Viscoplastic Properties of Sintered Silver Revealed by Indentation Testing and Modeling: Dependence on Temperature, Hydrostatic Pressure and Length-Scale

The viscoplastic steady-state behavior of porous sintered silver is determined at different temperatures using a combination of indentation testing and modeling. The microstructure of the sintered silver consists of agglomerates of silver particles and flakes, separated either by voids or by resin-rich regions. The characteristic length-scale of the microstructural heterogeneity is characterized with electron microscopy and image-processing methods. This study focuses on the dependence of creep behavior on hydrostatic stress and on length-scale, due to the heterogeneous morphology of the sintered material. Testing consists of constant-force indentations using spherical indenters of two different radii at three different temperatures: $25^{\circ} \mathrm{C}, 75^{\circ} \mathrm{C}$, and $125^{\circ} \mathrm{C}$. The indentation results were analyzed using an inverse-fitting approach using detailed global-local computational viscoplastic finite element analysis. The results show that indenters of different radii measure significantly different viscoplastic properties at each stress level and at each temperature. These differences are believed to be due to: (i) the porosity of the sintered material and (ii) the relationship of the indenter tip size to the characteristic length-scale of the heterogeneity of the sintered microstructure. The influence of the first factor is identified by the use of a Drucker-Prager creep model, which incorporates hydrostatic stress (since the inelastic deformation of porous materials is known to be dependent on the pressure). The aim of explicitly accounting for the effect of Factor (i) above is to isolate and identify the remaining difference observed due to length-scale effects (listed in Factor (ii) above), when interrogating the creep behavior with indenters of different radii.