(1. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130025, China) (2. College of Materials Science and Engineering, Jilin University, Changchun 130025, China)
Owing to high valence electron density and incompressibility of heavy transition-metal (TM), as well as the deformation resistance of light element (LE) atoms with strong covalent bonding, the compounds of TM and LE atoms exhibit hard (Vickers hardness>20 GPa), even superhard (Vickers hardness>40 GPa) characteristic. Based on that, The superhard/hard films are crucial and fundamental materials for applications in key technological fields, including cutting and drilling tools, wear-resistant coatings, and engine components. However, high hardness always brings with high brittleness (low toughness), which is in favor for cracks initiation and propagation. It might result in the materials fracture and decrease the reliability and service life of materials. Therefore, to deal with the conflict between hardness and toughness of superhard/hard films for achieving excellent comprehensive properties is full of challenges but also significant. The common methods and mechanisms of strengthening and toughening for superhard/hard films were reviewed in this paper. Furthermore, the other excellent properties of superhard/hard films obtained based on strengthening and toughening were introduced. Eventually, the urgent scientific problems and important research directions in the field of strengthening and toughening for superhard/hard films were put forword.