Impact of Heat Treatment on the Microstructure and Properties of Ti–Al Composite Coatings with Formation of a TiAl₃ Intermetallic Phase

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.     In the abstract, what is “Ti-Al₃ phase”? Is it TiAl₃ phase or Al₃Ti phase?

2.     Line 85, “(LPCS) )”??

3.     In the manuscript, “fig.” should be “Fig.”.

4.     In Line 140, “Structure examinations” should be “Phase identification”.

5.     For the XRD, what is the XRD patten is used for the phase TiAl3? D022, D023 or L12?

6.     In Lines 98-100, it is the heat treatment of without a protective atmosphere. Since either Al or Ti is very active to Oxygen, will this experiment induce a high amount of Oxygen to the Al-Ti system? If so, what is the effect of it on the coating formation, or to the related wear properties?

7.     From Fig.8 (The scale bar is not clear), the Ti3Al phase only forms at the outside layer of Ti powder. Both Ti and Al powders are clearly in the coating. In Fig. 10, where is the “formed TiAl3 layer”? Please label it in the image.

8.     In Fig.10d, what is the “solved diffraction”? Is it a standard terminology? Besides, what is the crystal structure of TiAl3 here?

9.     In Line 308, “TiAl3 particles”. In the former part, they are called layers, not particles.

10.  In the “Tribological testing”, is it any effect from abrasive wear? Do you make any XPS to identify the TiO2 or Al2O3?

11.  “This reduction in friction coefficient can be attributed to the increased hardness and presence of TiAl₃”. Such claim has the weak logic link. How to corelate the hard phase with the reduced friction coefficient? The hard phase is normally brittle. Will it be helpful to the wear?

Comments on the Quality of English Language

Many type errors or format problems. 

Author Response

Thank you very much for taking the time to review this manuscript and for your valuable comments, which will allow for more accurate phrasing of the research results in the future. All comments have been corrected in the text of the article. Below are the answers to each question.

1. In the abstract, what is the “Ti-Al₃ phase”? Is it the TiAl₃ phase or the Al₃Ti phase?
   Response: The article refers to the possibility of obtaining the TiAl₃ phase due to its superior mechanical properties. Studies using XRD methods and transmission electron microscopy confirmed the presence of the TiAl₃ phase. The recorded phase was identified as TiAl₃ with a tetragonal structure, with parameters a=b=3.82 Å, c=8.56 Å, and a zone axis of 010. These studies unequivocally confirmed the presence of the TiAl₃ phase.
2. Line 85, “(LPCS) )”??

Response: LPCS - low pressure cold spray, was explained earlier in the introduction, in line 32.

3. In the manuscript, “fig.” should be “Fig.”

Response: Corrected in the text.

4. In line 140, “Structure exams” should be “Phase identification”.

Response: Corrected in the text.

5. What XRD pattern is used for the TiAl₃ phase in the case of XRD? D022, D023, or L12?
Response: According to the literature, the XRD pattern for the TiAl₃ phase is D022 - a tetragonal structure. This has been added and explained in the text.

6. In lines 98-100, this is heat treatment without a protective atmosphere. Since both Al and Ti are highly reactive with oxygen, will this experiment result in a high amount of oxygen in the Al-Ti system? If so, what is its impact on coating formation or related wear properties?
Response: The authors were aware of the potential impact of an oxygen atmosphere leading to a high amount of oxygen. In the repeated experiments at the selected temperature and with the specified amount of modifying phases, no oxide phases were detected. In the future, research will be expanded to examine the effects of a protective atmosphere on the formation of this phase.

7. In Fig. 8 (the scale is unclear), the TiAl₃ phase forms only on the outer layer of the Ti powder. Both Ti and Al powders are clearly visible in the coating. Where is the “formed TiAl₃ layer” in Fig. 10? Please mark it in the image.

Response: The scale in Fig. 8 has been corrected. The entire area in Fig. 10 consists of the TiAl₃ phase, as it was created in the cross-section (FIB).

8. What is “resolved diffraction” in Fig. 10d? Is this standard terminology? Additionally, what is the crystal structure of TiAl₃ here?

Response: The description and solution of the diffraction registered using transmission electron microscopy have been corrected in the article.

9. In line 308, “TiAl₃ particles”. In the previous section, they are referred to as layers, not particles.
Response: Corrected in the text.

10. is there any effect of abrasive wear in the “tribological tests”? Did you perform any XPS calculations to identify TiO₂ or Al₂O₃?

Response: The tribological tests primarily focused on the sliding wear behavior of the Ti-Al composite coatings. While the observed wear was mostly plastic deformation, as mentioned in the article, there was also evidence of increased abrasive wear, especially in the Ti60Al40 OC samples. This suggests that abrasive wear contributed to the overall wear mechanism. However, we did not perform XPS to specifically identify the presence of TiO₂ or Al₂O₃. The surface analysis was limited to SEM and EDS, which confirmed the chemical composition of the coatings before and after heat treatment. If necessary, XPS could be considered in future research to further investigate potential oxidation products.

11. “This reduction in the coefficient of friction can be attributed to the increased hardness and presence of TiAl₃.” Such a statement has weak logical correlation. How can you connect a hard phase with a reduced coefficient of friction? A hard phase is usually brittle. Does this help with wear?
Response: You raise a valid point regarding the logical link between hardness and friction reduction. While it is true that a hard phase such as TiAl₃ is brittle, its contribution to wear resistance and the reduction of the friction coefficient can be explained through several factors.
Firstly, the increased hardness leads to a reduction in the contact surface area during sliding, as the ball in the tribological test indents the coating less. This smaller contact area reduces friction by lowering the interaction between the surfaces. Essentially, the harder surface deforms less under load, and the ball does not embed into the coating as deeply, which minimizes the overall resistance during motion.
Additionally, hardness also plays a role in reducing the wear of the ball itself, as described in the referenced work (coatings-3250790). The hard phase, particularly TiAl₃, helps to form a tribological film during sliding. This film can act as a protective layer, reducing direct contact between the sliding surfaces and thereby decreasing the friction coefficient. This behavior aligns with findings in other studies, where hard coatings have been shown to reduce both surface wear and friction due to their ability to form such protective layers. Thus, while TiAl₃ is a brittle phase, its presence in the aluminum matrix contributes to wear resistance and friction reduction through a combination of increased hardness, reduced contact area, and the formation of protective films during the sliding process.

Reviewer 2 Report

Comments and Suggestions for Authors

Review Report for: Impact of Heat Treatment on the Microstructure and Properties  of Ti-Al Composite Coatings with Formation of TiAl₃ Intermetallic Phase

“This paper proposes the regeneration of structural components by cold spray due to the degradation. The aim of this study was to examine the effect of heat treatment on the microstructure and properties of Ti–Al coatings. The investigations covered the effect of this modification on the microstructure, phase composition, hardness and tribological properties of composite coatings applied using the low-pressure cold spray method”

I do recommend the publication after take the notes in consideration.

Please find my notes in below:

-Line 77: No vendor was mentioned about the aluminum and titanium powder

- Figure 2, 14 and 15: the images are in low resolution and low quality, need to improve.

- Figure 6 and 7: The four SEM images have different working distance which may mislead the conclusion

- Figure 8: the authors have shown that three areas that were examined by EDS. The compositions analysis is displayed. Only on the interface, there is intermetallic phase. Does this mean the bulk side of Ti and bulk side of AI have different properties? What is the area fraction of the intermetallic relative to non-intermetallic phase.

-Line 11, “In response to environmental issues and the intensive degradation of parts the interest of, the civil and military aviation industries

Comments on the Quality of English Language

n/a

Author Response

Thank you very much for taking the time to review this manuscript and for your valuable comments, which will allow for more precise phrasing of the research results in the future. All comments have been addressed and corrected in the text of the article. Below are the responses to each point.

1. Line 77: There is no mention of the supplier of the aluminum and titanium powder.
   Response: Commercially available powders were used for the study, without information on the manufacturer.
2. Figures 2, 14, and 15: The images are low resolution and of poor quality, they should be improved.
   Response: The images have been improved.
3. Figures 6 and 7: The four SEM images have different working distances, which may be misleading.
   Response: The difference in WD (Working Distance) is due to the height of the samples and does not affect the analyzed image. For comparison purposes, the same BSD detector and identical magnification were used.
4. Figure 8: The authors showed that three areas were examined using EDS. The compositional analysis is displayed. The intermetallic phase appears only at the interface. Does this mean that the volume of the Ti side and the volume of the Al side have different properties? What is the surface fraction of the intermetallic phase compared to the non-intermetallic phase?
   Response: Only a qualitative elemental analysis was conducted in these areas, and the surface fraction of the intermetallic phase relative to the non-intermetallic phase was not analyzed. The presence of an intermetallic phase was confirmed at the interface between the titanium and aluminum phases. Additional XRD and electron diffraction studies confirmed the presence of the intermetallic phase, and its chemical composition described in Table 2 is consistent with the range of this phase shown on the Ti-Al equilibrium diagram.
5. Line 11: “In response to environmental issues and the intensive degradation of parts, which are of interest to the civil and military aviation industries..."
   Response: The sentence has been corrected.

Reviewer 3 Report

Comments and Suggestions for Authors

Although the article is original in terms of subject matter, there are many technical and stylistic errors and these errors need to be seriously analysed and evaluated. The experimental results obtained from a technical point of view should be analysed and explained in detail in the cause-effect relationship. Some of the errors/deficiencies I have identified in the article are given below; 

1)‘80 in the size range of 20-80 μm (fig. 2B) ,’ should be in the sentence (Fig. 1B).  Page2-Line 81

2) It should be corrected as Fig.X, not ‘fig.X’ in whole manuscript.

3) In Fig 4 and 5, the figure legends should be corrected to ‘Diffraction pattern...’.

4) The description of Fig.6 should be corrected to microstructure, not ‘morphology’.

5) In the Materials method section, which Al and which Ti series are used and the chemical composition of these materials should be given. It is insufficient to say that Al and Ti alloys were used in general.

6)In the experimental study, you stated that you performed hardness examinations at 3 different loads (10g- 500g-1000g), but in the experimental results there are only results obtained at 500g and 1000g. Why did you do the hardness examinations at three different loads? It would be better to determine only one load and make an examination on the values you made at this load. 

7) It is insufficient to explain the changes in hardness values only by porosity. More detailed explanation is required. (The effects of the formed phase on hardness and strength must also be mentioned)

8)It is not very appropriate to present the study with a single temperature value as ‘effect of heat treatment’. At least, it would be more appropriate to examine the changes occurring under different heat treatment conditions by changing the heat treatment times or heat treatment parameters.

Author Response

Thank you very much for taking the time to review this manuscript and for your valuable comments, which will help to further refine the research results in the future. All comments have been addressed and corrected in the text of the article. Below are the answers to each question.

1. "80 in the range of 20–80 μm (fig. 2B)" should be in the sentence (Fig. 1B). Page 2-Line 81.
   Response: This has been corrected in the text.
2. It should be corrected as Fig.X, not “fig.X” throughout the manuscript.
   Response: This has been corrected in the text.
3. In Figures 4 and 5, the figure captions should be corrected to "Diffraction pattern...".
   Response: This has been corrected in the text.
4. The caption for Figure 6 should be changed to "microstructure" instead of "morphology".
   Response: This has been corrected in the text.
5. In the Materials Methods section, you need to specify which series of Al and Ti were used and the chemical composition of these materials. Simply stating that Al and Ti alloys were commonly used is insufficient.
6. In the experimental study, you stated that you conducted hardness tests at three different loads (10 g, 500 g, 1000 g), but in the experimental results, only the results obtained at 500 g and 1000 g are provided. Why did you conduct hardness tests at three different loads? It would be better to specify only one load and provide the results obtained under that load.
   Response: The 10 g hardness test was conducted solely to determine the hardness of the TiAl₃ phase due to its very narrow area. Loads of 500 g and 1000 g were selected as they allowed consideration of the microstructural inhomogeneity and assessment of the influence of the TiAl₃ phase.
7. It is insufficient to explain the changes in hardness values solely by porosity. A more detailed explanation is required (the effect of the formed phase on hardness and strength should also be mentioned).
   Response: Yes, this is a very valid point. In addition to porosity, the uneven distribution of the formed TiAl₃ phase also affects the hardness changes. This has been added to the text.
8. Presenting a study with just one temperature value as the "effect of heat treatment" is not entirely appropriate. It would be more suitable to investigate the changes under various heat treatment conditions by altering the heat treatment time or parameters.
   Response: The article describes the effect of a single temperature value, which was chosen based on a literature review (cited in the article: T. Novoselova, S. Celotto, R. Morgan, P. Fox, W. Neill, Formation of TiAl intermetallics by heat treatment of cold-sprayed precursor deposits, J. Alloys Compd, Vol.436(1-2), pp.69-77, 2007.). Preliminary studies confirmed that within this range, it is possible to obtain the intermetallic phase for both analyzed compositions of the composite coatings. The authors aimed to determine the effect of this phase on the properties of the Ti-Al composite coatings.

Reviewer 4 Report

Comments and Suggestions for Authors

Ti-Al alloys are promising intermetallics for the production of heat resistant systems with low density, high mechanical properties and good corrosion resistance for industrial applications. The manuscript reflects interesting work on titanium aluminides, but its publication could be recommended after addressing several constructive points/comments listed below.

1) The manuscript looks more like a technology report than a scientific paper. The Conclusion section does not clearly state what new information or scientific knowledge has been gained in this paper. Conclusions 1 and 2 of the manuscript repeat information that is well known. What are the crystal chemical and physical rationales for expecting correspondence between theoretical conclusions and experimental results? On page 4 of the manuscript, line 147, there is the sentence: "According to the literature on the subject, the latter peaks indicate a phase expressed by the stoichiometric formula TiAl3". What does this sentence mean?

2) Based on the Al-Ti binary phase diagram, it could be seen that the ordered TiAl3 phase either has a very narrow range of homogeneity or corresponds to a tetragonal superstructure when there is no visible range of homogeneity. In this context, the obvious weakness of the work is that the crystal chemical parameters and the compositional range were not determined. Therefore, the data shown in Figures 4 and 5 cannot be used as a basis for concluding that the ordered intermetallic phase of composition TiAl3 exists in the system. In view of the experimental results, a brief discussion of the corresponding solid phase reactions has not also been included. The analysis leading to Figure 8 and Table 2 is not sufficiently detailed in the manuscript.

Author Response

Thank you very much for taking the time to review this manuscript and for your valuable comments, which will allow for more precise phrasing of the research results in the future. All comments have been addressed and corrected in the text of the article. Below are the responses to each question.

1. The manuscript looks more like a technical report than a scientific paper. The Conclusions section does not clearly state what new information or scientific knowledge has been gained in this article. Conclusions 1 and 2 repeat information that is already well-known. What are the crystalline, chemical, and physical premises for expecting consistency between theoretical conclusions and experimental results? On page 4 of the manuscript, in line 147, there is a sentence: "According to the literature on the subject, these latter peaks indicate a phase expressed by the stoichiometric formula TiAl₃." What does this sentence mean?

Response: The article was thoroughly reviewed and significantly improved. The Conclusions section has been extensively rewritten to address the critical feedback provided by the reviewer.

1. Based on the Al-Ti binary phase diagram, it can be observed that the ordered TiAl₃ phase has a very narrow homogeneity range or corresponds to a tetragonal superstructure when there is no visible homogeneity range. In this context, an obvious weakness of the work is that the chemical parameters of the crystal and composition range were not defined. Therefore, the data shown in Figures 4 and 5 cannot be used as a basis to conclude that an ordered intermetallic phase with the composition TiAl₃ exists in the system. Additionally, in light of the experimental results, there was no brief discussion of the corresponding solid-state phase reactions. The analysis leading to Figure 8 and Table 2 is not sufficiently detailed in the manuscript.

Response: The main goal of the study was to form the intermetallic TiAl₃ phase for different chemical compositions of Ti-Al composite coatings, subjected to additional heat treatment. The research conducted using EDS, XRD, and X-ray diffraction methods aimed to conclusively determine the presence of this specific intermetallic phase. Figure 8 and Table 2 have been described in more detail in the text. According to Table 2, a perfectly tetragonal structure of TiAl₃ was formed, based on both atomic and mass fractions, as confirmed by the Ti-Al equilibrium diagram.

Reviewer 5 Report

Comments and Suggestions for Authors

The problem of using aluminum and titanium powders as a hardening coating, leading to the formation of intermetallic phases Ai3Ni, is widely known and is studied in many publications and by many authors. However, the technology of processing the coating material is described in too much detail. Such detail is not required. It is desirable to construct histograms of particle size distribution, sort them so that very small and large fractions are removed. Sort the particles by shape and select the main ones. For this reason, I believe that this article requires significant revision.

Comments on the Quality of English Language

The English language in the article is full of certain shortcomings: prepositions are often used in sentences, there is a large number of articles, and the text uses inappropriate words. Significant revision of the English text is required.

Author Response

Thank you very much for taking the time to review this manuscript and for your valuable comments, which in the future will allow more accurate rephrasing of the research results. All comments have been corrected in the text of the article. Below are the answers to each question.

1. The problem of using aluminum and titanium powders as a hardening coating, leading to the formation of intermetallic phases Ai3Ni, is widely known and is studied in many publications and by many authors. However, the technology of processing the coating material is described in too much detail. Such detail is not required. It is desirable to construct histograms of particle size distribution, sort them so that very small and large fractions are removed. Sort the particles by shape and select the main ones. For this reason, I believe that this article requires significant revision.

Ad. 1. The article was re-checked, largely improved, and at the reviewer's suggestion. Particle size studies were carried out using a granulometer (Anton Paar 1190 laser analyzer), on the basis of which the particle size was precisely determined. The studies are described in the article.

2. The English language in the article is full of certain shortcomings: prepositions are often used in sentences, there is a large number of articles, and the text uses inappropriate words. Significant revision of the English text is required.

Ad. 2. The article was reviewed by a native English speaker.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript should be fine after minor editing of English language.

Comments on the Quality of English Language

The manuscript should be fine after minor editing of English language.

Author Response

Thank you very much for taking the time to review this manuscript again and for your valuable comments.

1. The manuscript should be fine after minor editing of English language.

Ad. 1. The article was checked for a fee by a native English speaker, professionally involved in translations. It should already meet the highest standards.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have made many corrections and improvements. Although my previous opinion was negative, the improvements they have made have changed my opinion on this issue. However, I still think that there are some simple mistakes in the article which should be corrected before publication. These are;

1)"For titanium powder, D10-D90 was 16-47 μm with a mean size of 51μm (Fig. 1B)"-There is an error here, if the D10 and D90 values are as stated, the D50 value must be a value between the D10 and D90 values. 

2) It is still not specified which serial Al alloy was used. Each of the Al alloys has very different properties and chemical compositions. Therefore, which alloy is used must be analysed and specified.

3) The answer you have given regarding the hardness measurement (10g load) is satisfactory and sufficient, but you should also state this in the article.

Author Response

Thank you very much for taking the time to review this manuscript again and for your valuable comments. All comments have been corrected in the text of the article. Below are the answers to each question.

1)"For titanium powder, D10-D90 was 16-47 μm with a mean size of 51μm (Fig. 1B)"-There is an error here, if the D10 and D90 values are as stated, the D50 value must be a value between the D10 and D90 values. 

Ad. 1. There was actually an error which has been corrected in the article.

2) It is still not specified which serial Al alloy was used. Each of the Al alloys has very different properties and chemical compositions. Therefore, which alloy is used must be analysed and specified.

Ad. 2. Yes, that's a serious mistake, the research used an aluminium alloy  AA1350, this was supplemented in the article.

3) The answer you have given regarding the hardness measurement (10g load) is satisfactory and sufficient, but you should also state this in the article

Ad. 3. Thank you for this valuable comment, it was also described in the text.

Reviewer 4 Report

Comments and Suggestions for Authors

The revised paper could be considered for publication in the journal.

Author Response

1. The revised paper could be considered for publication in the journal.

Ad.1. Thank you very much for these recommendations