Tribo-rheometry: from gap-dependent rheology to tribology

We describe a new triborheometry fixture that can be utilized with a commercial torsional rheometer in order to explore the coupled rheological and tribological properties of complex fluids and solid-liquid systems. The fixture is self-leveling and both the normal load and the sample gap can be monitored or controlled. At large gaps, the fixture imposes an approximately constant shear rate on the sample and the bulk viscometric properties of the fluid can be measured. However, as the gap between the plates is reduced, the measured viscosity function becomes gap-dependent. For gaps on the order of the surface roughness of the plates, the device is operated under a constant applied load and the tribological properties of the fluid-solid pair can then be measured. Using this new triborheometer fixture it is possible to obtain tribological information over a wider range of sliding velocities than is typically possible using conventional devices such as pin-and-disk systems. The data can be represented in the form of a classical Stribeck diagram or, by using a dimensionless gap-dependent shift factor, it is possible to construct a more general ’friction map’ of the gapand loaddependent effective viscosity (Luengo et al. Wear, 200, 1996). The capabilities of this system are illustrated using a number of different lubricant fluids, for a range of normal stresses and variations in surface properties such as the mean roughness.