Stress and Displacement Distribution of the Thermal Barrier Coatings in the Turbine Engine Combustion Chamber

Round 1

Reviewer 1 Report

My comments are as follows:

1.Poor preparation, poor language, not up to a level of peer-reviewed journal. Many sentences are incomprehensive. 
2. The findings are not significant.What is the contribution and novelty of this work? 
3. The modeling part is the core of this study, but it is lack of important information and explanation. The feasibility of the model is not explained? The geometric parameters and structural components of the model are not shown and explained? How about the meshing effect on the accuracy of the model?
4. The authors use a simple model to represent a 3-D model. Does it have enough accuracy? And based on what evidences? This point is very important and must be demonstrated.
5. What is the principle behind the simulation? What is the heat transfer model used in the modeling?
6. The paper states that the stress and displacement analysis is for enhancing the performance of the TBC system? How come? Please clarify?
7. Findings are not convinceable. In lines 135-136, “The results show that the TGO thickness is the most significant parameter for the performance of the TBC system.” What do you mean? You just present about the stress and displacement trend, and you conclude that.

Author Response

1.Poor preparation, poor language, not up to a level of peer-reviewed journal. Many sentences are incomprehensive. 

Ans: We have modified the structure and language of our paper for better understanding.

2. The findings are not significant. What is the contribution and novelty of this work? 

Ans: Common failure of turbine engine is the separation of thermal barrier coating on the turbine blade, but the turbine combustion chamber also works in the high temperature environment, although the stress and deformation of combustion chamber are very small, under cyclic high-temperature working conditions, stress accumulation and creep deformation may occur, which will affect the efficiency and life cycle of turbine engine. In this study, the effect of the working temperature, TGO and TBC thickness on the stress and deformation of combustion chamber under thermal loading have been analyzed, the analytical results can provide the research basis and design reference for fatigue life and creep research.

3. The modeling part is the core of this study, but it is lack of important information and explanation. The feasibility of the model is not explained? The geometric parameters and structural components of the model are not shown and explained? How about the meshing effect on the accuracy of the model?

Ans: This paper studied the effect of a factor on the wall stress and deformation of the combustor. Therefore, in order to save calculation time, a small turbine engine combustor with a diameter of 109 mm is selected as the model. With a 2D, 1 to 4 cylindrical model was used to the analysis. The model consists of TBC, TGO, substrate and BC, with initial thickness of 150um, 3um and 1000um respectively. The substrate and BC layer were combined to one block, because the material properties are similar. The maximum mesh size of TBC is 50um, that of TGO is 1.5um, and that of substrate and BC is 200um. The total number of the elements was 36,126.

4. The authors use a simple model to represent a 3-D model. Does it have enough accuracy? And based on what evidences? This point is very important and must be demonstrated.

Ans: In this paper, the combustion chamber is simplified as a thin-walled cylinder with a thickness of 1 mm. The stress is caused by the difference of thermal expansion coefficient between TBC, TGO and substate during thermal cycling. And the stress is mainly composed of radial and tangential parts. Therefore, the thin-walled cylinder can be simplified as a two-dimensional plane strain problem.

5. What is the principle behind the simulation? What is the heat transfer model used in the modeling?

Ans: The principle for the analysis method is calculating the stress and displacement distribution coupled with the temperature distribution due to heat conduction theory.

6. The paper states that the stress and displacement analysis is for enhancing the performance of the TBC system? How come? Please clarify?

Ans: The stress of TBC system is mainly caused by the difference of thermal expansion coefficient between the layers during thermal cycling load under the condition of constant TGO thickness. And the stress is mainly concentrated in TGO. When the stress reaches a certain value, it will cause the separation between TGO and the substrate, which will lead to the failure of TBC and TGO. Moreover, TGO is a brittle material composed of alumina, and the failure of TGO is judged by maximum principle stress theory. Therefore, to predict the failure of TBC, it is necessary to analyze the stress and deformation of TGO, and the main causes of stress and deformation can be found by changing the thickness of each layer and the operating temperature.

7. Findings are not convinceable. In lines 135-136, “The results show that the TGO thickness is the most significant parameter for the performance of the TBC system.” What do you mean? You just present about the stress and displacement trend, and you conclude that.

Ans: As you mentioned, it is unreasonable to define the TGO as the main influencing factor. Therefore, the conclusion is modified as follows:

Compared with figures 10, 11 and 12, the stress and deformation under the three thermal cycles increase with the increase of operating temperature and the thickness of TGO and TBC, except for the case of TGO thickness of 2um. This is because the increase of operating temperature will cause greater thermal stress, and the increase of TGO and TBC thickness will also increase the resistance to the substrate during thermal expansion, and then cause the increase of stress and deformation.

 

Reviewer 2 Report

The manuscript reports on a research that has used finite element analysis to investigate the influence of the thickness of TBC layer and the TGO on performance relative to maximum operating temperature. Having reviewed the manuscript, I found the research interesting and of scientific merit. I would, however, recommend some revisions before it can be published.

1) Introduction: The introduction fails to adequately cover the background. Also, it leaves the reader in dark about to the scientific questions that the research intends to answer. So, I recommend that in addition to a better review of existing art, the authors consider some introduction about the motivation of the research and why it is timely and significant. Why finite element method has been preferred over other approaches? how could this be investigated experimentally? what experiments and for what reason have been represented by finite element analysis?

2) Methods: A 2D model has been used for the analysis. How this represents the 3D case, or how does this relate to 3D structure? The methodology should provide some reasoning to justify the use of 2D model and support meaningful relation between 2D and 3D space. 

Also, please describe the boundary conditions as assumed in the model. 

3) Discussions: Only describing the curves produced by the model is not enough. The results should be thoroughly explained. For example, Fig. 12 shows that stress and displacement increase by thickness, but there is no discussion on why this happens, what if its effect on performance, how it can be leveraged in improvement of design, and how it compares to other materials? etc. 

Discussions are not adequate. The results should be discussed relative to each other and to existing literature, if any, to show how they compare. 

4) Conclusions: I would suggest a bullet-point list of main assumptions and findings of the study. Assumptions matter because the findings in a modelling study are built up on the assumptions and would not be meaningful without them. 

 

 

Author Response

The manuscript reports on a research that has used finite element analysis to investigate the influence of the thickness of TBC layer and the TGO on performance relative to maximum operating temperature. Having reviewed the manuscript, I found the research interesting and of scientific merit. I would, however, recommend some revisions before it can be published.

1. Introduction: The introduction fails to adequately cover the background. Also, it leaves the reader in dark about to the scientific questions that the research intends to answer. So, I recommend that in addition to a better review of existing art, the authors consider some introduction about the motivation of the research and why it is timely and significant. Why finite element method has been preferred over other approaches? how could this be investigated experimentally? what experiments and for what reason have been represented by finite element analysis?

Response : The introduction was modified for better understanding.

2. Methods: A 2D model has been used for the analysis. How this represents the 3D case, or how does this relate to 3D structure? The methodology should provide some reasoning to justify the use of 2D model and support meaningful relation between 2D and 3D space. 

Also, please describe the boundary conditions as assumed in the model. 

Response : In this paper, the combustion chamber is simplified as a thin-walled cylinder with a thickness of 1 mm. The stress is caused by the difference of thermal expansion coefficient between TBC, TGO and substate during thermal cycling. And The stress is mainly composed of radial and tangential parts. Therefore, the thin-walled cylinder can be simplified as a two-dimensional plane strain problem. Because the thickness of TGO is thin and the element is small, if it is replaced by 3D model, the number of elements will become too much, so the 3D model is simplified to 2D model under the condition of satisfying the calculation accuracy.

For the boundary conditions, the two sides of the analysis model were set as symmetry condition because the model is a 2D, 1 to 4 model. In this analysis, three cycles composed of heating time of 30s, maintaining time of 180s and cooling time of 30s were applied for the inner boundary.

3. Discussions: Only describing the curves produced by the model is not enough. The results should be thoroughly explained. For example, Fig. 12 shows that stress and displacement increase by thickness, but there is no discussion on why this happens, what if its effect on performance, how it can be leveraged in improvement of design, and how it compares to other materials? etc. 

Discussions are not adequate. The results should be discussed relative to each other and to existing literature, if any, to show how they compare. 

Response : As you mentioned, the conclusion is modified as follows:

Compared with figures 10, 11 and 12, the stress and deformation under the three thermal cycles increase with the increase of operating temperature and the thickness of TGO and TBC, except for the case of TGO thickness of 2um. This is because the increase of operating temperature will cause greater thermal stress, and the increase of TGO and TBC thickness will also increase the resistance to the substrate during thermal expansion, and then cause the increase of stress and deformation.

4. Conclusions: I would suggest a bullet-point list of main assumptions and findings of the study. Assumptions matter because the findings in a modelling study are built up on the assumptions and would not be meaningful without them. 

Response : According to your comment, we modified the conclusion part for better understanding.

 

Reviewer 3 Report

English language must be improved. The authors did not mention the name of the TBC. The table 1 describes its properties in comparison to the substrate, BC and TGO. This is a very short paper without significant in depth analysis. Only the simulation results have been presented.

Author Response

1. English language must be improved. The authors did not mention the name of the TBC. The table 1 describes its properties in comparison to the substrate, BC and TGO. This is a very short paper without significant in depth analysis. Only the simulation results have been presented.

Response : We have upgraded English expression according to your comments.

The descriptions for TBC, TGO and BC were in the introduction part.

Although the simulation model is simple compared to 3D model, but it can be used as a predictor before designing or manufacturing the TBC system.

Reviewer 4 Report

Ref.comments to the paper coatings-895896 titled as “Stress and Displacement Distribution of the Thermal Barrier Coatings in the Turbine Engine Combustion Chamber” written by the authors JianXin Wang , ZhenZhe Li , FengXun Li

It is known that the properties of new composites depend not only on the structure of the matrix material body, but also on its surface. In this regard, the study of the thermally resistant coatings significantly expands our knowledge in the field of components that strengthen the base material and create protection of the material from external influences. From this point of view, the paper is modern and actual.

 In the “Introduction” part the authors are shown the tentative construction of their coatings on the substrate. It connected with the bond coat (BC), the thermally grown oxide (TGO), and the thermal barrier coat (TBC) as well. In the section of “Finite element analysis method of TBC system” the thickness of the layers are presented with good advantage. In this accent I would like to ask the authors about the refractive index of the each layer. What are the values of the refractive indexes? It is important from the optical point of view to form the coatings step by step. Generally, refractive index of the subsequent layer must be consistent with the refractive index of the previous one through the square root! What are the coinciding conditions for the layers, which the author used in this paper?

In the part of “Analysis results and discussions” the authors clearly shown the displacement distributions of the TBC and TGO layers; they testified that the maximum displacements for the TBC and TGO were 0.69632 um and 0.69515 um. They found that the BC layer is the significantly affected region on the view of the displacement. Moreover, the authors presented the stress distribution of TBC treated after 3 cycles at 10000C with the TGO thickness of 3 um. It is interesting practical results.

In the analysis part the author are shown the experimental dependences of the stress on temperature and thickness of the constructed layers. Yes, these dependences are connected with the classical ones for the optical coatings placed on the glass or metal substrates. The authors are shown the results namely for their construction. I would like to ask the authors have they the comparative data with another researcher’s works published for the analogous layers?

In conclusion. This article is practically useful and quite interesting from the point of view of expanding the database of new coatings. However, this article shows only a clear practical orientation without at least a brief theoretical analysis, which can be performed using the ratios for the refractive index of all the layers used. Please update this article from these positions.

 

 

Comments for author File: Comments.pdf

Author Response

It is known that the properties of new composites depend not only on the structure of the matrix material body, but also on its surface. In this regard, the study of the thermally resistant coatings significantly expands our knowledge in the field of components that strengthen the base material and create protection of the material from external influences. From this point of view, the paper is modern and actual.

1. In the “Introduction” part the authors are shown the tentative construction of their coatings on the substrate. It connected with the bond coat (BC), the thermally grown oxide (TGO), and the thermal barrier coat (TBC) as well. In the section of “Finite element analysis method of TBC system” the thickness of the layers are presented with good advantage. In this accent I would like to ask the authors about the refractive index of the each layer. What are the values of the refractive indexes? It is important from the optical point of view to form the coatings step by step. Generally, refractive index of the subsequent layer must be consistent with the refractive index of the previous one through the square root! What are the coinciding conditions for the layers, which the author used in this paper?

Response : As you mentioned, the refractive index is very important for the layer coincidence. In the TBC system, the thickness of TBC can be measured according to the refractive index (like the surface emissivity of the high temperature TGO can be used to measure the thickness of TGO). In this paper, the finite element method is used to analyze the influence of various factors on the stress deformation, and the refractive index is not involved in the parameter setting. The overlap condition of each layer is to fix the nodes between the layers together, the disadvantage is that the separation of TGO cannot be simulated. However, the analysis results show that the stress of TBC system in combustion chamber is not enough to produce the separation, which does not affect the calculation accuracy.

2. In the part of “Analysis results and discussions” the authors clearly shown the displacement distributions of the TBC and TGO layers; they testified that the maximum displacements for the TBC and TGO were 0.69632 um and 0.69515 um. They found that the BC layer is the significantly affected region on the view of the displacement. Moreover, the authors presented the stress distribution of TBC treated after 3 cycles at 1000⁰C with the TGO thickness of 3um. It is interesting practical results.

Response : We checked the part again for confirming.

3. In the analysis part the author are shown the experimental dependences of the stress on temperature and thickness of the constructed layers. Yes, these dependences are connected with the classical ones for the optical coatings placed on the glass or metal substrates. The authors are shown the results namely for their construction. I would like to ask the authors have they the comparative data with another researcher’s works published for the analogous layers?

Response : There are many papers about the effect of temperature on TBC’s durability. In author’s previous paper on the deformation of surface groove of high temperature resistant materials, it is also found that the deformation around TGO increases with the increase of temperature. However, there are few papers about the influence of TGO and TBC thickness, and there are no papers about TGO and TBC thickness in turbine combustion chamber.

4. In conclusion. This article is practically useful and quite interesting from the point of view of expanding the database of new coatings. However, this article shows only a clear practical orientation without at least a brief theoretical analysis, which can be performed using the ratios for the refractive index of all the layers used. Please update this article from these positions.

Response : Since the analysis of refractive index is not involved in this paper, the conclusion modified in mechanical theory as follows:

Compared with figures 10, 11 and 12, the stress and deformation under the three thermal cycles increase with the increase of operating temperature and the thickness of TGO and TBC, except for the case of TGO thickness of 2um. This is because the increase of operating temperature will cause greater thermal stress, and the increase of TGO and TBC thickness will also increase the resistance to the substrate during thermal expansion, and then cause the increase of stress and deformation.

 

Round 2

Reviewer 1 Report

The revised manuscript was not significantly improved. The authors just simply answer the comments with little modifications. Therefore, I do not support the publication of this paper. Some modifications are needed.

1. Authors should explain the principle of the heat transfer model in the manuscript. Is the heat conduction theory suitable for solving this problem? Must be clarify?
2. The feasibility of the result was not demonstrated by experiment, at least with some references? Do the simplified 2-D model perform as good as the 3-D model? This must be demonstrated.
3. How about the meshing effect on the results. This question was asked before.
4. This paper has no significant findings. It simply presents stress-strain analyses.

Author Response

The revised manuscript was not significantly improved. The authors just simply answer the comments with little modifications. Therefore, I do not support the publication of this paper. Some modifications are needed.

1. Authors should explain the principle of the heat transfer model in the manuscript. Is the heat conduction theory suitable for solving this problem? Must be clarify? 

Response: This paper studies the stress and deformation of TBC systems under thermal cycle, so the temperature of each layer of each step is the same, but the thermal expansion coefficients mismatch between the different layers of the TBC system will cause thermal stress. At this time, the change of temperature can be achieved by assigning different temperatures to the element nodes, that is, the heat conduction phenomenon is not involved in this paper.

2. The feasibility of the result was not demonstrated by experiment, at least with some references? Do the simplified 2-D model perform as good as the 3-D model? This must be demonstrated. 

Response: In the author’s previous work, the deformation of cooling hole under thermal cycle is analyzed by using two-dimensional axisymmetric model, and the deformation of cooling hole measured under high temperature thermal cycle experiment is compared. The results of experiment and FEA are very consistent, which proves the feasibility of two-dimensional axisymmetric finite element calculation.

3. How about the meshing effect on the results. This question was asked before.

Response: As shown in the table below, the thickness distribution of TBC, TGO and substrate is relatively uniform compared with the size of its elements. Too big elements will affect the calculation results, but too small elements will also affect the finite element calculation, especially if the element of TGO is too small, the thermal expansion deformation is much greater than the size of the element, resulting in the error of finite element analysis and unable to calculate. In the author’s previous work, the size of TGO element in the FEA model of cooling hole (diameter of 1 mm) is about 1 um, and the calculated results are in good agreement with the experimental data.

	TBC	TGO	Substrate and BC
Thickness	150um	3um	1000um
Maximum mesh size	50um	1.5um	200um

 

4. This paper has no significant findings. It simply presents stress-strain analyses. 

Response: As you mentioned, this paper is relatively short, but as a few papers on thermal stress analysis of turbine combustor, it will provide inspiration and foundation for further research. In addition, the stress and deformation of turbine combustor are analyzed under different temperature, TGO and TBC thickness. These conditions represent different working conditions of turbine engine. With the increase of TBC thickness, the heat insulation effect will be improved, so the engine operating temperature can be increased. However, the increase of TBC thickness will reduce the penetration of external oxygen atoms, thus reducing the formation of TGO (thickness). Or the operating temperature of different substrate is different, and the thickness of TGO is different. Therefore, it is necessary not only to analyze the thermal stress under thermal cycle, but also to study the influence of various factors (temperature, TGO and TBC thickness, etc.) on the thermal stress.

Reviewer 2 Report

I appreciate the authors addressing review comments. My comments have been adequately addressed by the authors. I recommend the manuscript for publication, but I would recommend that the authors make a few very minor changes as given in the following before it is sent for publication: 

• The explanation about the validity of 2D modeling and the boundary conditions , as given by the authors in response to my question, should be included in the paper. BUT PLEASE MAKE SURE IT IS RE-WITTEN IN A PROPER LANGUAGE. The authors' response was as follows (please re-write it before incorporating in the paper): " In this paper, the combustion chamber is simplified as a thin-walled cylinder with a thickness of 1 mm. The stress is caused by the difference of thermal expansion coefficient between TBC, TGO and substate during thermal cycling. And The stress is mainly composed of radial and tangential parts. Therefore, the thin-walled cylinder can be simplified as a two-dimensional plane strain problem. Because the thickness of TGO is thin and the element is small, if it is replaced by 3D model, the number of elements will become too much, so the 3D model is simplified to 2D model under the condition of satisfying the calculation accuracy. For the boundary conditions, the two sides of the analysis model were set as symmetry condition because the model is a 2D, 1 to 4 model. In this analysis, three cycles composed of heating time of 30s, maintaining time of 180s and cooling time of 30s were applied for the inner boundary."
• The first statements in conclusion read as "The TBC systems were highlighted because of its wide uses in the various energy and power 171 systems. The detailed descriptions are as follows". Please revise this part with respect to writing quality (grammar use and word selection). Also,  please include a brief description of research motivation and significance (1-2 sentence each). 

Author Response

I appreciate the authors addressing review comments. My comments have been adequately addressed by the authors. I recommend the manuscript for publication, but I would recommend that the authors make a few very minor changes as given in the following before it is sent for publication: 

1. The explanation about the validity of 2D modeling and the boundary conditions , as given by the authors in response to my question, should be included in the paper. BUT PLEASE MAKE SURE IT IS RE-WITTEN IN A PROPER LANGUAGE. The authors' response was as follows (please re-write it before incorporating in the paper): " In this paper, the combustion chamber is simplified as a thin-walled cylinder with a thickness of 1 mm. The stress is caused by the difference of thermal expansion coefficient between TBC, TGO and substate during thermal cycling. And The stress is mainly composed of radial and tangential parts. Therefore, the thin-walled cylinder can be simplified as a two-dimensional plane strain problem. Because the thickness of TGO is thin and the element is small, if it is replaced by 3D model, the number of elements will become too much, so the 3D model is simplified to 2D model under the condition of satisfying the calculation accuracy. For the boundary conditions, the two sides of the analysis model were set as symmetry condition because the model is a 2D, 1 to 4 model. In this analysis, three cycles composed of heating time of 30s, maintaining time of 180s and cooling time of 30s were applied for the inner boundary."

Response: As you mentioned, 2D models and the boundary conditions have been added on paragraphs 1 and 5 of page 3.

 

2. The first statements in conclusion read as "The TBC systems were highlighted because of its wide uses in the various energy and power 171 systems. The detailed descriptions are as follows". Please revise this part with respect to writing quality (grammar use and word selection). Also, please include a brief description of research motivation and significance (1-2 sentence each). 

Response: As you mentioned the first statements in conclusion modified as follow:

“TBC has been widely used as coating material of turbine engine due to its excellent heat insulation performance. The research on TBC mainly focuses on the turbine blade, but the combustion chamber of turbine also works in high temperature environment. Although the stress of combustion chamber is small in thermal cycle, under cyclic high-temperature working conditions, stress accumulation and creep deformation may occur, which will affect the efficiency and life of turbine engine. The detailed descriptions are as follow”.