Stiction and Friction of Nano- and Microtextured Liquid Silicon Rubber Surface Formed by Injection Molding

Round 1

Reviewer 1 Report

Major comments:

1. For the introduction, more background on the PTFE are necessary.Can the author provide more background on the PTFE, i.e., the mechanical or other physical properties,  as a support for the broad use of PTFE in reducing friction and wear?

2. Most of the citations are pretty old, can the authors polish more on the instruction part and add some introduction of the current related research?

3. citation [11], there is a weird character in the end.

4. Citation format should be consistent, some of the journal names are italic while some are not.

5. considering the friction experiment result, can the author plot the error bar?

6.  Can the author include the whole name of PTFE, PEEK

7. Figure 11, it should be anisotropy instead of anistrop

8. Why were the 90-degree and 180 degrees chosen to test the anistropy property, if the choice of 45 is better than 180?

Author Response

1. For the introduction, more background on the PTFE are necessary. Can the author provide more background on the PTFE, i.e., the mechanical or other physical properties,  as a support for the broad use of PTFE in reducing friction and wear?

Thank you for hinting on this lack of information. We have added 2 state of the art paper that show the impressive abilities of ptfe-transfer layers.

2. Most of the citations are pretty old, can the authors polish more on the instruction part and add some introduction of the current related research?

We were reflecting a lot on this hint, and we have added 5 newer paper to all mentioned topics of all reviewers. But we still like to stick to the strategy e.g. to hold on old as well als new references e.g. to Schallamach (1971), go over to Fukahori (2010) and end up with a recent work of Zhibo (2021). In fact the variety of topics that could be discussed for a hyperelastic, viscoelastic, periodic or bioinspired structure will be endless and most of them are still strongly debated fields as for the formation of schallamach waves. When we do still miss some important topics, then it was not intentional, but a pragmatic consequence.

3. citation [11], there is a weird character in the end.

             Thank you. There was a nicely hidden error.

4. Citation format should be consistent, some of the journal names are italic while some are not.

Thank you, we have used an automatic citation format in our software. Maybe you refer to some lengthy italic formated names, but they are actually journal names.

5. considering the friction experiment result, can the author plot the error bar?

We have added the following description for this non-standard measurement procedure: “This method results in a measured mean value of three contacts, if the choice of the cut and the preparation has been made accordingly. For this purpose, the contacts of different molded specimens were compiled. The method itself is hardly susceptible to scattering due to the mild surface roughness of the sphere and the stabilized spherical contact. In addition, the stability of the friction values is ensured in advance by a series of 5 oscillation tests and compared with the subsequent sliding test with respect to an adhesion threshold (measured in both experiment types. A precise presentation would distract from the core of the paper and hope that the reader will find our referenced paper informative.”

6. Can the author include the whole name of PTFE, PEEK

             Thank you.

7. Figure 11, it should be anisotropy instead of anistrop

Thank you. We have used “anisotropic” and “anisotropy” instead.

8. Why were the 90-degree and 180 degrees chosen to test the anistropy property, if the choice of 45 is better than 180?

According to Zhibo, the contact between the sphere and the plate can generate Schallamach waves with different angles depending on the position with respect to the ball. That is why we did not choose an intermittent angle between 0° and 90°. The 180° measurements really aim at the anisotropy of a reversal of direction.

Reviewer 2 Report

The authors investigated the influence of different surface structures on the tribological behaviors of liquid silicone rubber. The functions of those structures with different dimensions were revealed, which is valuable for the research community. However, there is still some problems should be modified.

1.     Figure 3(b) and Figure 5(a) use the same image. One of the images should be replaced by a new image.

2.     Figure 3(c) was wrongly labelled as Figure 3(b).

3.     All the legend of y axes of those figures about friction properties in this manuscript is “friction (-)”, which should be replaces as “coefficient of friction”.

4.     Figure 7 shows the result of contact pressure distribution with non-structured substrate. The structured surfaces have influence on the stress distribution during friction, which have significant influence on the tribological behaviors. The distribution of contact pressure with different structured surfaces should be also presented as comparison, and the influence of contact pressure distribution should be also discussed combining related literatures. (Wear 271 (2011) 952–959; Carbon 167 (2020) 122–131)

Author Response

The authors investigated the influence of different surface structures on the tribological behaviors of liquid silicone rubber. The functions of those structures with different dimensions were revealed, which is valuable for the research community. However, there is still some problems should be modified.

 

1. Figure 3(b) and Figure 5(a) use the same image. One of the images should be replaced by a new image.

Now we have chosen a LSR surface for figure 5a. Thank you.

2. Figure 3(c) was wrongly labelled as Figure 3(b).

             Thank you.

3. All the legend of y axes of those figures about friction properties in this manuscript is “friction (-)”, which should be replaces as “coefficient of friction”.

Thank you. We have changed all figures.

4. Figure 7 shows the result of contact pressure distribution with non-structured substrate. The structured surfaces have influence on the stress distribution during friction, which have significant influence on the tribological behaviors. The distribution of contact pressure with different structured surfaces should be also presented as comparison, and the influence of contact pressure distribution should be also discussed combining related literatures.

We have compared the results with hyperelastic behavior of our simulated structure and found it to be different to that of plastic and elastic textures in literature. Consequently we have added fig 7c and the following phrase: “The hyperelastic deformation also leads to no relevant effect in surface buckling (height) but a dominant shear deformation in the surface. Elastomeric surfaces tend to form a full real contact, in contrast to thermoplastic, metallic or ceramic materials These calculated findings are different from results of plastic or elastic textures in literature [6,7].”

6: (Wear 271 (2011) 952–959, Finite element analysis of textured surfaces under reciprocating sliding,  Manel Rodríguez Ripoll, BojanPodgornik, Jože Vižintin

7: Carbon 167 (2020) 122–131), Enhancement of friction performance of fluorinated graphene and molybdenum disulfide coating by microdimple arrays

Reviewer 3 Report

The manuscript concerns research about stiction and friction of nano- and microtextured liquid silicon rubber surface formed by injection molding. Basic research is presented and the results may be helpful researchers in the future. The reviewer has some comments to the manuscript .

General thoughts:
1. The markers in Figs. 1, 2, 3 and 5 are poorly visible and illegible. Can you enlarge them?
2. Word "friction" should be capital "Friction" (line 430)

Detailed remarks:
1. What kind of steel was chosen for the substrate material? Has it been investigated how the grade of substrate material influences the adhesion of the silicone coating? (line 163-164)
2. According to what standards tribological test was carrying out? (line 210)
3. What does the given unit mm3 refer to? (line 224)
4. Can you add sample photos of droplets on the specimens? (line 244)
5. Can You show the specimens' surface after the tests? Has the surface in contact with the ball worn out in any way?

Author Response

The manuscript concerns research about stiction and friction of nano- and microtextured liquid silicon rubber surface formed by injection molding. Basic research is presented and the results may be helpful researchers in the future. The reviewer has some comments to the manuscript .

 

General thoughts:

1. The markers in Figs. 1, 2, 3 and 5 are poorly visible and illegible. Can you enlarge them?

We have doubled the size of the marker text.

2. Word "friction" should be capital "Friction" (line 430)

Thank you.

Detailed remarks:

1. What kind of steel was chosen for the substrate material? Has it been investigated how the grade of substrate material influences the adhesion of the silicone coating? (line 163-164)

The steel was a 100Cr6 (see Tribological Experiments). These balls have a very good finish, so to our experience the different surface energies e.g. for austenitic steel plays a role. Exactly a rough grade will change the friction behavior, that is why we keep that for specific wear measurements.

2. According to what standards tribological test was carrying out? (line 210)

The test is not standardized yet. Therefore the details were given.

3. What does the given unit mm3 refer to? (line 224)

             We have changed the information to:  (4 mm × 2 mm × 12 mm)

4. Can you add sample photos of droplets on the specimens? (line 244)

Sorry, we simply measure the contact angle as a standard method and do not make photos. For your interest, typical contact angles on LSR are 110° for water, 100° for ethylenglycol and 90° for dioodomethane. That gives already the information that LSR has a low surface energy and a very low polar part.

5. Can You show the specimens' surface after the tests? Has the surface in contact with the ball worn out in any way?

No, the surface of a nanostructured LSR surface is mirrorlike before and after the test. We did some wear experiments with rough 100CR6 counterparts in a ring-on-ring test, that gave a wear coefficient close to a NBR material but wear experiments on elastomers are tricky an beyond the scope of this paper.

Round 2

Reviewer 1 Report

My comments are all addressed, and I have no other questions. I recommend publication on "Applied Mechanics"

Reviewer 2 Report

This manuscript can be accepted in this form.