Influence of Ni Doping on the Microstructure and Mechanical Properties of TiB₂ Hard Coatings

Abstract

The unique crystalline structure and strong covalent bonding of TiB2 coating give it ultra-high hardness and chemical stability, but it also suffers from poor toughness and weak bonding with metal substrates. Herein, Ni-doped TiB2 coating was prepared by combining high-power pulse magnetron sputtering technology and radio frequency magnetron sputtering technology. Additionally, SEM, EPMA, XRD, XPS, nanoindentation instrument, scratch tester, high-temperature friction and wear testing machine were used to comparatively study the micromorphology, chemical composition, phase structure, mechanical properties, and tribological properties of TiB2-Ni coatings with different Ni contents. Results showed that the Ni-doped TiB2-Ni coating possessed a small amount of Ni element solidly dissolved in the TiB2 lattice, and the excess Ni element existed in the form of a metallic phase, forming a nanocomposite structure at the grain boundaries, which restricted the growth of TiB2 grains, and caused the refinement and nanometerization of the columnar grains of the coating. Besides, the Ni doping brought about a significant decrease in the hardness and elastic modulus of the TiB2 coatings. With the increase of the Ni content, the adhesion strength between the coating and the substrate and the toughness of coatings continuously increased, while the friction coefficient of the coating decreased first and then increased. Moreover, the wear morphology indicated the wear of the coating belonged to abrasive wear and adhesive wear. In conclusion, Ni doping improved the toughness of the TiB2-Ni coatings by generating Ni metal phase in the grain boundaries, which could absorb the fracture energy through deformation during crack expansion. The TiB2-Ni coating with a Ni content of 4.9% (atomic fraction) exhibited good hardness and toughness.

Keywords

• tib2 coatings|ni doping|nanocomposite|toughness|microstructure