Study of Anodic Film’s Surface and Hardness on A356 Aluminum Alloys, Using Scanning Electron Microscope and In-Situ Nanoindentation
Round 1
Reviewer 1 Report
The manuscript concerns aluminum oxide coatings fabricated on A356 aluminum alloys by a standard and well-known anodization method using oxalic acid as an electrolyte. The authors used standard anodization parameters and obtained the expected results, which were confirmed by scanning electron microscopy. The second part of the work presents the results of hardness measurements for one selected sample.
The manuscript meets the journal’s scope. Its quality is, however, low. It has many understatements, and the authors do not present any explanation or understanding of the observed phenomena. The experimental research is very confined, based on only a few measurements. The part of the work concerning the structure of samples and their thicknesses does not add anything new to the field and is only a repetition of experimental results published in other papers (e.g. see Sixiang Liu et al 2021 Nanotechnology 32 222001.). Additionally, the manuscript does not refer to the most recent literature and is written using poor quality language.
In my opinion, only the last part concerning hardness measurements is interesting and has a scientific value. However, it is currently a draft and a starting point for interesting research. This part requires a significant development, in-depth analysis, and the addition of next measurements (e.g. for reference samples) to make an important contribution to the scientific literature.
Specific comments:
1. The purpose of the work as defined by the authors was not achieved. As one can read in lines 247-248: „Our motivation in this work is to understand better the effect of microstructure of the anodized layers on their mechanical properties”. In my opinion, this motivation and purpose of work are valuable and significant, but the authors did not present any experiments or results that would address the research problem defined above. In order to investigate the influence of the microstructure on the mechanical properties, it is necessary to prepare a series of samples with different microstructure, and then carry out hardness tests for them. However, the authors performed hardness test for one sample only. Apparently, even this one measurement was carried out only once. Based on such limited research, no well-established conclusions can be drawn. Therefore, it can be concluded that the purpose of the work was not achieved and additional experiments are needed.
2. The experimental methods and conditions of measurements are not well described. The current description does not guarantee the reproducibility of the experiments. In particular, the following information is missing:
- Details of the paste used to remove the natural oxide layer should be added. What was its graininess? What was the roughness of the polished samples?
- In paragraph 2.2, please add the information about the temperature at which the anodization was carried out. How was the temperature measured and controlled?
- Please add in paragraph 2.2. information whether the electrolyte was stirred during anodization. If so, what parameters has been used?
- Please add information about the concentration of oxalic acid during anodization.
- In paragraph 2.2., please add information on the geometry of the cell used for anodization. What was the cathode size? What was its distance between the cathode and the anode?
- In paragraph 2.4., please add information on tip sizes used for indentation and materials used for their production.
- In the caption to Fig. 3, please add information about the process parameters used to fabricate the presented sample.
3. The authors refer to publications of historical importance (e.g. references No. 13, 21, 24, 25, 26), while omitting the latest publications on aluminum anodization. The reference list needs to be updated. In particular, the model of the formation of aluminum oxide during the anodization process has been studied in detail in recent years, therefore the description of the formation of the oxide layer must be based on publications from the 21st century.
Additionally, references No. 15 and 24 do not appear in the text at all, which should be corrected.
4. What do the authors mean by "electric shock"? It is a concept that is rarely used today in the context of anodization. Do the authors mean the electric potential barrier? Please clarify.
5. Lines 206-215. The authors compare the shape of the curves in Figure 5 to the shape of stress-strain curves. Such a comparison is completely unauthorized. This is comparing apples to pears and has no physical justification. At the end of this paragraph, the authors conclude "Thus, further studies on the anodizing process are needed to further understand the effect of current density." First, no conclusions can be drawn from such non-physical comparison. Secondly, the influence of the current density on the properties of aluminum oxide has already been studied many times in many scientific works, and it is a process that is quite well understood. I propose to delete paragraph 206-215 as a whole as it does not meet the criteria of scientific discussion.
6. Please comment, what are the uncertainties for the values presented in Figure 6.
7. Lines 244 – 246. The authors claim: „It should be noted that the hardness value (0.6±0.1 GPa) of the substrate, obtained in the present work is almost the same than that reported in [27].” This sentence is a misconception. Firstly, the work [27] presents studies of thin copper layers and it is not clear why the authors compare their results to results concerning completely different material. Without a detailed explanation of the purposefulness, such a comparison is pointless. Secondly, it is unclear what "almost the same" means. In my opinion, the use of such expressions in scientific articles should be avoided as they are imprecise. Thirdly, the hardness values of the Cu layers presented in [27] are not similar to the hardness values for the A356 aluminum alloy obtained by the authors. Please correct the text accordingly, referring to the above remarks.
8. The authors did not go deeper into the mechanisms that take place on the surface of the alloy and oxide layer. Despite results suggesting that the hardness depends on the distance from the surface, no question was asked about the causes of the observed dependence. The entire work presents only a description of the procedures carried out and shows the results, without an in-depth analysis or an overview of the physical phenomena occurring in the samples.
9. Most of the conclusions at the end of the paper are unauthorized or add nothing to the field. The conclusion No. 1 is obvious in the light of many previously published articles. Conclusion No. 2 is imprecise. It is not clear what sample and what parameters of anodization it refers to. Its usefulness is therefore questionable. Conclusion No. 3 is partially obvious (no one doubts the usefulness of SEM and in-situ nanoindentation methods) and partially it is not sufficiently supported by the results since the authors did not study abrasion resistance.
Taking into account the above remarks, my overall assessment of the article is low. It appears to be only a starting point for a good publication on hardness measurements for anodized aluminum alloys. In my opinion, the manuscript is messily, does not provide in-depth understanding of processes occurring in anodized aluminum, and it does not meet the journal's standards. Therefore, I recommend rejecting the article and encouraging the authors to thorough correction of the paper followed by a re-submission.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
In this work, the surface characteristics and hardness of the anodic film on the A356 aluminum alloy are investigated using SEM and in situ nanoindentation. Some corrections still need to be made, for example:
1. Title:
Using apostrophe -s is not formal in the title.
2. The Introduction section:
It is recommended to explain the effect of microstructural change by anodization on the distribution of absorbed hydrogen in the alloy. For example, in the following paper, it was shown that when hydrogen is absorbed from the environment, the sensitivity to hydrogen embrittlement increases depending on the microstructure.
https://doi.org/10.1016/j.ijhydene.2021.09.013
3. Experimental part:
-The nanostructure needed to be studied. How does anodization affect the nanostructure of the alloy?
-The process of anodization should be shown schematically.
-It has been reported that ultrasonic cleaning can affect the wear characterization of A356 aluminum alloys. Please explain this with reference to https://doi.org/10.3103/S1068375515040067
4. Results and Discussion section:
-Please explain the effects of the thickness of the anodized layer on the various properties of the sample, e.g., tensile strength, corrosion, ...
-Why does the hardness increase with anodizing?
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
Reviewer comments
I would like to introduce all my thanks for editors of Coatings journal. The purpose of this research is to understand the relationship between the substrate and the oxide layer by examining its microstructure and nano-hardness.
Comments of authors
1. Please mention the used phosphate bath and amount
2. An accurate description of the method and the values of the variables is required
3. Schematic diagram of the process adopted for development the nano-layer should be included in the Materials and Methods section.
4. Drawing a relationship between the formed thickness of the alumina layer and the hardness is required.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
I would like to thank the authors for the revision and for addressing all my comments and remarks. I believe the quality of the paper is much better now. In particular, adding new results and discussing them enriches the work and makes it more scientifically sound.
Nevertheless, I still claim that the paper does not provide in-depth understanding of processes occurring in anodized aluminum and the authors do not present sufficient explanation of the observed phenomena. I particular, I would like to reiterate my remark regarding the mechanisms that take place on the surface of the alloy and oxide layer. In my opinion, the authors did not explain enough why the hardness depends on the distance from the surface and there is no any overview of the physical phenomena occurring in the samples. During the last revision, the authors added only one sentence: “Considering the mechanism of the layer-formation showing the formation of solid oxide at the substrate, and then the porous structure outwards, it is understandable that the nanohardness decreases outwards along the layer.” In my opinion, it is not so ‘understandable’ since the authors have not presented the sample cross-section at magnification sufficiently good to claim that the porous layer is only close to the surface. However, it is known from other literature that the pores in the AAO layer pass through the entire oxide layer. Therefore, a significant change in the hardness of the sample with depth is interesting, puzzling and deserves deeper explanation. I suggest softening this sentence and admitting that the mechanism is not well known for the authors.
Author Response
We would like to thank the Reviewer for his very valuable comments in both review reports. We really learned a lot from the critical comments, which greatly improve the quality of our article. Regarding the latest comment of the Reviewer, we have changed the wording (see the yellow-highlighted text in page 10), admitting that the basic mechanism is not well known for us. Thus, only a hypothetical explanation remains from the previous sentence.
Reviewer 2 Report
1. the comments are not satisfactorily addressed. Therefore, this manuscript is not appropriate for publication.
2. Paper of low quality: no originality/novelty, no scientific soundness, nothing useful since many similar papers were already published.
Author Response
Thank you very much for reviewing our paper
Reviewer 3 Report
corrections have been done.
Author Response
Thank you very much for reviewing our paper
