title: Influence of thermal gradients on stress state of veneered restorations.
abstract: OBJECTIVE
  To assess transient and residual stresses within the porcelain of veneered restorations (zirconia and metal) as a result of cooling rate and porcelain thickness.
  
  
  METHODS
  Porcelain-on-zirconia (PZ) and porcelain-fused-to-metal (PFM) crowns were fabricated with 1 or 2mm of porcelain. Thermocouples were attached both internally and externally to the crowns to record transient temperatures. For fast cooling, the furnace was opened after the holding time and switched off. Slow cooling was accomplished by opening the furnace at 50°C below the glass transition temperature (Tg) of the material. An axially symmetric FEA model simulated thermal stresses. Time-dependent temperature equations from thermocouple readings were set as boundary conditions. Framework materials and the porcelain below Tg were considered to behave elastically. Visco-elastic behavior was assumed for porcelain above the Tg modeling properties as dependent on cooling rate.
  
  
  RESULTS
  Differences in residual stress were found for fast and slow cooled PZ and PFM crowns. Significant transient stress waves were observed within the porcelain when fast cooling through Tg. They are believed to be related to non-uniform volumetric changes originated from thermal gradients. Results were confirmed by modeling and physical testing of crowns containing a defect.
  
  
  CONCLUSIONS
  Residual stresses do not distinguish PZ from PFM. High magnitude transient stresses observed within the porcelain during fast cooling may explain clinical fractures involving internal defects. Stress waves may also originate internal micro-cracking which could grow under function. Therefore, slow cooling, especially for all-ceramic crowns with thick porcelain, is important to prevent thermal gradients and high-magnitude transient stresses.
authors.affiliation: University of Passo Fundo, Post-graduate Program in Dentistry, Passo Fundo, RS, Brazil. Electronic address: benetti.paula@gmail.com.
authors.forename: Paula
authors.initials: P
authors.lastname: Benetti
authors.affiliation: University of Connecticut, Department of Reconstructive Sciences, Farmington, CT, USA.
authors.forename: J Robert
authors.initials: JR
authors.lastname: Kelly
authors.affiliation: FMC Technologies, Houston, TX, USA.
authors.forename: Mauricio
authors.initials: M
authors.lastname: Sanchez
authors.affiliation: University of Passo Fundo, Post-graduate Program in Dentistry, Passo Fundo, RS, Brazil.
authors.forename: Alvaro
authors.initials: A
authors.lastname: Della Bona
mesh_terms: D003793:Dental Restoration, Permanent; D003799:Dental Stress Analysis; D003801:Dental Veneers; D020342:Finite Element Analysis; D008855:Microscopy, Electron, Scanning; D013314:Stress, Mechanical; D013696:Temperature
keywords: Cooling rate; Porcelain; Stress concentration; Thickness; Transient stresses; Zirconia