Comparison of Friction Behaviour of Titanium Grade 2 after Non-Contact Boriding in Oxygen-Containing Medium with Gas Nitriding

Round 1

Reviewer 1 Report

The composition of the article meets the journal's recommendations and includes the following sections: Abstract, Introduction, Materials and Methods, Results and discussion (divided into two sections), Conclusions and References. According to the Authors “the purpose of this work is to investigate the surface and tribological properties of titanium after non-contact solid boriding in an oxygen-containing environment and compare with the properties of titanium after gas nitriding, as one of the most well-known and used method”. In the reviewer opinion, some weak points should be improved before the paper can be considered for publication. The weaknesses are given below.

Please explain the following issues:

1.    Why the authors carried out tribological tests with parameters posted in the “2.4. Friction testing” section?

2.  How the applied force relate to Hertzian contact pressures (especially for steel counter-body)? Did you taken this into account?

3.      Why the authors not determine wear rates ?

4.     Why you determine wear resistance of commercially pure titanium Grade 2 samples with modified nitride (TiN, Ti₂N) and boride (TiB) compound layers based only on the friction coefficient and the analysis of the morphology of wear scars?

5.  Considering obtained results of tribological tests, I get the impression that in the Conclusions section these issues were described "in a neglected way". Only one conclusion?

Editing issues:

6.      The description under Figure 1 should be completed - what is shown on (a) and (b) ?

I proposed: Figure 1. Surface topography (a) and microstructure (b) of titanium Grade 2 in its initial state.

7.      Add scale bars on the Figure 12.

8.      There are a lot of typos in the article - examples below:

Lines 205-206:

Is: It indirectly indicates the formation of a saturated solid solution of oxygen in titanium Ti(N).

Should be: It indirectly indicates the formation of a saturated solid solution of oxygen in titanium Ti(O).

Lines 110-111:

Is: Then the gaseous nitrogen was pumped off and cooled in a vacuum (10^-3Pa) (Figure 2 b).

Should be: Then the gaseous nitrogen was pumped off and cooled in a vacuum (10^-3Pa) (Figure 2 a).

Line 279-280:

Is: Such an increase of the asymmetry coefficient occurs due to the growth of the nitdid layer,

Should be: Such an increase of the asymmetry coefficient occurs due to the growth of the nitrided layer,

9.      I recommend improve English expressions and related grammar - the paper should be checked by a native speaker.

Author Response

Dear Ms. Doreen Liu,

 

We would like to thank the Reviewers for the valuable remarks to our manuscript. We have revised the whole paper as recommended in the revision. The changes in the manuscript are written in yellow and red (grammar). You will find our answers and comments (text in red colour) to the Reviewers question bellow.

 

Yours Sincerely

 

Serhii Lavrys

 

 

Reviewer: 1

 

1. Why the authors carried out tribological tests with parameters posted in the “2.4. Friction testing” section?

 

We chose parameters of tribological investigations according to the analysis of literary sources where the authors ground their choice for titanium and its alloys for medical applications. For example, contact pressure was in the range of 0.88…4 MPa. We used tribo-pairs of nitrided and borided titanium with ultrahigh molecular weight polyethylene (UHMWPE) and 316L stainless steel. Tribo-pair with UHMWPE is used to evaluate the performance of a hip implant, and titanium-steel tribo-pair is used to evaluate the performance of bone fixators.

 

2. How the applied force relate to Hertzian contact pressures (especially for steel counter-body)? Did you taken this into account?

 

A very interesting and important remark.

In these studies, unfortunately, we did not take into account the relationship between the applied force and Hertzian contact pressure. In this work, we focused on a new method of boriding of titanium in comparison with nitriding. This work is our initial research, which makes it possible to conclude whether our proposed new method of non-contact boriding is promising and competitive (in terms of surface and tribological characteristics) compared to the more common and used method of nitriding. In our further studies, we plan to evaluate more widely the tribological characteristics of boride layers from the point of view of their medical application, where depending on the type of implant and its operating conditions, the parameters of tribological studies will also change. For example, tribo-pair with UHMWPE is used to evaluate the performance of a hip implant, and titanium-steel tribo-pair is used to evaluate the performance of bone fixators. This will be effect the choice of conditions for tribological tests (contact pressure, type of friction, medium (blood plasma (bone fixator) or bovine serum (hip joint implant). In our further research we will try to use tribomechanics approaches.

 

3. Why the authors not determine wear rates ?

 

As was above mentioned, we determined only the friction coefficient, which is due to our capabilities (during friction tests we did not have the opportunity to determine the wear rate). However, the wear rate is an important characteristic, and in further research we plan to determine it.

 

4. Why you determine wear resistance of commercially pure titanium Grade 2 samples with modified nitride (TiN, Ti₂N) and boride (TiB) compound layers based only on the friction coefficient and the analysis of the morphology of wear scars?

 

In these studies, we investigated only the frictional behaviour and evaluated the type of wear mechanism of titanium with nitride and boride layers. However, we did not estimate the wear resistance based on the weight loss.

 

5. Considering obtained results of tribological tests, I get the impression that in the Conclusions section these issues were described "in a neglected way". Only one conclusion?

 

We improved conclusions.

 

In this work, the comparison of friction behaviour of titanium Grade 2 after non-contact boriding in oxygen-containing medium with gas nitriding was studied. The main conclusions are as follows:

1. It was determined that the single-phase (Ti₂N) or two-phase (Ti₂N+TiN) compound layers was formed during nitriding of titanium at temperatures of 750°C or 900°C. The single-phase (TiB) compound layer was formed after non-contact boriding in oxygen-containing medium.
2. It was shown that non-contact boriding allows to form a homogeneous compound layer with better surface roughness (height, step and other parameters) compared to gas nitriding. According to the results of micro- and nanoindentation tests, boriding provides deeper hardened layer but lower surface hardness and elastic modulus.
3. It was found that borided titanium has better friction behaviour under dry sliding in tribo-pairs with stainless steel (friction coefficient is lower by ~ 1.5 times) and UHMWPE (friction coefficient is lower by ~ 2 times) in comparison with nitriding.
4. According to the SEM analysis, it was determined that formation of the compound layers on titanium leads to decrease of the adhesive component and increase of the abrasive one of the wear mechanism in tribo-pairs. Boriding due to better roughness provides the decrease of adhesive and abrasive wear compared to nitriding.

 

Editing issues:

6. The description under Figure 1 should be completed - what is shown on (a) and (b) ?

I proposed: Figure 1. Surface topography (a) and microstructure (b) of titanium Grade 2 in its initial state.

 

We changed the description under Figure 1 on: Surface topography (a) and microstructure (b) of titanium Grade 2 in initial state.

 

7. Add scale bars on the Figure 12.

 

The scale bars were added on the Figure 12.

 

Figure 12. Hardness distribution in surface layer of titanium in initial state (1) and after nitriding (2, 3) or boriding (4).

 

8. There are a lot of typos in the article - examples below:

Lines 205-206:

Is: It indirectly indicates the formation of a saturated solid solution of oxygen in titanium Ti(N).

Should be: It indirectly indicates the formation of a saturated solid solution of oxygen in titanium Ti(O).

Lines 110-111:

Is: Then the gaseous nitrogen was pumped off and cooled in a vacuum (10^-3Pa) (Figure 2 b).

Should be: Then the gaseous nitrogen was pumped off and cooled in a vacuum (10^-3Pa)(Figure 2 a).

Line 279-280:

Is: Such an increase of the asymmetry coefficient occurs due to the growth of the nitdid layer,

Should be: Such an increase of the asymmetry coefficient occurs due to the growth of the nitrided layer.

 

We corrected the text.

 

9. I recommend improve English expressions and related grammar - the paper should be checked by a native speaker.

 

The English was improved.

Author Response File: Author Response.docx

Reviewer 2 Report

1.      The title of “Surface characteristics and frictional behaviour of titanium with nitrided and borided compound layers” is not appropriate; it seems to investigate a complex method.

2.      Line 25-26 “Introduction：Titanium is well known because of its superior mechanical and anticorrosive properties, biocompatibility, which makes it an indispensable material for load-bearing biomedical applications (such as hip and knee joints, dental posts or heart pumps) ”. 

Comment:  Actually, Titanium has no superior mechanical properties, that is why surface modification is needed to enhance the hardness and wear resistance.

3.      Line 42-43 “Among the large number of diffusion methods, the most attractive and used method of increasing functional properties of titanium for medical applications is the method of gas nitriding.”

Comment: Can gas nitriding increase the functional properties of titanium for medical applications?

4.      Line 82:  The microhardness of titanium is 1.2±0.1 GPa  

Comment:  Please use HV to express the microhardness

5.      in Table 1  “Not more” is not a suitable expression,

6.      Please explain Why there exist no TiO2 while Boriding with Oxygen?  

7.      Line 205: it indirectly indicates the formation of a saturated solid solution of oxygen in titanium Ti(N)?  Comment: how can this be formed, why not TiO2?

8.      Line 223: “As a result, the surface topography repeats the grain boundaries, and the more pronounced the higher nitriding temperature.”  

Comment: How can you judge this phenomenon?

9.      the grammar of the manuscript needs to be carefully revised by a native English speaker．

eg. Line 61…….which effects the mechanical and fatigue properties negatively.

Author Response

Dear Ms. Doreen Liu,

 

We would like to thank the Reviewers for the valuable remarks to our manuscript. We have revised the whole paper as recommended in the revision. The changes in the manuscript are written in yellow and red (grammar). You will find our answers and comments (text in red colour) to the Reviewers question bellow.

 

Yours Sincerely

 

Serhii Lavrys

 

 

 

Reviewer: 2

 

1. The title of “Surface characteristics and frictional behaviour of titanium with nitrided and borided compound layers” is not appropriate; it seems to investigate a complex method.

 

The title of manuscript was changed on «Comparison of friction behaviour of titanium Grade 2 after non-contact boriding in oxygen-containing medium with gas nitriding».

 

2. Line 25-26 “Introduction：Titanium is well known because of its superior mechanical and anticorrosive properties, biocompatibility, which makes it an indispensable material for load-bearing biomedical applications (such as hip and knee joints, dental posts or heart pumps)”. Comment:  Actually, Titanium has no superior mechanical properties, that is why surface modification is needed to enhance the hardness and wear resistance.

 

We agree with the comment. We had in mind the high specific strength of titanium. So, in the "Introduction" section, we replaced the phrase «superior mechanical and anticorrosive properties» with «high specific strength and corrosion resistance».

 

3. Line 42-43 “Among the large number of diffusion methods, the most attractive and used method of increasing functional properties of titanium for medical applications is the method of gas nitriding.” Comment: Can gas nitriding increase the functional properties of titanium for medical applications?

 

Yes, gas nitriding can increase the functional properties of titanium and its alloys for medical applications.

For example, nitride layer formed after gas nitriding provides by 4-5 times higher hardness compared to untreated titanium alloy [1]. A better corrosion resistance and a significant decrease in ion release rates for the nitrided alloy (ion release of 1.41 ng/cm² compared to the 163.58 ng/cm² obtained for the commercially pure titanium at pH = 7.48 in artificial Carter-Brugirard saliva) was observed. The cytocompatibility was not compromised and the cell viability performed on human osteoblasts, fibroblastic cells, and epithelial cells was enhanced on the nitrided surface in comparison with the non-nitrided surface. These combined properties make the nitrided Ti-27Nb alloy a good candidate for dental implant applications. Nitriding of Ti-6Al-4V alloy at 850–900°C improved not only hardness, wear and corrosion resistance but also biocompatibility [2-4]. It was demonstrated that the cell growth and vitality on the nitrided alloy is higher than on untreated one [4].

 

1. Bédouin, Y., Gordin, D.-M., Pellen-Mussi, P., Pérez, F., Tricot-Doleux, S., Vasilescu, C., Drob, S.I., Chauvel-Lebret, D., Gloriant, T. Enhancement of the biocompatibility by surface nitriding of a low-modulus titanium alloy for dental implant applications. Biomed. Mater. Res. B Part B. 2018, 9999:9999, 1–8.
2. Jambagi, S C., Malik, V.R. A review on surface engineering perspective of metallic implants for orthopaedic applications JOM 2021, 73, 4349-4364.
3. Tamura, Y., Yokoyama, A., Watari, F., Kawasaki, T. Surface properties and biocompatibility of nitrided titanium for abrasion resistant implant materials. Dent. Mater. J., 2002, 21, 355–372.
4. Pohrelyuk, I.M., Tkachuk, O.V., Proskurnyak, R.V., Boiko, N.M., Kluchivska, O.Y., Stoika, R.S. Effect of thermodiffusion nitriding on cytocompatibility of Ti-6Al-4V titanium alloy. JOM 2016, 68, 1109–1115.

 

4. Line 82: The microhardness of titanium is 1.2±0.1 GPa. Comment: Please use HV to express the microhardness.

 

We agree with the comment. This sentence was removed from the to the section “Materials and Methods”. The surface microhardness of titanium is presented in Table 1.

 

5. In Table 1 “Not more” is not a suitable expression.

 

We agree with the comment and the appropriate corrections were made in the Table 1: “not more” was changed on “max”.

 

6. Please explain Why there exist no TiO₂ while Boriding with Oxygen?

 

The intensity of reaction of titanium with a boron-containing impregnating atmosphere depends on its gaseous components and with a reduction in oxygen partial pressure in the test range it decreases. A reduction in atmosphere activity is caused by weakening of the chemical reaction of oxygen with boron carbide, and consequently less intense formation of oxides that transport ions of interstitial elements from the atmosphere to the interface with the metal. The oxidation temperature for boron carbide is 500°C [Lavrenko, V. A., Pomytkin, A. P., Kislyj, P. S., & Grabchuk, B. L. (1976). Kinetics of high-temperature oxidation of boron carbide in oxygen. Oxidation of Metals, 10(2), 85–95. doi:10.1007/bf00614238 ], but consequently the concentration effect of oxygen in the system develops during heating to the isothermal soaking temperature.

A reduction in the intensity of oxidation of atmosphere components with a reduction in oxygen partial pressure weakens the intensity of removal of atomic boron through the vapor phase to the reaction zone, which is reflected in formation of titanium boride phases. With an oxygen partial pressure of 0.1 – 0.01 Pa the intensity of boride formation at the titanium surface is low, and this is indicated by the peak for titanium boride of low relative intensity and absence of titanium diboride reflections. With an oxygen partial pressure of 0.1 Pa only a weak reflection is recorded for the most intense reference line (101) for titanium diboride (TiB₂). At the same time, with a partial pressure of 1 Pa there is an increase in the concentration of boron in the reaction zone leading to predominance of reaction diffusion and consequently the intensity of forming boride phases increases. This is indicated by the more intense reflections for the titanium monoboride phase and the broad spectrum of titanium diboride lines. Here a reduction in the intensity of -titanium peaks points to a thicker boride film formed at the surface. The results obtained correlate with metallographic studies, indicating an increase in boride film thickness with a higher oxygen partial pressure.

 

7. Line 205: it indirectly indicates the formation of a saturated solid solution of oxygen in titanium Ti(N)? Comment: how can this be formed, why not TiO₂?

 

This was a mistake, instead of Ti(N) should be Ti(O) in the sentence. We made appropriate corrections.

Under such conditions of boriding (non-contact boriding above the backfill in oxygen-containing medium with partial pressure of oxygen ≈ 1 Pa), the internal energy of oxygen is sufficient for overcoming the energy barrier, dissolution into titanium and formation of a solid solution Ti(O) (diffusion layer) [Prytula, A.O., Pogrelyuk, I.N. Fedirko, V.N. Effect of impregnating atmosphere oxygen on boriding of titanium alloys. Met. Sci. Heat Treat., 2008, 50, 232–237; Kaplan, Y., Cetin Can, A., Ulukoy, A. A new medium for boriding of Ti6Al4V alloy for biomedical applications. Proc IMechE Part L: J Materials: Design and Applications, 2016, 0, 146442071666280].

 

8. Line 223: “As a result, the surface topography repeats the grain boundaries, and the more pronounced the higher nitriding temperature.” Comment: How can you judge this phenomenon?

 

As the temperature increases from 750 to 900°С, nitride formation at the grain boundaries intensifies, and as a result, we get a more obvious change on the surface (meaning the height of the nitride layer) between the grain boundary and the grain body.

 

9. The grammar of the manuscript needs to be carefully revised by a native English speaker. eg. Line 61 which effects the mechanical and fatigue properties negatively.

 

We made grammar corrections in the manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper "Surface characteristics and frictional behaviour of titanium 2 with nitrided and borided compound layers" is suitable for publication in Coatings Journal after some minor corrections.

The authors have performed very good work regarding the experimental analysis. The introduction is well written but needs some improvements. Please clearly add and mention some other coating techniques (lines 37-38).Suggested reference: 10.3390/coatings10121186.

Line 139: Please add the parameters of the XRD analysis and the SEM testing.

Line 180: Please add the ICDD files for all compounds identified.

Line 358. Please rewrite the name of the figure. You are talking about microstructure, and the images show an indentation test.

Summarize the conclusions in a more concise way.

Examine the reference template.

 

The rest is fine.

 

Author Response

Dear Ms. Doreen Liu,

 

We would like to thank the Reviewers for the valuable remarks to our manuscript. We have revised the whole paper as recommended in the revision. The changes in the manuscript are written in yellow and red (grammar). You will find our answers and comments (text in red colour) to the Reviewers question bellow.

 

Yours Sincerely

 

Serhii Lavrys

 

 

 

Reviewer: 3

 

1. The authors have performed very good work regarding the experimental analysis. The introduction is well written but needs some improvements. Please clearly add and mention some other coating techniques (lines 37-38). Suggested reference: 10.3390/coatings10121186.
2. https://doi.org/10.3390/coatings10121186
3. https://doi.org/10.3390/coatings10121216
4. https://doi.org/10.1016/j.mtla.2021.101302
5. https://doi.org/10.1016/j.surfcoat.2021.126968

 

We added References to section “Introduction” according to the recommendations:

1. Paleu, C.C., Munteanu, C., Istrate, B., Bhaumik, S., Vizureanu, P., Bal, M.S.¸ Paleu, V. Microstructural analysis and tribological behavior of amdry 1371 (mo–nicrfebsic) atmospheric plasma spray deposited thin coatings. 2020, 10, 1186.
2. Baltatu, M.S., Vizureanu, P., Sandu, A.V., Munteanu, C., Istrate, B. Microstructural analysis and tribological behavior of Ti-based alloys with a ceramic layer using the thermal spray method. 2020, 10, 1216.
3. Cura, A.C., Zuchua, J.I., Tribbia, L.T., Taravini, I. R., Decco, E.O.A. Sandblasted, acid etched and UV irradiated titanium surface for dental implants: In vitro and in vivo Mater. 2022, 21, 101302.
4. Özmeriç, N., Çakal, G.Ö._, Gökmenoğlu, C._, Özmeriç, Oduncuoğlu, B.F., Hacaloğlu, T., Kaftanoğlu, B. Histomorphometric and biomechanical evaluation of the osseointegration around micro- and nano-level boron-nitride coated titanium dental implants. Stomatol. Oral Maxillofac. Surg. 2022, 123, 694-700.

 

2. Line 139: Please add the parameters of the XRD analysis and the SEM testing.

 

The parameters of the XRD analysis and the SEM testing were added to the section “Materials and Methods”:

The phase composition of the surface layers was determined by X-ray phase analysis using a DRON-3.0 diffractometer in monochromatic CuKα radiation with Bragg–Brentano focusing. The voltage at the anode of the X-ray tube was 30 kV at a current of 25 mA. The diffraction patterns were registered with the help of a step scan in the 2q angles range – 30…80°. The scan step was 0.05°, exposure time – 5 s. Sietronix, Powder Cell 2.4 and FullProf softwares were used to define the locations of diffraction maxima of reflections and identify them according to the JCPDS–ASTM card-file. The analysis of surface topography was carried out by ZEISS EVO-40XVP scanning electron microscop (SEM) equipped with INCA Energy 350 energy dispersive X-ray (EDX) microanalysis and backscattered detector (BSD).

 

3. Line 180: Please add the ICDD files for all compounds identified.

 

The ICDD files for all identified compounds were added: Ti (44-1294); Ti₂N (77-1893); TiN (74-1214); TiB (73-2148).

 

4. Line 358. Please rewrite the name of the figure. You are talking about microstructure, and the images show an indentation test.

 

It was a mistake and the figure caption was changed.

 

5. Summarize the conclusions in a more concise way.

 

The Conclusions were improved.

 

In this work, the comparison of friction behaviour of titanium Grade 2 after non-contact boriding in oxygen-containing medium with gas nitriding was studied. The main conclusions are as follows:

1. It was determined that the single-phase (Ti₂N) or two-phase (Ti₂N+TiN) compound layers was formed during nitriding of titanium at temperatures of 750°C or 900°C. The single-phase (TiB) compound layer was formed after non-contact boriding in oxygen-containing medium.
2. It was shown that non-contact boriding allows to form a homogeneous compound layer with better surface roughness (height, step and other parameters) compared to gas nitriding. According to the results of micro- and nanoindentation tests, boriding provides deeper hardened layer but lower surface hardness and elastic modulus.
3. It was found that borided titanium has better friction behaviour under dry sliding in tribo-pairs with stainless steel (friction coefficient is lower by ~ 1.5 times) and UHMWPE (friction coefficient is lower by ~ 2 times) in comparison with nitriding.
4. According to the SEM analysis, it was determined that formation of the compound layers on titanium leads to decrease of the adhesive component and increase of the abrasive one of the wear mechanism in tribo-pairs. Boriding due to better roughness provides the decrease of adhesive and abrasive wear compared to nitriding.

 

6. Examine the reference template.

 

The References were presented according to the reference template.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript has been rewritten, having taken note of the reviewers' comments and suggestions. I recommend this paper for publication in present form.