Thermal induced deflection of a porcelain-zirconia bilayer: Influence of cooling rate

Abstract

Objective: To determine the thermal expansion of a porcelain (VM9) and tetragonal zirconia (Y-TZP) as well as the deflection upon re-heating and cooling of a bilayer fabricated from these two materials after slow and rapid cooling during initial fabrication.

Methods: The coefficient of thermal expansion (CTE) of bulk porcelain and Y-TZP as well as bilayer beam deflection was measured with a novel non-contact optical dilatometer. The influence of cooling rate during initial fabrication of the porcelain-zirconia bilayer and the bulk porcelain during subsequent heating and cooling is investigated. Specimens were heated to 900°C in the dilatometer, well in excess of the glass transition temperature (T_g) and softening temperature (T_s) of the porcelain.

Results: The thermal expansion of the porcelain above T_g exhibits a threefold increase in CTE over that observed below T_g. Observations of the bilayer deflection reflect the difference in the CTE of the component materials and enable T_g and T_s temperatures for the porcelain to be estimated. Initial cooling rate of the porcelain and porcelain-YTZP bilayer was found to have a profound influence on the subsequent response to slow reheating and cooling as well as the resultant residual deflection.

Significance: The estimation of the residual stress and potential for chipping of porcelain-zirconia dental restorative systems should not be based solely on thermal expansion data measured below T_g.

Keywords: Bilayer; Cooling rate; Fictive temperature; Porcelain; Structural relaxation; Thermal expansion; Zirconia.