Nanoindentation creep properties of lead-free nanocomposite solders reinforced by modified carbon nanotubes

In this research, creep properties and thermal behavior of Sn-3.5Ag-0.7Cu (SAC)/multi-walled carbon nanotubes (MW-CNTs) lead-free nanocomposite solders was studied by conducting the nanoindentation testing at different temperatures, in the range of 292-312 K. Solder materials were prepared by mechanical alloying followed by powder consolidation routes. Furthermore, nickel coating was electm-less plated on the surface of nanotubes to enhance their metallurgical compatibility with the SAC solder matrix. The content of Ni-coated MW-CNTs varied in the range of 0-0.2 wt%. Addition of Ni-coated nanotubes resulted in an improvement of the indentation creep behavior of SAC solder alloy in comparison with the non-coated agents. By increasing the fraction of Ni-coated MW-CNTs up to similar to 0.1 wt%, the creep resistance of solder nanocomposite was continuously enhanced. However, the higher contents of reinforcing agents led to the deterioration of creep behavior due to the aggregation of nanotubes and a considerable heterogeneous refinement of the microstructure. The activation energy for softening during localized deformation behavior of nanocomposite solders was estimated by two approaches based on Dorn constitutive and Lucas-Oliver creep models and found to be in the ranges of 8.8-18.2 kJ/mol and 17.3-48.4 kJ/mol, respectively. By changing in the activation energy for diffusion and sliding of grain boundaries, in dependency with the extent of grain structural refinement and generation of dislocations due to altering the content of nanotubes as the reinforcing agent, the time-dependent under-loading deformation phenomenon in terms of creep can be controlled. The experimental results revealed that modified carbon nanotubes which are located on sub-grain boundaries can reduce their chemical potential tremendously, suppress the sliding-based creep deformation up to an optimized content of similar to 0.1 wt%.