Indentation Creep Behavior of a Direct-Filling Silver Alternative to Amalgam
Hockin D. Xu, Kate Liao, F Eichmiller
Amalgam creep has been identified as a key parameter associated with marginal breakdown and corrosion. Recently, a consolidated silver powder was proposed as a mercury-free alternative to amalgam. The aim of the present study was to characterize the time-dependent deformation (creep) of consolidated silver in comparison to amalgam. The silver specimens were made by pressing a precipitated powder at room temperature at a pressure of 150 Mpa. The surface of the silver specimens was either polished or burnished. To examine creep in a local contact configuration, an indentation creep method was employed in which a Vickers indenter was loaded on the specimen surface at a load of 10 N with dwell times of 5 seconds (s), 10 s, 50 s, 102s, 103 s, 104 s, and 6x104 s. A bonded-interface technique was used together with optical and scanning electron microscopy to examine creep-induced subsurface deformation and damage. Indentation creep manifested as hardness decreasing significantly (family confidence coefficient = 0.95; Tukey's Multiple Comparison Test) with increased dwell time. The hardness value of amalgam at a dwell time of 6x104 s was reduced to 19% of the hardness value at 5 s, while that of the polished silver was reduced to 63%, and that of the burnished silver was reduced to 59%. Subsurface creep deformation in amalgam consisted of the shape change of the individual alloy particles, and the separation of matrix grains from each other, possibly due to grain-boundary sliding. The creep of the polished silver occurred by material densification and plastic flow, reducing the porosity and increasing the hardness, while the creep of the burnished silver occurred by the displacement of the burnished layer. These results suggest that the silver exhibits a higher resistance to long-term indentation creep than amalgam due to creep-induced subsurface work-hardening and densification. The hardness of silver approaches that of amalgam after prolonged indentation loading.