Friction at high normal pressures

Friction conditions between tool and workpiece in metal working are of the greatest importance to a number of factors such as force and mode of deformation, properties of the finished specimen and resulting surface roughness.It is shown, theoretically and experimentally, that the Amonton friction law expressed by τ = μq does not apply when normal pressure is higher than approximately the yield stress of the specimen; in this case it is necessary to consider the frictional stress as a function of normal pressure, surface topography, length of sliding, viscosity, and compressibility of the lubricant.The theoretical work was carried out by means of upper bound and slipline field analysis based on experiments with model surfaces in wax and metal. The theoretical model applied is one of multihole extrusion, the material beneath the valleys of the workpiece surface being extruded up towards the tool when the real area of contact exceeds a certain value. The effect of the trapped lubricant is to build up a back-pressure on the extrusion process.The experimental work was carried out with newly developed equipment enabling direct determination of the abovementioned function; construction and calibration of the equipment are described. The equipment allows determination of frictional stress on a surface with well-defined values of normal pressure, sliding length, and sliding velocity. The normal pressure can attain about 8 times the yield stress for commercially pure aluminium.The results obtained show reasonably good agreement between theory and experiment, and a dependence of the frictional stress on the sliding length, this dependence being a function of normal pressure.