Development of a methodology for the quantitative prediction of the damage in to the diffusion layers of reactive air brazed BSCF-Steel-Joints

钙钛矿型陶瓷如Ba0,5Sr0,5Co0,8Fe0,2O3-d （BSCF）是一种很有前途的氧传输膜材料，可用于氧气与空气的分离。膜技术在高温过程中供氧比其他空气分离技术更有效。为了在800°C左右的工作温度下可靠地工作，需要陶瓷和金属之间的气密连接。反应空气钎焊（RAB）被认为是一种适合这种材料组合的技术。由于材料在真空中的热力学不稳定性，其他钎焊技术很难在BSCF的情况下实现。在RAB的情况下，由于内部构建的CuO和BSCF之间的界面反应，银铜钎焊的润湿发生。这些反应是润湿所必需的。钎焊合金中cuo含量越高，润湿性越好，但同时陶瓷的显微组织也发生了变化，表现为晶粒三点处出现cu -co -氧化物并伴有微裂纹。此外，可以观察到钢和钎焊之间的反应层。力学试验表明，两种扩散层均降低了接头的力学强度。目的是开发一种定量预测反应空气钎焊陶瓷钢接头扩散层内部损伤的方法。因此，第一步应该是系统的热力学建模。由于陶瓷/钎焊合金/钢的复杂性，不可能对整个系统进行完整的建模，因此建模分为四个区域。有了这些数据，就应该对反应层和钎焊中的微观组织（晶粒结构、相的划分和分布）进行结构分析——在其时间发展上非常接近实际。计算得到的结构为预测接头质量和接头损伤发展的细观力学模拟提供了依据。因此，将特别考虑BSCF与银钎焊的渗透以产生BSCF界面质量的变化。通过仿真可以模拟钎焊接头、不同成分的钎焊合金和不同工艺参数之间的相互作用。下面的微观力学模拟是基于这些模拟的微观结构以及实验的二维和三维结构（3D EBSD）。微观力学模拟的目的是预测钎焊过程中损伤的发展。因此，可以针对特殊情况确定最佳钎焊工艺参数（Cu含量、温度、工艺）。该方法应适用于其他钎焊接头。例如，其他钙钛矿型陶瓷（如LSCF）也会出现类似的反应。采用这种方法，钎焊过程中的工艺参数可以直接与接头的机械质量相关。

Perovskite type ceramics like Ba0,5Sr0,5Co0,8Fe0,2O3-d (BSCF) are promising materials for the use as oxygen transport membranes (OTM) for oxygen separation from air. Supplying oxygen by membrane technology within a high temperature process can be more efficiently than other air separation techniques. For reliable working of the membranes at working temperatures of around 800°C, a gas tight joint between ceramic and metals is required. Reactive air brazing (RAB) was identified as a suitable technique for this material combination. Other brazing techniques are difficult to realize in the case of BSCF due to a thermodynamic instability of the material in vacuum. In the case of RAB, wetting of the silver copper braze occurs due to interface reactions between insito built CuO and BSCF. These reactions are required for wetting. The higher the CuO-content in the braze alloy is, the better the wetting is, although, at the same time a change of microstructure in the ceramic is visible, characterized by Cu-Co-Oxides at the grain triple points combined with micro cracks. Additionally, a reaction layer between steel and braze can be observed. Mechanical tests showed that both diffusion layers decrease the mechanical strength of the joints. The object is the development of a methodology for a quantitative prediction of damage inside of the diffusion layers of reactive air brazed ceramic steel joints.Hence, the first step should be a thermodynamic modelling of the system. Because an entire modelling of the overall system is not possible for ceramic / braze alloy / steel due to the complexity, the modelling is divided into four zones. With this data, structural analyses of the microstructure (grain structure, phase portion division and distribution) should be done in the reaction layer as well as in the braze - in its temporal development very near to reality. The calculated structures are a basis for the micromechanical simulation to predict the joint qualities and the damage development in the joints. Thereby, the infiltration of BSCF with the silver braze to generated variations of the qualities of the BSCF interface will be considered in particular. The interactions between the joining partners, the braze alloys of different composition and the different process parameters can be modelled by the simulation. The following micromechanical simulation is based on these simulated microstructures as well as on experimentally 2D and 3D structures (3D EBSD). The aim of the micromechanical simulation is the prediction of the damage development during brazing. Therefore, optimum brazing process parameters (Cu content, temperature, process) can be determined for the special case. The methodology should be transferable on other brazed joints. For example, similar reactions appear with other perovskite type ceramics (e.g., LSCF). With such a methodology, process parameters during brazing can be correlated directly with the mechanical qualities of the joints.