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 2 mm 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 T_g. Porcelains T_g 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 T_g 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). T_g of porcelains increase with faster cooling rates.

Significance

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.