Effect of Sn Content on Wettability and Interfacial Structure of Cu–Sn–Cr/Graphite Systems: Experimental and First-Principles Investigations

The co-addition of chromium (Cr) and tin (Sn) is known to enhance the wettability between copper (Cu) and graphite (C_gr), but the effect of Sn content remains poorly understood. This study aims to systematically investigate the influence of Sn content <i>a</i> (<i>a</i> = 0, 10, 20, 30, 40, 50, 80, 99 at. %) on the wettability, interfacial structure, surface/interface energy (<i>σ</i>_lv/<i>σ</i>_sl), and adhesion behavior of the Cu-<i>a</i>Sn-1Cr/C_gr system at 1100 °C. The experimental results show that as the Sn content increases, the equilibrium contact angle (<i>θ</i>_e) of the metal droplet shows a non-monotonic trend; the thickness of the reaction product layer (RPL, consisting of Cr carbides (Cr<i>_m</i>C<i>_n</i>)) gradually increases, accompanied by a decrease in the calculated adhesion work (Wadcal). A "sandwich" interface structure is observed, consisting of two interfaces: metal||Cr<i>_m</i>C<i>_n</i> and Cr<i>_m</i>C<i>_n</i>||C_gr. Sn content mainly affects the former. At metal||Cr<i>_m</i>C<i>_n</i>, Sn exists in various forms (e.g., Cu-Sn solid solution, Cu<i>_x</i>Sn<i>_y</i> compounds) in contact with Cr<i>_m</i>C<i>_n</i>. To elucidate the wetting and bonding mechanisms of metal||Cr<i>_m</i>C<i>_n</i>, simplified interfacial models are constructed and analyzed based on first-principles calculations of density functional theory (DFT). The trend of theoretically calculated results (<i>σ</i>_metal and <i>W</i>_ad) agrees with the experimental results (<i>σ</i>_lv and Wadcal). Further analysis of the partial density of state (PDOS) and charge density difference (CDD) reveals that charge distribution and bonding characteristics vary with Sn content, providing the microscopic insight into the nature of wettability and interfacial bonding strength.