Effect of Pressure and Temperature on Microstructure of Self-Assembled Gradient Al_xTi_1−xN Coatings

Round 1

Reviewer 1 Report

The manuscript present an interesting research. To start  with, few observations:

Lines:183, 184: "In the case of the T-coating deposited at a pressure of 2.3 kPa, (Figures 2a, 2b and 183 3a, 3b)" T coatings are presented in Figs. 2 ,3 (c,d), not (a,b).

Elaborate  on the period determination of the nano-lamellar structures and of the grain size in cubic and wurtzite structures. Just a reference [8] might be not sufficient.

Please comment on the linear increase of nano-lamellar period in T-coating at temperature increase, contrary to the constant values in p-coating at pressure increase.

Line 263, Figure 5 caption: "Representative STEM-EDX maps show qualitative Al, Ti and Cl element distributions", however in the text is written that Cl is not shown...please make it coherent

Line 266: "terminating cubic AlxTi1-xN grains, which are followed by polycrystalline Ti-rich and Cl-rich regions (indicated by solid outline arrows)." In the shown figure there are presented only Ti and Al rich regions. Cl rich region in a coating would be no good, and even if 2% is not implying Cl rich. Please rephrase.

 The combinatorial approach used seems to be OK for the pressure variation. Please comment on the possible influence of the high temperature attained as the deposition progresses on the structure of the already deposited coating at lower temperatures. 

The presentation of the work is hard to be followed, e.g. Fig. 7 is cited before Fig. 6.

Final and most important comment

I suggest to the authors to write a clean presentations of the facts obtained from various analysis carried out, without excessive interpretation. However, the main section should be the Discussion one, which should be extended, preserving the individual sections devoted to p and T coatings. In these sections should be included all the comments presented in the actual 3.1....3.3 sections. I would also suggest that the discussions start with CSnanoXRD Analysis. 

Author Response

Reviewer #1

The manuscript present an interesting research. To start  with, few observations:

Lines:183, 184: "In the case of the T-coating deposited at a pressure of 2.3 kPa, (Figures 2a, 2b and 183 3a, 3b)" T coatings are presented in Figs. 2 ,3 (c,d), not (a,b).
This has been amended.

Elaborate  on the period determination of the nano-lamellar structures and of the grain size in cubic and wurtzite structures. Just a reference [8] might be not sufficient.
We have included an explanatory sentence at line 246:
Essentially, the satellite reflections’ spacing is inversely proportional to the bi-layer spacing, in a relationship similar to small-angle scattering.
We did not attempt to evaluate grain size from XRD data, since the application of Scherrer’s formula is notoriously inaccurate and because it has to be expected that coherency strains and crystal defects present in the nano-lamellar structure would significantly change the obtained values. Grain size was therefore estimated from electron-microscopic data.

Please comment on the linear increase of nano-lamellar period in T-coating at temperature increase, contrary to the constant values in p-coating at pressure increase.
As written in the original manuscript, in the p-coating the period of nanolamellae is not exactly constant, there is still a slight variation. It is possibly conceivable that this is somehow connected to the smaller grain size of lamellar packets in the p-coating, as compared to the T-coating, but ultimately, we believe that the period is simply more susceptible to temperature than pressure, within the investigated deposition parameter space.

Line 263, Figure 5 caption: "Representative STEM-EDX maps show qualitative Al, Ti and Cl element distributions", however in the text is written that Cl is not shown...please make it coherent
We have amended the text at line 274 as follows:
Al, Ti and Cl maps…

Line 266: "terminating cubic AlxTi1-xN grains, which are followed by polycrystalline Ti-rich and Cl-rich regions (indicated by solid outline arrows)." In the shown figure there are presented only Ti and Al rich regions. Cl rich region in a coating would be no good, and even if 2% is not implying Cl rich. Please rephrase.

We have amended the text at line 271 as follows:
… Ti-rich and comparatively Cl-rich …

 The combinatorial approach used seems to be OK for the pressure variation. Please comment on the possible influence of the high temperature attained as the deposition progresses on the structure of the already deposited coating at lower temperatures.
We have added following paragraph:

We are aware of the possibility that the increasing of the working temperature during deposition process may have affected already deposited sublayers. On the other hand so far published results on thermal stability [11, 15] has shown only slight changes in the temperature interval which was selected in our study (750 – 860 °C). Therefore, we do not expect trends observed in our study to be significantly affected by our deposition strategy.

The presentation of the work is hard to be followed, e.g. Fig. 7 is cited before Fig. 6.
The position of figures 6 and 7 was swapped and the referring in text was changed accordingly.

Final and most important comment

I suggest to the authors to write a clean presentations of the facts obtained from various analysis carried out, without excessive interpretation. However, the main section should be the Discussion one, which should be extended, preserving the individual sections devoted to p and T coatings. In these sections should be included all the comments presented in the actual 3.1....3.3 sections. I would also suggest that the discussions start with CSnanoXRD Analysis.
We believe that there are no speculative interpretations present in the results section of the paper, but some comments on the results have been written directly in-place, to underline the plausibility of some of our observations that might otherwise seem non-intuitive.
For the detailed discussion of our observations, however, we think it to be most instructive to separate it into the two samples, rather than into the applied methods. In other papers this may be handled differently, but in our minds this seemed to be the more elegant approach.
Ultimately, we respectfully suggest that each reader will have a different view on this matter.

Reviewer 2 Report

The manuscript submitted for review meets the requirements of the periodical. The manuscript contains interesting and most current research into the coatings. The object of the investigations are Al_xTi_1-xN coatings

The issues pertaining to anti-wear coatings continue to be very relevant today as those coatings may be used to limit or eliminate the wear in the elements operating in various conditions; all new reports on those topics should therefore be published.

Abstract: The object of the investigations and the aim of the work have been given in the summary. The conclusions following from those investigations have been given in a satisfactory way. The summary has been written in accordance with the guidelines from the periodical.

Introduction: The authors explained the need for the investigations in question while referring to a relevant reference literature. The analysis of literature showing the examples of the use of PVD coatings for the protection of items against wear and tear is not in place, while the coatings are very widely used for the purpose. Practically speaking, they are utilised in all the areas of technology. It is worth paying attention to the following publications:

• The influence of selected PVD coatings on fretting wear in a clamped joint based on the example of a rail vehicle wheel set. Eksploatacja i Niezawodnosc. Volume 20, Issue 1, 2018, Pages 1-8,
• Assessment of the possibility of the application of a CrN+OX multi-layer coating to mitigate the development of fretting wear in a press-fit joint. Wear Volume 398-399, 2018, Pages 13-21,

• Failure analysis of the elements of a forced-in joint operating in rotational bending conditions. Engineering Failure Analysis,2020,118,104864.

Experiment details: The experiment details have been provided in an appropriate way. However, this requires some amendments:

• There is no information on the number of tests, samples etc.

• Please present in graphics how the wear tests were conducted.

Results and discussion: Test results match the assumptions made in the article. Wear and survey tests were performed in compliance with current standards.

Conclusion: The manuscript ends with a short conclusion in which the analyzed shells were compared. It is suggested that the conclusion should be completed with a proposal concerning further investigations.

Reference literature: The reference literature consists of 23 items. The authors quote current reference literature relevant for the topic of the article. Quoting complies with the guidelines from the periodical.

It is suggested that the manuscript be published in the journal “Coatings”.

Author Response

The manuscript submitted for review meets the requirements of the periodical. The manuscript contains interesting and most current research into the coatings. The object of the investigations are Al_xTi_1-xN coatings

The issues pertaining to anti-wear coatings continue to be very relevant today as those coatings may be used to limit or eliminate the wear in the elements operating in various conditions; all new reports on those topics should therefore be published.

Abstract: The object of the investigations and the aim of the work have been given in the summary. The conclusions following from those investigations have been given in a satisfactory way. The summary has been written in accordance with the guidelines from the periodical.

Introduction: The authors explained the need for the investigations in question while referring to a relevant reference literature. The analysis of literature showing the examples of the use of PVD coatings for the protection of items against wear and tear is not in place, while the coatings are very widely used for the purpose. Practically speaking, they are utilised in all the areas of technology. It is worth paying attention to the following publications:

• The influence of selected PVD coatings on fretting wear in a clamped joint based on the example of a rail vehicle wheel set. Eksploatacja i Niezawodnosc. Volume 20, Issue 1, 2018, Pages 1-8,
• Assessment of the possibility of the application of a CrN+OX multi-layer coating to mitigate the development of fretting wear in a press-fit joint. Wear Volume 398-399, 2018, Pages 13-21,
• Failure analysis of the elements of a forced-in joint operating in rotational bending conditions. Engineering Failure Analysis,2020,118,104864.

Citations number 3 was added.

Experiment details: The experiment details have been provided in an appropriate way. However, this requires some amendments:

• There is no information on the number of tests, samples etc.
  We do not understand what is meant by “tests”. The samples investigated for this article were subjected to microstructural and chemical analysis by electron microscopy and X-ray nanobeam diffraction. Since the preparation of samples for these kinds of characterizations is tedious and costly, it is standard procedure to prepare only one representative sample for each analysis. Specifically, for each of the two deposited coating types, two cross-sectional lamellae (one 50 µm thick slice for for X-ray diffraction and one 50 nm thick slice for TEM) were prepared from one single indexable cutting insert. In each deposition run, however, several indexable cutting inserts were coated.
• Please present in graphics how the wear tests were conducted.
  We do not understand the query. At no point in the paper have we mentioned wear tests. The article is about the influence of deposition process parameters on the formation of self-assembled nanostructures inside wear-resistant Al_xTi_1‑xN coatings and not about the actual wear behaviour of these coatings.

Results and discussion: Test results match the assumptions made in the article. Wear and survey tests were performed in compliance with current standards.
We did not perform any wear or survey tests. Has the reviewer read our manuscript at all?

Conclusion: The manuscript ends with a short conclusion in which the analyzed shells were compared. It is suggested that the conclusion should be completed with a proposal concerning further investigations. - OUTLOOK
We do not understand what is meant by “shells”. Again, the subject of our article is the microstructural and chemical analysis of nano-structured wear-resistant Al_xTi_1‑xN coatings. In the conclusions we have summarized our findings on the influence of deposition pressure and temperature on the formation of nano-structures inside the coatings and did not simply compare two different coating recipes.
Nevertheless, we have expanded the original conclusions sections by a short outlook:
“Based on existing knowledge about the relationships between structure and functional properties, our findings will enable the synthesis of wear-resistant coatings with increased performance and lifetime.”

Reference literature: The reference literature consists of 23 items. The authors quote current reference literature relevant for the topic of the article. Quoting complies with the guidelines from the periodical.

It is suggested that the manuscript be published in the journal “Coatings”.

Reviewer 3 Report

In this paper "Effect of pressure and temperature on microstructure of self-assembled gradient Al_xTi_1-xN coatings" Authors present an attempt to characterize of correlation between structural properties of Al-rich self-assembled nano-lamellar Al_xTi_1-xN coatings and process parameters used during their chemical vapor deposition (CVD). To measurements Authors prepared two gradient Al_xTi_1-xN coatings were prepared by a stepwise increase of temperature and pressure in the ranges of 750-860 °C and 1.56 to 4.5 kPa during the depositions at a constant composition of the gas mixture. Tests by X-ray nanodiffraction and electron microscopy were performed.

In my opinion this paper can be interesting to readers of Coatings journal. The paper is clearly presented. The paper contains until 7 figures – some figures are legible and good quality.

English of the paper is good – in my opinion the language of the paper should be a little improved. I am asking for corrections by a native speaker.

I find some mistakes for example:

• Please describe equipment used in the experiment – work development environment / work apparatus should be given – model of equipment (manufacturer, city, country).
• In my opinion Conclusions chapter should be a little changed. In this chapter there are no summary of all significant research results obtained by the Authors and written in the Results This chapter should be described in greater detail – it should contain more detailed information than that contained in the Abstract.
• Amount of references is also sufficient but some papers cited in the references 11 from all 23 are older then 5 years – these publications constitute over 47 % of all cited papers – this value should be less then 30 %.
• In the list of references I found 3 papers of the Authors of reviewed paper. Please indicate the differences in the studies presented in the cited articles and their relation to the presented topic.
• In the whole paper, you write the values in percent as for example 99.99% (for example line 100) – you should write and value with unit with spaces (99.99 %).
• Figures 2, 4 and 6 should be reduced (decreased).
• When the Authors mean – "In this work, ..." (for example line 17) the phrase "I this paper, ..." or "In this article ..." should be used, but not the word "… work …".

The results obtained are interesting and promising. The manuscript can be accepted for publication in Coatings journal after MINOR corrections.

Author Response

In this paper "Effect of pressure and temperature on microstructure of self-assembled gradient Al_xTi_1-xN coatings" Authors present an attempt to characterize of correlation between structural properties of Al-rich self-assembled nano-lamellar Al_xTi_1-xN coatings and process parameters used during their chemical vapor deposition (CVD). To measurements Authors prepared two gradient Al_xTi_1-xN coatings were prepared by a stepwise increase of temperature and pressure in the ranges of 750-860 °C and 1.56 to 4.5 kPa during the depositions at a constant composition of the gas mixture. Tests by X-ray nanodiffraction and electron microscopy were performed.

In my opinion this paper can be interesting to readers of Coatings journal. The paper is clearly presented. The paper contains until 7 figures – some figures are legible and good quality.

English of the paper is good – in my opinion the language of the paper should be a little improved. I am asking for corrections by a native speaker.

I find some mistakes for example:

• Please describe equipment used in the experiment – work development environment / work apparatus should be given – model of equipment (manufacturer, city, country).
  done (line 100)
• In my opinion Conclusionschapter should be a little changed. In this chapter there are no summary of all significant research results obtained by the Authors and written in the Results This chapter should be described in greater detail – it should contain more detailed information than that contained in the Abstract.
  We have extended the conclusions section starting at line 427:
  “…, while values range slightly higher at 9.2 – 10 nm.
  An intricate inter-dependence between deposition pressure, temperature, bi-layer thickness and in-plane film strain in the cubic phase has been observed and rationalized in terms of the stability of the growth process of the nanolamellar structures. Further investigations into the lamellar packets’ chemistry revealed the prevalence of hexagonal Al-rich terminating layers and corroborate the hypothesis of an oscillating growth reaction.”
• Amount of references is also sufficient but some papers cited in the references 11 from all 23 are older then 5 years – these publications constitute over 47 % of all cited papers – this value should be less then 30 %.
  Most importantly, the cited papers must be relevant to the article and therefore a “30 % rule” has to be viewed as a guideline rather than a strict rule. In the present case, the authoritative literature on PVD AlTiN coatings and especially their phase stability is older than 5 years, while research on CVD AlTiN coatings with self-assembled nanostructures has been carried out within the last 5 years or slightly more. Moreover, some of our references are to software and procedures that have been established before the 5-year cut-off.
• In the list of references I found 3 papers of the Authors of reviewed paper. Please indicate the differences in the studies presented in the cited articles and their relation to the presented topic.
  These papers (actually, there are five) represent the groundwork for the present study, detailing the morphology and growth process of the self-assembled structures and their dependence on the mixing ratio of precursor gases, as well as some of their mechanical properties and chemical/structural stability. The current article investigates in detail the influence of the previously unaddressed parameters of deposition temperature and pressure on the formation of the self-assembled structures.
• In the whole paper, you write the values in percent as for example 99.99% (for example line 100) – you should write and value with unit with spaces (99.99 %).
  done (line 103, add. lines 278, 279)
• Figures 2, 4 and 6 should be reduced (decreased).
  Here we don not agree with statement. Figure 2 is a stitched image containing high level of detailed information about microstructural response to deposition parameters changes. Any decrease of size would result in lost of the information. In Figure 4 are many labels. Here we want to make it easy to read for anybody. And finally Figure 6 is a HRTEM image. Decrease of size would lead to inability to resolve individual atomic columns. However, we have scaled down Figure 7.
• When the Authors mean – "In this work,..." (for example line 17) the phrase "I this paper, ..." or "In this article ..." should be used, but not the word "… work …".
  done (line 18, 92)

The results obtained are interesting and promising. The manuscript can be accepted for publication in Coatings journal after MINOR corrections.

Reviewer 4 Report

Main remarks.

1. The manuscript contains many illustrative materials, most of which are necessary to explain the results, but they should be optimized. In particular, Fig.1 should be replaced with a few words.
2. The authors examine and compare the two types of coating process, but just as the p-coating process was carried out at the same temperature, so the T-coating was carried out at only one pressure value. But this is absolutely not enough to make a correct comparison. It was necessary to carry out experiments with several values of the held parameters. Therefore, the conclusions and interpretation of the results should be changed to take into account the lack of data for a more complete analysis.
3. Figures 2 and 3 show the inverse relationship of the crystallization process with an increase in pressure or temperature, but the authors completely ignore this fact.
4. The crystal structure depends on the thickness (lines 364-369). For a more consistent conclusion about this, it is necessary to compare different samples with different parameters at a thickness comparable to the critical one. Obviously, this is the task of the following studies.
5. T-coating shows strains, but they are absent in p-coating. Why?
6. The term “oscillating chemical reactions” should be explained with respect to which variables and what is oscillating.

There are some technical remarks.

1. Authors should combine links, such as Refs. [1][2][3] as [1-3] or [1,2,3] lines: 37, 44, 52, 74, 87, 258, 274, 334,
2. Authors need to exact the limits of accelerating voltages at the line123: varying from 30V to 2 kV?
3. On line 152: Python is capitalized.

Author Response

Reviewer #4

Main remarks.

1. The manuscript contains many illustrative materials, most of which are necessary to explain the results, but they should be optimized. In particular, Fig.1 should be replaced with a few words.
   Since Fig. 1 is already described in the original text of the manuscript, we believe it does not need to be replaced. Moreover, it makes it easier to relate a particular coating thickness position to the corresponding pressure/temperature, at a glance and we would therefore prefer to keep Fig. 1 in the paper.
2. The authors examine and compare the two types of coating process, but just as the p-coating process was carried out at the same temperature, so the T-coating was carried out at only one pressure value. But this is absolutely not enough to make a correct comparison. It was necessary to carry out experiments with several values of the held parameters. Therefore, the conclusions and interpretation of the results should be changed to take into account the lack of data for a more complete analysis.
   While it is correct, that (generally speaking) a N-dimensional matrix of parameter-sets is necessary to study the complete effects of N variable parameters, we believe that in our case the exemplary investigation of two gradient coatings exhibiting only one varied parameter each, is enough to indicate trends and to draw some conclusions. We do not think that we h  ave overstated our conclusions.
   The next steps, would obviously include e.g. the further investigation of temperature at a certain interesting pressure, for instance in a region of microstructural instability. This, however, lies beyond the scope of the current manuscript. Moreover, we added the following passages to the conclusions in the revised manuscript:
   An intricate inter-dependence between deposition pressure, temperature, bi-layer thickness and in-plane film strain in the cubic phase has been observed and rationalized in terms of the stability of the growth process of the nanolamellar structures. (line 431)
   and
   To obtain a fuller understanding of the forming structures, additional variation studies at particularly interesting points in parameter space should be conducted, such as in phase/structure stability transition regions. (line 438)
3. Figures 2 and 3 show the inverse relationship of the crystallization process with an increase in pressure or temperature, but the authors completely ignore this fact.
   We do not understand the reviewer’s remark. We certainly did not ignore the apparently opposite effects of temperature and pressure. Just to make it even more clear, we have added a sentence explicitly stating this observation:
   In general, temperature and pressure seem to affect the coating inversely with respect to one another. (line 197).
4. The crystal structure depends on the thickness (lines 364-369). For a more consistent conclusion about this, it is necessary to compare different samples with different parameters at a thickness comparable to the critical one. Obviously, this is the task of the following studies.
   We agree and have included a sentence to the same effect in the conclusions section of the revised manuscript (see our reply to the reviewer’s comment #2).
5. T-coating shows strains, but they are absent in p-coating. Why?
   We do not understand the reviewer’s remark. Strains are present in both samples, but in both, there are also regions of low strain (cf. Fig. 3 and Section 3.2).
6. The term “oscillating chemical reactions” should be explained with respect to which variables and what is oscillating.
   This is, unfortunately, not something we could measure directly, but it can be supposed that spatial and temporal oscillations in reactant gas pressures and compositions are related to the nano-lamellar microstructure and chemistry of the coating. It is currently not clear, if these oscillations form spontaneously or if they are induced in some way by the deposition process.
   Since we possess no definitive knowledge about this, we would prefer this point to remain unspecified.

There are some technical remarks.

1. Authors should combine links, such as Refs. [1][2][3] as [1-3] or [1,2,3] lines: 37, 44, 52, 74, 87, 258, 274, 334,
   This has been amended.
2. Authors need to exact the limits of accelerating voltages at the line123: varying from 30V to 2 kV?
   This has been amended, it now reads “30 kV to 2 kV”.
3. On line 152: Python is capitalized.
   This has been amended.

Round 2

Reviewer 4 Report

I am sure that illustrative material could be optimized.