Environmental Barrier Coatings Made by Different Thermal Spray Technologies

Round 1

Reviewer 1 Report

Environmental barrier coating is key protective coating for advanced ceramics or ceramic composites used in severe environment. However, EBC coatings are usually made from oxide ceramics with high melting points. Therefore, it is difficult to coat them on the substrates by common techniques. So, thermal spray technologies are developed. In the paper, authors investigated different thermal spray technologies for EBC, which will provide important basis for making EBC. The paper could be accepted after minor revision.

Page 6: could you please give more details about the thermal cycling? The “four 20 h cycles at 1200°C” is quite confused. Figure 9: please check “Y2O2” and “monocl”, are they correct? Page 12: “1350 C” should be “1350 °C” Page 14: “Y2Si2O7” should be “Yb2Si2O7” Could you please label diffraction peaks in the XRD spectra for all the samples?

Author Response

Reviewer 1

Page 6: could you please give more details about the thermal cycling? The “four 20 h cycles at 1200°C” is quite confused.

The reviewer is right, we added that in the experimental part.

Figure 9: please check “Y2O2” and “monocl”, are they correct?

Y2O2 was corrected, monoclinic is right.

Page 12: “1350 C” should be “1350 °C”

changed

Page 14: “Y2Si2O7” should be “Yb2Si2O7” Could you please label diffraction peaks in the XRD spectra for all the samples?

“Y2Si2O7” was changed to “Yb2Si2O7”, Figure 12 b updated

Reviewer 2 Report

The authors fabricate various candidates of EBCs for ox/ox and SiC/SiC Ccomposites with variety of thermal spray techniques. The paper contains few deep academic insights or cutting-edge findings but provide practically useful information and suggestions for those who work for thermally sprayed T/EBCs. Thus the reviewer concluded that the paper should be published in this journal.

The reviewer found several minor grammatical mistakes and also would like to suggest concerns about the content of the paper as follows:

Line 55:

“summarized in. Figure 1.” Should be replaced with “summarized in Figure 1.”

Line 70:

“With” Should be replaced with “with”

Line 72:

Period should be added at the end of Eq. (3).

Figure 2:

“Mikrometer” Should be replaced with “Micrometer” Explanations written at the right side of torch phots are unclear; the reviewer suppose them carrier gasses, power and spraying distance. They authors should clearly show them, or they may be omitted.

Line 141:

The detail information of “O3CP” should be addressed.

Table 2:

The Raw of APS is extremely hard to distinguish the place of separation. They should be separated with lines or spaces.

Lines 203, 230

Coefficient of thermal expansion for GdZr2O7 should be clarified.

Line 230, :

“CTE” should be replaced with “TEC”.

Line 254, :

“arf” should be replaced with “of”.

Line 254 and Figure 7 :

Ra value for the cauliflower structure are different in the text and Figure 7.

Figure 9

“Cubic Y2O2” should be replaced with “Cubic Y2O3.”

Section 3.2

Conditions A-D are not identified.

Lines 299-300

Is the sentence for the explanation of the samples after heat exposure ?

Lines 377-385

The authors attribute the vertical cracks in APS Yb2SiO5 coatings to the large of Yb2SiO5. They discuss it based on the CTE values of crystalline materials of Yb2SiO5 and Yb2Si2O7 but the coating is amorphous. Generally speaking CTE of amorphous material is larger than that of crystalline materials but the Young’s modulus of amorphous is smaller than that of crystalline materials. The authors should discuss the thermal stress considering the above effect.

Figure 15

Designations of photos (a)-(d) seem wrong and missing.

Author Response

Comments and Suggestions for Authors

The authors fabricate various candidates of EBCs for ox/ox and SiC/SiC Ccomposites with variety of thermal spray techniques. The paper contains few deep academic insights or cutting-edge findings but provide practically useful information and suggestions for those who work for thermally sprayed T/EBCs. Thus the reviewer concluded that the paper should be published in this journal.

The reviewer found several minor grammatical mistakes and also would like to suggest concerns about the content of the paper as follows:

Thank you for your valuable remarks, we have corrected the text accordingly.

Line 55:

“summarized in. Figure 1.” Should be replaced with “summarized in Figure 1.”

Has been corrected

Line 70:

“With” Should be replaced with “with”

 Has been corrected

Line 72:

Period should be added at the end of Eq. (3).

We added a blank 

Figure 2:

“Mikrometer” Should be replaced with “Micrometer” Explanations written at the right side of torch phots are unclear; the reviewer suppose them carrier gasses, power and spraying distance. They authors should clearly show them, or they may be omitted.

That was changed and the following remark added: … showing plume temperatures and particle velocities, photos of the plumes plus conditions used (process gases, current in case of plasma processes, and stand-off distances), and photos of typical splats.

Line 141:

The detail information of “O3CP” should be addressed.

added (Oerlikon Metco, Wohlen, Switzerland) 

Table 2:

The Raw of APS is extremely hard to distinguish the place of separation. They should be separated with lines or spaces.

 That was done

Lines 203, 230

Coefficient of thermal expansion for GdZr2O7 should be clarified.

CTE of Gd2Zr2O7 was added in the text.

Line 230, :

“CTE” should be replaced with “TEC”.

We checked in the databases and both terms are used frequently, as we are using more often CTE we would like to keep it. 

Line 254, :

“arf” should be replaced with “of”.

The mistake was removed

Line 254 and Figure 7 :

Ra value for the cauliflower structure are different in the text and Figure 7.

 That was corrected

Figure 9

“Cubic Y2O2” should be replaced with “Cubic Y2O3.”

  That was corrected

Section 3.2

Conditions A-D are not identified.

That was clarified:

the condition D with hydrogen and long stand-off distance

Lines 299-300

Is the sentence for the explanation of the samples after heat exposure ?

Do not see a mistake

Lines 377-385: The authors attribute the vertical cracks in APS Yb2SiO5 coatings to the large of Yb2SiO5. They discuss it based on the CTE values of crystalline materials of Yb2SiO5 and Yb2Si2O7 but the coating is amorphous. Generally speaking CTE of amorphous material is larger than that of crystalline materials but the Young’s modulus of amorphous is smaller than that of crystalline materials. The authors should discuss the thermal stress considering the above effect.

We added the following sentence:

 A higher amorphous content of Yb₂SiO₅ would even further increase the CTE mismatch, however simultaneously due to a reduced Young´s modulus of the amorphous state, the effect on the stress level might be limited.

Figure 15: Designations of photos (a)-(d) seem wrong and missing

This was corrected

Reviewer 3 Report

The authors present a broad overview of several environmental barrier coatings with different thermal methods, outlining the strengths and challenges for each. In particular, the paper was focused on ensuring the deposition of highly crystalline and dense coatings that did not crack under thermal loading (during processing or subsequent heat treatments) that would prohibit their functionality for the desired application. Several important environmental barrier coating materials were explored, which is important for actual implementation of CMCs in gas-turbine engines.

The paper has a lot of important and useful information but requires significant reorganization to improve readability. The paper covers 6 different coating materials on multiple substrates and at least 4 spray processes. The results and characterization methods are not consistent between the different systems and it is hard to see why different methods were applied. In particular, the Yb2Si2O7 was different from the others and has already been previously discussed in other works. Furthermore, the methods and figures require significant clarification and description. The authors are also encouraged to edit the paper for grammar mistakes and to reduce excessive wordiness that limited the readability of the paper. More significantly, the paper reads as an assemblage of results and lacks a defining message. A list of considerations for spraying EBCs that summarize these findings would significantly improve the message.

Below are a list of revisions and questions for the authors: 

The focus is on the techniques ability to create dense and crack-free coatings. However, the abstract starts out describing the different EBC material needs for oxide and non-oxide CMCs. The authors should revise to reflect the topic of the paper. The introduction includes very general information about ceramics (page 1-40) and CMCs. Yet, the authors do not provide much description about the techniques or the needs of the EBCs, which is the focus of the paper. The authors can briefly describe the needs of the CMCs to allow more room to discuss the processes used that are more relevant to the paper’s conclusions. In particular, the benefits and challenges of each technique, which are currently distributed throughout the results discussion (for example, SPS on page 18 and HVOF on 16), should be first introduced in the introduction to guide readers. The authors provide the general equations for energy release rate but do not use these calculations elsewhere in the paper. Why is Gd2Zr2O7 being presented? It is a TBC rather than EBC material. Why were 6 materials chosen to compare? Are you presenting the optimized version for each system or just the first attempts? Do you think the coatings could be altered beyond what is presented here to avoid some of the issues (cracking, pores) discussed? This is needed to know how to compare the different coating results.  Please provide more clarification on the following methods: measuring the substrate temperature for the YbDS coatings, porosity measurements, high temperature XRD, calculations of crack density. Which samples went surface treatment? In discussion only a few discussed but not mentioned in the methods section. Also, it is unclear what you mean by “spraying” in that paragraph. For the porosity measurements, how much area of the sample was covered in the 10 SEM images? How did you determine your error with this method? Some of the coating methods had different processing parameters varied, like the surface treatment for APS or the spray distance in SPS. Why were only some varied? Is there other opportunities to optimize the coatings that were not tried? Can we compare the samples that are shown as they might be not optimized? Why was only APS used for the ox/ox CMCs, while the other coatings were sprayed with multiple techniques? These feel like two (or even 3) different papers. The section on YbDS was particularly different from the other sections and contained background information and results, while not all methods were described. Why should this be included here given that you have another paper on this material using these techniques? Why were some samples thermally cycled but not all? More information about the optimization is necessary to derive these conclusions stated in Table 4. Can you state where these techniques may be altered or if there are other possibilities? Furthermore, it was stated how the material properties are important for these techniques. How is that considered in your table? What is your main message? The paper feels like a collection of different findings rather than a single story of what must be considered when spraying EBCs. More specific to Text, Figures and Tables: Page 1, line 43: What is the benefit of oxide-based CMCs? Page 2, line 45: “…CMCs do not have to be protected against oxidation…” This sentence is misleading. Page 2, line 50: Please replace “hole” with “whole”. Table 2: recommend using YbDS rather than YDS – which is often used for yttria-based systems rather than Yb-based systems. Page 7, line 201: What does “bad wetting of the substrate” mean? Did this never adhere? If so, how could it delaminate (see next line). Page 14, line 318: wrong chemical formula (Y instead of Yb) Page 17: Why did you change the size of the sample to help improve the temperature but not discuss it in the paragraphs before? Could the results be included for comparison in Figure 12? Page 17: the discussion about cracking due to different phases wasn’t clear. HVOF has a lot of amorphous as well but didn’t crack but then the APS did. Please explain. Please reread the captions and check for correctness and clarity.  Figure 1: Please provide the data for Gd2Zr2O7 and Yb2SiO7, which are relevant to this study. Figure 2: Do the torches and splats correspond to the different techniques? If so – label them clearly. Figure 3: It was unclear to me whether the peaks with * were un-identified or just secondary peaks to the primary phase. Please clarify in the text and in the caption. Figure 4: Some of the views were along the fiber direction and some were perpendicular. Is there any difference in the cracking or delamination behavior based on these orientations? Also, is there a difference in thickness between the samples, which could explain the failure behavior? Figure 5: What is going on in the substrate in (c)? Figure 8: Why are the samples different thicknesses? Could that explain the change in fracture behavior? Figure 9: The legend refers to the power and distance but the caption discusses the secondary gas. Please correct both to include all 3. Figure 10: label all phases Figure 15: Please check that the labels are correct in the caption and in the text. Also, what value do the high magnfication images provide to the paper? Figure 16: Do you have error bars for the crack density? Please include horizontal lines that delineate the fields for all tables. It is hard to read Tables 1 and 2. Table 4: What do the symbols mean?

Author Response

The authors present a broad overview of several environmental barrier coatings with different thermal methods, outlining the strengths and challenges for each. In particular, the paper was focused on ensuring the deposition of highly crystalline and dense coatings that did not crack under thermal loading (during processing or subsequent heat treatments) that would prohibit their functionality for the desired application. Several important environmental barrier coating materials were explored, which is important for actual implementation of CMCs in gas-turbine engines.

 The paper has a lot of important and useful information but requires significant reorganization to improve readability. The paper covers 6 different coating materials on multiple substrates and at least 4 spray processes. The results and characterization methods are not consistent between the different systems and it is hard to see why different methods were applied. In particular, the Yb2Si2O7 was different from the others and has already been previously discussed in other works. Furthermore, the methods and figures require significant clarification and description. The authors are also encouraged to edit the paper for grammar mistakes and to reduce excessive wordiness that limited the readability of the paper. More significantly, the paper reads as an assemblage of results and lacks a defining message. A list of considerations for spraying EBCs that summarize these findings would significantly improve the message.

Thank you for your remarks, we tried to improve the paper according to your suggestions.

Below are a list of revisions and questions for the authors: 

The focus is on the techniques ability to create dense and crack-free coatings. However, the abstract starts out describing the different EBC material needs for oxide and non-oxide CMCs. The authors should revise to reflect the topic of the paper.

That was done

The introduction includes very general information about ceramics (page 1-40) and CMCs. Yet, the authors do not provide much description about the techniques or the needs of the EBCs, which is the focus of the paper. The authors can briefly describe the needs of the CMCs to allow more room to discuss the processes used that are more relevant to the paper’s conclusions. In particular, the benefits and challenges of each technique, which are currently distributed throughout the results discussion (for example, SPS on page 18 and HVOF on 16), should be first introduced in the introduction to guide readers.

Introduction was shortened with respect to general ceramics statements,

The spray techniques have been described in detail.

The authors provide the general equations for energy release rate but do not use these calculations elsewhere in the paper.

These equations were now used several times in the text

Why is Gd2Zr2O7 being presented? It is a TBC rather than EBC material. Why were 6 materials chosen to compare? Are you presenting the optimized version for each system or just the first attempts? Do you think the coatings could be altered beyond what is presented here to avoid some of the issues (cracking, pores) discussed? This is needed to know how to compare the different coating results. 

Gd2Zr2O7 (TEC 10.6 · 10-6 K-1) was considered as an interesting candidate for Ox/Ox CMCs as it was shown by Zhu et al. [6] that its recession rate in water vapor environment (1650°C at low velocity) is lowest among the pyrochlore zirconates, while general corrosion resistance (e.g. against CMAS) and processing is well known from TBC application. The TEC and reference to the corrosion study was added to the text.

The variety of different materials were chosen to show issues and chances of the proposed processing approaches. As mentioned above always options of further improvement remain.

[6] D. Zhu, D.S. Fox, N.P. Bansal, R.A. Miller, Advanced oxide material systems for 1650°C thermal/environmental barrier coating application, NASA Technical Report 20050199717, NASA, 2004.

Please provide more clarification on the following methods: measuring the substrate temperature for the YbDS coatings, porosity measurements, high temperature XRD, calculations of crack density.

The temperature measurement info is added under section 2.2 thermal spray facilities.

The porosity measurements, HT-XRD and crack density analysis are elaborated under section 2.4 characterization.

Which samples went surface treatment?

Only oxide based CMCs, Clarified in the abstract

 In discussion only a few discussed but not mentioned in the methods section.

In 2.3 the surface treatment by laser ablation is described

Also, it is unclear what you mean by “spraying” in that paragraph.

Spraying is a general term describing deposition by thermal spray, we added APS and HVOF for clarification

For the porosity measurements, how much area of the sample was covered in the 10 SEM images? How did you determine your error with this method?

Porosity measurement details are added in section 2.4 characterization. The error is defined as the standard deviation of the mean.

 Some of the coating methods had different processing parameters varied, like the surface treatment for APS or the spray distance in SPS. Why were only some varied? Is there other opportunities to optimize the coatings that were not tried? Can we compare the samples that are shown as they might be not optimized?

Some explanation was added:

The parameter used are based on long-year experience in the thermal spray of ceramics. Some further optimization trials have been made especially for EBC materials for oxide based CMCs but they are not shown here. As shown in the results, the coating microstructure appears rather dense and often crystalline, no efforts to further optimize the microstructure were made. In the case of YDS the APS coating properties were not satisfying, here also other techniques have been investigated.

Further comment: the term optimized is certainly rather flexible as it depends also on the skills of the scientist, so in fact there is hardly something like optimal, always options for further improvement remain

Why was only APS used for the ox/ox CMCs, while the other coatings were sprayed with multiple techniques? These feel like two (or even 3) different papers.

We tried to explain in the comment above, we also tried to improve the connection of the paper

The section on YbDS was particularly different from the other sections and contained background information and results, while not all methods were described. Why should this be included here given that you have another paper on this material using these techniques?

This manuscript was prepared for the special issue of EBCs in Coatings-MDPI and it aims to reflect the results of work performed in our institute on EBCs both for Ox-Ox as well as for SiC/SiC CMCs. As it is transparent from the references, some of the given results in this manuscript are already published, but additionally new data which add to the bigger picture is also included. YbDS inevitably needed to be incorporated for this purpose.

Why were some samples thermally cycled but not all?

We report only in the case of EBCs for oxide based CMC thermal cycling results. Here the most critical issue is the bonding which can be tested in thermal cycling tests, in the EBC for non-oxide CMCs more density and crystallinity are issues which are reported. Furthermore, the paper is already rather long. The following sentence was added:

Thermal cycling is seen as a tool to characterize the bonding in the case of the EBC for oxide based CMCs  and hence the efficiency of the surface treatment.

More information about the optimization is necessary to derive these conclusions stated in Table 4. Can you state where these techniques may be altered or if there are other possibilities? Furthermore, it was stated how the material properties are important for these techniques. How is that considered in your table? What is your main message? The paper feels like a collection of different findings rather than a single story of what must be considered when spraying EBCs.

We added some comments in the last paragraph of the conclusions.

A summary of the outcome reflecting the actual situation as seen in our institute is given in Table 4. So our message is that also other thermal spray methods than APS can be used to obtain promising EBCs. Certainly further improvements of processes can change the ranking in this evaluation.

Certainly, there are other possibilities, but these will be stated in future papers after we have investigated them.

More specific to Text, Figures and Tables: Page 1, line 43: What is the benefit of oxide-based CMCs? Page 2, line 45: “…CMCs do not have to be protected against oxidation…” This sentence is misleading.

Major benefits of oxide-based CMCs if compared with SiC-based CMCs is their stability in oxidizing environments and the relatively simple processing of CMCs with intentionally porous matrix.

Page 2, line 50: Please replace “hole” with “whole”.

Was deleted anyhow

Table 2: recommend using YbDS rather than YDS – which is often used for yttria-based systems rather than Yb-based systems.

That was changed.

Page 7, line 201: What does “bad wetting of the substrate” mean? Did this never adhere? If so, how could it delaminate (see next line).

A reaction of Gd2Zr2O7 with the alumina substrate wasn’t observed in our studies. This might be due to a lack of intimate contact between coating and substrate which may be caused by low wettability of the molten Gd2Zr2O7 causing only very small contact areas. The text was adapted.

Page 14, line 318: wrong chemical formula (Y instead of Yb)

Yes, Y is correct in case of the monosilicate.

Page 17: Why did you change the size of the sample to help improve the temperature but not discuss it in the paragraphs before? Could the results be included for comparison in Figure 12?

This point is clarified now in Section 3.3.
The sample dimensions are the same for Figure 12. In Figure 14b, as an additional example, an aps coating microstructure that was deposited at a higher temperature is shown. Some introduction for that sample is added now before Figure 14. But the details were already discussed after Figure 14.

Page 17: the discussion about cracking due to different phases wasn’t clear. HVOF has a lot of amorphous as well but didn’t crack but then the APS did. Please explain.

The cracking is not associated with the amorphous content in the manuscript but the Yb2SiO5/Yb2O3 phases, which have higher thermal expansion coefficients and hence lead to higher stresses. Vertical cracking occurs in the APS coating as it is rich in these phases with respect to the HVOF and VLPPS coatings.

Please reread the captions and check for correctness and clarity.  Figure 1: Please provide the data for Gd2Zr2O7 and Yb2SiO7, which are relevant to this study.

Data for Yb2SiO5 and Y2Si2O7 were added to Figure 1. Data for Gd2Zr2O7 was not included but only referenced in the text as conditions of studies differ too much.

Figure 2: Do the torches and splats correspond to the different techniques? If so – label them clearly.

This was added, in addition a mistake in the Figure (DJ2705 instead of DJ2700) was corrected

Figure 3: It was unclear to me whether the peaks with * were un-identified or just secondary peaks to the primary phase. Please clarify in the text and in the caption.

As mentioned in the text and caption, peaks with * could not be attributed to the desired phases of the corresponding feedstock and thereby correlate to the presence of secondary phases.

Figure 4: Some of the views were along the fiber direction and some were perpendicular. Is there any difference in the cracking or delamination behavior based on these orientations? Also, is there a difference in thickness between the samples, which could explain the failure behavior?

There was no influence of the fiber orientation on the delamination behavior observed.

As can be seen from as sprayed condition in Figure 4 coating thickness has been in similar range of 100µm except for Gd2Zr2O7 which was below. The delamination of the relatively thin coating at the interface supports the conclusion of low adhesion strength.

Figure 5: What is going on in the substrate in (c)?

The matrix of the industrial substrate FW12 consists of 85% Al2O3 plus 15% 3YSZ. This is less obvious from contrast in the other micrographs.

Figure 8: Why are the samples different thicknesses? Could that explain the change in fracture behavior?

The coatings of Gd2Zr2O7 for the various substrate conditions were produced in different spray campaigns. Variations in layer thickness in the range of ~30µm are not unusual for APS coatings. As the thickness is still in the same range as in case of the non-textured substrates in Figure 4 the suppression of delamination can clearly be attributed to the presence of surface structure.

Figure 9: The legend refers to the power and distance but the caption discusses the secondary gas. Please correct both to include all 3.

This was corrected

Figure 10: label all phases

In the discussion more details on the phases of the microstructural images are given. Only in the case of the rather fully crystalline material after 1350°C we can conclude on the phase from XRD. We also change the Figure 11 as the degree of crystallinity for 1350°C was missing.

Figure 15: Please check that the labels are correct in the caption and in the text. Also, what value do the high magnfication images provide to the paper?

The labels are correct, a remark why the high magnification is added is included: As seen especially in the higher magnification images bulk-like dense microstructure can be obtained

Figure 16: Do you have error bars for the crack density?

That was changed, also the method to determine the crack density is given in the characterization chapter: Crack density values of samples manufactured by suspension plasma spraying were calculated from the number of vertical cracks by using 15 SEM images (with x300 magnification and width of 600 mm) as well. 

Please include horizontal lines that delineate the fields for all tables. It is hard to read Tables 1 and 2.

The tables were changed, maybe some format adjustment is necessary

Table 4: What do the symbols mean?

This was added: (o reflects average, - (-- very) bad , + (++ very) good results with respect to the criteria)

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors have provided a detailed methods of the best practices for preparing EBC coatings. The paper is covers a lot of materials and spray techniques to guide future research.