Study of the Nature of the Destruction of Coatings Based on the ZrN System Deposited on a Titanium Alloy Substrate

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The submitted paper deals with the coating strength and the film-substrate adhesion strength of films based on ZrN system with the introduction of Ti, Nb and Hf. The coatings were deposited on the same titanium alloy substrate. It’s an interesting experimental work. The different film wear mechanisms developed during scratch testing were investigated for each examined coating case considering the remaining imprints.

 

My main remarks are the following:

 

1.                       The authors must make clear the novelty of this work in the text, since these coating systems have been thoroughly investigated in the past.

2.                       More details about the coatings used are required (coating thickness for each case, deposition parameters etc.)

3.                       For measuring the films’ hardness, nanoindentation were conducted with a maximum load of 200 mN. Depending upon the film thickness and the applied load, the effect of the substrate mechanical properties on the determined hardness may be critical and must be excluded.

4.                       In general, the developed film wear mechanisms during scratch testing can be attributed to insufficient film cohesion or/and insufficient film adhesion to its substrate. In this work, the authors based on the remaining scratch imprints try to correlate the type of developed wear with a specific reason i.e coating failure or bad adhesion. Some references showing that specific imprint geometries are associated with certain reasons would be useful.

Author Response

Comments and Suggestions for Authors

R: The submitted paper deals with the coating strength and the film-substrate adhesion strength of films based on ZrN system with the introduction of Ti, Nb and Hf. The coatings were deposited on the same titanium alloy substrate. It’s an interesting experimental work. The different film wear mechanisms developed during scratch testing were investigated for each examined coating case considering the remaining imprints.

A: The authors are grateful to the Reviewer for his attention to the article and valuable recommendations that help improve the quality of the manuscript.

My main remarks are the following:

 R: 1. The authors must make clear the novelty of this work in the text, since these coating systems have been thoroughly investigated in the past.

A: In this paper, not only is the fracture resistance of various coatings deposited on a titanium alloy substrate compared, but the nature of this fracture is also investigated depending on the compo-sition of the coatings. Considerable attention was paid to the issues of adhesion of coatings of various compositions and titanium substrate. The authors were unable to find a similar comparison of the fracture toughness of a large group (six coatings of different compositions) of coatings deposited on a titanium alloy substrate. It is possible that such a comparison, including an SEM study of cross-sections, has been made for the first time.

 

R: 2. More details about the coatings used are required (coating thickness for each case, deposition parameters etc.)

A: Included Figure 1 - structure of the studied coatings (TEM). The deposition parameters of the coatings are described in the Materials and Methods section.

R: 3. For measuring the films’ hardness, nanoindentation were conducted with a maximum load of 200 mN. Depending upon the film thickness and the applied load, the effect of the substrate mechanical properties on the determined hardness may be critical and must be excluded.

A: The author apologizes for the typo - the measurements were carried out on a nanoindentometer with a load of 20 mN in order to minimize the influence of the substrate on the measurement results. At least 20 measurements were carried out to obtain reliable data (excluding the influence of the phase of microparticles, cluster structure, surface microroughness and other factors).

R: 4. In general, the developed film wear mechanisms during scratch testing can be attributed to insufficient film cohesion or/and insufficient film adhesion to its substrate. In this work, the authors based on the remaining scratch imprints try to correlate the type of developed wear with a specific reason i.e coating failure or bad adhesion. Some references showing that specific imprint geometries are associated with certain reasons would be useful.

A: A number of relevant references to fundamental works describing various mechanisms of destruction during scratch testing have been added.  

Reviewer 2 Report

Comments and Suggestions for Authors

In this study, the fracture strength of coatings based on the ZrN system with the addition of Ti, Nb, and Hf was compared using a scratch test. These coatings were deposited on a titanium alloy substrate. The research holds significant practical value; however, some revisions are needed before publication:

1.The abstract can be appropriately expanded to highlight the importance, objectives, and potential contributions of the research to both academia and practical applications.

2.Lines 117-119 could include the cleaning time in the article.

3.Please provide a detailed description of how the average critical fracture value LC2 is determined in line 136.

4.Add the acoustic emission curve below the overall view of the scratch to help readers clearly understand the location of the LC1 value.

5.Include SEM images of all coatings in the text to help readers understand the differences in surface morphology among the various coatings.

6. Please discuss why the trends of LC1 and LC2 values differ.

Author Response

Comments and Suggestions for Authors

In this study, the fracture strength of coatings based on the ZrN system with the addition of Ti, Nb, and Hf was compared using a scratch test. These coatings were deposited on a titanium alloy substrate. The research holds significant practical value; however, some revisions are needed before publication:

A: The authors are grateful to the Reviewer for his attention to the article and valuable recommendations that help improve the quality of the manuscript.

R: 1.The abstract can be appropriately expanded to highlight the importance, objectives, and potential contributions of the research to both academia and practical applications.

A: The abstract has been supplemented.

R: 2.Lines 117-119 could include the cleaning time in the article.

A: Information added to the text of the article.

R: 3.Please provide a detailed description of how the average critical fracture value LC2 is determined in line 136.

A: Detailed description added to Materials and Methods section

R: 4.Add the acoustic emission curve below the overall view of the scratch to help readers clearly understand the location of the LC1 value.

A: Acoustic emission curves have been added for all samples, but in the case of coatings with a multilayer structure, the study of the scribing groove images is of greater importance, since there is also a destruction of the interlayer cohesive bond, which is identical in acoustic emission to the peeling of the coating from the base, while in reality such peeling does not occur. An additional explanation has also been added for the incomplete correspondence between the acoustic emission signal and the visually observed destruction of the coatings.

Coatings with a multilayer structure are destroyed during a scratch test by a mecha-nism that differs significantly from the destruction of monolayer coatings [77,78]. The in-terfaces between the layers provide a cohesive bond and can be destroyed by an indenter, while providing better resistance to destruction [78-81]. Thus, when scribing multilayer coatings, there is a repeated rupture of the cohesive bond between the nanolayers, and the acoustic emission signal that occurs during such a rupture is difficult to differentiate from that occurring when the entire coating is torn off the substrate. Thus, when analyzing the nature of destruction of multilayer coatings during a scratch test, it is worth considering first of all the results of visual observation, and not the acoustic emission signal. At the same time, the acoustic emission signal continues to play a certain role, allowing a better understanding of the nature of the coating destruction. (added to article)

R: 5.Include SEM images of all coatings in the text to help readers understand the differences in surface morphology among the various coatings.

A: Included Figure 1a - structure of the studied coatings (TEM). Also added are images of the surface morphology of the coatings (Figure 1b)

R: 6. Please discuss why the trends of LC1 and LC2 values differ.

A: The following fragment has been added to the text of the article: «It is worth noting the features of the practical significance of points LC1 and LC2 for coatings with a multilayer structure. Point LC1 characterizes the load at which the first signs of coating destruction appear. For multilayer coatings, this usually means the be-ginning of the destruction of cohesive bonds between individual layers. In this case, the coating usually retains its operability, continuing to perform its functional properties. Point LC2 characterizes the complete destruction of the coating in the scribing groove area. In this case, the coating is completely lost and the indenter moves directly in contact with the substrate.»

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

It is an interesting scientific paper suitable for publication in its current form.