Microstructure, mechanical, and tribological properties of Ag-free and Ag-doped VCN coatings

The aim of this work was a comparative study of the structure, mechanical, and tribological properties of VCN and VCN-Ag coatings. The VCN coatings were deposited by magnetron sputtering of V and C targets either in a gaseous mixture of Ar + 15%N-2 or in pure nitrogen. Silver was added into such coatings by simultaneous sputtering of metallic Ag target using an additional ion source. Microstructure and elemental composition of the coatings were studied by means of X-ray diffraction, transmission and scanning electron microscopy, atomic force microscopy, energy-dispersive spectroscopy, and Raman spectroscopy. The coatings were evaluated in terms of their mechanical properties, adhesion strength, intrinsic stress, fracture toughness, room-temperature friction coefficient, as well as wear resistance and fatigue strength. Incorporation of as much as 10-11 at.% Ag was found to cause significant changes in the coating structure and properties: (i) the columnar morphology changes to equiaxial one; (ii) the coating hardness and Young's modulus decrease from 22 to 26 to 15-17 GPa and from 260 to 270 to 220 GPa, respectively; (iii) the friction coefficient and wear rate increased slightly from 0.4-0.47 to 0.51-0.53 and from 1.5-1.6 x 10(-7) to 8.2-13.3 x 10(-7) mm(3)/Nm, respectively; (iv) the compressive stress decreased from 1.6 to 0.3-0.5 GPa. The Ag-doped VCN coatings withstood high elastic and plastic deformation during scratching with increasing load up to 50 N without adhesive failure and applied load as high as 1000 N for 10(5) cycles during dynamic impact tests without brittle fracture.