Formation of Protective Coatings on TZM Molybdenum Alloy by Complex Aluminosiliconizing and Application of a Preceramic Layer

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper investigates the influence of a two-stage composite protective coating deposition technique on the properties and characteristics of TZM molybdenum alloy. By applying a pre-ceramic coating based on silicon-boron modified polysilazane, the structure, thickness, chemical, and phase composition of the protective coating were analyzed. Thermodynamic calculations were conducted, and macroscopic and microscopic structural analyses of the coating after high-temperature oxidation were performed to compare its oxidation resistance. The research in this paper is of certain significance, but there are some deficiencies that need to be improved.

1. The title is not closely related to the main research content of this paper and needs to be rewritten. Additionally, the abbreviation "TZM" in the title should be replaced with its full name, and it should also be explained in the text when it first appears.
2. The structure of the abstract does not meet the requirements. It lacks an introduction to the research background and the significance of the research. Moreover, there are no quantifiable data in the abstract, and the research conclusions are not clear. Therefore, the abstract needs to be rewritten.
3. The keywords do not reflect the main research characteristics of this paper and are just random words. Please refine the keywords.
4. In the introduction section, the author references only 17 papers (it is recommended to increase it to 25). However, they are simply listed without in-depth analysis or comparison with the differences from this paper. Please explain what gaps this paper fills and what innovative work has been done.
5. In the last paragraph of the introduction, a section about the introduction of this paper, especially its innovation, should be added.
6. Figure 1 lacks dimension annotations. Please add them and check the entire text.
7. The abstract mentions "analyzing the structure, thickness, chemical and phase composition of the protective coating and conducting thermodynamic calculations," but there are no related thermodynamic calculations in the full text. Please add them.
8. Please explain how applying a pre-ceramic coating based on silicon-boron modified polysilazane eliminates the inherent defects (pores and cracks) of the coating.
9. What is the significance of Figure 9? What specific content should be observed?
10. The conclusion of this paper is quite lengthy. It is recommended that the author list the points one by one instead of narrating them together.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Molybdenum-based alloys, in particular TZM alloys, are promising high-temperature materials, but at temperatures above 700° C, they are susceptible to high-temperature corrosion.

In this article, T. Loskutova et al. present the results of a study of the protective properties of a multilayer composite coating designed to protect TZM alloys from oxidation at high temperatures. This coating was obtained in two stages: aluminosilication and application of a boron-doped Si-based preceramic coating. The resulting coating was characterized using SEM, EDS, DSC, TG methods, and its resistance to oxidation was measured. Although both of these protection methods have been tested repeatedly separately, the scientific novelty is beyond doubt, since the use of multilayer coatings has not been sufficiently studied.

 

Nevertheless, the following aspects of the work, in my opinion, require major revisions.

 

1) The composition of some of the proposed phases raises questions. For example, line 190, dark inclusions: when the mass fractions are converted to molar ratios, the result is Mo:Al = 0.73:0.88≈  1:1, while the authors assume the formation of the Al₆₃Mo₃₇ phase, in which the molar ratio is Mo:Al≈ 1:2. Also, on the Al-Mo phase diagram, the specified phase is formed at a significantly higher temperature (~1500° C).

 

2) In its current form, the XRD data does not make enough sense.

a) The authors should improve the quality of XRD pattern (Fig. 5), since its current format does not allow for high-quality extraction of information. The authors also do not explain the many reflections in the 32-47° range. Is this a signal or noise?

b) Why is there no XRD pattern of the sample before the preceramic coating was applied? This could confirm the phase composition of the coatings.

 

3) TG/DSC data also require improved presentation or interpretation.

a) Methodologically, what mass of samples were used for the study? This must be specified to assess the relevance of the data, i.e. the % of observed values relative to the total mass of the sample.

b) Figure 7 should be redrawn: please shift the origin so that TG is visible, and add ticks to the scales. Moreover, it is advisable to assign masses and DSC to different axes (right/left) to make it easier to perceive.

1. c) The interpretation of the exothermic effect (lines 304-310) is a rather bold statement that requires proof. The authors are advised to refer to the article [https://www.sciencedirect.com/science/article/pii/S0040609011000502] in which this statement was confirmed. Recording the DSC during cooling/heating cycles are also needed.

 

4) Regarding the discussion of the composition of the aluminosilicate coating, one part should be reduced, while the other, on the contrary, should be developed.

a) The description of the reasons for the appearance of nitrogen in the film is too detailed in the text (lines 208-216). First, no research was conducted during the work to confirm this. Second, in another work by this group of authors, to which they refer [11], the authors have already described the mechanism of nitrogen appearance.

At the same time, other issues are not given sufficient attention:

b) Why is the zone described in lines 224-228 saturated with nitrogen, unlike the rest of the film?

c) Why is the silicon content in this zone higher than in the dark zone, even though it is also on the surface and the SiCl₄ vapor pressure is very low according to the simulation?

 

5) The composition of the pre-ceramic coating also raises questions.

a) Why does the coating obtained from polyhydrosilazane contain no nitrogen at all (lines 262-264), although review [17] reports that coatings of Si-O-N composition are formed?

b) Please confirm the presence of boron in the pre-ceramic coating by any experimental method, e.g. IR/Raman. It seemed that the composition of the upper layer after oxidation (SiO₂, B₂O₃) was assessed only visually (lines 334-335).

 

6) It is methodologically unclear, what is the reason for the significantly shorter experiment time compared to other publications on this topic (24 hours, line 176. vs. 70-100 hours)? [https://doi.org/10.1016/j.apsadv.2025.100769], [17]. Please add an explanation to the text.

 

7) Since the authors are investigating protective coatings, it is necessary to provide detailed data for both the samples after oxidation to prove this function. In fact, the increase in mass (lines 325-330) may be due to a number of other factors, such as insufficient time for crack formation during the experiment or oxidation of crystalline silicon.

 

8) The data in Figure 8 are presented without the error bars, which are necessary for a clear understanding of the differences. Since this is key information for the article, the corresponding raw data on the primary samples should be added to the Supplementary Materials.

 

9) Figure 8 compares only two systems (Al+Si, Al+Si → PHPS), but this does not give the full picture. Add a sample with a single pre-ceramic coating (PHPS) applied to clearly show the effect of the dual-protectiveness.

 

10) Lines 194-195 state that "Oxygen is likely present in the form of dispersed aluminum oxide (Al₂O₃) inclusions," but no evidence of the dispersion aluminum oxide is provided (e.g., EDS mapping).

 

11) Experimental data:

a) It is not specified which software package was used to calculate the equilibrium, and there is no reference to the work describing the calculation method (line 154-159).

b) The errors in determining the elements on the EDS are not specified, which raises questions about the number of decimal places in the mass fractions of the elements throughout the text.

 

12) Quality of Presentation

a) Figure 2 and further in the text: The figure contains labels Spectrum 1, Spectrum 2, etc., but these labels are not used in the text, which means that the reader has to spend a long time figuring out which area is sulfur, which is light gray, etc. It would be useful to add a schematic image of the coating with labels for the phases related to this layer to improve the readability of the article.

b) The quality of the photographs is quite low and does not allow the reader to see most of the surface features described in the text.

c) Figure 4: The authors should add a diagram similar to the one proposed for Figure 2.

 

13) The Al-N phase diagram used in the text of the article is not presented in Ref. 19 (line 190).

 

14) Other comments

a) Lines 249-255: This paragraph is not the result of research and should be included in the "Introduction" section.

b) Lines 277-278: "It should be noted that the layer thickness depends on the viscosity of the polyhydrosilazane and the withdrawal rate of the samples." In general, the statement is quite obvious, but is it worth including it in this case? Or did the authors conduct a series of experiments? (See below on reproducibility).

c) Lines 286-288: Add a reference to Figure 1b in the text.

d) Figure 6b. The decimal separator must be a dot, not a comma.

 

P.S. The reviewer warrants that he/she has no affiliation with any of the works mentioned.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The author's conclusion still has not been revised as per the suggestion.

Author Response

Thank you for your valuable comments regarding the content of the work. The authors' conclusion has been revised in accordance with your suggestion.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors made some positive changes to the manuscript; however, these corrections are not sufficient and do not solve the key problems. Therefore, I will now repeat or clarify my previous comments keeping to the previous numbering.

 

1) Despite the changes made by the authors that improve the interpretation of the phase composition, it remains unclear. In fact, no evidence has been found for either "Al₈Mo₃ + AlMo₃" or "an intermetallic of the Al₆₃Mo₃₇ type" (line 260). X-ray diffraction and/or spot EDS data should be added to support this proposal. Other evidence is also welcome (see comment 2).

 

2) a) It is necessary to add comment in the article text regarding unidentified peaks (Fig. 6).

b) In the response to the Reviewer, the authors state that the main focus of the paper is on coatings obtained by two-stage processing. At the same time, 82 of the 203 lines of the Results and Discussion section are devoted to the composition of the coating obtained by the batch cementation method. In this regard, the reviewer considers it appropriate to request a more complete study of this layer in order to provide a fully characterized substrate for the second stage. If these data have already been presented by the authors earlier, then the descriptive part in the current manuscript should be shortened and a direct reference should be provided.

 

3) a) The mass of samples for both TGA and gravimetric analysis should be included directly in the experimental part.

с) The interpretation of the exothermic effect still requires appropriate support in the text of the manuscript. Please add relevant references.

d) The authors ignored the comment regarding DSC recording during heating/cooling, which indeed could confirm stability in an inert atmosphere.

 

4) a) Following the authors' answer, they are advised to elaborate more on the topic of diffusion processes specifically, rather than the mechanism of NH₄Cl decomposition, which lacks scientific novelty and was not investigated in this study.

b) Considering the zone saturated with nitrogen, the reviewer did not understand the relevance of durometry here. Secondly, the article text states that the light phase contains 17.75% nitrogen, not carbon (typo in the response or in the data?). The rest of the response to this comment should be included directly in the text of the manuscript.

 

5) a) If the electron microscope cannot detect nitrogen and boron, why does the article text specify nitrogen content (e.g., lines 261, 284, 295)?

 

7) At least microphotographs of cross-sections after oxidation should be added.

 

8) In this case, “raw data” referred to reproducibility data of the experiments, which are still not provided. Also, according to Figure 9, the relative error in oxidation time measurement is approximately 5%, resulting in an error of ±1 hour at the final points.

 

9) The data on the protective properties of the pre-ceramic coating without alumosiliconizing would more clearly demonstrate the advantages or disadvantages of the proposed composite coating compared to a single-component one. Otherwise, the main idea of the work is unclear, i.e. why use a more complex protective layer? Therefore, the requested data should be in the current work exactly.

 

10) The reviewer has no doubt about the thermodynamic possibility of Al₂O₃ formation under these conditions; the question concerns its dispersion. Therefore, this comment must be repeated: the dispersion of Al₂O₃ phase is steel not demonstrated. Are these individual small inclusions or extended zones? What is the size of these inclusions?

 

11) a) Unfortunately, the Astra Software Package cannot be found via Google, and article [23] is not publicly accessible. Therefore, the authors should add a link to the website or article where the method is described in detail, because currently the reviewer has questions regarding why a method based on entropy factor minimization is used instead of Gibbs free energy minimization.

 

12) a) We suggest the authors add markers Layer I, Layer II, etc., into the text to help readers navigate more easily.

 

13) The reviewer has access to the third edition of this book, where the given phase diagram is not present. Please indicate the page number where it can be found?

Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors provided satisfactory responses and corresponding revisions to the manuscript only to some of the previous comments.

Specifically, significant objections remain regarding Comments 1, 2, 8, and 9, which prevent the manuscript from being accepted for publication. Therefore, I am forced to repeat and/or reformulate these comments.

 

1) The phase composition of the samples remained partially unproven.

Specifically, the statement «However, the obtained results indicate that this zone contains six components, and the data show the presence of only the Al₆₃Mo₃₇ phase» remains unclear. It is impossible to find specific and relevant data in the manuscript proving the presence of such a phase (Al₆₃Mo₃₇). If this refers to micro-X-ray spectral analysis, then why is the molar ratio Mo:Al approximately 1:1 whereas in Al₆₃Mo₃₇ the Mo:Al ratio is about 2:1? Moreover, the authors have not provided additional data (XRD, XPS, etc.). Without additional data, the statement about the presence of the Al₆₃Mo₃₇ phase cannot be accepted.

The further assumption «This suggests that all components (O, Si, Ti, Zr) — or at least some of them — may stabilize the Al₆₃Mo₃₇ phase at lower temperatures, at least up to 1000 °C» can only be valid if comprehensive proof of the Al₆₃Mo₃₇ phase’s existence is supplied, since at present this assumption rests on a weak foundation, namely another assumption. It is completely unfounded. Publishing such speculative assumptions in a scientific journal is impossible.

 

2) Therefore, the methods used are insufficient to determine the phase composition of the presented coating. The authors' assertion that the XRD method is unavailable raises doubts, as only one diffraction pattern is present in the text of the article, and the method itself is quite common and fast.

 

8a) The reviewer still has not received an explanation of why the time measurement error, according to Fig. 9, is relative and reaches about 1 hour at the last points. Don't the authors have a more accurate instrument for measuring time?? Otherwise, this demonstrates a lack of qualifications in processing the primary data. Such results also cannot be published in a scientific journal.

 

8b) The data from five parallel experiments must be added in the Supplementary materials, including information on sample preparation and composition. This raises questions about the quality of the data and suggests a lack of statistical analysis. Currently it seems that only one coating sample was obtained and characterized. The results are then not scientific, but merely contrived. The authors' insistence on not providing data for all five parallel samples only makes the reviewer suspect unacceptable scientific manipulation. Please add the data to the Supplementary Materials. This is not difficult for any scientist if the data are real.

It is also necessary to include in the Materials and Methods section information about the number of parallel experiments conducted for multilayer coating deposition.

 

9) If the authors cannot or do not wish to add data from the mentioned article [27], it would be useful to refer to this article once again in the manuscript in the section describing the oxidation kinetics features (previously the article is mentioned in a different context) and to explicitly compare the multilayer coating with the pure pre-ceramic coating.  The reviewer does not imply that such a coating can be produced by a single method. However, given the purpose of the current manuscript, it is necessary that the advantages of the composite coating be clearly stated. The current manuscript doesn't show this, and therefore defeats its purpose. Therefore, a direct comparison is necessary.

 

Additionally, minor comments concern points 3a, 11, and 12:

3a) It is doubtful that both samples have such identical mass values to the fourth decimal place.

11) The reviewer suggests replacing the unavailable reference [23] with any of the alternatives proposed by the authors.

12) In the manuscript, there are mentions of Layer I and Layer II, but no marker or description of Layer III is provided.

Author Response

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Author Response File: Author Response.pdf