In general, a stationary value of the friction coefficient was
achieved after approximately 5–6 min when using the
biopolymer-based greases as lubricants, whereas commercial
samples needed longer times (8–10 min). The bar diagram shown
in Fig. 6 portrays the steady-state friction coefficient values
obtained by applying normal loads of 10, 20 and 40 N at constant
angular velocity of 10 rpm and temperatures of 25 and 125 °C, for
each grease used as lubricant in the tribo-contact. Samples such as
LI and CH display the same tendency at 25 and 125 °C, i.e. the
friction coefficient decreases with the normal load. This is not the
expected normal force influence in the boundary and mixed
lubrication regimes [54] but it is consistent with the higher
rheological resistance offered by the grease to the rotational
motion at lower normal loads [28]. Moreover, for both samples,
the friction coefficient values are even lower at higher temperature,
in accordance with the lower rheological resistance offered as
a result of the thermally-induced softening of greases. At the same
time, microstructure is better preserved at 125 °C in these two
samples, showing the higher values of the consistency index
(Table 2). As a consequence, it is presumed that at high temperature
the rheological resistance to the motion is reduced but the
entrainment of the whole lubricant into the contact takes place.
On the other hand, greases CA, CP and MC follow the former trend
at 25 °C, but completely the opposite at 125 °C. This effect may be
attributed to the fact that these microstructures are more strongly
affected by shear and temperature, as the lower values of the
consistency index suggest (Table 2), resulting in a significant
reduction of the effective viscosity and, therefore, in the lubricant
film thickness, which favours wear, especially at higher normal
loads, as discussed in the next section. In fact, this temperature is
rather close to the dropping point in sample CA (see Table 1) and
therefore a more significant softening and oil bleeding is expected.
This hypothesis is supported by the fact that a single Stribeck
master curve was obtained for CA and MC samples when using the
base oil viscosity to estimate the Stribeck parameter. Alternatively,
MC and, in lower extent, CP microstructures may release much
more oil at high temperature and normal loads, as observed after
performing the frictional tests, thus locally producing, in some
parts of the tribo-contact, an increase in the effective concentration
of the cellulosic thickening agent, which can interact with the
metallic contact surfaces, causing higher friction and wear