Application of hard coatings to ironing dies

In this study, in order to investigate the possibility of application of hard coatings to ironing die, especially in dry process, strip-ironing test was carried out by the strip-ironing type tribometer [1]. DLC-Si coating [2], which is improved adhesion strength by adding Si and using preliminary nitrided and ion etched substrate, and also CrN were tested to investigate tribo-characteristics. High tensile strength steel (SPFC590 by JIS) and brass (C2801-1/4H by JIS) were used as workpiece. The repeat ironing test of high tensile strength steel with DLC-Si coated tool was carried out. Alloy tool steel was used as substrate, and its surface texture was controlled into three types with lapping and grindings to evaluate its influence to damage of DLC-Si coating. Surface appearances of coatings, friction coefficient during ironing and surface roughness of workpiece were measured, and the variations in these parameters with repeat ironing were estimated. The friction coefficient of DLC-Si coating shows the lowest value, as shown in Figure 1 and galling is not observed. Some cracks occur in DLC-Si coating. The damage of DLC-Si coating and the surface roughness of workpiece increase with repeat ironing, and becomes higher in case of lapped die. However, the friction coefficient does not show significant change with repeat ironing.