Microstructures and properties of novel nanocomposite WNx coatings deposited by reactive magnetron sputtering

Novel nanocomposite tungsten nitrides (WNx) coatings were deposited on cemented carbides by direct current (dc) reactive magnetron sputtering under different deposition powers. The microstructures and related mechanical properties of the coatings were carefully investigated. At deposition power of 2 kW, the WNx coating is entirely composed of well-developed columnar fcc-W2N grains, showing the strongest (2 0 0)(W2N) preferred orientation, whereas the coatings deposited at other conditions are composed of nanocomposite structures consisting of nanocrystalline bcc-W and fcc-W2N embedded into amorphous-phase matrix, showing weak (2 0 0)(W2N) and (1 1 1)(W2N) preferred orientations. Moreover, there is an orientation relationship between bcc-W and fcc-W2N: (-1 0 -1)(W)//(1 1 -1)(W2N) + 2.1 degrees and [-1 1 1](W)//[0 1 1](W2N). The amorphous films with thickness of several nanometers always occur in a special periodic multilayer structure or in the large angle grain boundaries of bcc-W. Such amorphous films are resulted from the intensive bombardment of sputtered W atoms, which leads to the densification of the coating and further restrains the diffusion of W atoms for crystallization growth. Moreover, strong ion bombardment can interrupt the W-W metallic bond, and further restrain crystallization growth. Therefore, the nanocomposite structure shows the higher hardness and fracture toughness when compared with columnar W2N coating mainly due to the fine-grained strengthening and solution strengthening.