Effects of bias voltage on the structure, mechanical properties and tribological properties of TaBx films at elevated temperatures

Transition metal boride (TMBx) show the potential application as protective films at extreme environment due to their high hardness, but their low toughness limits reliability. In order to achieve both excellent mechanical and high wear resistances of the films, the microstructure of TaBx films deposited by magnetron sputtering were regulated by varing the substrate bias voltages. With the increase of the higher bias voltage, the B content of TaBx films decreases and the crystallinity of the TaBx films is enhanced. Meanwhile, the microstructure of TaBx films transforms from fibrous to dense columnar and then to granular structure with the increase of the bias voltage. The hardness and elastic modulue decrease then increase when the substrate bias voltage increases from 0 V to-240 V. The films exhibit a super high hardness of 86.07 GPa at the bias voltage increases of-180 V. The dislocation motion inhibited by the lattice defects lead to the super high hardness and a mild fracture toughness (KIC = 1.20 MPa m1/2) . For the high hardness and relatively moderate fracture toughness, the TaBx film deposited at a bias voltage of-180 V show a lower wear rate (-1.86 x 10-6,-5.22 x 10-6 mm3/Nm) at room temperature and 600 degrees C. Meanwhile, the hardness (77.5GPa) and thermal stability of-180 V films were mantained after annealing.