Nanomultilayer gradation strategy to improve mechanical properties of TiSiN/AlCrN hard coatings

Abstract

The structure design and optimization of multilayer coatings, which are utilized to surmount the trade-off between hardness and toughness, has been a current hot topic in the field of hard ceramic coatings. Herein, multi-layered TiSiN/AlCrN coatings with a constant and gradient sublayer thickness (Λ, modulation periods) ranging from 20 to 3.8 nm were prepared by the cathodic arc ion plating. The microstructure, mechanical properties, residual stress, and fracture toughness of four gradient structures were investigated systematically. All coatings exhibit a typical FCC crystal structure. In the coating with a monotonous decrease of modulation period (single gradient periodic decreasing structure, G2), the interface between the TiSiN layer and the AlCrN layer transformed from a partially semi-coherent interface at the bottom layers of Λ = 20 nm, to a fully coherent interface at the top layers of Λ = 3.8 nm. The coating with dual-gradient structures (modulation period increases first and then decreases, V2) demonstrated the highest hardness (37.6 ± 1.0 GPa), H/E* and H3/E*2 ratios (0.087 and 0.28 GPa), and bonding strength (75.3 N), as well as lowest friction coefficient (0.34) and wear rate (6.7 × 10−6 mm3/N m). The remarkable resistance to damage and toughness displayed by the V2 structure could be ascribed to its intrinsic capacity for effectively alleviating stress concentration and accommodating incompatibilities during the plastic deformation process. This work offers insights into employing gradient architecture design to enhance the strength and toughness of coatings.