It is well known that third bodies can heavily influence the tribological properties of surfaces. Sometimes the effect is beneficial and specifically designed, as in the case of solid lubricants or during the surface finishing of metals. In other cases, instead, the presence of particles is unwanted and could reduce significantly the performance and life of tribo-contacts. Therefore, a deep understanding of third-body friction is essential for the proper design of surfaces and lubricants. Despite several experimental works are available in the literature, a systematic understanding of the effect of surface roughness on third-body friction is still lacking.
In the present work, we employ the Discrete Element Method to simulate viscoelastic spherical particles within two surfaces with a sinusoidal roughness. Sensitivity analyses are performed by varying four main geometric quantities: the surface amplitude and wavelength, the clearance and the particle diameter. In addition, we study the effect of microscale parameters that determine the particle collisions (e.g. the local coefficient of friction and the coefficient of restitution) and of some macroscale operating conditions, such as the applied pressure and shear rate.
We show that the macroscopic frictional behaviour of the system is extremely dependent on the surface roughness amplitude and on the operating conditions, which drive the microscale particle dynamics. Instead, the effect of the roughness wavelength and of the other microscopic parameters is limited. We believe the results of our work could be of interest towards the design of new tribo-materials with improved and precisely engineered performance.