Correlation of microstructure and constitutive behaviour of sintered silver particles via nanoindentation

During the sintering process, a three-dimensional network-like porous structure forms as the consequences of atom diffusion and neck formation among silver particles. In this paper, two representative types of pressureless sintered silver particles, i.e., silver nanoparticles (AgNP) and silver microparticles (AgMP), are investigated to reveal the intrinsic correlation between microstructure and constitutive behaviour. The heterogeneous microstructure is found to result in distinguishing differences in terms of morphology and thermal stability. The difference of sintering mechanisms between these two types of silver pastes are observed by a scanning electron microscope and discussed. Regarding thermal stability, the effect of particle size and shape on the sintering temperature and mass loss are evaluated using the thermogravimetric analysis and the differential scanning calorimetry. The microstructural effect on the mechanical behaviour of both sintered AgNP and AgMP samples is measured by performing nanoindentations with a Berkovich indenter penetrated up to 2000 nm with different indentation strain rates. During indentation, Young's modulus and hardness are measured as the functions of penetration depth. By solving dimensionless equations, the parameters of a power-law constitutive model are obtained analytically based on the integrated work done in the loading stage and the contact stiffness in the unloading stage in the nanoindentation experiments. The correlation between microstructure and constitutive behaviour is elucidated by the comparison of sintered AgNP and AgMP but further investigations related to the size and shape effects of particles are expected.