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

在炉钎焊过程中，使用还原性工艺气体对工件表面进行脱氧是其与钎焊金属润湿性的先决条件，并决定了钎焊接头的成功和质量。虽然用氢或单硅烷还原天然氧化不锈钢表面的必要热力学条件是已知的，但到目前为止，这种反应的动力学还没有在原子尺度上进行研究。然而，后者对于该过程的一般理解是必不可少的，并且是钎焊技术进一步发展的关键。在这种情况下，使用单硅烷掺杂氮作为成本效益和资源节约的氢的替代品，这是最先进的炉钎焊技术，具有重要的科学和技术兴趣。提出的研究旨在发展对表面脱氧的物理化学机制的基本理解，当钎焊不锈钢在传送带炉使用氢和含有单硅烷的工艺气体。计划中的实验预计将提供钎焊过程中化学反应和表面条件的详细信息，这对于提高无钎焊工艺在较低工艺温度、坚固工艺和要求严格的不锈钢规格方面的进步至关重要。项目的出发点是对可能的反应进行热力学计算，并根据实际问题对氧化层形成的解析输运模型进行了规定和调整。这些理论考虑通过在传送带炉中覆盖典型工艺条件的表面反应的现场分析（也是时间分辨的）来验证，以便获得有关不锈钢表面区域在晶体结构、原子配位（键距、配位数）、化学键和原子扩散方面变化的动力学信息。为此，使用同步辐射进行了TR-XRD（时间分辨x射线衍射）和x射线吸收光谱（EXAFS/XANES）测量。这些测量过程中的实际炉况将在x射线实验的高温池中实现，该高温池是定制的，能够模拟实际钎焊过程中的条件。x射线测量是在多特蒙德的DELTA同步加速器光源进行的，该光源位于申请人的家庭机构附近，Frahm教授的工作组在那里操作两条x射线束线。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钎焊实验以及对热处理试样的非原位分析，将建立一个考虑钎焊过程中表面变化的相关物理化学方面的物理模型。

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.