Micromachining of optical features and their transfer via stamping on precious metals

加拿大皇家造币厂(RCM)是硬币制造和收藏业的领先者。今天，它为许多国家的金属货币盖章，并以其强大的研究和创新而闻名世界。为了保持铸币行业的领先地位，保持其竞争优势，并吸引其他国家在其金属货币上盖章，RCM正在研究新的防伪特征和新方法，以在硬币上产生光学设计和特殊视觉效果。在之前的NSERC-CRD资助中，我们证明了超快激光产生的表面形貌可以用来对金属着色。我们展示了固定颜色(即独立于视角)可以以符合行业要求的速度生产。虽然激光写入的速度与铸造货币硬币(即生产短系列的收集币)是一致的，但对于铸造金银柱和铸造率非常高的流通硬币来说，激光写入速度仍然不够。RCM的最终解决方案是使用他们目前的冲压工艺将这些光学效果转移到硬币上。NSERC-CRD项目的目标是评估RCM冲压工艺在转移贵金属(主要是银)上的光学效应方面的可行性。在冲压过程中，特征将首先用激光或高精度铣床在钢模上雕刻，然后用来冲压硬币，并将特征和产生的光学效果转移到硬币上。所寻求的光学效果是全息特征，其中颜色随视角和三维效果而变化。如果RCM成功地在贵金属上盖上光学特征，它将保持其在硬币制造行业的领导地位。

The Royal Canadian Mint (RCM) is a leader in the coin making and coin collection business. Today it stamps metal currencies for numerous countries and is renowned worldwide for its strong research and innovation. To remain a leader in the minting industry, to keep its competitive edge and to attract other countries to stamp their metal currencies, the RCM is now looking into new anti-counterfeiting features and new ways to produce optical designs and special visual effects on coins. In a previous NSERC-CRD grant, we demonstrated that surface morphologies produced by ultrafast lasers can be used to colourize metals. We showed that fixed colours (i.e. independent on viewing angle) can be produced at speeds in line with industry requirements. While laser writing speeds are in line with the minting of numesmatic coins (i.e. collector coins where short series are produced) they are still inadequate for minting bullions and for circulation coins where minting rates are very high. The ultimate solution for the RCM would be to use their current stamping process to transfer these optical effects on coins. The goal of this NSERC-CRD project is to assess the viability of the RCM stamping process in the transfer of optical effects on precious metals (mainly silver). In the stamping process, features will first be engraved on a steel die using a laser or high precision milling machine, which will then be used to stamp coins and transfer the features and resulting optical effects onto the coin. The optical effects sought are holographic features where colours change with viewing angle and three-dimensional effects. If the RCM were to successfully stamp optical features on precious metals, it would maintain its leadership position in the coin making industry.