Ion Charge State Distributions of Al and Cr in Cathodic Arc Plasmas From Composite Cathodes in Vacuum, Argon, Nitrogen, and Oxygen

Multielement cathodes are increasingly used for advanced coatings, yet most cathodic arc plasma measurements have been reported for pure element cathodes. In this contribution, we measure the charge state distributions of aluminum and chromium ions from Al–Cr composite cathodes of different Al to Cr ratios. The arc discharges are pulsed, with pulse duration of around 300
$\mu{\rm s}$
and currents of 175 A, operated at high vacuum and in gases with a pressure of up to 1.3 Pa of Ar,
${\rm N}_{2}$
, and
${\rm O}_{2}$
. For comparison with literature data, the measurements also included the plasma compositions of discharges using pure Al and Cr cathodes. As expected, the charge distributions are found to be affected by the cathode conditions, the type of gas, and the pressure of the gas into which the arc spot plasma is expanding. Generally, large effects of gas are observed when the pressure exceeded 0.1 Pa, which can be mainly associated with the ions' mean-free path with respect to charge exchange collisions. Differences between ions can be attributed to the energy- and species-dependent charge-exchange cross sections. Considering different cathode compositions, we found that Cr ions tend to have lower charge states from the composite cathodes compared with the pure element cathode, whereas Al ions are relatively unaffected by the cathode composition. Despite the wealth of detailed experimental results, it is difficult to discern trends and rules that could be generalized because measured data involve a convolution of cathode phenomena and gas collisional effects.