Characterization of MAO + Cu Composite Coatings on Aluminum Alloy

Round 1

Reviewer 1 Report

In my opinion, the manuscript is not ready for publication. First of all, the work lacks novelty. Some of the MAO coatings are few decades old thus cant be treated as a novel topic (as the authors suggest). There is also no information given in the Introduction section what is the novelty of their research.

The manuscript is hard to read and many editorial errors are to be found. Figure 1 should be removed, it is a part of ASTM standard and easily accessible. 

There is no description for abbrev MAO-1, MAO-2, etc. Is the variable factor the copper plating rate or the MAO operating voltage? Even in the conclusion section authors operate these abbreviations and do not provide information about the influence of the studied process.

Equation 1 lacks data. What was the size of the copper plating reaction rate, how was it calculated? what was the mass change for each studied coating? Why the plating rate was even measured since the authors do not provide any discussion regarding the obtained results?

What is the standard deviation for the coating thickness measured by authors, these surfaces appear to be very rough and heterogeneous? 

SEM in Fig. 5 does not provide much information, all can be determined from EDX maps. Instead, it would be more informative to show SEM cross-sections On the other hand, the information about colors labeled to each element is hard to read as infographics are low in quality/letters are small.

Where was the EDS from Table 4 measured? Is this a point analysis (where) or averaged map (what surface area was analyzed)

The language quality needs to be checked by the native speaker (beautification, coats, etc.).

Last but not least there is very little (often none) substantial discussion provided by the authors. Sometimes they only provide the reader without an explanation. What is the mechanism of the topography changes as described in p3 line 90-95?

What was the assumed effect of Cu plating on the binding strength test? Can it be assumed that the obtained differences originate primarily from the applied voltage for the MAO process? Again, there is no discussion of these results.

Based on Fig. 4 results I don't see much difference between MAO-1 and MAO-4. How have authors drawn conclusions that MAO-1 is the best?

Author Response

Dear Ms. Tamara Tuvic and reviewers,

Thank you very much for your letter and the comments on our paper. We have checked the manuscript and revised it according to your comments. A list of changes, comments and replies to comments are shown as follows.

1. Figure 1 should be removed, it is a part of ASTM standard and easily accessible. 

Answer: The original Figure 1 has been removed and changed to a process schematic.

2.There is no description for abbrev MAO-1, MAO-2, etc. Is the variable factor the copper plating rate or the MAO operating voltage? Even in the conclusion section authors operate these abbreviations and do not provide information about the influence of the studied process.

Answer: In this study, the MAO coating was adjusted. MAO-1, MAO-2, MAO-3, MAO-4 equal to the ceramic coating made in the operating voltage of 400V, 450V, 500V, 550V.

3.Equation 1 lacks data. What was the size of the copper plating reaction rate, how was it calculated? what was the mass change for each studied coating? Why the plating rate was even measured since the authors do not provide any discussion regarding the obtained results?

Answer: Corresponding data supplements are made in the article. Using the same electroless copper plating process, the weight change comes from the change of the MAO coating. While adding a result of the analysis of the copper plating rate.

4.What is the standard deviation for the coating thickness measured by authors, these surfaces appear to be very rough and heterogeneous? 

Answer:Thanks for your reminder. The thickness of MAO coating samples under different working voltages are 2.18±1.05 , 7.97±1.15 , 12.48±2.05 and 21.04±2.8μm, respectively. It has been supplemented at “3.1 Characterization of Morphology”.

5.SEM in Fig. 5 does not provide much information, all can be determined from EDX maps. Instead, it would be more informative to show SEM cross-sections On the other hand, the information about colors labeled to each element is hard to read as infographics are low in quality/letters are small.

Answer: For already been modified and a new part of the experiment of Figure 5.

6.Where was the EDS from Table 4 measured? Is this a point analysis (where) or averaged map (what surface area was analyzed)

Answer:Table 4 shows the map scan results of the selected area in Figure 5.

7.Last but not least there is very little (often none) substantial discussion provided by the authors. Sometimes they only provide the reader without an explanation. What is the mechanism of the topography changes as described in p3 line 90-95?

Answer: Added mechanisms to topography changes  “Micro arc oxidation reaction preferentially reacts at the defects with low internal resistance and oxygen release channels of the ceramic coating. In the process of continuous "melting → solidification → melting" of the ceramic coating,  the ceramic coating is thickened continuously, and the deposition power consumption of high resistance barrier layer required for MAO reaction on the surface of aluminum alloy samples continues to increase. However, a constant operating voltage does not increase the energy provided, causing the operating current to concentrate on the low resistance defect response and the micropores diameter to increase. ”

8.What was the assumed effect of Cu plating on the binding strength test? Can it be assumed that the obtained differences originate primarily from the applied voltage for the MAO process? Again, there is no discussion of these results.

Answer:

Added analysis and discussion in "3.2. Analysis of Binding Strength": “Copper penetrates into the internal pores of the ceramic coating, and the copper-plated layer and the MAO coating produce mechanical engagement, pinning or locking structures at the pores, and the combination is tight. Increasing the roughness of the ceramic coating and the size of the micropores provide a better bond for the adhesion of the copper coating on the surface of the ceramic coating..”

9.Based on Fig. 4 results I don't see much difference between MAO-1 and MAO-4. How have authors drawn conclusions that MAO-1 is the best?

Answer: There is no significant difference in the coefficient of friction in Figure 4(a), but combined with the results of the wear scar depth in Figure 4(b), it can be clearly seen that the depth of the MAO-4+Cu composite coating is shallower.

10.  Furthermore, we have revised the paper and marked by red coulour.  

Reviewer 2 Report

In this paper, PEO coated Al 6061surface was plated electroless with copper layer in order to improve functional properties. This significantly widens area of PEO coating applications and is of interest for Coating journal audience. The main part of the work demonstrates all necessary details; however, the Introduction should be significantly improved in accordance with questions listed below.

1. Introduction is poorly written.

Line 24-25. “and poor tribological properties of aluminum alloys, which caused a large loss to the economy [4,5].” – It is incorrect to speak about tribolocial properties of only one counter body. A pair should be considered, it is paired interaction. For instance, good tribological performance of Al can be found in pairs: Al-PTFE, alloy AlSi – cast iron (piston-cylinder), and many others. References 4, 5 – irrelevant here.

“The surface modification treatment technology used in industrial production” – examples?

Paragraph 2 (Line 29 etc.) sounds non-scientifically, too long and non informative. Following sentences are unclear:

1. “In theory, aluminum is more active than copper and can directly undergo displacement reactions.” – Are any doubts that it would work in practice? What are displacement reactions? Did you mean substitution? More details are necessary.
2. “At the same time, the electrode potential of aluminum is very low (−1.67 V), which makes it easy to lose electrons [11,12].” – Very low potential cannot make anything. Physically, lower potential means easier losing of electrons, by definition. Why does this obvious statement require 2 references?
3. “The substitution reaction between aluminum and zinc forms a zinc layer on the surface o...” – what does this mean? How can aluminium substitute zinc forming zinc?

The PEO process was not properly referenced. Paragraph 3 (line 49) to be expanded accordingly to the subject of the paper. PEO coatings on 6061 alloys, ways of copper incorporation as compounds or metal via PEO. References also should be given to features of bipolar PEO, which was employed in the work.

2. Table 1 has poor readability. Chemicals for different processes are mixed and confusing readers. Some gaps or horizontal lines are necessary between processes.

Author Response

Dear Ms. Tamara Tuvic and reviewers,

Thank you very much for your letter and the comments on our paper. We have checked the manuscript and revised it according to your comments. A list of changes, comments and replies to comments are shown as follows.

1. Line 24-25. “and poor tribological properties of aluminum alloys, which caused a large loss to the economy [4,5].” – It is incorrect to speak about tribolocial properties of only one counter body. A pair should be considered, it is paired interaction. For instance, good tribological performance of Al can be found in pairs: Al-PTFE, alloy AlSi – cast iron (piston-cylinder), and many others. References 4, 5 – irrelevant here.

Answer: Thanks for your reminder, has been changed into : “Unfortunately, serious damage due to the low hardness and abrasion resistance of aluminum alloys limited the extensive application. ”, and revised the references.

2. “The surface modification treatment technology used in industrial production” – examples?

Answer: Examples are supplemented in the first paragraph of the introduction.

3. Paragraph 2 (Line 29 etc.) sounds non-scientifically, too long and non informative. Following sentences are unclear:

“In theory, aluminum is more active than copper and can directly undergo displacement reactions.” – Are any doubts that it would work in practice? What are displacement reactions? Did you mean substitution? More details are necessary.

“At the same time, the electrode potential of aluminum is very low (−1.67 V), which makes it easy to lose electrons [11,12].” – Very low potential cannot make anything. Physically, lower potential means easier losing of electrons, by definition. Why does this obvious statement require 2 references?

“The substitution reaction between aluminum and zinc forms a zinc layer on the surface o...” – what does this mean? How can aluminium substitute zinc forming zinc?

Answer: We have made a comprehensive revision to the above question: “Aluminum electrode potential (-1.67 V) is lower than the potential for the copper electrode (+0.34 V), and copper ions can be reduced to copper, but electroless copper plating is usually carried out in an alkaline environment. Aluminum belongs to amphoteric oxide and reacts under alkaline conditions. Meanwhile, an oxide film is easily formed into the surface of aluminum alloy, which will affect the bonding force between aluminum and the metal deposited on the aluminum surface [10,11]. To chemical methods of preparing galvanized zinc coating (-0.76 V), and then electroless copper plating, to avoid Al directly reacted with OH-, can effectively enhance the bonding force.To chemical methods for preparing galvanized zinc coating, and then electroless copper plating, to avoid Al directly reacted with OH-, can effectively enhance the bonding force [13].”

4. The PEO process was not properly referenced. Paragraph 3 (line 49) to be expanded accordingly to the subject of the paper. PEO coatings on 6061 alloys, ways of copper incorporation ascompounds or metal via PEO. References also should be given to features of bipolar PEO, which was employed in the work.

Answer: The introduction of micro arc oxidation technology was beautified.has been changed into “The new environmentally friendly micro-arc oxidation (MAO) technology is used to prepare the oxide ceramic coating with good corrosion resistance. The replacement of the zinc coating by the ceramic coating will slow the shedding of the copper coating on the surface.” But there is no change to “copper incorporation as compounds or metal via PEO” because Copper will exist in the form of oxides. The existing literature has been told , for example:

[1] Yao XH, Zhang XY, Wu HB, et al. Microstructure and antibacterial properties of Cu-doped TiO2 coating on titanium by micro-arc oxidation. Applied Surface Science. 2014, 282: 944-947.

[2] Huang QL, LiuXJ, Zhang RR, et al. The development of Cu-incorporated micro/nano- topographical bio-ceramic coatings for enhanced osteoblast response. Applied Surface Science. 2019, 465: 575-583.

5. Table 1 has poor readability. Chemicals for different processes are mixed and confusing readers. Some gaps or horizontal lines are necessary between processes.

Answer: Modified Table 1, and added the experimental process in "2.1 Material Preparation".

6.  Furthermore, we have revised the paper and marked by red coulour.

Reviewer 3 Report

Dear Authors,

You have a great article with interesting research and findings. However, in my opinion, the following adjustments are needed.

1. Introduction, it is necessary to expand and add alternative polymer coatings, graphite coatings, which can also be used as an antifriction material, for example:

D.V. Mashtalyar, S.V. Gnedenkov, S.L. Sinebryukhov, I.M. Imshinetskiy, A.S. Gnedenkov, V.M. Bouznik, Composite coatings formed using plasma electrolytic oxidation and fluoroparaffin materials, Journal of Alloys and Compounds Volume 767, 2018, Pages 353-360

Y.M.Wang, B.L.Jiang, T.Q.Lei, L.X.Guo, Microarc oxidation and spraying graphite duplex coating formed on titanium alloy for antifriction purpose, Applied Surface Science Volume 246, Issues 1–3, 2005, Pages 214-221

          Alternative electrolytes for the MAO process, e.g. in molten salts

Alexander Sobolev, Tamar Peretz, Konstantin Borodianskiy, Fabrication and Characterization of Ceramic Coating on Al7075 Alloy by Plasma Electrolytic Oxidation in Molten Salt, Coatings 2020, 10(10), 993; https://doi.org/10.3390/coatings10100993

Alexander Sobolev, Michael Zinigrad, Konstantin Borodianskiy, Ceramic coating on Ti-6Al-4V by plasma electrolytic oxidation in molten salt: Development and characterization, Surface and Coatings Technology Volume 408, 25 February 2021, 126847

2. It is necessary to explain the designations of the names of the samples and what their difference is. For example MAO 2, at what voltage and currents are obtained.
3. It is also necessary to add a paragraph describing the MAO process briefly. Forming steps, current decreasing, voltage increasing, hard anodizing step, sparking step, etc. It is necessary to bring graphs of voltage, current and frequency of the amplitude signal.
4. The next step is the process of applying copper plating by chemical means, it is also necessary to describe it briefly. Materials, equipment and application sequence.
5. It is necessary to add X-ray diffraction analysis of the resulting coatings, after the MAO process and after the copper deposition. Since chemical analysis with an EDS detector does not provide an indication of phase composition, coating crystallinity or amorphousness.
6. After adding the above points to the article, it is necessary to change the conclusions and add a few words about changing the phase composition depending on the voltage and current of formation.

Author Response

Dear Ms. Tamara Tuvic and reviewers,

Thank you very much for your letter and the comments on our paper. We have checked the manuscript and revised it according to your comments. A list of changes, comments and replies to comments are shown as follows.

1. Added the content of the introduction, and made extensive revisions.

Answer:Application examples carried out in the introduction. For example “Pakiea uses laser beam treatment to introduce Ti and Fe onto the surface of AlMg5 aluminum alloy. Found that intermetallic phase rich in titanium and aluminum compound phase significantly improves the mechanical properties of the coating [5]. Mohamad used anodizing technology to prepare a graphite-containing composite oxide film on AA2017-T4 aluminum alloy. It was found that the addition of graphite reduced the surface porosity of oxide film, improved the mechanical and tribological properties of oxide film, extend the service life of the composite oxide film [1]. Sobolev uses Plasma electrolytic oxidation to perform surface treatment on 7075 aluminum alloy and found that the addition of oxide coating has significantly improved the corrosion resistance [6]”. 

2. It is necessary to explain the designations of the names of the samples and what their difference is. For example MAO 2, at what voltage and currents are obtained.

Answer: In this study, the MAO coating was adjusted, and the ceramic coatings obtained at different voltages were abbreviated as MAO-(1-4). The specific electrical parameter changes are shown in Figure 1 (b) voltage and Figure 1 (c) current.

3. It is also necessary to add a paragraph describing the MAO process briefly. Forming steps, current decreasing, voltage increasing, hard anodizing step, sparking step, etc. It is necessary to bring graphs of voltage, current and frequency of the amplitude signal.

Answer: Add “MAO adopts the constant voltage mode. When the voltage reaches 280V the aluminum alloy specimen cylindrical arc boundaries appear tiny silvery white, voltage increases lead to increased discharge phenomenon, the defects on the surface of the workpiece make the current distribution in an extremely unbalanced state, and the electrons preferentially converge to the defect micro-regions with lower resistance in the oxide layer, cause arcing walk back and forth on the sample surface. The prepared MAO-1 ceramic coating has a working voltage of 400V and a relatively low current. During the reaction, the surface of the ceramic coating remains only sporadic arcing.” at “2.1 Material Preparation ” .

The voltage and current changes during the reaction are shown in Figure 1(b-c).

4. The next step is the process of applying copper plating by chemical means, it is also necessary to describe it briefly. Materials, equipment and application sequence.

Answer: Added the corresponding supplement “After completion of the reaction samples were rinsed with deionized water, and then the MAO+Cu composite coating is prepared according to the process of Sensitization→Activation→Chemical copper plating.” The reaction process and specific process parameters shown in Figure 1 and Table 1.

5. It is necessary to add X-ray diffraction analysis of the resulting coatings, after the MAO process and after the copper deposition. Since chemical analysis with an EDS detector does not provide an indication of phase composition, coating crystallinity or amorphousness.

Answer: There are obvious copper characteristic peaks on the surface of the composite coating at 43.407°, 55.556° and 74.297°. However, due to the thin thickness of the MAO coating and the copper coating, XRD image is not satisfactory, so it does not appear in the article.

6. After adding the above points to the article, it is necessary to change the conclusions and add a few words about changing the phase composition depending on the voltage and current of formation.

Answer: Adding to the conclusion of the :“The surface of aluminum alloy was treated with MAO to form a ceramic coating mainly made of alumina. The higher operating voltage of constant pressure mode, the greater the thickness of the coating, the smaller the number of surface micropores, and the surface hardness first increases and then decreases. Electroless copper was filled into the pores of the MAO coating."

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors addressed the bulk of my comments. In my opinion, the discussion of the MAO coatings presented in the manuscript is still quite shallow, yet allows the reader to obtain elementary knowledge about the studied process.

Authors are kindly asked to provide in the experimental section information about the methodology to calculate average thickness. Judging by Fig 2l it seems not likely the average thickness to be only 2.8 um for MAO-4 coating.

Author Response

Dear Ms. Tamara Tuvic and reviewers,

Thank you very much for your letter and the comments on our paper. We have checked the manuscript and revised it according to your comments. A list of changes, comments and replies to comments are shown as follows.

1.The authors addressed the bulk of my comments. In my opinion, the discussion of the MAO coatings presented in the manuscript is still quite shallow, yet allows the reader to obtain elementary knowledge about the studied process.

Answer: Micro arc oxidation in the introductory part added: “ The instantaneous high temperature forms the basic phases of α-Al₂O₃ and γ-Al₂O₃ on the aluminum alloy surface.” at 2-60.

2.Authors are kindly asked to provide in the experimental section information about the methodology to calculate average thickness. Judging by Fig 2l it seems not likely the average thickness to be only 2.8 um for MAO-4 coating.

Answer: Thank your questions again, the problem for the numerical results with the results of your picture presented inconsistent, we re-measured the thickness of the MAO-4 sample with the TT260 eddy current thickness gauge. P4-111: “21.04 ± 2.8 μm” has been changed into “31.71± 4.37 μm”; At the same time, add the test method "and the average value of 15 random measurements is taken" at 3-88.

Reviewer 3 Report

Good work!!!

Author Response

Dear Ms. Tamara Tuvic and reviewers,

Thank you very much for your letter and the comments on our paper. We have checked the manuscript and revised it according to your comments. A list of changes, comments and replies to comments are shown as follows.

We have added the description of micro-arc oxidation: “ The instantaneous high temperature forms the basic phases of α-Al₂O₃ and γ-Al₂O₃ on the aluminum alloy surface.” at 2-60.