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

在使用还原工艺气体的炉钎焊过程中，工件表面的脱氧是其与钎焊金属润湿性的前提条件，并决定着焊接接头的成败和质量。虽然用氢或单硅烷还原天然氧化不锈钢表面所需的热力学条件是已知的，但这种反应的动力学到目前为止还没有在原子尺度上进行研究。然而，后者对于对该过程的一般理解是必不可少的，并且是进一步发展铜焊技术的关键。在这种背景下，使用掺氮的单硅烷作为替代氢的一种经济高效和节约资源的方法，具有重大的科学和技术意义。拟议的研究旨在加深对在传送带炉中使用含氢和含工艺气体的单硅烷钎焊不锈钢时表面脱氧的物理化学机理的基本理解。计划中的实验将提供焊接过程中化学反应和表面条件的详细信息，这些信息对于推进低工艺温度、稳健工艺和要求严格的不锈钢规格的无助焊剂焊接工艺至关重要。该项目的起点是可能反应的热力学计算，为此指定并针对实际问题调整氧化层形成的分析传输模型。这些理论考虑通过对传送带熔炉中典型工艺条件的表面反应的现场分析来验证，以获得有关不锈钢表面区在晶体结构、原子配位(键距离、配位数)、化学键和原子扩散方面变化的动力学信息。为此，利用同步辐射进行了时间分辨X射线衍射和X射线吸收光谱测量。在这些测量过程中，真实的炉子条件将在用于X射线实验的高温池中实现，该高温池是为能够模拟实际焊接过程中的条件而定制的。X射线测量是在多特蒙德的Delta同步辐射光源进行的，该光源靠近申请者的家庭机构，弗拉姆教授的工作组在那里运营着两条X射线光束线。计划在同步辐射设施SLS、Soleil和ESRF进行更多的实验。根据获得的衍射数据和在传送带炉中进行的补充焊接实验以及对热处理样品的异地分析，将开发一个物理模型，该模型考虑了焊接过程中表面变化的相关物理化学方面。

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.