Remote Thermoelastic Characterization of Candidate Structural and Protective Coatings for Lead-Bismuth Eutectic Cooled Nuclear Reactors

Round 1

Reviewer 1 Report

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The manuscript is presenting a method of contactless characterization of mechanical and thermal properties of protective coatings. After a clear introduction presenting the context and the preparation technique of the coating, as well as the aim of the paper, the sample are presented and characterized using SEM and XEDS technique. The methods used for the mechanical and thermal characterization are then quickly explained. The experimental setup allowing to perform the proposed measurement is then scrupulously described. The measured SAW dispersion curves are then presented and fitted to determine the Yong's modulus and the thickness of the coating. The Young's modulus is compared with another measurement done by nano-indentation showing good agreement. The thickness measurement is consistent with that measured using the SEM image. A topographic mapping of the thickness, of the Rayleigh velocity and of the density of the coating is also proposed, showing that the three coating parameters could not be extracted simultaneously. Finally, the thermal diffusivity of the coating layer is obtained thanks to a fitting of the (k,t)-signal of the thermal decay profile with a two-layered thermal model.The obtained value of the thermal diffusivity is consistent with the literature.

The presented methods and results would be of interest for the readers of Applied Sciences. My overall impression of the manuscript is good, although I would recommend the authors to add a section to present, even shortly, the used model for the fitting of the acoustic and the thermal experiments. The sample the authors have decided to characterize was far from being a simple sample and the obtained results are better than I would have expected, which shows that the used experimental is quite robust. The found misprints along the manuscript, together with my comments and questions are listed below.

Comments/Questions:

p. 2, l. 52: It is hard to feel from the introduction if all relevant references are included. Why the authors have chosen to cite the three papers [3-5] related to the characterization by laser ultrasonic inspection and not other one? I would suggest the authors to enrich the bibliography in the introduction for the reader to have a clear view of the background and of how their work is situated.

p. 2, l. 66: How the cross sections are made? Could the process play a role in the discrepancy between the estimated thicknesses obtained by SEM and FIB/SEM? What the authors meant by "FIB treated sample" at line 70?

Section 2, "Characterization approach": The presentation of the characterization approach is ok, although I felt a lack of explanation regarding the used model. I would recommend the authors to add here or later a short section to present the used model for the fitting of the acoustic and the thermal experiments. For instance, it was unclear to me until I read the caption of Figure 2 whether the model for the elastic guided waves considered the graded properties of the materials or not.

Section 3: while finishing the reading of that section, it was unclear to me wether the measurements with the slow and the fast photo detector was supposed to be conducted simultaneously or not. I would suggest the authors to add accordingly a clear statement related to this in the section 3, although I may have finished to understand in the following section that it was independent measurement.

The last part of Sec. 3 discussing the iterative position optimization is not crystal clear to me and I would suggest the authors to add some sentences in the main text to explain what is represented in Fig. 6.

p. 12, l. 297-301: Could the difference in the measured Young's moduli by the two approaches be associated with the graded properties, since the indentation probes the first 2 µm of the sample depth only, while the fitting gives an averaged value over the whole coating thickness?

p. 16, l. 374-384: Could the authors try to make that paragraph clearer? I did not indeed really understand why assuming 2 of the 3 fitting parameters to be known would lead to more consistent results.

Misprints/Proposed corrections:

p. 2, l. 41: please change the symbol between RRMS and 3 µm

p. 3, l. 82-83: please remove one of the two "in", the coma between "Table" and "1", and "as".

p. 3, l. 100-101: please delete here and everywhere else along the manuscript this kind of draft text: figurefigurefigure ...

p. 4, Fig. 1: in part (b) please change the color of the arrow and of the text to white

p. 5, Fig. 2: please change "frequency" to "angular frequency" and fix the issue with the caption.

p. 5, l. 134: the roughness was previously discussed with the value 3 µm. What has changed here? Could the authors explain this in the text?

p. 6, l. 158: please insert a white space (here and everywhere else along the manuscript where needed) between the number and the unit.

p. 7, l. 166: please consider removing the capital "T" of "Transforming".

p. 7, l. 183: please close the parenthesis between "axis" and "."

p. 8, Fig. 5: in the caption, please insert "polarizing" between "second" and "beam splitter".

p. 8, l. 194: please delete the extra ")."

p. 10, l. 246: please change "8(b)" to "8(a)".

p. 12, l. 296: please change the reference [CG1] to match a listed reference number in the bibliography.

p. 13, l. 318: please change "Figure 9" to "Figure 10".

p. 14, l. 336: please make a reference to the section presenting the coating/substrate model when it would have been added.

p. 14, l. 349: please correct for "Poisson's ratio".

p. 14, l. 349-350: please make sure about the wanted citation [14] or [1] and delete "citeweisenburger2008t9".

p. 15-16, Figs. 11-12: please consider enlarging the font size.

p. 16, l. 389: please insert a white space between "[21,22]" and "inferring".

p. 16, l. 394: please consider deleting "Rc" from this line as it is defined on the next line.

p. 17, l. 418: please provide a definition for "ISS".

p. 18, l. 426: please change "gray scale" for "color scale".

p. 18, l. 443: please remove the extra ")".

p. 19-20, Figs. 15-16: please consider to make only one figure out of those two.

p. 19, l. 470: please change "Table ??" for "Table 4".

p. 20, Fig. 16: in the caption, please change "an fit" for "a fit".

p. 20, l. 484; please change "an unique" for "a unique".

p. 22, Fig. 19: in the caption, please change "black" for "red".

References: please change the journal name in Refs. [3-5, 10] from all capital letters to the same style as the other references.

Author Response

see attached file

Author Response File: Author Response.pdf

Reviewer 2 Report

Laser ultrasonic based method is used to determine the thickness of the coating. Results are interesting, but more than that the methodology was significantly intriguing to me. The only thing that bothers me is slightly the language of the text in which some points are as a lecture format rather than a paper. Alongside with this, some figures can be improved by, for instance, enhancing the font sizes. Please find some point I noticed below:

 

-        Abstract is different with a technical summary. You may bring some point about the motivations. How about conclusions?

-        Line 37: superior could be replaced with other adjectives.

-        Line 41: what is ! upside down?

-        line 52: how about [3,4, and 5]?

-        Figure 11, 12 and 18, larger fonts?

 

I believe the experimental results are worthy to be published, but just the writing of the draft needs to be polished to be more native and understandable.  Minor revision is requested.

Author Response

see attached file

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors,

I would first want to thank you for your updated manuscript and for your answer to my questions.

My overall impression of the manuscript is good and I have only minor changes suggestions to make the manuscript suitable for publication. Since the resubmitted manuscript does not contain line numbers, I will refer to the page numbers only for my comments.

• p. 2: Please reformulate the following sentence that I failed to understand: "Often a focused pulsed laser beam was used to impulsively and locally stress, generating wideband surface acoustic waves.".

• p. 2: Please change the following sentence that is actually not a sentence (no verb) ! "By optically detecting the wave induces surface displacement [8], thickness [9][10], elastic and structural information [11][12][13][14][15].".

• p. 5: Please fix the caption of figure 2 where the font size is abruptly increased.

• p. 6: please add "and" in the following sentence: "..., and to references [42] and [18] for a full model..."

• p. 6: "to detect the normal surface displacements induced by the waves" The laser beam deflection technique is usually not proportional to the normal surface displacements but to the tangential component of the gradient of the normal displacement. Could you please explain why in your case the technique detect the normal displacement or change the sentence to be more accurate on what is actually measured? If it is not the normal displacement that is measured, please changed all references to normal displacement within the manuscript (including figure captions) with the correct quantity.

• p. 7: Please add a space between 0.5 and the unit in "(in steps of 0.5 µm)".

• p. 7: Please delete the coma and change it for "were" in: "The dynamic variations of the light intensities of the knife-edged probe beams were collected by two differential photodiodes:...".

• p. 9: Please consider putting the unit of the signal grating S(x,t) between parenthesis: "(Vpp)".

• p. 11: Please change "(b)" for "(a)" in "In figure 8(a), the temporal evolution of the normalized normal displacement...".

• p. 11: Please change "(a)" for "(b)" in "... domain, as is shown in Figure 8(b).".

• p. 11: When talking about the uncertainty of the extracted frequency, please precise how the width of the peak has been considered (width at half maximum, ... ?).

• p. 12: The uncertainty for the phase velocity accounts for the uncertainty on the frequency but not on the wavelength. Please consider explaining why in the main text.

• p. 12: Please delete "figurefigurefigure".

• p. 12: Please change cS for cT in "Via the relations cT=...".

• p. 12: Please change from "Young's modulus E1..." to "Young's moduli E1 and E2 of the coating and substrate materials, respectively, were derived...".

• p. 12-13: The discussion that starts by "In reality ..." and finishes on the next page at the end of the first paragraph ("..., we refer to [37].") should be considered to be moved at the end of the section 4, because it is explaining why the Young's modulus calculated with the SAW and obtained by nano-indentation are different. Please consider also adding a comment on the strong assumption about the Poisson's ratio (=0.3) in this discussion.

• p. 13: Please add a space between "calculated" and "[50]".

• p. 16: Please verify the numbering of the figures in the main text. To me, both referred figures in this page should be the same: Figure 11, whereas currently it is Figure 3 and Figure 12.

• p. 17: Please consider adding a reference to Figure 13 and remove the comma before "was" in the following sentence: "The value of RRMS = 2.38 µm was consistent with the roughness value of 2.18 µm found by white-light interferometry (see Fig. 13) [52, 53].

• p. 20: Please delete "figurefigurefigure" and delete the paragraph jump between "relative" and "fitting".

• p. 22: Please delete "figurefigurefigure"

Author Response

Dear reviewer,

thank you very much for the thorough revision. Please find hereby our replies to your valuable suggestions, followed by a version of the LATEX file with changes marked.

Sincerely yours,

Author Response File: Author Response.pdf