Influence of Friction-Reducing Powder-Compaction Tool Coatings on Green-Compact Properties

The most frequently used process for shaping metal powders is the two-sided pressing in dies. The properties of the green compact resulting from this pressing process, such as green density and strength, are decisive for the final density of the sintered component and thus for mechanical properties like tensile and fatigue strength. Green-compact properties, in turn, can strongly be influenced positively or negatively by the lubrication strategy applied during powder pressing. Friction-reducing tool coatings (e.g. diamond-like-carbon-based) offer the potential to improve green-compact properties by homogenizing density distribution, increasing green strength and reducing injection force. In addition, less admixed lubricant would be required resulting in resource savings. This paper presents first results of experimental investigations on the performance of friction-reducing coatings in die pressing. In particular, the influence of compaction pressure on the resulting density and the correlation between different lubrication strategies (conventional vs. coating) and green strength are studied. To interpret the results, force-displacement profiles were recorded during powder pressing and correlated with material properties. First findings indicate that the coatings alone cannot entirely replace the addition of lubricant. However, it was also found that a combination of both lubrication techniques improves wear behaviour and leads to a more homogeneous density distribution within the green compact. For future investigations, selective variations of lubricant content are planned in order to examine the behaviour of the coating in a more differentiated way.