Vacuum arc plasma from intermetallic and composite NbAl cathodes

Thin films and hard coatings synthesised by means of physical vapour deposition are nowadays applied in many different areas. A commonly used deposition technique is cathodic arc evaporation as it allows the synthesis of a wide variety of materials including metallic films, but also nitrides, carbides and oxides if a reactive background gas is used. The cathodes used for the deposition of these thin films or hard coatings typically contain 2 or 3 elements, in some cases even more. Investigations on the arc plasma properties, on the other hand, mainly focussed on single-element cathodes and only a few studies are available on the properties of multi-element cathodes. The present project aims to study intermetallic and composite cathodes in the system NbAl in order to find correlations between the plasma and material properties. Of particular interest is the so-called cohesive energy rule that was established for single-element cathodes including most of the metals in the periodic table. The binding energy or cohesive energy of the atoms in the cathode could be linked to properties of the cathodic arc plasma, e.g. mean charge state, electron temperature and ion energy. The study on the NbAl cathodes is designed to determine in which way the cohesive energy rule can be expanded to multi- element cathodes. Theoretical calculations of the cohesive energy of the intermetallic NbAl phases will be directly compared with the plasma properties when these cathodes are used in an arc discharge. Composite NbAl cathodes, where the phase configuration on the surface is altered due to the exposure to the plasma, will be investigated and the obtained correlations from the intermetallic cathodes will be applied. Further, it can be expected that the established correlations are readily adaptable for other material systems. In general, a sound understanding of the interdependence between the material properties of multi-element cathodes and the properties of the arc plasma using such cathodes is necessary for further improving the design of the cathodes and the arc sources with the final goal of synthesising new thin film or coating materials with new or improved properties.

The outcome of the current project provides a comprehensive overview of the cathodic arc plasma properties from NbAl dual-element cathodes. Even though such multi-element cathodes are frequently employed for the synthesis of thin films and coatings, the understanding of these processes is still limited. In particular changes in the phase composition occurring on the surface of the eroded composite cathodes due to the exposure to the plasma might alter the plasma properties. Such phase changes mainly include the formation of intermetallic phases within a modified layer on the surface of composite cathodes where the elements are typically present well separated. A detailed analysis of the plasma from composite and purely intermetallic NbAl cathodes revealed non-linear change in the plasma properties like discharge voltage, ion energy and mean charge state with changes of the cathode composition. A linear relation was expected based on calculations of the cohesive energies of intermetallic NbAl phases using ab initio methods and applying the so-called "cohesive energy rule" which was established for single-element cathodes correlating the cohesive energy of the cathode to its plasma properties. Therefore, the cohesive energy rule cannot be simply extended from single-element to multi-element arc cathodes, but other effects affecting the energy balance of the arc discharge need to be taken into account. The energy balance might, for example, be altered by the exothermic reaction of Nb and Al forming the intermetallic phases NbAl3 or Nb2 Al. This means that the phase composition and the formation processes of the modified layer on the surface of eroded multi-element cathodes play a more defining role for their arc plasma properties than it is the case for single-element cathodes. In addition, the temporal evolution of the plasma properties during pulsed arc discharges were studied in great detail applying a recently developed experimental setup allowing for a high time resolution. The obtained results show the importance of charge exchange collisions occurring after ignition of the arc plasma on the general plasma properties in vacuum conditions and in the presence of a background gas like Ar as it was studied in the present case. In general, the results of the project contribute to a better understanding of the cathodic arc plasma from multi-element cathodes aiding improvements in the design of such cathodes and the arc sources with the final goal of synthesising new thin film or coating materials with new or improved properties.