Ag/ZrO₂ Hybrid Coating for Tribological and Corrosion Protection of Ti45Nb Alloy in Biomedical Environments

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. The introduction mainly describes the applicability of Ti45Nb in implants, as well as the importance of surface modification with Ag/ZrO₂ to provide improved wear resistance. The author focused strictly on what is presented in the paper, rather than on a broader perspective on the topic. Therefore, I suggest expanding the introduction and specifically including the results of tribological studies conducted by other authors on the direct titanium alloy, as well as on modified surfaces for comparison.
2. In the case of implants, it's also worth at least mentioning a key tribological aspect, such as lubrication, in the introduction. Please refer to the paper where the authors analyzed the lubrication of a titanium implant with artificial synovial fluid, doi: 10.1016/j.triboint.2024.109562
3. Chapter 2.1 - Did the author analyze the chemical composition or it is based on the manufacturer's specifications?
4.  Bruker Contour GT-I 3D surface profilometer - please add measurement settings. Was profile or area roughness measured?
5.  coefficient of friction - please also add all measurement settings.
6.  Why material of Al₂O₃ (alumina) ball was used as the abrasive counter surface? Does such a countermaterial occur in the operating conditions of implants?
7. What is the accuracy of roughness measurements?
8. How many repetitions of all measurements were performed to present the results? Can the standard deviation be provided?
9. In the graphs of COF versus cycle count, why was COF monitoring stopped at this number of cycles? Could changes be noted in the long term?
10. Why is the surface after the friction test not shown?

Author Response

Comment 1: The introduction mainly describes the applicability of Ti45Nb in implants, as well as the importance of surface modification with Ag/ZrO₂ to provide improved wear resistance. The author focused strictly on what is presented in the paper, rather than on a broader perspective on the topic. Therefore, I suggest expanding the introduction and specifically including the results of tribological studies conducted by other authors on the direct titanium alloy, as well as on modified surfaces for comparison.

Response 1: Thank you for your valuable observation. Accordingly, the Introduction section has been revised to include additional literature on tribological performance of unmodified Ti-based alloys and modified surfaces. I prepared the paragraph regarding with my comment and added it in the revised manuscript as follows:

Page 2, 1. Introduction

“In addition to β-type Ti45Nb alloy, various titanium-based surfaces have been systematically investigated in terms of their tribological behavior. For instance, it has been shown that the phase composition of titanium alloys plays a critical role in determining their wear mechanisms and frictional stability. A comparative study on α (T50), α+β (Ti-6Al-4V), and β (Ti-5553) titanium alloys under steel ball contact revealed that different oxide phases form during sliding, governing distinct wear regimes. While β-type Ti-5553 exhibited a single abrasive stage dominated by TiO₂-anatase formation, α and α+β alloys transitioned to additional regimes involving TiO₂-rutile and Fe-oxide layers, which led to fluctuations in the coefficient of friction and wear severity [14]. Li et al. [15] reported that uncoated β-Ti exhibited severe adhesive wear in dry contact due to its low surface hardness, whereas surface nitriding significantly improved the wear resistance. Similarly, Yilmazer et al. [16] demonstrated that Ti-Nb-Zr substrates coated with TiO₂ showed reduced wear track width and lower coefficient of friction compared to uncoated alloys. Moreover, recent studies by Frutos et al. [17] and Çomaklı et al. [18] have shown that oxide, nitride, and bioceramic coatings enhance the tribocorrosion behavior of β-type Ti alloys, but often lack antibacterial functionality. These findings highlight the need for hybrid coating strategies that offer both mechanical durability and biological compatibility. In addition to mechanical and electrochemical stability, lubrication is a crucial tribological parameter in biomedical implants, especially in synovial joint environments where surface interactions with biological fluids significantly influence wear mechanisms. The wettability and lubrication behavior of implant surfaces directly affect the formation and stability of boundary layers, which in turn co-determine friction and wear performance. Recent studies have emphasized the importance of mimicking physiological lubrication conditions during in vitro assessments. For instance, Wieczorek et al. investigated the lubrication behavior of electro-discharge machined titanium implant surfaces using artificial synovial fluid and demonstrated that microtextured surface features and discharge energy significantly influenced contact angle and surface free energy, thereby affecting lubrication regimes and frictional response [19].”

Comment 2: In the case of implants, it's also worth at least mentioning a key tribological aspect, such as lubrication, in the introduction. Please refer to the paper where the authors analyzed the lubrication of a titanium implant with artificial synovial fluid, doi: 10.1016/j.triboint.2024.109562

Response 2: I thank the reviewer for this helpful suggestion. Indeed, lubrication plays a critical role in determining the tribological performance of implant surfaces. In response, I revised the Introduction section1 to include a brief discussion of lubrication effects in joint implant environments, citing the article recommended by reference 19.

Page 2, 1. Introduction

“In addition to mechanical and electrochemical stability, lubrication is a crucial tribological parameter in biomedical implants, especially in synovial joint environments where surface interactions with biological fluids significantly influence wear mechanisms. The wettability and lubrication behavior of implant surfaces directly affect the formation and stability of boundary layers, which in turn co-determine friction and wear performance. Recent studies have emphasized the importance of mimicking physiological lubrication conditions during in vitro assessments. For instance, Wieczorek et al. investigated the lubrication behavior of electro-discharge machined titanium implant surfaces using artificial synovial fluid and demonstrated that microtextured surface features and discharge energy significantly influenced contact angle and surface free energy, thereby affecting lubrication regimes and frictional response [19].”

Ref. 19: Peta, K.; Bartkowiak, T.; Rybicki, M.; Galek, P.; Mendak, M.; Wieczorowski, M.; Brown, C.A. Scale-Dependent Wetting Behavior of Bioinspired Lubricants on Electrical Discharge Machined Ti6Al4V Surfaces. Tribol. Int. 2024, 194, 109562.

Comment 3: Chapter 2.1 - Did the author analyze the chemical composition or it is based on the manufacturer's specifications?

Response 3: Thank you for your attention to this detail. The chemical composition of the Ti45Nb alloy was based on the manufacturer's specification sheet provided with the raw material. However, to validate the substrate, I also performed a preliminary EDS analysis prior to coating. The results were consistent with the nominal atomic ratios of ~69.2% Ti and ~30.1% Nb.

Comment 4: Bruker Contour GT-I 3D surface profilometer - please add measurement settings. Was profile or area roughness measured?

Response 4: I appreciate this technical request. In the study, surface roughness measurements were performed using area-based scanning mode (5× objective lens) over a 500 µm × 500 µm region. The data was collected using vertical scanning interferometry (VSI) mode. The results presented are Ra (arithmetical mean height) values extracted from area roughness data. These details are included in Section 2.3 as follows:

Page 4, 2.3. Coating Characterization

“The surface roughness measurements were performed using a Bruker Contour GT-I 3D optical profilometer (Contour GT-I, Bruker, USA) operating in Vertical Scanning Interferometry (VSI) mode, which provides a vertical resolution of 0.01 nm and a lateral resolution of 0.2 µm under the selected scanning parameters. Area-based scans were acquired using a 5×objective lens over a 500 µm×500 µm region. To ensure statistical robustness and repeatability, each sample surface was scanned at five randomly selected locations, with three repeated scans per region. The reported Ra values represent the mean ± standard deviation across these measurements, as summarized in Table 2.”

Comment 5: coefficient of friction - please also add all measurement settings.

Response 5: Thank you for pointing this out. The coefficient of friction (COF) was recorded under the following test parameters in Section 2.3:

Page 4, 2.3. Coating Characterization

“The tribological tests were conducted using a “ball-on-flat” reciprocating motion configuration on a Bruker UMT-2 multifunctional mechanical tester. A normal load of 2 N was applied, with a stroke length of 5 mm and a reciprocating frequency of 1 Hz, over a total of 1000 cycles, corresponding to an approximate sliding distance of 100 mm. Tests were performed at ambient temperature (23 ± 2°C) under two different environmental conditions: dry friction and Hank’s Balanced Salt Solution (HBSS) to simulate physiological conditions. All test parameters were kept constant and standardized across samples to ensure comparability. Throughout the tests, the coefficient of friction (COF) was continuously recorded in real time. After testing, the width of the wear scars was analyzed using scanning electron microscopy (SEM), and the wear volume and wear rate were calculated in accordance with the ASTM G133 standard.”

Comment 6: Why material of Al₂O₃ (alumina) ball was used as the abrasive counter surface? Does such a countermaterial occur in the operating conditions of implants?

Response 6: This is an insightful observation. I prepared the paragraph regarding with my comment and added it in the revised manuscript as follows:

Page 5, 2.3. Coating Characterization

“The Al₂O₃ (alumina) ball with a diameter of 6 mm was used as the counter surface. Al₂O₃ was selected due to its chemical inertness, high hardness, and dimensional stability, which allow for highly reproducible and controlled testing conditions. Although Al₂O₃ does not replicate the mechanical behavior of bone or cartilage, it is routinely employed in standardized tribological protocols, such as ASTM G133, to simulate severe yet consistent wear scenarios that facilitate comparative evaluation of surface performance. Moreover, alumina-based ceramics have been utilized in various implant systems, including ceramic-on-metal and ceramic-on-polymer couples, underscoring their relevance in preclinical wear assessments of biomedical materials [25]. All characterization measurements (hardness, roughness, electrochemical tests and wear tests) were repeated three times and each value presented is the average of three independent replicates.”

Comment 7: What is the accuracy of roughness measurements?

Response 7: I thank the reviewer for this technical query. The Bruker Contour GT-I profilometer used in this study operates in vertical scanning interferometry (VSI) mode and provides a vertical resolution of 0.01 nm and a lateral resolution of 0.2 µm under the selected scanning conditions. To increase the statistical reliability of the surface roughness measurements, each surface was scanned at five different locations, with three replicate scans per area. The reported Ra values represent the mean ± standard deviation of these measurements, ensuring high reproducibility and confidence in the data. The Ra values, along with their standard deviations, are reproduced in Table 2, and the values are updated in the revised manuscript. I prepared the paragraph regarding with my comment and added it in the revised manuscript as follows:

Page 4, 2.3. Coating Characterization

“The surface roughness measurements were performed using a Bruker Contour GT-I 3D optical profilometer (Contour GT-I, Bruker, USA) operating in Vertical Scanning Interferometry (VSI) mode, which provides a vertical resolution of 0.01 nm and a lateral resolution of 0.2 µm under the selected scanning parameters. Area-based scans were acquired using a 5×objective lens over a 500 µm×500 µm region. To ensure statistical robustness and repeatability, each sample surface was scanned at five randomly selected locations, with three repeated scans per region. The reported Ra values represent the mean ± standard deviation across these measurements, as summarized in Table 2.”

Table 2. The Ti45Nb alloy and Ag/ZrO₂ hybrid coated Ti45Nb alloy sample; thickness, average surface roughness and hardness values.

Comment 8: How many repetitions of all measurements were performed to present the results? Can the standard deviation be provided?

 Response 8: This is a valuable claim for assessing data robustness. All characterization measurements (hardness, roughness, electrochemical tests and wear tests) were performed in triplicate and each value presented in the paper is the average of three independent replicates. Standard deviation values for thickness, hardness, roughness and wear rates have been added to Tables 2 and 5 in the revised paper to better reflect the reproducibility of the results. These values have also been revised in the manuscript.

Table 2. The Ti45Nb alloy and Ag/ZrO₂ hybrid coated Ti45Nb alloy sample; thickness, average surface roughness and hardness values.

Table 5. Average coefficient of friction and wear rate values of uncoated and Ag/ZrO₂-coated Ti45Nb samples tested under a 2 N wear load in dry and HBSS environments.

Page 5, 2.3. Coating Characterization

“All characterization measurements (hardness, roughness, electrochemical tests and wear tests) were repeated three times and each value presented is the average of three independent replicates.”

Comment 9: In the graphs of COF versus cycle count, why was COF monitoring stopped at this number of cycles? Could changes be noted in the long term?

Response 9: I thank the reviewer for this important observation. Tribological tests were limited to 1000 cycles (approximately 100 mm sliding distance) in accordance with ASTM G133 guidelines, which are widely used to evaluate the wear behavior of biomedical coatings. In addition to being compatible with standard methods, the relatively low thickness of the Ag/ZrO₂ hybrid coating (~1.8 µm) also influenced the choice of test duration. Prolonged sliding beyond this range could result in complete removal of the thin coating layer, making it difficult to distinguish between coating-dominant and substrate-dominant wear mechanisms. Therefore, the selected number of cycles was designed to characterize the initial wear regime without causing complete delamination or significant substrate exposure. In summary, the number of cycles was kept low to allow for characterization of the wear type due to the low coating thickness.

Comment 10: Why is the surface after the friction test not shown?

Response 10: Thank you to the commenter for this relevant observation. Post-wear surface morphology is critical for understanding the dominant wear mechanisms and supporting the friction behavior observed in COF profiles. In the paper, representative SEM images of the worn surfaces after friction tests under both dry and HBSS conditions are available in Figure 6. These images show the wear scar morphology for both uncoated and Ag/ZrO₂ coated Ti45Nb samples.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, Ag/ZrO₂ hybrid coating prepared by sol-gel method on β-type Ti45Nb alloy was applied by spin coating technique, the microstructural, mechanical, electrochemical and tribological properties of the surface were evaluated in a multi-dimensional manner. The research topic of the paper is valuable, and the structure of the paper is well organized. There are the following issues that need to be addressed.

(1) What is the basis for determining a series of parameters such as content when preparing Hybrid Solution Preparation?

(2) In Section 2.2 Spin Coating Process, are the preparation parameters used optimized? How to ensure that the prepared coating has excellent performance? Explanation should be provided in the manuscript.

(3) In Section 2.3, please provide information on the manufacturer and origin place of the equipment and instruments used.

(4) In Figure 6, the labels for figures b, c, and d are missing.

(5) Some of the fonts in the figures are too small, such as the numbers in the coordinates of Figure 5.

(6) The Ag/ZrO2 ₂ hybrid coating prepared by the author has shown excellent performance in various tests. It is recommended that the author conduct a thorough analysis of the mechanism involved. After all, the paper is not an experimental report.

Author Response

Reviewer 2

In this paper, Ag/ZrO₂ hybrid coating prepared by sol-gel method on β-type Ti45Nb alloy was applied by spin coating technique, the microstructural, mechanical, electrochemical and tribological properties of the surface were evaluated in a multi-dimensional manner. The research topic of the paper is valuable, and the structure of the paper is well organized. There are the following issues that need to be addressed.

Comment 1: What is the basis for determining a series of parameters such as content when preparing Hybrid Solution Preparation?

Response 1: Thank you for this insightful question. The requested information was added to the revised manuscript as follows:

Page 3, 2.1. Material and Hybrid Solution Preparation

“The mixture was stirred on a magnetic stirrer for 2 h until a homogeneous solution was obtained. The solution components and molar ratios used were determined based on experience gained from previous sol-gel studies on Ag/ZrO₂ hybrid systems in the literature. In particular, a ratio of 1 mmol Zr(OC₃H₇)₄ and 0.5 mmol AgNO₃ (Zr:Ag = 2:1) has been reported to ensure homogeneous dispersion of silver nanoparticles in solution while preventing excessive agglomeration and uncontrolled precipitation [20,24]. Furthermore, the amounts of acetic acid and water used in the hydroxylation step (0.1 mL and 0.05 mL, respectively) were optimized for controlled hydrolysis of the Zr precursor and stable formation of the gel structure [21]. These content parameters were chosen in accordance with preliminary experiments to support surface integrity, particle distribution, and phase stabilization during film formation. Thus, both the tetragonal phase formation of ZrO₂ and the controlled surface migration of the Ag phase were achieved.”

Comment 2: In Section 2.2 Spin Coating Process, are the preparation parameters used optimized? How to ensure that the prepared coating has excellent performance? Explanation should be provided in the manuscript.

Response 2: Thank the reviewer for raising this important point. Yes, the spin coating parameters were optimized through a series of preliminary experiments aimed at achieving uniform film thickness, good adhesion, and crack-free surface morphology. The spin speed (3000 rpm), number of layers (3 cycles), and pre-baking protocol (100°C for 5 min) were selected based on our prior experience and empirical screening, where different speeds (1000–4000 rpm) and cycle counts (1–5 layers) were tested. Performance optimization was confirmed via XRD crystallinity integrity, SEM and EDS surface morphology after drying. The dual-stage annealing was specifically adopted to stabilize the tetragonal ZrO₂ phase and promote the surface migration of Ag, which is critical for achieving the hybrid character and enhanced functionality. A brief description of this optimization procedure has now been included in Section 2.2 of the revised manuscript.

Page 4, 2.2. Spin Coating Process

“To ensure optimal film quality and functional performance, the spin coating parameters were systematically optimized through a set of preliminary trials. Various spin speeds (1000-4000 rpm), coating cycles (1-5 layers), and pre-baking durations (3-10 min at 80-120°C) were evaluated to identify the conditions that provided the most uniform, crack-free, and adherent coatings. The final parameters (3000 rpm spin speed, 3-cycle coating, and 5-minute pre-baking at 100°C) were selected based on the resulting film’s morphological consistency, XRD crystallinity, and SEM-EDS surface integrity. Furthermore, the dual-step annealing protocol (300°C/1 h and 450°C/30 min) was specifically implemented to induce tetragonal phase stabilization of ZrO₂ and to facilitate Ag nanoparticle segregation toward the surface, thereby enhancing the hybrid coating functionality. These optimization steps ensured that the final coating possessed dense microstructure, enhanced adhesion, and reproducible electrochemical and tribological performance.”

Comment 3: In Section 2.3, please provide information on the manufacturer and origin place of the equipment and instruments used.

Response 3: Thank you for this valuable suggestion. The requested information has now been added to Section 2.3 of the manuscript as follows:

Page 4, 2.3. Coating Characterization

“The morphological analysis and elemental distributions of the coated surfaces were determined by field emission scanning electron microscope (FEI Quanta FEG-450, Thermo Fisher Scientific, USA) and energy dispersive X-ray spectroscopy (EDS; FEI QUANTA 250, Thermo Fisher Scientific, USA). Crystal structures were analyzed by X-ray diffraction (Empyrean XRD system, Malvern Panalytical, Netherlands) scanning in the range of 20°-90°(2θ) using Cu-Kα beam (λ=1.5406 Å). In addition, the surface hardness of the samples was determined using the universal mechanical tester (UMT-2, Bruker Corporation, USA), which allows for high accuracy assessment of mechanical performance at the micro scale.”

“Measurements were performed in the GAMRY Reference 3000 Potentiostat/Galvanostat/ZRA (Gamry Instruments, USA) using a three-electrode cell system at a constant temperature of 37°C; Ag/AgCl was used as the reference electrode and a graft rod was used as the counter electrode.”

Comment 4: In Figure 6, the labels for figures b, c, and d are missing.

Response 4: Thank the reviewer for pointing out this oversight. The figure labels for parts (b), (c), and (d) in Figure 6 were added.

Comment 5: Some of the fonts in the figures are too small, such as the numbers in the coordinates of Figure 5.

Response 5: Thank you for noting this formatting issue. Figure 5 has been re-rendered using a higher resolution and increased font sizes for axes labels, legends, and tick marks to improve readability. The revised figure 5 is given below.

Comment 6: The Ag/ZrO₂ hybrid coating prepared by the author has shown excellent performance in various tests. It is recommended that the author conduct a thorough analysis of the mechanism involved. After all, the paper is not an experimental report.

Response 6:

Page 7, 3.1. Surface Analysis

“The increase in surface hardness observed following the Ag/ZrO₂ hybrid coating is largely due to the high structural hardness of the ZrO₂ ceramic phase, which is dominant in the coating. In particular, the tetragonal phase, stabilized at low temperatures, provides high resistance to plastic deformation and increases the mechanical strength of the surface [27]. Additionally, it is estimated that the silver nanoparticles within the coating increase microdensity by filling intergranular micropores within the microstructure, thus contributing to structural integrity. The resulting compact, multiphase film structure limits the propagation of cracks that may occur under applied mechanical loads and ensures that the surface exhibits crack-resistant behavior [24].

Page 9, Section 3.2: Electrochemical Corrosion Analyses

“This increase in corrosion resistance can be explained by the combined effects of both physical and electrochemical mechanisms. In Ag/ZrO₂-coated samples, the dense ZrO₂ matrix forms an effective physical barrier against chloride and oxygen ion ingress, preventing the initiation of corrosion reactions. At the same time, silver particles within the coating can improve passivation behavior by regulating electron exchange at the surface. The high cathodic Tafel slope coefficient in the coated samples confirms that oxygen reduction reactions are suppressed and a more stable passive film is formed. These results demonstrate that the coating not only provides physical protection but also positively contributes to electrochemical processes [28, 29].”

Page 12, 3.3. Tribological Properties

“The contribution of the Ag/ZrO₂ hybrid coating to tribological performance can be explained by both physical surface modification and tribochemical interactions. The highly crystalline ceramic nature of ZrO₂ enables the surface to resist plastic deformation and limits surface depressions that may occur during wear. Especially in HBSS media, this hybrid structure increases the stability of a tribochemically formed passive layer on the surface, and this layer creates a barrier against both mechanical frictional effects and tribocorrosion processes induced by corrosive ions [29,33]. As a result of these synergistic mechanisms, lower friction coefficients, narrower wear scars, and more predictable surface behavior are achieved in both dry and physiological environments.”

Author Response File: Author Response.pdf