Confirmation of pressure dependence of coefficient of friction by means of direct observation of micro contact behaviors at interface between tool and workpiece - tribology in sheet metal forming III

摘要

Determination of the relationship between the coefficient of friction and the average pressure was previously investigated for application to numerical process simulation. The author found experimentally that the coefficient of friction was constant in the low pressure range, while in the high pressure range, it decreased with increasing average pressure. In the present work, a flat tool drawing apparatus which consists of a transparent die made of quartz, a microscope with a CCD camera and a video system is newly developed in order to directly observe in situ the contact behavior at the interface. The model workpiece material chosen is commercially pure annealed aluminum sheets having two rough surface levels. The flat tool drawing experiments are carried out at a constant speed of 0.14 mm/s at eleven levels of average pressure within a range from 19 to 72 MPa. Paraffinic mineral oil having a viscosity of 1460mm{sup}2/s with 5 % oleic acid is used as the lubricant. The results obtained are as follows. (1) At low average pressure, the coefficient of friction is constant and the regime is characterized by the isolated contact area according to the boundary lubrication model. (2) At medium average pressure, the coefficient of friction decreases with increasing average pressure and the regime is characterized by closed lubricant pools in which the hydrostatic pressure is generated according to the hydrostatic-boundary lubrication model. (3) At high average pressure, the coefficient of friction also decreases with increasing average pressure and the regime is characterized by oil permeation into the real contact area according to the boundary, hydrostatic and micro-plasto-hydrodynamic lubrication model.