Effect of Vacuum Annealing on Microstructure and Hot-Salt Corrosion Behavior of CoNiCrAlY/YSZ/LaMgAl11O19 Double-Ceramic Coating
, Yingchao Sun 2,†, Ling Xiang 3, Zhiguo Wang 1,*, Suying Hu 1,* and Zhiwen Xie 1
Round 1
Reviewer 1 Report
The study examines the effect of vacuum annealing on the microstructure
and hot salt corrosion behavior of the LMA double ceramic coating. The purpose of the study is clear. The material and method are appropiate. Figures of SEM photos should be larger beacuse the structures marked with arrows are very poorly visible (with the exception of Figure 1.). These illustration should be changed. Literature is sufficient.
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Reviewer 2 Report
The authors report here the effect of annealing temperature on microstructure and hot salt corrosion behaviour of a double ceramic coating. The scientific soundness of the paper is good. While I think that the results are somewhat unsurprising and straightforward, I feel it is worth being published and may be used in the future as a reference material for the impact of annealing in double ceramic coated materials. I would therefore recommend the manuscript for publication after the authors address the minor comments below.
Line 41: Doesn’t need “the”
Line 67: “Sentence should be “The GH199 superalloy was used as the substrate.”.
Line 74: Should be “purity” not “pure”
Line 74: “used” should be after were
Table 1 : There are two sample columns. One of these needs to be removed. Are these all the same composition?
Line 82: “carried out in a muffle furnace”
Line 88: It seems odd to refer to the sensitivity of the balance as 10-4 and would be better to state either 0.0001 g or to 4 decimal places.
Line 89: there shouldn’t be a space on either side of – in X-ray
Line 99: “The diffraction strength of the LaMA peak in sample S1 hardly changes,”
Line 103: “LaMA phases are also detected in the LMA layer, indicating a significant recrystallization”
Line 123: “fraction LaMA phase in LMA layer indicates a significant amorphous recrystallization during”
Line 149: Change “expand” to “expanded” or “extended”
Line 160: Replace “he” with “the”
Equation 2: The Cl in the equation is not balanced.
Line 177: “a large fraction…”
Line 185: “It has been…”
Line 189: Change “result” to “results”
Author Response
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Reviewer 3 Report
The development of thermal barrier coatings (TBCs) with good corrosion resistance is of interest to many applications.
This paper studied the effect of vacuum annealing on the microstructure and hot salt corrosion resistance of the APS sprayed and annealed coatings made of CoNiCrAlY/YSZ/LaMgAl11O19. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to analyze the coatings' chemical and microstructure.
It seems that the vacuum annealing improves to a certain degree the corrosion resistance of the TBC, but there are some problems due to cracks formation, which acts as micro-channels for corrosion propagation within the microstructure of the TBC.
The paper is original, but there are important aspects to improve before recommending it for publication.
- No discussion on the results presented in Figure 3 is provided (no discussions on corrosion resistance of the samples).
- How did the authors choose the annealing procedure?! There is any indication for the annealing procedure in the existing literature?
- The chemical composition and mechanical properties of the GH199 superalloy are missing.
- When using a combination of top and bonding layers in multilayer coatings, the hardness of each APS deposited layer must be determined before and after each annealing process. Which layer has the highest micro-hardness and for which sample?
Author Response
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Reviewer 4 Report
The paper deals with the effect of vacuum annealing and hot salt corrosion of double ceramic coating CoNiCrAlY/YSZ/LaMgAl11O19 on the superalloy GH199. The results yielded that the sprayed coating exhibited an unsatisfactory anti-corrosion performance and the LMA layer suffered from severe fracture and corrosion degradation after the hot salt corrosion test. Vacuum annealing induced an apparent recrystallization of amorphous phase in LMA layer triggering severe volume shrinkage and initiation of micro-crack which were enhanced with the increase of annealing temperature from 900 °C to 1200 °C. The annealed coating with higher fraction of LaMA phase showed better resistance to hot salt corrosion, but the salt mixture diffuse simultaneously along the micro-cracks and eventually penetrate into the YSZ layer and cause its erosion.
The results of the paper are interesting especially in term of application of rare earth hexaauminates (LnMgAl11O19 (Ln = La, Nd or Gd)) for an application as thermal barrier coatings (TBCs). The perfromance of studied double ceramic coating CoNiCrAlY/YSZ/LaMgAl11O19 is rather unsatisfactory, but presented results are valuable and helps to suggest further development in the research area of TBCs.
The paper is clearly written and understandable to the readers. Some English sentence formulation should be improved. The reviewer miss an analysis of corrosion products which would make the study more complete and enhance its impact (local chemical EDS analysis in SEM of the surfaces cross-sections (line profile or maps), X-ray diffraction of corroded surfaces).
The reviewer suggest the paper for publication with incorporation of the following questions and recommendations.
The questions which should be answeared and incroporated to the text of the manuscript:
Q1: What are the conditions for phase transformation of LaMA phase from amorphous LMA? It‘s needed to provide more details from quoted refferences [13, 15, 20-22 ] than given in the text (lines 56-59).
Q2: The experiment. What was the motivation for selection of the annealing temperatures 900 and 1200°C? Please, answer it.
Q3: The experiment. What was the size (dimensions) of the samples for annealing and corrosion tests?
Q4: What were the conditions of X-ray diffraction measurement? Should be better described (angle, time, power, software, database, the penetration depth of measurement).
Q5: Better description of corrosion test is needed than given on the lines 82-92. What was the corrosion time and temperature? By which way were the specimens cleaned?
Q6: What is the definition of LaMA phase (line 95)? It should be better described (crystal lattice, formula,…).
The reviewer’s recommendations/suggestions:
Line 13: The gap between lines in the text.
Line 17: The word “phase” is missing in the sentence “Vacuum annealing induced…”.
Line 79: Please, use “were shown” instead of “are shown”.
Line 112-115: The reviewer suggest to use the term “width of micro-cracks interfaces“ as the used terminiology “the width value of micro-crack“ is rather inconvenient.
Line 126: The reviewer suggest to use the term “nucleation“ rather than “sprouting“ for the micro-cracks.
Line 159: The term “strong effect“ is exaggerated based on the presented results. Please, omit it.
Line160: The mistake in the definite article in the sentence: “…behaviour of the sprayed double ceramic coating.“ Please correct it.
Line 201-205: The word “phase” is missing in the sentence “By contrast, the top LMA layer…” and also in the following sentence “The recrystallization…”.
Line 204: The term “excellent resistance“ is exaggerated based on the presented results. Please,use rather “improved“.
Author Response
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Round 2
Reviewer 3 Report
The authors responded to all the questions and amended the paper as suggested. I recommend this paper for publication as it is.
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Reviewer 4 Report
Thank you for your revisions. The reviewer suggests now the manuscript of the paper for publication.
I found only those minor mistakes, so please, correct them in final version.
Line 60: missing space between sentences "°C.These...".
Line 114-116: the sentence "To reduce measurement errors, the weight and average of three measurement results for each layer were taken as the final measurement results." in not clear at all and should be reformulated. The analysis of the results/data obtained should be clearly described. Which results were therefore considered as the final results?
Author Response
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