Abstract
The material used to produce one baht coin in the Royal Thai Mint has been changed from cupronickel (75% Cu and 25% Ni) to nickel-electroplated steel, not only for economic reasons but also for the durable properties of the latter. As a result, the higher hardness value of the new material (110 HV30 versus 80 HV10) combined with the high accuracy requirements of the coin detail have reduced the durability and lifetime of coining tools. The number of coins produced by a single coining tool was reduced from approximately 300,000 to only 40,000 coins. However, the tool life can be increased if proper selections of tool material and hard thin film coating technique are made. Two types of tool material were selected: cold working tool steel, 0.97% C and warm working tool steel, 0.5% C. CrN, TiN and TiCN coatings were examined using physical vapour deposition (PVD). It was determined that the type of tool material, the type of coating material, the coating temperature and the coating thickness influenced the lifetime of a coining tool. For the conditions studied, a 3-µm-thick TiN coat on warm working tool steel produced under a coating temperature of 400°C provided the maximum tool life up to 1.22 million strokes.
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