The composition demixing effect has been found often in alloy coatings deposited by
cathodic arc ion plating using variousalloy
cathode targets. The characteristics of composition demixing phenomena were summarized. Beginning with the ionization zonenear the surface of the cathode target, a physical model in terms of the ions generated in the ionization zone and their movement in theplating room modified by bias electric field was proposed. Based on the concept of electric charge state, the simulation calculation ofthe composition demixing effect was carried out. The percentage of atoms of an element in coating and from the alloy target was de-monstrated by direct comparison. The influences of the composition change of the alloy target and the bias electric field on the compo-sition demixing effect were discussed in detail. It is also proposed that the average charge states of the elements may be used to calcu-late the composition demixing effect and to design the composition of the alloy target.