Influence of the Nanoclay Concentration and Oil Viscosity on the Rheological and Tribological Properties of Nanoclay-Based Ecolubricants

Round 1

Reviewer 1 Report

This work brings results on the rheology and tribology behavior of mixtures of two vegetable oils using nanoclays of two types as additives, at varied concentrations.

The text is well written, but some questions have come out during reading, and thus, should be addressed before going to next steps.

1. In the sliding friction experiments, what are the Hertz contact pressure corresponding to the 10 N and 40 N, considering each one of the three ball-on-plate contacts? The Hertz contact pressure is an indicative of the lubrication regime acting in the contacts during sliding and thus, should be informed.
2. For how much time was the sliding kept for the friction coefficient data capture, in each rotation? How was the wear state at that moment? The condition adopted for taking the friction data should be clarified.
3. Figure 2: Y axis is n (flow index), it needs correction.
4. Line 257-258: The statement “In addition, this result might provide an explanation to the better storage stability exhibited by the Pangel B20-based ecolubricants”, is confuse. Clarify “this result” and the relation of “better storage stability” to “this result”.
5. As for the Results, reference data for rheology, friction coefficient and wear are missing. In order to prove the occurrence of any improvement or change in the rheology or tribology behavior of the lubricant by the inclusion of the nanoparticles in the base oil, corresponding data of the mixtures without additives must be presented as references.
6. Figure 5: It is very hard to get the comparison about the contribution of the two additives in modifying friction, in the way the friction data are seen in these four graphs. Plot the friction coefficient data as a function of the parameter that is really related with the lubrication regime, and vastly used in the literature (viscosity*velocity/contact pressure). In this way, the reader is able to look at the friction data in a correct Stribeck-like curve, and so get the real modifications caused by the nanoclay additives at the varied concentrations. In doing this the explanations/assumptions given in lines 280 to 325 will become more objective in order to depicting the really relevant findings.
7. Line 330: how do the authors know that the lubrication regime was of mixed type with 40 N and 16 mm/s? The basis taken to state the tests occurred in mixed lubrication regime should be informed.
8. Figures 6 and 7, what is the meaning of the error bars shown along the friction coefficient curves? Dispersion of the real friction coefficient data? Is the straight line an average value, if so, average of how much data? These points need to be clarified.
9. Why two friction curves in the middle condition in Figures 6 and 7, needs clarification.

Author Response

The authors would like to thank all the useful comments made by the reviewer. All of them were carefully taken into consideration in the revised version of the manuscript.

1. In the sliding friction experiments, what are the Hertz contact pressure corresponding to the 10 N and 40 N, considering each one of the three ball-on-plate contacts? The Hertz contact pressure is an indicative of the lubrication regime acting in the contacts during sliding and thus, should be informed.

Answer 1: We totally agree with the reviewer. Hertzian pressure values have been calculated and introduced on both page 4 line 147 and page 8 line 268, according to the reviewer recommendation (see revised version of the manuscript). On page 8, it has been indicated that Hertzian pressures were referred to each plate.

2. For how much time was the sliding kept for the friction coefficient data capture, in each rotation? How was the wear state at that moment? The condition adopted for taking the friction data should be clarified.

Answer 2: In Figures 5 and 6 (Stribeck curves), the friction data were captured every 3 seconds during 300 s, in order to obtain enough information within the entire sliding velocity range studied (velocity ramp from 1 to 50 rpm). Every run was carried out using a new set of steel plates and ball bearing. Moreover, upon finalizing the test, no evidence of wear was observed at the contact points on the plates’ surfaces.

Regarding the wear tests were, they were run for 1800 s at the rotational speed of 0.667 rps (40 rpm). The friction coefficients were captured every 9 seconds, and every test was run using also a new plates-ball set (with the same roughness and without previous damage). We selected these conditions to generate enough wear at the mixed friction regime.

Both data sampling conditions have been specified in the revised version, in section 2.4.

3. Figure 2: Y axis is n (flow index), it needs correction.

Answer 3: Thanks a lot for this comment. Y axis of Figure 2 was corrected in the revised version of the manuscript.

4. Line 257-258: The statement “In addition, this result might provide an explanation to the better storage stability exhibited by the Pangel B20-based ecolubricants”, is confuse. Clarify “this result” and the relation of “better storage stability” to “this result”.

Answer 4: Thanks a lot for this appreciation. This sentence has been deleted in the revised version, since it does not provide any relevant information to this study.

5. As for the Results, reference data for rheology, friction coefficient and wear are missing. In order to prove the occurrence of any improvement or change in the rheology or tribology behavior of the lubricant by the inclusion of the nanoparticles in the base oil, corresponding data of the mixtures without additives must be presented as references.

Answer 5: The new version of the manuscript shows in section 2.1 the dynamic viscosity at 25 ºC of both neat vegetable oils used in this study. Also, the friction curves of both neat vegetable oils at 10 and 40 N have been introduced in Figure 5 as reference; in the revised version, the results in Figure 5 have been split into Figures 5 and 6 according to the comment from reviewer 2. Figure 7 shows the wear scar diameters and friction curves for the neat vegetable oils from wear tests.

6. Figure 5: It is very hard to get the comparison about the contribution of the two additives in modifying friction, in the way the friction data are seen in these four graphs. Plot the friction coefficient data as a function of the parameter that is really related with the lubrication regime, and vastly used in the literature (viscosity*velocity/contact pressure). In this way, the reader is able to look at the friction data in a correct Stribeck-like curve, and so get the real modifications caused by the nanoclay additives at the varied concentrations. In doing this the explanations/assumptions given in lines 280 to 325 will become more objective in order to depicting the really relevant findings.

Answer 6: Although the edited curves show more or less the same behaviour, we agree with the reviewer, and the sliding speed in the abscissa axes of Figures 5 and 6 have been replaced with the Stribeck parameter (viscosity*velocity/contact pressure).

7. Line 330: how do the authors know that the lubrication regime was of mixed type with 40 N and 16 mm/s? The basis taken to state the tests occurred in mixed lubrication regime should be informed.

Answer 7: A sliding velocity within the mixed friction regime was selected according to the curves shown in Figure 5. The evolution of the friction coefficient clearly shows a decay (mixed friction) in a sliding velocity interval that includes the chosen velocity of 16 mm/s.

8. Figures 6 and 7, what is the meaning of the error bars shown along the friction coefficient curves? Dispersion of the real friction coefficient data? Is the straight line an average value, if so, average of how much data? These points need to be clarified.

Answer 8: As the reviewer says, the straight lines are the average values among replicates. This has been clarified in section 2.4. The error bars show the dispersion associated to the experimental data. The legends of Figures 8 and 9 have been modified accordingly.

9. Why two friction curves in the middle condition in Figures 6 and 7, needs clarification.

Answer 9: According to the central composite design with two variables, the central point needs replication (see the central composite design scheme shown in Figure 1). However, as suggested by the reviewer, it may confuse the readers in Figures 7 and 8. Therefore, we have selected one of them as representative curve of the middle point, as we have done with the wear picture. 

Reviewer 2 Report

This is a well written and organized paper.   There is a lot of data related to the friction and wear performance of the fluids.  However, the data is all combined in Figures 5, 6 and 7.   Each of these Figures has multiple panels and multiple graphs / sets of data in each panel.  it would be easier to understand the data, if these three figures are separated into multiple figures with less data in each figure.

In addition the experimental design was well organized in a Box-Wilson design.  These means that a rigorous statistical analysis of the friction and wear data could be performed.  It would be good to show a statistical analysis that would explicitly describe friction and wear as a function of nano-clay type, concentration and oil viscosity. 

Author Response

The authors would like to thank all the useful comments made by the reviewer. All of them were carefully taken into consideration in the revised version of the manuscript.

This is a well written and organized paper. There is a lot of data related to the friction and wear performance of the fluids. However, the data is all combined in Figures 5, 6 and 7.   Each of these Figures has multiple panels and multiple graphs / sets of data in each panel.  It would be easier to understand the data, if these three figures are separated into multiple figures with less data in each figure.

Answer 1: Thanks a lot for this appreciation. For the sake of clarity, Figure 5 has been separated into two Figures, one for each nanoclay. Moreover, the results at the two different normal forces studied are displayed in separate plots.

In addition, the experimental design was well organized in a Box-Wilson design.  These means that a rigorous statistical analysis of the friction and wear data could be performed.  It would be good to show a statistical analysis that would explicitly describe friction and wear as a function of nano-clay type, concentration and oil viscosity. 

Answer 2: As the reviewer says, the Box-Wilson design has enabled a rigorous statistical analysis of the friction, wear and rheological data. However, the complexity of the tribosystem studied is such that affected the quality of the regression analysis, yielding not so high regression coefficients. Thus, friction coefficient’s adjustment showed R square around 0.84396 for Pangel B20 and 0.82842 for Cloisite 15 A. Respect to the F parameter, their values were 18 and 37, respectively; wear diameters’ adjustment showed R square around 0.83366 for Pangel B20) and 0.4286 for Cloisite 15 A. Respect to the F parameter, their values were 24 and 1, respectively. We provide comments on this issue on page 12, lines 375-377. That is the reason why we consider more suitable showing the tribological data as in Figures 8 and 9, instead of in the form of surface fitting models. Moreover, Figures 8 and 9 will allow the readers to better understand the position of each sample on the central composite design according to the influence of both variables studied (see supplementary Figure)

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Good work