Mechanical and Tribological Properties of Polytetrafluoroethylene Composites Modified by Carbon Fibers and Zeolite

Round 1

Reviewer 1 Report

The paper entitled “Mechanical and Tribological Properties of Polytetrafluoroethylene Composites Modified by Carbon Fibers and Zeolite” is concerned the effect of combined fillers (carbon fibers and zeolite) on the mechanical, tribological properties, and structure of polytetrafluoroethylene.

The manuscript is well written, results are nicely presented. However, the discussion was not convincing enough. Besides, the following problems are still existing:

1. It is suggested to supplement the microstructure of the prepared material.
2. According to the author, the density of PCMs with a content of 1-5 wt.% CF remains at the level of the initial PTFE, while when the CF content reaches 8%, the density of the composites decreases. Why? In my opinion, the density value changes only in the last digit, within the error.
3. The oblique section (or longitudinal section) of the sample after wear is recommended to be supplemented to illustrate the role of the fiber in the wear process.

Author Response

Dear Reviewer,

We deeply appreciate the time you spent on reviewing our paper and valuable recommendations you made. All the comments are considered and corresponding changes are made to the manuscript body text. Detailed point-by-point answers are attached.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript addresses the effects of carbon fibers and zeolite fillers within a PTFE matrix on the material structure as well as mechanical and tribological properties. It was reported that the tensile strength and elongation at break was retained at a content of 1–5 wt.% of carbon fibers and 1 wt.% zeolite while the compressive stress increased by 53%, and the yield stress increased by 45% compared to oure PTFE. Moreover, the wear resistance was increased by a factor of 810 while maintaining a low COF, especially for higher fiber content (8-10 wt.%).

Basically, the topic and results are substantial enough to be of interest for the research community. Furthermore, the paper and its methods are well built. Before it can be accepted, the following points should be addressed:

1. The language of the paper should be checked. Some expressions are misleading. For example, the expression "low wear resistance in friction" does not really make sense.
2. References to figures and tables should be formulated in a passive way (since a figure/table is not able to actively do anythin; instead, something is shown in a figure by the authors).
3. In order to be able to estimate the stress in the tribo-test, the nominal pressure should be specified
4. I assume that the tribometer was operated in rotational sliding mode. In this case, the wear radius should also be given in order to be able to estimate how many cycles or to what sliding distance the testing time corresponded to.
5. Were there any noticeable aspects in the running-in behavior or in the friction curve over time? Possibly the progressions of the different samples (from which the mean values were calculated) could be provided in a Fig. or an Appendix.
6. The wear rate as a tribological parameter is - like friction - very volatile and replicate measurements are essential. Have replicate tests been carried out? If so, the scatter bars should also be specified for wear (Fig. 5a, Fig. 9).
7. The authors only considered the wear of the PCM discs. However, a tribological system always includes counterbody wear as well. It may be that the main body features less wear, but the mating body more. The authors should supplement corresponding investigations and/or discuss on this aspect.

Author Response

Dear Reviewer,

We deeply appreciate the time you spent on reviewing our paper and valuable recommendations you made. All the comments are considered and corresponding changes are made to the manuscript body text. Detailed point-by-point answers are attached.

Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript the preparation of polymer composites consisting of PTFE reinforced with glass fibers and zeolite is presented. The prepared materials are mechanically and tribological tested. The manuscript it generally well written. There are several sections in the manuscript where I believe more information must be provided. Therefore, I suggest major revision

• Page 1 line 32: PAO₆ should be PAO 6
• Page 2 line 73: What is 80 G
• The authors should give more information on how the density was measured as the readers do not have the GOST standard readily available.
• In Figure 3 besides the strong peak at 2θ of ~18°, other peaks can be seen at 2θ values of 31°, 37°, 41°. The authors should improve the quality of the Figure, maybe make a couple of subfigures so all the peaks can be seen. Furthermore it would be good to present how the crystallinity was calculated, how were the peaks fitted, what lineshapes were used etc..
• The authors should discuss the potential effect of the addition of the zeolite towards the crystallinity of the composites.
• Regarding the tribological tests:

1. The authors should state how many repetitions were performed per sample.
2. What is the typical sliding radius during testing, I guess a unidirectional pin-on-disk device was used ?
3. What is the standard deviation of the values in Figure 5 a) ?

• The authors wrote: “As can be seen from Figure 5 (a), the addition of combined fillers to PTFE increased 183 the wear resistance by order of magnitude compared to the original polymer . The results of comparing the PTFE/CF/Zt composites with the original PTFE showed a decrease in the wear rate by a factor of ~190 –810”. How did the authors calculate the decrease in the wear rate by a factor of 190-810? As far as I can see the best performing composite has an ~5 times lower wear rate compared to the original PTFE (~0.5 x 10^-6 vs ~2.5 10^-6 mm³/Nm).
• The worn surface of the polymer composites had glass fibers on it. Did these glass fibers abrade the steel counterbody, i.e. did the authors analyze the counterbody for any sign of wear or morphological changes of its surface after sliding against the polymer composites.

Author Response

Dear Reviewer,

We deeply appreciate the time you spent on reviewing our paper and valuable recommendations you made. All the comments are considered and corresponding changes are made to the manuscript body text. Detailed point-by-point answers are attached.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have addressed most of my comments. Despite stating in the response letter that remark #7 has been corrected in the text, I don't see it in the manuscript. My concern was that it is unclear for which body the wear rate was determined. Was it for the PCM pin and/or for the steel disk? It may be that the pin featured less wear, but the disk more. Therefore, the wear should be considered for the base and counter body and the wear rate should be given for both (wear rate of the pin and wear rate of the disk). This should then also be reflected in the discussion.

Author Response

Thank you for your comment! The wear rate was determined for the pin (for polymer composite materials).  There is almost practically no wear on the steel disc because the disc is made of sufficiently strong 45-grade steel. The tribological properties of PTFE are attributed to the relatively easy sliding between the lamellae or fibrils, resulting in a low coefficient of friction but also a high wear rate [Ye, J.; Burris, D.; Xie, T. A Review of Transfer Films and Their Role in Ultra-Low-Wear Sliding of Polymers. Lubricants 2016, 4, 4, doi:10.3390/lubricants4010004.]. Fillers introduced into PTFE reduce the subsurface crack propagated, and part of the normal load is transferred to the solid filler [Blanchet, T.A.; Kennedy, F.E. Sliding Wear Mechanism of Polytetrafluoroethylene (PTFE) and PTFE Composites. Wear 1992, 153, 229–243, doi:10.1016/0043-1648(92)90271-9.]. In addition, wear of the PTFE-based polymer composite formation a transfer film on the surface of the steel counterbody, which also contributes significantly to wear reduction. Thus, the film formed on the surface of the composite and the counterbody protect them from wear. The friction surface of the composites was investigated in this work.

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Thank you for addressing my concern. I'm satisfied and recommend that the paper can now be accepted in its present form.