Analysis of thermal distributions in veneered zirconia and metal restorations during firing.
OBJECTIVE
The present work evaluated the thermal behavior of porcelain-metal and porcelain-zirconia restorations during fast and slow firing and cooling.


METHODS
All-ceramic (porcelain on zirconia) and porcelain-fused-to-metal (PFM) molar crowns were fabricated with 1 or 2mm porcelain thickness. Thermocouples were attached to the cementation (T1) and occlusal (T4) surfaces of the restoration and embedded at the framework-porcelain interface (T2) and inside the porcelain (T3) to acquire temperature readings by time. Slow heating was set as 45°C/min and fast heating as 140°C/min. For fast cooling, the furnace was opened immediately after the holding time. Slow cooling was effected by opening the furnace when it reached 50°C below the Tg. Porcelains Tg were calculated for each cooling rate.


RESULTS
Slow heating rate was measured at T4 as being 30°C/min while fast heating at T4 was 100°C/min. The measured cooling rates within the porcelain (T2) around the Tg range were 20°C/min and 900°C/min for slow and fast cooling, respectively. During slow cooling, similar temperatures were found for both zirconia and metal crowns. Remarkable temperature gradients were observed for the fast cooled all-ceramic crown (T1-T4=100°C) and, of lower magnitude for PFM (T1-T4=30°C). Tg of porcelains increase with faster cooling rates.


CONCLUSIONS
Slow cooling appears to be especially important for all-ceramic crowns to prevent high magnitude thermal gradients, which could influence cracking and fracture of the porcelain.
Dental School, University of Passo Fundo, Brazil. Electronic address: benetti.paula@gmail.com.
Paula
P
Benetti
John Robert
JR
Kelly
Alvaro
A
Della Bona
D003793:Dental Restoration, Permanent; D003801:Dental Veneers; D015040:Zirconium / Q000737:chemistry*
D015040:Zirconium
Metal; Temperature gradients; Thermal analysis; Veneering porcelain; Zirconia