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for the arc behavior part of this study. Arc current, Iarc =
25–200 A d.c. was applied between the cathodes and the
grounded anode. Each cathode had its own welding power
supply and trigger mechanism. The cathode-spot-produced
plasma jet entered the deposition chamber via 54-mm
diameter holes in the anode, coaxial with each cathode. An
axial d.c. magnetic field, B = 12 mT, was produced by
three magnetic coils connected in the same direction, and
positioned co-axially with the system axis. The axial
magnetic field was intended to confine the cathode spot
motion on the front cathode face [27, 28] as well as to
guide the plasma flow from the cathode spots via the anode
aperture to the ion current probe. The front surfaces of the
cathodes were situated midway between coils 1 and 2.
For most of the coating deposition study, a 1/8 torus
magnetic macroparticle filter was inserted between the
plasma gun and the vacuum chamber (Fig. 1c). A d.c.
magnetic field of B = 12 mT was applied (by five coils) to
the straight and curved parts of the duct. An ion probe or
substrate holder was positioned at an axial distance of
150 mm from the end flange to which the plasma gun or
the 1/8 torus filter was connected, i.e., approximately
midway between coils 2 and 3 in Fig. 1b, and between
coils 4 and 5 in Fig. 1c.
In both configurations, before arc ignition, the vacuum
chamber was pumped down to an initial residual pressurelower than 0.01 Pa. The arcs were operated in vacuum
(background pressure less than 0.01 Pa) and in an oxygen
or oxygen ? argon background pressure of P = 0.1–
1.3 Pa. After each experiment with oxygen, the cathode
was cleaned of oxides by igniting an arc in vacuum [29].
The arc parameters (arc current and voltage, ion current,
etc.) were continuously recorded [29, 30].
The total ion saturation current at the probe, Ip, was
measured with a 130-mm diameter flat disk probe centered
on the duct axis and oriented normal to the plasma flux.
A negative d.c. bias voltage, Vb =-50 V was applied to
the probe with respect to the grounded anode to ensure Ip
saturation, while minimizing the probability of igniting
cathode spots on the probe [28].
Al2O3–ZrO2 coatingswere deposited on siliconwafer and
WC–Co cutting insert substrates. The cutting insert surfaces
were polished to a mirror-like finish, and the substrates werecleaned with alcohol before deposition. Prior to deposition,
the substratewas heated to the desired substrate temperature,
Ts, with a 1-kW halogen lamp placed within the substrate
holder [31]. Ts was measured using a thermocouple situated
beneath the coated sample and regulated using a feedback
control system [31]. During deposition, Ts sometimes
increased due to the surface bombardment by energetic ion
flux; however, this increase was not more than 20. The
coatings were deposited while simultaneously operating the
Al and Zr cathodes with Iarc = 75 and 100 A, respectively,
in an Ar ? O2 mixture with total pressure PAr=O2
= 1.06 Pa
(PAr = 0.26 Pa, PO2
= 0.80 Pa). In this research study, Ts,
was varied between 50 and 500 C and Vb between -50 and
-200 V. The deposition time was 360 s. After deposition,
the coating thickness, t, was measured using an Alpha-step
profilometer. The coating’s electrical resistivity, q, was
measured by applying a silver paste on a controlled area of
the coatings surface. q was then measured by ohmmeter
between the silver paste and the Si substrate. The substrate
q was neglected.
The coating’s cross-sectional morphology was studied
using scanning electron microscopy (SEM). The coating’s
composition was analyzed using SEM combined with