being us, the sliding velocity. This parameter takes into account
the fluid film formation between the two surfaces in a tribocontact
and discriminates the lubrication regimes as boundary,
mixed lubrication or (elasto)hydrodynamic [49]. The Stribeck
curve is widely used to describe the lubrication regimes of Newtonian
lubricating oils, including bio-based oils [47,50–53]. However,
this approach is not widely reported for lubricating greases
as a consequence of the non-Newtonian character and the
unknown properties of the active lubricant inside the contact [16].
Lu and Khonsari [54] and Gonçalves et al. [55] proposed the
inclusion of the viscosity of the base oil, assuming that under
highly stressed conditions, like those achieved in a tribological
contact, the microstructure of the grease is almost destroyed and
the main responsible for lubrication is the base oil