Hard and tough (NbTaMoW)Nx high entropy nitride films with sub-stoichiometric nitrogen

(NbTaMoW)Nx films with different x value were prepared by reactive magnetron sputtering in a mixed Ar/N2 atmosphere at 575 K. The films deposited at fN (fN=N2/Ar+N2) of 0–25% correspond to the N/Me composition ratio x of 0–0.72. The film doping with less than ~8 at% N have a BCC solid solution structure and shows an improved Hardness (H) and Elastic modulus (E), while it transforms to FCC structure when x is above 0.16. (NbTaMoW)Nx films at a wide composition ranges are FCC structure for x = 0.16–0.72. (NbTaMoW)Nx film with fN= 10% (x = 0.48) presents the combination of high H (30.8 GPa) and H/E (0.11), high fracture toughness and excellent wear resistance (5 ×10−7 mm3/Nm). The antisite defects of transition metal on the N sublattice (TMN) in sub-stoichiometric nitride films are responsible for the high hardness and the enhanced toughness. All the (NbTaMoW)Nx films with sub-stoichiometric nitrogen exhibit higher d-t2g(Me) - d-t2g(Me) metallic states compared with p(N)- d-eg(Me).