Determination of the Coefficient of Friction of Highly Filled Rubber over Polished Steel, Depending on Roughness, Lubrication, and Contact Pressure

Friction tests were performed on a disc made of oil- and gasoline-resistant rubber during reciprocating sliding on a polished steel surface with lubrication. The tests were carried out at room temperature for roughness values of 0.63 and 1.27 microns, lubrication with water, MDPN-C oil, and castor oil, and a pressure range of 0.5–20 MPa. Cyclic programs with stepwise varying constant pressure levels and pressure changes proportional to the displacement in the friction pair were considered. The dependence of the coefficient of friction on pressure in the range of 0.5–20 MPa for both oils is well approximated by the inverse power law, differing in this range by a factor of 10–20. The coefficient of friction when lubricated with castor oil is 2–5 times lower (depending on pressure) compared to its value when lubricated with MDPN-C oil, and the latter is 1.5–8 times lower when lubricated with water. In a test with a constant pressure level, the coefficient of friction does not stabilize, and its value increases with roughness for castor oil and MDPN-C oil (at low pressures), decreases for water and MDPN-C oil (at high pressures), demonstrating dependence on the wettability of the rubber with grease and the drainage properties of the counterbody surface. In the proportional pressure change test, the friction coefficient is reached, stabilizes, and increases with roughness. The test data is approximated for use in numerical calculations in the design of friction units.