The Effects of the Finishing Polish Process on the Tribological Properties of Boride Surfaces of AISI 4140 Steel

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript investigates the effect of polishing processes on the tribological properties of borided AISI 4140 steel surfaces. The results may provide a good reference for optimizing the application of borided surfaces in high-demand friction systems. The research topic and findings of the manuscript align with the journal's scope and it is worth publishing after necessary revisions.

Here are suggestions for revision:

1. It is suggested that not using a period at the end of the title.
2. The layout of the figures is recommended to be optimized. This suggestion applies to all figures. For the multi-figure, descriptions should be added to both the figure and the caption. Currently, the readability of the figures is insufficient.
3. What is the reason for the choice of parameters for the polishing process used? Why was a "finish polished process (FPP) for 25 minutes using a slurry of Al2O3 of 0.5μm at 10% w/w" chosen? The selection of the polishing process is crucial as it is the premise for all characterization studies.

Author Response

Good afternoon, reviewer, attached the answers.

 

 

1. It is suggested that not using a period at the end of the title.

The period was eliminated from the title.

 

1. The layout of the figures is recommended to be optimized. This suggestion applies to all figures. For the multi-figure, descriptions should be added to both the figure and the caption. Currently, the readability of the figures is insufficient.

 

The layout and some characteristics of the figures were applied to improve readability of the figures.

 

1. What is the reason for the choice of parameters for the polishing process used? Why was a "finish polished process (FPP) for 25 minutes using a slurry of Al2O3 of 0.5μm at 10% w/w" chosen? The selection of the polishing process is crucial as it is the premise for all characterization studies

These parameters were selected due to these were used during the AISI 4140 preparation before of the thermochemical process (except for the abrasive solution). Nowadays, one project is dedicated to analyst the effects of each of these parameter in the boride layer of different steels.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors submitted a very interestgin topic about producing a FeB/Fe2B coating on the surface of AISI 4140 specimen to increase wear stability of these materials. The authors used a high variation of measurement methods, not just tribological test, but also analytical devices to define how this coating works under tribological loads. The topic is interesting and worth to be published.

However, some hints and modification requests are mentioned. From my point of view, the biggest problem with this submitted manuscript is the understandability. The authors usually writing in long sentences, some paragraphs are almost 1 page long and it is very easy to loose the interest towards the topic. Some pictures are too big, some are too small, some pictures are too difficult to read and understand, because the picture is not enough sharp. In some cases, the numbers and letters on the pictures are unreadable. Furthermore, the authors add result information about their measurements in the text, instead of using a table-form presentation where these numnbers can be understood more easily and the numbers itself can be easily compared. 

I gathered my questions and modification requests into the following list. 

1. Line 119: The boriding process should be described in details, not just referring to a citation. The paper would be more complete, more individual so. I tried to collect information about boriding process from chapter 1 and 2 as well, multiple times, but it is still unclear for me.
2. Line 150: use min instead of m, m means meter, not minute.
3. Please check your whole manuscript for such mistakes: ZrO2, Al2O3, the 2 and 3 should be in subscript. There are similar mistakes in the Abstract as well.
4. Line 136, measuring the boride layers. Did you prepare sections from the samples to acquire these images? If yes, please add it to the text including section preparation.
5. Question: The whole introduction is about  using boriding process for extreme conditions like high or low temperatures. However, the tribological tests are performed under room temperature. Firstly, can you explain, why? Secondly, can you define room temperature, is it 20°C)? How can you ensure that the specimens are not overheated by the heat generated by the frictional losses?
6. Question: how many independent tribological measurements were carried out with each variations? If only one, how can you ensure that no 
7. Line 165-167, please use spaces regurarly, these lines are not sophistical. Furthermore, this information would be better to present in a table-form to ensure higher visibility and understandability instead of using abbreviations like P (peak) and V (valley). I would reconsider presenting the data information between lines 161 and 189 completely, the reader can easily be lost in the data.
8. Figure 1 is too far away from its first mentioning. This figure is too big, and too difficult to see the numbers on them, so please rearrange and rethink this figure to ensure its visibility. Possibly separate the a, b, c and d, because these images require two complete pages, and the reader has to jump continuously between the sentences and the images to watch the images and read the sentences.
9. Line 174: until this line, there was no result information about wear rate, it should come later in the tribological results chapter. Or is it an another wear rate?
10. Line 178, reduced the abrasion wear... is it refered to the cited literature sources (if yes, the sentence should be rewritten into it can reduce..., because it is just a hypothesis, not a measured and proved fact), or to the tribological results chapter (if yes, than it is bad, because we were not informed about the tribological results until this line).
11. Figure 1 a) and b), please rethink these images to ensure its visilibity. The numbers on the pictures are sometimes gloomy (make sure the images are as sharp as possible), or they are not visible because of the background color (e.g. 1123K, P, 1273K, Po). Can you add the information, what paramters did you use acquiring these SEM images (magnification, acceleration voltage, etc.)?
12. Figure 1 a), this magnification (shown with orange arrow) is covering a significant amount of the original image. Please rethink these image presentation to avoid this covering process.
13. Figure 1 d), use thicker lines for the error bars of the red data, it is almost unseeable, even at 180% magnitude on the PC.
14. Figure 3. Please ensure that the name of the Figure and the Figure itself is not divided into two pages. Furthermore, I recommend to mark the area on the lower magnification pictures, where the higher magnifications are taken. The pictures are not coherent, some has the 50 micron mark, some don't.
15. Please rearrange Figure 4, too big images, you could use the complete width of the A4 paper. It is very strange that 3 images consume a complete page, including a page change with a very high space holder at the bottom of page 9.
16. From my points of view, Figure 5 b) is unnecessarily too much information. A proper evaluation including table-formed or diagram-formed illustration would be much more beneficial. Suggestion: wear depth. The diagrams have different Y-axis minimum and maximum values, they are difficult to compare. The 4th vertical column is not properly explain what is presented there (10, 15, 20 N and the 4th is?)
17. Figure 5 c), it would be better to place 1123K and 1123K-P next to each other (and so on with the other variations) to ensure proper comparison on the diagram as well.
18. Question: what is the purpose of Figure 5 a)? I do not see any, it is not mentioned in the text either.
19. Friction force: one possible explanation of the friction increase can be the lack of heating during the tribotest. During the tribotest started at room temperature, the heat generated by the friction is not transported via cooling media or lubricant, which means the heat stays between the contacting surfaces of the specimens increasing its temperature, and modifying the tribosystem significantly, especially in the presence of the measured high COF values. The problem is that this is just a hypothesis, because the authors did not consider this during their methodology, and they did not measure the contact temperature during their test.
20. Figure 6, it would be better to present COF only instead of friction force, because the 10, 15 and 20 N load cannot be compared based on friction force, only according to COF.
21. The question rises here as well: How many independent measurements were carried out in the case of each variations? If only one, how could you calculate the error bars on Figure 6?
22. Figure 6, the pictures should be labeled with a) - e), as it was done in the previous Figures correctly.
23. Figure 6, what is the difference between 1st row and AISI 4140 measurements? Which indivicual measurement was used to produce AISI 4140 diagram from the previous ones?
24. Figure 6, please check the labels of the diagram, 2273K was written instead of 1273K, and so on.
25. Line 414-417. The list with 1-2-3 number should be reallocated. It would be more understandable if it would not be in the continuous sentences, rather like a real list.
26. Line 419, the authors continuously used CoF, this line contains Cof. Please be transparent.
27. Please add some possible future steps for further research and development purposes.

Author Response

Good afternoon, reviewer, attached the answers.

The authors submitted a very interestgin topic about producing a FeB/Fe2B coating on the surface of AISI 4140 specimen to increase wear stability of these materials. The authors used a high variation of measurement methods, not just tribological test, but also analytical devices to define how this coating works under tribological loads. The topic is interesting and worth to be published.

However, some hints and modification requests are mentioned. From my point of view, the biggest problem with this submitted manuscript is the understandability. The authors usually writing in long sentences, some paragraphs are almost 1 page long and it is very easy to loose the interest towards the topic. Some pictures are too big, some are too small, some pictures are too difficult to read and understand, because the picture is not enough sharp. In some cases, the numbers and letters on the pictures are unreadable. Furthermore, the authors add result information about their measurements in the text, instead of using a table-form presentation where these numnbers can be understood more easily and the numbers itself can be easily compared. 

I gathered my questions and modification requests into the following list. 

1. Line 119: The boriding process should be described in details, not just referring to a citation. The paper would be more complete, more individual so. I tried to collect information about boriding process from chapter 1 and 2 as well, multiple times, but it is still unclear for me.

The next text was added to the manuscript:

 

1. López-Perrusquia et al. [32], M. Y. García-Santibañez et al. [33]

 

The next image was included in the manuscript

 

 

 

 

1. Line 150: use min instead of m, m means meter, not minute.

 

The text in the manuscript was modified to: 30 min

 

 

1. Please check your whole manuscript for such mistakes: ZrO2, Al2O3, the 2 and 3 should be in subscript. There are similar mistakes in the Abstract as well.

 

The text in the manuscript was revised and fixed.

 

 

1. Line 136, measuring the boride layers. Did you prepare sections from the samples to acquire these images? If yes, please add it to the text including section preparation.

 

The next text was included in the manuscript:

 

To study the boride layer morphology, the samples were cut using a diamond cutting disc and then polished with sandpaper and a 5 μm Alumina solution to achieve a mirror-like surface finish. After polishing, the boride samples were treated with a solution containing 2% nitric acid in ethanol for 3 seconds.

 

1. Question: The whole introduction is about using boriding process for extreme conditions like high or low temperatures. However, the tribological tests are performed under room temperature. Firstly, can you explain, why? Secondly, can you define room temperature, is it 20°C)? How can you ensure that the specimens are not overheated by the heat generated by the frictional losses?

 

The increase in temperature resulting from contact and sliding operations during the friction process affects the friction coefficient, wear rate, and lubrication dynamics. However, measuring temperature changes at the contact point is quite complex. As a result, many studies on tribological properties often overlook reporting these temperature changes, as well as related heat transfer and energy consumption.

The next text was included in the manuscript:

(around 273 K)

 

 

 

 

 

1. Question: how many independent tribological measurements were carried out with each variations? If only one, how can you ensure that no 

 

All samples were characterized at least three times each, providing sufficient data for statistical analysis.

 

 

1. Line 165-167, please use spaces regurarly, these lines are not sophistical. Furthermore, this information would be better to present in a table-form to ensure higher visibility and understandability instead of using abbreviations like P (peak) and V (valley). I would reconsider presenting the data information between lines 161 and 189 completely, the reader can easily be lost in the data.

 

The next table was included in the manuscript

 

Table 1. Variation of peaks and valleys size of sawtooth-like morphology, and roughness before and after finish polishing process (FPP) of boride surfaces of AISI 4140.

 

Treatment temperature (K)	Morphology		Roughness (Rq)
	Peak (μm)	Valley (μm)	Before FPP (μm)	After FPP (μm)
1123	40.6±2.4	12.8±3.2	0.6±0.1	0.4±0.04
1173	48.2±3	23.18±4.2	0.9±0.3	0.7±0.08
1223	115.4±3.5	87.9±5	1.1±0.5	0.97±0.03
1273	127.2±7	80.2±9	2.6±1	1±0.07

 

 

 

 

 

1. Figure 1 is too far away from its first mentioning. This figure is too big, and too difficult to see the numbers on them, so please rearrange and rethink this figure to ensure its visibility. Possibly separate the a, b, c and d, because these images require two complete pages, and the reader has to jump continuously between the sentences and the images to watch the images and read the sentences.

 

The layout of the figures was fixed to improve the improve the relation between the text and the figures.

 

 

 

1. Line 174: until this line, there was no result information about wear rate, it should come later in the tribological results chapter. Or is it an another wear rate?

 

The wear rate reported in these lines are of the reduction of the finish polishing process, while the wear rate of the sliding test are reported in the tribologicla results section.

 

 

 

1. Line 178, reduced the abrasion wear... is it refered to the cited literature sources (if yes, the sentence should be rewritten into it can reduce..., because it is just a hypothesis, not a measured and proved fact), or to the tribological results chapter (if yes, than it is bad, because we were not informed about the tribological results until this line).

 

The reduction of the abrasive wear in this section is the reduction in the abrasive wear of the finish polishing process.

 

 

 

1. Figure 1 a) and b), please rethink these images to ensure its visilibity. The numbers on the pictures are sometimes gloomy (make sure the images are as sharp as possible), or they are not visible because of the background color (e.g. 1123K, P, 1273K, Po). Can you add the information, what paramters did you use acquiring these SEM images (magnification, acceleration voltage, etc.)?

 

The images were modified to:

 

 

The nest text was included in the manuscript: (at 20 kV and X500)

 

 

 

1. Figure 1 a), this magnification (shown with orange arrow) is covering a significant amount of the original image. Please rethink these image presentation to avoid this covering process.

 

 

The figure 1a was fixed to:

 

1. Figure 1 d), use thicker lines for the error bars of the red data, it is almost unseeable, even at 180% magnitude on the PC.

 

The figure 1 d was update to:

 

 

 

1. Figure 3. Please ensure that the name of the Figure and the Figure itself is not divided into two pages. Furthermore, I recommend to mark the area on the lower magnification pictures, where the higher magnifications are taken. The pictures are not coherent, some has the 50 micron mark, some don't.

 

 

The layout of the manuscript was fixed

The Figure 4 was modified to: The brown squares mark the zones where the SEM images were taken.

 

 

The nest text was included in the manuscript: (20 KV at X120 and X500).

 

 

 

 

1. Please rearrange Figure 4, too big images, you could use the complete width of the A4 paper. It is very strange that 3 images consume a complete page, including a page change with a very high space holder at the bottom of page 9.

 

The layout of the figure 4 was fixed

 

 

 

 

1. From my points of view, Figure 5 b) is unnecessarily too much information. A proper evaluation including table-formed or diagram-formed illustration would be much more beneficial. Suggestion: wear depth. The diagrams have different Y-axis minimum and maximum values, they are difficult to compare. The 4th vertical column is not properly explain what is presented there (10, 15, 20 N and the 4th is?)

 

The standardization of the Y axis was not possible due to the difference of roughness screamed the wear track profiles and increase the difficulty to be analyzed.

The graphic longitudinal wear track profile was eliminated.

 

 

 

 

 

1. Figure 5 c), it would be better to place 1123K and 1123K-P next to each other (and so on with the other variations) to ensure proper comparison on the diagram as well.

 

The order used to illustrate the width of the worn zones on the different samples was chosen to emphasize the variation in values for the 1123K, 1173K, 1223K, and 1273K samples, as well as the stable values observed in the 1123K-P, 1173K-P, 1223K-P, and 1273K-P samples.

 

1. Question: what is the purpose of Figure 5 a)? I do not see any, it is not mentioned in the text either.

 

The Figure 6a was eliminated of the manuscript.

 

 

1. Friction force: one possible explanation of the friction increase can be the lack of heating during the tribotest. During the tribotest started at room temperature, the heat generated by the friction is not transported via cooling media or lubricant, which means the heat stays between the contacting surfaces of the specimens increasing its temperature, and modifying the tribosystem significantly, especially in the presence of the measured high COF values. The problem is that this is just a hypothesis, because the authors did not consider this during their methodology, and they did not measure the contact temperature during their test.

 

The increase in temperature resulting from contact and sliding operations during the friction process affects the friction coefficient, wear rate, and lubrication dynamics. However, measuring temperature changes at the contact point is quite complex. As a result, many studies on tribological properties often overlook reporting these temperature changes, as well as related heat transfer and energy consumption.

 

 

 

1. Figure 6, it would be better to present COF only instead of friction force, because the 10, 15 and 20 N load cannot be compared based on friction force, only according to COF.

 

While the coefficient of friction (CoF) is a crucial piece of data in a tribological system, we believe that the friction force graphs enhance our understanding of the tribological phenomena observed during sliding operations. They also illustrate how these phenomena change in response to variations in the contact systems.

 

 

1. The question rises here as well: How many independent measurements were carried out in the case of each variations? If only one, how could you calculate the error bars on Figure 6?

 

All samples were characterized at least three times each, providing sufficient data for statistical analysis.

 

1. Figure 6, the pictures should be labeled with a) - e), as it was done in the previous Figures correctly.

 

The images were labeled to:

 

 

 

 

1. Figure 6, what is the difference between 1st row and AISI 4140 measurements? Which indivicual measurement was used to produce AISI 4140 diagram from the previous ones?

 

The first row of graphics is the friction force behavior of the boride AISI 4140 surfaces, while the second row is the friction force of the AISI 4140 unboride surface.

 

 

1. Figure 6, please check the labels of the diagram, 2273K was written instead of 1273K, and so on.

 

The Figure 6 was fixed.

 

 

1. Line 414-417. The list with 1-2-3 number should be reallocated. It would be more understandable if it would not be in the continuous sentences, rather like a real list.

 

The text was fixed to:

1) The increment of the roughness value with the increment of the temperature treatment.

2) The top layer presented a lower stress resistance due to its contests of the Fe₂B phase.

3) The increment of the coating thickness that reduced the surfaces deformation during the sliding tests (see Figure 8).

 

1. Line 419, the authors continuously used CoF, this line contains Cof. Please be transparent.

 

The “CoF” laber was changed to “Cof”.

 

 

1. Please add some possible future steps for further research and development purposes

 

The next was included in the manuscript:

Upcoming projects: To improve the tribological properties of the AISI 4140 boride surface, the characteristics and properties of the boride AISI 4140 surfaces at different temperatures and tribocorrosion wear resistance.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors!

Thank you very much for considering my previous comments. I think, the overall readability of the manuscript has improved significantly.

Some further hints:

• please use the whole width of the page in the case of Figure 2 a) and b), to increase the size of the images and ensure proper visiblity

My previous comments:

Comment 9:

Reviewer: Line 174: until this line, there was no result information about wear rate, it should come later in the tribological results chapter. Or is it an another wear rate?

Author: The wear rate reported in these lines are of the reduction of the finish polishing process, while the wear rate of the sliding test are reported in the tribologicla results section.

R: Please add this information to the text as well.

Comment 10: 

R: Line 178, reduced the abrasion wear... is it refered to the cited literature sources (if yes, the sentence should be rewritten into it can reduce..., because it is just a hypothesis, not a measured and proved fact), or to the tribological results chapter (if yes, than it is bad, because we were not informed about the tribological results until this line).

A: The reduction of the abrasive wear in this section is the reduction in the abrasive wear of the finish polishing process.

R: Please add this information to the text.

Comment 16:

R: From my points of view, Figure 5 b) is unnecessarily too much information. A proper evaluation including table-formed or diagram-formed illustration would be much more beneficial. Suggestion: wear depth. The diagrams have different Y-axis minimum and maximum values, they are difficult to compare. The 4th vertical column is not properly explain what is presented there (10, 15, 20 N and the 4th is?)

A: The standardization of the Y axis was not possible due to the difference of roughness screamed the wear track profiles and increase the difficulty to be analyzed. The graphic longitudinal wear track profile was eliminated.

R: I understand that the standardization of the Y axis is not possible, however the images are very decisive in this case. Maybe you could highlight it in the description of the Figure to avoid any missunderstanding.

Comment 19:

R: Friction force: one possible explanation of the friction increase can be the lack of heating during the tribotest. During the tribotest started at room temperature, the heat generated by the friction is not transported via cooling media or lubricant, which means the heat stays between the contacting surfaces of the specimens increasing its temperature, and modifying the tribosystem significantly, especially in the presence of the measured high COF values. The problem is that this is just a hypothesis, because the authors did not consider this during their methodology, and they did not measure the contact temperature during their test.

A: The increase in temperature resulting from contact and sliding operations during the friction process affects the friction coefficient, wear rate, and lubrication dynamics. However, measuring temperature changes at the contact point is quite complex. As a result, many studies on tribological properties often overlook reporting these temperature changes, as well as related heat transfer and energy consumption.

R: The increase of the temperature is usually overlooked, yes. But this extra temperature has a very high impact, especially in tribosystems with low ambient temperatures (like in this case, room temperature). Usually, there are three options to avoid this comversation: a) measure the contact temperature, b) increase the ambient temperature to a higher value, where this extra temperature is relative not so high, c) use lubricant to cool down the contact surfaces. My question is here, how can you be sure that your measured friction force increase over time (for example AISI 4140 unborided under 20N) is a normal process and not just because of the overheating of the contact surface during the time duration of the test?

Comment 20:

R: Figure 6, it would be better to present COF only instead of friction force, because the 10, 15 and 20 N load cannot be compared based on friction force, only according to COF.

A: While the coefficient of friction (CoF) is a crucial piece of data in a tribological system, we believe that the friction force graphs enhance our understanding of the tribological phenomena observed during sliding operations. They also illustrate how these phenomena change in response to variations in the contact systems.

R: I understand that friction force increases when the normalforce was increased. However, the main purpose of the coefficient of friction value is that different normal force values are becoming comparable, which is important. I did not request to delete Figure 7 d), just to calculate the CoF in each measured time and plot it instead of friction force. Doing that, the shape of the curve will not be changed, but the comparision becomes possible. 

 

Author Response

 

Thank you very much for considering my previous comments. I think, the overall readability of the manuscript has improved significantly.

Some further hints:

• please use the whole width of the page in the case of Figure 2 a) and b), to increase the size of the images and ensure proper visibility

 

The Figure 2 a) and b) were updated.

My previous comments:

Comment 9:

Reviewer: Line 174: until this line, there was no result information about wear rate, it should come later in the tribological results chapter. Or is it an another wear rate?

Author: The wear rate reported in these lines are of the reduction of the finish polishing process, while the wear rate of the sliding test are reported in the tribologicla results section.

R: Please add this information to the text as well.

The text in the results and discussion section changed from wear rate to wear rate because most wear tracks did not overcome the roughness valleys on the treated surfaces.

Comment 10: 

R: Line 178, reduced the abrasion wear... is it refered to the cited literature sources (if yes, the sentence should be rewritten into it can reduce..., because it is just a hypothesis, not a measured and proved fact), or to the tribological results chapter (if yes, than it is bad, because we were not informed about the tribological results until this line).

A: The reduction of the abrasive wear in this section is the reduction in the abrasive wear of the finish polishing process.

R: Please add this information to the text.

The sentence in the text was modified to: the wear produced by the finish polishing process on the surfaces [8,10,37-39]

Comment 16:

R: From my points of view, Figure 5 b) is unnecessarily too much information. A proper evaluation including table-formed or diagram-formed illustration would be much more beneficial. Suggestion: wear depth. The diagrams have different Y-axis minimum and maximum values, they are difficult to compare. The 4th vertical column is not properly explain what is presented there (10, 15, 20 N and the 4th is?)

 

A: The standardization of the Y axis was not possible due to the difference of roughness screamed the wear track profiles and increase the difficulty to be analyzed. The graphic longitudinal wear track profile was eliminated.

R: I understand that the standardization of the Y axis is not possible, however the images are very decisive in this case. Maybe you could highlight it in the description of the Figure to avoid any missunderstanding.

The next text was included in the manuscript:

Figure 6 exhibits the profiles of the wear tracks produced during the sliding tests on the boride AISI 4140 steel surfaces at 1123K, 1173K, 1223K, and 1273K with and without the finish polishing process. The wear track profiles show a variation of the Y (μm) axis due to the increment of roughness value with the increment of the temperature treatment and the material accumulation formed by the plastic deformation of the asperities and deposition of the debris of the fractured aspirates. This material accumulation produced a tribolayer that protects the boride surfaces during the sliding tests, avoiding the overcome lowest valleys of the roughness during sliding tests.

Comment 19:

R: Friction force: one possible explanation of the friction increase can be the lack of heating during the tribotest. During the tribotest started at room temperature, the heat generated by the friction is not transported via cooling media or lubricant, which means the heat stays between the contacting surfaces of the specimens increasing its temperature, and modifying the tribosystem significantly, especially in the presence of the measured high COF values. The problem is that this is just a hypothesis, because the authors did not consider this during their methodology, and they did not measure the contact temperature during their test.

A: The increase in temperature resulting from contact and sliding operations during the friction process affects the friction coefficient, wear rate, and lubrication dynamics. However, measuring temperature changes at the contact point is quite complex. As a result, many studies on tribological properties often overlook reporting these temperature changes, as well as related heat transfer and energy consumption.

R: The increase of the temperature is usually overlooked, yes. But this extra temperature has a very high impact, especially in tribosystems with low ambient temperatures (like in this case, room temperature). Usually, there are three options to avoid this comversation: a) measure the contact temperature, b) increase the ambient temperature to a higher value, where this extra temperature is relative not so high, c) use lubricant to cool down the contact surfaces. My question is here, how can you be sure that your measured friction force increase over time (for example AISI 4140 unborided under 20N) is a normal process and not just because of the overheating of the contact surface during the time duration of the test?

 

The next text was included in the manuscript:

Results section

The temperature significantly affects the tribological performance of sliding surfaces due to the increase or decrease in heat energy, which can alter the characteristics of the contact surfaces. Sliding operations generate heat energy that is influenced by factors such as pressure, motion, and friction force. Some authors, including Maalekian et al.[54] and Chey et al. [55], propose a formula to approximate the heat energy produced during the sliding operation::

 

Where q is the heat energy generated during the rubbing operation for a friction stress (τfric) at the average sliding speed (v). The τfric is determinate using the average friction coefficient (μ) and the normal force P applied during the sliding operation. However, the variation in the contact temperature depends eater of the material characteristics of the surfaces in contact.

Figure 8 illustrates a theoretical approximation of the heat energy produced during sliding tests involving AISI 4140 steel, AISI 4140 boride surfaces, and AISI 4140 boride surfaces that underwent a finishing polishing process (FPP). The Figure 8 shows a general trend of increasing heat energy with the increase in normal force for all sliding tests, with higher values observed at a 20 N applied load [55-58]. The AISI 4140 surfaces exhibited lower heat energy production due to the unboride surface having lower μ values at 10, 15, and 20 N. Nonetheless, the heat energy produced during the sliding tests and the surrounding atmospheric conditions altered the characteristics of the AISI 4140 surface, leading to oxidation in the worn zone [59-61]. The heat energy q generated during sliding tests was greater on boride surfaces compared to unboride surfaces, especially noted on the 1123K sample of the boride surfaces without the FPP. The 1123K and 1123K-P samples exhibited the highest q values at 10 and 20 N applied loads due to their higher μ values, both for the boride samples with and without the FPP process. Additionally, the q value decreased with an increase in treatment temperature on the boride surfaces, indicating that greater boride layer thickness and roughness reduced frictional stress during the sliding operations. Furthermore, the lower q value for boride surfaces that underwent FPP demonstrated improved performance, suggesting that the FPP altered the frictional behavior positively.

                                                                         

 

 

Figure 8 Theoretical aproximation of the heat energy produced on; AISI 4140 steel surface, boride AISI 4140 steel boride surfaces and boride AISI 4140 steel surfaces with the finish polishing process (FPP).

Discussion section

4) The modification of the surfaces characteristic of the boride treatment with the in-crement of the temperature of the boride thermal treatment reduced the frictional stress (τfric), reducing the heat energy generated by the rubbing operations.

Additionally, the FPP operation alters the heat energy generated on the boride surfaces, resulting in a more stable variation of the q value as the treatment temperature increases. This behavior may be attributed to the more stable structural properties and roughness characteristics observed in the samples treated with the FPP operation. These samples exhibit less than 11% variation in roughness and show a greater presence of the FeB phase.

 

Comment 20:

R: Figure 6, it would be better to present COF only instead of friction force, because the 10, 15 and 20 N load cannot be compared based on friction force, only according to COF.

A: While the coefficient of friction (CoF) is a crucial piece of data in a tribological system, we believe that the friction force graphs enhance our understanding of the tribological phenomena observed during sliding operations. They also illustrate how these phenomena change in response to variations in the contact systems.

R: I understand that friction force increases when the normalforce was increased. However, the main purpose of the coefficient of friction value is that different normal force values are becoming comparable, which is important. I did not request to delete Figure 7 d), just to calculate the CoF in each measured time and plot it instead of friction force. Doing that, the shape of the curve will not be changed, but the comparision becomes possible. 

The figure was modified to:

Author Response File: Author Response.pdf