Study on the Wear Modes of PVD Films Using Different Concentrations of Al₂O₃ Abrasive Particles and Textured Rotating Balls

Reviewer #1

Comment/Suggestion

Action done

This work intends to correlate the effect of different testing times (500, 1000, and 1500 cycles) and different concentrations of 3 μm Al2O3 abrasive slurry (25, 35, and 45 g/100 ml) on the micro abrasion resistance of a TiN thin coating film, using balls of AISI 52100 steel whose texture and roughness were prepared by 60 seconds chemical etching. The rotation speed of each test was 80 rpm, applying a normal load of 2 N.  There are some detailed suggestions as follows:

Major problems:

The introduction is too long (>3 pages) and should be simplified. It is suggested that the introduction directly focus on the main content of the paper, introduce the current progress at home and abroad, and then put forward your own views. Finally, you can briefly introduce your work and innovation.

We remove the redundant information of the introduction section.

Regarding the motivation of the manuscript it is already detailed in the introduction section as follow: “Titanium nitride - TiN hard coating deposited by sputtering has brought one of the most significant technological advances in the development of modern tools. In the early years of PVD hard coating, TiN was just about the only coating developed in 1980 [44]. Although many other PVD hard coatings were later developed in the market, TiN coating is still today one of the most important since it is a simple coating system, easily to be produced with high rehabilitee. Despite of the immense tribological studies done on this coating system, the studies regarding their abrasion performance using micro-scale abrasion resistance is still scarce. Essentially tests conducted against counterparts with induced roughness, tests duration and abrasive concentrations slurry on the wear mechanism and specific wear rate are still missing in the literature”

After the reviewer comment we decreased the introduction and improved the motivation of the manuscript either in the introduction and abstract sections.

2. The textured AISI 52100 226 steel balls are important in this work. However, there is no 3D figures or SEM figures about them. Please add them in the revised paper.

We understand the reviewer point of view.  We would like to clarify that such attack to the ball surface to conduct micro-scale abrasion tests was optimized previously and the image has been published. Thus, in order to avoid copyright issues (since at the moment we just have a 3D profile of it) we decided to add a sentence in the manuscript to reference a work previously developed by the same authors, where an image of the texture of the sphere with the same attack time is presented.

3. Section 3.7:  "SEM balls analysis" does not analyze the underlying reasons.

Thank you for stopping our mistake.

Other problems:

1. Please improve the quality of figures, especially Figure 1 and 2.

Thank you for the comment. The images were improved.

2. row270: missing half a bracket.

Thank you for stopping our mistake. we corrected the problem.

3. row329: "S" should be "s", which is the same with that above.

Thank you for the comment. The change has been made.

4. Please revise the format of references according to the latest published manuscript. 

Thank you for the comment. The change has been made.

5. The test work should be more clearly represented by a flow chart in Section 2.

We could not clear understand the reviewer comment. We believe that the comment is regarding figure 3 representing the scratch test apparatues. After the reviewer comment we removed figure 3, since it brings ambiguous information.

Based on the above problems, this paper isn't suitable to publish in its current vision. A minor revision is my opinion.

Reviewer #2

Comment/Suggestion

Action done

The work carried out by the author on the wear modes of thin film using varying abrasive particle concentration on textured rotating balls are interest of the research community. But it is required to incorporate the suggestions/comments in further improvement of the quality of work.

1. The author should add 1-2 lines about application of work in abstract.

Thank you for your comment. The application was already referred in the introduction part. After the reviewer comment we highlight the application either in the introduction and abstract sections.

2. The abstract should be quantitative in terms of improvement.

In our opinion the abstract should contain the application, research problem/gap, application and the main results achieved. This is what we have in the currently abstract of the manuscript.

We should highlight that contrarily to what is normally observed in the literature, regarding wear modes, where rolling is promoted with increasing abrasive concentration, in this work grooving took place instead. This is caused by the use of rough balls on the experiments which due to the incorporation of abrasive particles in the vales of the ball promotes abrasion mechanism. The higher the abrasive concentration the higher the grooving mechanism since more particles are available to scratch the surface.

This is the most important conclusion of thi work and is already summarized in the abstract section.

However, if the reviewer believes that we should refer in the abstract the morphology, adhesion, structure of the coatings we will do it.

3. In what point of view, the author has chosen less number of cycles.

This was to avoid the formation of wider and deep wear scars.

4. Research gap followed by objective should be elaborate for better understanding of researchers.

The research gap is already presented in the introduction part of the manuscript as follow:

Titanium nitride - TiN hard coatings deposited by sputtering has brought one of the most significant technological advances in the development of modern tools. In the early years of PVD hard coating, TiN was just about the only coating developed in 1980 [44]. Although many other PVD hard coatings were later developed in the market, TiN coating is still today one of the most important since it is a simple coating system, easily to be produced with high rehabilitee. Despite of the immense tribological studies done on this coating system, the studies regarding their abrasion performance using micro-scale abrasion resistance is still scarce. Essentially tests conducted against counterparts with induced roughness, tests duration and abrasive concentrations slurry on the wear mechanism and specific wear rate are still missing in the literature.

After the reviewer comment we slight adjusted the text to make it more clear to readers.

5. The scientific justification of columnar structure if missing in the section 3.1.

The morphology of the films depends on many deposition parameters (deposition pressure, distance of the targets to the substrate, level of ionization if applicable, bias applied to the substrates, deposition temperature, etc.) and on different processes triggered during deposition (mainly the shadowing effect). Thus, the explanation for formation of the columnar morphology is difficult to access and is not possible to be given in this manuscript. Independently of that, the majority of the sputtering films displayed a columnar morphology as the one presented in this manuscript.

Nonetheless, the authors would like to highlight that this coating was deposited without eating the substrates/chamber and may be the cause for the columnar morphology. In such condition the low mobility of the arriving atoms at the surface will stay at arrival positions, triggering the shadowing effect and  growing a columnar morphology.

6. For finding out adhesion between the substrate and coating, the cross hatch test is widely used. What is author viewpoint to select the Rockwell test for the same.

According to the literature, the adhesion and cohesion of thin-solid films deposited by sputtering can be either evaluated by scratch tester and Rockwell tests; i.e. currently tests presented in the manuscript. Of course, one of the methods is enough to evaluate adhesion/cohesion of films and there is no need to use both techniques. If the reviewer believes we should just show one of the results, we will do it. 

7. The heading should be conclusions. Further, the author can add few lines about the future scope of the work. Also, the author should quantify the conclusions in terms of improvements.

We deleted some redundant information from the conclusions section and improved the conclusions. Regarding the future work the authors would like to clarify the following: this work was design to study the effect of different testing times, and different concentrations of abrasive slurry (25, 35 and 45 g/ 100 ml), on wear resistance of a TiN film, using an AISI 52100 steel balls whose texture and roughness were prepared by chemical etching.  The results are discussed in terms of effect of the roughness of the ball, concentration of abrasive slurry on the dominant wear mechanisms and specific wear rate of the coating. The conclusion addressed in this work are clear based on the motivation. A further work may be conducted by reducing the load so that the alumina particles are aggregated to the texture created in the sphere in order to have a more abrasive effect. Moreover the studying of the influence of  different roughnesses of the ball on the wear response and wear modes may be also conducted. Authors decided to partially disclose this information in the end of the results/discussion section..

Reviewer #3

Comment/Suggestion

Action done

1. After each terminology firstly appear in this paper, it necessary to explain, for example V.

Thank you for your comments. We verified all the terminologies appearing in the manuscript for the first time

2. The description of Figure 6 (b) and (d) is not seen in the text.

Thanks for your comments. A sentence was added to explain the evolution of wear on the surface of the sphere taking into account the different numbers of cycles and concentrations.

3. What is the “itis” in line 362?

Thank you for stopping our mistake; we corrected it.

4.The authors declared that “Increasing the abrasive concentration showed to reduce the specific wear rate of …”. Why? Is it not related to other reasons? For example, high number of cycle.

Such explanation is already given in the manuscript. -            Increasing the abrasive concentration showed to reduce the specific wear rate of the film as it promotes grooving wear mechanism which is less effective in removing material as compared to the mix rolling/grooving mode detected in the condition tested with lower abrasive concentration.

The reviewer should kindly note that rolling/three body abrasion wear is more effective in removing material than grooving wear mode. I reference justifying this is also provided in the discussion part.

5.In the film hardness measurement experiment, the depth of the indentation is related to the thickness of the film and the substrate. Typically the indentation depth is kept below 10% of the film thickness to prevent impact from the substrate. However, what is the film thickness of the research?

Thanks for your comment. The thickness of the film can be seen in current figure 3. The film thickness was introduced at the experimental procedure.