Effect of Vacuum Heat Treatment on the High-Temperature Oxidation Resistance of NiCrAlY Coating
Round 1
Reviewer 1 Report
The manuscript entitled Influence of Vacuum Heat Treatment on High-Temperature Oxidation Resistance of NiCrAlY Coatings is one of the novels approaches to study the thermal stability and behavior of NiCrAlY coatings produced by high-velocity oxygen fuel spraying (HVOF). MCrAlY coatings are widely used in power generation, jet engine and turbine components, and other applications where temperature and creep resistance are required. So far, improving the high-temperature oxidation resistance of NiCrAlY coatings has mainly focused on processing by particle bombardment, adding RE or other elements. However, the stage of final treatment of the applied coatings is also of great importance, e.g. vacuum VHT heat treatment, the effects of which are presented in the article. The authors investigate the mechanism of phase transitions, microstructure stability, and oxidation resistance of NiCrAlY coatings after various VHT processes in vacuum heat treatment (temperature range from 1000 to 1200 °C). The results confirmed the interesting effects that take place in NiCrAlY coatings after the VHT process. The authors indicated that with increasing VHT temperature, the content of the yttrium in the surface of the coatings increases, and the porosity of the coating is gradually eliminated (sintering effect), which has a positive effect on the resistance of the NiCrAlY coating to oxidation. Moreover, the dense microstructure of the coatings after VHT at 1200 °C contains less internal oxides, which ultimately means better resistance to oxidation. This was certainly confirmed by 21 hours of oxidation in air at 1100 °C. The authors also confirmed that the addition of yttrium (Y) can improve the morphology of coating oxides in thermal cycles. Cross-sectional observations showed a more homogeneous structure of the coating after VHT at 1200 °C. The microstructure, phase composition, and resistance to oxidation of NiCrAlY coatings (as-sprayed and after VHT) were systematically examined. The results of the tests and observations were documented using SEM, EDS, XRD, and the oxidation test (mass gain analysis). Here, I appreciate the use of more than one research technique to study the phase composition and microstructure of coatings. Considering the strengths above, though, as I read the manuscript I found some errors which I would be appreciated when correcting by the Authors.
1. The experimental part of the article should be re-edited. It is not clear whether the authors examined the coatings with the substrate (FeCrAl alloy) or the coatings removed from the substrate.
„The coating was automatically separated from the substrate when it reached a certain thickness. It was then cut into 15 mm×15 mm×0.5 mm pieces by using a surface grinder and a cutter. The coating surface was polished to metallic luster by using sandpaper (360 mesh)”.
If the authors examined only the coating, they should define the preparation of the samples more clearly.
The authors also did not clearly define the abbreviations VHT1000, VHT1100, and VHT1200 (the abbreviation VHT1200s also appears – Line 135). These are certain samples after a vacuum heat treatment temperature, but for the readers, it needs to be clarified.
2. Some sentences are unclear please re-edit the fragments and sentences.
Line 31:
The advantages of high-temperature oxidation resistance of MCrAlY coatings have been developed [6].
Line 156:
Fig. 7 shows TGO on the surface of oxide samples. The as-sprayed and VHT coatings formed a dense TGO layer. The average thickness of as-sprayed coating is about 8 μm. Is it a TGO layer? The TGO thickness of three VHT coatings is all about 5 μm.
Line 203:
When the VHT process at a cooling stage (＜950 ℃), the phase transform of γ+β→γ’+α will be conducted [30]. Therefore γ’-Ni3Al was generate in VHT coaings (Fig. 4 and 9).
3. Occasionally there are word errors in the article
Line 9:
is: high-velocity oxygen fuel spray (HVOF)
should be: high-velocity oxygen fuel spraying (HVOF)
Line 204:
is: was generate in VHT coaings
should be: was generated in VHT coatings
Line 18, 130:
is: lameller morphology
should be: lamellar morphology
Line 51:
is: the parameters of thermal spray
should be: the parameters of thermal spraying
Line 182:
is: After heat treated at different
should be: After heat treatment at different
Line 226:
is: During oxidation test, the following reactions can occur at high temperaturę stage (＞1000 ℃) in VHT coaings [14,30,35]:
should be: During the oxidation test, the following reaction can occur at high temperaturę stage (＞1000 ℃) in VHT coatings [14,30,35]:
4. Some captions under the figures and tables require clarification
Line 79:
is: Figure 1. Microstructure of the powder.
should be: Figure 1. The microstructure of the NiCrAlY powder used for HVOF spraying.
Line 81:
is: Figure 2. XRD result of feedstock powder.
should be (example): Figure 2. XRD diffractogram of feedstock NiCrAlY powder.
Line 83:
Figure 3. Cross-sectional microstructure of NiCrAlY powder at (a) low and (b) high magnifications.
Magnifications should be precisely given
Line 92:
is: Table 2. Spray parameters of HVOF.
should be (in the example): Table 2. HVOF spraying parameters of NiCrAlY coatings
Line 113:
is: of the samples was studies by energy spectrometer
should be: of the samples was studied by energy spectrometer
5. I suggest changing some keywords (temperature, microstructure) to words that more precisely relates to the issues discussed in the article.
6. Please add space between the units, Line 11, 12
Generally, the article requires editorial refinement and proofreading at some points. The issues presented in the article are important from the point of view of TBC coating technology and suitable for publication in the Coatings journal, but to increase the scientific value, the article requires corrections and should be re-edited by the authors. I recommend the paper for publication in Journal Coatings (ISSN 2079-6412) after minor revision.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
It is an interesting paper. Improving the properties of the bond coat is a very important issue when using Thermal Barrier Caotings (TBC). The authors very clearly determined the aim of the study. The paper is well written - clear motivation, explanations. However, some improvements should be introduced:
1. line 89 “The coating was automatically separated from the substrate when it reached a certain thickness” Please explain how it was done, what does it mean automatically?
2. how many samples were in each batch of VHTs?
3. was the porosity calculated from image 4 or from larger area or other samples?
4. Fig. 5 shows the weight change up to 21h. Can a trendline be adjusted and its function specified?
After making minor corrections to the text, I recommend the article for publication.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments about ZHENG et al.
Effect of Vacuum Heat Treatment on the High-temperature Oxidation Resistance of NiCrAlY Coating
General comments: Interesting work. A few remarks
1. Abstract, and key words: OK
2. Introduction: OK. No comments
3. Materials and methods
Line 89 To separate the coating from the substrate, you mention a certain thickness is required. Which one?
Line 95 and following. You mention vacuum in the furnace. Which kind of residual atmosphere is still present? Air, oxygen, inert gas?
Line 102 and following. In the oxidation test, which kind of atmosphere do you use? Oxygen, air? I guess it is at the atmospheric pressure?
Line 191 and following. I did not understand the meaning of table 4. Does it mean that there is diffusion of the different elements to the surface but against which one? Please can you clarify this point?
4. Discussion
Interesting one. Is it possible to use the different potential chemical reactions between the components for explanation of the described behavior?
5.Conclusion: OK
Author Response
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Author Response File: Author Response.pdf
Reviewer 4 Report
Thank you very much for your interesting paper. Relationship between oxidation behavior and VHT condition of the NiCrAlY alloys were investigated in detail and the oxidation mechanism of their samples were proposed from the experimental results. So, this paper will be useful if the authors explanation is correct. However, there were not enough data in order to explain the mechanism you proposed properly. Therefore, I would like to ask you to revise this paper with taking account of the following comments.
1. Please add the symbol for upper arrow lines in Fig. 3 (b). In addition, upper arrow lines also seem to indicate white area. If not, please correct the positions of the upper lines.
2. Please correct the positions of the arrow lines properly in Fig.4. In addition, please add the feature (color) of the phases.
3.From the results of Fig.6, you explained that VHT1200 contained the least amount of internal oxide. I understand that weight gain difference among them shown in Fig.5 was not due to difference of amount of TGO but due to difference of amount of internal oxide. However, there is no data show oxygen concentration in the NiCrAlY coating except TGO. (Fig.6 is not sufficient in order to prove your explanation.) So, I would like to ask you to show the oxygen concentration of them. If possible, please show the mapping results obtained by EPMA.
4. In Fig,7, if there are some differences among the TGOs of VHT1000, 1100 and 1200, please tell the difference.
5. In Fig.8, white particles on the surface became large as the VHT temperature was raised. Please tell what the white particles are.
6. In Fig.9, according to your explanation, “The diffraction of θ-Al2O3 was not observed in the as-sprayed coating.” However, you proposed that θ-Al2O3 made oxide diffusion and caused internal oxidation (P.10, l.237-243). How to confirm the existence of θ-Al2O3?
That is all.
I would very much appreciate your contribution.
Author Response
Please see the attachment. Thanks very much for your helpful comments.
Author Response File: Author Response.pdf
Round 2
Reviewer 4 Report
Thank you very much for your revision.
Since this paper was revised properly along the reviewers' comments, I would like to accept this paper as publishable one.
I would very much appreciate your contribution.
