Cathode-spot arc coatings: Physics, deposition and heating rates, and some examples

Metallic plasma production in cathode-spot arcs is concentrated in minute areas near the cathode surface which produce fully ionized energetic plasma jets directed away from the cathode surface. Theories explaining cathode spot operation vary from a series of micro-explosions to a quasi-steady state model with acceleration either caused by an ambipolar diffusion induced potential hump or jet-engine like expansion of the hot concentrated plasma away from the minute ionization zone adjacent to the cathode spot into the surrounding vacuum. The plasma jets carry an ion current which is a fixed fraction (7% - 12%) of the arc current. The spatial distribution of the plasma jet peaks normal to the cathode surface. The spatial distribution can be modified by varying the aperture geometries, and the jets can be collimated using magnetic fields. Deposition rates can be estimated using data on the spatial distribution and ion current fraction. The heat flux to the substrate will always include an ionic component, dominated by the kinetic energy of the ions (50 - 150 eV), and the ionization energy of the ions. If the substrate is connected as the arc anode, an additional electron component is added to the energy flux, which depends on the electron temperature and is typically twice as large as the ionic component. If a background gas is present, the plasma jets tend to push the gas away from the cathode to a distance increasing with arc current and decreasing with gas pressure, whereas the ion flux reaching a remote substrate is generally attenuated. Production of oxide and nitride coatings require the simultaneous arrival at the substrate surface of fluxes of the metallic and gaseous substances, though many details of the flow mechanics are not known at present. Coating examples with deposition rates varying from 0.1 to 70 μm s^-1 for remote and close substrates respectively are presented.