Surface Characterization and Performance Evaluation of Nitrogen Implanted Coinage Dies

Abstract

Very high fluence implantation of 14N+ ions was used to promote the formation of a nitride layer on the surface of steel coinage dies used by the Portuguese Mint. Die steel samples were studied before and after implantation at 90 keV plus 30 keV for two nominal fluences, 5.0x10^17 at/cm^2 and 1.0x10^18 at/cm^2, and for two implantation temperatures, room and liquid nitrogen temperature. Surface characterization was carried out by ultramicrohardness indentation, Atomic Force Microscopy (AFM), and Rutherford Backscattering Spectrometry (RBS). A significant increase (factor two) in depth reached by the implanted nitrogen was observed for LN2 implantations, which, to our knowledge, was never reported before. Results point out the benefit of nitrogen implantation at room temperature with a nominal fluence of 5.0x10^17 at/cm^2, which raised the hardness of the dies by approximately 40% (from 6.2 \pm 0.5 to 8.7 \pm 0.7 GPa). Nitrogen implanted dies were used to mint circulating two cents euro coins and their performance was assessed, showing that when compared to Vautrol coated dies (standard coating used by the Portuguese Mint), the crack density was lower and similar wear resistance was observed.