Formation and Characterization of Ti-Al Intermetallic and Oxide Layers on Ti6Al4V as Interlayers for Hydroxyapatite Coatings

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The reviewed work concerns the production of intermetallic layers on the surface of the Ti6Al4V alloy by EDM. The publication is interesting from the point of view of functionalization of the Ti alloy surface in terms of mechanical and tribological properties. However, there are several inconsistencies in the work that should be clarified and commented on.

1. In the title of the paper, the authors used the phrase that the obtained layers were intended to increase hydroxyapatite adhesion, although they did not present any research results that would confirm such statements. Discussing the results in this regard in the light of the literature data (point 4) is insufficient. There are numerous reports in the literature regarding the study of HAp-type layers on Ti6Al4V alloys obtained using simple chemical/electrochemical methods in SBF or Hank's solution (soaking in biological solutions) [https://doi.org/10.1016/j.vacuum.2022.111390, https://doi.org/10.1007/s10856-007-3285-1].
2. The quality of Figures 5 and 8 should be improved.
3. The estimated layer thickness after heat treatment at 600°C/3h was approximately 8.2 micrometers. This result is surprising compared to the unmodified layer. In such a case, some increase in layer thickness could be expected; why did this not occur?
4. How was the Al layer thickness controlled in the EDM process?
5. There is no information in what atmosphere the thermal treatment at 600^oC/3h was carried out?
6. Based on XRD measurements, oxide phases TiO₂, Al₂O₃ and intermetallic phases were identified, which appeared after subsequent stages of surface modification. Therefore, what is the actual structure (construction) of the layers from the surface to the substrate (oxides/intermetallic compounds/transition zone – mixture of compounds/substrate)?
7. An interesting result is shown in Figure 6, in particular the Ti distribution in the layer after thermal treatment. What is the mechanism migration of Ti towards the surface?
8. The authors of the paper suggest that: "These results confirm the formation of hard intermetallic compounds (e.g., γ-TiAl, TiAl₂, TiAl₃), along with oxide (TiO₂, Al₂O₃) and nitride (TiN) phases, which collectively contribute to the enhanced mechanical robustness and wear resistance of the treated surfaces. Such improvements are critical for load-bearing biomedical applications where long-term mechanical durability and resistance to surface degradation are essential.” This finding should be closely correlated with the specific structure of the obtained layers, as components that increase biocompatibility, corrosion resistance, and other factors that shape mechanical properties. Therefore, which structural elements are assigned to the above-mentioned properties?

Author Response

We thank the Reviewer for the careful reading and constructive comments. Below we respond point-by-point and indicate the revisions we have made or will make in the manuscript.

Comment 1. In the title of the paper, the authors used the phrase that the obtained layers were intended to increase hydroxyapatite adhesion, although they did not present any research results that would confirm such statements. Discussing the results in this regard in the light of the literature data (point 4) is insufficient. There are numerous reports in the literature regarding the study of HAp-type layers on Ti6Al4V alloys obtained using simple chemical/electrochemical methods in SBF or Hank's solution (soaking in biological solutions)[https://doi.org/10.1016/j.vacuum.2022.111390, https://doi.org/10.1007/s10856-007-3285-1].

Response 1) We fully agree with the reviewer’s observation and appreciate this constructive remark, which has helped us clarify one of the novel aspects of our work.

Our main goal was not to demonstrate hydroxyapatite (HAp) deposition directly, but rather to develop and characterize a Ti-Al intermetallic/oxide interlayer that can act as a functional interface for future HAp coatings. This interlayer is obtained through an EDM-based process, which differs fundamentally from conventional chemical or electrochemical methods typically used for HAp formation in SBF or Hank’s solution. The novelty of the present study therefore lies in using EDM surface alloying to create a graded Ti-Al-O system with mechanical and chemical properties tailored for direct HAp deposition using the same EDM-assisted approach in future experiments.

To avoid any unsubstantiated claims, we have revised the title as follows:

Formation and Characterization of Ti–Al Intermetallic/Oxide Layers on Ti6Al4V as Interlayers for Hydroxyapatite Coatings.

In the Introduction and Discussion, we have reworded the relevant sections to state clearly that the present work focuses on engineering a suitable surface for HAp adhesion, not on measuring adhesion itself. A brief paragraph has been added at the end of the Introduction, indicating that in-vitro deposition and adhesion tests will be performed in future work using the same EDM-based process.

We also expanded the literature review to include the study suggested by the reviewer (Vacuum 2022). This papers is now discussed to (i) illustrate how oxide-rich Ti surfaces (TiO₂, Al₂O₃) promote apatite nucleation in conventional soaking methods, and (ii) to contrast those results with our electro-discharge method, which creates similar bioactive oxide phases through a single-step surface modification route rather than post-treatment in biological solutions.

Added to the end of the Introduction: The present study aims to develop and characterize Ti–Al intermetallic/oxide surface layers on Ti6Al4V alloy using the EDM process and subsequent thermal/thermochemical treatments. These modified layers are designed as intermediate interlayers to enhance compatibility with hydroxyapatite (HAp) coatings, rather than to demonstrate direct HAp deposition. The investigation focuses on structural, chemical, and mechanical aspects that determine the suitability of such surfaces for future biofunctionalization. Experimental evaluation of HAp adhesion will be the subject of subsequent work.

Added in Discussion Section: Literature reports indicate that oxide-rich Ti surfaces, particularly those containing TiO₂ and Al₂O₃ phases, exhibit enhanced wettability and facilitate apatite nucleation when exposed to simulated body fluids or Hank’s solution [Vacuum 2022]. The intermetallic/oxide layers produced in this work therefore create a favorable chemical and structural foundation for subsequent HAp coating, although direct adhesion testing was beyond the current study’s scope.

Comment 2. The quality of Figures 5 and 8 should be improved.

Response 2. We regenerated both figures at high resolution and improved readability:

• Figure 5 (cross-sections): replaced with 300 dpi micrographs.
• Figure 8 (microhardness): replaced with 300 dpi image.

Comment 3. The estimated layer thickness after heat treatment at 600°C/3h was approximately 8.2 micrometers. This result is surprising compared to the unmodified layer. In such a case, some increase in layer thickness could be expected; why did this not occur?

Response 3. The apparent reduction (from ~12.0 µm to ~8.2 µm) is consistent with: (i) Al→Ti interdiffusion that consumes the Al-rich top layer to form compact Ti–Al intermetallics beneath the free surface; (ii) oxidation/densification at 600 °C (formation of TiO₂/Al₂O₃) that can reduce the measured metallic/intermetallic thickness while creating a thin oxide top film not easily distinguished from mounting resin in BSE contrast; and (iii) limited polishing-induced spall of a microcracked rim. We have added this explanation in Section 3.2 and now clarify that thickness refers to the continuous, adherent intermetallic/modified zone, not including any discontinuous surface fragments. We also added a note that we measured thickness at multiple locations and now report mean ± SD.

Added in Section 3.2, after reporting the thickness results:

The slightly lower thickness observed after heat treatment (~8.2 µm) compared to the as-deposited layer (~12.0 µm) can be attributed to several phenomena: (i) mutual diffusion of Ti and Al at 600 °C, leading to the formation of dense Ti–Al intermetallics and the consumption of the Al-rich outer zone; (ii) oxidation and densification of the surface, which form a compact TiO₂/Al₂O₃ layer and reduce the measurable metallic thickness; and (iii) partial removal of a porous rim during polishing. Therefore, the reported value refers to the continuous, adherent intermetallic zone rather than any loose or discontinuous surface fragments.

Comment 4. How was the Al layer thickness controlled in the EDM process?

Response 4. Thickness was controlled by specific energy input and pass count: constant power (5 W), traverse speed (120 mm/s), a criss-cross toolpath over 20×13 mm, and 20 repeated passes (specific processing time 17.5 s cm⁻²). Electrode rotation (200 rpm) and diameter (3.01 mm) were kept constant. We now state that we calibrated thickness vs. pass count from cross-sections and mass gain; the average thicknesses reported derive from that calibration. These process-control details are consolidated at the end of Section 2.2.

Added in Section 2.2

The thickness of the deposited Al layer was controlled by adjusting the specific energy input and the number of discharge passes. All samples were processed under constant power (5 W), electrode rotation speed (200 rpm), traverse speed (120 mm s⁻¹), and 20 repeated criss-cross passes, corresponding to a specific processing time of approximately 17.5 s cm⁻². A calibration curve of layer thickness versus number of passes, obtained from cross-sectional SEM measurements and mass gain data, was used to verify reproducibility. The resulting layer thickness before heat treatment was 12.0 ± 0.6 µm.

Comment 5. There is no information in what atmosphere the thermal treatment at 600^oC/3h was carried out?

Response 5. We have clarified the atmospheres in Section 2.3:

• Thermal treatment: conducted in a laboratory muffle furnace in air; samples were placed face-down in Al₂O₃ powder to limit direct oxidation and ensure uniform heating.
• Thermochemical treatment: carried out in a sealed retort embedded in a solid reactive pack (urea, NH₄Cl, graphite, Al₂O₃) to generate N-/C-bearing species; the same 600 °C/3 h/controlled cool schedule was used.

Comment 6. Based on XRD measurements, oxide phases TiO₂, Al₂O₃ and intermetallic phases were identified, which appeared after subsequent stages of surface modification. Therefore, what is the actual structure (construction) of the layers from the surface to the substrate (oxides/intermetallic compounds/transition zone – mixture of compounds/substrate)?

 Response 6. Based on XRD, EDS maps, and cross-sections, we now include a schematic stratigraphy and a short paragraph in Section 3.3/Discussion:

• After Al deposition (untreated): very thin oxide/nitride skin (Al₂O₃, TiN traces), Al-rich modified zone with initial Ti–Al intermixing, then diffusion zone (Ti-rich α+β with Al/V gradient), then substrate.
• After thermal treatment (600 °C): TiO₂/Al₂O₃ top film (crystalline; XRD), underlying Ti–Al intermetallics (TiAl₃/TiAl₂ with γ-TiAl beneath), graded diffusion zone, substrate.
• After thermochemical treatment: TiN-dominant surface with persistent TiO₂/Al₂O₃, then γ-TiAl + TiAl₂/TiAl₃, then diffusion zone, substrate.

We explicitly reference the phase assignments in Fig. 7 and the elemental gradients in Fig. 6 that support this construction.

Added in Section 3.3 (Results – XRD/EDS discussion)

Based on the combination of XRD, EDS, and cross-sectional analyses, the layer structure can be schematically described as follows:

• After Al deposition (EDM): a thin oxide/nitride film (Al₂O₃, TiN traces), an Al-rich modified zone with initial Ti–Al intermixing, a Ti–Al diffusion zone, and the Ti6Al4V substrate.
• After heat treatment (600 °C/3 h): a compact surface oxide layer composed of TiO₂ and Al₂O₃, an intermetallic zone containing TiAl₃, TiAl₂, and γ-TiAl phases, followed by a Ti-rich diffusion zone and the substrate.
• After thermochemical treatment: a TiN-containing top layer together with TiO₂/Al₂O₃, underlain by γ-TiAl and TiAl₂/TiAl₃ phases and a gradual transition to the substrate.

This stratified architecture indicates the progressive transformation from pure Al deposits to complex oxide/intermetallic systems with diffusion-bonded interfaces.

Comment 7. An interesting result is shown in Figure 6, in particular the Ti distribution in the layer after thermal treatment. What is the mechanism migration of Ti towards the surface?

Response 7. We have added a mechanistic note: at 600 °C, interdiffusion between Ti (substrate) and Al (deposit) proceeds with unequal intrinsic diffusivities. Ti can diffuse outward along defects and newly formed intermetallic grain boundaries; at the same time, the oxygen chemical potential at the free surface drives Ti to form TiO₂, effectively “pulling” Ti toward the surface (a classical outward Ti flux under oxidizing conditions). The net effect, together with the Kirkendall imbalance, produces the Ti enrichment observed near the surface in the heat-treated state. This explanation is inserted where Fig. 6 is discussed.

Added: The enrichment of Ti near the surface after the 600 °C treatment (Figure 6) results from the unequal diffusion rates of Ti and Al at this temperature. Ti exhibits a higher diffusivity along grain boundaries and defects in the growing intermetallic layer and can migrate outward under the oxygen potential gradient present during oxidation. Concurrently, the formation of TiO₂ at the surface consumes Ti and promotes its flux toward the surface, producing a classical Kirkendall-type effect. This mechanism explains the Ti concentration increase observed in the heat-treated layer.

Comment 8. The authors of the paper suggest that: "These results confirm the formation of hard intermetallic compounds (e.g., γ-TiAl, TiAl₂, TiAl₃), along with oxide (TiO₂, Al₂O₃) and nitride (TiN) phases, which collectively contribute to the enhanced mechanical robustness and wear resistance of the treated surfaces. Such improvements are critical for load-bearing biomedical applications where long-term mechanical durability and resistance to surface degradation are essential.” This finding should be closely correlated with the specific structure of the obtained layers, as components that increase biocompatibility, corrosion resistance, and other factors that shape mechanical properties. Therefore, which structural elements are assigned to the above-mentioned properties?

Response 8. We added a dedicated subsection in the Discussion that explicitly correlates phases/structures to properties:

• HAp nucleation/biocompatibility: TiO₂ (anatase/rutile) and Al₂O₃ top films provide hydroxylated surfaces that promote protein adsorption and apatite nucleation in SBF/Hank’s (supports future HAp coating). (New literature added - paper suggested by the Reviewer; also see existing refs on TiO₂ bioactivity)
• Corrosion resistance (physiological media): compact TiO₂/Al₂O₃ acts as a passive barrier; the γ-TiAl/TiAl₂/TiAl₃ intermetallic underlayer reduces galvanic mismatch to HAp and limits interdiffusion.
• Hardness & wear: the Ti–Al intermetallics and TiN (thermochemical route) explain the ~3× increase in surface hardness (up to ~1057 HV) and expected wear resistance; however, excessive oxide/intermetallic brittleness in the purely heat-treated state correlates with the lower scratch critical load and observed delamination. The thermochemical state balances hardness with adhesion (near-baseline critical load). Figures 8-10 are cross-referenced.

We close by underscoring that, consistent with the Reviewer’s point, direct HAp adhesion tests will be included in future work; our present claim is limited to engineering a suitable interlayer and demonstrating the mechanical/chemical preconditions for durable HAp coatings.

Added: The functional properties of the modified surfaces can be directly related to their microstructural constituents. The outer TiO₂ and Al₂O₃ films are responsible for high chemical stability and serve as bioactive sites capable of hydroxylation, which facilitates protein adsorption and subsequent apatite nucleation in physiological environments [Vacuum 2022]. The underlying Ti–Al intermetallic phases (γ-TiAl, TiAl₂, TiAl₃) contribute to the significant increase in hardness (up to ~1057 HV) and wear resistance, while maintaining metallic bonding continuity with the Ti6Al4V substrate. In the thermochemically treated variant, the presence of TiN further enhances hardness and abrasion resistance. Together, these phases provide a graded mechanical and chemical transition from oxide/ceramic top layers to the ductile Ti substrate, which is essential for durable performance and long-term biocompatibility in load-bearing biomedical applications.

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript investigates the formation of Al coatings and subsequent heat treatment on Ti-6Al-4V. The finding that these processes enhance the surface hardness is meaningful and potentially valuable for practical applications.
However, the discussion section is insufficiently to the standards of a scientific paper. I recommend the authors to further elaborate their discussion and interpretation of results.

(1) Please include carbon (C) and nitrogen (N) in the EDS analysis results shown in Figure 6. Since the X-ray diffraction peaks of TiN and TiO are located close to each other, distinguishing these phases requires careful consideration. Therefore, it is important to confirm the presence of nitrogen by EDS analysis.

(2)In Figure 7, please explain why TiN was formed in samples (2) and (3). In addition, please discuss the changes in the reaction products depending on the heat treatment conditions, and provide a metallurgical explanation for the differences in the phases formed.

(3) L353 "The thermodynamic stability of this phase under thermal and chemical stress highlights its potential role in anchoring subsequent coatings while contributing to surface passivation."

Please provide the references that support this claim.
 

Author Response

We thank the Reviewer for the careful reading and constructive comments. Below we respond point-by-point and indicate the revisions we have made or will make in the manuscript.

Reviewer comment (1): Please include carbon (C) and nitrogen (N) in the EDS analysis results shown in Figure 6.

Author response: The SEM–EDS system used in this work was equipped with a high-performance SDD detector, which is fully capable of detecting light elements. However, the limitations are intrinsic to the EDS technique itself rather than to the detector. In our case, the samples were embedded in bakelite, and therefore the carbon signal could not be meaningfully interpreted - it originates mainly from the mounting resin rather than from the modified surface layer. For this reason, carbon was intentionally excluded from the analysis to avoid misleading results.

Regarding nitrogen, it is expected to be present only in trace amounts (especially in the heat-treated sample) and may therefore remain below the quantitative detection limit of the spectrometer, particularly when measured in the presence of heavy elements such as Ti and Al. Although nitrogen was not clearly detected by EDS, its existence in the modified surface is strongly supported by complementary evidence:

• The XRD patterns display distinct diffraction peaks at approximately 36° and 42.6° (2θ) that match the (111) and (200) planes of TiN (JCPDS 38-1420), which cannot be explained by TiO or Ti₂O phases.
• The formation of TiN is thermodynamically expected under the applied EDM conditions, since the air dielectric decomposes into reactive nitrogen species within the discharge plasma.
• During the thermochemical treatment, the urea/NH₄Cl mixture releases NH₃ and CN fragments that also act as nitrogen donors, further promoting TiN formation.
• The absence of strong oxygen-only peaks in these regions supports the interpretation that TiN, rather than TiO, is the dominant phase.

Accordingly, the combined evidence from XRD, process chemistry, and phase stability confirms the formation of TiN, even though nitrogen could not be quantitatively resolved in the EDS spectra.

Reviewer Comment 2: In Figure 7, please explain why TiN was formed in samples (2) and (3). In addition, please discuss the changes in the reaction products depending on the heat treatment conditions, and provide a metallurgical explanation for the differences in the phases formed.

Response 2: Sample (2) – heat treatment at 600 °C/3 h (air): The weak TiN reflections observed after heat treatment are attributed to a residual nitrided skim generated during EDM in a nitrogen-bearing environment (air plasma and discharge by-products). At 600 °C in air, nitrogen uptake is limited; thus, heat treatment primarily stabilizes pre-existing Ti-N species and promotes simultaneous oxidation to TiO₂/Al₂O₃ near the free surface, while Al and Ti interdiffusion yields Ti-Al intermetallics beneath. Sample (3) – thermochemical pack at 600 °C/3 h: In contrast, the reactive pack (urea/NH₄Cl/graphite/Al₂O₃) decomposes to release NH₃/HNCO/N-bearing species, enabling active nitriding. Under these conditions, TiN is thermodynamically favored over Ti₂N at 600 °C for extended times, producing the stronger TiN peaks measured, together with TiO₂/Al₂O₃ and γ-TiAl/TiAl₂/TiAl₃ in the sub- surface [1,2].

Metallurgical rationale for phase differences inserted in the text: The observed sequence reflects (i) chemical potentials of O and N at the surface (air vs N-releasing pack), (ii) diffusion kinetics of Ti and Al (promoting intermetallic formation: TiAl₃ → TiAl₂ → γ-TiAl with increasing Ti activity), and (iii) parabolic oxidation of Ti to TiO₂ in air. Thus, sample (2) develops a TiO₂/Al₂O₃-capped intermetallic architecture with only trace TiN, whereas sample (3) exhibits a TiN-containing top layer over Ti-Al intermetallics, forming a duplex ceramic/metallic gradient conducive to hardness and wear resistance.

[1] Bakhtiari-Zamani, H.; Saebnoori, E.; Bakhsheshi-Rad, H.R.; Berto, Corrosion and Wear Behavior of TiO2/TiN Duplex Coatings on Titanium by Plasma Electrolytic Oxidation and GasNitriding. Materials 2022, 15, 8300. https://doi.org/10.3390/ma15238300

[2] Zhang, ; Zeng, W.; Zi, Z.; Chu, P.K. Corrosion resistance of TiN coated biomedical nitinol under deformation. Mater. Sci.  Eng.  C  2009, 29(5),  1599–1603. https://doi.org/10.1016/j.msec.2008.12.022

Comment (3): L353 "The thermodynamic stability of this phase under thermal and chemical stress highlights its potential role in anchoring subsequent coatings while contributing to surface passivation."

Response 3: we replaced the phrase with: The thermodynamic stability and chemical inertness of TiN under physiological and tribological environments, together with its role as a diffusion-barrier interlayer, highlight its potential to anchor subsequent ceramic coatings (e.g., HAp) and to contribute to surface passivation and corrosion/wear resistance.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Taking into account the responses received from the authors of the work and the proposed additions to the publication, I believe that it can be published in present form.

Reviewer 2 Report

Comments and Suggestions for Authors This paper has been appropriately revised.