Study on Preparation of Nano-CeO₂ Modified Aluminized Coating by Low Temperature Pack Aluminizing on γ-TiAl Intermetallic Compound

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article presents rather interesting data on the production of protective coatings on the TiAl intermetallic compound and their properties. However, there are the following comments on the content of the article.

1. Table 2 shows only two components of the penetrant, but there are no other components - CeO2 and NH4Cl, which are mentioned in the text.
2. On page 3, line 126, it is stated that CeO2 added aluminized coating has good adhesion to the substrate. This statement is not based on measurements and in fact, it is not shown in which case the adhesion is higher – in the presence of CeO2 or in its absence.
3. Page 4, in 1st paragraph after Fig. 1 we read “This is because rare earth elements have very strong chemical reactivity. At medium temperatures, they decompose…”. But rare earth elements do not decompose themselves, perhaps the authors wanted to say that compounds (in this case CeO2) of rare earth elements decompose. However, the fact that CeO2 breaks down into Co and O2 under these conditions is not at all obvious. Further, the statement "Moreover, the adsorbed rare earth atoms can change their diameters and enter the matrix" requires proof, possibly in the form of references to literary sources. In general, the entire paragraph under consideration, together with Fig. 2 looks unconvincing and it is unclear how CeO2 molecules can provide a channel for the infiltration of aluminum atoms. Additionally, in Fig. 2 it is not clear what GB is.
4. In Fig. 3E, the XRD diagram is illegible. It needs to be improved.
5. The first sentence in the Conclusion section needs to be reformulated, instead of "this formula", it is necessary to specify which formula is meant and helps to get coating.
6. In Conclusion section we read that CeO2 promotes the diffusion of aluminum atoms. However, this statement looks not proven. If the authors insist on this, it is necessary to provide additional explanations and evidence.
7. In Conclusion section it is claimed that the aluminized sample is oxidized according to a parabolic law. However, this is not visible in Fig. 6 (red lines and dots). Rather, the oxidation proceeds according to a linear law.
8. The list of references does not include DOIs for journal articles. It's inconvenient for the reader.

 

Author Response

Response to Reviewer 1 Comments

Point 1: Table 2 shows only two components of the penetrant, but there are no other components - CeO2 and NH4Cl, which are mentioned in the text.

Response: Thank for your important suggestion. We have modified the table 2.

Table 2. The formula components and the proportion of each component of the diffusion agent in the powder embedding aluminizing process

Raw material	Al	Al2O3	NH4Cl	CeO2
Formula1(wt%)	30	66	3	1
Formula2(wt%)	30	67	3	0

Point 2: On page 3, line 126, it is stated that CeO₂ added aluminized coating has good adhesion to the substrate. This statement is not based on measurements and in fact,it is not shown in which case the adhesion is higher – in the presence of CeO₂ or inits absence.

Response: Thank for your suggestion. We have deleted this statement aboout the adhesion. It is correct that we can ont show derectly the high adhesion thothough the figure 1. In fact, this atatement is based the literature below. To avoid ambiguity,we choose to deleted.

14. Cho D W ,Kim I .Formation of pegs during hightemperature oxidation of Fe₃Al containing yttrium[J]. Metallurgical and Materials Transactions A, 2000, 31(6): 1685-1687.

Point 3: Page 4, in 1st paragraph after Fig. 1, rare earth elements do not decompose themselves, perhaps the authors wanted to say that compounds (in this case CeO₂) of rare earth elements decompose. However, the fact that CeO2 breaks down into Ce and O under these conditions is not at all obvious

Response: Yes. Thank you for pointing out our error. We have modified the paragraph by citing more references. In fact, we want to elaborate the mechanism of the action of rare earth oxides.

NH₄Cl decomposes into HCl and Cl₂ at 450℃. HCl reacts with CeO₂ as follows[]:

2CeO2+2HCl = Ce2O3+Cl2+H2O (1)

Ce2O3+6HCl = 2CeCl3+3H2O (2)

Ce2O3+3Cl2 = 2CeCl₃+3/2O2

Then CeCl₃ diffuses onto the surface of the substrate , is reduced to active Ce atoms. Rare earth elements Ce deposited on the surface layer can activate the coating surface layer. As the atomic radius of Ce is larger than that of nickel atoms, the atoms infiltrated into the alloy surface cause to a serious distortion of the surrounding lattice and reduce the free surface energy^[17], which is beneficial to the adsorption and diffusion of Al, resulting in forming a high Al concentration layer on the surface of the alloy in a short time. The high Al concentration surface layer provides a very high concentration gradient for Al to quickly spread to the inside.

16. PJG, GL N, EA B, Chlorination of lanthanum oxide.[J].The journal of physicalchemistry.A,2012,116(9):2062-70.https://doi.org/10.1021/jp210457r.
17. West G ,Perkins J ,Lewis M .The effect of rare earth dopants on grain boundary cohesion in alumina[J].Journal of the European Ceramic Society,2006,27(4):1913-1918.

Point 4: In Fig. 3E, the XRD diagram is illegible. It needs to be improved.

Response: Thank you for your detail advice. The figure 3 has been improved as suggested. The figure 3 has become figure 2, because we deleted figure 2.

Figure 2. Surface morphologies of aluminized coating of γ-TiAl intermetallic compound which at 600℃ for different soaking time and XRD patterns

Point 5: The first sentence in the Conclusion section needs to be reformulated, instead of "this formula", it is necessary to specify which formula is meant and helps to get coating.

Response: Thank you for your detail advice. The introduction has been rewritten as suggested. In fact, this formula is our formulation for preparing this coating. In oeder to make it clearer still, we rewritten the following  sentence.

This formulation of the coating, namely 30%Al、66%Al₂O₃、3%NH₄Cl、1%CeO₂, has successfully achieved aluminizing coating on γ-TiAl intermetallic compounds at 600℃, and the coating thickness continuously increased with the increase of aluminizing time.

Point 6: In Conclusion section we read that CeO₂ promotes the diffusion of aluminum atoms.However, this statement looks not proven.

Response: Thank for your important suggestion. We have already elaborated and supported this point in the line 129-132. In this paper, we conducted experiments with two formulations under the same external conditions, namely a 3-hour aluminizing at 950℃. The first formulation did not contain CeO₂, while the second formulation contained 1% CeO₂. We provided the surface and cross-sectional morphology diagrams of the coatings(Figure 1). From the figure 1, it can be seen that the aluminum alloy coating with 1% CeO₂ has a thicker thickness, is more dense and more continuous. In fact ,according to the following reference, under the temperature of 950℃, the diffusion rate of Al atoms in the nickel-based system is significantly higher than that of nickel atoms. Therefore, we can dedue that the presence of CeO₂ promotes the diffusion of Al atoms into the matrix.

15. Yamane T ,Katayama K ,Minamino Y , et al.Reaction Diffusion in Diffusion Couples Ni-Al SolidSolution/Intermetallic NiAl and Ni/Intermetallic NiAl[J].Solid State Phenomena,2007,698(127-127):77-82.

Point 7: In Conclusion section it is claimed that the aluminized sample is oxidized according to a parabolic law. However, this is not visible in Fig. 6 (red lines and dots). Rather, the oxidation proceeds according to a linear law.

Response: The figure 6 has become figure 5, because we deleted figure 2. According to the parabolic relationship between oxidation weight gain and time proposed by Wagner's theory(according to the following referrence ), the growth of the oxide film in our experiment did indeed conform to the parabolic theory. However, in our experiment, the determination of oxidation kinetics was carried out using a discontinuous measurement method, so the parabolic curve in the measurement results was not obvious. In the field of high-temperature oxidation, oxidation kinetic curves are expressed in this way. The linear interconnection represents the changing trend of weight gain, and does not mean that measured value lies in linear line. Wehave listed several references below:

https://doi.org/10.3390/coatings13010022.  Figure 3a.

https://doi.org/10.3390/coatings13081336.  Figure 10.

https://doi.org/10.1016/j.corsci.2023.111491. Figure 7.

https://doi.org/10.1016/j.corsci.2023.111339 Figure 3a

In subsequent experiments, we also improved the experimental method, such as adopting a continuous measurement method, and the parabolic results would be more obvious.

Wagner C .Theoretical Analysis of the Diffusion Processes Determining the Oxidation Rate of Alloys[J].Journal of The Electrochemical Society, 2019, 99(10): 369-369.

Point 8: The list of references does not include DOIs for journal articles. It's inconvenient for the reader.

Response: Thank you for your detail advice. The list of references has been

rewritten as suggested.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors of the article “Study on preparation of Nano-CeO2 modified aluminized coating by low temperature pack aluminizing on γ-TiAl intermetallic compound” present their findings on the effects of including rare earth Cerium to the aluminized coating process. The findings are interesting, and  the article can be recommended for publication after a moderate number of  technical and formatting/language points are addressed. The detailed reviews for the overall article are listed below:

 

 

Article formatting and English language/writing feedback

• Please revise the formatting of the references at the end of the document to be consistent with the journal guidelines.
• General use of articles ‘a’ and ‘the’ needs to be reviewed and corrected throughout the document.
• There are many instances in the article where the authors seem to write “A1” (as in the letter A followed by the number 1) instead of “Al” to denote aluminum. Please correct this.
• Oxford commas should be included throughout the article (e.g., Lines 13, 33, 34, 35, 47, 58, etc.)
  
  
• Line 17: please remove ‘it; before ‘easy to obtain’.
• Line 36: please remove ‘a’ before ‘continuous’
• Line 55: please remove capitalization of ‘Al’ in ‘Swadzba et al.’.
• Line 81-85: Change to past tense.
• Line 94-104: Change to past tense.
• Line 109-111: Please combine the last 2 sentences of the paragraph and revise the wording.  
• Line 120-122: The order of this sentence is a little confusing. Please revise to make it clearer.

 

 

Technical feedback

• Line 17: The abbreviation ‘PBR’ is not defined anywhere in the abstract or the manuscript. I assume it is intended to mean Pilling Bedworth Ratio. Please define the abbreviation in the abstract, and also cover its relevance within the main body of the manuscript.
  
  Also, the authors state that the value for Al2O3 is moderate. But what is the value? And what makes it moderate? And why is a moderate value desirable/easy to obtain?
  
  
• Line 39-40: Please provide literature reference(s) to provide more information on the decrease in oxidation resistance with temperatures above 800C.
• Line 65: 1273K’ Please do not mix between temperature units and stay consistent with either Celsius or Kelvin throughout the document.
• Line 73: Please correct ‘austenized’ to ‘austenitized’.
• Lines 76-77: The readers do not have context to know the meaning of the “14^th five-year plan”. Since this is not directly related to the technical information listed in the article, the sentence can be removed. Otherwise, it can be reworded to be made more generally to convey the fact that “ lower aluminizing temperatures also help with decreasing energy consumption during the process”.
• Line 88: Please specify some information about the condition of the substrate material. Details on the production method, source, or manufacturer, would be greatly appreciated.
• Figure 1: For the ease of the reader, please include a measurement in 1C similar to the one 46μm in 1D.
• Line 135: Please clarify what is meant by “medium temperatures” by including the range of temperatures in the manuscript.
• Line 137: Please provide literature references to support the phenomenon of rare earth atoms changing their diameter.
• Lines 138-142: Please provide references for the rare earth mechanisms being described here.
• Line 149: The sentence “It is known that the substrate is a γ-TiAl intermetallic compound.” is not needed and can be removed.
• Line 150-155 and Figure 3: EDS is not a typically reliable method for providing quantitative estimates of the weight percentages of elements. The more reliable way of confirming the specific intermetallic phase being observed is through XRD, which the authors included in Figure 3 E. Also, the wt.% for number 1 do not add to 100%, while those for rows 2 and 3 do add to 100%.
  
  Moreover, the surface element analysis wt.% values do not match the expected stoichiometric ratios of the Ti-Al intermetallics specified in the beginning of the article (line 30), even when accounting for the presence of other elements (Mn and Ce). In particular, the proportion of Al in these regions seems to be quite high. The expected weight percent values are listed below.

Phase	Molecular wt.	Ti wt.%	Al wt.%
TiAl	74.85	63.95%	36.05%
Ti3Al	170.588	84.18%	15.82%
TiAl3	128.812	37.16%	62.84%

If the authors do want to include EDS data in the study, then elemental maps would be far more useful, as they would provide a good qualitative way to highlight  potential locations of heterogeneities in elemental distribution.

• Lines 169-171: The authors claim that the optimal aluminizing duration is 5 hours. How was this optimum determined? Did the authors perform mathematical optimization? If so, what was the objective function used? Given that 5 hours was the longest duration tested for the study, how do the authors know that aluminizing for even longer would not lead to even better results than the “optimal” 5 hours. If there was no actual optimization performed, please remove this sentence.
• Figure 5: Similar to earlier comments on Figure 3, the quantitative EDS table should be replaced with colored elemental maps representing the distribution of each of the elements. Also, in 5C, it is not possible to read the legends for the three line scans provided. Please make sure to include larger text in the figure or clarification in the figure caption.
• Figure 7 and 8: Similar Feedback as Figures 3 and 5.
• Please specify the purpose of adding Nickel coating to the surface either in the main text or in the caption for 7C (to preserve/protect the surface layer during cross sectioning and sample preparation for SEM observation). Also, please specify the type of Nickel coating method used.
• Lines 284-304: Some more discussion connecting the experimental findings of the presented study to the claims of TiAl oxidizing to form Al2O3 would significantly improve this section.
• The article requires significantly more citations to strengthen the technical merit of the work being presented.

Author Response

Response to Reviewer 2 Comments

Point 1: Please revise the formatting of the references at the end of the document to be consistent with the journal guidelines.

Response: Thank you for your detail advice. The list of references has been

rewritten as suggested.

Point 2: General use of articles ‘a’ and ‘the’ needs to be reviewed and corrected throughout the document.

Response: Thank you for your detail advice. The article has been rewritten as suggested.

Point 3: There are many instances in the article where the authors seem to write “A1” (as in the letter A followed by the number 1) instead of “Al” to denote aluminum. Please correct this.

Response: Thank you for your detail advice. The article has been rewritten as suggested.

Point 4: Oxford commas should be included throughout the article (e.g., Lines 13, 33, 34, 35,47, 58, etc.)

Response: Thank you for your detail advice. The article has been rewritten as suggested.

Point 5: The manuscript also needs some minor formal editing:

-Line 17: please remove ‘it; before ‘easy to obtain’.

-Line 36: please remove ‘a’ before ‘continuous’

-Line 55: please remove capitalization of ‘Al’ in ‘Swadzba et al.’.

-Line 81-85: Change to past tense.

-Line 94-104: Change to past tense.

-Line 109-111: Please combine the last 2 sentences of the paragraph and revise the wording.

-Line 120-122: The order of this sentence is a little confusing. Please revise to make it clearer.

Response: Thank you for your detailed advice. I have made the suggested revisions.

Point 6: Line 17: The abbreviation ‘PBR’ is not defined anywhere in the abstract or the manuscript. I assume it is intended to mean Pilling Bedworth Ratio. Please define the abbreviation in the abstract, and also cover its relevance within the main body of the manuscript. Also, the authors state that the value for Al2O3 is moderate. But what is the value?And what makes it moderate? And why is a moderate value desirable/easy to obtain?

Response: Thank you for your detailed advice. I have made the suggested

revisions. The Pilliing - Bedworth Ratio (PBR) refers to the ratio of the volume of an oxide to the volume of the metal consumed in forming that oxide. It is an important criterion for judging the integrity of an oxide film and one of the main factors causing growth stress within the oxide film. When the PBR value is less than or equal to 1, the formed oxide film is difficult to cover the substrate; when the PBR value is greater than or equal to 2, the oxide film is prone to cracking and cannot provide protective effects. Therefore, only when the PBR value is greater than 1 and less than 2, the oxide film has better integrity and stronger anti-oxidation protection. Among common oxides, the PBR value of aluminum oxide is between 1 and 2. so aluminum compound coatings are the most widely used.

Point 7: Line 39-40: Please provide literature reference(s) to provide more information on the decrease in oxidation resistance with temperatures above 800C.

Response: Thank you for your detailed advice. I have made the suggested revisions. Wu et al. [3] have reported when the service temperature of TiAl alloy is higher than 800℃, the main oxidation products on TiAl alloys are non-protective rutile TiO₂ or TiO₂ + Al₂O₃ mixed scale which have greatly limit their further applications.

3. Wu L ,Wu J ,Wu W , et al.High temperature oxidation resistance of γ-TiAl alloy with pack aluminizing and electrodeposited SiO₂ composite coating[J].Corrosion Science,2018,14618-27.

Point 8: Line 65: 1273K’ Please do not mix between temperature units and stay consistent with either Celsius or Kelvin throughout the document.

Response: Thank you for your detailed advice. I have made the suggested revisions.

Point 9: Line 73: Please correct ‘austenized’ to ‘austenitized’.

Response: Thank you for your detailed advice. I have made the suggested revisions.

Point 10: Lines 76-77: The readers do not have context to know the meaning of the “14^th five-year plan”. Since this is not directly related to the technical information listed in the article, the sentence can be removed. Otherwise, it can be reworded to be made more generally to convey the fact that “ lower aluminizing temperatures also help with decreasing energy consumption during the process”.

Response: Thank you for your detailed advice. I have made the suggested revisions in the lines 81-82.

Point 11: Line 88: Please specify some information about the condition of the substrate material. Details on the production method, source, or manufacturer, would be greatly appreciated.

Response: Thank you for your detailed advice. I have made the suggested revisions. The matrix material is γ-TiAl-based alloy, provided by the Titanium Alloy De-partment of the Institute of Metal Research, Chinese Academy of Sciences. The nomi-nal chemical and EPMA-detected composition of γ-TiAl intermetallic compounds (wt%) is shown in Table 1.

Point 12: Figure 1: For the ease of the reader, please include a measurement in 1C similar to the one 46μm in 1D.

Response: Thank you for your detailed advice. I have made the suggested revisions.

Figure 1. Effect of the addition ofCeO₂ on aluminized coating of γ-TiAl intermetallic compound at 950℃ for 3h: (A) surface of addition of 0% CeO₂; (B) surface of addition of1% CeO₂; (C) cross-section of addition of 0% CeO₂; (D) cross-section of addition of 1% CeO₂.

Point 13: -Line 135: Please clarify what is meant by “medium temperatures” by including the range of temperatures in the manuscript.

-Line 137: Please provide literature references to support the phenomenon of rare earth atoms changing their diameter.

-Lines 138-142: Please provide references for the rare earth mechanisms being

described here .

Response: Thank you for your detailed advice.In fact,we have modified this paragraph in lines.What id more important, we want to clarify the mechanism of the action of CeO₂. In the following reference, it is mentioned that RE segregation at grain boundaries can significantly reduce the free surface energy.

17. West G ,Perkins J ,Lewis M .The effect of rare earth dopants on grain boundary cohesion in alumina[J].Journal of the European Ceramic Society,2006,27(4):1913-1918.

Point 14: Line 150-155 and Figure 3: EDS is not a typically reliable method for providing quantitative estimates of the weight percentages of elements. The more reliable way of confirming the specific intermetallic phase being observed is through XRD, which the authors included in Figure 3 E. Also, the wt.% for number 1 do not add to 100%, while those for rows 2 and 3 do add to 100%.

Response: Revised as suggested. We have added it to the manuscript. The figure 3 has become figure 2, because we deleted figure 2.

Figure 2. Surface morphologies of aluminized coating of γ-TiAl intermetallic compound which at 600℃ for different soaking time

In fact, we observed the existence of phases through XRD. EDS was used to verify the matching of elemental composition and to check if the doped element Ce could be detected. It might be because the addition amount of CeO2 was very small in the position 1 that EDA failed to detect it.

Point 15: The surface element analysis wt.% values do not match the expected

stoichiometric ratios of the Ti-Al intermetallics specified in the beginning of the article (line 30), even when accounting for the presence of other elements (Mn and Ce).

Response: Thank you for your detailed advice. Figure 2 (original figure 3) shows the surface morphology and elemental distribution analysis of γ-TiAl after aluminizing at 600℃. The high aluminum content on the surface is normal, indicating that the aluminizing effect at this location is good. Research shows that at 600℃(https://doi.org/10.4028/www.scientific.net/SSP.127.77), the diffusion rate of nickel atoms is less than that of aluminum atoms, and with the presence of Mn and Ce elements, the nickel content on the surface is significantly lower than the aluminum content.

Point 16: Lines 169-171: The authors claim that the optimal aluminizing duration is 5 hours. How was this optimum determined? Did the authors perform mathematical optimization? If so, what was the objective function used? Given that 5 hours was the longest duration tested for the study, how do the authors know that aluminizing for even longer would not lead to even better results than the “optimal” 5 hours. If there was no actual optimization performed, please remove this sentence.

Response: Thanks for pointing this problem out. After only conducting 3 sets of experiments (1 h, 3 h and 5 h), it is indeed impossible to directly conclude that 5 hours of aluminizing is the optimal aluminizing time. In fact, what we intended to convey is that 5 hours of aluminizing can result in a thicker coating compared to 1 h and 3 h. Also, considering the issue of energy consumption, we chosed 5 hours as the optimal aluminizing time. However, for the sake of rigor, we have revised it to the following sentence.

Therefore, in this experiment,we chosed 5 hours as the embedding aluminizing dura-tion when the aluminizing temperature is 600℃.

Point 17: Figure 5: Similar to earlier comments on Figure 3, the quantitative EDS table should be replaced with colored elemental maps representing the distribution of each of the elements. Also, in 5C, it is not possible to read the legends for the three line scans provided. Please make sure to include larger text in the figure or clarification in the figure caption. Figure 7 and 8: Similar Feedback as Figures 3 and 5.

Response: Thank you for your detailed advice. In fact, The elemental distribution characteristics presented by the elemental distribution map are not obvious, especially for elements with low content. It is more appropriate to use the elemental table.

Point 18: Please specify the purpose of adding Nickel coating to the surface either in the main text or in the caption for 7C (to preserve/protect the surface layer during cross sectioning and sample preparation for SEM observation). Also, please specify the type of Nickel coating method used.

Response: Revised as suggested. We have added it to the manuscript.

When inlaying samples, a nickel layer is electroplated on the surface of the coating to protect the alumina layer from cracking during subsequent processing in a plating solution of 200 g/L NiSO₄·6H₂O, 40 g/L H₃BO₃, 40 g/L NiCl₂, and 120 g/L C₆H₅Na₃O₇·2H₂O.

Point 19: Lines 284-304: Some more discussion connecting the experimental findings of the presented study to the claims of TiAl oxidizing to form Al₂O₃ would significantly improve this section.

Response: Thank you for your detailed advice. In fact, the main statement is about the high-temperature oxidation mechanism of TiAl intermetallic compounds in lines 284-304, which is due to the main oxidation products on TiAl alloys are non-protective rutile TiO₂ or TiO₂ + Al₂O₃ mixed scale which have greatly limit their further applications. However, the coating prepared in our experiment did indeed improve this situation. Since the phase generated after aluminizing with TiAl intermetallic compound powder is the aluminum-rich TiAl₃ phase, a single, continuous, and dense protective aluminum oxide film can be formed on the surface during oxidation. This has been stated in lines 314-321.

Point 20: The article requires significantly more citations to strengthen the technical merit of the work being presented.

Response: Thank you for your detailed advice. We have added the following references.

4. Wu L ,Wu J ,Wu W , et al.High temperature oxidation resistance of γ-TiAl alloy with pack aluminizing and electrodeposited SiO₂ composite coating[J]. CorrosionScience,2018,14618-27.https://doi.org/10.1016/j.corsci.2018.10.031
5. Cho D W ,Kim I .Formation of pegs during hightemperature oxidation of Fe₃Al containing yttrium[J]. Metallurgical and Materials Transactions A, 2000, 31(6): 1685-1687. https://doi.org/10.1007/s11661-000-0180-5
6. Yamane T ,Katayama K ,Minamino Y , et al.Reaction Diffusion in Diffusion Couples Ni-Al SolidSolution/Intermetallic NiAl and Ni/Intermetallic NiAl[J].SolidStatePhenomena,2007,698(127-127):77-82.https://doi.org/10.4028/www.scientific.net/SSP.127.77
7. PJG,GL N,EA B, Chlorination of lanthanum oxide.[J].The journal of physicalchemistry.A,2012,116(9):2062-70.https://doi.org/10.1021/jp210457r.
8. West G ,Perkins J ,Lewis M .The effect of rare earth dopants on grain boundary cohesion in alumina[J].Journal of the European Ceramic Society,2006,27(4):1913-1918. http://doi.org/10.1016/j.jeurceramsoc.2006.07.001

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article describes and intends to aluminize the gamma-TiAl coatings for thermal barrier coating applications by doping with CeO2. This topic, being highly relevant, has been described in an insignificant manner by the authors in the introduction and subsequently in the discussions. The experimental procedure does not seem to be coherent and involves sentences like a laboratory manual. Readability of the article is highly questionable; hence, it is difficult to apprehend the novel findings from the study.  What is the significance of Fig. 2, and the reason behind its poor representation? The reviewer would not recommend the article for publication.

Comments on the Quality of English Language

Language is not appropriate, and serious grammatical flaws throughout the manuscript.

Author Response

Response to Reviewer 3 Comments

Point 1: The article describes and intends to aluminize the gamma-TiAl coatings for thermal barrier coating applications by doping with CeO₂. This topic, being highly relevant, has been described in an insignificant manner by the authors in the introduction and subsequently in the discussions.

Response: Thank you for your review. In this paper, the main aim is to obtain a modified aluminum oxide coating for steel, which provided protection at a preparation temperature of 600℃. The key point is that we added CeO to the formula, which compensates for the problem of slow aluminum atom diffusion caused by too low temperature. This not only reduces energy consumption but also updates the preparation temperature for the aluminum oxide coating. We also presented the entire text based on the experimental process and results.

Point 2: The experimental procedure does not seem to be coherent and involves sentences like a laboratory manual. Readability of the article is highly questionable.

Response: Thank you for your detailed advice. I have made the suggested revisions.

Point 3: What is the significance of Fig. 2, and the reason behind its poor representation?

Response: In fact, the figure 2 aimed to illustrate that due to the deviation of Ce atoms from the crystal boundary, lattice distortion occurs, thereby becoming a rapid diffusion channel for aluminum atoms. To avoid causing ambiguity, we have delect this figure.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors mostly took into account the comments on the first version of the article, provided appropriate explanations and made corrections to the manuscript. However, regarding the comment 5, the first sentence in the Conclusions was not rewritten, although this is stated in the reply to the reviewer. This means that in Conlusions, instead of "This formulation", an explanation should be given, for example, such as “This formulation,namely the use of the 30%Al-66%Al2O3-3%NH4Cl-1%CeO2 composition, successfully deposited aluminide coatings on…” and further along the text.

The second point concerns the mentions of NiAl alloy in the text. The phrase “Given that Al diffusion rates in nickel-based systems exceed Ni diffusion at 950 ºС [15], Ce presence is confirmed to enhance Al interdiffusion into the substrate” in lines 135-136 is unclear. What does Ni have to do with it, if we are talking about a titanium-based TiAl alloy? Line 148 mentions Ni atoms again. In addition, Ref [15] concerns the NiAl alloy, but not TiAl. If the authors assume that similar processes are taking place in both the NiAl and TiAl cases, then it is necessary to write about it.

Author Response

Response to Reviewer 1 Comments

Point 1: The first sentence in the Conclusions was not rewritten, although this is stated in the reply to the reviewer.

Response: Thank for your detail review. We have added this revised description in this paper.

This formulation,namely the use of the 30%Al-66%Al₂O₃-3%NH₄Cl-1%CeO₂ composition, successfully deposited aluminide coatings on γ-TiAl intermetallic compounds at 600℃ for 5 h.

Point 2: The second point concerns the mentions of NiAl alloy in the text. The phrase “Given that Al diffusion rates in nickel-based systems exceed Ni diffusion at 950 ºС [16], Ce presence is confirmed to enhance Al interdiffusion into the substrate” in lines 135-136 is unclear. What does Ni have to do with it, if we are talking about a titanium-based TiAl alloy.

Response: Thank for your important suggestion. This analysis addresses coating growth kinetics by comparing aluminizing mechanisms between TiAl and NiAl alloys. Similar to NiAl systems, deposited Al atoms diffuse into the matrix. Concurrently, elevated Ti content in the substrate drives outward Ti diffusion from high to low chemical potential regions per Fick's law—consistent with Ti6Al4V aluminization schematics [16]. Crucially, given that Al diffusion rates in nickel-based systems exceed Ni diffusion at 950 ℃, so the thickness of the coating mainly depends on the difussion of Al. In our experiment, it can see that the coating doped with CeO₂ is thicker than that without CeO₂ added,which indirecttly indicates that, Given that Al diffusion rates in nickel-based systems exceed Ni diffusion at 950 ℃, Ce presence is confirmed to enhance Al interdiffusion into the substrate. Considering the rigor, we have added these statements to this paper.

 

Figure 2. Schematic representation of aluminizing process on Ti6Al4V alloy

 

16. K.M. Doleker, T. Yener, A. Erdogan, F, etal. Effect of Si and Cr on formation of aluminide coatings on Ti6Al4V alloy by low temperature aluminizing: Wear and oxidation behavior[J]. Surface and Coatings Technology, 509 (2025) 132207.https://doi.org/10.1016/j.surfcoat.2025.132207

 

Point 3: Line 148 mentions Ni atoms again. In addition, Ref [15] concerns the NiAl alloy, but not TiAl. If the authors assume that similar processes are taking place in both the NiAl and TiAl cases, then it is necessary to write about it.

Response: Thank for your important suggestion. We have modified the line 148 about Ni and reference [17](original 15) as following. In order to avoid confusion, we have delete the content related to Ni.

17. H. Mehrer, Diffusion in Intermetallics[J]. Materials Transactions, JIM, 37 (1996). https://doi.org/10.2320/matertrans1989.37.1259

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The revisions are not up to the mark for the questions and points suggested by the reviewer.

Author Response

Response to Reviewer 3 Comments

Point 1: The revisions are not up to the mark for the questions and points suggested by the reviewer.

Response: Revised as suggested. We have continue to revise the questions and viewpoints according to the previous version.

The following reply below provides a more detailed answer to the first question. The specific revisions have been submitted in the revised manuscript.

Point 2: This topic, being highly relevant, has been described in an insignificant manner by the authors in the introduction and subsequently in the discussions.

Response: We appreciate your suggestion and have added clarifying statements. The introduction establishes γ-TiAl alloys' aerospace relevance (excellent corrosion resistance, low density, high specific strength) while noting their critical limitation: inadequate oxidation resistance above 800°C due to non-protective oxide scales. This necessitates protective coatings. We review common high-temperature coating systems (alumina, modified alumina, multilayers, thermal barriers) and cite Hao et al.'s demonstration of significantly enhanced oxidation resistance via Al₂O₃/Y₂O₃ composite coatings—inspiring our rare earth oxide approach. Existing aluminizing methods (850–1050°C) risk substrate degradation, motivating our focus on low-temperature aluminizing enhanced by rare earth additives. The discussion section analyzes γ-TiAl oxidation mechanisms, highlighting Kirkendall void formation at bilayer oxide interfaces and TiO₂-induced scale spallation. Contrastingly, aluminized specimens (surface Al: 85.63%) formed dense, adherent Al₂O₃ scales (Figures 8–9). Phase evolution studies reveal: TiAl₂ > TiAl₃ > Ti stability above 800°C, with Al diffusivity ranking TiAl > TiAl₃ > TiAl₂. At 800°C, scales comprised primarily Al₂O₃ and residual TiAl₃ (Figure 8); at 900°C, accelerated oxidation produced minor TiO₂ and promoted TiAl₃ decomposition to TiAl (Figure 9).

The following reply provides a more detailed answer to the first question. The specific revisions have been submitted in the revised draft.Point 3:. The experimental procedure does not seem to be coherent and involves sentences like a laboratory manual.

Response: Thank for your detailed suggestion. We have added some experimental procedure. The general experimental process is as follows.

Firstly, we selected titanium-aluminum alloy as the matrix, Subsequently, we cut, grind, clean and sandblast the sample. Aluminum diffusion was achieved via pack cementation (Figure 1). Pre-mixed powder components (excluding NH₄Cl activator) were ball-milled to designated wt.% ratios, dried at 400°C for 2h under inert atmosphere, then blended with NH₄Cl. Specimens embedded within this mixture in corundum crucibles (positioned at 1/3–1/2 height) underwent 600°C heat treatment in a muffle furnace followed by air cooling. Extracted samples were ultrasonically cleaned; cross-sectional specimens received protective Ni electrodeposition (NiSO₄·6H₂O 200 g/L, H₃BO₃ 40 g/L, NiCl₂ 40 g/L, Na₃C₆H₅O₇·2H₂O 120 g/L) to preserve alumina integrity during sectioning. Oxidation resistance was evaluated through 20h isothermal tests at 800°C/900°C in static air. Coating microstructure was characterized by XRD and FE-SEM/EDS analysis of surface/cross-section morphology pre-/post-oxidation.

Figure 1. Schematic cross-section view of an assembled pack box

Point 4: The experimental procedure does not seem to be coherent and involves sentences like a laboratory manual.

Response: Thank for your detailed suggestion. We have corrected the grammatical issues in the entire text.

Point 5: Readability of the article is highly questionable; hence, it is difficult to

apprehend the novel findings from the study.

Response: In this paper, we chose to add 1% CeO₂ to the powder for embedding to lower the aluminizing temperature of γ-TiAl. The specific experimental results are as follows:

1. After pack aluminizing at 950°C for 3 hours, the coating with 1% CeO₂ exhibited greater thickness than its undoped counterpart, demonstrating that cerium oxide enhances Al diffusion
2. The experiment successfully deposited a ~30-μm-thick coating on γ-TiAl at 600 °C for 5 h using a 30% Al–66% Al₂O₃–3% NH₄Cl–1% CeO₂ mixture.
3. Analysis of the coating's surface morphology and XRD pattern revealed a dense structure devoid of obvious through-thickness cracks, primarily composed of TiAl₃ phases
4. The base material and aluminized samples underwent isothermal oxidation tests at 800℃ and 900℃ for 20 hours each. Mass change measurements revealed that the aluminized specimens exhibited negligible mass gain, with oxidation kinetics significantly lower than those of the base material.
5. Examination of the post-oxidation surface morphology and cross-sectional microstructure revealed an intact Al₂O₃ film, free from cracks or apparent spallation

Point 6: What is the significance of Fig. 2, and the reason behind its poor representation?

Response: We initially proposed that orginal Figure 2 illustrates CeO₂'s mechanism: dislocating at grain boundaries induces lattice distortion, thereby creating rapid diffusion channels for Al atoms. However, due to the absence of experimental validation, we have omitted this figure to maintain rigorous scientific standards.

Author Response File: Author Response.pdf