Investigation of Programmable Friction with Ionic Liquid Mixtures at the Nano- and Macroscales

Round 1

Reviewer 1 Report

Gatti et al present a study into the friction and wear behavior of glass/steel (nanoscale) and steel/steel (macroscale) interfaces lubricated with ionic liquid mixtures and placed under electric bias. Application of negative potential results in lowering of the friction and the wear. This is attributed to the increased presence at the contact of cations with long alkyl chains that can provide lubrication. The effect of electrical bias on friction and wear is less pronounced for the macroscale experiments than for the nanoscale experiments. The authors suggest that the presence of larger and sharper asperity peaks in the macroscale contacts can break down the lubrication mechanism. 

The manuscript is very complete in terms of its introduction and the level of detail with which the methods such as AFM experiments are described. The interpretation of the data is reasonable, I recommend publication after the following points have been addressed through revisions in the manuscript:

-Relatively large voltages are applied in the experiments. The authors mentioned that this also resulted in measurable currents. What currents have been measured? How much energy dissipation does this correspond to? Can this energy heat the ILM (locally)?

-Electrolysis of water can easily take place at 4V: was this observed by increased current upon the addition of water? Are other electrochemical reactions expected?

-Line 375: Hertz theory ignores roughness which is subsequently used by the authors as an explanation for less friction reduction in the macroscopic experiments. It would be better to see the Hertz calculation as a lower limit for the stress at the interface. 

-Relating to the significant contact pressures; is extrapolation of the experimentally measured pressure viscosity relation needed? To what extend are the 200 bar (2e7 Pa) representative for what happens in the contact (more than 2e8 Pa)?

-In the ball-on-three pins experiment is the normal force equally shared by the three contacts? The variation in wear between the pins should illustrate this. 

-How would the authors characterize the low wear behavior compared to applications that run in the boundary regime; is this an improvement?

-Line 335: in the plate-plate geometry the shear rate varies across the measurement cell; maybe that should be mentioned.

-The scale dependence of roughness is discussed in 3.3. As explained by the authors larger RMS is observed at larger scales. It would be good to refer to recent work on using the power spectral density to describe roughness; for example Jacobs, T.D.B., Pastewka, L. & Guest Editors. Surface topography as a material parameter. MRS Bulletin 47, 1205–1210 (2022). https://doi.org/10.1557/s43577-022-00465-5

Typo's:

112 However,... sententence does not make sense.

389: traces, not trances

for the scale dependence of roughness, consider referring to psd descriptions Pastewka/Jacobs (3.3 wear analysis)

Author Response

Please see attachement.

Author Response File: Author Response.docx

Reviewer 2 Report

I enjoy reading this paper. However, the authors should address the following comments and revise the paper accordingly before it is accepted.

1. Could authors explain why ionic liquid is environmentally friendly? I suppose applying voltage is also a kind of energy consumption.

2. The paragraph from line 101 has too much information which is not related to why the authors did this work, for example, the hypothesis of the mechanisms.

3. The authors should show more details about what was done in this work and a summarized sentence is also needed in the end of the introduction.

4. The authors should show raw data and explain how to calculate the adhesion force at line 200.

5. I suggest the authors moving the theoretical equations in Materials and Methods to Supplementary Information.

6. At line 361, is there any reason for stopping the viscosity measurements at 50 C? If not, the authors can try higher temperature to see is there is any stable value.

7. As the colloidal bead was bonded to the cantilever using epoxy, the authors should take some SEM images on the bead to make sure there is no epoxy on the bead surface.

8. What is the reason for choosing the colloidal tip not the regular sharp AFM tip?

9. Why are the friction loops shown in Figure 2 don’t follow the stick-slip model? If it is due to the topography, the authors should show the height results. In addition, how did authors get the force from trace and retrace curves to get the friction force as they changed with the distance.

Major revision.

Author Response

Please see attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The author addressed all my comments and I suggest accepting the manuscript.