Preparation of Superhydrophobic Hydroxyapatite Coating on AZ31 Mg Alloy by Combining Micro-Arc Oxidation and Liquid-Phase Deposition
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsHydroxyapatite was coated on oxidized Mg-Al ally and its structure, mechanical and electrochemical properties were estimated in the submitted study. My comments are as follows.
- P5 Figure 1: It’s difficult to see scale bars on the photographs.
- P7 Figure 2: Why calcium element observed for the sample MAO.
- P7 Figure 3: LPD coating was deposited on MAO layer. But the thickness and structure of MAO are different for (a) and (b). Why is it?
- P8 Figure 4 and other places on the text: MAO is surface oxidized layer of magnesium. So this layer is not coating. It is oxidized layer not coating.
- P11 : Concerning estimation of the adhesion, the delaminate place is important for discussion. Where were the delaminate places of LPD and MAO in figure 8(a)? And for MAO-LPD samples Is the adhesion of this study enough to use for medical field?
Author Response
1.Comment: 1. P5 Figure 1: It’s difficult to see scale bars on the photographs.
Reply: We would like to thank the reviewer for the insightful comment. Based on the reviewer's suggestion, we have populated the bar to make it clearer.
2.Comment: P7 Figure 2: Why calcium element observed for the sample MAO.
Reply: We sincerely thank the reviewer for this valuable suggestion. The amount of calcium in the EDS energy spectrum of the MAO coating is very low. The results of our tests show less than one percent of calcium, which may be due to instrumental errors. Our base magnesium alloy has a very small amount of magazine calcium, so we did not remove the calcium that could cause errors when checking the test results.
3.Comment: P7 Figure 3: LPD coating was deposited on MAO layer. But the thickness and structure of MAO are different for (a) and (b). Why is it?
Reply: Micro-arc oxidation process is an in-situ oxidation, in-situ growth process, the interface between the coating and the substrate is not a uniform straight line, which is presented as an occlusal curve, resulting in the surface of the micro-arc oxidized coatings are presented as a high and low undulation state. The liquid-phase deposition process is a dynamic equilibrium process, redeposition process, there will be a small amount of micro-arc oxidation coating dissolution, the overall rate of deposition is greater than the rate of dissolution. Therefore, the liquid phase deposition layer can be observed on the surface of the micro-arc oxidized coating.
4.Comment: P8 Figure 4 and other places on the text: MAO is surface oxidized layer of magnesium. So this layer is not coating. It is oxidized layer not coating.
Reply: We sincerely apologize for the difficulties encountered by the reviewer. We highly value and appreciate your insightful comments. There is no consensus on whether micro-arc oxidation is a coating or a layer. The vast majority of the literature describes the micro-arc oxidized layer as a coating. Only a very small number of literature will micro-arc oxidation coating described as film, according to the literature research results, the physical deposition process generated by the majority of the film described as film, chemical reaction generated by the majority of the film described as coating. In this paper the micro-arc oxidized coating is a layer of the coating in the composite system. But the micro-arc oxidized coating itself is also a layer consisting of a dense inner layer, and a loose and porous outer layer. So our work is still called as coating.
- Comment: P11 Concerning estimation of the adhesion, the delaminate place is important for discussion. Where were the delaminate places of LPD and MAO in figure 8(a)? And for MAO-LPD samples Is the adhesion of this study enough to use for medical field?
Reply: The in vivo application of biomedical materials is a complex process. Coated materials are evaluated even more rigorously for their application due to their own special characteristics. At present, biomaterials obtained by plasma spraying of titanium alloy surfaces to construct bioactive coatings have been used in orthopedics, and the bonding strength between the coating and the substrate is much less than that of our system. The biggest problem of magnesium alloy coating system as a biomedical material is still the corrosion problem.
(1) Le Yu, Tomas M. Silva Santisteban, Qinqing Liu, Changmin Hu, Jinbo Bi, Mei Wei, Effect of three-dimensional porosity gradients of biomimetic coatings on their bonding strength and cell behavior, J Biomed Mater Res. 2021;109:615–626.
(2) X. Zhou, Raj Siman, Lin Lu, Pravansu Mohanty, Argon atmospheric plasma sprayed hydroxyapatite/Ti composite coating for biomedical applications, Surface & Coatings Technology 207 (2012) 343–349.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe comments are attached.
Comments for author File: 
Comments.pdf
Author Response
1.Comment: Can the authors elaborate on the proposed mechanism for the formation of the “dandelion flower-like” and “bar-like” HA structures as NaHâ‚‚POâ‚„ concentration increases? Are these morphologies consistent with previous literature? And is the reported morphology uniform across the full sample area, or are there localized variations (e.g., clustering, cracking, or incomplete coverage)?
Reply: We sincerely thank the reviewer for this valuable suggestion. There are no reports in the literature on the construction of HA coatings on the surface of micro-arc oxidized coatings using a similar method, but there have been reports on the construction of HA coatings on the surface of magnesium alloys using a similar method.
(1) Huawei Yang, Kada Xia, Taolei Wang, Junchao Niu, Yiming Song, Zuquan Xiong, Kui Zheng, Shiqing Wei, Wei Lu, Growth, in vitro biodegradation and cytocompatibility properties of
nano-hydroxyapatite coatings on biodegradable magnesium alloys, Journal of Alloys and Compounds 672 (2016) 366-373.
(2) Masanari Tomozawa, Sachiko Hiromoto, Yoshitomo Harada, Microstructure of hydroxyapatite-coated magnesium prepared in aqueous solution, Surface & Coatings Technology 204 (2010) 3243–3247.
(3) Seo-Young Kim, Yu-Kyoung Kim, Moon-Hee Ryu, Tae-Sung Bae & Min-Ho Lee, Corrosion resistance and bioactivity enhancement of MAO coated Mg alloy depending on the time of hydrothermal treatment in Ca-EDTA solution, Scientific Reports, 7: 9061.
(4) Hui Tang, Dezhen Yu, Yan Luo, Fuping Wang, Preparation and characterization of HA microflowers coating on AZ31 magnesium alloy by micro-arc oxidation and a solution treatment, Applied Surface Science 264 (2013) 816– 822.
2.Comment: On page 5, the authors write ‘In this study, Mg2+ ions are released from the PEO layer due to the dissolution of MgO.’ From where did this PEO layer came and why it has not been introduced fully?
Reply: The process of liquid-phase deposition is a dynamic process of dissolution of the existing film layer and growth of new film layer deposition. In our experimental program, we used mild deposition conditions to reduce the dissolution of the original film layer. The idea of our work is derived from hydrothermal synthesis, where the dissolution of the original film layer is also observed.
(1) Zhixin Ba, Yongmin Wang, Tianyi Sun, Yongqiang Jia, Lingling Zhang, Qiangsheng Dong, Preparation and properties of hydrophobic micro-arc oxidation/layered double hydroxide composite coating on magnesium alloy, Surface&CoatingsTechnology475(2023)130113.
(2) Jinhe Dou, Jing Wang, Huancai Li, Yupeng Lu, Huijun Yu, Chuanzhong Chen, Enhanced corrosion resistance of magnesium alloy by plasma electrolytic oxidation plus hydrothermal treatment, Surface & Coatings Technology 424 (2021) 127662.
(3) Hao Zhang, Kun Liu, Mengmeng Lu, Lin Liu, Yanzhe Yan, Zhuangzhuang Chu, Yuran Ge, Tao Wang, Jing Qiu, Shoushan Bu, Chunbo Tang, Micro/nanostructured calcium phytate coating on titanium fabricated by chemical conversion deposition for biomedical application, Materials Science & Engineering C 118 (2021) 111402.
- Comment: On page 6, the authors mention ‘This suggests the formation of carbonate substituted hydroxyapatite, aligning with previous reports.’. Can they cite their works here?
Reply: Based on the reviewers' suggestions, we have added the cited references
- Comment: How many contact angle measurements were taken per sample, and what was the standard deviation? Were measurements made in multiple regions to account for surface heterogeneity?
Reply: Each sample was measured five times in different areas, and because the samples themselves were relatively homogeneous, the results of the five measurements were relatively close. We chose the middle value as its final measurement result.
- Comment: Can the authors relate specific resistance improvements to pore sealing seen in SEM? Could the authors comment on the long-term stability of these coatings in corrosive environments based on EIS data?
Reply: Thanks to the reviewers for their suggestions, this is a very interesting and important endeavor. We used the form of fitted circuits to illustrate the variation pattern of resistance and capacitance of each layer, which is the data directly related to the number and size of holes in the layers. From the resistance and capacitance data, it can be seen that the liquid-phase deposition layer significantly improves the resistance of the loose and porous outer layer of the micro-arc oxidation, which is directly related to the liquid-phase deposition layer into the micro-arc oxidation film layer.
- Comment: What is the practical implication of the observed two- to four-order magnitude increases in resistance for biomedical applications? Would be nicer to have a conclusion here.
Reply: An increase in the resistance value indicates an increase in corrosion resistance. Here two- to four-order magnitude increases. It shows that the corrosion resistance of magnesium alloy is significantly enhanced after modification. This is very important for magnesium alloy as a biomedical material. However, due to the different casting, forging, and grades of magnesium alloys, there is no data standard for the resistance of magnesium alloys in simulated body fluids. In our work, we have demonstrated that our composite coating can significantly enhance the corrosion resistance of magnesium alloys.
- Comment: Was the pH of the SBF solution monitored during the immersion period, and did it vary between samples?
Reply: Due to the corrosion of magnesium alloys and the reaction between oxides and water, the pH of the simulated solution is increasing during the immersion process, and this is also an important parameter reflecting the corrosion rate of magnesium alloys. In the later long-term immersion of composite coatings and animal implantation tests, we back to focus on the rate of hydrogen release, as well as pH value changes.
- Comment: How might the differences in mineral morphology (e.g., cluster vs. flower vs. irregular) impact cell adhesion, proliferation, or osteointegration in a biological context?
Reply: Differences in mineral morphology (e.g., agglomerates vs. flowers vs. irregularities) certainly have an effect on cell proliferation and growth. Surface morphology and roughness directly affect cell adhesion and growth. However, there is no direct data to show which surface morphology is more suitable for cell adhesion and growth: agglomerates vs. flowers vs. irregularities.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsUncertainties were revised. The manuscript is acceptable level.
