An experimental model for mixed friction during running-in

Mechanical engineering often uses fluid lubrication to limit friction. Even in the presence of a lubricant, metallic contact between the sliding surfaces may readily occur. In running-in, the oil film thickness can be so thin that contact arises at the summit of asperities, thus, increasing both the friction coefficient and wear. Such regimes are the so-called mixed or boundary lubrication. In these situations, the friction coefficient varies continuously and it is therefore, necessary to calculate the friction coefficient at any given moment. Given two rough parallel surfaces in a lubricated environment, a model is herein proposed whereby the friction coefficient is controlled by a single hydrodynamic parameter. This experimental analysis seeks to take into account a wide variety of factors influencing the conditions in which the contact operates: functional parameters (normal load, sliding speed, viscosity), the contact pair’s mechanical properties (elastic modulus, Poisson’s ratio) and surface microgeometry (as expressed by the standardised roughness parameters). This extended friction model predicts tribological behaviour in lubrication regimes: thus, by judicious choice of materials (according to the most appropriate mechanical properties and surface roughness) one can reduce the running-in period. To confirm the model, different material pairs have been tested. The tests have been conducted using a pin on disc apparatus in a 100 Neutral solvent (100 NS) oil at 20 ◦C. Experimental results totally confirmed the model. © 2002 Elsevier Science B.V. All rights reserved.