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  • Erick Yair Vargas-Oliva1,
  • Carolina Hernández-Navarro2 and
  • Violeta Guzman-Ayon3
  • et al.

Reviewer 1: Anonymous Reviewer 2: Alexander Malkin

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Title: Synthesis and characterization of Titanium layer with fiber- like morphology on HDPE by plasma treatment

This work investigates the fabrication of a titanium (Ti) coating with fiber-like morphology on high-density polyethylene (HDPE) substrates using DC magnetron sputtering combined with plasma etching. The manuscript addresses an important challenge in improving the mechanical and tribological performance of HDPE for demanding applications. The topic is relevant to the Coatings journal readership, and the combination of surface etching with Ti deposition is technically sound. The results on hardness, adhesion, scratch resistance, and tribological behavior are systematically presented.

  1. The wear rates are described qualitatively, but numerical wear rate values (e.g., mm³/N·m) for each load would strengthen the results section.
    • Plasma treatment parameters like ( Exact etching duration before Ti deposition, Exact etching duration before Ti deposition, Deposition time for Ti coating) , are insufficiently defined for reproducibility.:
  2. Sample size and statistics are absent. The number of replicates for hardness, scratch, and tribological tests is not specified, nor is any statistical significance testing reported.
  3. The tribological test per ASTM G133 lacks key parameters:
    • Exact stroke length and speed of reciprocating motion.
    • Counterface material hardness verification for ZrO₂ ball.
    • Ambient environmental control (humidity, temperature).
  4. The explanation that the reduced electrical resistance is “attributed to Ti fiber layer” lacks supporting evidence for percolation conduction or continuous metallic path formation. Cross-sectional conductivity mapping (e.g., conductive AFM) would strengthen this claim.
    • Shore D hardness (ASTM D2240) is a bulk-scale indentation and not optimal for thin coatings. The 6-unit increase (61±3 → 67±2) could be due to experimental scatter rather than true coating reinforcement. Instrumented microindentation (e.g., Vickers or nanoindentation) would be more suitable to isolate coating hardness from substrate effects.
  5. Coefficient of friction (CoF) results are only shown in figures; mean ± standard deviation values are not tabulated for clarity.
  6. XRD analysis shows α-Ti and HDPE peaks , but no discussion of potential TiO₂ formation despite the O content (8–10%) in EDS results. This could influence tribological behavior.
  7. SEM images appear low contrast, making fiber morphology less distinguishable.
  8. The study does not include direct side-by-side tribological and scratch test results for untreated HDPE, preventing accurate quantitative improvement claims. Without controls, it is unclear how much of the observed improvement is due to Ti fiber morphology versus simply having a metallic layer.

 

Author Response

Reviewer 01

This work investigates the fabrication of a titanium (Ti) coating with fiber-like morphology on high-density polyethylene (HDPE) substrates using DC magnetron sputtering combined with plasma etching. The manuscript addresses an important challenge in improving the mechanical and tribological performance of HDPE for demanding applications. The topic is relevant to the Coatings journal readership, and the combination of surface etching with Ti deposition is technically sound. The results on hardness, adhesion, scratch resistance, and tribological behavior are systematically presented.

 

  1. The wear rates are described qualitatively, but numerical wear rate values (e.g., mm³/N·m) for each load would strengthen the results section.

The text was modified to:

The coefficient of friction (CoF) increases with the increment of applied load, showing CoF values of 0.33±0.04, 0.35±0.05 and 0.38±0.03 for the test at 1, 2 and 3 N, respectively. The increment in the CoF was produced by the increment in the elastic deformation of the surfaces in front of the indenter during the rubbing operation. While, the wear rate (K) presented a lower value produced by the tests at 2 and 3 N tests than at 1 N, with K values of 6.3±0.8, 4.9±0.7 and 5.6±0.7 x10-13 m3N-1m-1 respectively.

 

  1. Plasma treatment parameters like (Exact etching duration before Ti deposition, Exact etching duration before Ti deposition, Deposition time for Ti coating), are insufficiently defined for reproducibility:
    1. Sample size and statistics are absent. The number of replicates for hardness, scratch, and tribological tests is not specified, nor is any statistical significance testing reported.
    2. The tribological test per ASTM G133 lacks key parameters: Exact stroke length and speed of reciprocating motion. Counterface material hardness verification for ZrO₂ ball. Ambient environmental control (humidity, temperature).

The text was modified to:

Before the Ti deposition step, a DC plasma etching of 5 min, the substrate surfaces were performed using the same parameters as in the sputtering process. The samples were structurally, elementally, and morphologically characterized using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and scanning electron microscopy (SEM). For the morphological analysis of untreated HDPE surfaces, a thin gold (Au) coating was applied via DC sputtering for 20 seconds to prevent charging during SEM imaging.

 

The variation in the electrical resistivity of the treated and not treated polymer surfaces was measured using two points technique with two points of three size pyramid covered with gold layer at 2 mm of distance. The hardness was determinate using Shore D on treated and not treated HDPE surfaces according to ASTM D2240, the hardness test has carried out 4 time on different coated and uncoated zones. Scratch resistance was evaluated using a scratch tester with loads ranging from 0 to 10 and 40 N, creating tracks of 5 mm length over 1 minute, using an Al2O3 ball with a diameter of 3 mm (based in ASTM D7027-CTR UMT2), the scratch test was repeated 3 time on different coated zones. The tribological properties were examined using sliding tests configuration with a linear stroke length of 10 mm at 30 RPM of the motion motor, producing 1 go-back reciprocating motion cycle each 2 second, applying loads of 1, 2, and 3 N for 1800 seconds with a spherical pin of ZrO2 measuring 5 mm in diameter according to ASTM G133, the wear tests were repeated 3 time on different coated zones. The friction force was recorded in real-time, and wear tracks were characterized using an optical profilometer and SEM for wear track on HDPE treated surfaces and optical microscopy for the wear track on spherical pin of ZrO2.

 

  1. The explanation that the reduced electrical resistance is “attributed to Ti fiber layer” lacks supporting evidence for percolation conduction or continuous metallic path formation. Cross-sectional conductivity mapping (e.g., conductive AFM) would strengthen this claim.

Due to the time to send the revision and the restriction to some specific devises to determine these properties, the comment about the electrical resistance was removed of the manuscript.

 

  1. Shore D hardness (ASTM D2240) is a bulk-scale indentation and not optimal for thin coatings. The 6-unit increase (61±3 → 67±2) could be due to experimental scatter rather than true coating reinforcement. Instrumented microindentation (e.g., Vickers or nanoindentation) would be more suitable to isolate coating hardness from substrate effects.

Although the nano and microindentation are used to measure the layer hardness on metallic and ceramic substrates, the measure of hardness modification on coated surfaces was done using Shore D methods due to this method is the basic method to measure the polymer hardness.  

 

  1. Coefficient of friction (CoF) results are only shown in figures; mean ± standard deviation values are not tabulated for clarity.

The text was modified to:

The coefficient of friction (CoF) increases with the increment of applied load, showing CoF values of 0.33±0.04, 0.35±0.05 and 0.38±0.03 for the test at 1, 2 and 3 N, respectively. The increment in the CoF was produced by the increment in the elastic deformation of the surfaces in front of the indenter during the rubbing operation. While, the wear rate (K) presented a lower value produced by the tests at 2 and 3 N tests than at 1 N, with K values of 6.3±0.8, 4.9±0.7 and 5.6±0.7 x10-13 m3N-1m-1 respectively.

 

  1. XRD analysis shows α-Ti and HDPE peaks, but no discussion of potential TiO₂ formation despite the O content (8–10%) in EDS results. This could influence tribological behavior.

The formation of titanium oxide phase was considerate during the characteristic analysis, nevertheless the XRD patter did not exhibited the main peaks of anatase TiO2 phase, while the formation of rutile and brookite phase of TiO2 is low possible.

 

  1. SEM images appear low contrast, making fiber morphology less distinguishable.

The images in the manuscript was update to:

  1. The study does not include direct side-by-side tribological and scratch test results for untreated HDPE, preventing accurate quantitative improvement claims. Without controls, it is unclear how much of the observed improvement is due to Ti fiber morphology versus simply having a metallic layer.

Although studying the tribological phenomena of the coated and uncoated surfaces should increase the clarity of the influence of the surfaces' properties, this work shows the initial results to understand the evolution and changes of the polymeric surfaces treated by plasma processes.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript under review is devoted to the study of a thin film formed through plasma treatment by titanium onto the surface of high-density polyethylene.Products of this kind are of particular interest for special applications.In this case, the authors carefully studied the composition of the formed film, as well as the mechanical and tribological characteristics of the material.

The work as a whole is performed quite qualitatively, and the presented manuscript contains sufficiently detailed information based on a thorough methodological approach.

The most interesting thing is that the deposited titanium (according to the authors’ opinion) forms a fibrillar morphology, and titanium with the molecular composition of the coating occupies at least 91%.

 

Some comments.

  1. It would be interesting to indicate what applications could be used for the obtained titanium coated films.
  2. In Fig. 1a nothing is visible.It requires either replacement, or removal, or explanation.
  3. Section 3/3 is called “Mechanical properties”. Meanwhile, it includes electrical properties too. Please, take it into account.
  4. Important point: really I do not see fibrillary morphology in the presented pictures, but only oval particles. Meanwhile, the authors consider this as an important point, stressing it by the title. Please, give more clear and convincing photos
  5. Conclusion should be slightly reconsidered and shortened. Conclusions should not repeat the content of the study but only scientific results and/practical recommendations. In this aspect, the last conclusion about a task of the future study is doubtful, since you had not to present experimental data without search for optimizing the technological regime. I advise to delete this point

Author Response

Reviewer 2

  1. It would be interesting to indicate what applications could be used for the obtained titanium coated films.

This text was included to the manuscript:

The formation of a Ti layer with a fiber-like morphology could increase the use of HDPE in the osseointegration and catalytic application.

  1. In Fig. 1a nothing is visible. It requires either replacement, or removal, or explanation.

 

The Fig. 1a was changed to:

  1. Section 3/3 is called “Mechanical properties”. Meanwhile, it includes electrical properties too. Please, take it into account.

The comment about the electrical resistance was removed of the manuscript.

  1. Important point: really I do not see fibrillary morphology in the presented pictures, but only oval particles. Meanwhile, the authors consider this as an important point, stressing it by the title. Please, give more clear and convincing photos

The SEM images were updated to:

  1. Conclusion should be slightly reconsidered and shortened. Conclusions should not repeat the content of the study but only scientific results and/practical recommendations. In this aspect, the last conclusion about a task of the future study is doubtful, since you had not to present experimental data without search for optimizing the technological regime. I advise to delete this point

The last conclusion was erased.

 

Author Response File: Author Response.pdf