Hartmetallschichten - Historie und Perspektiven

The current most important hardmetal compositions can be described as a series starting with plain WC-Co(Ni), with a stepwise replacement by Cr3C2, and ending with Cr3C2-NiCr. This series is characterised by complex interactions between both carbides during feedstock powder production, leading to the formation of new phases depending on the WC/Cr3C2 ratio. Which new phases are formed also depends on the carbon content as a second parameter. Initial development of hardmetal coating compositions occured in the 1950's-1960's together with DGS coating solutions. Since 1980, corresponding to the period of the increasing importance of HVOF, only a few commercial powders with variations of the hard material composition have entered into the market. During this time the main development activities have focused on the use of more complex binder alloys. A common feature has been the apparent empirical approach to hardmetal composition development, rather than the systematic consideration of the interaction between the constituents during feedstock powder production and the changes in chemical and phase composition during spraying. In this regard, a lot of optimization potential has yet to be discovered, even for compositions based on WC and/or Cr3C2. In particular, Cr3C2-based compositions are very promising. The lower process temperature and higher particle velocity of HVAF results in less carbide dissolution in the binder and very dense coatings. However, the expectation that this is connected with lower carbon loss and oxygen content of the coating is misleading due to the formation of water vapor from the combustion of hydrocarbon or hydrogen fuels in the HVAF process, as well as in the HVOF process. High particle velocities can lead to additional carbon loss by rebounding of carbide grains, even from a&s feedstock powders. Overall, highly wear resistant HVOF- and HVAF-sprayed hardmetal coating solutions are now an industrial standard. Although the profile of process temperature and particle velocity of these systems is advantageous, future developments might consider the use of alternative heat sources to overcome the water vapor issues generated by combustion of liquid/gas fuels. From the range of alternative carbides, TiC is the most promising alternative in many applications, where the unique properties of WC are not irreplaceable. For many wear applications it has already been shown that TiC-based compositions are competitive when properly alloyed. While currently hindered by the high feedstock production cost for small batch sizes, this situation can suddenly change due to factors such as new legislative regulations, combined with possible shortcuts in the supply of tungsten and cobalt. In any case, it must always be considered that a sudden substitution is practically impossible and will always require long-term development efforts.