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:
As Gonçalves et al. pointed out [55], the bleed oil viscosity is
not affected by the type and concentration of the thickener and,
according to Palacios and Palacios [56], grease viscosity tends to
reach the base oil viscosity at extremely high shear rates.
Obviously, this rule cannot be directly followed when using
polymeric additives able to modify the base oil viscosity [57].
Moreover, the role of the thickener in the tribo-contact is not
considered in Eq. (3). Accepting the inclusion of the base oil
viscosity in the Stribeck parameter, among all the lubricating
grease samples studied only CA and MC display a simple tribological
behaviour which acceptably leads to a unique Stribeck master
curve by superimposing the frictional curves obtained at different
temperatures and normal loads (Fig. 4a and b). This means that
base oil viscosity and normal load govern film formation in these
two greases, as expected for a Newtonian lubricant. In the
remaining formulations, a single Stribeck curve cannot be
obtained. In particular, when using CP sample as lubricant, the
shape of the friction coefficient vs. sliding speed curve did not
significantly change by modifying normal force or temperature,
displaying maximum and minimum values of the friction coeffi-
cient at approximately the same values of the sliding velocity. An
intermediate behaviour was observed in the case of CH sample.
Stribeck curves calculated at different normal load and temperature
conditions overlap each other only in the elastohydrodynamic
region, whereas a scatter in the friction coefficient values, generally
decreasing with the applied normal force, was found in the
mixed lubrication regime (Fig. 4c). Therefore, in these cases, the
role of the thickener in the film formation seems to be more evident
and greatly influences the frictional behaviour.