Tribocorrosion Performance of WC-12Co HVOF-Sprayed Coatings Reinforced with Carbon Nanotubes

Round 1

Reviewer 1 Report

The manuscript “Tribocorrosion performance of WC-12Co HVOF-sprayed coatings reinforced with carbon nanotubes” by M. H. Staia, A. Mejias, S. Kossman and E. S. Puchi-Cabrera is dealing with the study of the influence of carbon nanotubes (CNTs) addition on the tribocorrosion performance of WC-12Co HVOF-sprayed coatings. The manuscript is well written, the results obtained are significant for the research line under question, and clearly presented and discussed.

Nevertheless, some points should be clarified.

Line 108 Usually, the term “multiwalled” is used in the literature, and subsequently the abbreviation MWCNTs for multiwalled carbon nanotubes.

Line 232 – 233 “As explained in section Error! Reference source not found., before wear…” The sentence should be checked.

A relevant issue when MWCNTs are used as reinforce component if the MWCNTs maintain their outstanding mechanical properties after incorporation in the coatings. Also, it is not clear from the text about the interface MWCNT/coating, whether it was direct contact or with intermediate oxide layer? The mechanism of the load transfer between MWCNT and coating material is important for analysis of the tribological behaviour. During the tribocorrosion tests, in the MWCNTs so called “glove effect”, when the internal layers (nanotubes) slide with respect of the outer nanotube, might be present. These issues should be addressed in the manuscript.

Moreover, for characterization of the tribological behaviour of the coatings, the nanohardness tests can be valuable tool. If the authors considered this technique in their study?

Author Response

Dear Editor,

Thank you for processing and reviewing our work. Please find here below the responses to the reviewers’ comments. For the revised manuscript, please see the attachment. 

Reviewer No. 1:

1. Reviewer’s comment : Line 108 Usually, the term “multiwalled” is used in the literature, and subsequently the abbreviation MWCNTs for multiwalled carbon nanotubes.

Answer 1: The abbreviation CNTs has been replaced by MWCNTs in the whole document to refer to the multiwalled carbon nanotubes as it is commonly used in the literature.

2. Reviewer’s comment: “As explained in section Error! Reference source not found., before wear…” The sentence should be checked.

Answer 2: Yes, you are right. It has already been corrected in the manuscript (line 250-251 in the revised manuscript).

3. Reviewer’s comment: A relevant issue when MWCNTs are used as reinforce component if the MWCNTs maintain their outstanding mechanical properties after incorporation in the coatings.

Answer 3: We agree with Reviewer No. 1. Indeed, the outstanding properties of the MWCNTs have led many researchers to use them as reinforcement materials. The literature survey of these studies are mentioned in our previous publications (see references [14,16] of the present manuscript). 

4. Reviewer’s comment: Also, it is not clear from the text about the interface MWCNT/coating, whether it was direct contact or with intermediate oxide layer?

Answer 4: The details of the blending process, as well as the results of the particle size distribution analysis before and after the dispersion process were described in our previous work [14]. The morphological and chemical analysis was carried out by using Field Emission Scanning microscope equipped with an Energy Dispersive X-ray Spectrometer (EDS) and micro-Raman analysis and no detection of any oxide at interface between coating and MWCNTs was found.

Since, a thorough investigation was reported in ref [14], a detailed characterization falls beyond the scope of this study. Accordingly, we have mentioned it in the experimental procedure in relation to the samples preparation (lines 138-145 in the revised manuscript).

5. Reviewer’s comment: The mechanism of the load transfer between MWCNT and coating material is important for analysis of the tribological behavior. During the tribocorrosion tests, in the MWCNTs so called “glove effect”, when the internal layers (nanotubes) slide with respect of the outer nanotube, might be present. These issues should be addressed in the manuscript.

 Answer 5:  It was shown in ref. [16] that MWCNTs reinforcement increased the wear resistance by forming inter-splat bridges and acting as a lubricant, whereas the improvement in hardness was attributed to their deformation resistance. Additionally, it has been determined that the fracture toughness increased for the WC_CNT coating as compared to WC_CONV coating by approximately 30%, which also explained the improvement in the wear resistance of the reinforced material. Therefore, the  H³/(E*)² ratio should be considered since it is a measure of the resistance to plastic deformation of the coating. In the above ratio, H represents the hardness and E* the plain strain elastic modulus.

In our case, taking into consideration that the addition of MWCNTs did not give rise to any significant change in the elastic modulus, the hardness alone can be used as a reference parameter for anticipating the wear behavior.

Related to the “glove effect” indicated by Reviewer No. 1 or to the phenomenon usually referred to as “telescoping effect” [26], yes, it is true that inner tubes of MWCNTs are susceptible to be pulled-out of the outer tube by tensile stresses. However, in our case, in the reinforced coating there is a random orientation distribution of these MWCNTs, which minimizes this effect (this point has been addressed in lines 424-438 in the revised manuscript).

6. Reviewer’s comment: Moreover, for characterization of the tribological behaviour of the coatings, the nanohardness tests can be valuable tool. If the authors considered this technique in their study?

Answer 6: We agree with Reviewer No. 1. In fact, hardness, apparent yield stress and fracture toughness calculated from instrumented indentation tests were previously evaluated and reported in https://doi.org/10.1080/02670844.2018.1529285  (reference [16] in the manuscript). These values of hardness and fracture toughness were considered in the discussion of the manuscript. This issue was not detailed addressed in the present work.

Reviewer 2 Report

Comments of crystals-2249016

 

The main weaknesses of the manuscript:

 

1.     The title reads well and is appealing for the readership of Crystals.

2.     The individual paragraphs of the introduction read disconnected. Please smoothen the transition of the individual paragraphs. The novelty of the study must be better worked out.

3.     The carbon nanotubes cannot be seen in SEM image (Fig. 3), it is necessary to re-do this graph and illustrate in detail.

4.     What are the respective coating thicknesses? On which base?

5.     No information about the mixing procedure has been provided.

6.     No information about the dispersion stability of the carbon nanotubes has been presented.

7.     The Optical analysis (Fig. 6) is rather superficial. Please try to present better and more discussion.

8.     Conclusions must be shortened and streamlined.

Author Response

Dear Editor,

Thank you for processing and reviewing our work. Please find here below the responses to the reviewers’ comments. For the revised manuscript, please see the attachment.

Reviewer No. 2 :

1. Reviewer’s comment: The individual paragraphs of the introduction read disconnected. Please smoothen the transition of the individual paragraphs. The novelty of the study must be better worked out.

Answer 1:  The introduction has been modified and some sentences and connectors have been added to improve the readability of this section. The last paragraph of the introduction has been slightly modified to highlight the novelty of this research work (lines 54-56, 112 and 116-118 in the revised manuscript).

2. Reviewer’s comment: The carbon nanotubes cannot be seen in SEM image (Fig. 3), it is necessary to re-do this graph and illustrate in detail.

Answer 2: After the HVOF-sprayed coating were deposited by using, for example, the 36h-milled MWCNTs-reinforced conventional feedstock powder, the MWCNTs imbedded in the metal matrix composite were not distinguishable. Since the samples were subjected to tribocorrosion tests, unfortunately, it is not possible to illustrate such details.           

3. Reviewer’s comment: What are the respective coating thicknesses? On which base?

Answer 3: The thicknesses of coatings were measured by using SEM micrographs and are reported in Table 1. Accordingly, HVOF-sprayed coating WC_AS-RECEIVED  achieved a thickness of 453 ± 5 μm, whereas the WC_CONV coating and the WC_CNT coating  achieved a thickness of 540 ± 5 μm and
520 ± 6 μm, respectively. The HVOF coatings were deposited on SAE 1020 steel substrates. This last missing information has been pointed out in the manuscript (lines 147 and 150 in the revised manuscript).

4. Reviewer’s comment: No information about the mixing procedure has been provided.

Answer 4: The mixture processing has been clarified in the text (lines 138-145 in the revised manuscript).

5. Reviewer’s comment: No information about the dispersion stability of the carbon nanotubes has been presented.

Answer 5: The details of the blending process, as well as the results of the particle size distribution analysis before and after the dispersion process were described in our previous work [14]. The morphological and chemical analysis was carried out by using Field Emission Scanning microscope equipped with an Energy Dispersive X-ray Spectrometer (EDS) and micro-Raman analysis and no detection of any oxide at interface between coating and MWCNTs was found.

Since, a thorough investigation was reported in reference [14], a detailed characterization falls beyond the scope of this study. However, it has been mentioned it in the experimental procedure in relation to samples preparation (lines 138-145 in the revised manuscript).

6. Reviewer’s comment: The Optical analysis (Fig. 6) is rather superficial. Please try to present better and more discussion.

Answer 6: The revised manuscript has been modified considering this remark, and a broader discussion about the analysis from the optical profilometry images has been addressed (lines 341-351 in the revised manuscript).

7. Reviewer’s comment: Conclusions must be shortened and streamlined.

Answer 7: This section of the manuscript has been modified to shortly expose the relevant conclusions of this research work (lines 521-546 in the revised manuscript).

Reviewer 3 Report

Paper: Tribocorrosion performance of WC-12Co HVOF-sprayed coatings reinforced with carbon nanotubes present very interesting experimental results among with proper conclusions. The authors manage to evaluate the tribocorrosion performance of a WC-12Co coating reinforced with multiwall CNTs (using HVOF process) and compare it with that of the unreinforced coating deposited under similar process conditions with analogous feedstock powders in terms of phase composition and porosity. One of the main realizations of the authors in this paper is an innovative in-house designed and manufactured tribocorrosion cell (TC) conceived for this kind of samples. Thus, the computation of the wear volumes and wear constants of the different coatings tested in a 3.5% NaCl solution allowed an understanding of the role played by the reinforcement of CNTs on the tribocorrosion performance of these cermet materials. 

Few minor corrections can be performed in order to improve the quality of the manuscript: 

- how was the Coating porosity (%) from Table 1 determined ? or the coating thickness ? were the results experimentally determined with a software program ?  

- please indicate the type of the electrolyte: NaCl electrolyte , (how much NaCl is in solution?) 3.5%

- L189: mention if metallization was used for SEM experiments 

- L232: clarify the Section number: Error! Reference source not found.,  

- re-structure the Conclusions section 

 

 

 

 

Author Response

Dear Editor,

Thank you for processing and reviewing our work. Please find here below the responses to the reviewers’ comments. For the revised manuscript, please see the attachment.

1. Reviewer’s comment: how was the Coating porosity (%) from Table 1 determined ? or the coating thickness ? were the results experimentally determined with a software program ?  

Answer 1:  The thicknesses of coatings were measured by using SEM micrographs and are reported in Table 1. Accordingly, HVOF-sprayed coating WC_AS-RECEIVED  achieved a thickness of 453 ± 5 μm, whereas the WC_CONV coating and the WC_CNT coating  achieved a thickness of 540 ± 5 μm and
520 ± 6 μm, respectively. The HVOF coatings were deposited on SAE 1020 steel substrates. This last missing information has been pointed out in the manuscript (lines 147 and 150 in the revised manuscript). The porosity values were determined by ten measurements performed within the coating cross-section by using an image analyzer software.

2. Reviewer’s comment: please indicate the type of the electrolyte: NaCl electrolyte , (how much NaCl is in solution?) 3.5%

 Answer 2:  The electrolyte employed was a 3.5% NaCl electrolyte, this is pointed out in line 179 of the revised manuscript.

3. Reviewer’s comment: L189: mention if metallization was used for SEM experiments.

 Answer 3: Non, metallization was not used for SEM experiments.

4. Reviewer’s comment: L232: clarify the Section number: Error! Reference source not found.

 Answer 4:  Yes, you are right. It has already been corrected in the manuscript (line 250-251 in the revised manuscript).

5. Reviewer’s comment: re-structure the Conclusions section.

 Answer 5:  This section of the manuscript has been already restructured (lines 521-546 in the revised manuscript).

Reviewer 4 Report

The authors have studied the corrosion behavior of HVOF WC-12Co sprayed coating reinforced with carbon nanotubes (CNTs). The authors chose the corrosion medium as 3.5wt%NaCl. The results are interesting but there are minor issues.

1. Introduction should be improved concerning corrosion of new alloys, composites and processes on corrosion resistance. Some important references should be added, e.g., https://doi.org/10.1016/j.surfcoat.2014.11.023;  https://doi.org/10.3390/ma15238531

2. Figure 1 should be improved. The various parts of corrosion set-up are not clearly visible.

3. Please check the current density in Fig. 4. It is too high. Is it in mA or micro Amps? please check. 

4. What is the mechanism of corrosion protection in CNT reinforced matrix?

5. Please add scale bars in Figure 6. 

6. Figure 9 should be clubbed together for better representation.

7.  Conclusions should be in bullet format. Only important conclusions should be presented. 

8. Additionally, English should be improved.

 

Author Response

Dear Editor,

Thank you for processing and reviewing our work. Please find here below the responses to the reviewers’ comments.

Reviewer No. 4:

1. Reviewer’s comment: Introduction should be improved concerning corrosion of new alloys, composites and processes on corrosion resistance. Some important references should be added, e.g., https://doi.org/10.1016/j.surfcoat.2014.11.023; https://doi.org/10.3390/ma15238531.

 Answer 1:  The introduction has been improved and some sentences have been added to point out other corrosion techniques to evaluate the corrosion performance of metal matrix composites and the references https://doi.org/10.1016/j.surfcoat.2014.11.023 and  https://doi.org/10.3390/ma15238531 have been included (lines 90-93 in the revised manuscript).

2. Reviewer’s comment: Figure 1 should be improved. The various parts of corrosion set-up are not clearly visible.

Answer 2: The figure 1 has been improved and the components of the tribocorrosion cell are clearly shown (line 190 in the revised manuscript).

3. Reviewer’s comment: Please check the current density in Fig. 4. It is too high. Is it in mA or micro Amps? please check.

 Answer 3: The unit of the current density has been checked and it is expressed in mA. This have been correctly rewritten in revised manuscript.

4. Reviewer’s comment: What is the mechanism of corrosion protection in CNT reinforced matrix?

 Answer 4: The present results indicates that the presence of MWCNTs in addition to WC will slightly enhance the dissolution of the Co matrix due to the galvanic couple that is formed between the matrix and the mixture of WC and MWCNTs. Thus, the selective dissolution of the matrix would give rise to the presence of a somewhat greater quantity of isolated constituents. Therefore, the addition of MWCNTs is not likely to improve the corrosion behavior of these coatings (this has been included in lines 449-453 in the revised manuscript).

5. Reviewer’s comment: Please add scale bars in Figure 6.

Answer 5: The scale bars have been added to the 2D optical profilometry images (line 336 in the revised manuscript)

6. Reviewer’s comment: Figure 9 should be clubbed together for better representation.

Answer 6: Thank you for this advice. However, authors decided to present the wear-track profiles in three separate figures to better highlight the overall profile and the reference points defining the width of the wear-tracks. It has been an intended choice in the visualization of data (line 501 in the revised manuscript).

7. Reviewer’s comment: Conclusions should be in bullet format. Only important conclusions should be presented.

Answer 7: The revised manuscript has been modified considering your observation (lines 521-546 in the revised manuscript).

8. Reviewer’s comment: Additionally, English should be improved.

 Answer 8: The manuscript has been carefully read and revised. Some sentences have been modified to improve the readability of the document.

Round 2

Reviewer 1 Report

The authors have addressed appropriately the reviewer´s comments. The manuscript can be published now.

Reviewer 2 Report

Comments of crystals-2249016

The manuscript is well revised and can be published in present form.