A Review on In Situ Mechanical Testing of Coatings

Round 1

Reviewer 1 Report

1. The subject of the article is of current interest and in line with the profile of the journal.
2. The authors examined the main articles/works identified and developed considerations appropriate to a review article.
3. Probably for editing reasons, there are some expressions in the paper that seem to have resulted from the unwanted breaking of some findings. For example, “Because the traditional post-processing or post-mortem assessment approaches such as fractography analysis fail to provide a detailed understanding on the dynamic deformation and damage mechanisms of coatings ’material under varied loading conditions.” (Abstract), “While for traditional tensile and bending testing, the initiation of defects such as micro-voids, cracks propagation and the evolution of deformation and damage mechanisms can’t be monitored.”, in Introduction, “While the characteristic X-rays are utilized to define the specimen elemental composition through Energy Dispersive Xray (EDX) analysis.” (rows 96-98), “While in the dynamic TEM (DTEM) technique, the deformation phenomena occurred during high-speed mechanical testing can be efficiently captured, since DTEM has time resolution in the order of tens of nanoseconds [19].” (chapter 2 of the article). In principle, concepts other than those in the article should not be used in the abstract. However, the concept of "post-mortem assessment" is mentioned only in the abstract and I do not think it is a formulation in a scientific spirit.
4. When an abbreviation is used for the first time, an explanation must be included immediately after the abbreviation. Such an explanation seems to be missing in the case of some abbreviations, for example in the case of “OM/SEM”, “FE-SEM”, “HEA”, etc.
5. The paper included numerous figures taken from the works of other researchers. Are there agreements of those who own the intellectual property rights on those figures?
6. It is necessary to write “GYbZ” instead of “GybZ” (row 560).
7. The wording "The toughness of the coatings was assessed by measuring their strain tolerance during the experimental trials." (rows 605-606), “Summary of this research work can be allocated in Table 1.” (row 713), “The limits of current in-situ techniques include limiting temperature of in-situ SEM testing is max. of 600 ° C, TEM technique requires electron transparent and ultrathin samples, DIC testing temperature achieved up to 1600 ° C but no insight on the interior of the material, AE events require complex analysis and affected by acoustic noise.” (rows 724-728), “SEM testing temperature Max. of 600 ° C.” (in Table 1) seem a bit confusing.
8. In the Introduction, the authors could briefly present the historical evolution of research on the subject.
9. There are relatively few critical or comparative assessments in the article. Critical or comparative assessments of the various opinions or findings made by other researchers could be included in the article and not just descriptions of what other researchers have achieved.
10. The works on the paper subject and with a large number of citations should not be missing from the analysis and the list of bibliographical references. This could also include:
11. a) S. Liu, J. M. Wheeler, P. R. Howie1, X. T. Zeng, J. Michler, and W. J. Clegg. Measuring the fracture resistance of hard coatings. Appl. Phys. Lett. 102, 171907 (2013); https://doi.org/10.1063/1.4803928 (cited by 81);
12. b) E. Grigore, C.Ruset, K.Short, D.Hoeft, H.Dong, X.Y.Li, T.Bell. In situ investigation of the internal stress within the nc-Ti2N/nc-TiN nanocomposite coatings produced by a combined magnetron sputtering and ion implantation method. Surface and Coatings Technology, 200, 1–4, 2005, 744-747 (cited by 52);
13. c) Z. Qu, K. Wei, Q. He, R. He, Y. Pei, S. Wang, D. Fang. High temperature fracture toughness and residual stress in thermal barrier coatings evaluated by an in-situ indentation method. Ceramics International, 44, 7, 2018, 7926-7929 (cited by 28);
14. d) H.T.Liu, L.W.Yang, X.Sun, H.F.Cheng, C.Y.Wang, W.G.Mao, J.M.Molina-Aldareguia. Enhancing the fracture resistance of carbon fiber reinforced SiC matrix composites by interface modification through a simple fiber heat-treatment process. Carbon, 109, 2016, 435-443. https://doi.org/10.1016/j.carbon.2016.08.047 (cited by 41);
15. e) Wang, W., Zhang, C., Zhang, ZW., Yi-Cheng Li, Muhammad Yasir, Hai-Tao Wang &

 Lin Liu. Toughening Fe-based Amorphous Coatings by Reinforcement of Amorphous Carbon. Sci Rep 7, 4084 (2017). https://doi.org/10.1038/s41598-017-04504-z (cited by 15), etc.

11. Although English is in principle used correctly, the authors may pay more attention to editing the article and expressing it in English. For example, the wording "understanding of the dynamic deformation" seems to be more correct instead of "understanding on the dynamic deformation" (in Abstract), "... the ongoing need ... has made this approach ...", instead of “... the ongoing need ... have made this approach ...” (in Introduction), “A schematic representation to outline the logical flow of in-situ monitoring technique which includes sample preparation”, instead of "A schematic representation to outline the logical flow of in-situ monitoring technique which include sample preparation", etc.

One can write "Young's modulus" instead of "young's modulus" (row 152), "inside a SEM", instead of "inside an SEM" (row 176), "the tensile experimentation primally uses smoothly shaped specimens", instead of “the tensile experimentation primally use smooth shaped specimens” (lines 469-470), “As mentioned earlier, Zhu et al. [48] ​​evaluated the thermomechanical loading effect”, instead of “As mentioned earlier, Zhu et al. [48] ​​to evaluated the thermomechanical loading effect”, etc.

More attention should be paid to the use of the comma: for example, a comma is not required for "It was demonstrated that, the heat" (rows 221-222), "Figure 14 (a), shows" (row 435), etc. A dot is not required after the title of a subchapter (row 334). It is not necessary to use capital letters in the case of the concept “Unirradiated Tensile sample” (in the legend of Figure 12).

A blank space must be placed between the number and the unit of measurement in the case of the '”96s” (row 450).

The t symbol used in subchapter 3.3 could be written using italics.

The bibliographic reference [68] is missing the volume number (20).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear authors,

when I read the abstract of your review, I was very excited to read the paper and to learn about new techniques or insights in coatings and get “deep insights on the mechanical behaviour and material failure with remarkable range and resolution of length scales, microstructure and loading conditions.“

However, after reading the whole paper, I was disappointed. Many of the investigations shown here could also be revealed by ex-situ tests. This is more or less the case for most of the results. The focus of a review should lay on a critical description of the state of the art of experiments where you can get additional information by in-situ investigations you could not get from ex-situ-testing.

It starts with the introduction where you can´t find any discussion about open questions which could not be answered by ex-situ testing. There is no summary of the state of the art for ex-situ and the expectations on in-situ testing. There are no problems and limitations of the different techniques discussed.

The whole paper is more or less a list of different results about in-situ (if necessary or not) investigations on coatings. This could also be an interesting lineout for a review, however, in this case it should not be organized by methods but by open questions or problems of coatings. And in this case, title and abstract are misleading and the organization of the paper is confuse.

The whole paper looks more or less like the introduction of a master or PhD thesis. This impression is supported by the relative low number of cited papers which is with 70 references actually less than for a normal scientific paper in the same length.

Summarizing: For a review the requirements are very high. In my view, the paper cannot fulfill the high scale for such a comprehensive paper in the actual state. In case you decide to make a comprehensive revision of the paper (which will take several weeks, if you make it seriously) here are some minor comments:

Lines 9-12: Please check, if this sentence is complete

Keywords: Why do you write some techniques with capital letters and others not?

Lines 47-52: “Microscopes are normally … final failure.”: This is a repetition.

Lines 60-62: “While for traditional tensile and … can’t be monitored.” If you perform these test in-situ (as you describe very detailed in the review) you can monitor these processes.

Line 41-67: The whole paragraph is not organized very well.

Lines 76: DIC systems (with lowercase letter)

Line 79: Why do you consider the AFM as an in-situ testing tool and not a microscope (Atomic Force Microscope). And a general question: Why do you mention and show in Fig. 1 AFM and nanoindentation but don´t show any result obtained by these techniques without any comment.

Line 85: ´Do you mean micro-tensile samples?

Lines 89-95: You don´t have to explain how a SEM works, this is the most common technique in materials science. You don´t explain the other techniques also. However, it would be interesting, what´s about the resolution and contrasts (information) you can get.

Line 97: “…Energy Dispersive Xray (EDX)…”: X-ray

Lines 110-112: In my view, the TEM is useful for some very special investigations only. For coatings you typically need a substrate which is quite thicker than the specimen and then the whole sample is no more transparent for electrons. You only show one example. Therefore, you should mention this as marginal phenomenon.

Line 119: “…during high speed mechanical testing”: In most cases, not high speed testing is the problem but the high speed of the physical processes even during nearly static loading.

Line 129-131: “…permitting monitoring of coating response under loading [1].”: In special cases. Normally, you also get failure AE-events from the substrate.

Lines149-151: “…failure mechanism arises within the interior of materials…” What do you mean by "interior" Interior of the substrate or coating? And: In most cases (SEM top-view), you also have no insight in the coating, just on the surface.

Lines 156-157:” Furthermore, to differentiate between varied mechanisms occurred inside materials due to…”: Not only inside materials is important. Especially for coatings: What happens at the interface?

Lines 191-192: “For instance, the NiAl-GNP composite coatings exhibited 60% improved tensile strength and 25% higher ductility…”: Of the coating? Or the whole sample? In case of coating only: How was is tested? In case of whole: What´s about the substrate and the thickness of substrate and coating?

Line 200: “In addition, the effects of post heat treatments…”: Post treatment means after the coating (not loading), you should mention here.

Fig. 6: More information needed about what is shown in the figure.

Lines 257-258: “It was found that, the saturation crack density reduced with rising gold layer thickness.”: This is very important: for ultra-thin layers the sample preparation by conductive layer have great influence on the results.

Line 290: Start a new paragraph after figure 8.

Fig. 8: What are the different particles? What´s about the gray matrix in the cathode? The cracks develop in this matrix which is obviously not present for the anode.

Lines 313-314: ”The evolution of strain fields during in-situ tensile testing of TBCs is illustrated in Figure 9.”: What´s about the AE results? Do they correspond to the cracking events? This would be the interesting question for a review.

Lines 363-364: “Also, the ductility and strength of the coated Zr-4 specimens were enhanced as compared to uncoated specimens.”: For all these results: Why is here in-situ SEM necessary? All of these results could also be revealed by ex-situ tests. This is more or less the case for most of the results shown here.

Lines 386-388: “…with the use of AE signals to measure DBTT.”: What are the results from the AE measurements?

Lines 392-393: “The ER response increased because of the progression of damage.”: Too short to understand the results and why in-situ-testing is helpful.

Lines 393-394: “The strain fields capture by DIC attained good agreement with AE energy analysis…”: Which AE? Is this related to Ray et al.?

Line 386-394: This is a good example for the inflationary use of abbreviations. Please reduce the abbreviations to the words you find more than 4 times in the paper in near passages.  

Lines 416-417: “Although the materials strength is increased, the sample fractured in a brittle manner at a very small strain about 2%.”: Actually, in Fig. 13, the fracture strain for the irradiated specimens is between 10 % and 26%

 Lines 461-462: “…and crack propagated through the phase interface, resulting in final rupture.“: This would be of interest for a review: How could you measure substrate and coating in the TEM?

Line 473-475: “Also, more investigations on the behaviour of coatings’ material during elevated temperature (more than 800 °C) and high deformation rates using SEM technique should be made.”: And this would also be interesting: How to perform SEM-measurements at such high temperatures, where you have a lot of thermal electrons emitted and the detectors become blind. You just mention this in the summary, this should be discussed already here (Actually while reading, I had the question here).

Fig. 17: These are just SEM or optical images but not DIC. They may be used for DIC but they are not.

Line 560: “GybZ“: should be GYbZ.

Fig. 19: Bad quality of the figure

Line 643: “…respectively as shown in Figure 23.”: Is this from experiment or FE modelling?

Line 656-659: “It was also suggested that the strain evaluation using in-situ tensile testing can be used as a qualitative measure for the fracture toughness of hard coatings instead of stress intensity in front of the crack tip at crack initiation.”: This would be of fundamental interest. And this could be one point where you really need in-situ-observation to find the point of crack initiation. Could you please provide more information about that?

Line 699: “…when the temperature exceeded 800 °C…”: I the summary you write above 600°C. See also my comment about lines 473-475

Line 701: “achieve”: This should be achieved

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Dear authors,

after the comprehensive revision of your paper, it is now more suitable for a review paper. Especially in the introduction where the different techniques are introduced, some critical evaluations of these techniques are provided. The classification and the state of the art is now more in focus and has been revised or even been written newly. There are many references added and the descriptions of the various experiments and their results are much more precisely now.

However, I still think, that many of the experiments presented here, could also be performed ex situ. At least this is critically commented now. And it shows, that it is not so easy to create in-situ experiments for many reasonable questions due to the small size of the coatings in one dimension.

All minor comments are corrected, commented, or could be destroyed by your argumentation. However, I think, that the revisions were necessary in many cases.

Therefore, I can recommend the manuscript for publication now.

Best regards
Your reviewer