Raymond L. Boxman

Macroparticle contamination in cathodic arc coatings: generation, transport and control

The cathode spot of a vacuum arc generates a spray of liquid droplets directed almost parallel to the cathode surface as well as a highly ionized plasma jet directed normal to the cathode surface. Theories include droplet ejection by the reaction force of back-streaming ions on the underlying microscopic liquid pool and formation from explosive debris. The droplets have an exponentially decreasing size distribution and velocities ranging from 10 to 800 m s-1. During their motion in the arc, the macroparticles can be further accelerated and deflected, obtain a negative charge, be heated to temperatures of around 2000 °C and evaporate. The macroparticle mass emission rate from the cathode increases with increasing arc current and average cathode surface temperature and decreases with increasing cathode material melting temperature. Cathodes with gaseous surface layers have less macroparticle erosion than clean cathodes. Droplet production can be reduced by maintaining as low a temperature as possible on the cathode surface near the cathode spots by providing effective cooling, by operating at low cathode current densities, by using magnetic fields to provide for directed rapid cathode spot movement and, in reactive deposition, by operating a poisoned cathode. Macroparticle inclusions can be reduced by substrate biasing and by concentrating the plasma flow with magnetic fields and can be eliminated completely by using a curved magnetic plasma duct.

Recent progress in filtered vacuum arc deposition

Abstract During this decade significant advances have been made both in the understanding and implementation of filtered vacuum are deposition. Rigid rotor models have been analyzed statistically, and new models which treat the mutual influence of the electrons and ions on each other self-consistently, take into account the centrifugal force on the ions, and take into consideration collisions, have been formulated. It was shown that the plasma transport efficiency is limited by drifts caused by the centrifugal force and by the electric field generated by charge separation in the plasma. For a range of magnetic fields strengths for which the ions are not magnetized, i.e., confined to a Larmor radius less than the duct radius, the transport efficiency for Cu plasma is about 10%, and depends only weakly on the magnetic field strength. Increased transmission is found when the ions are magnetized, reaching about 50% for a 36–60 mT field in typical configurations. The plasma transport efficiency and spatial distribution has been measured over a large parameter range, and correlated with the various theories. The plasma beam may be approximated as a Gaussian distribution which is displaced in the B × G direction, where G is in the direction of the centrifugal force, while a displacement in the plane of symmetry is surprisingly found in the − G direction. The total convected ion current decreases exponentially with distance from the toroidal filter entrance. Macroparticle transport within the magnetic filter has been analyzed, and it has been shown that electrostatic reflection from the walls can occur if the magnetic field is weak. Filtered arc sources with improved throughput performance and novel geometries have been built, and are now available commercially. The range of coatings deposited with FVAD has been expanded to include metals, oxides, and nitrides, as well as diamond-like carbon. In several cases, coatings having the highest quality reported in the literature have been fabricated with the FVAD technique, and one commercial application has been reported.

2D expansion of the low-density interelectrode vacuum arc plasma jet in an axial magnetic field

The two-dimensional expansion of a current carrying plasma jet in the interelectrode gap of a vacuum arc with an axial magnetic field is analysed by finding the steady state solution of the fully ionized plasma in the hydrodynamic approximation. Two models are presented: (1) expansion into a duct with known geometry and (2) free jet expansion. The first approach models the plasma jet expansion with a conical shape. In the second model the geometric position of the free boundary was determined by the free hydrodynamic jet expansion into vacuum without and with the influence of a magnetic field. In the case of plasma expanding into a conical guide, it was found that the flow field in the near-axis region does not depend on the cone angle for cone angles . The radial velocity becomes comparable to the axial velocity due to the expansion, depending on the cone angle and the initial axial velocity. A model of the free boundary plasma expansion was developed, based on the jet-like (i.e. axial velocity larger than the radial velocity) plasma flow in the vacuum arc near the cathode spot. The free jet boundary was calculated by solving the equations for the normal and tangential velocity components at the free boundary. It was found that the plasma jet had a conical shape, and for axial distances 3 - 4 times greater than the initial jet radius, the radial velocity becomes comparable with the axial velocity if no magnetic field is imposed. Imposition of a magnetic field reduces the radial component of the plasma velocity. The streamline angle is about for a 0.001 T magnetic field and about for a 0.01 T magnetic field. The plasma remains quasi-neutral in all regions except in the space charge boundary layer, where an outward directed electric field appears for low magnetic fields, and an inward directed field is present for strong magnetic fields.

The interaction between plasma and macroparticles in a multi‐cathode‐spot vacuum arc

The interaction between the interelectrode plasma and macroparticles (droplets) produced by a multitude of cathode spots in a vacuum arc between Cu electrodes is analyzed, using previous experimental measurements of the macroparticle size distribution and erosion rate and a flowing plasma model. The effect of the plasma on the macroparticles is considered by treating the macroparticles as floating probes and calculating the particle, momentum, and energy fluxes to them. It is found that slow macroparticles are significantly deflected form their original trajectories owing to ion bombardment, and that steady‐state macroparticle temperatures of 2000–2600 K are obtained from the balance of the energy influx (primarily from ion bombardment) with the evaporative outflux. The effect of the macroparticles on the plasma is considered by calculating the production rate of neutral atoms and ions originating from macroparticle evaporation, by examining the possibility of occlusion of the discharge path by macroparti...

Principles and applications of vacuum arc coatings

The development of vacuum arc coatings, commencing a century ago with Thomas Edison and continuing through the recent development of industrial-scale batch coating machines, is reviewed. Most of the work exploited the high ionization, plasma production rate, and ion energy intrinsic in the cathode spot arc to deposit metals, diamondlike carbon Si, and, with the presence of a background gas, various ceramics. Deposition rates of up to 400 mu m/s were achieved in pulsed operation. Various techniques were developed to control the motion and location of the cathode spots and to reduce the macroparticle contamination of the coatings. Hot electrode vacuum arc modes were investigated recently as well. Simple models for the plasma transport to the substrate based on known properties of the cathode spot plasma jets are presented, as well as a description of current industrial practice. >

Magnetic constriction effects in high‐current vacuum arcs prior to the release of anode vapor

A model is developed describing the interelectrode plasma generated by a multi‐cathode‐spot cathode as a conducting fluid flowing from the cathode to the anode. The model is analyzed numerically for a sample physical situtation consisting of a 3‐kA Cu vapor arc between 25‐mm‐diam electrodes separated by 9 mm, in which case the mass density and fluid velocity are found to be 5×−5 kg/m3 and 7×103 m/s, respectively. The fluid flow is analyzed and a constriction caused by the magnetic pinch force is found to develop near the anode. A constriction in the current flow is also calculated, caused primarily by the Hall current.

Twenty-five years of progress in vacuum arc research and utilization

Progress in understanding and applying vacuum arcs is reviewed. Laser diagnostics have demonstrated the existence of micron-sized regions in the cathode spot plasma having electron densities exceeding 10/sup 26/ m/sup -3/. The expanding plasma produces a highly ionized jet whose ions typically have charge states of 1-3 and energies of 50-150 eV. Gas dynamic and explosive emission models have been formulated to explain cathode spot operation. In cases where the arc is constricted at the anode, forming an anode spot, or the anode is thermally isolated, forming a hot anode vacuum arc, material emitted from the anode may dominate the interelectrode plasma. Evaporation from liquid droplets may also provide a substantial component of the plasma, and the presence of these droplets can have deleterious consequences in applications. The vacuum arc has been extensively utilized as a plasma source, particularly for the deposition of protective coatings and thin films, and as a switching medium in electrical distribution circuit breakers.

Theoretical study of plasma expansion in a magnetic field in a disk anode vacuum arc

The low-density plasma flow in an axial magnetic field to a disk-shaped anode in a vacuum arc was studied theoretically using a two-dimensional model. The plasma expansion was modeled using the sourceless steady-state hydrodynamic equations, where the free boundary of the plasma was determined by a self-consistent solution of the gas-dynamic and electrical current equations. The anode was modeled as a current and plasma collector, which does not influence the plasma flow field. Magnetic forces from both the azimuthal self-magnetic field, and the imposed axial magnetic field were taken into account. It was found that the self-magnetic field does not substantially influence either the plasma jet shape, density, velocity, or the current density distribution for arc currents I⩽200 A. On the other hand, the plasma jet angle (α0) at the starting plane and the radial plasma density gradient force in the expansion region do have a strong influence on the plasma and current flow. The mass and current flow in a 500...

Interferometric measurement of electron and vapor densities in a high‐current vacuum arc

Electron and vapor densities in a 0.4–4.0‐kA copper vapor (vacuum) arc were investigated by means of optical interferometry. Nonsimultaneous measurements were made with a visible wavelength (0.63 μm) and an infrared wavelength (10.6 μm) to separate the electron and vapor contributions. The electron contribution to the index of refraction was found to dominate at both wavelengths. An upper limit to the ratio of copper vapor density to electron density of 2.8 could be set as a result of the 0.63‐μm measurement. The integral of the electron density over the optical pathlength was measured at various lateral positions of the arc by means of the 10.6‐μm interferometer; the Abel transformation was applied to these results to obtain the radial profile of the electron density. A diffuse electron density profile is observed, with the axial densities ranging from 1 × 1014/cm3 at 0.4 kA to 2 × 1015/cm3 at 4 kA. At the transition from the diffuse or quiescent mode to the high‐voltage or constricted mode, which occurr...

Model of the anode region in a uniform multi‐cathode‐spot vacuum arc

A model is developed for the anode region of a uniform multi‐cathode‐spot vacuum arc. A first‐order approach is used, in which the plasma up until the anode sheath is assumed to be produced solely by a multitude of cathode spots and is described by a zero‐order plasma model previously developed, and the influence of the anodic emissions on this plasma is assumed initially to be negligible. Calculations of the electron mean‐free‐path and the Debye length indicate that the anode sheath may be modelled as collisionless. The anode potential is calculated by imposing the requirement of current continuity, and it is found that the overabundant supply of random electron current forces the anode to assume a negative potential with respect to the adjacent neutral plasma with a magnitude of approximately (1/3)(kTe/e)(vT/vd), where Te is the electron temperature, vd is the electron drift velocity, and the thermal velocity vT is defined by (kTe/2πme)1/2. The magnitude of the electric field at the anode surface is als...