China Banknote Printing and Minting Corporation —Graphite-like Coating (GLC) Technology in Coin Dies

China

Traditional commemorative coin dies utilize the PVD process, coated with a TiN coating. During the stamping process, product surfaces frequently encounter defects such as “fogging” and “flash lines,” which severely affect production efficiency and die life. The inner hole of the collar die used for forming the product edges is processed with abrasive grain machining and manual polishing. The edges of the product, particularly the serrated sections, are prone to wear. Additionally, lubricating the edge of the blank with oil is prone to causing surface oil stains, affecting the appearance of the product and service life of the collar die.
For the first time, China Banknote Printing and Minting Corporation applied Graphite-like Coating (GLC) to circulating commemorative coin dies and collar dies. By conducting optimization of the coating structure and the coating process, the adhesion of the coating and the die was improved. This enabled the application of GLC coating in coinage production, effectively enhancing the surface quality of ordinary commemorative coin products.

GLC thin film is a special type of diamond-like amorphous carbon film, predominantly featuring SP² hybridization (SP3 content ≤ 20%). It possesses both high hardness and a low friction coefficient (0.05 ~ 0.09). When in contact with steel materials, it does not cause catalytic effect, making it an excellent anti-friction and anti-wear coating. Beyond its unique tribological performance and ultra-low friction, GLC coating features high hardness, superior wear resistance and robust self-lubricating properties.
When friction occurs between GLC coating and metal substrates, sp²-hybridized carbon layers easily shear off the coating surface and adhere to the metal counterpart to form a transfer film, separating the two friction surfaces. On one hand, friction mainly takes place between the transfer film and carbon coating interface, minimizing direct contact between metal and coating.
On the other hand, the formation of transfer film redistributes contact stress and improves load-bearing capacity, improving the load-carrying capacity of the coating during service.

The GLC coating newly introduced by China Banknote Printing and Minting Corporation is manufactured on equipment supplied by Teer Coatings. The coating system adopts a chromium underlayer with an additional chromium nitride (CrN) support interlayer. The multi-layer composite architecture balances hardness and toughness, and significantly strengthens coating-substrate adhesion. This significantly mitigates the drawbacks of traditional coatings during stamping, including die sticking, abrasive wear and surface contamination.

The GLC coating process is based on magnetron sputtering technology. It utilizes carbon targets and chromium targets, using argon and nitrogen as the processing gases. The processing is a dry vacuum process, generating no wastewater or water pollution. The target materials (carbon and chromium) are non-toxic or possess low toxicity, resulting in lower occupational health risks. Furthermore, waste gas emissions are virtually zero. The entire production process is clean and pollution-free, making it a green and environmentally friendly technique. Its promotion and application in the coinage industry will have a highly positive significance.