Project Details
Projekt
In-situ investigations of the physicochemical mechanisms of surface activation of stainless steels during heat treatment applying brazing-process-like conditions in reducing process gases
Subject Area
Coating and Surface Technology
Term
from 2015 to 2019
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 268192580
The deoxidation of work piece surfaces in a furnace brazing process using reducing process gases is the pre-condition for its wettability with braze metal and determines the success and the quality of the resulting brazed joints. While the necessary thermodynamic conditions for the reduction of native oxidized stainless steel surfaces with hydrogen or monosilane are known, the kinetics of such reactions has not been investigated up to now on the atomic scale. However, the latter is essential for a general understanding of the process and is key for further developments in brazing technology. In this context, the use of monosilane doped nitrogen as a cost efficient and resource saving alternative to hydrogen, which is state of the art in furnace brazing, is of mayor scientific and technologic interest.The proposed research aims at developing a fundamental understanding of the physicochemical mechanism of surface deoxidation, when brazing stainless steels in a conveyor belt furnace using hydrogen and monosilane containing process gases. The experiments planned are expected to provide detailed information of the chemical reactions and surface conditions during brazing, which are essential for the advancement of fluxless brazing processes with regard to lower process temperatures, robust processes and demanding stainless steel specifications.The starting point of the project are thermodynamic calculations of possible reactions, for which analytical transport models of oxide layer formation are specified and adjusted for the actual problem. These theoretical considerations are validated by in situ analysis of surface reactions - also time resolved - covering typical process conditions in a conveyor belt furnace, in order to obtain information about the kinetics of changes in the surface region of stainless steels with respect to crystal structure, atomic coordination (bond distances, coordination numbers), chemical bonding and atomic diffusion. For this purpose TR-XRD (Time Resolved X-ray Diffraction) and X-ray absorption spectroscopy (EXAFS/XANES) measurements using synchrotron radiation are performed. Realistic furnace conditions during these measurements will be realized in a high temperature cell for X-ray experiments, which is custom-made to be able to mimic the conditions in the actual brazing process.The X-ray measurements are performed at the DELTA synchrotron light source in Dortmund, which is located close to the home institutions of the applicants, and the working group of Prof. Frahm operates two X-ray beamlines there. Additional experiments are planned at the Synchrotron Radiation Facilities SLS, SOLEIL and ESRF.Based on the diffraction data obtained and complementary brazing experiments in a conveyor belt furnace with ex-situ analysis of the heat treated specimen a physical model will be developed, which takes into account the relevant physicochemical aspects of surface changes in brazing processes.
DFG Programme
Research Grants
Co-Investigators
Professor Dr. Dirk Lützenkirchen-Hecht; Professor Dr.-Ing. Kai Möhwald
