Phase composition and mechanical properties of homostructure NbN nanocomposite coatings deposited by modulated pulsed power magnetron sputtering

Homostructure NbN nanocomposite coatings with cubic delta-NbN phase and hexagonal delta'-NbN phase were deposited by modulated pulsed power magnetron sputtering (MPPMS) under varied nitrogen flow rate f(N2) from 15% to 30%. Low f(N2) favored the formation of delta'-NbN phase with (100) preferred orientation and delta-NbN phase with (200) preferred orientation, while high f(N2) favored the formation of delta-NbN phase with (200) preferred orientation. The NbN coatings were characterized as the nanocomposite coatings with delta'-NbN nanocrystallites embedded into delta-NbN matrix. The hardness and modulus of NbN coatings went up to 36 GPa from 30 GPa and 460 GPa from 366 GPa as f(N2) increased to 20% with residual compressive stress from 0.47 GPa to 1.93 GPa, then decreased to 29 GPa and 389 GPa with residual compressive stress of 1.01 GPa as f(N2) further increased. Meanwhile, the toughness of coatings increased with the content of d'-NbN phase showing an abrupt change of NbN coating toughness under the f(N2) of 30%. The homostructure NbN nanocomposite coatings with both enhanced hardness and toughness were achieved by forming nanocomposite structure at low f(N2). The results showed that the essential conditions of the design criterion for hard and tough coatings should be arranged in the impact of weight as follows: 1) the designed microstructure, 2) the compressive residual stress, 3) high H/E and H-3/E-2, 4) high W-e.