Effect of Ag and Ti Addition on the Deformation and Tribological Behavior of Zr-Co-Al Bulk Metallic Glass

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper reports on the effect of Ag and Ti addition on the mechanical and tribological behaviour of  a Zr56Co28Al16 BMG. With the addition of a small amount of Ag and Ti, it is shown to be able to improve the thermal stability, wear resistance and tribological performance of the BMG. Some of the results are worthy for publication, although the work is not particularly innovative or in-depth.  The specific comments are:

1.       How were these three alloys, Zr56Co28Al16, Zr56Co23Al16Ag5, and Zr56Co23Al16Ti5 are selected for investigation? Will the behaviour of the BMG be affected by the addition of different amounts of Ag and Ti?

2.       The authors found a compacted tribo-layer (marked by point 8 in Fig. 3(h), and point 9 in Fig. 3(i)). However, it is suggested to show more clearly such layer as well as the peel-off zones of the Zr-Co-Al-Ti BMG.

3.       Does the Zr56Co23Al16Ti5 BMG exhibit good potential for bioimplant? How does it compare with existing bioimplant materials? 

Author Response

Responses to Reviewers

 

Effect of Ag and Ti Addition on the Deformation and Tribological Behavior of Zr-Co-Al Bulk Metallic Glass

 

The authors thank the editor for the opportunity to revise our manuscript. We also thank the reviewer for their constructive comments and suggestions that helped us improving the manuscript. The reviewer comments and our responses are included below, and the changes are highlighted in yellow in the revised manuscript

 

                                                    Response to Reviewer 1 Questions

 

Comments and Responses

 

Question 1: How were these three alloys, Zr56Co28Al16, Zr56Co23Al16Ag5, and Zr56Co23Al16Ti5 are selected for investigation? Will the behaviour of the BMG be affected by the addition of different amounts of Ag and Ti?

 

Response: Thank you for the questions and comments. Zr₅₆Co₂₈Al₁₆ was chosen as a model bulk glass former in the Zr-Co-Al ternary system. Indeed, different amounts of Ag and Ti addition may have varying effects on the glass forming ability (GFA). Replacing 5 at% of Co with Ti in one case and Ag in another gave the best combination of glass forming ability and thermal stability for the corresponding quaternary alloy systems. The width of the supercooled liquid region increased from 48 K in case of the ternary Zr-Co-Al BMG to 74 K for Zr-Co-Al-Ag BMG and 54 K for Zr-Co-Al-Ti BMG, indicating improvement in processing window of the amorphous alloys with the addition of the fourth element. A short justification for alloy selection is added in the revised manuscript.

 

Question 2: The authors found a compacted tribo-layer (marked by point 8 in Fig. 3(h) and point 9 in Fig. 3(i)). However, it is suggested to show more clearly such layer as well as the peel-off zones of the Zr-Co-Al-Ti BMG

 

Response: Thank you for the comments . Fig3(h) and 3(i) have been changed in the revised manuscript to clearly highlight the compacted tribo-layers and peel-off zones. The compacted tribo-layers are marked by dashed oval symbols and peel-off zones are marked by dashed rectangular symbols in Figs 3(h) and 3(i) and explained in the figure caption of the revised manuscript.

 

Question 3: Does the Zr56Co23Al16Ti5 BMG exhibit good potential for bioimplant? How does it compare with existing bioimplant materials?

 

Response: Thank you for the question. Indeed, the Zr56Co23Al16Ti5 BMG exhibits excellent potential for bioimplant applications. The absence of toxic elements such as Ni and Be along with its high wear resistance and mechanical strength makes it a promising candidate for biomedical applications.

 

When compared with conventional bioimplants such as Ti-based alloys (Ti-6Al-4V) and Co-Cr alloys, Zr56Co23Al16Ti5 BMG offers many advantages. Ti-based alloys exhibit good corrosion resistance and biocompatibility, but they tend to suffer from wear-related issues, especially under dynamic loading conditions. Co-Cr alloys exhibit excellent wear resistance but may cause metal ion release and biocompatibility concerns. Our study shows that the Zr56Co23Al16Ti5 BMG exhibits significantly lower friction and wear rates due to the formation of a protective tribo-layer, suggesting enhanced durability for bioimplant applications.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In a manuscript under the title “Effect of Ag and Ti Addition on the Deformation and Tribological Behavior of Zr-Co-Al Bulk Metallic Glass” submitted to the Metals journal, the author presents an oxide tribo-film makes protector of the metal contact for Zr-Co-Al Bulk system adding small amount of Ag or Ti. The manuscript is written well, with additives effect and the reason of higher performance than the additive free system.

 

My opinion as a review is that the quality of this manuscript, which is submitted as an article, recommends it for publication in the Metals journal, following a minor revision.

 

 

Here is the list of my questions:

 

Question (1): where is Ag or Ti

Where are Ag and Ti located within the structure? Are they uniformly distributed throughout the bulk, or do they exist as substitutions at Co sites? And also please describe how was this determined?

 

Question (2): why only one concentration of Ag and Ti and how to decide the atomic ration of Ag and Ti

For the compositions of Zr₅₆Co₂₃Al₁₆Ag₅ and Zr₅₆Co₂₃Al₁₆Ti₅, what analytical method was used to determine these composition ratios? Are these ratios based on the initial mixing proportions rather than the actual composition of the sintered BMG?

 

Question (3): relationship among thermal properties, mechanical properties, and tribological properties

There is no significant difference among the three samples in terms of glass transition temperature, undercooling temperature, or buckling strength behavior. Regarding the differences in friction coefficients and the preferential oxidation of AgO or TiO (which function as solid lubricants), please provide details on whether frictional energy is involved.

 

Question (4): characters of debris

To explain the relationship between the functioning of AgO and TiO as solid lubricants and the wear scar depth, could you add further discussion supported by analysis or characterization data of the wear debris?

 

Question (5): bonding energies and oxidation energies

Could you present, in a list, the estimated binding energies for Zr₅₆Co₂₈Al₁₆, Zr₅₆Co₂₃Al₁₆Ag₅, and Zr₅₆Co₂₃Al₁₆Ti₅, as well as the energies associated with oxide film formation and the binding energies of the oxides?

 

Question (6): positioning of figures

In figure 5, b) and c) have different passion with others. Please adjust them. Please add same scale graphs of inside wear tracks. Present graphs in c), f), and i) indicated different scale and difficult to understand the differences with inside and outside of the wear tracks.

 

Comments for author File: Comments.pdf

Author Response

Responses to Reviewers

 

Effect of Ag and Ti Addition on the Deformation and Tribological Behavior of Zr-Co-Al Bulk Metallic Glass

 

The authors thank the editor for the opportunity to revise our manuscript. We also thank the reviewer for their constructive comments and suggestions that helped us improving the manuscript. The reviewer comments and our responses are included below, and the changes are highlighted in yellow in the revised manuscript

 

                                   Response to Reviewer 2 Questions

 

Question 1: Where are Ag and Ti located within the structure? Are they uniformly distributed throughout the bulk, or do they exist as substitutions at Co sites? And also, please describe how was this determined?

Response: Thank you for the question. The distribution of Ag and Ti in Zr₅₆Co₂₃Al₁₆Ag₅ and Zr₅₆Co₂₃Al₁₆Ti₅ was analyzed using energy-dispersive X-ray spectroscopy (EDS). The results confirm that both Ag and Ti are uniformly distributed throughout the alloys and there is no evidence of elemental segregation. Below are the EDS mappings over ~ 50 μm x 50 μm area showing uniform elemental distribution of Ag in Zr-Co-Al-Ag BMG and Ti in Zr-Co-Al-Ti BMG. However, short-range order (SRO) and medium-range order (MRO) calculations to know the atomic positions and substitutions requires molecular dynamics simulations, which is well beyond the scope of this paper.

   Distribution of Ag in Zr-Co-Al-Ag                            Distribution of Ti in Zr-Co-Al-Ti

       

 

Question 2:  why only one concentration of Ag and Ti and how to decide the atomic ration of Ag and Ti. For the compositions of Zr₅₆Co₂₃Al₁₆Ag₅ and Zr₅₆Co₂₃Al₁₆Ti₅, what analytical method was used to determine these composition ratios? Are these ratios based on the initial mixing proportions rather than the actual composition of the sintered BMG?

Response: Thank you for the questions and comments. Zr₅₆Co₂₈Al₁₆ was chosen as a model bulk glass former in the Zr-Co-Al ternary system. Indeed, different amounts of Ag and Ti addition may have varying effects on the glass forming ability (GFA). Replacing 5 at% of Co with Ti in one case and Ag in another gave the best combination of glass forming ability and thermal stability for the corresponding quaternary alloy systems. The width of the supercooled liquid region increased from 48 K in case of the ternary Zr-Co-Al BMG to 74 K for Zr-Co-Al-Ag BMG and 54 K for Zr-Co-Al-Ti BMG, indicating improvement in processing window of the amorphous alloys with the addition of the fourth element. A short justification for alloy selection is added in the revised manuscript.

 

Alloys with nominal composition of Zr₅₆Co₂₈Al₁₆, Zr₅₆Co₂₃Al₁₆Ag₅, and Zr₅₆Co₂₃Al₁₆Ti₅ were synthesized by arc-melting the mixture of high-purity constituent elements. They were re-melted six times under a Ti-gettered Ar atmosphere to ensure uniformity in composition. The actual composition measured by EDS analysis as an average over 10 different locations on the sample was within ± 0.2 at% of the nominal composition, suggesting uniform elemental distribution.

 

Question (3): relationship among thermal properties, mechanical properties, and tribological properties. There is no significant difference among the three samples in terms of glass transition temperature, undercooling temperature, or buckling strength behavior. Regarding the differences in friction coefficients and the preferential oxidation of AgO or TiO (which function as solid lubricants), please provide details on whether frictional energy is involved.

Response: Thank you for the comments. While there is no significant difference in thermal and mechanical properties among the three alloys, their tribological behavior differs significantly due to friction-induced oxidation and tribo-layer formation. That is the novelty of the current work suggesting that alloy development needs to be geared towards specific applications. The formation of TiO₂ in Zr-Co-Al-Ti significantly reduces friction and wear, markedly improving its tribological performance and stability even with just 5 at% Ti addition. Ag₂O in Zr-Co-Al-Ag provides some friction reduction, but its lower thermal stability limits its long-term effectiveness under prolonged wear conditions. The Zr-Co-Al ternary BMG, lacking Ag or Ti, does not form an effective tribo-layer, leading to high friction and predominantly abrasive and adhesive wear mechanisms.

 

Question (4): To explain the relationship between the functioning of AgO and TiO as solid lubricants and the wear scar depth, could you add further discussion supported by analysis or characterization data of the wear debris?

Response: Thank you for the excellent comments. Detailed wear debris characterization was done and included as Figure 4 in the revised manuscript along with the corresponding discussion. Figure 4 shows the SEM images of the wear debris for the three BMGs studied at 10N load. Figure 4(a) shows that the wear debris from Zr-Co-Al primarily consists of relatively small and dispersed particles. The fine debris suggests material removal due to direct metal-metal contact. In contrast, Figures 4(b) and 4(c) show the wear debris for Zr-Co-Al-Ag and Zr-Co-Al-Ti, which exhibit more clustered and compacted debris. This morphology is characteristic of oxidative wear, where wear debris aggregates due to the formation of a protective oxide layer. The presence of oxygen-rich debris suggests that Ag and Ti promote the formation of a tribo-layer, which reduces severe material removal by acting as a solid lubricant.

 

Question 5: Could you present, in a list, the estimated binding energies for Zr₅₆Co₂₈Al₁₆, Zr₅₆Co₂₃Al₁₆Ag₅, and Zr₅₆Co₂₃Al₁₆Ti₅, as well as the energies associated with oxide film formation and the binding energies of the oxides?

Response: Thank you for the comments. Detailed XPS measurements and corresponding analysis is beyond the scope of the current work. The table below provides the binding energy values from literature for TiO_2, Ag₂O and ZrO₂ which are the main surface oxides influencing the tribological behavior of our BMGs.

 

Oxide	Binding Energy (ev)	Reference
TiO2	~459	1, 2
Ag2O	~367	3
ZrO2	~ 181	4

 The higher binding energy of TiO₂ explains why Zr₅₆Co₂₃Al₁₆Ti₅ exhibit better wear resistance and a lower COF due to the formation of a more stable and protective tribo-layer. The lower binding energy of Ag₂O suggest that Zr₅₆Co₂₃Al₁₆Ag₅ may exhibit reduced adhesion and a less durable oxide layer potentially leading to high wear rate compared to Zr₅₆Co₂₃Al₁₆Ti₅. Since ZrO₂ is present in all compositions, it provides moderate oxide stability.

 

Question 6: In figure 5, b) and c) have different passion with others. Please adjust them. Please add same scale graphs of inside wear tracks. Present graphs in c), f), and i) indicated different scale and difficult to understand the differences with inside and outside of the wear tracks.

Response: Thank you for the comments. Same scale bar has been added to the wear track images, and Figure5 has been modified to improve the clarity in differentiating Raman spectroscopy analysis between inside and outside wear tracks where   indicates outside the wear track and     indicates inside the wear track for the three BMGs studied.

 

References:

[1] Akira Sasahara, Tatsuya Murakami, Masahiko Tomitori,XPS and STM study of TiO2(110)-(1 × 1) surfaces immersed in simulated body fluid,Surface Science,Volume 668,2018,Pages 61-67,ISSN 0039-6028, https://doi.org/10.1016/j.susc.2017.10.022.

[2] Eaimsumang, Srisin & Prataksanon, Piyachat & Pongstabodee, Sangobtip & Luengnaruemitchai, Apanee. (2020). Effect of acid on the crystalline phase of TiO2 prepared by hydrothermal treatment and its application in the oxidative steam reforming of methanol. Research on Chemical Intermediates. DOI:10.1007/s11164-019-04031-8

[3] XPS International LLC. "Silver Oxide (Ag₂O) XPS Data." XPS Data Library, 2023. Available at: https://www.xpsdata.com/ag2o.pdf.

[4] Köck, E.-M.; Kogler, M.; Götsch, T.; Schlicker, L.; Bekheet, M.F.; Doran, A.; Gurlo, A.; Klötzer, B.; Petermüller, B.; Schildhammer, D.; et al. Surface Chemistry of Pure Tetragonal ZrO2 and Gas-Phase Dependence of the Tetragonal-to-Monoclinic ZrO2 Transformation. Dalton Trans. 2017, 46, 4554–4570, doi: https://doi.org/10.1039/C6DT04847A

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

In general, the comments are adequately addressed by the authors.