Surface behaviour testing of phosphate—stearate coated steel parts

Abstract

Two recently developed tests are used in this work in order to simulate contact and friction conditions in metal working processes. The upsetting—sliding test and the indentation—rotation test are able to measure the frictional properties induced by various lubricants or steel surface treatments during the actual forming operation. In this paper, the interest is focused on friction measurement and seizure performance of steel phosphate coatings strengthened by localized high contact pressures. In the upsetting—sliding test, the indenter slides along the workpiece surface with a given penetration. Depending on the penetration and on the indenter geometry, a more or less strengthened plastic zone appears in the contact surface vicinity. Indenter normal load and tangential force values are related to mean contact pressure and mean friction stress at the contact surface. The experimental surface behaviour is then identified using friction stress—contact pressure curves. In the indentation—rotation test, the workpiece rotates while in contact with a fixed semi-cylindrical slider. Curves of friction force versus workpiece revolutions are related to an increase in coating damage and to seizure. The sensitivity of the upsetting—sliding test is illustrated by friction stress—contact pressure curves obtained for different phosphate—stearate coatings, indenter velocities and workpiece initial roughness. Phosphate—stearate coating performance until seizure is finally obtained from the indentation—rotation test and results from both tests are compared and discussed.