Correlation between fracture characteristics and valence electron concentration of sputtered Hf-C-N based thin films

Abstract Hard protective coating materials based on transition metal nitrides and carbides typically suffer from limited fracture tolerance. To further tune these properties non-metal alloying – substituting C with N – has been proven favorable for magnetron sputtered Hf-C-N based thin films. A theoretically predicted increase in valence electron concentration (from 8.0 to 9.0 e/f.u. from Hf C to Hf N) through nitrogen alloying lead to an increase in fracture toughness (KIC obtained during in-situ SEM cantilever bending) from 1.89 ± 0.15 to 2.33 ± 0.18 MPa·m1/2 for Hf0.43C0.57 to Hf0.35C0.30N0.35, respectively. The hardness remains close to the super-hard regime with values of 37.8 ± 2.1 to 39.9 ± 2.7 GPa for these specific compositions. Already the addition of small amounts of nitrogen, while sputtering a ceramic Hf C target, leads to a drastic increase of nitrogen on the non-metallic sublattice for fcc single phased structured HfC1-xNx films, where x = N/(C + N). The here obtained results also provide experimental proof for the correlation between fracture characteristics and valence electron concentration.