Design, Preparation and Properties of Ti-B-N Nanocomposite Coatings

TiB2 coating comprises a large number of ionic and covalent bonds, conferring it with excellent properties such as high melting point, high hardness, and good oxidation and corrosion resistances. However, its application to cutting tool surfaces is limited due to high brittleness. When doped with N atoms, TiB2 coating forms a nanocomposite structure with improved toughness. However, the hardness of the resulting coating is significantly impaired by the abundant amorphous BN (a-BN) phase. The addition of metal ions and reactive N-2 increases the proportion of hard nitrides and improves the coating hardness. However, the addition of N-2 increases the amount of soft a-BN phase, which largely negates the strengthening effect. To further improve the mechanical properties of Ti-B-N coating, a series of Ti-B-N coatings were prepared by pulsed direct current magnetron sputtering in this work. The content of softphase a-BN in the coating was reduced by decreasing the flow of reactive gas N-2. Meanwhile, the amount of hard TiB2 phase was increased by increasing the sputtering power of the TiB2 target. Consequently, a noncrystalline (nc)-(Ti2N, TiB2)/a-BN nanocomposite coating with significantly improved toughness and strength was formed. The influence of TiB2 target sputtering power on the composition, microstructure, and mechanical and tribological properties of the Ti-B-N coatings were systematically investigated by EDS, TEM, SEM, XRD, and nano-indentation, scratch, and ball-on-disk tribological testings. As the sputtering power of the TiB2 target increased, the microstructure of Ti-B-N coatings gradually evolved from nc-Ti2N/a-BN to hexagonal-close-packed TiB2/a-BN, and the nanohardness also increased gradually. The particle size on the coating surface was significantly increased, and all Ti-B-N coatings were uniform and compact without pinholes and other defects. The coating with highest hardness of about 33.8 GPa was achieved under a sputtering power of 2.4 kW at the TiB2 target. This coating also exhibited the lowest friction coefficient (0.55), lowest wear rate (2.1x10(-4) mu m(3)/(N.mu m)), and best wear resistance