Properties of TiN/CrN superlattice hard coatings deposited by reactive magnetron sputtering

Nanostructured TiN/CrN superlattice hard coatings were deposited using a reactive closed field unbalanced magnetron sputtering in the presence of additional anode for higher plasma ionisation degree at temperatures lower than 240 degrees C. Coatings with three different bilayer periods (Lambda) of 16 nm, 20 nm and 24 nm with applying a moderate substrate bias were obtained. The effect of bilayer period on the mechanical and tribological properties of the coatings was investigated by atomic force microscopy, nanoindentation, micro scratch and ball-on-disk wear tests. All the coatings possessed enhanced hardness, decreased coefficient of friction, improved adhesion and higher toughness in comparison with single-layer and gradient-composition hard coatings. The highest hardness value of about 30 GPa and elastic modulus of 360 GPa were measured for the coating with bilayer thickness of 24 nm. The friction coefficient against the diamond tip was within the range 0.2 for Lambda = 20 nm to 0,208 for Lambda = 16 nm. The calculated wear rates against 3/16 inch alumina ball were within the range from 1,045.10(-6) mm(3)/N.m for Lambda= 24 nm to 1,955.10(-6) mm(3)/N. m for Lambda = 16 nm. The static coefficient of friction mu(SS) against polished SS plate was mu(SS) = 0,268 for Lambda = 24 nm, up to mu(SS)= 0,301 for Lambda = 16 nm, as the reference TiN layer possesses mu(SS) = 0,218. The wetting angle of the coatings against deionised water was between 91 degrees for Lambda = 16 nm to 85 degrees for Lambda = 24 nm.