2O3/GNPs coating by using Al2O3/GNPs powders and the conventional Al2O3 coating by using commercial Al2O3 powders. In this way, the effect of the GNPs addition on the electrical conductivity of Al2O3 coating would be revealed directly. Inspired by your comment, we have added this explanation, as highlighted from line 130 to 131 in the revised manuscript as follow:
…For comparison, commercial fuse-crushed Al2O3 powders with a size range of 15-45 μm were also used for coating deposition to investigate the effect of GNPs incorporation on the electrical conductivity of Al2O3 coating.
4. Line 116 “GNPs were evenly dispersed in an aqueous organic binder (polyvinyl alcohol) to form a slurry” What amount of PVA? Ratio between Al2O3:GNPs:PVA? It was for powder prepared by spray drying. What binders for coatings? Ratio?
For your first query, as you suggested, we have added the detailed ratio between Al2O3, GNPs, and PVA, as highlighted in line 117 and 118 in the revised manuscript as follow:
…The proportion (in wt. %) of Al2O3, GNPs, and PVA in the slurry was 98.65%, 1%, and 0.35%, respectively…
For your second query about the binder for coatings, there is no binders within the plasma-sprayed coatings. It is well known that, the splats within the plasma-sprayed coatings are generally bonded by strong mechanical interlocking effect. Therefore, there are no extra binders being added within plasma-sprayed coatings (Li, C.J. et al., J. Therm. Spray Technol. 2002, 11, 365-374).
5. Line 112 “…length of 5-10 μm and thickness of less than 10 nm (Figure 1)”. The length of 5-10 μm can be supported by Figure 1a, but Figure 1b cannot confirm that “thickness of less than 10 nm”. Thus, Figure 1b could be removed or changed by better image with another scale.
Thanks for your kind and valuable suggestion. Since the thickness of GNPs is too small to be detected by SEM method, inspired by your suggestion, we have removed Figure 1b. In the late future, we will try to do more precise characterization for GNPs by TEM method.
6. Why 850 and 1280oC were chosen as heat treated temperatures for powder?
There is no particular reason to choose these two heat treatment temperature. Our aim is to select some gradually increased temperature to investigate the effect of powder heat treatment on GNPs survival and electrical conductivity of Al2O3/GNPs coatings. 850 and 1280 oC were just chosen as two typical and model temperature for powder treatment. In fact, the experiments presented in our manuscript is sufficient to support our opinion that higher heat treatment temperature for Al2O3/GNPs powders enhanced the GNPS survival and electrical conductivity of Al2O3/GNPs coating. It is speculated that a higher powder heat treatment temperature may bring higher electrical conductivity of Al2O3 coatings. Inspired by your comment, we have added our next plan of further increasing powder heat treatment temperature, as highlighted at the last of section 3.4 (from line 494 to line 497) in the revised manuscript as follow:
…Furthermore, heat treatment at a temperature higher than 1280 oC would be applied to the Al2O3/GNPs powders to investigate whether an Al2O3/GNPs coating with more GNPs survival and higher electrical conductivity could be achieved.
7. What is deposition time?
During plasma spraying process, the deposition of a molten droplet after impacting onto substrate or pre-deposited splats involves spreading, followed by rapid cooling and solidification. In general, the time required for a splat spreading is about 0.5~1 μs. For the latter cooling and solidification process, considering the cooling rate of ceramic splats and metallic splats are ~104-106 oC/s and 106-108 oC/s, respectively, the time needed for splat cooling and solidification is usually ~10-20 μs (R. McPherson, Surf. Coat. Technol. 1989, s39-40, 173-181; M. Vardelle, J. Therm. Spray Technol. 1995, 4, 50-58). Thus, the total deposition time of a plasma-sprayed splat is about dozens of μs.
8. In Line 134 “ Each coating has a thickness of ~300 μm”. But in Line 177 “free-standing coating samples with a … thickness of ~600 μm.” Why did you use different thickness?
Thank you for your valuable comment. Here is the reason why the thickness of the coating used for electrical conductivity test is different from that used for microstructure characterization: During the preparation process of the sample used for electrical conductivity test, a relative thick coating with a thickness of ~1 mm was deposited on substrate at first. Then, the substrate was grind off with emery papers to obtain a free-standing coating without substrate for test. Considering the uncertain thickness loss in the substrate grinding process, an initially thick coating is always required. The 600 μm is the thickness of the free-standing coating after substrate was removed. So, there is no significant reason for us to obtain this thickness. In general, the electrical conductivity test basically has no specific requirement for the coating thickness and the electrical conductivity of a coating is independent of the coating thickness (J. Van Herle, et al., J. Mater. Sci., 1994, 29, 3691-3701). Therefore, the electrical conductivity of the Al2O3/GNPs coating in the current work tested by using a coating with a thickness of 600 μm is reasonable.
9. Line 168 “Prior to test, all the free-standing coating samples without substrate were ground into powders for TGA test.” How free-standing coating samples for TGA were obtained? How it was removed from substrate? How it was grounded?
As you suggested, we have added the detailed experimental process, as highlighted from line 185 to line 187 in the revised manuscript as follow:
…Prior to test, the substrate was ground off with emery paper to obtain a free-standing coating. Then, these free-standing coatings were ground into powders with a mortar and pestle for TGA test.…
10. Figure 2 needs to be also presented with the scale 200nm as Figure 3 for comparison and to see the influence of spray-coating process of structure.
Thanks for your kind and valuable suggestion. As you mentioned, the higher magnification images of Figure 2 (the surface morphologies of AG powders) have been presented in Figure 3 (with scale of 200 nm) in our original manuscript. The influence of heat treatment conditions for AG powders on the powder microstructure, i.e., the different sintering effect, GNPs microstructure, and so on, has been clearly presented in Figure 3. So, we think Figure 3 is sufficient to demonstrate our aim.
11. How cross-section was prepared? How steel was cut without damage of covered film on top?
As you suggested, we have added the detailed experimental process for the preparation of a cross-section sample, as highlighted from line 147 to line 151 in the revised manuscript as follow:
…After plasma spraying process, in order to obtain a cross-section coating sample, the round coating samples were cut in the middle along the direction perpendicular to the coating surface with a precision sample cutting machine. After that, the sample was mounted, ground with SiC emery paper up to 2000 grit, then mirror-polished to a 0.02 μm finish with silica polishing agent.
For your second query, it is inevitable to bring some damage to the coating immediately adjacent to the cutting position. Therefore, during the metallographic sample preparation process, the sample was carefully grind down to some deep depth to get rid of the damage region.
12. Line 235 “exhibited highest porosity of ~6.8%, which was much higher than the 3.4% and 1.6% …” It was not mentioned how density and porosity was calculated. It needs to be shown here because typically inserting of graphene increases the porosity. Discuss it and show comparison with other papers. As usually, powders lost graphene with increasing of temperature and, respectively, AG coating (that used already treated powder) can present higher density for film with higher-temperature treated powder because no graphene will be there.
(a) Thank you for your kind suggestion. We have already presented the detailed method to calculate the coating porosity in line 165 (the updated line numbers in the revised manuscript) in our original manuscript as follow:
…To quantitatively characterize the interface bonding within Al2O3 and three types of Al2O3/GNPs composite coatings, the apparent porosities of the four types of coatings were measured by image analysis method conducted on their cross-sectional images. Ten images taken randomly were used to obtain the average porosity value for each coating…
As you suggested, we have added discussion and comparison with other paper, as highlighted from line 260 to line 262 in the revised manuscript as follow:
…Noteworthy, the porosity of 6.8% for the present AG coating is considerable to the 5.6% porosity of the reported plasma-sprayed Al2O3/0.5 wt. %GNPs composite coating by using spray-dried Al2O3/GNPs powders without further powder heat treatment [20]..…
(b) In the case of your second query, there may be some misunderstanding about the effect of GNPs on the coating porosity. On one hand, the AG powders were heat-treated under argon atmosphere; thus, there are basically very little GNPs loss during powder heat treatment process. This is also indicated by our TGA test results shown in Figure 9 (the detailed TGA results analysis could be found in our response to the latter query 16) that the GNPs contents within the AG, AG850, and AG1280 powder feedstock were 0.93%, 0.88%, and 0.86%, respectively, which were slightly lower than 1 wt. % that added into the initial Al2O3/GNPs mixture before spray drying. This issue has already been discussed in detail in section 3.3 in our original manuscript. On the other hand, our manuscript has revealed that powder heat treatment enhances the GNPs survival within the resultant Al2O3/GNPs coatings. This has been proved by our TGA results shown in Figure 9, Figure 10, and Table 3 that the retention percentage of GNPs within coating increased in the following sequence: 12.9% for AG coating < 28.4% for AG850 coating < 37.2% AG1280 coating. The decreased coating porosity with increased powder heat treatment temperature is primarily due to the enhanced thermal conduction of well-sintered powders and the high thermal conductivity of GNPs survived within coating. This issue has also been discussed in detail in the last paragraph of section 3.2 in our original manuscript as follow:
…In the case of the plasma-sprayed AG, AG850, and AG1280 coatings, with the increase of heat treatment temperature for powders, the occurrence of sintering necks within the nano-structured alumina particles enhanced thermal conduction throughout agglomerated droplets; thus, high melting degree of alumina was obtained during in-flight process. This promoted the sufficient filling of voids by molten droplets upon impacting on substrate or pre-deposited lamellae. Consequently, decreased porosity, improved interface bonding and coating density was achieved, e.g., for the AG 1280 coating. In contrast, for the AG powders without heat treatment, there were no sintering necks but only many pores existing among the nano-structured alumina particles, hindering their thermal conduction during in-flight process and thus low melting degree of AG powders was obtained. Finally, after impacting onto substrate, high porosity and poor interface bonding was obtained for the AG coating. The well-molten of AG1280 droplets would facilitate the survival of GNPs within coating and this will be discussed in the next section. Noteworthy, when compared to the plasma-sprayed Al2O3 coating deposited by plasma spraying of commercial fuse-crushed Al2O3 powders, the AG1280 coating exhibited a much denser microstructure, less porosity, and stronger adhesive strength. This is likely attributed to the well-dispersion of GNPs within AG1280 coating and the contribution of high thermal conductive GNPs (thermal conductivities of GNPs and Al2O3 are ~5000 and 10 W/m×K, respectively) to heat transfer during in-flight process of powders..…
13. Lines 259-260 “Evidently, for the AG coating, large amount of partially-molten (PM) regions were presented within the coating, indicating the low melting degree of AG powders during in-flight process”. It was mentioned that AG was prepared without heat treatment: How appears “partially-molten (PM) regions”? What is the temperature of coating process at what “low melting degree” can appear? What happens during the process? What do you mean “in-flight process”? What is the melting temperature of Al2O3?
As we have demonstrated in our manuscript, plasma-sprayed coatings are formed by successive impact of molten or semi-molten droplets, followed by flattening, rapid cooling, and solidification processes. The schematic of plasma spraying process is shown in the following schematic a. In general, a copper-based nozzle and tungsten electrode act as anode and cathode, respectively. Solid powders are continuously injected into plasma jet to be heated until impacting onto substrate, this process is defined as in-flight process. The plasma jet temperature adjacent to the nozzle exit is highest, which is always higher than 10000 oC (McPherson, R. Thin Solid Films, 1981, 83, 297-310). Thus, plasma spraying is available to melt any materials that has a physical melting point under an appropriate spraying parameters. However, the radial temperature gradient of plasma jet is large; thus, during in-flight process, the solid powders injected into the plasma jet center would be melted readily to obtain a fully-molten droplet; powders that not be injected in the jet center, i.e., at the jet edge, would be partially melted to form a partial/semi-molten droplet with low melting degree. Therefore, after droplets impacting on substrate, the fully-molten droplets and semi/partial molten droplets would deposit as fully-molten and partially-molten regions, respectively, within the resultant coating. In addition, the melting temperature of Al2O3 is 2054 oC.
14. Line 261 “poor thermal conduction of AG powders without heat treatment”. Why did you mention it? Graphene has high enough thermal conductivity. Indicate please the thermal conductivity GNPs and Al2O3.
As we have demonstrated and explained in line 335 (the updated line numbers in the revised manuscript) in our original manuscript as follow: “…In the case of the plasma-sprayed AG, AG850, and AG1280 coatings, with the increase of heat treatment temperature for powders, the occurrence of sintering necks within the nano-structured alumina particles enhanced thermal conduction throughout agglomerated droplets; thus, high melting degree of alumina was obtained during in-flight process…” In contrast, for the AG powders without heat treatment, there is no sintering neck between nano-structured alumina particles but many pores exist among them, which hinder thermal conduction of AG powders during in-flight process. Inspired by your comment, we have added more explanation about this issue, as highlighted from line 342 to line 347 in our revised manuscript as follow:
…In contrast, for the AG powders without heat treatment, there were no sintering necks but only many pores existing among the nano-structured alumina particles, hindering their thermal conduction during in-flight process and thus low melting degree of AG powders was obtained. Finally, after impacting onto substrate, high porosity and poor interface bonding was obtained for the AG coating.…
In addition, as you suggested, the thermal conductivities of GNPs and Al2O3 are added, as highlighted in line 352 in the revised manuscript as follow:
…This is likely attributed to the well-dispersion of GNPs within AG1280 coating and the contribution of high thermal conductive GNPs (thermal conductivities of GNPs and Al2O3 are ~5000 and 10 W/m×K, respectively) to heat transfer during in-flight process of powders.
15. Line 322 “.. plasma spraying of commercial fuse-crushed Al2O3 powders” Where is data for that samples?
For the plasma-sprayed Al2O3 coating (marked as Al2O3 coating) deposited by plasma spraying of commercial fuse-crushed Al2O3 powders, its microstructure, porosity value, and adhesive strength were presented in Figure 4a and 5a, Figure 6, and Table 2.
16. Line 342 “the GNPs contents within the AG, AG850, and AG1280 feedstock were slightly lower than 1 wt. % that added into the initial Al2O3/GNPs mixture before spray drying.” What do you mean “slightly lower than 1 wt. %”? Based on Figure 9a mass loss for AG powder during heating is very big: 0.93 wt% after 850 oC. Thus, powder treated at 850 and 1280 oC can have only 7 % of GNPs after treatment? And further TGA analyses of these samples (reheating) show lower mass loss because graphene already disappears and it well correlated with the following decreasing: 0.88 wt% (for 850 oC) and 0.86 wt % (for 1280 oC). But why tendency of increasing mass loss appears for coating? Make it clear. Do you use high amount organics/binders/etc for preparation of film by spray-coating?
Thanks for your kind comment. The principle of TGA test is to measure the mass loss of sample under high temperature and oxidizing atmosphere to obtain the content of GNPs retained within samples. That is, GNPs retained within powders or coatings would combust under TGA test conditions, leading to mass loss of sample during TGA test; thus, the amount of GNPs remained within samples could be measured through testing the mass loss of sample during TGA test. Therefore, the weight loss occurring between 500 and 900 oC in TGA results represents the GNPs remaining in the feedstock or coating but not the GNPs loss during powder heat treatment or thermal spraying process. So, the 0.93%, 0.88%, and 0.86% mass loss presented in TGA results (Figure 9) represent the GNPs amount retained within the AG, AG850, and AG 1280 powders, respectively, which were slightly lower than 1 wt. %.
This issue has been clearly and detailed explained and discussed in section 3.3 in our original manuscript as follow: “…Evidently, for each type of feedstock and coating, their weight loss during heating process of TGA test appeared in two temperature regions. The one is the weight loss between ~100-500 oC, which was considered to be primarily owing to the burning of organic adhesives added in spray drying process together with the evaporation of residual water within powder. The other is the weight loss between 500-900 oC, which is a typical combustion temperature range for graphene [22, 23]. Thus, the weight loss occurring between 500 and 900 oC in TGA results represents the GNPs remaining in the feedstock or coating. It is indicated from Figure 9 that GNPs were successfully survived from the high-temperature plasma jet and retained in the three types of Al2O3/GNPs coatings. In addition, the GNPs contents within the AG, AG850, and AG1280 feedstock powders (Figure 9a) were 0.93 wt. %, 0.88 wt. %, and 0.86 wt. %, respectively, which were slightly lower than 1 wt. % that added into the initial Al2O3/GNPs mixture before spray drying.…”
Likewise, as can be seen from Figure 9, the GNPs amount retained within AG, AG850, and AG 1280 coatings were 0.12 wt. %, 0.25 wt. %, and 0.32 wt. %, representing retention percentage of 12.9%, 28.4%, and 37.2% compared to the GNPs amount within their corresponding feedstock powders (0.93%, 0.88%, and 0.86%, respectively). Thus, the GNPs survival and retention within plasma-sprayed Al2O3/GNPs coatings increased with the increase in powder heat treatment temperature. This issue has also been demonstrated and explained in section 3.3 in our original manuscript as follow (the highlighted sentence is the added explanation to make our demonstration more clear):
Line 375: “…Apparently, the GNPs amounts within the resultant plasma-sprayed AG, AG850, and AG 1280 coatings were 0.12 wt. %, 0.25 wt. %, and 0.32 wt. %, respectively (Figure 9b), which were lower than that in the corresponding feedstock…
Line 384: …The retention percentages of GNPs for the three types of plasma-sprayed Al2O3/GNPs coatings (the ratio of GNPs amount within coating to that within their corresponding feedstock powders) were calculated and the results are shown in Figure 10 and listed in Table 3. Evidently, the GNPs retention after plasma spraying process increased as follow: 12.9% for AG coating < 28.4% for AG850 coating < 37.2% AG1280 coating. This results revealed that powder heat treatment significantly increases the survival of GNPs within plasma-sprayed Al2O3/GNPs coatings. In addition, higher heat treatment temperature endows more preserve of GNPs after high-temperature plasma spraying process.…”
For your query about binders/organics/etc, the proportion of binder has been added in line 117 in our revised manuscript as follow:
…The proportion (in wt. %) of Al2O3, GNPs, and PVA in the slurry was 98.65%, 1%, and 0.35%, respectively…
17. Line 358 “for flame-sprayed YAS/GNPs coating” At what temperature? What ratio/ammount between YAS and GNPs?
We have demonstrated the spraying temperature in line 394 (the updated line numbers in the revised manuscript) in our original manuscript. Inspired by your comment, to make our demonstration more convincing, we have added related reference about the spraying temperature, as highlighted from line 394 to line 395 in the revised manuscript as follow:
…Taking account of the harsh processing condition and much higher temperature (~10000 oC) [31] during plasma spraying than that of the flame spraying (~3000 oC) [23], much more GNPs would be survived by using heat-treated agglomerated powders…
As you suggested, we have added the ratio of GNPs, as highlighted in line 393 in the revised manuscript as follow:
…Noteworthy, the GNPs retention of 37.2% for the AG1280 coating is considerable to that of the reported 40% for flame-sprayed YAS/2.3 wt.% GNPs coating [23]…
18. Line 353 “12.9% for AG coating < 28.4% for AG850 coating < 37.2% AG1280 coating” Where did you take these values? In Figure 9 another data can be seen. Correct it please.
Thank you for your kind comment, the issue relating to the GNPs retention amount within coating has been detailed explained and responded in the above query 16. As mentioned in response to query 16, the data presented in Figure 9 are the GNPs amount retained within feedstock powders (Figure 9a) and within coatings (Figure 9b). The 12.9%, 28.4%, and 37.2% are the retention percentages (the ratio of GNPs amount within coating to that within their corresponding feedstock powders) of GNPs within AG, AG850, and AG1280 coatings, respectively. The method how we obtained these retention percentages has also been explained from line 385 to line 386 in the revised manuscript as follow:
The retention percentages of GNPs for the three types of plasma-sprayed Al2O3/GNPs coatings (the ratio of GNPs amount within coating to that within their corresponding feedstock powders) were calculated and the results are shown in Figure 10 and listed in Table 3. Evidently, the GNPs retention after plasma spraying process increased as follow: 12.9% for AG coating < 28.4% for AG850 coating < 37.2% AG1280 coating. This results revealed that powder heat treatment significantly increases the survival of GNPs within plasma-sprayed Al2O3/GNPs coatings…
So, the discussion presented in our manuscript is correct.
19. Line 354 “powder heat treatment significantly increases the survival of GNPs within plasma-sprayed Al2O3/GNPs coatings”. Are you sure: 1 wt% of GNPs before treatment of powder and mass loss 0.93 wt% after 850 oC (Fig.9a)?
As replied in query 16, the mass loss of 0.93% shown in the TGA results (Figure 9a) represents the GNPs amount retained within the AG powders. Also, the detailed explanation about our opinion “powder heat treatment significantly increases the survival of GNPs within plasma-sprayed Al2O3/GNPs coatings” has been explained in the above query 16 and been demonstrated in section 3.3 in our original manuscript as follow:
…The retention percentages of GNPs for the three types of plasma-sprayed Al2O3/GNPs coatings (the ratio of GNPs amount within coating to that within their corresponding feedstock powders) were calculated and the results are shown in Figure 10 and listed in Table 3. Evidently, the GNPs retention after plasma spraying process increased as follow: 12.9% for AG coating < 28.4% for AG850 coating < 37.2% AG1280 coating. This results revealed that powder heat treatment significantly increases the survival of GNPs within plasma-sprayed Al2O3/GNPs coatings. In addition, higher heat treatment temperature endows more preserve of GNPs after high-temperature plasma spraying process.…
20. Line 358 “Taking account of the harsh processing condition and much higher temperature (~10000 oC) during plasma spraying (3 000oC)” Refs. Did you use 10 000 oC? What is the temperature during plasma spraying in the current work? Why you mentioned 10 000 oC?
The 3000 and 10000 oC are typical jet/flame temperature during conventional flame spraying and plasma spraying process, respectively, under general processing parameters. Although we have not measured the specific temperature of plasma jet in the current work, it is reasonable to qualitatively compare the processing temperature of flame spraying and plasma spraying since the coatings presented in the current work were fabricated by using conventional plasma spray device and common parameters. Inspired by your comment, we have added references relating to the temperature of flame spray and plasma spray, as highlighted in line 394 and line 395 in the revised manuscript as follow:
Taking account of the harsh processing condition and much higher temperature (~10000 oC) [31] during plasma spraying than that of the flame spraying (~3000 oC) [23], much more GNPs would be survived by using heat-treated agglomerated powders.
21. Line 415 “…the electrical conductivity of Al2O3/GNPs coating increased from 1.24 × 10-9 S m-1 of Al2O3 coating to 18 × 10-6, 7.68 × 10-5, and 2.06 × 10-2 S m-1 for the AG, AG850, and AG1280 coating, respectively, i.e., ~3 orders, 4 orders, and 7 orders of magnitude higher than that of Al2O3 coating”. The concentration of conductive GNPs cannot be more than 1 wt % in all AG coatings it was written in experimental part. According to Figure 9a quantity of GNPs in powder decreased with increasing of temperature. Thus, quantity of GNPs in Al2O3/GNPs 1280 oC coating is lower than that in initial AG coating. Why conductivity is higher? Unclear.
As we have demonstrated in section 3.3 and responded in the above query 16, the amount of GNPs retained within AG, AG850, and AG1280 coatings were 0.12 wt. %, 0.25 wt. %, and 0.32 wt. %, respectively (Figure 9). Thus, the GNPS survival and retention increased with the increase in the powder heat treatment temperature, i.e., the GNPS retention amount increased as follow: AG coating < AG850 coating < AG1280 coating. The detailed reason relating to the effect of powder treatment on the electrical conductivity of Al2O3/GNPs coatings has been demonstrated in section 3.4 in our original manuscript as follow:
Line 455:…That is, the electrical conductivity of Al2O3/GNPs coating increased from 1.24 × 10-9 S m-1 of Al2O3 coating to 1.18 × 10-6, 7.68 × 10-5, and 2.06 × 10-2 S m-1 for the AG, AG850, and AG1280 coating, respectively, i.e., ~3 orders, 4 orders, and 7 orders of magnitude higher than that of Al2O3 coating. This is mainly owing to the improved interface bonding and well-survived GNPs within the Al2O3/GNPs coatings by using AG powders with high heat treatment temperature. It has been well-reported that the electrical conductivity of plasma-sprayed coatings is primarily depended on their inter-lamellar bonding [20, 34-36]. In general, the current conduction would be interrupted by the un-bonded interfaces, leading to poor electrical conductivity of plasma-sprayed coatings than that of the corresponding bulk materials. In terms of the AG1280 coating, its high density, low porosity, and well inter-lamellar interface bonding contributed to its smooth electrical conduction. Moreover, the GNPs trapped between lamellae acted as bridges, favoring electrical conduction between lamellae. The well-dispersed GNPs may also form a connected network throughout the entire coating, allowing the current flow across the GNPs-GNPs contacts [23]. In contrast, in the case of the AG coating, its weak interface bonding and little preserved GNPs resulted in the low electrical conductivity…
22. Figure 9 should be reorganized as only powders and only coatings.
Thanks very much for your kind and valuable suggestion. As you suggested, we have reorganized Figure 9 as only powders (Figure 9a) and only coatings (Figure 9b) in the revised Figure 9.
Based on the presented information and according to the rules for Reviewers the current manuscript can be Reconsider only after major revision (control missing in some experiments).
Round 2
Reviewer 1 Report
I read the revised manuscript and the reply letter carefully. Authors' response to my comments is appropriate, therefore, I suggest that the paper be accepted as it is.
Thank you very much for your recognition of our article.
Reviewer 2 Report
Much better
Thank you very much for your recognition of our article.
