In Situ High-Temperature Tensile Fracture Mechanism of PS-PVD EBCs
Round 1
Reviewer 1 Report
This paper presents an experimental study of a fibre-reinforced SiC ceramic composite, coated with a PS-PVD coating system. The coating system was thermally and mechanically stressed. In situ techniques were applied during loading. I can suggest this paper for publication in MDPI materials after addressing the following points:
- Please add some additional information about the temperature history during testing: how long did you hold the specimen at the desired temperature? Additionally, the discussion should be elaborated, especially by discussing the findings in Fig. 10 and 11, which play a major role for identifying the failure mechanism in Fig. 12.
- For a PS-PVD process, I would expect a columnar microstructure of the thermal barrier coating - close to a PVD TBC. However, Fig. 5 shows a splat-like structure, as found in APS coatings. This is typically the case for high feed rates during the process. Why was this parameter set chosen for this study?
- The resolution and distortion in some pictures is very noticeable and should be improved (especially Fig. 4, Fig. 7).
- The scales in Fig. 10 and Fig. 11 are the same in each subfigure. However, the white marking highlight that some pictures are higher resolution versions. Please check these scales carefully. Additionally, some subfigures don't seem to be a higher resolution of another subfigure (e.g. Fig. 11 (a) and (c), Fig. 10 (b) and (d)).
- Fig. 12 shows a schematic sketch of the failure mechanism. Unfortunately, this paper does not present evidence for the horizontal crack formation and propagation (delamination) at the SiC/Si interface and the kinking of the vertical cracks (Fig. 12, part 3 and 4). Additional micrographs of the cross-section of the coatings is necessary to support this failure mechanism.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript presents the fabrication of multilayer coating on SiC-based substrate and the influence of temperature and mechanical loading on the whole coated material. In my opinion, there are some points that should be elucidated or improved and the study requires a major revision before publication.
- The authors should explain why they selected for the study that kind of layers and why they were applied in that order. The properties of the control sample, temperature and loading influence, should be presented.
- The substrate was preheated at 500, 1000 and 830 °C – the temperature values were selected experimentally or based on literature?
- The main weakness of the manuscript is lack of the discussion, any comparison to other reports and results.
- The report does not contain any chemical and structural data. It should be elucidated how the layer deposition process influences the selected material and the description and sources of the materials should be implemented to the experimental part of the study.
- Figure 1 is not correctly labeled, e.g. Fig. 1c is labeled as Si but in the text of the manuscript as a mullite layer. Moreover, in the Figure there is Yb2SiO5 compound labeled and in the text Yb2SiO7. The SEM images should be presented in the order in which the layers were subsequently applied on the substrate and with the same magnification to allow comparison.
- Figure 2 is not mentioned in the manuscript. In my opinion, the figure is not necessary and should be removed or moved to the Supplementary part.
- Abstract should be rewritten to clarify the coating fabrication process and aims of the study, e.g. in the current version EBC acronym is explained two times in one sentence.
- The beginning of the Results and Discussion part should be also rewritten to explain what is “substrate sample” and subsequent layers of “EBC” – a short description. Moreover, Figures 10 and 11 mentioned matric fibers, which were not described in the text of the manuscript (substrate characterization). The first paragraph is about the microstructure, not about the properties.
- Paragraph 3.4 describes the failure mechanism without any data connected to chemical composition chances or physical phenomena that appeared in the material. In my opinion, it is only observation/scheme of cracks formation. This part and also image caption do not present any temperature and load values on cracks formation – it is not dependent on the above-mentioned conditions? Moreover, there is an expression “after a period of time” it should be elucidated because any properties and processes were not be studied as a function of time.
Author Response
please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
In situ high temperature tensile fracture mechanism of PS-PVD EBCs
Authors: Dongling Yang, Junling Liu, Jungui Zhang, Xinghua Liang, Xiaofeng Zhang
In the paper Si/mullite/Yb2Si2O7 composite coatings were deposited using PS-PVD on SiC fiber-renforced SiC ceramic matrix composites as environmental barrier coatings (EBCs). Crack formation in the coatings was tested under various loads for samples heated up at different temperatures.
The use of rare earth silicates as EBCs is reported in many papers.
I have a few questions:
- What is the novelty of the work?
- The paper should contain information related to other techniques of rare earth silicate coating (including Yb2Si2O7 coating) manufacturing.
- The paper should also contain explanation why Yb2Si2O7 was chosen? Why is it better than other RE silicates (for example Lu2Si2O7 or others)?
- What was the thickness of coatings obtained?
- Was the porosity of coatings similar?
- Was the adhesion of coatings to the substrate tested?
- How do these results relate to the results obtained in other works by other authors?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 4 Report
Coatings on turbine blades are fractured by erosion and mechanical loads at high temperatures. The article investigates the influence of temperature and mechanical tensile loads on the failure mechanism of Si/Mullite/Yb2Si2O7 coatings. These studies were carried out in situ by scanning electron microscopy. Such studies are extremely rare/ This is an indisputable merit of the work.. Powder coatings were applied using the plasma spray-physical vapor deposition method. In the initial state, multilayer coatings had big concentration of microcracks, which were formed under the influence of phase transformations and differences in the thermal expansion coefficients of the phases. This predetermined the brittle mechanism of coating failure during tensile tests. The study was carried out at a good methodological level. The results obtained are scientifically substantiated. The coating based on Si/Mullite/Yb2Si2O7 is brittle fractured even when tested in an electron microscope. It can be assumed how this coating will be catastrophically destroyed during the exploitation of the turbine.
I advise the authors to make the following changes in the article:
1.Rewrite section 4. Conclusions. This Conclusions should contain a list of obtained scientific results. At the same time, “researched, discussed, manufactured” are unacceptable. For this reason, the following text “Multilayer Si/Mullite/Yb2Si2O7 EBCS composite coatings were prepared on SiC fiberreinforced SiC ceramic matrix composites (SiCf/SiC CMC) by a PS-PVD method. In this study, the failure behavior of coated specimens under thermal mechanical coupling was investigated, and the failure mechanism was discussed. The thermal failure of EBCS coatings were tested at 766oC with 0-340N loading tension." should be removed from this part 4. The Conclusions should contain only a few items, for example 1. Multilayer Si/Mullite/Yb2Si2O7 EBCS composite coatings are characterized by microporosity and the presence of microcracks resulting from phase transformations and differences in the coefficients of thermal expansion of the phases. These coating defects are not eliminated by preliminary and subsequent heat treatments of the initial powders and coatings. 2. The results show no obvious crack propagation is found on the coating surface under the condition of no tension load with the temperature of the coated sample rises from room temperature to 766 oC, which indicate that the influence of temperature on the failure of the sample is negligible. Next, the authors should rewrite the section according to this example.
- In accordance with the relevance of this study, the authors proposed a coating based on multilayer Si/Mullite/Yb2Si2O7 EBCS composite as promising for aero-engines and turbine engines in the fields of aviation, aerospace, military and civil technology. It is necessary to express your scientific opinion on whether such a coating is promising in these areas of technology or not. Otherwise, it is necessary to change the introduction to the article and reduce the practical significance and leave only the scientific significance.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Most of the comments were addressed. However, the following points need to be fixed before publication:
- The scale bars in Figure 10 and 11 are still unchanged and incorrect.
- line 230: 'indicating that the failure mechanism from the coated
specimens differed from the single 230 temperature factor.' - meaning
of this part of the sentence is unclear.
- line 225, misplaced number '2'
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
I think the paper can be published now
Author Response
Thanks
