Browsing by Author "Rauch, Nicole"

Filter results by typing the first few letters
Now showing 1 - 1 of 1
  • Thumbnail Image
    ItemOpen Access
    High temperature spreading kinetics of metals
    (2005) Rauch, Nicole; Rühle, Manfred (Prof. Dr. Dr. h.c. )
    Due to its great technological importance, a large body of empirical knowledge has been accumulated on the wetting of molten metals and oxides. However, in most of the systems the results are ambiguous and even inconsistent and the theoretical analysis of high-temperature spreading lags far behind the progress made for low-temperature systems. This is in part due to the inherent complexity of high-temperature systems for which the substrates cannot be described as rigid and insoluble and spreading is usually accompanied by adsorption, triple line ridging, interdiffusion or interfacial reactions. In addition the high-temperature experiments are challenging since they require temperature and atmosphere control. In this PhD work a drop transfer setup combined with high speed photography has been used to analyze the spreading of Ag on polished polycrystalline Mo and single crystalline Mo (110) and (100) substrates. The drop transfer setup provides a unique opportunity to systematically analyze isothermal spreading and avoid the complications related to melting and equilibration. The Ag-Mo system has been selected due to its simplicity. There are not interfacial reaction products, the mutual solubilities are very small and the melting point of the substrate is much higher than the experimental temperature. The objective of this work was to unveil the basic phenomena controlling spreading in metal –metal systems. The observed spreading kinetics were compared with current theories of low and high temperature spreading such as a molecular kinetic model and a fluid flow model. Analyses of the data reveal that the molecular model does describe the fastest velocity data well for all the investigated systems. Therefore, the energy which is dissipated during the spreading process is a dissipation at the triple line rather than dissipation due to the viscosity in the liquid. A comparison of the determined free activation energy for wetting of ΔG≈95- 145kJ/mol with literature values allows the statement that the rate determining step seems to be a surface diffusion of the Ag atoms along the triple line. In order to investigate possible ridge formation, due to local atomic diffusion of atoms of the substrate at the triple during the spreading process, grooving experiments of the polycrystalline Mo were performed to calculate the surface diffusities that will control ridge evolution. Ridge formation can influence the spreading kinetics and was reported for several high-temperatures system for example such as glass on Mo and Al on Al2O3. The analyses of this work showed that a ridge formation at the fastest reported wetting velocities was not possible if there is no initial perturbation for a ridge. If there was an initial perturbation for a ridge the ridge had to be much smaller than 1nm in order to be able to move with the liquid font. Therefore ridge formation does not influence the spreading kinetics for the studied system and the chosen conditions. SEM, AFM and TEM investigations of the triple line showed that ridge formation does also not occur at the end of the wetting experiment when the drop is close to equilibrium and the wetting velocity is slow. The spreading was recorded for up to one hour.