Slurry Aluminizing of Nickel Electroless Coated Nickel-Based Superalloy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper studies the nickel electroless coated nickel-based  superalloy, aiming to improve the surface properties. But some problems should be reivsed. 

1. In the abstract, some major results are missing, and the author should add the effect results of different parameters on surface properties of superalloys.
2. In the introduction, why to realize the surface properties of superalloys by coating, and the reason should be dicussed in detail.  In addition, the comparison among these coating technologies should conducted to give the importance.
3. The layer thickness of coated surface should be analyzed to illustrate the parameter effect.
4. The deposit mechanism of Ni electroless layer shoud be given by comparing them with other technologies.
5. The phase transition effect from heating treatment should also be analyzed, and the relationship  between phase and microstructure should be anlyzed.
6. The properties of coated layers including hardness should be analyzed.

Comments on the Quality of English Language

The paper studies the nickel electroless coated nickel-based  superalloy, aiming to improve the surface properties. But some problems should be reivsed. 

1. In the abstract, some major results are missing, and the author should add the effect results of different parameters on surface properties of superalloys.
2. In the introduction, why to realize the surface properties of superalloys by coating, and the reason should be dicussed in detail.  In addition, the comparison among these coating technologies should conducted to give the importance.
3. The layer thickness of coated surface should be analyzed to illustrate the parameter effect.
4. The deposit mechanism of Ni electroless layer shoud be given by comparing them with other technologies.
5. The phase transition effect from heating treatment should also be analyzed, and the relationship  between phase and microstructure should be anlyzed.
6. The properties of coated layers including hardness should be analyzed.

Author Response

Comments 1: In the abstract, some major results are missing, and the author should add the effect results of different parameters on surface properties of superalloys.

Response 1: Thank you for this comment. We have completely revised the abstract.

 

Comments 2: In the introduction, why to realize the surface properties of superalloys by coating, and the reason should be discussed in detail. In addition, the comparison among these coating technologies should conducted to give the importance

Response 2: Thank you for the suggestion. We are not entirely sure about the specific point you would like us to address. In the introduction, we already wrote that “However, their resistance against corrosion/oxidation is limited and thus protective aluminide coatings are often applied on the surface [4–7]. Many processes are commonly employed to elaborate aluminium diffusion coatings”. We have repeated again “protective” in the sentence “Many processes are commonly employed to elaborate protective aluminium diffusion coatings”

Comments 3: The layer thickness of coated surface should be analyzed to illustrate the parameter effect.

Response 3: The thickness of the coating is described and discussed in figures 7 and 8 and the associated paragraphs.

 

Comments 4: The deposit mechanism of Ni electroless layer should be given by comparing them with other technologies.

Response 4: The electroless deposition mechanism is described in the article of our co-authors (Ref 28).

 

Comments 5: The phase transition effect from heating treatment should also be analyzed, and the relationship between phase and microstructure should be analyzed.

Response 5: The effect of phase transitions induced by the heat treatment, as well as the relationship between phase evolution and microstructure, have been analyzed and discussed in Sections 3.4 and 4 of the revised manuscript. In particular, we describe how the increase in temperature from 700 °C to 1080 °C promotes the transformation of the δ-Ni₂Al₃ phase into β-NiAl, as confirmed by both XRD and EDS analyses (see Figs. 3–4). This phase transition strongly affects the microstructure, leading to the development of a precipitate-free β-NiAl outer layer and to the formation of Kirkendall-type porosity due to unbalanced interdiffusion between Ni and Al. Moreover, the relationship between the diffusion-controlled phase transformations and the final coating morphology is further detailed in the Discussion (Section 4) and summarized schematically in Figures 6–7.

 

Comments 6: The properties of coated layers including hardness should be analyzed.

Response 6: We thank the reviewer for this helpful comment. The present work focuses primarily on the formation mechanisms and microstructural evolution of the aluminide coatings rather than on their mechanical characterization. Therefore, hardness measurements were not included at this stage, but it could be great to add measurements in future works.

 

4. Response to Comments on the Quality of English Language

Point 1: The English could be improved to more clearly express the research.

Response 1: We have improved the syntax of some sentences. The changes made are highlighted in the revised version.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for the opportunity to review your manuscript on slurry aluminizing applied to an electroless nickel pre-layer on René N5. The work shows that the Ni pre-layer steers the formation toward a low-activity (LA) microstructure, that the Ni thickness controls the thickness of precipitate-free β-NiAl, and that Kirkendall porosity is associated with the δ-Ni₂Al₃ → β-NiAl transition, being mitigated by reducing the ramp from 20 to 5 °C·min⁻¹. I propose the following changes:

• Add a minimal block of functional testing isotermal and cyclic oxidation (e.g., 1000–1100 °C, TGA), adhesion (scratch or four-point bend), and thermal-shock testing. At present the superiority of the LA layer is inferred from microstructure; these tests would demonstrate it.
• Include advanced phase/texture and chemistry confirmation EBSD (phase/orientation) to corroborate δ-Ni₂Al₃ at 700 °C and its transformation to β-NiAl at 1080 °C; local TEM across the porous band to document vacancy nucleation during δ→β; and WDS to refine the assignment of TCP precipitates (W/Ta/Mo/Re).
• Apply thermo-kinetic modelling use CALPHAD/DICTRA to simulate Al/Ni profiles, fluxes, and vacancies during δ→β for 20 vs 5 °C·min⁻¹ and, if possible, complement with DSC or in-situ XRD. The difference in the Ni slope in the EDS profiles (Fig. 5) suggests more balanced interdiffusion with the slower ramp; modelling would close the proposed Kirkendall mechanism.
• Assess external validity repeat a representative condition (e.g., intermediate Ni electroless + 5 °C·min⁻¹ ramp) on another Ni-base superalloy (René 80, CMSX-4) and/or on a complex geometry to consolidate external validity. The current study uses single-crystal René N5 (Table 1) and flat coupons.
• Specify key methodological detail: Ar purity, residual O₂/dew point, and pre-purge in TGA; the method/precision for slurry loading (weighing before/after, mg·cm⁻², uncertainty); and grit-blasting parameters (mesh, pressure, standoff, time) to avoid surface artefacts.

The manuscript makes a useful contribution to coating engineering; with these additions, your conclusions would gain in quantitative robustness and transferability.

Author Response

Comments 1: Add a minimal block of functional testing isothermal and cyclic oxidation (e.g., 1000–1100 °C, TGA), adhesion (scratch or four-point bend), and thermal-shock testing. At present the superiority of the LA layer is inferred from microstructure; these tests would demonstrate it.

Response 1: The main objective of this study was to understand the mechanisms of coating formation with electroless nickel and slurry aluminizing, particularly Kirkendall porosity formation. Oxidation testing of these optimized coatings will be the subject of another complete article to be submitted.

Comments 2: Include advanced phase/texture and chemistry confirmation EBSD (phase/orientation) to corroborate δ-Ni₂Al₃ at 700 °C and its transformation to β-NiAl at 1080 °C; local TEM across the porous band to document vacancy nucleation during δ→β; and WDS to refine the assignment of TCP precipitates (W/Ta/Mo/Re).

Response 2: We appreciate this suggestion. The δ-Ni₂Al₃ phase at 700°C and β-NiAl at 1080°C were confirmed by X-ray diffraction (Fig. 3.a and text for Fig. 4.d) and by EDS analysis showing the Al/Ni ratios (Fig. 4.d and Fig. 5.c-d). We agree that EBSD, TEM, and WDS would provide more details about crystallography and chemistry. However, these techniques were not available in our laboratory during this study. The TCP precipitates rich in refractory elements (W, Ta, Mo, Re, Cr) are consistent with the bright contrast in BSE images and with previous works on aluminized Ni-based superalloys (references [34-36]).

 

Comments 3: Apply thermo-kinetic modelling use CALPHAD/DICTRA to simulate Al/Ni profiles, fluxes, and vacancies during δ→β for 20 vs 5 °C·min⁻¹ and, if possible, complement with DSC or in-situ XRD. The difference in the Ni slope in the EDS profiles (Fig. 5) suggests more balanced interdiffusion with the slower ramp; modelling would close the proposed Kirkendall mechanism.

Response 3: We agree that CALPHAD/DICTRA modelling would help to explain the difference in diffusion between 20 and 5°C/min heating rates. Unfortunately, we do not have access to the computational tools and expertise needed for this type of simulation. The same limitation applies for in-situ XRD experiments during heating. However, our EDS results clearly show more balanced interdiffusion with slower heating rate, as demonstrated by the softer Ni gradient at 5°C/min compared to 20°C/min (Fig. 5). This experimental evidence, combined with the strong reduction in porosity, supports our proposed mechanism.

 

Comments 4: Assess external validity repeat a representative condition (e.g., intermediate Ni electroless + 5 °C·min⁻¹ ramp) on another Ni-base superalloy (René 80, CMSX-4) and/or on a complex geometry to consolidate external validity. The current study uses single-crystal René N5 (Table 1) and flat coupons.

Response 4: This is a good suggestion. In this study, we used single-crystal René N5 flat samples to establish mechanisms under controlled conditions. However, we recognize that testing on other alloys (René 80, CMSX-4) and complex geometries is necessary to confirm industrial applicability. The advantage of electroless deposition is that it can coat complex shapes uniformly, which makes this validation important. We have mentioned this limitation in the revised manuscript and proposed it as future work.

Comments 5: Specify key methodological detail: Ar purity, residual O₂/dew point, and pre-purge in TGA; the method/precision for slurry loading (weighing before/after, mg·cm⁻², uncertainty); and grit-blasting parameters (mesh, pressure, standoff, time) to avoid surface artefacts.

Response 5: We thank the reviewer for this comment. We have added the following details in Section 2 as follow “All heat treatments were conducted under high-purity argon atmosphere (Alphagaz 1 grade from Air Liquide, 99.999% purity, residual O₂ < 3 ppm, H₂O < 3 ppm) in a thermobalance (TGA-92, SETARAM). The TGA chamber was systematically purged for 30 minutes at 20 mL·min⁻¹ before each treatment to minimize oxygen contamination. The amount of slurry deposited on each sample was determined by weighing the samples before and after spraying using an analytical balance with a precision of ±0.1 mg. The amount of slurry sprayed was then calculated as the mass gain divided by the sample surface area and expressed in mg·cm⁻² with the uncertainties reported in Table 3. The slurry residue left over the aluminized surface after annealing was manually grit-blasted with alumina particles (220 mesh, pressure 1 bar) in a SANDMASTER FG-94 apparatus. The blasting was performed at grazing angle to ensure uniform removal of the slurry residue without damaging the underlying coating. Cross-sectional observations confirmed that this grit-blasting procedure did not cause any surface damage to the aluminide coatings.

4. Response to Comments on the Quality of English Language

Point 1: The English is fine and does not require any improvement.

Response 1: Thank you, we have, however, improved some sentences. The changes made are highlighted in the revised version.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Slurry Aluminizing of Nickel Electroless Coated Nickel-Based  Superalloy is very interesting paper. The mechanisms of formation of low-activity nickel aluminide coatings on nickel-based superalloy from a pre-deposition of a pure nickel electroless layer and subsequent slurry aluminizing were studied. Minor improvements are required.

Line 15, 16: The mechanisms of formation of low-activity nickel aluminide coatings on nickel-based superalloy from a pre-deposition of a pure nickel electroless layer and subsequent slurry aluminizing were studied (In what temperature interval)

Line 26:  Increasing the amount of slurry (what is chemical composition of slurry) increases the overall coating thickness

Line 125: In table 3 you can add  also the unit: Heating rate (°C/min)  Dwell (°C/1h)

Page 5: Coatings 2025, 15, x FOR PEER REVIEW 2 of 14 (please to change: 2 of 5; the correct value: 5 of 14. Please to check the number of all pages)

Line 168: However, the microstructure of the two coatings depends on the amount of slurry sprayed on (in what reaaction time)

Page 6: Coatings 2025, 15, x FOR PEER REVIEW 3 of 14 (Please to change it in: 6 of 14)

Line 247: In essence, while the EDS profiles of  Al and of the alloying elements (such as…) are quite similar for both heating rates,

Line 269: making aluminium (powder) the limiting reactant

Conclusion:

Line 333, 334: Adjustments  in the thickness of the Ni-electroless pre-deposit and the quantity of slurry allows to control the overall coating thickness ( in what range?)

General question:

Line 278: Kirkendall-type porosity appears at the interface due to the unbalanced flow  of outward Ni atoms! Can you explain the influence of diffusion of oxygen on porosity  in this system?

Author Response

Comments 1: Line 15, 16: The mechanisms of formation of low-activity nickel aluminide coatings on nickel-based superalloy from a pre-deposition of a pure nickel electroless layer and subsequent slurry aluminizing were studied (In what temperature interval).

Response 1: Thank you for this suggestion. We have added the two temperatures studied in the abstract.

 

Comments 2: Line 26: Increasing the amount of slurry (what is chemical composition of slurry) increases the overall coating thickness

Response 2: We do not consider it necessary to detail the chemical composition of the slurry in the abstract because the number of characters is limited in the abstract.

 

Comments 3: Line 125: In table 3 you can add also the unit: Heating rate (°C/min) Dwell (°C/1h).

Response 3: Thank you for this suggestion. We have added the unit to the heating rate column. However, we believe that keeping the last column (dwell) unchanged ensures better clarity.

Comments 4: Page 5: Coatings 2025, 15, x FOR PEER REVIEW 2 of 14 (please to change: 2 of 5; the correct value: 5 of 14. Please to check the number of all pages)

Response 4: Thank you for this kind suggestion. The issue has been fixed accordingly.

 

Comments 5: Line 168: However, the microstructure of the two coatings depends on the amount of slurry sprayed on (in what reaction time)

Response 5: We have added the dwell time at 1080°C in the sentence: “However, the microstructure of both coatings depends on the amount of slurry sprayed under the same heat treatment conditions (i.e., 1080 °C for 1 hour).”

 

Comments 6: Page 6: Coatings 2025, 15, x FOR PEER REVIEW 3 of 14 (Please to change it in: 6 of 14)

Response 6: The issue has been fixed accordingly.

 

Comments 7: Line 247: In essence, while the EDS profiles of Al and of the alloying elements (such as…) are quite similar for both heating rates,

Response 7: We have changed this sentence for “In essence, the EDS profiles of both Al and Ni exhibit notable differences between the two heating rates in the outermost 20 µm of the coating, while the profiles of the other alloying elements (W, Re, Mo, Co, Cr and Ta) remain quite similar.”

 

Comments 8: Line 269: making aluminium (powder) the limiting reactant

Response 8: We have added your suggestion, thank you.

Comments 9: Line 333, 334: Adjustments in the thickness of the Ni-electroless pre-deposit and the quantity of slurry allows to control the overall coating thickness (in what range?)

Response 9: We have added “and the microstructure” at the end of the sentence.

General question: Line 278: Kirkendall-type porosity appears at the interface due to the unbalanced flow of outward Ni atoms! Can you explain the influence of diffusion of oxygen on porosity in this system?

Response to general question: Regarding the potential role of oxygen in the formation of Kirkendall porosity, we believe that oxygen is not responsible for the observed voids based on several experimental observations. First, all heat treatments were conducted under controlled argon atmosphere in a thermobalance, which significantly limits oxygen partial pressure and minimizes oxidation phenomena. More importantly, the strong correlation between heating rate and porosity formation contradicts an oxygen-driven mechanism. Indeed, reducing the heating rate from 20 to 5°C/min substantially decreases void formation, whereas a slower heating rate would increase the exposure time to any residual oxygen trace, potentially increases porosity if oxygen were the main contributor. Additionally, no porosity was observed during the formation of the δ-Ni₂Al₃ phase at 700°C, despite this temperature being sufficient for aluminum oxidation if oxygen were present in significant amounts. The voids appear exclusively during the δ-Ni₂Al₃ to β-NiAl transformation, which is consistent with the well-documented unbalanced diffusion mechanism between Al and Ni in this system. The softer compositional gradients observed with slower heating rates, as evidenced by EDS profiles, further support that porosity is controlled by diffusion kinetics rather than oxidation phenomena. These observations collectively indicate that the Kirkendall porosity formation is primarily governed by the intrinsic diffusion imbalance between aluminum and nickel during phase transformation, with negligible contribution from oxygen under our experimental conditions.

 

4. Response to Comments on the Quality of English Language

Point 1: The English is fine and does not require any improvement.

Response 1: Thank you, we have, however, improved some sentences. The changes made are highlighted in the revised version.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscrip has solved some problems, and it can be published.

Comments on the Quality of English Language

The revised manuscrip has solved some problems, and it can be published.