Modeling of the surface coverage and application to the calculation of friction on surfaces contaminated by particles

Abstract

This paper deals with the modeling of the coverage of surfaces contaminated by fine particles, the objective being the prediction of the skid resistance of road surfaces when it rains after a long dry period. The research methodology is based on the identification of particles' flows in the tribological circuit composed of the particles (3rd body) and the tire and the road (1st bodies). Experiments are conducted in laboratory where sliding friction is measured between a rubber pad (simulating a tire tread block) and a sandblasted aluminum surface (simulating a microtextured road surface) covered by particles. The test program includes particle concentrations representative of deposits of particles on the road surface at different dry periods and different particle size fractions. The test protocol consists in repeating passages of a rubber pad on the test surface and visualizing the particles’ movements by means of high-speed cameras. Two particles' flows are identified: particles ejected from the contact and those raised by the rubber pad then fall back to the surface. Similarities are established with the removal and deposition of gas species in vapor-phase lubrication. An analytical model was derived to express the surface coverage as a function of the number of passages and two parameters (values between 0 and 1) called respectively the ejection and recirculation ratios. The proposed model is included in a linear rule-of-mixtures equation for the calculation of friction. Calculated friction coefficients compare favorably to experimental data and the model's parameters are determined. A master curve for different particles' sizes is obtained when relating the surface coverage to the mass of particles. Relationships between the ejection and recirculation ratios, the particles' characteristics and the surface texture are presented. Discussions are made in terms of transposition of the model to real road surfaces.