Review Reports

Round 1

Reviewer 1 Report

The paper describes the structural changes of a Ti surface due to bombardment with steel balls. The structure is characterised by SEM, TEM and XRD. The relation to mechanical properties also discussed. The paper is well structured, the English is satisfactory, however I pointed out some deficiencies in Enlish (see below). I also have some technical questions:

1. The initial polishing of the Ti plates may also cause structural changes at the surface. How was this effect excluded?

Thank you for your instructive question. Although the initial polishing of the Ti plates may also cause structural changes at the surface， this change is very small and can be ignored. The impacts uninterruptedly input compressive stresses and work hardenings to the surface region of the work-piece, which can refine the crystal particle to the orders of nanometer.

2. Could the authors give the velocity/energy of the bombarding steel balls? Also the flux (ball/s and/or energy/s on unit area) would also be useful information.

Thank you for your instructive advice. We have revised this part as follows:

The facility of MSP includes a high-frequency engine with the rate of 3800 rad/min and output power of 1800W, a linkage device made up of eccentric gear and linkage, a vibration piston and piston chamber composed of special processed Cr12MoV steel.

3. define the abbreviation SMAT in the abstract.

Thank you for your careful work, and we are very sorry for this mistake. We have changed the paper in revised manuscript as follow：

By comparing with the nanocrystalline layer produced by surface mechanical attrition treatment (SMAT)

4. comments on the English:

generally replace:

grains size → grain size

grains refinement → grain refinement

craters formation → crater formation

compring with → compared with

uninterpreted → continuous

line 16 and lines 118-119:  prolong → increase; obtained stability of → saturated at

line 85: protective air → protective gas

line 127: of → to

line 162 to introduce → introduces

line 215: kinetics → kinetic

line 123: delete   “a decrease in the”

Thank you for your careful work, and we are very sorry for this mistake. We have changed the paper in revised manuscript.

Reviewer 2 Report

The topic of the paper is not really new but it has the merit of showing the rather precise results of an " innovation " or extension of SMAT treatment as MSP treatment.

1. Figure 6 : scale of colors is missing. The conclusion could be strengthened with a better comparison between SMAT and MSP.

Thank you for your careful work. We had studied the description about the figure; the surface 3D morphologies of three kinds of specimens can be clearly seen according the scale of colors.

2. From line 259 to line 269 : It seems that this part is not fulfilled

Thank you very much for pointing out this in our manuscript, and we are very sorry for this mistake. We have revised this part as follows:

Author Contributions: Data curation, Yao Quantong; Funding acquisition, Tong Weiping; Investigation, Wang Ge; Methodology, Gao Jing; Writing – original draft, Fu yuzhu.

Acknowledgments: This research was supported by the National Natural Science Foundation of China (U1810109).

Conflicts of Interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Reviewer 3 Report

The manuscript (MS) focuses on the impact of applying the mechanical shot peening (MSP) technique for producing a nanocrystalline layer on a large size pure titanium plate. The results are interesting for readers and researchers. However, the authors used some of the published data in their previous works. In order to avoid any misunderstanding, I suggest replacing the published items with the unpublished ones or at least a citation in the figure caption and/or text should be provided. Below, there are also some comments which need to be addressed before publication:

1. Line 19: Please introduce SMAT abbreviation.

Thank you for your careful work, and we are very sorry for this mistake. We have changed the paper in revised manuscript as follow：

By comparing with the nanocrystalline layer produced by surface mechanical attrition treatment (SMAT)

2. Line 34 & 35: The authors may introduce the abbreviations used in “USSP [9], HESP [10], ECAP [11], HPT [12], SMGT [13], SMAT [14], etc. …”.

Thank you for your careful work, and we are very sorry for this mistake. We have changed the paper in revised manuscript as follow：

a variety of SNC methods, such as ultrasonic shot peening (USSP) [9], high energy shot peening (HESP) [10], equal-channel angular pressing (ECAP) [11],high pressure torsion (HPT) [12], surface mechanical grinding treatments (SMGT) [13], SMAT [14], etc.

3. Line 98-100: The authors wrote, “The foil samples prepared for TEM observation were prepared by grinding and mechanical polishing by an ion beam thinner (Gatan Model 691).” It seems that the word prepared in the first place should be removed: “The foil samples for TEM observation were prepared by grinding and mechanical polishing by an ion beam thinner (Gatan Model 691).” Besides, I suggest that the authors should avoid writing similar sentences used in their previous research works such as references 22 & 23.

Thank you very much for pointing out this in our manuscript, and we are very sorry for this mistake. We have revised this part as follows:

The foil samples for TEM observation were prepared by grinding and mechanical polishing by an ion beam thinner (Gatan Model 691)

We have carefully revised the grammar of the manuscript.

4. Line 120: Can the authors discuss the following sentence with mentioned references in detail: Previous studies have reported that the main deformation form of the pure titanium is the mechanical twins [15, 24-26]?

Thank you very much for pointing out this in our manuscript.

The pure titanium is a hexagonal close-packed structure, and generally twin deformation is used as a form of initial deformation. As the amount of deformation accumulates, the slip deformation occurs. Therefore, under the metallographic microscope, only mechanical twins can be seen, and slip deformation can be observed in the area where the dislocation density is highly accumulated. It has been reported that the main deformation form of the pure titanium is the mechanical twins in previous studies [15, 24-26]

5. Line 140: The article “the” before the adjective “similar” should be omitted: “layer produced by SMAT is the similar to that of the MSP shown in Figure 2(b).” Moreover, it is not clear what the authors mean by writing this sentence: “In addition to the thickness, the nanocrystalline layer produced by SMAT is the similar to that of the MSP shown in Figure 2(b). The thicker nanocrystalline layer can be attributed to the longer SNC processing time of SMAT.” Do you mean except the thickness, the observed layers in both techniques are similar? Or something else?

Thank you very much for pointing out this in our manuscript, and we are very sorry for this mistake. We have revised this part as follows:

In addition to the thickness, the nanocrystalline layer produced by SMAT is similar to that of the MSP shown in Figure 2(b). The thicker nanocrystalline layer can be attributed to the longer SNC processing time of SMAT.

The observed layer thicknesses observed in both techniques are approximately similar.

6. Line 143: Figure 2a was previously reported in reference no. 23 “Large-Scale Synthesis of Nanostructured Nitride Layer on Ti Plate Using Mechanical Shot Peening and Low-Temperature Nitriding”. The authors should avoid using the published figures unless mentioning recompiled from [23] in the figure caption.

Thank you very much for pointing out this in our manuscript, and we are very sorry for this mistake. We have revised this part.

7. Line 190: Figure 4a-d was also previously shown in reference no. 23. Thus, authors should mention in the figure caption.

Thank you for your instructive advice. We are very sorry for this mistake. We have revised this part.

Reviewer 4 Report

The reviewed manuscript presents the results of studies on the use of the mechanical shot peening (MSP) method for producing a nanocrystalline layer on the surface of a large size titanium substrate. The presented results suggest that microstructure and properties of the nanocrystalline layer produced by MSP method was clearly better in comparison to layers produced using surface mechanical attrition treatment (SMAT) method.

In my opinion, the reviewed manuscript may be accept to publicaton in Coatings after minor revisions.

The paper requires the following amendments:

1. Frequent use of acronyms without their proper explanation makes the content of the manuscript difficult to read by persons, which are interested in this topic, but do not engage in this type of studies; e.g. Abstract: SMAT is not explained,

Thank you for your careful work, and we are very sorry for this mistake. We have changed the paper in revised manuscript as follow：

Abstract: By comparing with the nanocrystalline layer produced by surface mechanical attrition treatment (SMAT)

Introduction: a variety of SNC methods, such as ultrasonic shot peening (USSP) [9], high energy shot peening (HESP) [10], equal-channel angular pressing (ECAP) [11],high pressure torsion (HPT) [12], surface mechanical grinding treatments (SMGT) [13], SMAT [14], etc.

Results: Surface roughness is usually measured by Ra or Rq, where Ra refers to the arithmetic mean of the absolute values of the offsets between the highest point and the lowest point in the contour, and Rq is the root mean square of the absolute values of the offsets between the highest point and the lowest point, the values of two have the same trend with the same trend.

2. Introduction: What is the newelty of the presented research?HNO

Thank you for your careful work. Up to now, surface nanotechnology can only process small-sized samples, but for large-size samples such as large titanium plates and other samples, surface nanocrystallization cannot be achieved, so surface nanotechnology such as SMAT cannot be applied in industrial applications. This thesis provides a novel method of surface nanometerization, which can realize the surface nanometerization of large-size titanium plates, which is conducive to the industrial application of surface nanometerization technology.

3. Experimental: Should be HNO₄ and H₂O

Thank you for your careful work. It’s not HNO₄ and H₂O, which corrodes iron-based materials. Kroll regeant: 5% HF, 5% HNO₄, and 90% H₂O for about 30 s at room temperature.

Round 2

Reviewer 3 Report

Due to the changes made, the paper can be published in the present form.