Papers by Author: Harald Behrens

Abstract: Residual stress measurements directly in the coated cutting edge are not possible with X-ray diffraction (XRD) due to the diameter of the X-ray beam. On the other hand, Raman microscopy enables measurements on the micrometer scale. Parameter variations in the PVD process were used to provide different residual stress states in (Al,Ti)N coatings on carbide cutting tools. They were examined by XRD in regions that can be reliably measured. The same area was then examined by Raman microscopy to determine the relationship of Raman peaks to the residual stress. Local high-resolution Raman measurements were then taken at the cutting edge and further influences on the Raman peak position besides residual stresses were excluded. In order to analyze the relationship between Raman peak shift and residual stress state, measurements were performed during a bending load. Finally, an outlook on further investigations is given.

Abstract: Diffusion and solubility of sulphur have important effects on the degassing of silicate melts. Both properties are closely related to the structural incorporation of sulphur in the melt. Depending on the oxygen fugacity, sulphur can be present as sulphide (S2-), sulphite (S4+) or sulphate (S6+). Sulphates play an important role in the industrial production of glasses especially in the fining process. The decomposition products of sulphate amass in bubbles which ascend and homogenize the melt. Structural incorporation of sulphur in glasses is studied by XANES (X-ray Absorption Near Edge Spectroscopy). Diffusion of sulphur is investigated in simple silicate systems using the diffusion couple technique. First diffusion profiles were measured in sodium trisilicate glasses by electron microprobe. The results indicate that sulphur diffusivity in high temperature melts is close to the Eyring diffusivity calculated from viscosity data.

Abstract: Hydrous silicate glasses with different compositions (LiAlSi4O10, CaMgSi2O6, CaMgSi2O6 - CaAl2Si2O8 and (Ca,Ba,Sr)O – SiO2) containing up to 3 wt% H2O were synthesized in platinum capsules at high temperature and pressures from 100 to 500 MPa in an internally heated gas pressure vessel. The water content of the glasses was analyzed by Karl-Fischer titration. Using infrared microspectroscopy the homogeneity of water distribution in the samples was checked and concentrations of OH- groups and H2O molecules were estimated. Frequency-dependent ionic conductivity of the glasses was investigated using impedance spectroscopy. It is shown that the proton conductivity is usually orders of magnitude lower than the partial conductivity of alkalis in silicate glasses. Proton conductivity in hydrous barium silicate glass was found to be more than one order of magnitude higher than in an aluminosilicate glass, supporting that the concentration of non-bridging oxygen is crucial for proton conduction in glasses.

Abstract: Knowledge on diffusion of water in glasses and melts is important for glass sciences as well as for Earth sciences. The ranges of water contents differ widely in both research domains. Industrial glass contain typically less than 0.1 wt% H2O whereas natural glasses may even contain up to 10 wt% H2O. Dissolved water strongly modifies physical and chemical properties of the melts and, hence, water diffusion studies at low water contents often cannot be transferred to high water contents and vice versa. Pressure effect on water diffusion, on the other hand, is small so that high pressure data can be applied also at ambient pressure without significant error. At low water contents the assumption of constant water diffusivity is often justified whereas at high water contents the H2O diffusivity increases strongly (often linearly or exponentially) with water content. Combining experimental data over a wide range of water contents allows getting deeper understanding of the mechanisms of water diffusion and of the effect of dissolved water on melt structure. In this paper an overview is given on experimental investigations on water diffusivity in aluminosilicate and silicate systems. Effects of base compositions and water content on water diffusivities are discussed. New experimental results for water diffusion in soda lime silica glass, float glass and borosilicate glass are presented.