Structure–property relationships in single- and dual-phase nanocrystalline hard coatings

The structure of hard coatings deposited by PVD (physical vapor deposition) strongly depends on the growth parameters. In this work, the influence of microstructure and chemical composition on mechanical properties was investigated in detail for several nanocrystalline hard coatings. Single-phase TiN, CrN and TiB2 as well as dual-phase CrN–Cr2N, TiN–TiB2 and TiC–TiB2 coatings were prepared by reactive or non-reactive unbalanced dc magnetron sputtering, respectively. The hardness of the coatings investigated will be discussed with respect to the average grain size and residual biaxial stresses. By optimizing both nanostructure and biaxial stresses of stoichiometric TiN coatings, their microhardness (H) could be improved from 33 to 56 GPa, whereas the reduced Youngʹs modulus (E*) changed from 402 to 480 GPa. Thus, the ratio H3/E*2 which is representative for the resistance to plastic deformation could be increased from 0.222 to 0.806 GPa. For Cr–N coatings, H3/E*2 values up to 0.521 GPa were obtained, depending on their chemical composition and nanostructure. The nanocrystalline dual-phase coatings TiN–TiB2 and TiC–TiB2 even yielded H3/E*2 values up to 1.332 and 1.575 GPa, respectively. Comparing the different single- and dual-phase coatings investigated, the establishment of correlations between structure and mechanical properties enables the development of advanced coatings with high hardness and high resistance against plastic deformation. In addition, this work shows the tremendous importance of a controlled deposition process for optimizing coating properties.