Abrasion Behaviour of Natural and Composite Polytetrafluoroethylene Seal Materials Against Stainless Steel in Lunar Regolith Conditions

Round 1

Reviewer 1 Report (Previous Reviewer 2)

Comments and Suggestions for Authors

In this paper, the tribological performance of various PTFE materials against Steel under pin-on-disc conditions were tested and the phenomena was analysed. There is some overlap with their previous publication.

PTFE-based materials and composite used in tribology are well documented, however, the authors should state why they chose the packed materials which perform rather poorly which has also been proven in this work.

> In section 2.2, what is the meaning of a rotating disc (n=0.61/s)? Please give the rotation speed and diameter of the tracks.

> For the analysis of debris and grains on the steel disc, like fig 3, the authors should indicate which is lubricate or grain in the picture.  Similar work should be done in other figures and photos. In this way, readers should have a clear understanding.

In section 7.2, figure 7 appears in front of the other figures and Figure 5 was not used. Additionally, the figures and tables were not properly sequenced and poorly referred to in the text, such as there are two figure 7, no figure 9 and table 6.

 

> In Table 3, the data in the first line and the third line are exactly the same!

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

Line 71 - it is fixed

Lines 186, 187 - it is fixed

Lines 207, 208 - it is fixed.

Limitations and prospects of the study are described in lines 393-398. 

Author Response

Thank you for your comments and critical remarks.

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

This paper aims to present and compare different types of PTFE polymers used for seals, especially when exposed to two types of lunar simulants. Although the study is interesting and such research is important for the specific adjustments of different types of space equipment, there are some issues that need to be clarified before further consideration and publication:

Section 'Abstract':

- When highlighting the results, please clearly indicate the materials used. For example, which are barded and block pins in the summary?

- Surface roughness and electron microscopy are both only analyses of the worn surface, only the way and the result of the analysis are different.

- ''Analysis of friction resistance, wear, surface roughness and electron microscopy were used to evaluate tribological properties'' needs to be rephrased to clarify which parameters were measured and which methods were used to evaluate the results

- What is the main message of the article? Which material is better for the seals, which dust is more problematic. Please explain the main results.

Section "Introduction":

·      The introduction lacks a lot of literature explaining the *adaptation of the material of the space machines; *typical seals materials or innovative materials *some quantitative definitions of space dust and materials. For example, tables, comparisons, specifics…all should be based on relevant scientific literature.

·      There are some inadequate expressions, e.g. 'planetary bodies' Line33, 'smallest matter' Line45; 'hereinafter' Line 153… please check again carefully;

·      Are these 'seals' or 'sealing agents' that you are analysing, please standardise

·      What type of seals did you try to characterise the materials for, as this will determine the tribological test parameters (e.g. normal load, frequency, speed....)

Section 'Materials and methods' section:

1.Materials

· Please include any data necessary to clearly describe the materials used and the test procedure. Some data are presented in the Results section (e.g. pim dimensions) that do not belong in the Results section

· Include a table with the representative mechanical properties of the materials used for the pin and disc

· How were the specimens prepared? Cemented, polished, cleaned before each test? What was the roughness prior to testing?

2.Methods

· Please give name, model, manufacturer, Country and set-up for all machines used (e.g. pin on disc device??)

· What is meant by 'coupon level' Line106

· There should be a scheme for measuring Dp. How did you decide on the location of the measurement? Was it only at one point or?

· Why did you measure for different durations?

· How did you decide on the normal load and speed? Were there specific criteria?

· Were your tests 'pin' or 'block' on dick? Based on figure 1? How did you cut your pin samples?

· What was the initial contact area on the tested material pairs? Point, line, circular?

· How did you attempt to standardise the amount of dust uniformly covered with abrasive simulants in a procedure Line107-108

Section 'Results':

The entire results section should be carefully reorganised, e.g. by adding additional sections (e.g. by dust type) to allow better presentation and comparison of results

 · The whole result section should be carefully reorganized, maybe in additional section ( for example On the basis of Figure 2, the COF for Pc was measured at 110 m, so that the other results cannot be used for other material pairs.

· Are the results in Figures 1 and 4 average values from all measurements?

· Figure 3 is unclear, add the magnification and scale so that they are readable and add magnification and scale in the same way for all microscopic images: Figure 5,6,7,8,9

· Renumber Figure 8

· Figure 3 and Figure 8 should be the same size, as well as 5 and 9

· Figure 9: Change the size so that all images are the same size

· Please standardise the captions in the images and in the text and check the entire manuscript again. There are sooo many inconsistencies; For example:

LHS1/LMS1…in Line181/Line182 ; but LHS-1/LMS-1…....in Line224/Line225;

Pn pin / pin Pn…Line 281/Line 283

· Coefficient of friction or Coefficient of friction or COF or CoF ?

· Labels horizontal/vertical ( Figure 2/4)

· Figures 6 and 7 can be combined (also in size)

· Why do not you calculate the specific wear? How can you compare the wear at different times? Of course, the degree of penetration  changes if the experiment takes longer. Is there a specific correlation between time and material? Critical time?

· How did you measure the surface roughness? Machine, method, repeatability? In Table 4, are there no deviations in the results? Why did you measure all these parameters? What information did you gain from them?

 Section 'Conclusions':

 If the Discussion section is included in the Conclusions, a more in-depth discussion of all figures and tables in the Results section is required.

At the end mention 'soil laboratory conditions'. This should be made clear in the introduction or at the beginning of the methodology, and all measurement parameters used should be systematically listed and defined by the mentioned DIN standard.

At the very end you mention 'space conditions' – vacuum and temperature: what exactly are the parameters expected in space (for your application)?

How do other researchers for space applications deal with this (reference)?

How do you expect the results to change with a different experimental procedure?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

Dear authors, thank you for carefully answering most of the questions which were often given as comments and suggestion. Although you have answered them, not a lot of your conclusions have been added to the manuscript.

By my opinion, methodology is now explained more clearly.

The results, are still not systematized which makes it difficult to understand and compare them.

For example:

3.1. On line friction : 3.1.1. Influence of simulant powder – if there is no influence, than comment it

And proceed in same order 3.1.1. Ln/3.1.2. Lc/3.1.3.Pn/3.1.3.Pc

3.2. Wear: 3.2.1. Influence of simulant powder – if there is no influence, than comment it  and proceed in same order

Wear of : Ln/3.2.2. Lc/3.2.3.Pn/3.2.3.Pc

3.3. Surface analyses: 3.3.1. Ln/3.3.2. Lc/3.3.3.Pn/3.3.3.Pc

* Table 3: what about Pn/Pc?

Table 4: what about LMS-2?

*Table 5: be consistent with results - order

But the biggest concern is about the Figures:

Figure 3 – unclear, unknown magnification and scale from picture.

Figure 5 – not uniform scale;

Figure 6- without scale on picture;

Figure 7 = Figure 3, unclear, no magnification, no scale on picture;

Figure 8 – different magnification, conditions….

These 'technical' changes make the paper more systematized and better for reading and understanding, only than the study uniqueness is highlighted. 

Kind regards

Author Response

"Please see the attachment"

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper aimed to study the abrasive wear properties and sealing performance of different sealant pin materials in lunar regolith conditions. However, it is concerned that the research problems were not properly addressed by the testing methods. In general, the wear behaviour of the sealant materials is directly related to their sealing performance, i.e. good sealing performance would effectively prevent the infiltration of the large abrasive dusts/particles. Hence, to assess or compare the wear properties of sealant materials, it is important to use the real sealing structure. For instance, in Reference 4 (Delgado et al), the researchers investigated using rotary seal rig test set-up.  In Reference 5 (Matsumoto et al), the tests were conducted by using the collar seal-shaft test apparatus. In this paper, the tests were carried out under three-body or two-body abrasive wear tests, which shouldn’t be used to assess the wear problems of sealant materials in real applications. Also, the environment conditions such as vacuum were also not considered in the research design.

In addition, there were some format/edit issues. For example, lines 167-180 should be removed.  

Reviewer 2 Report

Comments and Suggestions for Authors

This paper has used too much data from previously published work in Materials (10.3390/ma17174240).

For example, Figure 1, some curves in Figure 2/Figure 4, the data in Table 4 and some SEM photos. 

To publish, the authors need some novel results and ideas.

Comments on the Quality of English Language

English is generally ok.

Reviewer 3 Report

Comments and Suggestions for Authors

1. State the purpose of the article in the introduction

2. Perhaps the text in lines 167 – 181 is not relevant to the text of the article.

3. Why was the data in the study 60 in line 191

4. State the value “a lower sliding resistance” at 199

5. State the numerical value “but with a more uniform value” in line 220

6. Describe the limitations of the study and its future prospects in the Conclusions section

7. Delete “The results are interpreted and concluded” in line 403

8. Delete lines 407 and 408