Real area of contact and friction stress at high pressure sliding contact

An experimental investigation of the plastic deformation of the surface asperities of a soft rough specimen surface in sliding contact with a hard smooth tool surface is described. The experimental equipment allows independent variation of the normal pressure and sliding length. The normal pressure was varied from 0.5 to 2.5 times the yield stress of the specimen material and the sliding length was varied from 0 to 20 mm. Experiments with soap-coated phosphated specimen surfaces showed that the real area of contact and the friction stress were related to normal pressure and sliding length as expected from theoretical considerations. Measurements of the change of the asperity slopes were also in agreement with theory. Experiments with dry friction showed a steadily increasing contact area with increasing sliding length; this explains the fact that the coefficient of friction can rise to a very high value, increasing with decreasing normal pressure. On the basis of the experimental results a new slip-line field was developed taking the slope of the asperities into consideration.