ICMCTF2000 Session B1-1: Evaporation and Arc Technologies

The highly ionized, energetic, metal vapor plasma jet produced by cathode spots in vacuum arc is conventionally used to produce coatings. However, the presence of macroparticles (MP's) degrades the film quality. In this paper, a new form of vacuum arc, the Hot Refractory Anode Vacuum Arc (HRAVA), is presented as a plasma source with highly reduced MP contamination.

Arcs were sustained between the parallel end surfaces of a pair of cylindrical electrodes. The cathode was water-cooled and constructed from Cu, while the anode was thermally isolated graphite or Mo. Arc currents of 175-340 A were sustained for periods of up to 120 s. Cu coatings were deposited on steel substrates placed 100 mm from the electrode axis along the inter-electrode gap mid-plane. A shutter controlled when the substrate was exposed to the plasma. The coating rates were measured by weighing, and the coating surface was examined by optical microscopy.

The arcs started as conventional cathode spot vacuum arcs, and deposited material from the cathode spot jets onto the anode. The arc heated the anode to a sufficient temperature so that at a later stage, metal was re-evaporated from the hot refractory anode. In the steady-state, all of the material emitted from the cathode was eventually transported radially in the form of an expanding plasma jet.

Films formed at the beginning of the arc, when it operated in the cathode spot mode, were heavily contaminated with MP's - in a 30 s exposure to a 175 A arc, the density of MP's with diameters of 3-50 µm was about 750 mm ^-2. However, when a 30 s exposure began 30 s after arc initiation, by which time the arc was in the HRAVA mode, the MP density was reduced to about 1 mm ^-2. The HRAVA deposition rate varied from 1.8 to 2.6 µm/min for arc currents of 175-340 A. Over this same current range, the ionization fraction increased from 31% to 45%, while the estimated extractable ion current to arc current fraction increased from 9.1% to 9.9%. These results indicate that the efficiency of the HRAVA in terms of mass of deposited material per unit charge transfer in the arc is comparable to conventional cathode spot vacuum arc deposition while having greatly reduced MP contamination, and several times greater than filtered vacuum arc deposition.

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). The 5x8x22 mm bar samples 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 5 mTorr 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 of 110-130° C. Two deposition rates: slow 2.3-3 nm/s and fast 20-23 nm/s were used. The injected dc transverse current was in the range of 0 - 40 A. Surface microhardness was measured by Vickers micro-indentation, using a 25 g load. Film structure was examined by X-ray diffractometry, scanning electron microscopy (SEM) using scattered electrons (SE), back scattered electrons (BSE), and energy dispersive spectroscopy (EDS).

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. XRD patterns indicated a cubic NaCl type TiN phase in thick films (4-4.5 µm), while the cubic TiN phase was absent in thinner films (~0.4 µm). EDS spectra indicated that approximately the same ratio of Ti and N atoms existed in both films. No X-ray diffraction lines of TiN were observed, and this sagest existence of nanocrystalline or amorphous structure of the thin films. In comparison with films deposited without TCI, the microhardness of thick TiN films (~4 µm) obtained with 10 A of transverse current was larger by a factor of 1.3, whereas the microhardness of thinner films (~0.4 µm) was up to a factor of 1.7 larger with 20 A of transverse current.

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 400° C. Substrate bias voltage (V) was varied in the range of -40 to -600 V. Coating structure and properties were studied both on the sample surface normal to the plasma flux, and on its side surfaces. Structure and phase composition were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Microhardness and coating adhesion to the substrate were studied using Vickers micro-indentation and scratch tests, respectively.

The TiN coatings consisted of oriented columnar grains, which had a single-phase cubic structure. The microstructure and preferred crystal orientation were the same on the front and side surfaces of the samples. The deposition rate to the front surface was to 2-4 times higher than that to the sides. The microstructure and grain orientation depended on the substrate bias voltage. At V=-40 V, the coating consisted of a mixture of (200) and (111) oriented grains, at V=-200 V and higher a strong (111) grain orientation was observed. At V=-40 V, equiaxed grains of 30-50 nm average size were observed near the coating-substrate interface, while the bulk of the coating consisted of columnar grains with 60-70 nm average size. With increasing the negative bias voltage, the average grain size increased up to 120-250 nm at V=-400 V. With increasing V from -40 to -600 V, the average deposition rate decreased from 22 to 12.5 nm/s on the face surface, and from 9 to 3 nm/s on the side surface, and the average superficial microhardness decreased from 27 to 20 GPa.

Hard coatings such as TiN, Ti(C,N), TiAlN and others are deposited by most different ion assisted PVD - processes. These coatings are widely used on cutting tools to reduce wear to improve lifetime and to boost productivity when applied at higher feed and speed. Their application on components still lags significantly behind the cutting tools.

Independent from the PVD - process used for the application of the coatings the cycles for the deposition of such coatings last many hours. paragarphFor this reason the development of a high rate arc evaporation process was initiated to increase the deposition rates and to significantly reduce the deposition time. At the same time the established qualities of the coatings had to be maintained.

This development is not limited to the arc evaporation source concerning its: 1) design, 2) geometry and 3) control, but all other relevant peripheral elements of the vacuum coating systems were taken into consideration such as: 1) multiaxis rotatable substrate carrier, 2) arc evaporation power supply, 3) bias power supply and 4) control software. The goals of this development are: 1) to reduce the deposition time, 2) to reduce the cost of production, 3) to reduce the cost per coated part and 4) to engineer an improved plasma surface treatment process.