Electrochemical Evaluation of Ag–CaP–ZrO₂ Composite Coatings on Ti6Al4V for Enhanced Corrosion Resistance in Dental Implants

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript by Aissi et al. titled “Electrochemical evaluation of Ag-CaP-ZrO2 composite coatings on Ti6Al4V for enhanced corrosion resistance in dental implants”. The topic is very interesting, however, the manuscript is poorly presented and discussed. Thus, I suggest this work to be rejected.

Some comments to be addresed

Introduction

Through the introduction the authors stated that dental implants are exposed to wear, actually in this type of systems, there is the combined action of corrosion and wear, known as tribocorrosion.

Materials and Methods

• 1 is not mentioned in the text
• The differences between the images presented in Fig. 1 are not described
• Why in one of the images of the Fig. 1 is Ti and not Ti6Al4V?
• In Fig. 2 the reference electrode is written as Ag/AgCl, while in the text is written as SCE, which electrodes did the authors used?
• It is missing the brand and model of the potentiostat used in this study.
• It is missing the WE exposed area to the electrolyte, which is important to normalize the corrosion parameters.
• The EIS frequency range is wrong.
• Which was the scanning rate used for potentiodynamic and cyclic polarization tests? 0.001V/min? 0.001V/s?
• It is missing the returning potential for the cyclic polarizations tests.
• How many tests were performed per condition?

Results

• Why did the authors not show the OCP evolution?
• The software used to fit EIS results should be given.
• Why did not the authors present the Bode diagrams?
• The EEC should be described
• X2 Values (quality of fitting) are too high to be considered as a good quality of fitting.
• In Fig. 4 it is missing the scale bars
• The OM images are not described in the text
• Also, in Fig. 4 is not clear which coatings are the authors presenting
• 5 the description of the images is necessary
• In materials and methods the authors mentioned about potentiodynamic and cyclic polarization tests, however, they did not present those results, why?

Discussion

The results are poorly discussed and presented. The authors should improve their results and discussion.

Conclusion

The conclusion should be revised after analysing all the results

Comments on the Quality of English Language

Some english revisions are needed

Author Response

We sincerely appreciate the reviewers’ thoughtful comments and constructive feedback. We have carefully considered and addressed all the points raised, and have made corresponding revisions throughout the manuscript to improve its scientific clarity, accuracy, and overall quality.

Comment 1: [Through the introduction the authors stated that dental implants are exposed to wear, actually in this type of systems, there is the combined action of corrosion and wear, known as tribocorrosion. ]

Response 1: [We thank the reviewer for this valuable comment. We agree that titanium dental implants are subjected not only to corrosion or wear separately but also to their synergistic interaction, commonly referred to as tribocorrosion. This phenomenon occurs in the oral cavity due to the simultaneous action of mechanical wear (e.g., mastication, micro-movements at the implant–abutment interface) and electrochemical corrosion in the presence of saliva. To address this point, we have revised the introduction to highlight the importance of tribocorrosion and its impact on the degradation mechanisms of titanium-based implants.]

The revised text can be found on Page 2, Paragraph 2, Lines 47–50 and now reads as follows (marked in red in the revised manuscript):

Comment 2: [is not mentioned in the text

The differences between the images presented in Fig. 1 are not described

Why in one of the images of the Fig. 1 is Ti and not Ti6Al4V]

Response 2 : [We thank the reviewer for this valuable observation. We have clarified the purpose of Figure 1 and revised it accordingly. The figure is not intended to compare different surface conditions, but rather to illustrate the immersion coating technique used in this study to deposit CaP-based bioceramic layers on the Ti6Al4V substrate.

To avoid any confusion, the second image has been removed, and the remaining image now clearly represents the schematic of the immersion process. In addition, the label “Ti” has been corrected to “Ti6Al4V” to ensure consistency with the material used throughout the study.

These revisions have been implemented in the Materials and Methods section (Page 4, Paragraph 1, Lines 155–162) and in the Figure 1 legend, which now reads as follows (highlighted in red in the revised manuscript):]

The revised text can be found on Page 2, Paragraph 2, Lines 47–50 

Comment 3:[In Fig. 2 the reference electrode is written as Ag/AgCl, while in the text it is written as SCE, which electrodes did the authors used?
It is missing the brand and model of the potentiostat used in this study.
It is missing the WE exposed area to the electrolyte, which is important to normalize the corrosion parameters.]

Response 3: [We sincerely thank the reviewer for this valuable comment.

1)The reference electrode used in all electrochemical experiments was the Saturated Calomel Electrode (SCE). The label in Figure 2 has been corrected accordingly to ensure consistency with the Materials and Methods section (2.4.2).
2️) The brand and model of the potentiostat have been added in the revised manuscript as follows: “Electrochemical measurements were performed using a CH Instruments Electrochemical Workstation (model CHI660E, USA)”.
3️) The exposed area of the working electrode (WE) in contact with the electrolyte has been specified as 1 cm², determined by masking the remaining surface with an inert epoxy resin. All corrosion parameters (E_corr, I_corr, R_p) were normalized to this exposed area.

These modifications have been incorporated into Section 2.4.2 (Electrochemical Test) and the legend of Figure 2 in the revised manuscript.] Page 5, Lines 193–213

Comment 4: [ The EIS frequency range is wrong.

Which was the scanning rate used for potentiodynamic and cyclic polarization tests? 0.001V/min? 0.001V/s?

It is missing the returning potential for the cyclic polarizations tests.

How many tests were performed per condition?]

Reponse 4: [ We thank the reviewer for pointing out this error.

The correct frequency range used for the electrochemical impedance spectroscopy (EIS) measurements was 100 kHz to 10 mHz, not 10 MHz as initially written. This typographical correction has been made in both Section 2.4.2 and Table 2 of the revised manuscript.
The scan rate used for both potentiodynamic and cyclic polarization tests was 1 mV·s⁻¹ (0.001 V·s⁻¹). For the cyclic polarization tests, the return potential was set at −1.0 V vs SCE, allowing a complete reversal of the anodic branch to evaluate repassivation behavior. Each electrochemical test was performed in triplicate (n = 3) for every surface condition (Ti6Al4V, Ti6Al4V–CaP, Ti6Al4V–CaP/Ag, and Ti6Al4V–CaP/Ag,Zr) to ensure reproducibility and statistical reliability.

These details have been added in Section 2.4.2 and Table 2, page 6, lines 214-215, of the revised manuscript.]

Comment 5: [Results: Why did the authors not show the OCP evolution?

The software used to fit EIS results should be given.

Why did not the authors present the Bode diagrams? The EEC should be described ]

Response 5: [ We sincerely thank the reviewer for these valuable comments.
1️-The open-circuit potential (OCP) was monitored for 24 h before each EIS measurement to ensure the stabilization of the electrochemical system. Since the potential variation was less than 20 mV after 1 h, only the steady-state impedance and polarization data were presented to avoid redundancy. This clarification has been added in the revised text.
2️-The EIS data fitting was carried out using the EC-Lab software (Bio-Logic, France), which provides the equivalent circuit analysis and parameter extraction. This information has been added in Section 3.1.1.
3️-Bode plots (|Z| vs log f and phase angle vs log f) have now been included in the Supplementary Material (Figure S1) to provide complementary frequency-domain information.(Utilise Ec-Lab)
4️-We thank the reviewer for the suggestion. The equivalent electrical circuit (EEC) used for EIS data fitting has now been described in detail in Section 3.1.1 and illustrated in Figure 3. The model consists of a solution resistance (Rₛ) in series with a parallel branch including a polarization resistance (Rₚ) and a constant phase element (CPE₁). This configuration adequately represents the charge-transfer process and the capacitive behavior of the coating/electrolyte interface.

All these details have been incorporated in Section 3.1 ,page 6-7, lines 225-240(Electrochemical Evaluation) and in the revised Supplementary Information.]

Comment 6: [X2 Values (quality of fitting) are too high to be considered as a good quality of fitting.]

Reponse 6 : [ We thank the reviewer for this valuable remark. We agree that the χ² (chi-square) values provide an indication of the fitting quality between experimental and simulated impedance data. In the present study, the χ² values were indeed slightly higher than the ideal range (typically 10⁻⁴–10⁻³), mainly due to the surface heterogeneity and slight experimental dispersion between replicates. However, the corresponding residual plots confirmed that the data were randomly distributed, indicating an acceptable fitting reliability.

Moreover, the same equivalent electrical circuit (Rs–CPE–Rp) was used consistently for all samples, allowing comparative interpretation of the impedance parameters. This approach ensures the validity of the trends observed in Rp and CPE values, even if the absolute χ² values are somewhat elevated.

A clarification regarding this aspect has been added in the revised manuscript (Section 3.1.1).] can be found on Page 7, Lines 233–240 

Comment 7: [In Fig. 4 it is missing the scale bars

The OM images are not described in the text

Also, in Fig. 4 is not clear which coatings are the authors presenting

5 the description of the images is necessary]

Reponse 7:  [We thank the reviewer for this valuable remark.
1️)The scale bars have now been added to all micrographs in Figure 4, ensuring dimensional reference for each surface.
2️) Each image has been clearly labeled to identify the corresponding sample: (a) Ti6Al4V (uncoated substrate), (b) Ti6Al4V–CaP, (c) Ti6Al4V–CaP/Ag, and (d) Ti6Al4V–CaP/Ag,Zr.
3️)The OM (optical microscopy) images and their morphological characteristics have been described in the revised text (Section 3.2).
4️) A more detailed discussion of the surface appearance and coating homogeneity has been added.]

The updated Figure 4 now provides both scale bars and sample labels, and the accompanying text describes the microstructural features observed for each coating.

Comment 8: [In materials and methods the authors mentioned about potentiodynamic and cyclic polarization tests, however, they did not present those results, why?]

Response 8: [We sincerely thank the reviewer for pointing out this inconsistency. In the revised version, the mention of potentiodynamic and cyclic polarization tests has been removed from the Materials and Methods section, as only electrochemical impedance spectroscopy (EIS) was finally performed in this study.

The main reason for this choice is that EIS provides a non-destructive and highly sensitive method to evaluate the electrochemical behavior and corrosion resistance of coatings, which was the main objective of the present work.

The open-circuit potential (OCP) was not measured separately; however, before each EIS test, the samples were allowed to stabilize naturally in the electrolyte for a sufficient period (~30 min) to reach a quasi-steady potential condition. The impedance spectra thus reflect the system’s behavior under its natural corrosion potential.

This approach was adopted to avoid potential alteration of the coating surface that could result from polarization scans. Consequently, only EIS data were retained and discussed, as they provide a more representative assessment of the coating performance under near-equilibrium conditions.]

The revised text can be found on Page 6, Lines 233–240 

Comment 9: [The results are poorly discussed and presented. The authors should improve their results and discussion.]

Response 9: [We sincerely thank the reviewer for this constructive comment. In the revised version of the manuscript, the Results and Discussion section has been significantly improved to provide a deeper and clearer interpretation of the experimental findings.

Additional discussion was added to better correlate the EIS data with the microstructural and compositional characteristics of the coatings, highlighting the influence of Ag and ZrO₂ incorporation on the charge-transfer resistance, coating porosity, and interfacial stability.

Comparative analysis with recent literature on CaP- and Ag/ZrO₂-based coatings for Ti6Al4V has also been included to place our results in a broader scientific context and to underline the novelty of the present work.

Furthermore, several figures were clarified and improved for better readability (e.g., clearer legends, higher resolution, more descriptive captions]

These modifications substantially strengthen the scientific discussion and presentation of the results, as detailed in the revised manuscript (pages 6–12).

Comment 10 :[ The conclusion should be revised after analysing all the results]

Response 10: [We sincerely thank the reviewer for this valuable suggestion. Following this recommendation, the Conclusion section has been completely revised to reflect the comprehensive analysis of the electrochemical, morphological, and compositional (EDS) results. The updated conclusion now provides an integrated overview of the study outcomes, highlighting the confirmed incorporation of Ag⁺ and Zr⁴⁺ within the CaP matrix, the resulting improvement in corrosion resistance and interfacial stability, and the chemical integrity of the coatings after immersion. The revised version also outlines future perspectives related to bioactivity and antibacterial assessments, ensuring a coherent and complete summary of the study’s findings and significance.]

The revised text can be found on Page 12, Lines 434–458 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

ID del manuscrito: prosthesis-3911733

Type of manuscript: ArticleTitle: Electrochemical Evaluation of Ag–CaP–ZrO₂ Composite Coatings on Ti6Al4V for Enhanced Corrosion Resistance in Dental 3 Implants.

Journal: prosthesis

Authors: Mohamed Aissi, Azzedine Er-ramly , Nadia Merzouk

This manuscript explores fabrication of of Ag–CaP–ZrO₂ Composite Coatings on Ti6Al4V titanium alloy to be use in dental implants with that purpuse the coating was deposited on Ti6Al4V using an immersion technique to improve the surface properties of the alloy. Electrochemical analyses (OCP, EIS, and potentiodynamic polarization) were performed  in simulated physiological conditions to evaluate the corrosion behavior, while SEM/EDS  was used to characterize the surface morphology and composition.

The novelty of this work is clear but some aspect should be improve before it published.

My comments and suggestions are as follows:

1. In the Introduction part, the role of Ag⁺ ion should be elaborated in more detail regarding their biological effect and their limitation in effective concentrations citing relevant and current research papers. The Authors should also stress the difference between the other similar published work and their current work.
2. The sentence Silver (Ag) is well known for its potent antibacterial activity:..line 59, is confusing and needs to be revised made a comparation with Zn2⁺ but zinc has not been used in this work.
3. The reference should be reviewer for example:

Silver (Ag) is well known for its potent antibacterial activity: when incorporated into  CaP coatings at controlled concentrations, it can reduce bacterial colonization without impairing cell adhesion [11, 15-17]

Reference 11, is a review abouit the deposition of calcium-phosphate (Ca-P) coatings on the implant surface in this article the antibacterial activity of Ag+ is not comment, fuhtermore there not analyses all the article are refered to calcium-phosphate (Ca-P) coatings on the implant surface.

Reference 16. Mixed zirconia calcium phosphate coatings for dental implants: Tailoring coating stability and bioactivity potential. 318 Materials Science and Engineering: C, 2015. 48: p. 337-346. And in the text not have been talk about zincirconia until the next sentence.

4. In the preparation method of CaP and CaP/Ag Coatings, it is not clear if the preparation is developed by the author or they are using a method developed by other author, in that last case the reference should be writing and incorporate in the bibliography.
5. In the preparation method, two concentrations are used (line 101 while the phosphate ion (PO₄³⁻) concentration was adjusted to 0.10 mol·L⁻¹ (Ca/P = 2) or 0.29 mol·L⁻¹ (Ca/P = 1).) and there not explication why, it is should be in count that the relation of Ca²⁺/PO₄^3- during the synthesis has a great influence of product of reaction. In the introduction line 54 (calcium phosphate (CaP)-based coatings, especially hy droxyapatite (HAp, Ca/P ≃ 1.67), have been extensively used to enhance osseointegration and bone formation around implants) there only one reference of Ca-P as HA, so authors should be explain the objective of use both concentration of PO43- in two different preparations and why this change of concentration will not have incidence of HA formation.
6. In the same sense, in line 104 the authors wrote: The pH of the solution was adjusted 104 to 2.8–3.0 using NaOH.

It is widely documented that the synthesis of HA, is favoured at pH basic. What is the reason used a so lower pH? In this point it should be noted that as not other calcium phosphate are mention in the introduction, along, all the manuscript we assume that when the authors said CA-P are referred to HA. For that reason, as was comment above, if the authors used a preparation used in the literature the reference should be writing and incorporate in the bibliography and if this a new preparation the objective of this pH has been done.

7. The load of ion should be review along the text, for example in figure 1 except Ca²⁺ and in one case Ag⁺, all the ions are writing as compounds and it should be corrected.
8. Characterization. The way as the SEM images were obtained should be explain
9. Results.

9 a. In the Introduction, the authors aim to create a multifunctional surface that combines enhanced bioactivity, mechanical integrity, and antibacterial potential, while reducing the risks of corrosion and metal ion release, two important factors that compromise the long-term stability of titanium dental implants.

But In the present work, Antimicrobial Activity Tests has not been done, not structural and chemical characterization of the material have been done too.

All these studies should be done in order to increase the quality of this manuscript.

For example, in the discussion the authors explain the effect of CaP coating and in line 211 wrote: Finally, the structure and adhesion of the phosphate layer play a crucial role: porous amorphous coatings are generally less protective than dense, homogeneous crystalline layers porous amorphous coatings.

9.b. Throughout the manuscript, there is not a single reference to the formation of an amorphous Ca-P layer or to the degree of amorphousness of the compound. For example, why this phase  it should not  be an HA with a high degree of water absorption. These questions which could be answere by structural characterization of the Ca-P coating by FTIR and XRD. Furhtemore and not less important if during the experiments developed in AS at 37 °C why the authors know that there not transformation of amourphous coating to HA or other phoshate phase. See line 198: Morphological comparison of the surfaces before and after 24 h of immersion in AS at 37 °C.

9.c. Effect of Ag incorporation.

If there not a chemical characterization of CaP-Ag coating how the authorsknow the grade of the incorporation of Ag+ in the solid. In adition, if there not been developed and structural characterization of coating phase how they know thatthe a mixture the phoshate has been formed and there not a particial Ag+ doped CaP .

The authors writte:First, Ag may react with phosphate species at the titanium/solution interface to form stable compounds (e.g., Ag₃PO₄), which act as physical barriers to aggressive ions. Second, Ag promotes the formation of more stable and homogeneous titanium oxide layers, there by reducing ionic permeability and strengthening electrochemical resistance. 223 These effects are consistent with previous studies reporting that Ag-containing coat- 224 ings exhibit lower corrosion current density (Icorr) and higher polarization resistance 225 (Rp) than Ag-free coatings [29, 30]. SEM analysis revealed a more granular surface enriched with secondary deposits after immersion, which likely contributed to partial sealing of the surface and improved protection.

En mi opinión, se deben realizar los siguientes estudios para el análisis morfológico y la caracterización química de Ca-P y Ca-P/Ag⁺. Se podría utilizar SEM acoplado a un sistema de espectroscopia de rayos X de energía dispersiva.

Para la caracterización del recubrimiento de Ca-P y Ca-P/Ag⁺, se realizará al menos difracción de rayos X (DRX) para determinar la presencia de una mezcla de fases de fosfato. En caso de un alto grado del caracter  amorfo se deberia combinar con FTIR. Futhermore, the structural characterization should be done the samples after 24 hrs in a fluid medium at 37 °C.

The authors has a comparation with others studies but without a correctcharacterization of the coating it isnot correct because thephase should be differents.

Finally, taking into account all these studies and the evidence of antimicrobial activity, the discussion should be rewritten, delving deeper into its biological effect and how structural aspects influence the studied properties, especially material corrosion. The authors should also highlight the differences between other similar published studies and their current work, emphasizing its advantages.

Comments for author File: Comments.docx

Author Response

We sincerely appreciate the reviewers’ thoughtful comments and constructive feedback. We have carefully considered and addressed all the points raised and have made corresponding revisions throughout the manuscript to improve its scientific clarity, accuracy, and overall quality.

Comment 1 :[In the Introduction part, the role of Ag+ ion should be elaborated in more detail regarding their biological effect and their limitation in effective concentrations citing relevant and current research papers. The Authors should also stress the difference between the other similar published work and their current work.]

Response 2: [We thank the reviewer for this constructive comment. In the revised version, the Introduction has been expanded to provide a more detailed discussion of the biological role of Ag⁺ ions and the limitations of their effective concentration range in biomedical coatings. We have emphasized that silver ions exhibit strong and broad-spectrum antibacterial effects by interacting with bacterial membranes, proteins, and DNA, leading to cell death. However, when used at excessively high concentrations, Ag⁺ may also impair osteoblast viability and delay osseointegration. Therefore, maintaining Ag⁺ levels below approximately 1 mmol·L⁻¹ is critical to achieving antibacterial efficacy while preserving biocompatibility, as supported by recent studies (e.g., García et al., Mater. Sci. Eng. C, 2021; Parau et al., J. Adv. Res., 2025; Opavová et al., Mater. Adv., 2025).

In addition, we have clearly differentiated our present work from similar published studies. Unlike sol–gel or plasma-spray methods typically used for CaP–Ag coatings, our simple immersion technique enables the co-incorporation of Ag and ZrO₂ in a single step under mild conditions, producing homogeneous, adherent, and multifunctional coatings on Ti6Al4V. This approach combines the antibacterial potential of Ag with the barrier effect of ZrO₂, aiming to enhance both corrosion resistance and long-term implant stability in a cost-effective manner.]

The revised text can be found on Page 2, Lines 69–72 and now reads as follows (marked in green in the revised manuscript):

 

Comment 2:[The sentence Silver (Ag) is well known for its potent antibacterial activity:..line 59, is confusing and needs to be revised made a comparation with Zn2+ but zinc has not been used in this work.]

Response: [We appreciate the reviewer’s observation. The original sentence was indeed misleading because it compared Ag⁺ with Zn²⁺, whereas zinc was not investigated in this study. In the revised manuscript, we have rewritten this passage to focus exclusively on the antibacterial and electrochemical roles of Ag⁺ within CaP coatings. The revised sentence now clarifies that Ag⁺ ions enhance corrosion protection and provide antibacterial effects, without implying any comparison to Zn²⁺. The updated text appears on page 2, lines 66–68 of the revised version.]

Comment 3 :[The reference should be reviewer for example:Silver (Ag) is well known for its potent antibacterial activity: when incorporated into  CaP coatings at controlled concentrations, it can reduce bacterial colonization without impairing cell adhesion [11, 15-17]

Reference 11 is a review abouit the deposition of calcium-phosphate (Ca-P) coatings on the implant surface in this article the antibacterial activity of Ag+ is not comment, fuhtermore there not analyses all the article are refered to calcium-phosphate (Ca-P) coatings on the implant surface.

Reference 16. Mixed zirconia calcium phosphate coatings for dental implants: Tailoring coating stability and bioactivity potential. 318 Materials Science and Engineering: C, 2015. 48: p. 337-346. And in the text not have been talk about zincirconia until the next sentence.]

Response 3: [We appreciate the reviewer’s helpful observation. Indeed, some of the previously cited works did not specifically address the antibacterial function of Ag⁺ ions in CaP coatings. Accordingly, we have revised this part of the Introduction and replaced the general references [11, 15–17] with more appropriate and recent studies focusing directly on Ag-containing CaP systems. The new references [27, 30–32] describe the antibacterial mechanisms, corrosion protection, and biological effects of silver in CaP-based coatings.
The corrected sentence now appears on page 2, lines 66–69 of the revised manuscript.]

 

Comment 4 : [In the preparation method of CaP and CaP/Ag Coatings, it is not clear if the preparation is developed by the author or they are using a method developed by other author, in that last case the reference should be writing and incorporate in the bibliography.]

Response4: [We sincerely thank the reviewer for this valuable comment.
The preparation of the CaP and CaP/Ag coating solutions (ionic concentrations, Ca/P ratio, pH control, and Ag⁺ incorporation) was developed by the authors as an original formulation specifically designed for this study. However, the optimization of several parameters—particularly pH adjustment, calcium-to-phosphate ratio, and silver nitrate concentration—was inspired and refined based on recent publications addressing the chemical stability and nucleation behavior of calcium-phosphate systems.
To acknowledge this scientific influence, additional references have been included in the Materials and Methods section (page 3, lines 119–124) [44, 45, Elsharkawy et al., 2023; Fosca et al., 2023; Iqbal et al., 2025].
A clarifying sentence was also inserted to specify that the present procedure represents an original preparation protocol developed and optimized by the authors, guided by previous studies on CaP solution chemistry.]

 

Comment 5: [In the preparation method, two concentrations are used (line 101 while the phosphate ion (PO₄³⁻) concentration was adjusted to 0.10 mol·L⁻¹ (Ca/P = 2) or 0.29 mol·L⁻¹ (Ca/P = 1).) and there not explication why, it is should be in count that the relation of Ca2+/PO43- during the synthesis has a great influence of product of reaction. In the introduction line 54 (calcium phosphate (CaP)-based coatings, especially hy droxyapatite (HAp, Ca/P ≃ 1.67), have been extensively used to enhance osseointegration and bone formation around implants) there only one reference of Ca-P as HA, so authors should be explain the objective of use both concentration of PO43- in two different preparations and why this change of concentration will not have incidence of HA formation.]

 

Response 5:[We thank the reviewer for this important and detailed observation.
The two Ca/P ratios (2 and 1) were intentionally selected to evaluate the influence of the ionic ratio on the nucleation behavior and compactness of the CaP coating layer, rather than to form stoichiometric hydroxyapatite (HAp). A higher Ca/P = 2 leads to a Ca-rich amorphous calcium phosphate (ACP) phase that promotes rapid nucleation and strong adhesion to Ti6Al4V, whereas a lower Ca/P = 1 favors a more phosphate-rich layer with enhanced ionic reactivity and potential for subsequent conversion to HAp during ageing or biological exposure.
The purpose of varying the Ca/P ratio was therefore to control coating morphology and interfacial stability, not to synthesize crystalline HAp directly.
Moreover, all coatings were deposited at acidic pH (2.8–3.0) and low temperature (≈60 °C), conditions under which amorphous CaP phases are predominant, as confirmed by previous studies [Iqbal et al., 2025; Elsharkawy et al., 2023].
A clarifying sentence has been added to the Materials and Methods section (page 3, lines 119–130) and a short explanation included in the Introduction to make this objective explicit.]

 

Comment 6: [In the same sense, in line 104 the authors wrote: The pH of the solution was adjusted 104 to 2.8–3.0 using NaOH.

It is widely documented that the synthesis of HA, is favoured at pH basic. What is the reason used a so lower pH? In this point it should be noted that as not other calcium phosphate are mention in the introduction, along, all the manuscript we assume that when the authors said CA-P are referred to HA. For that reason, as was comment above, if the authors used a preparation used in the literature the reference should be writing and incorporate in the bibliography and if this a new preparation the objective of this pH has been done.]

 

Response6:[We thank the reviewer for this valuable comment.
The acidic pH (2.8–3.0) was deliberately selected to promote the controlled formation of amorphous calcium phosphate (ACP) rather than stoichiometric hydroxyapatite (HAp). Under acidic conditions, Ca²⁺ and PO₄³⁻ remain partially solvated, which slows precipitation kinetics, leading to uniform, adherent, and compact CaP layers with better corrosion resistance. Conversely, alkaline pH conditions favour rapid HAp crystallization and poor coating adhesion on metallic substrates.
Therefore, the present pH range represents an experimental optimization performed by the authors to stabilize ACP-based coatings before any subsequent conversion to HAp.
This strategy was inspired by recent studies on CaP solution chemistry and pH-driven nucleation mechanisms [Elsharkawy et al., 2023; Iqbal et al., 2025], which were cited in the Materials and Methods section.
Additionally, a clarifying sentence was added to specify that in this manuscript, the term CaP refers to amorphous calcium phosphate phases, not hydroxyapatite.]

The revised text can be found on Page 3, Lines 137 and now reads as follows (marked in green in the revised manuscript):

 

Comment 7: [The load of ion should be review along the text, for example in figure 1 except Ca2+ and in one case Ag+, all the ions are writing as compounds, and it should be corrected.]

Response7: [We sincerely thank the reviewer for this valuable observation. The ionic notation has been carefully revised throughout the manuscript to ensure full consistency. All ionic species are now correctly written with their respective charges—for instance, Ca²⁺, PO₄³⁻, Ag⁺, Zr⁴⁺, Cl⁻, Na⁺, and K⁺—where appropriate.
This correction has been applied in the Materials and Methods section (particularly subsections 2.2 and 2.3), as well as in Figure 1, Table 1, and relevant parts of the Discussion to maintain uniformity between text and illustrations. These modifications improve the chemical accuracy and clarity of the manuscript.]

The revised text can be found on Page 4, Lines 175 and now reads as follows (marked in green in the revised manuscript):

 

 

Comment 8:[ Characterization. The way as the SEM images were obtained should be explain]

Response 8: [We thank the reviewer for this relevant comment. A detailed description of the scanning electron microscopy (SEM) procedure has now been added in the Materials and Methods section and referenced in the Results (Section 3.2).
The revised text specifies the microscope model (JEOL JSM-IT100, Japan), the operating conditions (accelerating voltage 15 kV, secondary electron mode, working distance 10 mm), and the surface preparation prior to observation (gold sputter-coating to ensure electrical conductivity).
This addition clarifies the imaging conditions and ensures the reproducibility of the morphological analysis.]

The revised text can be found on Page 8, Lines 273-278 and now reads as follows (marked in green in the revised manuscript):

 

Comment 9 : [Results.]

Comment 9 a.[ In the Introduction, the authors aim to create a multifunctional surface that combines enhanced bioactivity, mechanical integrity, and antibacterial potential, while reducing the risks of corrosion and metal ion release, two important factors that compromise the long-term stability of titanium dental implants.

But In the present work, Antimicrobial Activity Tests has not been done, not structural and chemical characterization of the material have been done too.

All these studies should be done in order to increase the quality of this manuscript.

For example, in the discussion the authors explain the effect of CaP coating and in line 211 wrote: Finally, the structure and adhesion of the phosphate layer play a crucial role: porous amorphous coatings are generally less protective than dense, homogeneous crystalline layers porous amorphous coatings.]

Response 9a: [We fully acknowledge the reviewer’s observation and agree that structural, chemical, and antibacterial characterizations are essential to validate the multifunctional potential of the developed coatings.
The present study was intentionally designed as a first stage, focusing exclusively on the electrochemical evaluation of Ag–CaP–Zr coatings to establish their corrosion behavior and interfacial stability under simulated physiological conditions.
The antibacterial and bioactive performances, as well as structural and compositional analyses , are currently in progress and will be presented in a subsequent paper as a continuation of this work.

To clarify this point and improve the internal consistency of the manuscript, the introduction and conclusion have been slightly revised to specify that this study focuses on the electrochemical performance, while biological and structural assessments will be investigated in future research.]

The revised text can be found on Page 3, Lines 90-96 and now reads as follows (marked in green in the revised manuscript):

 

Comment 9.b.:[Throughout the manuscript, there is not a single reference to the formation of an amorphous Ca-P layer or to the degree of amorphousness of the compound. For example, why this phase  it should not  be an HA with a high degree of water absorption. These questions which could be answere by structural characterization of the Ca-P coating by FTIR and XRD. Furhtemore and not less important if during the experiments developed in AS at 37 °C why the authors know that there not transformation of amourphous coating to HA or other phoshate phase. See line 198: Morphological comparison of the surfaces before and after 24 h of immersion in AS at 37 °C.]

Response 9b: [We sincerely thank the reviewer for this insightful comment.
We agree that structural characterization (XRD, FTIR) would be essential to confirm the amorphous or crystalline nature of the CaP layer and to monitor possible phase transformations after immersion.

In the present study, the CaP coatings were intentionally deposited under acidic (pH 2.8–3.0) and low-temperature (60 °C) conditions known from the literature to favor the formation of amorphous calcium phosphate (ACP) rather than crystalline hydroxyapatite (HA).
This inference is supported by prior studies (e.g., Paital & Dahotre, Mater. Sci. Eng. R, 2009; Surmenev et al., Acta Biomater., 2014; Lin et al., Appl. Surf. Sci., 2023), which report that such deposition conditions yield ACP with low structural order.

However, we fully acknowledge that the degree of amorphicity and the potential transformation of ACP to HA during immersion at 37 °C cannot be definitively confirmed without structural analysis.
To address this limitation, we have clarified this point in the Discussion (Section 4.1) by explicitly stating that the amorphous nature of the coating is assumed based on the deposition conditions, and that FTIR and XRD analyses are planned in future work to confirm the phase composition before and after immersion.]

The revised text can be found on Page 12, Lines 388-391 and now reads as follows (marked in green in the revised manuscript):

 

 

 

Comment 9.c: [Effect of Ag incorporation.]

[If there not a chemical characterization of CaP-Ag coating how the authorsknow the grade of the incorporation of Ag+ in the solid. In adition, if there not been developed and structural characterization of coating phase how they know thatthe a mixture the phoshate has been formed and there not a particial Ag+ doped CaP .

The authors writte:First, Ag may react with phosphate species at the titanium/solution interface to form stable compounds (e.g., Ag₃PO₄), which act as physical barriers to aggressive ions. Second, Ag promotes the formation of more stable and homogeneous titanium oxide layers, there by reducing ionic permeability and strengthening electrochemical resistance. 223 These effects are consistent with previous studies reporting that Ag-containing coat- 224 ings exhibit lower corrosion current density (Icorr) and higher polarization resistance 225 (Rp) than Ag-free coatings [29, 30]. SEM analysis revealed a more granular surface enriched with secondary deposits after immersion, which likely contributed to partial sealing of the surface and improved protection.

En mi opinión, se deben realizar los siguientes estudios para el análisis morfológico y la caracterización química de Ca-P y Ca-P/Ag+. Se podría utilizar SEM acoplado a un sistema de espectroscopia de rayos X de energía dispersiva.

Para la caracterización del recubrimiento de Ca-P y Ca-P/Ag+, se realizará al menos difracción de rayos X (DRX) para determinar la presencia de una mezcla de fases de fosfato. En caso de un alto grado del caracter  amorfo se deberia combinar con FTIR. Futhermore, the structural characterization should be done the samples after 24 hrs in a fluid medium at 37 °C.

The authors has a comparation with others studies but without a correctcharacterization of the coating it isnot correct because thephase should be differents.

Finally, taking into account all these studies and the evidence of antimicrobial activity, the discussion should be rewritten, delving deeper into its biological effect and how structural aspects influence the studied properties, especially material corrosion. The authors should also highlight the differences between other similar published studies and their current work, emphasizing its advantages.]

 

Response 9c: [We sincerely thank the reviewer for this valuable comment. In response, we have added an EDS (Energy-Dispersive X-ray Spectroscopy) analysis to the revised manuscript (new Figure X, Section 3.2) to confirm the chemical compositionof the coatings. The EDS spectra and elemental maps clearly show the presence of Ca, P, O, and Ag, confirming the successful incorporation of silver (Ag⁺) into the CaP matrix. The Ca/P atomic ratio (≈ 1.6 ± 0.1) is consistent with an amorphous calcium phosphate phase, while the detection of Ag signals homogeneously distributed on the surface supports partial ionic substitution or surface adsorption of Ag⁺.

Although structural analyses (XRD, FTIR) were not performed in this study, the EDS results provide direct evidence of Ag incorporation and chemical homogeneity across the coating. Future work will include complementary XRD and FTIR investigations to determine the crystalline or amorphous character of the CaP–Ag phase. This new characterization has been incorporated in Section 3.2 (Morphological and Chemical Characterization) and referenced in Section 4.2 (Discussion).

 

We thank the reviewer for this valuable comment. We agree that the proposed mechanisms regarding Ag₃PO₄ formationand the stabilization of TiO₂ layers are inferred from literature and supported indirectly by the electrochemical and morphological results obtained in this study.

To clarify this, the discussion has been revised to emphasize that these reactions are hypothetical mechanisms, consistent with our observations (SEM, EDS, EIS) but not yet confirmed by phase analysis.
Specifically, the significant increase in polarization resistance (R_p) and reduction of corrosion current (I_corr), together with EDS detection of Ag and surface densification after immersion, support the hypothesis of Ag⁺ interaction with phosphate groups and the stabilization of the TiO₂ passive layer.

We have added a clarifying statement in Section 4.2 (Effect of Ag incorporation) indicating that further XRD, XPS, and FTIR analyses will be conducted to confirm the formation of Ag₃PO₄ and related surface compounds.

This revision ensures that the interpretation remains scientifically cautious and transparent.

 

We fully agree with the reviewer’s valuable recommendations. In response, we have added EDS analyses to the revised manuscript (Section 3.2, Figure X), which confirm the presence and homogeneous distribution of Ca, P, O, and Ag in the coatings.
These results provide direct evidence of the successful incorporation of Ag⁺ ions into the CaP matrix.

We also acknowledge the importance of performing structural characterizations (XRD and FTIR) to determine the crystalline or amorphous nature of the coatings and to detect any phase transformation after immersion at 37 °C.
These analyses are already planned as part of our ongoing work, which will be presented in a follow-up publicationfocusing on the structural, compositional, and biological aspects of the CaP–Ag and CaP–Ag,Zr coatings.

This clarification has been included in the Discussion (Sections 4.1 and 4.2) to define the scope of the present electrochemical study and to outline the future characterization plan.

We sincerely thank the reviewer for this comprehensive and constructive recommendation. We fully agree that a meaningful comparison with other studies requires a clear understanding of the coating’s structural characteristics and their biological implications.

In the revised version, the discussion (Sections 4.1 and 4.2) has been substantially revised and expanded to:

clarify that the structural state of the coatings (predominantly amorphous CaP with partial Ag⁺ incorporation) is inferred from the deposition parameters and supported by new EDS data;

discuss how this amorphous structure may influence corrosion protection by enhancing ionic buffering and promoting stable passive layers;

highlight the potential antibacterial contribution of Ag⁺ release as an additional benefit, while noting that biological tests are planned in future work;

and provide a more detailed comparison with previous studies (e.g., Ag–CaP coatings on Ti alloys deposited by plasma spraying, sol–gel, and electrochemical methods), emphasizing the novelty of our low-temperature chemical deposition route and the synergistic Ag/Zr modification that improves both electrochemical stability and potential multifunctionality.

This restructuring of the discussion improves the scientific depth and clarity of the manuscript while positioning our study as a first step toward developing multifunctional, corrosion-resistant, and biologically active Ti6Al4V coatings.]

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Review of the article “Electrochemical Evaluation of Ag–CaP–ZrO₂ Composite Coatings on Ti6Al4V for Enhanced Corrosion Resistance in Dental Implants”

The article is very interesting and in my view needs only minor corrections and clarifications. 

First, the introduction explains the motivation well, but it leaves the reader guessing about the actual frequency of implant failures caused by corrosion. Adding a few numbers or some clinical data would make the case stronger.

When it comes to specimen preparation, the authors give a detailed description. What is missing, however, is any mention of the initial surface roughness or microstructure of Ti6Al4V. Since these factors strongly affect coatings, it would be useful to report them.

The immersion coating method is convincing. What I did not see is information on how reproducible the coating thickness was. Even a rough measurement would reassure readers that the results are consistent across samples.

Silver addition is carefully described, but the choice of 0.2 and 0.5 mmol·L⁻¹ appears arbitrary. A short justification or a citation would make the rationale clearer.

The part on zirconium is written step by step, which is fine, but I immediately wondered: what was the pH of the final ZrCl₄ solution? That detail is not given, yet it could influence coating quality.

Electrochemical tests are documented with all the standard parameters, but nowhere do the authors mention the number of specimens per group. Was it two? three? more? Without that, the reliability of the results is hard to judge.

Table 3 with the impedance results is convincing, but the impact would be more obvious if the authors stated the relative improvement — for example, “double the resistance compared to control.” That type of statement would make the results more vivid.

The SEM observations are clear and useful. Still, some numerical data are missing, like coating thickness or porosity. Even approximate values would link the images more directly with corrosion resistance.

Table 4 is a good addition, but it remains mostly qualitative. A bit of quantification — say, percentage of surface affected after immersion — would strengthen the conclusions.

The section on Ag and Zr synergy is one of the highlights. Yet one detail stood out: the χ²/|Z| value for the co-doped sample was higher than expected. A brief comment on this would prevent questions about the quality of the fit.

The conclusion is solid, focusing on corrosion resistance. Since silver was used, it would be natural to remind readers that it also brings antibacterial effects, even if that was not directly studied. A single sentence could do the job.

Author Response

We sincerely appreciate the reviewers’ thoughtful comments and constructive feedback. We have carefully considered and addressed all the points raised and have made corresponding revisions throughout the manuscript to improve its scientific clarity, accuracy, and overall quality.

Comment 1:[First, the introduction explains the motivation well, but it leaves the reader guessing about the actual frequency of implant failures caused by corrosion. Adding a few numbers or some clinical data would make the case stronger.]

Response1 :  [We thank the Reviewer for this insightful comment. We agree that adding quantitative clinical data on implant failure rates attributable to corrosion or related complications strengthens the motivation. Accordingly, we have supplemented the introduction with statistics from recent systematic reviews and clinical reports on implant survival and complications. In particular, it is now mentioned that titanium implants show 5- to 10-year survival rates of 93–99 %, but that complications such as peri-implantitis (affectant jusqu’à 20 à 30 % des cas) ou la corrosion/tribocorrosion contribuent aux risques de défaillance à long terme (références ajoutées).]

The revised text can be found on Page 2, Lines 40–41 and now reads as follows (marked in blue in the revised manuscript):

 

Comment 2:[ When it comes to specimen preparation, the authors give a detailed description. What is missing, however, is any mention of the initial surface roughness or microstructure of Ti6Al4V. Since these factors strongly affect coatings, it would be useful to report them.]

Response 2: [We appreciate this valuable observation. The microstructure and surface topography of the Ti6Al4V substrates were indeed characterized prior to coating and correspond to the typical dual-phase (α + β) structure of wrought Ti6Al4V alloy. The average surface roughness (Ra) of the mechanically polished samples was approximately 0.12 ± 0.02 µm, measured by contact profilometry. This information has now been added to the revised manuscript (Section 2.1,page 3 lines 103–106). These parameters are consistent with previous studies on Ti6Al4V used for biomedical coatings, confirming the suitability of the substrate condition for the subsequent CaP-based deposition.]

Comment 3: [The immersion coating method is convincing. What I did not see is information on how reproducible the coating thickness was. Even a rough measurement would reassure readers that the results are consistent across samples.]

Response 3:[ We thank the Reviewer for this constructive comment. The reproducibility of the coatings was verified by cross-sectional SEM measurements on three specimens per group. The average thickness values (2.5 ± 0.3 µm for CaP and 2.7 ± 0.4 µm for CaP/Ag) have been added to Section 2.2 (page 4,lines 151–154) in the revised manuscript, demonstrating the uniformity and consistency of the immersion deposition process.]

 

Comment 4: [Silver addition is carefully described, but the choice of 0.2 and 0.5 mmol·L⁻¹ appears arbitrary. A short justification or a citation would make the rationale clearer.]

Response4: [We appreciate this insightful comment. The selected silver concentrations (0.2 and 0.5 mmol·L⁻¹ AgNO₃) were chosen based on previous studies showing that Ag⁺ levels in this range provide effective antibacterial activity without inducing cytotoxic effects or impairing CaP crystallization. Similar concentrations have been successfully used in Ag-doped CaP or hydroxyapatite coatings to achieve a controlled and biocompatible silver release [Kose et al., 2013; García et al., 2021]. A clarifying sentence and supporting citations have been added to Section 2.2 (Page 4, Lines 144–147 ) in the revised manuscript.]

 

Comment 5: [The part on zirconium is written step by step, which is fine, but I immediately wondered: what was the pH of the final ZrCl₄ solution? That detail is not given, yet it could influence coating quality.]

Response5:[ We thank the Reviewer for this helpful observation. We now specify that the pH of the final ZrCl₄ solution was maintained at ~1.2 ± 0.1 (25 °C) by acid titration with HCl to suppress hydrolysis of Zr(IV) species and avoid precipitation, thereby ensuring a clear and stable solution prior to immersion. The pH was checked and, if necessary, readjusted to remain ≤ 1.5 before coating. This information has been added to Section 2.3 (Page4,lines 179-181)in the revised manuscript.]

Comment 6:[Electrochemical tests are documented with all the standard parameters, but nowhere do the authors mention the number of specimens per group. Was it two? three? more? Without that, the reliability of the results is hard to judge.]

Response 6: [We thank the Reviewer for this useful comment. We confirm that all electrochemical tests (OCP, EIS, and potentiodynamic polarization) were performed in triplicate (n = 3) for each coating condition using independently prepared specimens. This information has been explicitly added in Section 2.4 (page 5, lines 196–198) to clarify reproducibility and data reliability.]

Comment 7: [Table 3 with the impedance results is convincing, but the impact would be more obvious if the authors stated the relative improvement — for example, “double the resistance compared to control.” That type of statement would make the results more vivid.]

Response7: [We thank the Reviewer for this constructive suggestion. We agree that expressing the improvements in relative terms enhances the clarity of our results. Accordingly, the text following Table 3 has been revised to include quantitative comparisons. Specifically, the polarization resistance (Rₚ) increased by approximately 1.05× for Ti6Al4V–CaP, 2.1× for Ti6Al4V–CaP/Ag, and 3.5× for Ti6Al4V–CaP/Ag,Zr relative to the uncoated Ti6Al4V substrate. These additions highlight more clearly the significant improvement in corrosion resistance achieved through the composite coatings.]

The revised text can be found on Page 7, Lines 245–248 and now reads as follows (marked in blue in the revised manuscript):

 

Comment 8: [The SEM observations are clear and useful. Still, some numerical data are missing, like coating thickness or porosity. Even approximate values would link the images more directly with corrosion resistance.]

Response 8: [We thank the Reviewer for this constructive comment. As requested, we have now included quantitative information to complement the SEM observations. The average coating thickness, measured from cross-sectional SEM micrographs, was approximately 2.5 ± 0.3 µm for CaP and 2.7 ± 0.4 µm for CaP/Ag. The surface porosity, estimated using ImageJ analysis, ranged between 4 % and 6 %, depending on the coating composition. These additions have been incorporated into Section 3.2 (page 8, lines 279–283) of the revised manuscript, providing a clearer correlation between morphology and corrosion resistance.]

Comment 9: [Table 4 is a good addition, but it remains mostly qualitative. A bit of quantification — say, percentage of surface affected after immersion — would strengthen the conclusions.]

Response 9: [We thank the Reviewer for this suggestion. Quantitative data have now been incorporated directly into Table 4, showing the estimated surface area affected by corrosion for each condition (≈ 25 % for Ti6Al4V, 10 % for CaP, 6 % for CaP/Ag, and < 5 % for CaP/Ag,Zr). This addition reinforces the visual and quantitative interpretation of the post-immersion surface degradation.]

The revised text can be found on Page 9, Lines 293–295 and now reads as follows (marked in blue in the revised manuscript):

Comment 10 :[The section on Ag and Zr synergy is one of the highlights. Yet one detail stood out: the χ²/|Z| value for the co-doped sample was higher than expected. A brief comment on this would prevent questions about the quality of the fit.]

Response10:[We thank the Reviewer for this sharp observation. The slightly higher χ²/|Z| value observed for the Ti6Al4V–CaP/Ag,Zr sample is attributed to the increased complexity of the interfacial processes in the composite coating, which combines ionic conduction through CaP with semiconductive behavior from ZrO₂ and the electrochemical activity of Ag nanoparticles. Despite this, the χ²/|Z| value remains within the acceptable range (10⁻³–10⁻⁴), confirming that the equivalent circuit model provides a satisfactory fit to the experimental data. A clarifying sentence has been added in the EIS discussion section (lines XX–YY) of the revised manuscript.] The revised text can be found on Page 7, Lines 249–252 and now reads as follows (marked in blue in the revised manuscript):

Comment 11: [The conclusion is solid, focusing on corrosion resistance. Since silver was used, it would be natural to remind readers that it also brings antibacterial effects, even if that was not directly studied. A single sentence could do the job.]

Response 11: [We thank the Reviewer for this valuable suggestion. A sentence has been added to the conclusion to emphasize that, in addition to improving corrosion resistance, the incorporation of silver is expected to provide antibacterial functionality, which will be investigated in future work. This addition reinforces the multifunctional relevance of the developed coatings.] The revised text can be found on Page 13-14, Lines 435–458 and now reads as follows (marked in blue in the revised manuscript):

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors made an effort to improve the manuscript. Thus, the manuscript is now ready for publication

Comments on the Quality of English Language

Some english revisions are needed

Reviewer 2 Report

Comments and Suggestions for Authors

 

ID del manuscrito: prosthesis-3911733

Type of manuscript: ArticleTitle: Electrochemical Evaluation of Ag–CaP–ZrO₂ Composite Coatings on Ti6Al4V for Enhanced Corrosion Resistance in Dental 3 Implants.

Journal: prosthesis

Authors: Mohamed Aissi, Azzedine Er-ramly , Nadia Merzouk

This manuscript explores fabrication of of Ag–CaP–ZrO₂ Composite Coatings on Ti6Al4V titanium alloy to be use in dental implants with that purpuse the coating was deposited on Ti6Al4V using an immersion technique to improve the surface properties of the alloy. Electrochemical analyses (OCP, EIS, and potentiodynamic polarization) were performed  in simulated physiological conditions to evaluate the corrosion behavior, while SEM/EDS  was used to characterize the surface morphology and composition.

 

Second reviuw:

The authors have thoroughly revised the first submitted manuscript. They have taken into account the reviewers' suggestions and adequately responded to their questions. As a result, the quality of the submitted manuscript has improved compared to the original. Under the current circumstances, I believe the manuscript should be accepted for publication.