Sintered SiC and SiC/SiC composites are of great interest owing to their notable physical and mechanical properties at high temperature and under irradiation. Therefore, they are materials of choice for power generation (fabrication of heat exchanger, heat exchanger/reactor, fuel cladding for fission reactors, solar absorbers…). Brazing is a promising technique for the assembly of complex structures of SiC-based materials to be used in energy production under severe environments. In this context, the research activity done in the past 30 years at CEA/Liten on the brazing of sintered silicon carbide and composite SiC/SiC is presented by addressing scientific and technological aspects, with some examples of SiC-based joined components.
Generally good wetting, low reactivity and formation of strong interfaces are necessary conditions to successfully braze ceramics. However, brazing SiC-based materials presents two main problems, especially for applications at high temperature requiring brazes with high melting points. First, the strong reactivity between SiC and brazes containing elements such as Cu, Ni, Fe, Co or Ti, leads to degradation of SiC and composites during brazing (dissolution of SiC, formation of brittle compounds). Second, the difference in thermal expansion coefficient between SiC and the braze induces high residual stresses that may be detrimental for the integrity of the assemblies. Therefore, CEA developed* the BraSiC^® process well suited to the brazing of SiC that means achieving good wetting and adherence on SiC, the reactivity of several metals with SiC being suppressed by adding silicon to metals. Thus, more than 20 alloys based on silicon and/or silicides allow brazing of SiC for a wide variety of applications. The mechanical properties of the joints formed depend essentially on the composition of the braze and the thickness of the joints. Note, in the case of composites, that the infiltration of the liquid braze into the composite porosities may occur during brazing. This phenomenon is also considered to control the joining process (proper filling of the joint by the braze, no degradation of SiC fibers).
The methodology used to study and implement brazing requires the following steps: first wetting investigation with the sessile drop technique, second brazing experiments with physico-chemical and mechanical characterizations of the joints, and finally develop reliable mechanical resistance criterion. In some cases, mock-ups were also manufactured and tested. Thus, this approach was used in different projects to develop heat exchangers and nuclear fuel cladding for fission reactors.
*This work was also done in collaboration with Nikos Eustathopoulos and Fiqiri Hodaj from SIMAP laboratory (Université Grenoble Alpes) as PhD advisers.
This project has been founded by the French government under the framework “Plan de relance”. The obtained results have been achieved through a collaborative work between CEA and Framatome.