Dual characterization of boundary friction thanks to the harmonic tribometer: Identification of viscous and solid friction contributions

All the usual characterization of an interface submitted to shear is generally achieved thanks to an experiment using a pair of two loaded solids in continuous sliding. So, the friction coefficient value can be determined. But, only this friction value is quite poor information and is not able to reveal the complexity of the interfacial processes occurring in the sliding contact. A novel method has been developed at LTDS, allowing us to identify both the velocity-dependent and the solid friction contributions. It is based on a contact submitted to constant normal load, one solid being supported by a 1D mechanical oscillator, under the form of an elastic bi-blade, able to return to its equilibrium position through damped oscillations. The way the amplitude is decaying during this elastic recovery is recorded. A mechanical model has been developed, in order to fit experimental data, allowing us to identify two different contributions: the velocity-dependent contribution (typically viscous damping), and the solid-like contribution (typically solid friction). This technique has been applied here to a lubricated sphere-on-plane contact. The contacting surfaces are made of AISI 52100 polished steel. Two model products are tested: pure glycerol and 1,3-buthylene glycol. First results are presented, showing the capabilities of this technique for giving a new insight on the mixed lubrication regime description.