Influence of Deposition Parameters on Structural and Electrochemical Properties of Ti/Ti₂N Films Deposited by RF-Magnetron Sputtering

Round 1

Reviewer 1 Report

The paper deals with the deposition of TiN thin films on steel-carbon substrates using RF magnetron sputtering from the titanium target. The authors' intention was to investigate structural, morphological and anti-corrosive properties of coated substrates.

First of all, I have to mention that the English language of the manuscript is very hard to read. The sentences are very choppy and complicated. Consequently, it is very difficult to follow the author's thoughts and understand the text correctly. Moreover, I found plenty of typos in the text. Here are some examples of bad English: page 2 - They are methods..., radius frequency, page 3 - gasses, page 4 - applicants.

The main shortcoming of the presented work is the fact that the authors discuss the properties of TiN material in the paper, but the coatings are more or less TiOx thin films and moreover strongly sub-stoichiometric. The main problem is a very poor background vacuum in order of 10-2 mbar which is insufficient for the deposition of good-quality TiN thin films. I would recommend improving the deposition experiments as the fundament for high-level scientific results. It should also be noted that a large number of scientific studies on magnetron deposition of TiN thin films have been published so far. So, the novelty of the work is questionable and is not properly presented in the manuscript.

Therefore, I think it is not possible to publish the manuscript in its present form.

Author Response

Dear Editor,

Coauthors and I very much appreciated the encouraging, critical, and constructive comments on this manuscript by the reviewer. The comments have been very thorough and useful in improving the manuscript. We strongly believe that the comments and suggestions have increased the scientific value of revised manuscript by many folds. We have taken them fully into account in revision. We are submitting the corrected manuscript with the suggestion incorporated the manuscript. The manuscript has been revised as per the comments given by the reviewer, and our responses to all the comments are as follows:

Response to Reviewer 1 Comments

1. First, I have to mention that the English language of the manuscript is very hard to read. The sentences are very choppy and complicated. Consequently, it is very difficult to follow the author´s thoughts and understand the text correctly. Moreover, I found plenty of typos in the text. Here are some examples of bad English: page 2-They are methods…, radius frequency, page 3- gasses, page 4-applicants.

Response- Thank you for appreciating comments. We have reviewed the manuscript and have been modified (pages 1,2):

1. a) Abstract

“The titanium nitride (TiN) films have good mechanical properties such as high hardness and chemical stability, giving TiN good wear and corrosion resistance. The properties of films deposited by PVD techniques are determined by their structure, microstructure, composition, and morphology that depend on the deposition parameters such as substrate temperature, vacuum pressure, and the distance between the target and the substrate. The influence of these parameters has been studied one at a time. This work studied the structure, morphology, composition, and electrochemical behavior of Ti/TiN films deposited by RF-magnetron sputtering on AISI 1060 carbon steel, such as a function of the target to substrate distance, power of the RF source, and substrate temperature. The film structure was analyzed by X-ray diffraction (XRD), the morphology of cross-section by SEM, the semi-quantitative composition by EDS, and the electrochemical properties was studied by Open Circuit Potential, Potentiodynamic Polarization, Electrochemical Impedance Spectroscopy techniques. The films showed two phases of Ti and Ti₂N. The SEM-EDS exhibited the formation model growing according to Stranskis-Krastanov type (layer + island). The substrate temperature (400°C) strongly influences the electrochemical properties”.

1. b) Introduction

“The AISI 1060 carbon steel is mechanical engineering and general-purpose. In mechanical engineering, this grade of steel is often used in automotive parts requiring high wear resistance, such as axles, half shafts, pistons, gears, clutch discs, and solid railway wheels. This steel is commonly provided in an untreated or normalized state. Through the application of hard films to parts applicable to the industry, it is possible to increase their hardness, decrease the coefficient of friction, and increase the resistance to wear and fatigue, thereby obtaining an extension of the useful life of the part, which can mean an improvement in the production of a company. Financial investment caused by corrosion phenomena in AISI 1060 carbon steel is a significant problem faced in the industry. Therefore, the aim of hard films, in addition to providing the above properties, is to reduce costs by applying them to low-value materials that can be used as substitutes for others that may have high corrosion resistance but are usually very expensive [1‒3]. The industrial demand for high-performance tool steel has led to new hot work tool steels with improved mechanical properties, such as high toughness, tensile strength, resistance to softening at elevated temperatures, resistance to heat checking, and wear resistance [4]. For this reason, the cutting tool steel contains between 0.6% to 1.5% carbon, like D2, M2, H13, and AISI 1060 to 1090 [5‒7]”.

2. The main shortcoming of the presented work is the fact that the authors discuss the properties of TiN material in the paper, but the coatings are more or less TiOx thin films and moreover strongly sub-stoichiometric. The main problem is a very poor background vacuum in order of 10^-2 mbar which is insufficient for the deposition of good-quality TiN thin films. I would recommend improving the deposition experiments as the fundament for high-level scientific results. It should also be noted that a large number of scientific studies on magnetron deposition of TiN thin films have been published so far. So, the novelty of the work is questionable and is not properly presented in the manuscript.

Response- Thank you so much for your precious suggestion. We added the justification of research

“In many applications, the components are exposed to corrosive environments; therefore, the hard films deposited on the steels must outstand the corrosive attack. The corrosion resistance of coated tools is a function of the microstructure and a reduced number of pinholes and defects in the growth film [15]. Optimizing the deposition parameters by RF- magnetron Sputtering allows obtaining a dense film with low permeability. The influence of these parameters has been studied one at a time, but there are few comprehensive studies. The work was made a study the influence of nitrogen to argon ratio, substrate temperature, RF-input power, substrate argon and nitrogen pressure, and substrate- target distance on the structure and electrochemical properties of Ti/TiN films”.

Author Response File: Author Response.pdf

Reviewer 2 Report

Although the reviewer could not be an expert in English, the text of the article is very difficult to understand in many places. Just one example from the introduction.
"They are methods of surface modification and structural modification; when the methods of surface modification are used for increasing the hardness of cutting tools, the surface methods like Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD)."
And the introduction itself does not meet the standards accepted in journals - there is no formulation of the purpose of the work.

Lots of misprints. For example: "steel carbon", "the chamber vacuum is spherical sharp".... The reviewer recommends that the authors copy the text into Google translator, which he himself often does just that.

Now a few remarks on the content.
2.1.Deposition conditions (parameters is better)
"The residual vacuum into the chamber was around 2.1 × 10-3 mbar, and the working vacuum was 8.7." There is obviously wrong statement. May be this is again typo. 
Also here a paragraph on the preparation of sample-substrates is needed.
2.2. Details (coating, sample...) characterization 
Paragraph on SEM-EDX is strongly needed. 

3.1. XRD Ti/TiN films
It is not clear why the authors call the coating Ti/TiN. On fig. 1 no titanium lines. (about EDX see below). 
The reviewer did not understand what experiments were carried out and how many. In Fig. 1a, it is not clear what the other parameters were. The same can be said about the rest of the charts. Figure 1 needs to be redraw in such way that the evolution of the titanium nitride and titanium oxide lines is more obvious. 

3.2.2. Composition (chemistry) by EDS
From fig. 3 shows that the spectrum was taken at a beam energy of more than 12 kV. The coating thickness is 0.156+-0.49 microns (by the way, this is an incorrect metrological record. 0.15+-0.05 is correct). Why are there no k-lines of steel elements in the spectrum? And what program was used to calculate the composition. A strong line is present around 1.7 keV. What's this? Looks like k-a silicon.

3.2. (it seems 3.3 should be) Electrochemical behavior of TiN films.

The reviewer considers it appropriate to consider the rest of the article after the authors respond to the above comments.

Author Response

Dear Editor,

Coauthors and I very much appreciated the encouraging, critical, and constructive comments on this manuscript by the reviewer. The comments have been very thorough and useful in improving the manuscript. We strongly believe that the comments and suggestions have increased the scientific value of revised manuscript by many folds. We have taken them fully into account in revision. We are submitting the corrected manuscript with the suggestion incorporated the manuscript. The manuscript has been revised as per the comments given by the reviewer, and our responses to all the comments are as follows:

Response to Reviewer 2 Comments

1. Although the reviewer could not be an expert in English, the text of the article is very difficult to understand in many places. Just one example from the introduction. “They are methods of surface modification and structural modification; when the methods of surface modification are used for increasing the hardness of cutting tools, the surface methods like Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD).” And the introduction itself does not meet the standards accepted in journal-there is no formulation of the purpose of the work.

Response- Thank you for observation. We are embarrassed by English grammar.

The manuscript was reviewed using Grammarly under the license version. Grammarly

2. Lots of misprints. For example: “steel carbon”,”the chamber vacuum is spherical sharp”…The reviewer recommends that the authors copy the text into Google translator, which he himself often does just that.

Response- Thank you so much for your precious suggestion.

We have considered the observation and we reviewed detail.

3. Now a few remarks on the content.

2.1 Deposition conditions (parameters is better) “The residual vacuum into the chamber was around 2.1  10-3 mbar, and the working vacuum was 8.7.” There is obviously wrong statement. May be this again typo. Also here a paragraph on the preparation of sample-substrates is needed.

Response- Thank you for observation. We have reviewed section 2.1 Deposition conditions and reorganized the information (page 3).

“The TiN films were deposited onto carbon steel substrates (AISI 1060) by reactive RF-magnetron sputtering, using a Ti target of 50.8 mm in diameter and 63.5 mm in thickness (Sigma-Aldrich® 99.99 %). The argon (Ar‒99.998 %, grade 5.0) and nitrogen (N₂‒99.99% high-purity) mixed gas in a commercial RF-magnetron sputtering chamber (Trinus Vacuum, Spain). The residual pressures in the chamber ranged from 1.3 to 2.3  10^-3 mbar using a molecular-turbo pump by a primary rotatory pump, and the working pressures (mixture Ar/N₂) were from 4.3 to 8.7  10^-3 mbar, into spherical geometry chamber-vacuum of 1.1 m³ volume. The flow rates of Ar and N₂ gasses were controlled separately by mass flow controllers (Alicat Scientific & Cole-Palmer, USA), using fluxed standard cubic centimeter per minute (sccm) unit. The metallic substrate was previously prepared with a dimension of 30  30  2 mm; it was degreased, gritted with SiC papers (up to 1500) and polished with a 0.3 µm aluminum oxide solution, cleaned with water, rinsed with alcohol-acetone, and dried at room temperature according to ASTM G1-03 [23]. Before deposition films, it uses 10 minutes of pre-sputtering to clean the system with the shutter closed. Posteriorly, the deposition time in overall samples was 10 min for Ti films, whereas 110 min was used to obtain TiN films. Table 1 shows sputtering conditions used to develop the different sets of experiments and obtain Ti/TiN bilayered films through five-level each parameter deposition varying the power (100‒175W), target-to-substrate distance (10.5 ‒ 16.5 cm), the temperature on substrate (300 ‒450°C), and different Ar/N₂ ratio relation (16/4 ‒46/13). So, under a previous review of literature proposing to study variables, the RF-power uses minimum other parameters such as 300°C, 10.5 cm, and Ar/N₂ ratio of 16/4. After deposition, the sample came to the XRD laboratory analyzing the diffraction pattern considering the best cubic crystalline structure of TiN films (considering diffraction peak related to cubic TiN (111) phase preferred growth orientation). Likewise, all parameters’ constant conditions were obtained using the same methodology, such as temperature on the substrate, targe-to-substrate, and flow of Ar and N₂ gases”.

2.2 Details (coating, sample…) characterization

Paragraph on SEM-EDX is strongly needed.

Response- Thank you for the comment. We added the characterization of SEM/EDS (page 4)

“The microstructure and elemental analysis of sputtered films were observed by Scanning Electron Microscopy on Energy Dispersive Spectrometer mode SEM/EDS (JSM- 7610F, JOEL, Tokyo, Japan) at 15 keV of operating voltage”.

3.1XRD Ti/TiN films

It is not clear why the authors call the Ti/TiN. On fig. 1 no titanium lines (about EDX see below).

The reviewer did not understand what experiments were carried out and how many. In Fig. 1a, it is not clear what the other parameters were. The same can be said about the rest of the charts. Figure 1 needs to be redrawn in such way that the evolution of the titanium nitride and titanium oxides lines is more obvious.

Response- Thank you for observation. We have reviewed section 3.1 XRD Ti/Ti₂N films, and we have replated results-discussion (page 4).

“This deflection corresponds to the formation of Ti₂N tetragonal crystal structure (141/amd spatial group), with planes (112), (200), (105), (204), (220), and (312) in Bragg reflections observed at ~36°, ~41°, ~57°, ~61°, ~62° and ~76° peaks diffraction angles respectively [24], corresponds to JCPDS card No. 01- 080-3438 chart. Moreover, it observes the presence of Iron Oxide (Fe₃O₄) corresponding to diffraction peaks ~30°, ~35°, and ~62° with planes (220), (311), (440) respectively, according to JCPDS card No. 19-629 chart [25]. Additionally, it can be observed on ~35°all films that a weak reflection of Ti pure due to the deposition of the buffer layer correspond to the α-Ti phase [26].

• Composition (chemistry) by EDS

From fig. 3 shows that the spectrum was taken at a beam energy of more than 12 . The coating thickness is 0.156 +- 0.49 microns (by the way, this is an incorrect metrological record, 0.15 +-0.05 is correct). Why are there no k-lines of steel elements in the spectrum? And what program was used to calculate the composition. A strong line is present around 1.7 keV. What´s this? Looks like-a silicon.

Response- Thank you for observation.

We have reviewed section 3.2.1 Thickness and morphology analysis on page 9 and found that the measurement of means 0.156 ± 0.049 μm thickness originally. However, we modified it under the recommendation, is saying, 0.150 ± 0.05 μm. Also, we have replaced the spectrum of elemental Ti/Ti₂N deposited on AISI 1060 by EDS. Here shows k-line of Fe (iron, substrate).

3.2 (it seems 3.3 should be) Electrochemical behavior of TiN films.

Response- Thank for the perceptive observation. We done a mistake

“Corrected”

Round 2

Reviewer 1 Report

The authors improved the manuscript, which is now suitable for publication.

Author Response

Thank you!

The best regards,

Dr. Andrés González-Hernández

Reviewer 2 Report

The reviewer believes that the authors have not corrected the comments. As before, English does not meet the standards of an English-language magazine. The places of work of the 3rd and 4th authors are not indicated.
By content.
1. Figure 3 shows the EDX spectrum. The intense iron line indicates that the thin film program must be used to calculate the film composition. Well, for example, an old but still used GMRFilm. At the same time, the data of the so-called k-factors of the titanium, oxygen, and nitrogen lines used should be given. This is the reviewer's requirement. the composition of the coating is actually the subject of the article.
2. The reviewer is not sure that the authors correctly indicated the base pressure. It is too high for a turbo molecular pump to be used. Nevertheless, if this is still the case, then the results of the work should be recognized as having no practical value.

The reviewer believes that the quality of the deposition process and the methodically correct analysis of the deposited films are the focus of the presented work. It is for this reason that the reviewer believes that in this form the article cannot be accepted for consideration.

Author Response

Dear Reviewer,

Thank you so much for the comments on our manuscript; We attached the responses to the correct observations...

The best regards,

Andrés González-Hernández

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

The reviewer carefully read the revised version of the paper. First of all, the authors should correct three typos.
1. Section 2.1 does not mention that the films were deposited by magnetron sputtering.
2. And the last sentence of this section should probably end like this: labeled with an asterisk.
3. In Figures 1, iron should be indicated instead of AISI1060 in the designations of diffraction peaks.

By content.
The reviewer understands that the main result of the work is presented in sections 2.1 and 3.3., while in sections 3.1 and 3.2 the reader should receive information about what material (coating) was studied.

The title of section 3.1 seems unfortunate to the reviewer. The diffraction results show that the coating contains titanium and titanium nitride phases. But the EDS data, although, as the authors note, are semi-quantitative, nevertheless indicate that the coating is mainly titanium. This is possible if the growth rate at the stage of sputtering only in argon was much higher than at the stage of reactive sputtering. Or maybe the coating includes finely crystalline phases of nonstoichiometric titanium nitrides... The authors should write a few sentences about this. Of course, the answer to the question about the thickness of the upper layer (which, as the authors suggest, is stoichiometric titanium nitride) would be given by the use of quartz microbalance.

The reviewer believes that within the framework of this article, the authors failed to properly characterize the coatings they synthesized. One of the possible options to make the text of the article acceptable for publication is to simply indicate the presence of crystalline phases and leave the obtaining of quantitative data on the composition for further work. Moreover, the microscope indicated by the authors allows this to be done. It is specially designed for analysis at low electron energies down to hundreds of eV, which will allow removing iron from EDS spectra.

Author Response

Dear Editor and Reviewer:

Coauthors and I very much appreciated the reviewer's encouraging, critical, and constructive comments on this manuscript. The comments have been extensive and valuable in improving the manuscript. We strongly believe that the comments and suggestions have increased the scientific value of the revised manuscript by many folds. We have taken them fully into account in the revision. We are submitting the corrected manuscript with the suggestion incorporated into the manuscript.

The reviewer carefully read the revised version of the paper. First of all, the authors should correct three typos.

1. 
   Section 2.1 does not mention that the films were deposited by magnetron sputtering.

Response: Thank you for your observation. In section 2.1 deposition conditions we added the text (page 3)

“The films were deposited by RF-magnetron sputtering”

2. 
   And the last sentence of this section should probably end like this: labeled with an asterisk.

Response:Thank you for your comment. We are sorry for the mistake. The correct sentence is:

“labeled with an asterisk”

3. In figures 1, iron should be indicated instead of AISI1060 in the designations of diffraction peaks.

Response:Thank you for the suggestion. We have been replaced AISI 1060 as:

“Iron” was designed as the diffraction peaks of substrate

By content.
The reviewer understands that the main result of the work is presented in sections 2.1 and 3.3., while in sections 3.1 and 3.2 the reader should receive information about what material (coating) was studied.

Response: Thank you for the observation. We have been added the following from sections 2.1 to 3.3:

From the section 2.1, it mentioned Ti/Ti₂N films were obtained by RF-magnetron sputtering.

The title of section 3.1 seems unfortunate to the reviewer.

Response:Thank you for the observation.  We have been modified it (page 5):

“3.1 Structure studies by XRD”

The diffraction results show that the coating contains titanium and titanium nitride phases. But the EDS data, although, as the authors note, are semi-quantitative, nevertheless indicate that the coating is mainly titanium. This is possible if the growth rate at the stage of sputtering only in argon was much higher than at the stage of reactive sputtering. Or maybe the coating includes finely crystalline phases of nonstoichiometric titanium nitrides... The authors should write a few sentences about this. Of course, the answer to the question about the thickness of the upper layer (which, as the authors suggest, is stoichiometric titanium nitride) would be given by the use of quartz microbalance.

 

Response: Thank you for the suggestion. We have been added the follows text (page 5):

“A few of Ti/Ti₂N films not showed clear diffraction peaks, indicating a low crystallinity, but most show peaks corresponding to crystalline phases of Ti and Ti₂N, probably due at the nonstoichiometric of titanium nitrides”

“The relatively low flow of nitrogen (Ar/N₂ = 2.28)) used to deposit most coatings produces the Ti₂N phase instead of TiN; for the set 5, the ratio Ar/N₂ was even lower, ranging from 3.53 to 11.5”

The reviewer believes that within the framework of this article, the authors failed to properly characterize the coatings they synthesized. One of the possible options to make the text of the article acceptable for publication is to simply indicate the presence of crystalline phases and leave the obtaining of quantitative data on the composition for further work. Moreover, the microscope indicated by the authors allows this to be done. It is specially designed for analysis at low electron energies down to hundreds of eV, which will allow removing iron from EDS spectra.

Response: We appreciate your feedback.

We have reviewed the EDS results, and, unfortunately, the elemental chemical composition was made using low electron energies (15 keV), so it was impossible to reduce the k signal of iron. Therefore, according to your suggestion, we have decide eliminated the EDS spectra, leaving the composition analysis for future work.

Author Response File: Author Response.pdf