Evolution of conventional hard coatings for its use on cutting tools

The investigation of hard materials started already in the 1930s by depositing TiC, although it took until the 1950s that coatings were deposited on steel substrates and further 20 years till the first commercial TiC coatings on hardmetal tools were available. The development of coatings was deemed necessary because conventional hardmetal tools were no longer cost-effective for its use in the new generation of metal working machines. However it was not until the introduction of TiN and Ti(CN) that the machines could be used to their full capability. Further progress for hard coatings was the development of CVD processes for industrial deposition of other compounds like Al2O3, ZrN, Zr(C,N), Ti-Zr-C, TiB2, various TaxCy- and CrxCy-layers. Parallel to CVD the development of PVD coatings started in the 1980s. The next step was to control the microstructure of the hard coatings. The crystal size could be varied by changing the deposition temperature and adding impurities. Medium temperature (MT) and Plasma CVD processes were developed. For improving cutting tool performances various coatings were combined to multilayers like TiN/MT-TiCN/Al2O3/TiN. The newest investigations are interested in nano-structured coatings mixing two phases within one layer. (Al,Ti)N-coatings with high Al-content show great potential and are one example developed during the last years.