Determination of friction law in metal forming under oil-lubricated condition

Frictional behaviour under oil-lubricated conditions is studied in compression of a steel sheet by a DLC coated tool. The average frictional stress increases linearly with the average pressure first, then decreases and finally increases linearly again. This tendency is not affected by the lubricant viscosity, lubricant thickness and the stress state of the specimen. It is concluded that the decrease of the frictional stress with increasing average pressure is due to the decrease of the ratio of real contact area, which is caused by the lubricant pressure in closed pools. A new law of friction under oil-lubricated condition is proposed.