Surface Roughness and Its Effect on Adhesion and Tribological Performance of Magnetron Sputtered Nitride Coatings
, Lazar Kovačević 1,*, Vladimir Terek 1, Zoran Bobić 1, Aleksandar Miletić 1, Branko Škorić 1, Miha Čekada 2 and Aljaž Drnovšek 2Round 1
Reviewer 1 Report
Comments and Suggestions for Authors1) Improvise the abstract by adding the novelty of the current work
2) Add x-ray diffraction (XRD) to allow for determining these properties of the sputtered material and also calculate the lattice parameters.
3) Kindly account for the substrate related effect on the surface roughness
4) The average grain size of the deposition is to be added for the effectiveness of the article.
5) What mechanisms could potentially cause a notable increase in wear for the TiAlN and TiAlN/CNx coatings when tested with the coarse ground surface (with a roughness average of around 530 nm)? Were there any changes in the morphology or composition identified on the worn surfaces?
6) What further research do you suggest to better understand the mechanisms driving the observed sensitivities and to develop coatings with more consistent performance across varying surface roughness levels?
7) Considering your research results, what suggestions would you propose to enhance the tribological performance of these coatings in real-world scenarios with varying surface roughness? Are there any particular surface preparation approaches that might reduce the observed sensitivity, particularly for the AlTiN/TiN nanolayer coating?
Author Response
1) Improvise the abstract by adding the novelty of the current work
Response 1) Thank you for the comment. We added one sentence to the start of the abstract.: Reports on the influence of surface roughness on adhesion and tribological performance of contemporary nitride coatings with different layer designs are still scarce in the literature. Therefore, in this study, we evaluated the behavior of…
On the other side, the novelty of the work is also indicated by the sentence: “No general rule could be established about the effect of surface roughness on tribological behavior and adhesion of different hard coatings.”
2) Add x-ray diffraction (XRD) to allow for determining these properties of the sputtered material and also calculate the lattice parameters.
Response 2) Given that the paper deals with the behavior of three contemporary coatings of different chemical compositions, which are already known in the literature, we don`t see how the coatings' texture and lattice parameters can be brought in relation with their different behavior. In our opinion, XRD results are not necessary for understanding the behavior of different coatings. Introducing the XRD results would just additionally lengthen the paper and affect its readability and conciseness.
3) Kindly account for the substrate related effect on the surface roughness
Response 3) Section 3.3. is devoted to the samples surface topography and to the differences between the substrates and coated samples topography. In the 1st paragraph of Section 3.3 we explained the change in surface topography and in the 2nd paragraph of Section 3.3 we explained the changes in surface roughness parameters. However, to better convey this information the 2nd sentence of Section 3.3. is modified to: The coating deposition altered the sample surface topography obtained by the machining processes employed in substrates preparation.
4) The average grain size of the deposition is to be added for the effectiveness of the article.
Response 4) Grain size is in conjunction with the mechanical properties of the investigated coatings. We included indentation load-related mechanical properties of the investigated coatings so the differences between the coatings should be observed from that standpoint. We don’t see how the grain size of the coatings could be linked with the effect of surface roughness without some thorough microscopy evaluation. That research would deserve a separate investigation and a paper.
Additionally, in scientific writing, the incorporation of results that are not crucial for the explanation of the observed behavior or trends is highly discouraged. Therefore, similarly, as in the case of incorporating the XRD results, we think this will just unnecessarily lengthen the manuscript and make it more complicated.
5) What mechanisms could potentially cause a notable increase in wear for the TiAlN and TiAlN/CNx coatings when tested with the coarse ground surface (with a roughness average of around 530 nm)? Were there any changes in the morphology or composition identified on the worn surfaces?
Response 5) It was already explained at the end of 5th paragraph in section 3.5 : “On the rougher surfaces the interaction of the counter-ball and surface ridges…” But to better convey the observed behavior we also added a sentence regarding the topography and morphology of these wear tracks : “Since the topography and morphology of TiAlN and TiAlN/CNx coarse-ground wear tracks are comparable with those of the smoother samples (Figure 7. and S.2) it is suggested that the wear mechanism is the same.” The chemical composition of the wear tracks was not evaluated in this study but surely should be done in some future investigations.
6) What further research do you suggest to better understand the mechanisms driving the observed sensitivities and to develop coatings with more consistent performance across varying surface roughness levels?
Response 6) Thank you for this remark, we should include some sentences on this topic. Two sentences are added at the end of the Results and Discussion section: Thus, future investigations should establish the exact correlation between the mentioned parameters and the coating behavior. Such results would be beneficial in minimizing the effect of surface roughness in coatings' practical applications. In addition, to confirm the observed behavior of nanostructured coating on rough surfaces, future investigations should include a few different kinds of these coatings. Finally, in order to reveal the wear mechanism of the coatings with different roughness, the wear tracks should be evaluated by different kinds of high-resolution electron microscopy techniques performed on their cross sections, such as focused ion beam and transmission electron microscopy. The results of such investigations could lead to the establishment of procedures and treatments that can improve the coating performance on rougher surfaces.
7) Considering your research results, what suggestions would you propose to enhance the tribological performance of these coatings in real-world scenarios with varying surface roughness? Are there any particular surface preparation approaches that might reduce the observed sensitivity, particularly for the AlTiN/TiN nanolayer coating?
Response 7) This investigation did not answer completely why some coatings have higher sensitivity of their tribological performance on the surface roughness. As addressed in the answer to your previous remark, an additional more thorough investigation is needed to obtain information that can lead to suggestions for problem solving. We added one sentence at the end of the Results and discussion section, which is linked with previous two: The results of such investigations could lead to the establishment of procedures and treatments that can improve the coating performance on rougher surfaces
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article is devoted to an important topic and is of interest for practical applications of the results obtained. The structure of the article is clear and well organized, the conclusions are logical. There are comments on the experimental part of the work: 1. The authors mention 4 targets used for deposition of TiAlN and the nanolayer AlTiN/TiN layers. Three of them are listed in the table 2. What is the fourth target? 2. The text should explicitly describe how the AlTiN/TiN multilayer structure was obtained if "...coatings were produced engaging all four targets during the whole deposition process...".
Author Response
The article is devoted to an important topic and is of interest for practical applications of the results obtained. The structure of the article is clear and well organized, the conclusions are logical. There are comments on the experimental part of the work:
Thank you for your valuable comments.
- The authors mention 4 targets used for deposition of TiAlN and the nanolayer AlTiN/TiN layers. Three of them are listed in the table 2. What is the fourth target?
Response 1. As can be seen in the 2nd sentence of 2nd paragraph of the Experimental section, the used deposition device is equipped with 4 cathodes where 4 targets can be placed and used during the deposition. Therefore, for the TiAlN single-layer coating, 4 TiAl targets were used simultaneously. Nanolayered AlTiN/TiN coating was deposited using 2 TiAl and 2 Ti targets simultaneously. Finally, for the deposition of TiAlN/CNx 3 TiAl targets and 1 C target were used, first 3 TiAl targets were engaged to deposit the bottom TiAlN layer and then only 1 C target to deposit the top CNx layer. However, if all deposited coatings are considered, only 3 different types of targets are used: TiAl, Ti, and C. Therefore, Table 2 shows 3 types of targets and corresponding powers used for each target type during the deposition.
A more detailed explanation of the deposition process is included in the 2nd paragraph of the Experimental section.
- The text should explicitly describe how the AlTiN/TiN multilayer structure was obtained if "...coatings were produced engaging all four targets during the whole deposition process...".
Response 2. The answer given to your previous remark also addressed this one. In the field of PVD coatings deposition, it is well-known how does the deposition of multilayer structure works. Therefore, in our opinion, exhaustive explanations in this regard are not needed. The explanation implemented in the 2nd paragraph of the Experimental section should be sufficient to understand how a nanolayered structure formed.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe publication examines the tribological properties of coatings. The results presented in the publication are valuable and complementary to this field. However, the publication has many shortcomings that need to be addressed by the authors:
1. The name is complicated. It should be changed to be fluid, reflecting the essence of the publication.
2. In the literature, you would like logical sequence and comparative analysis and problem emphasis.
3. The novelty and purpose of the research must be clearly defined.
4. The introduction lacks fluency.
5. The authors are suggested to include a more detailed scheme of the experiment and describe the conditions of the experiment (what gas, flow, etc.) in the description of the methodology.
6. It is necessary to accurately describe the dynamics of changes in the thickness and chemical composition of coatings. This section of the results is complicated because it is very scattered.
7. The change in the hardness of the coatings is discussed very abstractly. It is necessary to add numerical values and to discuss the patterns of change and influencing factors.
8. It is recommended to measure the roughness with a profilometer. Roughness measurement graphs should be included to specifically show the actual roughness values. The occurrences and influencing factors of coarseness variation need to be discussed.
9. The research is valuable, but the discussion of the results is very abstract and complicated.
10. The conclusions should be expanded.
A review of English fluency/correctness is recommended.
A review of English fluency/correctness is recommended.
Author Response
The publication examines the tribological properties of coatings. The results presented in the publication are valuable and complementary to this field. However, the publication has many shortcomings that need to be addressed by the authors:
- The name is complicated. It should be changed to be fluid, reflecting the essence of the publication.
Response 1. The title of the paper is shortened. Now the paper title is: Surface roughness and its effect on adhesion and tribological performance of magnetron sputtered nitride coatings
- In the literature, you would like logical sequence and comparative analysis and problem emphasis.
Response 2. The literature used for the preparation of this manuscript was chosen to address the few main problems considered in this investigation. First the aspects of surface roughness and its influence on coating performance. The second is the coatings' scratch adhesion and its dependence on surface roughness. Third is the modeling of the scratch test and the necessity of further experimental research and the fourth is the coatings' tribological performance related to surface roughness. Respectfully but we don’t see the issue with this sequence, this is the sequence used also in the results and discussion section.
In the literature review, comparative analyses were given for the cases where applicable. In other cases, the knowledge gap is indicated which directly leads to the motivation for performing this investigation. In the introduction section at the end of every paragraph, after the addressed topic, we emphasized what is still the problem in the field and addressed the reasons for further investigation. This is the case for the last sentences in the paragraphs: 3, 4, and 5 of the Introduction section.
- The novelty and purpose of the research must be clearly defined.
Response 3. Thank you for your remark, we added two sentences at the end of the introduction section.
The purpose of this study is to reveal the effects of surface roughness on scratch adhesion, scratching direction and tribological performance of different hard coatings. As such, this study gives novel information on the behavior of contemporary PVD coatings of different layer designs.
- The introduction lacks fluency.
Response 4. In authors' opinion, the sequence of the topics in the Introduction is logical and well-connected, the findings obtained in the literature review converge to the needs and motivations for conducting further investigation in the field. However, to improve the readability we made a lot of changes in the introduction section.
- The authors are suggested to include a more detailed scheme of the experiment and describe the conditions of the experiment (what gas, flow, etc.) in the description of the methodology.
Response 5. The types of the used gases and the flow rates are already given in Table 2.
The coatings are produced by a closed-field magnetron sputtering system which is a well-known configuration in the field of PVD coatings deposition. Nevertheless, in the 2nd sentence of the 2nd paragraph of the Experimental section, the configuration of the magnetron sputtering unit is described sufficiently. Additionally, in the same paragraph in the 5th sentence, it is noted that the details about the system can be seen in references 28 and 29. The inclusion of the schemes of the deposition systems would unnecessarily lengthen the paper and reduce its conciseness.
- It is necessary to accurately describe the dynamics of changes in the thickness and chemical composition of coatings. This section of the results is complicated because it is very scattered.
Response 6. The thicknesses of the investigated coatings and their constituting layers are for polished samples given in Table 3. It is well known that in the case of PVD deposition by industrial devices, the thickness of a specific coating does not vary on a sample nor on the samples with different roughness. The procedure employed to measure the chemical composition of the coatings by the EDS is explained in the experimental section.
To improve the readability of this section, the paragraph in section 3.1 is divided into smaller ones.
- The change in the hardness of the coatings is discussed very abstractly. It is necessary to add numerical values and to discuss the patterns of change and influencing factors.
Response 7. The missing numerical values of hardness are implemented into the text. The change of the hardness value with the indentation load for TiAlN/CNx coating was already explained previously, but we added explanations about the most pronounced phenomena for other coatings, at the start of the Section 3.2.
- It is recommended to measure the roughness with a profilometer. Roughness measurement graphs should be included to specifically show the actual roughness values. The occurrences and influencing factors of coarseness variation need to be discussed.
Response 8. The coating roughness is indeed measured by 3D tactile profilometry. The procedures employed for these measurements are in great detail given in the 4th paragraph of the Experimental section. 3D topography images obtained by 3D tactile profilometry are presented in Figure 3 and the values of surface roughness parameters are given in Table 4. The whole section 3.3 is devoted to the topography and roughness of the investigated samples.
- The research is valuable, but the discussion of the results is very abstract and complicated.
Response 9. Thank you for your comment. The investigated tribological phenomena are complex and the behavior of the coatings depends on a lot of parameters and properties. Therefore, the explanations cannot be simple. To obtain answers to the questions raised in this study a more thorough characterization of the tribo-tracks is needed. Anyways, we are open to hearing your constructive suggestions for correcting the discussions in the manuscript.
- The conclusions should be expanded.
Response 10. The conclusions section is already too long. It contains six bullets and almost stretches on the whole page. The conclusions already contain the main findings obtained from the conducted investigation, so its expansion would require repetition of the topics already addressed during the discussion of the results or to include some trivial findings about the results.
Reviewer 4 Report
Comments and Suggestions for AuthorsThe authors studied the substrate roughness effect on different types of PVD-nanocoating. They reported that no general rule can be established on the studied roughness effect on the tribological behavior and adhesion properties of the nanocoating.
The study is comprehensive. However, it is advised that the authors improve on the significance of the study and strongly highlight its main contribution to literature. Since the study resulted in a null hypothesis, the authors should also focus on explaining why such conclusion is reached based on the experimental measurements.
Other comments on the manuscript are given below:
Comment #1:
It is mentioned that the scratch adhesion test was repeated 3-5 times. Does this mean that the authors prepared 3-5 samples each or just repeated the test at 3-5 different locations on the same sample? Please clarify.
Comment #2:
If the authors did prepare 3-5 samples each, please provide the standard deviation of the coating thickness measured.
Also, did the authors get similar thickness for the different substrates (with different roughness)? Please clarify.
Comment #3:
Improve the y-axis numbering for Figure 2(a). Reduce the number of values on the y-axis. E.g., start from 800 and end at 4000, with an increment of 500.
Also, for Figure 2, the hardness is measured using which substrate (which roughness?)?
Comment #4:
Why did the authors select the use of Al2O3 ball for the reciprocating sliding tests? Please justify in the manuscript.
Comment #5:
What is the parameter on the secondary y-axis in Figure 8? Please include the axis label.
Also, please revise figure 8 in the similar manner following Comment #2, especially for the secondary axis.
Comment #6:
Why is the tribological properties for AlTiN/TiN coating highly dependent on its surface roughness? Include the reason in conclusion.
Comments on the Quality of English Language
The language is acceptable and would benefit from some editing and careful proofreading.
Author Response
The study is comprehensive. However, it is advised that the authors improve on the significance of the study and strongly highlight its main contribution to literature. Since the study resulted in a null hypothesis, the authors should also focus on explaining why such conclusion is reached based on the experimental measurements.
Thank you for your comments. In the 8th paragraph of the Section 3.5 few sentences already existed regarding the contribution to the literature, but additional sentences are also added to this paragraph and to the end of this section.
Other comments on the manuscript are given below:
Comment #1:
It is mentioned that the scratch adhesion test was repeated 3-5 times. Does this mean that the authors prepared 3-5 samples each or just repeated the test at 3-5 different locations on the same sample? Please clarify.
Response # 1: This means that the scratch tests were repeated 3-5 times on different locations of the same sample. To clarify a sentence is added to the end of the 1st paragraph, and one sentence is modified in the 6th paragraph of the Experimental section.
Comment #2:
If the authors did prepare 3-5 samples each, please provide the standard deviation of the coating thickness measured.
Response # 2: The coating thickness is determined only on polished samples by performing 3 measurements. This is implemented in a sentence in 3rd paragraph of the Experimental section.
The deviations of these measurements are implemented in Table 3.
Also, did the authors get similar thickness for the different substrates (with different roughness)? Please clarify.
Response: This remark is addressed in the previous part of the comment #2, and a sentence is added to the 3rd paragraph of the Experimental section: Since all samples during the PVD deposition are subjected to the same conditions it is expected that the coating thickness does not vary significantly between the samples of different roughness. Therefore, the ball-cratering was performed only on the polished coated samples.
Besides mentioned, the ball cratering method is not precise for the determination of the coating thickness on samples with larger roughness. For these purposes, more sophisticated techniques would be required. Therefore, this is a method widely accepted and the most commonly used in the field of PVD coating, usually used for the determination of the coating thickness on mirror polished samples.
Comment #3:
Improve the y-axis numbering for Figure 2(a). Reduce the number of values on the y-axis. E.g., start from 800 and end at 4000, with an increment of 500.
Response # 3: Thank you for the remark, this is implemented in the Figure 2.
Also, for Figure 2, the hardness is measured using which substrate (which roughness?)?
Response : The hardness measurements were conducted on polished samples. To clarify, this was implemented in the sentence from 3rd paragraph of the Experimental section: Mechanical properties of the coatings were evaluated also on polished samples by the nanoindentation technique using H100C (Fisherscope) device and employing a Vickers prism.
Comment #4:
Why did the authors select the use of Al2O3 ball for the reciprocating sliding tests? Please justify in the manuscript.
Response # 4: Three sentences are added to the 6th paragraph of the experimental section, and these are: Al2O3 was selected as a counter material because of its high hardness, wear resistance, and high inertness. As such, consistent and reproducible results of abrasive wear are en-sured. Additionally, it is a standard material in coatings wear testing which allows easy comparison with other investigations from the field.
Comment #5:
What is the parameter on the secondary y-axis in Figure 8? Please include the axis label.
Also, please revise figure 8 in the similar manner following Comment #2, especially for the secondary axis.
Response # 5: The secondary axis is the coefficient of friction (COF). The axis label is implemented on the first diagram, because of the problem with space in the image. We modified the image, according to the suggestion.
As indicated in the Experimental section. The tribo-test were performed for 1000 and 2000 sliding cycles, without other repetitions, which is added to the sentence of the Experimental section. But the average values nor the deviations could not be calculated.
Comment #6:
Why is the tribological properties for AlTiN/TiN coating highly dependent on its surface roughness? Include the reason in conclusion.
Response # 6: We did not find an answer to such behavior, but based on the obtained results we discussed and postulated some mechanisms that can lead to such behavior of the coating. To address this remark, we added the following sentence to the conclusions: The reasons for such behavior were not identified in this study, but a few mechanisms were postulated. These include the specific interaction of the tribo-film with surface asperities and susceptibility to cohesive cracking.
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors have made substantial changes to the publication, so the publication can be accepted.
