Research Description:

Analysis of Residual Stress in Monolithic and Bilayer Dental Ceramics

It is well known that structural flaws may be points of origin for fracture of dental ceramic prostheses such as crowns and bridges.  Flaws that generally originate in the fabrication process are exposed to residual stress.  Zones affected by compressive and tensile stress are distributed throughout each specimen. The contribution of these residual stresses to prosthetic failure has not yet been clearly established.

Objective: Test the hypothesis that monolithic and bilayer dental ceramic specimens subjected to slow cooling will produce less global residual compressive stress than dental ceramic monolithic and bilayer ceramic specimens subjected to fast cooling.

Methods: Monolithic ceramic veneer and bilayer core/veneer ceramic bars (e.max Ceram and e.max Press, Ivoclar Vivadent, Schaan, Liechtenstein) were fabricated and fired according to the manufacturer’s specifications.  Each specimen was either fast cooled (FC) by quenching in air or slow cooled (SC) by cooling in the furnace after terminating power.  The specimens (40 mm x 2 mm x 4 mm) were polished through 1 µm alumina abrasive.

Vicker’s microindentations were produced using a microhardnesss indenter (Micromet 3 Buehler, Lake Bluff, IL) along rows at distances of 200, 300, 400, 600, and 800 µm from the surface using a 9.8 N load.   Indentations were made in the center 20 mm to avoid end stress concentration approximately every 500 microns to avoid overlapping residual stress fields.  Lateral and vertical crack diameters (2c) propagating from the indentations were captured as individual images that were measured and recorded using the Buehler OmniMet Capture System. Residual stress calculations are based on the Kese and Rowcliffe (2000) equation for microhardness with respect to the specimen cooling treatment (FC or SC).  Calculations will be performed and recorded to establish the residual stresses at their respective distances from the surface for each the cooling rates used in this study.