INDENTATION CREEP-BEHAVIOR OF A DIRECT-FILLING SILVER ALTERNATIVE TO AMALGAM

Amalgam creep has been identified as a key parameter associated with m arginal breakdown and corrosion. The aim of this study was to evaluate the time-dependent deformation (creep) of a novel silver filling mate rial as an alternative to amalgam. We made the silver specimens by pre ssing a precipitated powder at room temperature to a density that can be achieved in clinical hand consolidation. The surface of the silver was either polished or burnished. To examine local contact creep and t he effect of surface finishing, we used an indentation creep method in which a Vickers indenter was loaded on the specimen surface at a load of 10 N with dwell times of 5 sec to 6 x 10(4) sec. We used a bonded- interface technique to examine subsurface creep mechanisms. The flexur al strength (mean +/- SD; n = 10) was 86 +/- 20 MPa for amalgam, 180 /- 21 MPa for polished silver, and 209 +/- 19 MPa for burnished silver -values which are significantly different from each other (family conf idence coefficient = 0.95; Tukey's multiple-comparison test). Indentat ion creep manifested as hardness number decreasing with increased dwel l time. With dwell time increasing from 5 sec to 6 x 104 sec, the hard ness number of amalgam was reduced by approximately 80%; that of the p olished silver and the burnished silver was reduced by only 40%. Subsu rface creep in amalgam consisted of the shape change of the alloy part icles from spherical to elongated shapes, and the separation of matrix grains from each other, possibly due to grain-boundary sliding. Creep of the polished silver occurred by densification reducing porosity an d increasing hardness; that of the burnished silver occurred by the di splacement of the burnished layer. These results suggest that, due to creep-induced subsurface work-hardening and densification, the consoli dated sliver exhibits a higher resistance to indentation creep than do es amalgam. The hardness number of silver approaches that of amalgam a fter prolonged indentation loading.