Microstructure, mechanical and tribological properties of novel multi-component nanolayered nitride coatings

Novel multi-component nanolayered nitride coatings composed of four alternating layer constituents in a modulation period (Λ), i.e., TiN, Mo2N, ZrN and AlN, were synthesized using the unbalanced closed-field magnetron sputtering technique. The microstructural evolution and mechanical properties of the coatings were investigated over a wide range of the modulation period. X-ray diffraction analysis indicated that at the smallest Λ of 1.8 nm, all the layer constituents formed a single cubic structure, with a lattice constant near the average of the individual constituents. In addition, a moderate (200) preferred orientation was observed in this condition. With increasing modulation period, the preferred orientation diminished and the cubic structure evolved to form hexagonal AlN layers, followed by the formation of crystallographic equilibrium structures of the individual constituents at Λ larger than 20 nm. These microstructural changes had a direct influence on the hardness (H) and Young's modulus (E) of the coatings. The maximum hardness of 34.0 GPa, which was considerably higher than the rule-of-mixture hardness of the constituent layers, was obtained at Λ = 31.1 nm. The coatings exhibited lower Young's modulus and significantly higher H3/E2 ratios than the monolayered TiN coating. Dry sliding tests demonstrated that these coatings had a lower coefficient of friction and higher dry sliding wear resistance than TiN coating.