To measure friction on the die-workpiece interface, sheet specimens of soft copper were first subjected to plastic compression in plane strain, and then one of the platens was slid in the direction of sheet width. With lower viscosity lubricants and lower sliding speeds, depressed flat portions and discrete oil pits were evident on the worked surfaces. With lubricants of higher viscosity under pressure and higher sliding speeds, “microscopic plasto-hydrodynamic interactions” operated to diminish oil pits and to increase the thickness of boundary films on the flat portions. The frictional stress increased almost logarithmically with the product of viscosity under pressure and sliding speed.

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