Cathodic arc plasma of AlCr in reactive atmospheres

Hard coatings synthesised by means of physical vapour deposition are nowadays applied in many different areas comprising the protection of cutting and forming tools as well as engineering components in various forms. A commonly used deposition technique is cathodic arc-evaporation as it allows for synthesising a huge variety of materials including metallic coatings, but also nitrides, carbides and oxides if a reactive background gas is used. In the last years, a major progress has been made by alloying well established coatings like TiN or CrN with other elements in order to improve their properties. The cathodes used for the deposition of such coatings typically contain 2 or 3 elements, in some cases even more. Investigations on the arc plasma properties, on the other hand, mainly focused on single-element cathodes in vacuum conditions. Only a few studies are available on the properties of multi-element cathodes or the influence of reactive gases on the plasma. The present project aims to study dual- element cathodes in the system AlCr since they are used to deposit the state-of-the-art hard coatings AlCrN and AlCrO. These studies will pioneer the understanding of the influence of reactive background gases on the cathode erosion, plasma properties and thin film growth. By correlating the results obtained from cathode, plasma and coating investigations, a comprehensive scientific basis of physical vapour deposition processes of multi-element coatings can be developed. Such information is necessary for the general scientific chain synthesis-structure- properties of coatings grown by plasma-based deposition processes like cathodic arc-evaporation which, in turn, enables a more efficient development of new coating materials with improved properties.

The outcome of the current project provides a comprehensive overview on the cathodic arc plasma properties from AlCr dual-element cathodes in different gas atmospheres, namely Ar, N2 and O2 . Even though such plasmas are frequently employed for the synthesis of thin films or coatings, the understanding of these processes is limited. Most of the insights originate from investigations in vacuum conditions. The new results clearly show that the presence of a background gas strongly influences the plasma properties as the ion charge states and the ion energies are reduced with increasing gas pressure. This affects the growth conditions of the thin films which might not be as energetic as widely assumed, especially at higher gas pressures. The typically applied dc mode of the cathodic arc plasma has been studied in detail to serve as a reference for the synthesis of AlCrN, AlCrO or similar thin films.Another important aspect was the use of dual-element AlCr cathodes vs. single-element Al and Cr cathodes. The influence of the cathode composition on the plasma properties is less pronounced than the influence of the background gas, but a sound understanding is necessary to further improve the deposition process. In particular, phase changes on the cathode surface due to intermixing of Al and Cr and due to reactions with the background gas in the case of N2 and O2 are of interest. The obtained results will serve as a basis for future studies to reveal more details about the interrelation between the material properties of metallic cathodes and the plasma properties in the arc plasma from these cathodes. With such knowledge an economic synthesis of thin films and coatings with optimised properties is enabled.