Engineering Copper Adhesion on Poly-Epoxy Surfaces Allows One-Pot Metallization of Polymer Composite Telecommunication Waveguides

Round 1

Reviewer 1 Report

The Manuscript covers the interesting topic of innovative and scalable processes for metallization. Where they involved the sequential direct liquid injection metalorganic chemical vapor deposition process as a cost-effective and scalable solution. The article has a discrete structure, well written, and the data support the author's claims. I would suggest this paper for publication with a minor revision. To improve the paper, it is desirable to address the following comments:

1. Although the scaleable process is reported, the Preliminary CFRP waveguide assembly made of 60 mm-long has a non-uniform layer issue, which shows the uncontrollability of the process at a large scale with a larger SD. 
   The author should explain the reason for such large fluctuations. 
   Provide some evidence of real-time measurement of thickness i.e., SEM images or etc.
   If possible add the reproducibility data in the manuscript.
2. High-resolution images are required, as some images look inappropriate 
3. references style needed to re-check

Author Response

We thank the reviewer for his(her) time dedicated to our work ans the constructive comments. We respond to them point by point hereafter

1. Although the scaleable process is reported, the Preliminary CFRP waveguide assembly made of 60 mm-long has a non-uniform layer issue, which shows the uncontrollability of the process at a large scale with a larger SD. 
   The author should explain the reason for such large fluctuations. 
   Provide some evidence of real-time measurement of thickness i.e., SEM images or etc.
   If possible add the reproducibility data in the manuscript.

This remark reveals that the corresponding paragraphs in the initial version of the manuscript were not written in a comprehensive way. Indeed, the surface finishing of the CFRP material in terms of uniform chemical, energetic and morphological characteristics must be improved on purpose during the fabrication of this material. The aim is to allow dense and uniform nucleation of Cu. Also, the deposition process of the Cu film on the inner surface of long tubes with narrow section (it is recalled that the bottom part of figure 4 refers to a 300 mm-long wave guide) has to be optimized considering multiscale and multiphysics computational modeling of the process based on an appropriately defined chemical kinetics mechanism. Both these actions are out of the scope of the present work; they must be implemented in further steps towards the implementation of metalized CFRP waveguides in representative operating environment.

The entire section of the manuscript was rewritten as follows to be more explicite:

In order to identify the reasons of these high insertion losses, we sectioned the 300 mm waveguide into its four constitutive rectangles. The top part of Figure 4c presents a photograph of the bottom inner surface of the waveguide. Visual inspection of the coated CFRP surface reveals that the coating is patterned following the weaving of the composite material. The diagram below the photograph is the corresponding thickness of the film measured at 31, one centimeter equidistant points along the sectioned CFRP plate. The reported uncertainties for each point correspond to thickness variations along the width of the plate at a given length. It is recalled that the thickness was determined by X-ray fluorescence and was verified in SEM cross sectional micrographs as the ones shown in figure 2. We observe that the thickness varies between 5±3 and 1.5±1.5 µm, with a gradual decrease from the gas inlet to the gas outlet sides. Resistivity measurements reveal that it evolves in a similar way, from 18 µOhm.cm in the gas inlet side, to 69 µOhm.cm at the gas outlet one. In addition, observation of both the rectangle plate and the entire waveguide prior sectioning reveals uncoated spots localized at the edges.

These observations illustrate non-optimal control over both the Cu deposition process and the fabrication process of the CFRP waveguide. The latter should focus on the internal surface finishing prior metallization to ensure uniform chemical, energetic and morphological characteristics. The deposition process requires dedicated design of the DLI-MOCVD reactor...

1. High-resolution images are required, as some images look inappropriate 

The suggestion was checked and we rely on the editor’s requests.

1. references style needed to re-check

The references were double checked and numerous corrections were applied.

Reviewer 2 Report

Please see attached report

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for his(her) time spent to our manuscript and for the constructive comments. We considered all of them and replied hereafter point by point.

1. The authors in the manuscript presented methodologies to deposit metals on CFRP. The reviewer thinks the authors can further polish the article by addressing the following issues before the manuscript can be published in Coatings. Since the author has published a similar work in ref. 13, what is the major scientific novelty of this manuscript?

The presented methodologies aim at depositing metals on the internal surface of long CFRP waveguides with narrow section. The results presented in reference 13 refer to an exploratory screening of surface pretreatments of flat CFRP plates for the deposition of Cu films. Neither the ozone pretreatment nor the use of the dimethoxyethane solvent were investigated in that publication. The outcome of the screening was that UV pretreatment maintains satisfactory adhesion with low surface roughness of the CFRP plate and appropriate electrical resistivity of the Cu film. Such directional pretreatment cannot be adapted to the inner surfaces of such a waveguide. However, the work reported in reference 13 serves as a basis for the development of the present approach. We clarify this difference in the appropriate place in the manuscript:

"Our previous studies led to the development of a direct liquid injection metalorganic chemical vapor deposition (DLI-MOCVD) process for forming copper coatings on flat CFRP coupons [13]. Among the different pretreatments we investigated, an ultraviolet, UV radiation based one maintained satisfactory adhesion with low surface roughness of the CFRP plate and appropriate electrical resistivity of the Cu film of 4.3 μΩ*cm. However, such directional pretreatment cannot be adapted to the inner surfaces of a waveguide..."

2. The key term in the ‘title’ is ‘adhesion’, but the author did not provide a single quantifiable measure (e.g. adhesion force, adhesion energy, etc.) for adhesion throughout the manuscript. In line 207, it seems the authors did some adhesion measurement but did not provide any experimental details or quantitative results. What do these data look like and how did you process them?

Insight in the interfacial adhesion was obtained by the cross-cut test, conforming to the ISO 2409 norm. For this, perpendicular incisions (6 × 6) spaced by 2 mm form a grid on the film, on top of which the tape is applied. After 1 min, the tape is manually peeled at a constant rate and under a 60 ° angle. The corresponding scale ranges from class 0 to class 5, from the strongest to the weakest adhesion, respectively. This procedure is reported in the experimental section.

3. Line 31-33 ‘Compared to Al, …, lower’, CFRP is a range of materials with various grades but not ‘A’ particular material with a set property, how did the authors come up with such a comparison? Also, there is no reference.

We agree with the referee on the fact that CFRP is a family of materials; the CFRP in the present work was composed of a thermosetRTM6 resin (Hexcel^®) and was tuned on purpose to meet specifications of space applications. The mean density of CFRP is around 1.8 g/cm3, to be compared with 2.7 g/cm3 for Al. In view of this difference, we replaced the term “two times lower (density)” by “significantly lower”. We assume that the generic data provided in this very introductory part may not need dedicated references.

4. How were the estimations in table.1 calculated?

The term “Estimated” we used in the caption of Table 1 is erroneous. We performed wettability measurements and we determined the surface energies by applying the Owens-Wendt model, as we mention in the text. We replaced in the caption of Table 1 the sentence “Estimated surface energies of pristine and pretreated surfaces” by “Surface energies of pristine and pretreated surfaces”.

5. What do the XPS spectra look like?

The XPS investigation of pristine and pretreated CFRP surfaces was part of our work reported in reference [13]. In that publication, we developed a dedicated methodology to analyze the XPS spectra and we use it in the present work. We replaced in the text the sentence “The surface energies and their polar and dispersive components are presented in Table 1, along with atomic O/C ratios determined by X-ray photoelectron spectroscopy (XPS)” by the one “The surface energies and their polar and dispersive components are presented in Table 1, along with atomic O/C ratios determined by X-ray photoelectron spectroscopy (XPS) following a procedure presented in [13]”.

6. Line 205, how was the thickness estimated?

If the reviewer refers to the 1 µm thickness of the film where blisters start to appear, it was estimated by dedicated experiments where the deposition was stopped immediately after the first blisters appeared, the sample was taken out of the reactor and the thickness was measured by X-ray fluorescence.

Round 2

Reviewer 2 Report

Agree to publish