  Al$_{x}$Cr$_{1-x}$ composite cathodes with Al contents of x = 0.75, 0.5 and
0.25 were exposed to cathodic arc plasmas in Ar, N$_2$ and O$_2$ atmospheres
and their erosion behaviour was studied. Cross-sectional analysis of the
elemental distribution of the near-surface zone in the cathodes by scanning
electron microscopy revealed the formation of a modified layer for all cathodes
and atmospheres. Due to intermixing of Al and Cr in the heat-affected zone,
intermetallic Al-Cr phases formed as evidenced by X-ray diffraction analysis.
Cathode poisoning effects in the reactive N$_2$ and O$_2$ atmospheres were
non-uniform as a result of the applied magnetic field configuration. With the
exception of oxide islands on Al-rich cathodes, reactive layers were absent in
the circular erosion zone, while nitrides and oxides formed in the less eroded
centre region of the cathodes.
