Size and Speed Effects From Lubricant Breakdown in Plastic Compression

A size dependence of the average pressure for reducing thin, geometrically similar, lubricated disks of aluminum has been associated with the development of lubricant-film breakdown at the periphery. In general, the greater amount of interface sliding (a maximum at the periphery) of larger disks led to earlier breakdown and higher stresses. However, with certain liquid lubricants the size-effect reversed with increasing reduction, the larger disks requiring the smaller stress. This reversal has been related to the presence of a lubricant reservoir, entrapped across the interface, that increases with sample size, supplying the peripheral region and eventually controlling the progress of breakdown. The effects of speed on pressure requirements were attributed to such factors as strain-rate dependence of lubricant shear strength, junction growth across the interface, and changes in the size of lubricant reservoirs.