Journal of Dental Research, Vol 77, 1991-1998, Copyright © 1998 by International & American Associations for Dental Research Online Journals
Indentation creep behavior of a direct-filling silver alternative to amalgam
H. H. Xu, H. Liao and F. C. Eichmiller
Paffenbarger Research Center, American Dental Association Health Foundation, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
Amalgam creep has been identified as a key parameter associated with
marginal breakdown and corrosion. The aim of this study was to evaluate the
time-dependent deformation (creep) of a novel silver filling material as an
alternative to amalgam. We made the silver specimens by pressing 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 the 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 6x10(4) sec. We used a bonded-interface technique to
examine subsurface creep mechanisms. The flexural 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 confidence coefficient = 0.95; Tukey's
multiple-comparison test). Indentation creep manifested as hardness number
decreasing with increased dwell time. With dwell time increasing from 5 sec
to 6x10(4) sec, the hardness number of amalgam was reduced by approximately
80%; that of the polished silver and the burnished silver was reduced by
only 40%. Subsurface creep in amalgam consisted of the shape change of the
alloy particles 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
and increasing hardness; that of the burnished silver occurred by the
displacement of the burnished layer. These results suggest that, due to
creep-induced subsurface work-hardening and densification, the consolidated
silver exhibits a higher resistance to indentation creep than does amalgam.
The hardness number of silver approaches that of amalgam after prolonged
indentation loading.
