The frictional behaviour of greases analysed as lubricants in the
steel–steel ball-on-plates contact was first characterized by
applying a rotational speed sweep. Fig. 3 shows the friction
curve was not observed at 125 °C when applying the commercial
lubricating greases used as benchmarks (Fig. 3b and d). On the
contrary, the different lubrication regimes are still apparent at
125 °C for the model biopolymer-based greases studied.
The classical Stribeck curve relates the friction coefficient (μ)
with the dimensionless Hersey number, which is used to describe
the combined influence of dynamic viscosity (η), rotational speed
(Ω) and the normal contact load applied (FN) in journal bearings.
Instead, in similar geometries and conditions to that used in this
work, the so-called hydrodynamic Stribeck parameter is defined as