Microstructure simulation of reactive air brazing
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Ceramic oxygen-conducting membranes can selectively separate oxygen from the air at temperatures above 750 °C and an applied oxygen partial pressure gradient. If this oxygen is used as a combustion gas, e.g. in the context of oxyfuel combustion, it is possible to separate and store the resulting CO2 or to convert it chemically. For the industrial implementation of this membrane technology, a reliable, gas-tight and high-temperature-stable joining of the ceramic membranes to metal components must be developed.
In previous projects, different brazing processes for the ceramic Ba0.5Sr0.5Co0.8Fe0.2O3- d (BSCF) could already be tested. Only reactive brazing (RAB - Reactive Air Brazing) of BSCF using Ag-CuO solders to the austenitic steel X15CrNiSi25-21 can potentially meet the high requirements. However, new phases and micro-cracks formed at the ceramic interface during brazing, which deteriorate the mechanical properties.
Now, the follow-up project will simulate how and why this reaction layer forms. The findings will be used to subsequently avoid the reaction layers or to design them advantageously by adapting the process.
Development of a methodology for the quantitative prediction of damage in the diffusion layers of reactive brazed BSCF steel composites
A preliminary thermodynamic database for the solder and BSCF ceramics containing the elements Ag, Ba, Co, Cu, Fe, Sr, and O was constructed. Initial calculations for the reaction between the solder and the BSCF ceramic could be performed (see Figure 1).Copyright: © IWM
Figure 1: Calculated isothermal section a) at 970 °C, 0.21 atm O2 in Ag-Co-Cu-O and b) at 1000 °C in the Ba-Fe-O system for the BaO-FeO-Fe2O3 region.
- The chemical composition and phase structure of the grain triplets in reactively soldered BSCF have been identified. These are three fcc phases (BSCF, Co3O4 and CoO) and the monoclinic CuO phase with edge solubility for Co/Cu (see publication 1).
- The grain coarsening in BSCF in the reaction zone to solder and the observation of triple point phases can be explained by the "wetting" of the grain boundaries by a thin (nanometer scale) melt film of liquid solder. The physics of wetting in this case corresponds to the phenomenon of "grain boundary premelting".
- Phase field simulations on the grain scale reproduce the accelerated grain coarsening with the formation of stretched grains in the BSCF in the region of the reaction zone to the solder.
Figure 2: Comparative representation of elongated grain coarsening by a) optical microscopy and b) EBSD image of a wetting sample BSCF with Ag-14CuO and c) by phase field simulation with varied grain boundary mobility.
- Thermal expansion coefficients of the microscopic phases in the triple point of reactive soldered BSCF ceramics could be determined by dilatometry of macroscopic powder replicas. The process sequence of pressing powders, sintering, homogenization at a suitable temperature according to the phase diagram, quenching, phase control by XRD and dilatometry was suitable for this purpose (publication ongoing).
- Phase-specific model parameters were determined using inverse multiscale models. The fracture stresses of the pure BSCFs are overestimated by the inverse modeling, as indicated by the extreme value analysis of the pores in the BSCFs. The highly varying internal structure of the triple point phase results in a large number of possible thermal expansion coefficients and elastic constants as a function of temperature. These model parameters determine the quantitative level of local stress in the form of residual stresses. In combination with the fracture stresses of the BSCF, they determine the crack initiation in the model.
- The micromechanical model for the simulation of the process-related residual stresses shows high residual stresses at the BSCF-TPP phase boundary (see Figure 3).
Top: RVE simulation determined residual stresses II. Type of RVEs A, B and C for different parameters of the triple point phase ((a) single phase Co3O4 and b) multiphase TPP in Figure 5a)) over time and cooling curves;
Bottom: Exemplary stress plot for RVE B with parameters of the TPP single phase Co3O4.
- The micromechanical simulation showed that for an advantageous design of the reaction layers (1) the size of the triple point phases should be small, (2) the shape should be spherical and (3) as little Co3O4 as possible should be formed.
- In wetting experiments, no microstructure coarsening at all was observed below a CuO content of 1-3 mol%. Presumably, CuO-poor melts can no longer sufficiently wet the grain boundaries, the triple point phases do not form and do not put the BSCF under tensile stress. Therefore, advantageous process pa-rameters are: t Löt = 6 min, TL Löt =950-970 °C, CuO content x in mol%: 1 ≤ x <3.
- Various diffusion barriers to reduce chromium diffusion from the me-tallic joining partner into the ceramic were applied to the metallic joining partner before brazing. This significantly increased the strength after 1000 h of aging at 850 °C. The coating concepts are probably transferable to other reactive-brazed ceramic-metal composites and can thus make an important contribution to the industrial acceptance of reactive brazing (see Figure 4).
- By applying suitable diffusion barriers, the decomposition of the BSCF ceramic due to chromium poisoning can be avoided. The long-term stable microstructure of the joined BSCF ceramic is an important prerequisite for its use as an oxygen transport membrane (see Cr mapping in Figure 4).
Figure 4: Increase of composite strength after 1000 h aging at 850 °C by pre-oxidation of the metallic joining partner with simultaneous suppression of chromium poisoning in the BSCF.
- L.C. Ehle, S. Richter, S. Herzog, C. Broeckmann, J. Mayer, Identification of Cu-Co-oxide phases of reactive air brazed Ba0.5Sr0.5Co0.8Fe0.2O3-δ - Ag-14CuO joints by EBSD, EPMA and TEM diffraction, IOP Conf. Ser.: Mater. Sci. Eng. 891 (2020) 12012.
- S. Herzog, G. Boussinot, A. Kaletsch, M. Apel, C. Broeckmann, Microstructure coarsening in Ba0.5Sr0.5Co0.8Fe0.2O3-δ during reactive air brazing, Journal of the European Ceramic Society 42, Issue 13 (2022), pp. 5842-5850.
- S. Herzog, A. Kaletsch, C. Broeckmann, Reduced strength degradation of reactive air brazed BSCF membranes by pre-oxidation of metallic components, J. Mat.Sci.Eng. A, 857 (2022) https://doi.org/10.1016/j.msea.2022.143993
The joint project with access. E.V. with the number 392944287 was funded by the German Research Foundation (DFG).