Phase formation and decomposition in polycrystalline films

The proposed work deals with phase formation and solid state reactions in rapidly solidified materials with a focus on two component polycrystalline metal films. Sputtered coatings of this type recently gained significant technological importance as coatings for plain bearings in diesel engines. Many other applications nowadays also use polycrystalline films wherever possible and develop towards high vacuum deposition because of cost reduction and simplification of the processes. Additionally many coatings consist of multi - component materials. They combine partners of very divergent physical properties to meet complex profiles defined for their mechanical, electrical or thermal performance. The generally desired high deposition rate and low deposition temperature favor the formation of metastable alloys with new and superior properties. Therefore the long term structure stability is an important issue. This project focuses on sputter deposited polycrystalline films of Aluminum - Tin. They were selected not only because of their practical use as tribological layer, but also because they can serve as a perfect model system to study film formation and evolution of the microstructure in two component films for the following reasons: The alloy is thermodynamically immiscible in the solid. Aluminum and Tin have different crystallographic structures and can selectively be dissolved to study their 3-D arrangement in the film. The low melting points of both materials allows to cover a wide temperature range in respect to the melting temperature during and after deposition.Phase decomposition and long term stability of films deposited at various deposition conditions shall be investigated by Scanning Auger Measurement (SAM), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). The combination of these methods will yield informatoion about the influence of various decomposition processes on the surface chemistry, the surface morphology and the bulk phase distribution in the material.

The present state of materials preparation includes several techniques which generate conditions where solids are formed far away from thermodynamic equilibrium. These include rapid quenching techniques with extremely high cooling rates from the melt as well as deposition techniques where a coating is formed by a direct transition from the vapor to the solid phase. Miscibility and de-composition in these materials are key issues for resistance against severe environments (elevated temperatures, high pressures or corrosive ambient), long time stability and safe mechanical performance. The Project dealt with the sputter deposition of the thermodynamically immiscible material system Aluminium (Al) - Tin (Sn). Magnetron sputtering is a technique where a coating of several m thickness is formed directly from the vapor phase at deposition rates of up to 1 m/min. The energy of the impinging particles can be varied by a proper choice of the deposition parameters. Coatings produced at this parameter-set currently are used as tribological layers of bearings in high performance engines. Therefore an investigation of mechanisms which might lead to coating decomposition is of high interest because of the extreme pressures and temperatures present in the above application. Special emphasis was given to the transport of Sn on inner and outer surfaces of polycrystalline Al. The high mobility of Sn, which was confirmed within this project, is a pathway to the formation of growth instabilities (whiskers), but can also be viewed as beneficial due to the formation of a very thin, but tribologically active wetting layer. Within the project the detailed mechanisms of wetting layer formation were investigated by a joint approach which combines morphological analysis by Scanning Force Microscopy and surface chemical analysis by Scanning Auger Measurements. Ways to suppress or enhance wetting layer formations have been found which might be useful for future tribological applications.