Corrosion and Tribological Performance of Diamond-like Carbon-Coated ZK 60 Magnesium Alloy
, Iván Fernández 2, José Antonio Santiago 2, Pablo DÃaz-RodrÃguez 2, Miguel Panizo-Laiz 3,4, Joseba Esparza 5, José F. Palacio 1,5, Gonzalo G. Fuentes 1,5, Iñaki Zalakain 1 and José Antonio GarcÃa 1,*Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors utilized HiPIMS to deposit DLC coatings for the purpose of protecting ZK60 magnesium alloy. The coatings were characterized in terms of their mechanical, tribological properties, as well as corrosion behavior. However, further efforts should be made to clarify the scientific novelty of the study.
1. In Figure 3, please use either uppercase or lowercase letters only for labeling the tests. Please specify the test 1 to 3.
2. The DLC coatings is less than 1 μm. Is it better to be called thin film?
3. It would be beneficial to display the entire scratch track. In the scratch test, critical loads of Lc1 to Lc3, or Lc1 to Lc4, Lc1 to Lc5 can all be seen in the reported work. So please define the physical meaning of Lc1 to Lc3 employed in this work first.
4. Considering the thin DLC coatings, the indentation depth played a crucial role in obtaining accurate hardness measurements. Could you please specify the indentation depth?
5. What is the residual stress level of the coatings?
6. What was the rationale behind using alumina balls in the friction test?
Comments on the Quality of English LanguageStill need revision.
Author Response
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Reviewer 2 Report
Comments and Suggestions for Authorszthis manuscript investigates the corrosion and tribological behaviors of the ZK 60 magnesium alloy by diamond like carbon coatings. The research topic is interesting and can be helpful for the engineering. The analysis and discussion are also suitable. The reviewer thinks it can be considered if the authors can well respond the comments as follow:
(1) More introductions about the ZK60A-T65 should be involved for the materials section, including the properties and advantages for the perchase.
(2) For the discussion section, the authors should better make some more literatures for the validation.
(3) For the corrosion analysis section, some more literatures in the micro and macro scale results could also be considered as follow: Spatial Distribution of Corrosion Products Influenced by the lnitial Defects and Corrosion-Induced Cracking of the Concreteï¼›Impact of defects in steel-concrete interface on the corrosion-induced cracking propagation of the reinforced concrete.
(4) The language was sometimes over tedious. The authors should be better make the manuscript more concise.
Comments on the Quality of English LanguageSome improvements are required.
Author Response
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Reviewer 3 Report
Comments and Suggestions for AuthorsThis work focused on the tribological and corrosion behaviors of DCL coatings coated on magnesium alloys by HiPMS, which is an interesting topic of research. However, several concerns about the experimental design, the formal analysis of results as well as discussions must be solved before it can be published.
1. They author has stated the significance of interlayers for the mechanical and anti-corrosion properties of DLC coatings on magnesium alloys. During the experimental design, a interlayer-free DLC coating should be prepared as a control sample.
2. The structure of the coating is not clear. Hydrogenated DLC was prepared on Cr/WC interlayer, while hydrogen-free DLC was deposited on Cr/CrC interlayer, please explain in detail the choice of such structures of coatings.
3. Please specify the significance of WC or CrC load support layer. Control samples could be much helpful to highlight the significance of such a load support layer.
4. The interlayer could enhance the bonding strength of DLC coatings by minimize the mismatch of mechanical properties between DLC layer and Mg substrate. Thus it is preferable to provide the mechanical properties of the interlayer. Nanoindentation tests on the inclined cross-sections could clarify this point.
5. It is recommended to summarized the coating deposition parameters in a table. Pulsing time of steps 2 and 3b (lines 170 and 180) is significantly different with step 4b (lines 187) with more than 6 orders of magnitude, but they all are HiPMS processes. Please check it.
6. Calotest was used to measure the thickness of coatings. Please provide the thickness of interlayers, especially the thickness of load support layers because the thickness of load support could significantly influence the bonding strength and mechanical properties of DLC layers on top. High resolution SEM observation on the cross-section could be used to double-check the thickness measurements.
7. The thickness of a-C coating is significantly higher than that of the a-C:H, please explain why.
8. The bonding strength of a-C:H is much higher than that of a-C, is it due to the lower thickness? Or the different load support layer? Or the difference of internal stresses? Please explain the reason in detail.
9. The author stated that a-C:H coating has higher content of graphite. However, the hardness of a-C:H is higher than that of the a-C. Could it be due to higher internal stress in the a-C:H coating? Please clarify it.
10. Tribological tests 1-3 have no significant difference, except the sliding velocities (approximately 0.25, 0.28 and 0.35 m/s). However, the COF and wear resistance show huge difference. Please explain this point in detail.
11. In table 4, the H3/E2 value of a-C is higher than that of the a-C:H, but the wear resistance of a-C is much lower that that of a-C:H, which is inconsistent with reported works. Please explain in detail.
12. Figure 10 is misleading, which has an impression that a-C:H has highest K value. Please remake this figure.
13. There is no scale bar or dimension in Figure 11.
14. Tafel fitting results summarized in Table 6 is obviously wrong. According to Figure 12, uncoated Mg should have lowest jcorr instead of a-C:H. Please double check your results.
15. The results of hydrogen evolution tests were not reported.
16. The EDS results in Figure 15 were obviously wrong. The content of Mg in figure 15a should not be such high according to the shape of spectra. The author stated that EDS analysis were performed on the corrosion products, but I am afraid not, since there is no sign of O or Mg in the spectra.
17. The English needs to be improved.
Based on above comments, I do think that the manuscript was not well organized, the design of experiments and analysis of experimental results were not scientific sound. Therefore, this manuscript is not suitable to be published on Coatings in the present form. I would like to recommend the author the resubmit the manuscript after addressing the above comments.
Comments on the Quality of English LanguageThe manuscript is difficult to read. English needs to be improved.
Author Response
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Reviewer 4 Report
Comments and Suggestions for AuthorsThis work is focused on corrosion and tribological performance of DLC coated ZK 60 Magnesium alloy. It shows some interesting results and discussions. It can be considered for publication after addressing the following comments:
1) In the abstract section, introductory sentences (Magnesium alloys are considered very interesting materials for applications such as automotive industry, aircraft and biomedical implants because of their good mechanical properties, biocompatibility, and low density. The combination of their properties is an advantage over other materials in terms of weight saving, resource availability and sustainability. However, their poor corrosion and wear resistance limits their real application. This handicap has led to the need to develop surface modification strategies, among which are PVD coatings.) should be deleted. The abstract can start with (In this work, Diamond Like…).
2) What is new about this research? This should be added to Abstract.
3) Please correct:
- Line 65 "Different strategies have been studied along last years to improve the corrosion resistance and mechanical properties of mg alloys…" as "Different strategies have been studied along last years to improve the corrosion resistance and mechanical properties of Mg alloys…".
- Line 84 "(hardness properties of diamond). [25,26]." as "(hardness properties of diamond) [25,26].".
- Line 85 "of these coatings; high hardness," as "of these coatings: high hardness,".
- Line 92 "with different elements [28,29] , depositing " as "with different elements [28,29], depositing".
- Line 108 "Mg alloys; DC magnetron sputtering [12]" as "Mg alloys: DC magnetron sputtering [12]".
- Line 110 "Sputtering (HiPIMS) [39,40].Among" as "Sputtering (HiPIMS) [39,40]. Among".
- Line 151, 171, 175, 265 "m3" as "m3", "W/cm2" as "W/cm2", "cm2" as "cm2".
- Line 188 "The deposition rateobtained" as "The deposition rate obtained".
- Line 200 "ordered(D-band) and graphite" as "ordered (D-band) and graphite".
- Line 430 “the Uncoated samples showed grooves” as “The uncoated samples showed grooves”.
4) - Line 136. Please correct “information that was provided by the manufacturer is presented in table?.” Table???
5) - Line 136. It may be better to present the parameters used for various substrate pre-treatments in a table.
6) - Line 270. “This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.” Delete.
7) - Line 294 Please correct. “the circumferences that can be seen in figure 2.” as “the circumferences that can be seen in figure 4.”
8) in Figure 4 and 5, please correct the scale bar, values are not read.
9) Please correct Fig. 12 - Current Density (A/cm2) as Current Density (A/cm2).
Author Response
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Reviewer 5 Report
Comments and Suggestions for AuthorsThe aim of this work was to improve the tribological and corrosion properties of magnesium alloys by depositing a-C and a-C:H coatings on the magnesium alloy ZK60 through high-power impulse magnetron sputtering (HiPIMS) with positive pulses. The a-C:H presented better results in tribological and corrosion tests, but the corrosion protection of both coatings is insufficient and must be improved by using thicker layers or multilayer structures. The authors conducted original research using the prospective HiPIMS coating method. The results obtained are interesting for expanding the practical application of magnesium alloys with good mechanical properties and biocompatibility. This work can be recommended for publishing in Polymers after some minor revisions listed below:
Line 66. “mg alloys” should be changed to “Mg alloys”.
Line 136. “in table” should be changed to “in table 1”.
Line 191. There is no reference to Fig.2 in the text.
Line 270. This paragraph must be removed from the manuscript.
Line 289. Explain why during tribotest 1 (2000 cycles) the a-C coating was completely delaminated and during tribotest 3 (3000 cycles) not.
Line 296. It would be easier to compare the effects of the coatings if they were of the same thickness. Why did you study coatings that differed more than twice in thickness?
Line 314. Explain what are the critical loads Lc2 and Lc3?
Line 495. “hydrogen coating” should be changed to “hydrogenated coating”.
Line 557. The statement “The a-C layer presented a thickness of 900 nm while the a-C:H one 400 nm.” is not a conclusion. It is just a fact that depends on the growth time.
Comments on the Quality of English Language
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Author Response
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Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsIn order to mitigate the high saturated vapor pressure of magnesium alloys, it is crucial to deposit a thin film or coatings at low temperatures. However, in the revised manuscript, the authors have not adequately emphasized the novelty of their work. Additionally, there are still typos present in the manuscript, and important experimental details are still missing.
Comments on the Quality of English LanguageTypos still exist in the manuscript
Author Response
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Reviewer 2 Report
Comments and Suggestions for AuthorsThe reviewer thinks it's OK now.
Comments on the Quality of English LanguageThe reviewer thinks it's OK now.
Author Response
The reviewer has accepted the paper for publication, so we would like to thank him for his advice and time spent in reviewing our work.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors have addressed several concerns proposed during 1st round of peer-review. However, there are still several concerns remain unaddressed. Please solve following problems before this manuscript can be published:
1. Figures 7(a) and 7(d) are duplicated.
2. The only difference of tribological conditions between test 1-3 is the sliding velocity. Test 2 has the highest sliding speed while Test 3 has the lowest one. However, the change of COF did not follow this trend. Please specify the reason.
3. Test 2 is the most demanding wear condition as stated by the authors. However, the wear rate of a-C:H coated sample under Test 1 is the highest among the three conditions. Please give a detailed and reasonable explanation.
4. The scale bar and values in Figure 11 is hard to distinguish.
5. In lines 535-536, the authors stated that the load support capacities of CrC and WC are similar. Is there any experimental evidence or reference to support this claim? Nanoindentation tests on WC and CrC, which could give their mechanical properties, might help.
6. The authors attribute the difference of bonding strength to the different internal stresses of two coatings. Please give the data of internal stresses, which could be measured with XRD residual stresses analysis technique.
7. The author ascribe the lower COF of a-C:H to the sp2 C-H bond. Therefore, XPS analysis on the wear tracks as well as wear scars on the sliding balls could be very helpful.
Based on above concerns, I think that the authors need to provide more detailed studies (or relevant references) of the mechanical properties and tribological behaviors of the coated samples.
Author Response
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Reviewer 4 Report
Comments and Suggestions for AuthorsDear authors, thanks for the revision. The revised paper can be published as is.
Author Response
The reviewer has accepted the paper for publication, so we would like to thank him for his advice and time spent in reviewing our work.
