Review Reports - An Experimental Investigation of Controlled Changes in Wettability of Laser-Treated Surfaces after Various Post Treatment Methods

Round 1

Reviewer 1 Report

The effect of the post-processing including vacuum treatment, low temperature annealing, ambient air storage, and polyethylene bag of laser surface structuring of Ti-6Al-4V samples on their surface structure and wettability was studied, and some interesting results can be found in the manuscript. But there exist some shortcomings in the manuscript:

(1) The results should be discussed more deeply so that the manuscript does not look like an experiment report.

(2) Only EDS and Raman spectroscopy are not enough for understanding the composition and microstructure of the surface of the sample treated by different post-processing, and XPS, IR, AFM, etc should be used to analyze the surface composition and microstructure.

(3) Some results which were shown in the tables were used to graph the figures again. The results should be used once in the whole manuscript.

(4) The discussions in the manuscript looks like the conclusions. Some of them should be modified and found the conclusions. And the discussion must be greatly improved so that the reason and mechanism of the influence of the post-processing and laser treatment processes should be shown to the readers more clearly.

(5) The figures should be modified.

(6) The description for the experimental details and the results should be modified so that the readers can know them more clearly and easier.

Author Response

Dear reviewer,

We would like to thank you for your time and comments, which will definitely help to improve the quality of the manuscript. We've tried our best to respond to all of your comments, and we've tried to take all of your suggestions to improve the quality of the manuscript.

Below you will find our responses to your questions.

The results should be discussed more deeply so that the manuscript does not look like an experiment report.
- We tried to modify the discussion session. We have added relevant references and commented deeply on all points.
Only EDS and Raman spectroscopy are not enough for understanding the composition and microstructure of the surface of the sample treated by different post-processing, and XPS, IR, AFM, etc should be used to analyze the surface composition and microstructure.
- Detailed microstructure analysis was performed on the laser confocal microscope Keyence VK-X1000. Chemical behavior of the samples was better described according to relevant literature.

(3) Some results which were shown in the tables were used to graph the figures again. The results should be used once in the whole manuscript.

We went through the manuscript again and consolidated the results so that they were not duplicated.

We hope that the modification in discussion were satisfactory and all phenomena were well described.

(5) The figures should be modified.

We tried to modify figures, and add some more according to other reviewers suggestions.

(6) The description for the experimental details and the results should be modified so that the readers can know them more clearly and easier.

Discussion was modified and more complex supported by the relevant literature

Author Response File: Author Response.docx

Reviewer 2 Report

The subject of the article is extremely important for science and industry. I see in this the great substantive value of the research conducted. This allows you to analyze not only the material itself, but also the wettability of the surface. This phenomenon is difficult to establish. The reasons are different. Therefore, the very process of the scale of the phenomenon under given operating conditions of technical devices is extremely difficult. This is also influenced by the oil pressure on the surface. It is true that the wettability itself is tested, but the introduction of a pressure condition makes the whole cognitive process difficult. I would introduce more examples about the laser treatment process itself. I would give an example of the practical use of this technology. This is of great importance in the internal combustion engine industry in particular. The wettability of piston rings is considered in many respects. This is one of the most difficult issues for these scientists. I would recommend inserting a few entries (I will leave this to the author's discretion).

Wróblewski, P., Technology for Obtaining Asymmetries of Stereometric Shapes of the Sealing Rings Sliding Surfaces for Selected Anti-Wear Coatings, Event: SAE Powertrains, Fuels & Lubricants Meeting, SAE Technical Paper 2020-01-2229, SAE International USA, 2020, https://doi.org/10.4271/2020-01-2229

Soveja A., Jouvard J. M, Grevey D .: Mul-tiphysics modeling metal surface laser teksturowanie pojedynczego efektu uderzenia. Fragment z obrad Konferencji Użytkowników COMSOL 2007 Grenoble. 8. Dobrzański LA,

these may be other items

standardize the text scale in figures 2

A valuable finding is that the surface roughness does not affect as significantly as the surface chemistry of the material sample. The research was carried out correctly. The discussion would require a more detailed reference to individual research results. Some conclusions in the discussion can be given in conclusions.

Applications should be more detailed. What has been achieved on the basis of the conducted research. What was the contribution to the given field, i.e. surface topography. What is the scientific value. The presented conclusions are not very precise. A major correction is required here.

Author Response

Dear reviewer,

Below you will find our responses to your questions.

Thank you for your relevant informations, we well see the possibilities of surface functionalization, especially for combustion engine industry. We have used according to your recommendation for our paper more appropriate literature [14] RYK, G. a I. ETSION. Testing piston rings with partial laser surface texturing for friction reduction. Wear, 2006, č. 261, s. 792–96. DOI:10.1016/j.wear.2006.01.031.

these may be other items

standardize the text scale in figures 2

Figure 2 was modified to unified the text scale.

The discussion section was extended and some missing references were added.

Applications were better discussed and we now hope, that all conclusion are precisely described.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper presents up-to-date and interesting study. However, there are some key issues which still need to be addressed:

Introduction:
- There are some important review paper missing in this section: https://doi.org/10.1016/j.cirp.2013.05.008 and 10.1177/0954405416661583
- The effect of certain microtexture type should be better described with a reference to existing literature
- superhydrophic effect can usually be achieved by multiscale structure, i.e, micro and nanoscale geometric features which mimic the natural topographic structure of lotus leaf. A short reference to this effect and multiscale analysis should be given: https://doi.org/10.1016/j.cirp.2018.06.001 This effect is probably noticed by the authors in reference [18] - Huerta-Murillo et al. - which used the combination of nano and femtosecond pulses and created desired geometric features at different dimensional level. This has to be carefully addressed in this section.
- Applications for laser microstructured surfaces should be wider described (at least 5-10 more examples should be given).
Materials and methods:
- Why titanium alloy was used here? Is it intended for medical purposes? Why not stainless steel e.g. 316L? What about it processing performance using laser?
- Are you sure that nanosecond laser allows removal of material via evaporation? Usually pico and femtosecond lasers are used laser microstructurization. A brief description of physical phenomena occurring in your process should be provided.
- Was the beam Gaussian or Top-hat shape? What were the dimension of the beam - radius?
- Please explain if the microstructure was achieved by using linear row-like trajectory of scanning or some other?
- Was there only one sample for each postprocessing? This sounds like a huge limitation of your study.
- Since you use focus variation microscopy and take areal images I would strongly advise to use more complex analysis of your results. Please try to use appropriate areal parameters according to ISO 25178 - check feature, hybrid and functional parameters. Some parameters are intended to characterize tribological behaviour of surfaces and might be applicable in your study.
- What about statistical analysis - ANOVA?
Results
- Please provide the images of the representative droplets for each postprocessed surfaces in the paper.
- Please provide profile - cross-section and indicate the width and depth of the valley? created by the laser linear scanning.
- Please do provide ANOVA to support your observations.
- What kind wetting state was noted: Cassie-Baxter, Wenzel?
Discussion
- The discussion is very limited. It should explain the reason why certain hydrophobic behaviour was noted for particular postprocessing. This should be carefully discussed with already known effects. A clear reference to already known wetting models should be given.
Conclusion
- This should be improved once the discussion and results section are amended
Other: Please reformat the literature to comply with MDPI citation style.

Author Response

Dear reviewer,

Below you will find our responses to your questions.

The paper presents up-to-date and interesting study. However, there are some key issues which still need to be addressed:

Introduction:

There are some important review paper missing in this section: https://doi.org/10.1016/j.cirp.2013.05.008 and 10.1177/0954405416661583

Mentioned references and many others were added into the introduction and discussion.

The effect of certain microtexture type should be better described with a reference to existing literature

Some references and explanation relating to microstructures type were added into the introduction.

superhydrophic effect can usually be achieved by multiscale structure, i.e, micro and nanoscale geometric features which mimic the natural topographic structure of lotus leaf. A short reference to this effect and multiscale analysis should be given: https://doi.org/10.1016/j.cirp.2018.06.001 This effect is probably noticed by the authors in reference [18] - Huerta-Murillo et al. - which used the combination of nano and femtosecond pulses and created desired geometric features at different dimensional level. This has to be carefully addressed in this section.

Some references and explanations were added according to this point.

Applications for laser microstructured surfaces should be wider described (at least 5-10 more examples should be given).

We tried to extended this topic in introduction.

Materials and methods:

Why titanium alloy was used here? Is it intended for medical purposes? Why not stainless steel e.g. 316L? What about it processing performance using laser?

Titanium alloy was used due to its application in medicine, aerospace and industry. There is no limitation laser machining of Ti6Al4V, in addition this alloy has standard ablation behaviour.

Are you sure that nanosecond laser allows removal of material via evaporation? Usually pico and femtosecond lasers are used laser microstructurization. A brief description of physical phenomena occurring in your process should be provided.

Yes, you are right that the main evaporation effect is observed during femtosecond and short picosecond laser ablation. During nanosecond laser ablation, the dominant effect is surface heating to melting temperature followed by vaporization. We improve this point in the discussion and we supported this point by relevant literature.

Was the beam Gaussian or Top-hat shape? What were the dimension of the beam - radius?

Beam profile was Gaussian with the diameter of 0.15 mm. More information about used laser source are described in 2.2.

Please explain if the microstructure was achieved by using linear row-like trajectory of scanning or some other?

Yes, the microstructure was fabricated by lines in distance of 0.05 mm. After scanning of hole sample, the degree of hatching move by 45 degrees and repeat. Totally four scans were applied.

Was there only one sample for each postprocessing? This sounds like a huge limitation of your study.

Yes, only one sample was used for each post-process method. But on the each sample, there were 3 structures in three repetitions.

Since you use focus variation microscopy and take areal images I would strongly advise to use more complex analysis of your results. Please try to use appropriate areal parameters according to ISO 25178 - check feature, hybrid and functional parameters. Some parameters are intended to characterize tribological behaviour of surfaces and might be applicable in your study.

Thank you for this idea, we added the appropriate areal parameters as you suggested to the manuscript. We also discussed this results with appropriate literature.

What about statistical analysis - ANOVA?

All measured values were statistically significant so on, all of the values were considered.

Results

Please provide the images of the representative droplets for each postprocessed surfaces in the paper.

Images of the representative droplets were added into the paper.

Please provide profile - cross-section and indicate the width and depth of the valley? created by the laser linear scanning.

Cross sections of the samples were added into the paper.

Please do provide ANOVA to support your observations.

All measured values were statistically significant so on, all of the values were considered.

What kind wetting state was noted: Cassie-Baxter, Wenzel?

According to our observations, there were Wenzel state.

Discussion

The discussion is very limited. It should explain the reason why certain hydrophobic behaviour was noted for particular postprocessing. This should be carefully discussed with already known effects. A clear reference to already known wetting models should be given.

The discussion session was extended and relevant literature in this field was considered.

Conclusion

This should be improved once the discussion and results section are amended

We tried to improve both sections to be clear and according to relevant literature.

Other: Please reformat the literature to comply with MDPI citation style.

Literature was reformatted according to MDPI citation style.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Some interesting results can be found in manuscript, and the manuscript have been modified. But the conclusions should be further modified.

Author Response

Dear reviewer,

We would like to thank you for your time and comments, which will definitely help to improve the quality of the manuscript. We have tried to improve our conclusion to include all the important points from the manuscript. We now hope that our manuscript is ready for publication in Materials journal.

Thank you,

Tomas Primus

Author Response File: Author Response.pdf

Reviewer 3 Report

Thank you very much for improving your paper.

I have a final comment:

Please improve the quality of figure 2 - please make the same vertical scale (z-axis) in all three figures. Some isometric view is also needed.

If you performed ANOVA for your results it should be clearly stated in the text. P-values should be given either in the text or in table (e.g. Table 4).

Conclusion section should be extended. Some future work can be mentioned too.

Author Response

Dear reviewer,

We would like to thank you for your time and comments, which will definitely help to improve the quality of the manuscript. We have tried to improve our conclusion to include all the important points from the manuscript and to include also future work. Next points which you mentioned were also modified.

Please improve the quality of figure 2 - please make the same vertical scale (z-axis) in all three figures. Some isometric view is also needed.
- The samples were scanned again to have same vertical scale for all of them.
If you performed ANOVA for your results it should be clearly stated in the text. P-values should be given either in the text or in table (e.g. Table 4).
- We added a few sentences about our ANOVA analysis and P values. This information was added before Table 4.

We now hope that our manuscript is ready for publication in Materials journal.

Thank you,

Tomas Primus

Author Response File: Author Response.pdf