Structural and mechanical properties of compositionally gradient CrNx coatings prepared by arc ion plating

Compositionally gradient CrNx coatings were fabricated using arc ion plating by gradually increasing N2 flow rate during the deposition process. The effect of substrate bias, ranging from 0 to −250 V, on film microstructure and mechanical properties were systematically investigated with XRD, SEM, HRTEM, nanoindentation, adhesion and wear tests. The results show that substrate bias has an important influence on film microstructure and mechanical properties of gradient CrNx coatings. The coatings mainly crystallized in the mixture of hexagonal Cr2N, bcc Cr and fcc rock-salt CrN phases. N2 flow rate change during deposition results in phase changes in order of Cr, Cr + Cr2N, Cr2N, Cr2N + CrN, and CrN, respectively, along thickness direction. Phase fraction and preferred orientation in CrNx coatings vary with substrate bias, exerting an effective influence on film hardness. With the increasing of bias, film microstructure evolves from an apparent columnar structure to a highly dense one. The maximum hardness of 39.1 GPa was obtained for the coatings deposited at a bias of −50 V with a friction coefficient of 0.55. It was also found that adhesion property and wear resistance of gradient CrNx coatings were better than that of homogeneous CrN coatings.