Effect of transverse current injection during vacuum arc deposition of TiN

Transverse current injection (TCI) is a technique in which an electrical current is imposed parallel to the surface of a workpiece exposed to processing or to a service environment. In this work, TCI was applied during vacuum arc deposition of TiN film on WC cemented carbide substrates (90% WC, 8% Co, 2% TaNbC). Bar samples with dimensions 5×6.5×20 mm were mounted on a holder that provided electrical contacts at their ends, and a thermocouple to measure sample temperature. Prior to deposition, the substrates were heated in vacuum up to 150°C by a current of 100 A. The TiN coatings were obtained by vacuum arc deposition of Ti plasma in a 0.67-Pa nitrogen background. Films with different thickness were obtained by controlling of the plasma flow density. The arc current was 250 A, and substrate temperature during deposition was in the range 110–130°C. Two deposition rates,: 2.3–3 and 20–23 nm/s were used. The injected d.c. transverse current was in the range 0–40 A. Surface microhardness was measured by Vickers microindentation using a 25-g load. Film structure was examined by X-ray diffractometry (XRD), scanning electron microscopy (SEM) using scattered electrons (SE), back scattered electrons (BSE), and energy dispersive spectroscopy (EDS). EDS spectra indicated that approximately the same ratio of Ti and N atoms existed both in thick (4–4.5 μm) and in thinner films (∼0.4 μm). XRD patterns indicated a cubic NaCl-type TiN phase in the thick films, while this phase was absent in the thinner films. In comparison with films deposited without TCI, the maximum microhardness of thick TiN films (∼4 μm) was larger by a factor of 1.3 with 10 A of transverse current, whereas the maximum microhardness of thinner films (∼0.4 μm) was up to a factor of 1.7 larger with 20 A of transverse current. TiN coatings deposited at 110–130°C with TCI were uniform and did not contain delaminating fragments, whereas coating deposited using the same deposition parameters but without TCI contained delaminating fragments.