Content of review 1, reviewed on July 26, 2019

In the actual world context of excessive pollution, there is an increasing demand for environmentally-friendly lubricants.

In this regard, the authors utilized a synthetic ester base oil, mainly composed by trimethylolpropane trioleate (TMPTO). This lubricant is over 90% biodegradable. In order to improve its tribological, thermophysical and reological properties, the TMPTO was additivated with hexagonal boron nitride (h-BN) nanoparticles at 0.50, 0.75 and 1.0 wt%., also a environmental friendly product.

Tests on viscometer, densimeter, and ball-on-disk tribometer were carried out. Friction coefficient was monitored during tribological tests. The wear of disk samples and mating balls was evaluated with a 3D profilometer, a Scanning Electron Microscope (SEM) and confocal Raman microscopy.

I appreciate the in-depth investigation carried out by the authors and I consider this paper as interesting. In order to augment the clearness of presentation, I have several suggestions.

  1. An image of the tribometer with some details would be welcome.

  2. Indicate the computed value of lubrication parameter, λ, and also the value of the arithmetic roughness of balls, omitted at line 165.

  3. Had the authors any reason or any recommendation to choose a non-hardened AISI 420 steel (194 HV) for the disk samples? In my opinion, this is the cause of obtained high plastic deformations and wavy wear tracks on disk samples.

  4. An important decrease of friction coefficient for 0.75 % h-BN was obtained, but the transversal area of the wear track width rested almost unchanged in comparison with 0.5% h-BN. Also the increase of friction coefficient over the neat oil for 1% h-BN, but with diminished wear value was not completely elucidated. More comments must be added for Figure 7.

  5. Microscopic pictures of Figure 8 are realized with different techniques. In the upper left figure, it can be seen an emphasized colored wear track, but in the lower left figure the wear track is in blurred gray color, difficult to observe. The same remark for the right side images of Figure 8, as they were lightened in different ways. If possible, take similar pictures of wear tracks.

  6. The link of supplementary materials is no more active (www.mdpi.com/xxx/s1). It returns the message "Error 404 - File not found".

Source

    © 2019 the Reviewer (CC BY 4.0).

References

    M., L. d. R. J., G., G. M. J., P., C. M. J., R., L. E., I, P. J., Luis, L., Josefa, F. 2019. Tribological and Thermophysical Properties of Environmentally-Friendly Lubricants Based on Trimethylolpropane Trioleate with Hexagonal Boron Nitride Nanoparticles as an Additive. Coatings.