The Influence of Nitrogen Flow Rate on the Structure and Properties of Mo-Hf-Y-Si-B-N Coatings

Round 1

Reviewer 1 Report

The author prepared the Mo-Hf-Y-Si-B-N coatings by magnetron sputtering under  different nitrogen flow rate. Mechanical properties of the coatings are rather poor, hardness up to 16 GPa and wear rate to 5x10^-6 mm3/Nm, but promising is thermal resistance.   Despite the good structuring of the article, many parts of them should be corrected.

My overall impression is that while I am not questioning the technical correctness of coating characterization, with some doubts, the scientific novelty is not presented.

The statement in Conclusions "An increase in the proportion of Si-N bonds with an increase in nitrogen flow rate contributed to an increase in optical characteristics: the coating obtained at a consumption of 37.5 sccm N2 was characterized by a maximum transmittance of 12%. " is unsupported by presented in manuscript investigations.

Formal aspects:

-       The English language should be corrected.

-       Graphic materials are presented in good quality.

-       Statistical errors for elements are needed in Table.

-       The abstract contain many obvious statements, as " The results showed that the introduction of nitrogen into the working atmosphere leads to a decrease in the concentration of target elements in plasma and an increase in the concentration of Nn+ ions." and should be corrected.

-       References are adequate, but authors should consider reduction of autocitations 9 References on 41 all.

-             Please explain the sense of addition of other elements to SiCBN when it results in decrease in thermal resistance from 1700°C (SiCBN [7,8]) to 1300°C (HfYSiCBN [10], HfHoSiCBN [11])?

Experimental:

1.       There is no complete and understandable description of the coating deposition process:

-    partial pressure of nitrogen,

-    base pressure in the vacuum chamber,

-    ion sputtering (cleaning) other parameters, pressure, bias,

-    substrate-magnetron source distance,

-    bias voltage during deposition,

2.       Abbreviations - KEF, VK-100?

3.       The roughness results are not presented in the manuscript, due to it the sentence "The roughness was 107 determined using an optical profilometer WYKO NT1100 (Veeco, NY, USA)." have to be removed.

4.       The transmittance results are not presented in the manuscript, due to it the sentence " The  transmittance (T) was measured in the range of 350–950 nm on a KFK-3 spectrophotometer. " have to be removed.

Results:

1.       The paragraph beginning from "During non-reactive..." and finishing "...depleted in target ions" - lines 135-144. What is the conclusion from this paragraph? The decrease in Metal peaks intensity with introduction of nitrogen is obvious.

2.       The deposition rate is relatively high. In many References it is significantly lower, even 10 - 30 nm/min. Please explain why here it is so high?

3.       The presence of Si3N4 diffraction line (blue diamond symbol in Fig. 3a) is speculation, because it overlap the Al2O3 line. The XPS results studies should be added for MoB, MoSi2 and Si3N4 presence confirmation.

4.       What is a phase(s) for diffraction lines positioned at 30°-35° at 40°-45° for coating formed at nitrogen flow rate 37.5 sccm. By the way abbreviation - sccm - should be explained.

5.       " The decrease in the E can be associated with an increase in the content of the amorphous component (a-Si3N4) in nitrogen-containing coatings  ..." But simultaneously the hardness should be reduced?

6.       In the sentence "According to XRD data, all coatings were X-ray amorphous (Fig. 3)." instead Fig. 3 should be Fig. 3a.

7.       " The maximal values of W = 57%, H/E = 0.098, and H3/E2 = 0.157 GPa were achieved at an  N2 flow rate of 25 sccm, which may be due to the high level of internal compressive stresses in reactive nitrogen-containing coatings [18]." Please present the stress results in investigated coatings to confirm this statement.

8.       Lines 223-225. Is it conclusion from the own studies or citation? If citation it shouldn't be on the beginning of this paragraph about friction.

9.       Line 243. Instead 37.4 should be 37.5.

10.     What is the roughness of substrates and coatings?

11.    The sentence " decrease in wear resistance with an increase in nitrogen consumption can 258 be associated with a high level of residual stresses and low adhesive strength."  is speculation, both  results of stress and adhesion investigations were not presented in manuscript.

12.    Why the wear rate of the substrate is investigated? By the way - what substrate - silicon, alumina, steel? As I see the subject of manuscript  is Mo-Hf-Y-Si-B-N coatings.

13.    As I see the Ks and Kc are similar (Fig. 6a.b) in the time 30-90 min. Why?

14.    What is the wear depth? In Fig. 6, except one sample (5s, 0 sccm), I see the substrate in the center of the sample. It means that authors didn't investigate the coating wear but coating-substrate system wear.

15.    What is the difference in Vw and Kc symbols of wear rate?

16.     What is physical explanation of  -Dm/S (Fig. 7a)?

17.     In conclusion are obvious statements, as " Plasma analysis showed that during non-reactive deposition in plasma, the maximalconcentration of target ions Mon+, Sin+, B+, 325 Hfn+, Yn+ and Ar+ gas is observed.",  "The introduction of nitrogen into the working medium leads to an increase in the concentration of Nn+ ions and a decrease in the concentration of ions of the target components." and should be removed or corrected.

18.    Conclusions - "The introduction of nitrogen contributed to the formation of an additional Si-N bond." - it is speculation, XPS analysis should be presented.

19.    Conclusions - " An increase in the proportion of Si-N bonds with 333 an increase in nitrogen flow rate contributed to an increase in optical characteristics: the coating obtained at a consumption of 37.5 sccm N2 was characterized by a maximum transmittance of 12%." this is not confirmed in presented results.

Author Response

Please see the attachment.

Author Response File: Author Response.doc

Reviewer 2 Report

The manuscript is very well-written and can be accepted after minor revisions.

1. The novelty of the work is not clear. 

2. Please add critical discussion on the subject and how the application of the current work can be realized.

3. The conclusions are significantly longer. Please add concise conclusions.

4. Please add a paragraph discussing the limitation of the current study. How the future work can further improve the study?

5. The selection of alloy design and working parameters should be justified.

6.  Please discuss other methods than magnetron sputtering that can achieve similar sorts of coatings.  

7. Why magnetron sputtering was opted?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Table 1 - please correct rounding to proper form.

Fig. 4 N1s, Mo3p - please note that the Si-N and B-N lines do not give the profile shown.

Symbols of wear rates - Vw and Kc - although the authors explained in Response how these parameters differ, there is no such explanation for readers in the article. Additionally, the authors write in the Response that "the Kc parameter describes the rate of wear of the coating under abrasive conditions and takes into account the thickness of the coating". What formulas were used to calculate these parameters? What is the importance of the Kc parameter if, to a large extent, the wear concerns the substrate.

What influence does the type of materials: substrate and ball have on the Kc value? Anyway, the authors probably do not consider this study significant because there is no summary of these results in the conclusions.

Why does Kc decrease with test duration? After all, the properties of the coating do not change.

"An increase in nitrogen flow rate to 25 and 37.5 sccm led to an increase in the friction coefficient to 0.98 and 1.0, respectively." It is very small difference. What is the accuracy of friction coefficient calculations?

What is the wear depth in ball-on-disc test?

I insist on removing the mention of transmittance from the Materials and Methods and Conclusions section because it is not presented and discussed in the Results section.

"It was shown in [47] that a high level of residual stresses in coatings reduces their adhesive strength and affects the nature of failure." In [47], adhesive strength is not mentioned, but it actually documents the correlation between wear rate and internal stress (Fig. 6), which is inconsistent with the results presented in this paper.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript is revised satisfactorily 

Author Response

Thank you for your positive response on our manuscript. 

Round 3

Reviewer 1 Report

My comments to R2.

Fig. 4 N1s, Mo3p - please show the binding of the last added gray line.
The authors did not respond to my comment from the previous opinion: "Additionally, the authors write in the Response that "the Kc parameter describes the rate of wear of the coating in abrasive conditions and takes into account the thickness of the coating." What is the importance of the Kc parameter if the wear is largely on the substrate? I don't see any discussion of this in the manuscript. Additionally, where in the presented formulas (2-4) is the thickness shown directly?

I asked earlier "Why does the Kc decrease with the duration of the test? The Reply Authors argue that the wear diameter changes over time, which in conjunction with the formula gives a different Kc value. So the question is whether it is a constant or a function Kc(t)? Would a change in the distance (and thus the time of the test) to determine the wear rate (Vw) change this rate? What is this time dependent factor? How to use it? By the way, in line 132-134 there is no friction method given, ball-on-disc? reciprocal?

Thank you for Fig. 6c. It shows that the wear depth of the coating obtained at a nitrogen flow rate of 37.5 sccm is 0.55 μm. But the thickness of the coating is less, it is only 0.5 μm. This means that the wear rate of the coating and substrate was calculated. The wear rate of the coating obtained at a nitrogen flow rate of 37.5 sccm is unknown. Please re-measure it or remove these results from the text.

Author Response

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Author Response File: Author Response.pdf

Round 4

Reviewer 1 Report

What bonds is for grey line in Fig. 4 N1s, Mo3p?

I cannot agree with the authors that the  wear rate Vw depends on time. Of course, at different stages of friction, the wear rate may be different, but wear is a process stretched over time. Just like we do not write about the coefficient of friction based only on the initial stage but on the stable stage of the friction process.

Probably the reason for the discrepancies in the assessment of the wear coefficients Kc and Vw is the measurement methodology adopted by the authors. It does not make much sense to measure the wear of coating-substrate systems and conclusions about the wear of the coating on this basis. Please note the title of the manuscript "The influence of nitrogen flow rate on the structure and properties of Mo-Hf-Y-Si-B-N coatings" COATINGS, not the coating-substrate system. The wear of two different materials in such system can be extremely different and a simple calculation of the wear index Kc does not reflect the actual wear resistance of the coating. In my opinion, the wear rate should be calculated for the coating itself as a fairly homogeneous material.

If the thickness of the coating is 0.5 µm and the depth of abrasion is 0.55 µm, it means only that the substrate material has been subjected to the counterpart in the friction test and has also been abraded. The wear rate calculated on this basis applies to the coating-substrate system, not the coating. It can be assumed that due to the difference in the hardness of the coating and the substrate, the wear rate of the substrate is higher. What was the reason for inserting the symbol ">" in Fig. 6 (>11.3)? The coating wear rate is rather lower than the calculated value for the coating-substrate system.

There is probably a mistake in formula (3), instead of "a4" it should be? "a⁴".

What is the distance S taken for Ks calculations? Is it the same as in the case of the Kc rate? This would result from formulas (4), but these are different distances. As the coating is abraded, the substrate is not in contact with the counterpart. This does not mean that Ks = 0, the rate is not defined because of 0 in the denominator of the formula (4). This means that probably the only correctly determined parameter from the set presented in Fig. 7 is the one described as (0 sccm, 5s). The others are unbelievable.

Author Response

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Author Response File: Author Response.pdf

Round 5

Reviewer 1 Report

The use of the same distance S for the calculations of Kc and Ks parameters is an obvious factual error. The whole discussion based on these results is unbelievable, lines 316-339. Therefore, I cannot accept the manuscript for publication.

Author Response

Thank you for your attention and your time. Because you and I did not agree on abrasion wear, the abrasion resistance methodology and results have been removed. We are interested in the rapid publication of the results. In the future, we will be more attentive to the results of this type of test, and we will carry out verification calculations. We will take into account your critical comments when conducting experiments regarding the measurement methodology and processing of results.

Round 6

Reviewer 1 Report

After removing the disputed part of the article, it can be accepted for printing with minor revision. Please check the References carefully and remove from the list items not mentioned in the article, e.g. [28,29].