Doctor of Philosophy
Faculty of Engineering
Yasbandha, H., Surface engineering of coinage dies, Doctor of Philosophy thesis, Faculty of Engineering, University of Wollongong, 2001. http://ro.uow.edu.au/theses/1838
In this work, the feasibility of applying surface engineering technologies to enhance life of coinage dies has been investigated. This project was essentially conducted in four stages or phases.
The first phase of this investigation involved microstructural characterisation, fractographic examination and, hardness measurement of six different tool steels, as part of the screening program to select the best tool steel(s) for further processing. The steels were characterised, in terms of size, shape and morphology of carbides, inclusions, and porosity. The steels were screened according to the requirements at Royal Australian Mint (RAM) for coinage dies e.g. cleanliness and homogeneity of microstructure free from large carbides, pores, cleanliness, hobbability, polishability, impact strength, coinability, cost, and suitability for the surface coating/
In general, the most uniform and homogeneous structure was observed in the Viking steel (double electro-slag refined shock resistant tool steel supplied by ASSAB), which had no trace of banding and was largely free from undissolved primary carbides. This steel was very clean and only a few non-metallic inclusions were noted and was deemed to meet the stringent standards of cleanliness, homogeneous structure free from large carbides, pores, and high impact strength required for coinage dies. Furthermore, the fact that Viking tool steel is capable of being tempered at high temperature (540°C), thus allowing the application of surface engineering technologies such as Plasma Immersion Ion Implantation (PI3) and Physical Vapor Deposition (PVD) coating, rendered it the most suitable contender for proof dies.
The Viking tool steel was subsequently treated/coated by PI3 (a non-line-of-sight nitrogen implantation process developed by Australian Nuclear Science and Technology Organisation and referred to as PI3) and Titanium Nitride (TiN) coated by PVD processes, using a commercially available process (Balzers) and a prototype system generically referred to as "Filtered Arc Deposition System (FADS)" in phases II and respectively. In phases II and III, a thorough microstructural and tribological characterisation was conducted to find the optimum processing parameters for PI3 treatment and TiN coating.
It has been demonstrated that the PI3 process is capable of implanting nitrogen ions concentrations and depths required for surface engineering of tool steels. Wear tests conducted using a ball-on-disc tribotester demonstrated that implantation induces improvements in wear behaviour of Viking tool steel. Specifically, whilst a typical disc would wear by a grooving/oxidation mode a typical implanted one would experience very mild polishing wear and exhibits great resistance to scratching. A degree of roughening was observed on all PI3 treated samples, primarily associated with the sputtering effect of high energy impact of nitrogen ions
Tribological behaviour (friction and wear) of TiN coatings applied by the Filtered Deposition (FAD) and Balzers processes were studied using a ball-on-disc tribotester. In general, TiN coatings experienced more wear at low load than the un-coated specimen. No protective oxide layer was observed on TiN coating at low load and a polishing/burnishing wear mechanism was operating. In contrast, at higher load a protective oxide film was formed and resulted in a very low wear rate. This protective oxide layer was, in turn, supported by the TiN coating. Therefore, even at higher load the oxide film was not ruptured and extremely low friction-low wear regime was maintained throughout the test. An extensive coating optimisation programme established the influence of substrate bias voltage on coating adhesion and properties. The best properties for FADS coatings were obtained at a 100 volts bias voltage
The final phase (IV) of this investigation was concerned with the evaluation and performance assessment of surface engineered coinage dies to produce production (circulating) and proof (numismatics) coins. Optimum process parameters for PI3 treated and TiN coatings were utilised to treat/coat 5 Cents production and 10 Cents proof dies. Extensive coining trials in comparison with the standard hard chromium plating enabled the performance ranking of each surface treatment/coating
In the case of production dies, the main life determining damage and/or failure mechanism can be generally classified as substrate cracking (fatigue), abrasive (ploughing) and adhesive (galling or material transfer) wear. Substrate cracking was dominant mode of failure. Cr-plated dies were prone to cracking, and exhibited moderate ploughing (abrasive wear) and galling (adhesive wear). The high internal stress, crazed pattern and poor adhesion of Cr-plating could all combine to reduce its resistance to cracking. In addition, the moderate hardness of Cr-plating incurred moderate resistance to ploughing
PI3 treated dies, although are harder than Cr-plated dies, still showed severe ploughing, due to a large amount of exposed inclusions on the surface of the dies. TiN coated dies produced by Balzers process, exhibited pin holes and macroparicles and generally yielded a performance inferior to Cr plating.
In the case of proof dies, a significant improvement (almost by a factor of 5 times) the die life was obtained for coating deposited by FADS compared with Cr plated dies. These coatings replicated the original finish and had high hardness and good adhesion which substantially reduced the extent of abrasive and adhesive wear processes. However, it was found that the FADS process employed in this work was rather irreproducible and inconsistent. It is believed that future improvements and modifications in design, process monitoring and control should enable deposition of coatings with repeatable and predictable properties, hence, resulting in significant reduction in producing proof coins.