This work was carried out under the supervision of Céline PASCAL (PM), Fiqiri HODAJ (TOP), and Anni ANTONI (TOP).The thesis was conducted in collaboration with Schneider Electric, under the supervision of Sophie Roure and Corinne Durand.
The electrical contact materials investigated in this thesis are based on a multilayer assembly consisting of an Ag-C composite tip and a Cu-P brazing alloy, joined to a solid copper substrate that has been pre-coated with a thin layer of silver (Ag-C | Cu-P | Ag | Cu). This assembly is produced using a resistance sintering-joining process, which combines the sintering of the Ag‑C | Cu-P composite and its bonding to the substrate into a single operation, driven by Joule heating generated through the simultaneous application of electric current and mechanical force.
Initially, the influence of process parameters (current intensity, cycle duration, and applied force) was studied through microstructural characterization and shear testing performed on prototype assemblies. Experimental observations were complemented by numerical simulations of the process using COMSOL.
Subsequently, the behavior of these assemblies under thermal stress was investigated using two complementary approaches. On one hand, tests under extreme conditions were conducted on industrial assemblies integrated into circuit breakers, subjecting them to current intensities far exceeding nominal operating conditions. On the other hand, thermal aging treatments were carried out to evaluate the system's microstructural evolution. These studies highlighted significant microstructural transformations, revealing the crucial role of interactions between the silver and the Cu-P brazing alloy.
Finally, to better understand the physicochemical mechanisms governing these changes, fundamental studies were conducted on the Cu-7P alloy. Investigations into the wetting behavior and interfacial reactivity of the Cu-7P alloy on copper and silver substrates revealed excellent wetting, with contact angles around 10° - even on oxidized copper substrates. In parallel, model diffusion couples (Cu-7P | Ag) were studied at various temperatures to analyze the interactions between the solid Ag and Cu-7P phases and the liquid Ag-Cu-P phase formed during heat treatment. Analysis of the associated mechanisms and kinetics provides greater insight into the phenomena involved in the brazing process and the in-service evolution of these multilayer assemblies.