Bias voltage and incidence angle effects on the structure and properties of vacuum arc deposited TiN coatings

TiN coatings were deposited on WC-Co bar substrates using a vacuum arc plasma gun connected to a cylindrical plasma duct in which an axial magnetic field was imposed. During deposition, the cathode arc current was 200 A, nitrogen pressure was 0.67 Pa, and the substrate temperature was 420 °C. Substrate bias voltage (V_bias) was varied in the range of -40 to -600 V. The coating structure and properties were studied both on the sample face surface, i.e. normal to the plasma flux, and on its side surfaces. The structure and phase composition were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Microhardness and scratch critical load were studied using Vickers micro-indentation and scratch tests, respectively. The TiN coatings had a single-phase cubic δ-TiN structure and consisted of oriented columnar grains. No difference was observed in the preferred grain orientation and grain size at the substrate-coating interface for the coatings deposited on the face and the substrate side surface for every studied V_bias. The deposition rate decreased both on the face and the side surfaces, while the ratio between deposition rates on the face and the side surfaces increased from three to seven times when |V_bias| increased from 40 to 600 V. With increasing |V_bias|, the preferred orientation of the columnar grains changed from a mixture of (200) and (111) at -40 V to a strong (111) at -200 and -400 V. At -600 V, (111) remained dominant, while the (220) orientation also appeared. Increasing |V_bias| increased the grain size on the coating surface. A zone of equiaxed grains was observed near the substrate-coating interface, whose thickness increased with increasing |V_bias|. Possibly, the grain size growth was a thermal effect due to an increase in ion beam heating with increased |V_bias|. The grain size on the side surfaces was smaller than that on the face. The coating surface roughness and friction coefficient were smaller on the side surfaces than those on the face, while no differences in microhardness were observed.