Effect of surface-modified nanoparticles on microstructure and hardness of low-temperature solder alloys

We focus on the role of surface modification of nanoparticles employed as reinforcements in Sn–52mass%In composite alloys (the alloys), and how these surface differences affect the microstructure and mechanical properties of the alloys. Microscopic observation revealed that the addition of surface–modified ZrO₂ nanoparticles (ZrO₂ NPs) by NiO (NiO/ZrO₂ NPs) led to a finer γ-phase spacing compared to alloys containing unmodified ZrO₂ NPs. This is attributed to enhanced heterogeneous nucleation promoted by the surface modification. Nanoindentation tests showed that the microhardness of the alloys was increased by 5.7 MPa upon NP surface modification, suggesting microstructural refinement caused by surface modification and subsequent changes in the free energy of heterogeneous nucleation. Density functional theory calculations demonstrated that NiO/ZrO₂ surfaces exhibit lower ensemble average of adsorption energies for the alloys adatoms than that exhibited by ZrO₂ surfaces, with energy minima aligning with the lattice periodicity of the β–phase or γ–phase. These findings suggest that the surface modification induces a crystallization preference during solidification, thereby improving the alloy properties. This work highlights the impact of NP surface modification-induced changes in nucleation energetics on the solidification behavior and mechanical properties of the alloys, providing insights for electronic packaging material design.