Friction characteristics and material transfer tendency in metal powder compaction

The friction characteristics and material transfer tendency between metal powder and die tool material in metal powder compaction play an important role in the production of near-net-shape components of high density. A natural step to further increase the green density and simplify the sintering process is to reduce the amount of internal lubricant in the powder since the volume fraction of an organic lubricant will result in a significant contribution to the resulting porosity. However, this will significantly increase the adhesive contact and thus the friction between the die and the powder/green body during the powder compaction process. As a result, the compaction and ejection forces as well as the wear rate of the die and punch surfaces will increase. Consequently, improved knowledge concerning the friction mechanisms prevailing at the metal powder/die tool material interface is needed. The present paper will present data regarding the influence of type of tool and coating material on the friction characteristics and material transfer tendency during simulated powder compaction of a water atomized plain iron powder under no or starved lubrication conditions using two different laboratory tribo tests. Tool materials investigated include ingot cast tool steel, powder metallurgy (PM) tool steel and TiAlN and DLC-type PVD coatings. Post-test characterization using scanning electron microscopy and energy dispersive X-ray spectroscopy was used to analyse the tribo surfaces and especially the tendency to material transfer and tribo film formation. The results show that the material transfer tendency is mainly controlled by strong adhesive metal–metal contacts and that a PVD coating showing intrinsic low-friction properties and a smooth surface topography may significantly reduce the interaction between the mating surfaces promoting a stable friction and a low material transfer tendency.