TiAl-Based Oxidation-Resistant Hard Coatings with Different Al Contents Obtained by Vacuum-Pulse-Arc Granule Melting

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This article provides good results on detailed material analysis in obtaining a high content < 50% of Al coating in oxidation resistance of TiAl-based alloy. A vacuum-arc melting granule technique on Ti₅₀Al₄₄Nb _4.9Mo₁B _0.1 (at.%) granules was proposed for Ti₂AlNb ortho-alloys. Few comments are as follows:

1.       The abstract needs to rewrite for clarity in some areas, for example, in lines 10-12, these need to rewrite “…pulse-arc Melting of 100-μm thick granule layers or granules /powder mixture was executed by vacuum-pulse-arc discharge technique, coatings with a composition of Ti₅₄Al₄₀Nb₅Mo_0.9B_0.1 (O₁C_0.8Fe_0.2) and Ti₄₃Al₄₉Nb₅Mo_0.6B_0.1 (O₁C₁Fe _0.3) respectively in a thickness of 50-60 μm were obtained on the surface of the Ti₅₀Al₂₄Nb₂₅V₁ alloy.” Lines 12-13 also need to rewrite.

2.       How do you control well on the Al content with an aluminum content < 50% in your material preparation particularly in granules mixed with aluminum powder?  Need to briefly mention in Materials and Methods section.

3.       In Table 2, need to label at .% in pt.x, the locations coming from which Figures in SEM images, i.e. pt.1-3 from Fig 5 and Pt.4-7 from Fig. 8.

Author Response

Reviewer 1:

This article provides good results on detailed material analysis in obtaining a high content < 50% of Al coating in oxidation resistance of TiAl-based alloy. A vacuum-arc melting granule technique on Ti50Al44Nb 4.9Mo1B 0.1 (at.%) granules was proposed for Ti2AlNb ortho-alloys. Few comments are as follows:

1. The abstract needs to rewrite for clarity in some areas, for example, in lines 10-12, these need to rewrite “…pulse-arc Melting of 100-μm thick granule layers or granules /powder mixture was executed by vacuum-pulse-arc discharge technique, coatings with a composition of Ti54Al40Nb5Mo0.9B0.1 (O1C0.8Fe0.2) and Ti43Al49Nb5Mo0.6B0.1 (O1C1Fe 0.3) respectively in a thickness of 50-60 μm were obtained on the surface of the Ti50Al24Nb25V1 alloy.” Lines 12-13 also need to rewrite.

Author response:

Thank you for the comment. We have clarified and modified the abstract according to your suggestions:

“…Using original vacuum pulse-arc melting of 100-μm thick granule layers, coatings with different Al/Ti ratio and a thickness of 50-60 µm, were obtained on the surface of the Ti₅₀Al₂₅Nb₂₅ alloy. Granules Ti₅₀Al₄₄Nb_4.9Mo₁B_0.1 (at. %), 20-60 μm in size, were employed. To vary Al content, initial granules and their mixture with Al powder were used.”

2. How do you control well on the Al content with an aluminum content < 50% in your material preparation particularly in granules mixed with aluminum powder? Need to briefly mention in Materials and Methods section.

Author response:

We planned to produce coatings with 50 at.% of Al. To achieve that we prepared a mixture with higher Al content (apporx. 55 at.%) since a part of Al powder smears onto walls of cylinder.

The Materials and Methods section was modified: “To vary Al content in the coatings, either only granules or a mixture of granules with Al powder (size 10-16 μm, 5 at.% residual O₂) processed in a ball mill were used, which resulted in approximately 50 at.% of Al content in the coating.”

3. In Table 2, need to label at .% in pt.x, the locations coming from which Figures in SEM images, i.e. pt.1-3 from Fig 5 and Pt.4-7 from Fig. 8.

Author response:

Thank you for the comment. Table 2 was modified according to your suggestion.

Reviewer 2 Report

Comments and Suggestions for Authors

 

In this paper, the authors studied the oxidation resistance of TiAl-based coatings with high Al content. The coatings were deposited on Ti₂AlNb alloys using vacuum pulse-arc melting of metallic powders/powder mixtures. The work is interesting and worthy of publication subject to revision. The following comments should be considered:

1.The paper is very technical. Several abbreviations are left undefined in the manuscript, e.g., PVD (physical vapor deposition), ESD (electro spark deposition), SHS (self-propagating high temperature synthesis), etc. Although some of these are well-known acronyms, the authors should explain all abbreviations at their first mention in the manuscript.

2. In lines 104-107 the authors describe the choice of starting powders used to prepare the coatings. However, the Al powder contained a large amount of residual oxygen (5 at. %, line 107). It could have influenced the oxidation resistance results since some Al₂O₃ might have already been present in the as-deposited coatings. Could you please explain the presence of oxygen in this starting material? Have you not been able to get rid of it?

3.The as-prepared and as-oxidized materials are characterized by several experimental methods. However, no results are presented for the oxidation kinetics. It is a major handicap. In fact, the authors studied only two oxidation times: 10 and 100 h. It is not clear whether parabolic oxidation has been observed or not. It is very difficult to judge the long-term oxidation resistance of the coatings from two oxidation times only. It would greatly improve the quality of the manuscript if some kinetic data, e.g. specific weight gains of coated and uncoated alloys at several different annealing times, were provided.

4.The oxide scale observed after 100 h of oxidation is relatively complex. A thin external alumina scale was formed (Fig. 10 b) which might have provided a sufficient protection. However, the inner oxide layer was heterogeneous as it was a mixture of Al₂O₃ and TiO₂. You should provide a scale bar in Figs. 10b, c, and d to clearly identify the thickness of the external alumina scale. Furthermore, you should provide the EDS element maps for the uncoated alloy and for the coating oxidized at 10 h to yield a reasonable comparison. Fig. 8 does not provide any chemical information. As such, it is difficult to judge the homogeneity and chemical composition of the oxide scales.

5.The mechanism of formation of the external alumina scale (Fig. 10b) should be adequately discussed. Results should be compared with previous studies.

Author Response

Reviewer 2:

In this paper, the authors studied the oxidation resistance of TiAl-based coatings with high Al content. The coatings were deposited on Ti2AlNb alloys using vacuum pulse-arc melting of metallic powders/powder mixtures. The work is interesting and worthy of publication subject to revision. The following comments should be considered:

1.The paper is very technical. Several abbreviations are left undefined in the manuscript, e.g., PVD (physical vapor deposition), ESD (electro spark deposition), SHS (self-propagating high temperature synthesis), etc. Although some of these are well-known acronyms, the authors should explain all abbreviations at their first mention in the manuscript.

Author response:

Thank you for the comment. We have provided all abbreviations during their first mention in the manuscript.

2. In lines 104-107 the authors describe the choice of starting powders used to prepare the coatings. However, the Al powder contained a large amount of residual oxygen (5 at. %, line 107). It could have influenced the oxidation resistance results since some Al2O3 might have already been present in the as-deposited coatings. Could you please explain the presence of oxygen in this starting material? Have you not been able to get rid of it?

Author response:

Oxygen in the initial powders is due to the intrinsic oxide present of the surface of Al powder. We could not get rid of it, and the Al content in the coatings is up to 1.0 at.%. We agree that residual oxygen might have influenced the oxidation resistance results somehow, however, we did not study that matter.

3.The as-prepared and as-oxidized materials are characterized by several experimental methods. However, no results are presented for the oxidation kinetics. It is a major handicap. In fact, the authors studied only two oxidation times: 10 and 100 h. It is not clear whether parabolic oxidation has been observed or not. It is very difficult to judge the long-term oxidation resistance of the coatings from two oxidation times only. It would greatly improve the quality of the manuscript if some kinetic data, e.g. specific weight gains of coated and uncoated alloys at several different annealing times, were provided.

Author response:

Thank you for comment. We agree that kinetics investigation would significantly enhance the quality of the manuscript, however, the aim of the paper was the development of a new deposition technology, and oxidation part was to demonstrate the technology capabilities.

4.The oxide scale observed after 100 h of oxidation is relatively complex. A thin external alumina scale was formed (Fig. 10 b) which might have provided a sufficient protection. However, the inner oxide layer was heterogeneous as it was a mixture of Al2O3 and TiO2. You should provide a scale bar in Figs. 10b, c, and d to clearly identify the thickness of the external alumina scale. Furthermore, you should provide the EDS element maps for the uncoated alloy and for the coating oxidized at 10 h to yield a reasonable comparison. Fig. 8 does not provide any chemical information. As such, it is difficult to judge the homogeneity and chemical composition of the oxide scales.

Author response:

Thank you for the valuable suggestion, we believe that it may solidify the oxidation behavior results, however, such a detailed investigation was outside the scope of the manuscript and will be realized in future studies along with kinetics investigations.

Scale bars we added to the Fig. 10b, c, and d.

5.The mechanism of formation of the external alumina scale (Fig. 10b) should be adequately discussed. Results should be compared with previous studies.

Author response:

Thank you for this comment. The mechanism of external Alumina layer was additionally discussed in the end of Section 3.2:

“The formation of a thin surface layer of alumina can be explained as follows. The oxide layer is mostly based on a mixed oxide of titanium and aluminum, formed due to the close values of the Gibbs free energy [40]. This oxide does not act as an effective barrier to oxygen diffusion [41], [42]. The oxidation resistance of TiAl intermetallics can be improved by alloying with Zr, Nb, Mo, Hf, Ta, W, and Re [17].

At the initial moment of oxidation, a mixed oxide of TiO2 and Al2O3 is formed. As the oxide thickness increases, oxygen atoms penetrate the mixed oxide and, as a result of counter-diffusion, react with the more active Al, forming Al2O3 within the oxide layer.

The introduction of Nb into TiAl reduces the probability of Al2O3 formation compared to TiO2 [43]. As a result, the formation of Al2O3 within the oxide layer is suppressed and Al atoms have the opportunity to reach the surface and form a dense surface oxide.”

Reviewer 3 Report

Comments and Suggestions for Authors

The study titled "contents obtained by vacuum-pulse-arc granule melting" is aimed at the production of Ti54Al40Nb5Mo0.9B0.1 (O1C0.8Fe0.2) and Ti43Al49Nb5Mo0.6B0.1 (O1C1Fe0.3) coatings with different thicknesses on Ti50Al25Nb25 material. The study contains remarkable findings. It seems to have high originality for readers. However, the hesitations listed below must be eliminated.

1.        The originality of the work should be well emphasized in the paragraph on line 91 of the introduction section. The motivation for the study should be explained in concrete terms.

2.        In line 106., Was chemical contamination or oxidation analysis performed on the powders after the "arc plasma spheroidization of SHS" process?

3.        The "scheme of melting process" representation in Figure 2 (g) is very well thought out, but the figure does not fully make sense.

4.        In line 170, "Individual cracks are seen in the sections, some of which extend to the surface. It is noteworthy that these cracks do not spread deep into the sublayer." According to the statement, it would be appropriate to mark the existence of the cracks in question on Figure 2. The possible outcome that causes this needs to be interpreted.

5.        The definition and properties of the orthopahse or ortho alloys mentioned in the article need to be well defined. What could be the approach to originality in this work?

6.        Ti, Al, Nb intermetallics revealed by XRD. An attempt has been made to prove it through phase diagrams in Figure 4. References need to be added to these diagrams.

7.        Figure 5 shows crack-like conditions under the coating. Comments can be made about this.

8.        In the "3.2. Annealing and Oxidation" section, information can be given about the standard enthalpies of the compounds of the resulting phases.

9.        It would be appropriate to enrich the "Results" section with items. More results have been achieved.

Author Response

Reviewer 3:

The study titled "contents obtained by vacuum-pulse-arc granule melting" is aimed at the production of Ti54Al40Nb5Mo0.9B0.1 (O1C0.8Fe0.2) and Ti43Al49Nb5Mo0.6B0.1 (O1C1Fe0.3) coatings with different thicknesses on Ti50Al25Nb25 material. The study contains remarkable findings. It seems to have high originality for readers. However, the hesitations listed below must be eliminated.

1. The originality of the work should be well emphasized in the paragraph on line 91 of the introduction section. The motivation for the study should be explained in concrete terms.

Author response: Thank you for the comment. The introduction was rewritten to emphasize the originality and motivation: “The originality of this work lies in the development of a non-contact pulsed melting method for the formation of Al-rich coatings on Ti-Al-Nb alloy. This method is unique in that it avoids the use of traditional arc melting techniques, which can lead to undesirable defects and inhomogeneities in the coating. The use of granules containing Nb, Mo, and B ensures that the composition of the deposited layer is well-controlled, while the mixing of pure Al powder with the granules allows for the intentional increase in Al content in the coating. This approach is particularly beneficial for applications where high Al content is required for improved oxidation resistance or mechanical properties.”

 

2. In line 106., Was chemical contamination or oxidation analysis performed on the powders after the "arc plasma spheroidization of SHS" process?

Author response:

 

3. The "scheme of melting process" representation in Figure 2 (g) is very well thought out, but the figure does not fully make sense.

Author response:

We believe that scheme of melting process in Figure 2 is essential and helps the readers to understand the interaction of arc discharge and granules and how it affects the resulting coating formation.

4. In line 170, "Individual cracks are seen in the sections, some of which extend to the surface. It is noteworthy that these cracks do not spread deep into the sublayer." According to the statement, it would be appropriate to mark the existence of the cracks in question on Figure 2. The possible outcome that causes this needs to be interpreted.

Author response:

Thank you for the comment. The cracks in the Figure 2 were marked; possible outcomes of their formation were discussed as well. ” The observed cracks can negatively affect the oxidation resistance.”

5. The definition and properties of the orthopahse or ortho alloys mentioned in the article need to be well defined. What could be the approach to originality in this work?

Author response:

We used commercially available ortho-alloy based on Ti₂AlNb orthophase (90%) with inclusions of bcc β₂ phase. Orthorhombic alloys have higher values of low- and high-temperature specific strength and ductility compared to α2-alloys (Ti₃Al) and γ-alloys (TiAl).

6. Ti, Al, Nb intermetallics revealed by XRD. An attempt has been made to prove it through phase diagrams in Figure 4. References need to be added to these diagrams.

Author response:

Thank you for the comment; we added all necessary references to phase diagrams.

7. Figure 5 shows crack-like conditions under the coating. Comments can be made about this.

Author response:

The cracks observed after oxidation in Fig. 5 are those demonstrated for the initial as-deposited coatings and may potentially deteriorate the oxidation resistance upon very long-term annealing of more than 1000 h.

8. In the "3.2. Annealing and Oxidation" section, information can be given about the standard enthalpies of the compounds of the resulting phases.

Author response:

Thank you for the comment. We have added necessary values of Gibbs free energy for resulting phases. “The formation of a thin surface layer of alumina can be explained as follows. The oxide layer is mostly based on a mixed oxide of titanium and aluminum, formed due to the close values of the Gibbs free energy (-780 and -850 kJ/mol, respectively) [40,41].”

9. It would be appropriate to enrich the "Results" section with items. More results have been achieved.

Author response:

The aim of the present work was to demonstrate the novel non-contact coating deposition technology. The potential of the proposed technology was approved by depositing oxidation resistant TiAlNb-based coating and provided results are sufficient to exhibit its prospectives. Further detailed studies of the oxidation resistance of TiAlNb-based coatings will be conducted in future.      

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors answered my previous comments in the cover letter. The discussion has been improved. The paper is acceptable for publication in its current form.

Reviewer 3 Report

Comments and Suggestions for Authors

All the requested corrections were made in the study titled "TiAl-based oxidation-resistant hard coatings with different Al contents obtained by vacuum-pulse-arc granule melting". The study is acceptable in current state.