title: Residual stresses in Y-TZP crowns due to changes in the thermal contraction coefficient of veneers.
abstract: OBJECTIVE
  To test the hypothesis that the difference in the coefficient of thermal contraction of the veneering porcelain above (αliquid) and below (αsolid) its Tg plays an important role in stress development during a fast cooling protocol of Y-TZP crowns.
  
  
  METHODS
  Three-dimensional finite element models of veneered Y-TZP crowns were developed. Heat transfer analyses were conducted with two cooling protocols: slow (group A) and fast (groups B-F). Calculated temperatures as a function of time were used to determine the thermal stresses. Porcelain αsolid was kept constant while its αliquid was varied, creating different Δα/αsolid conditions: 0, 1, 1.5, 2 and 3 (groups B-F, respectively). Maximum (σ1) and minimum (σ3) residual principal stress distributions in the porcelain layer were compared.
  
  
  RESULTS
  For the slowly cooled crown, positive σ1 were observed in the porcelain, orientated perpendicular to the core-veneer interface ("radial" orientation). Simultaneously, negative σ3 were observed within the porcelain, mostly in a hoop orientation ("hoop-arch"). For rapidly cooled crowns, stress patterns varied depending on Δα/αsolid ratios. For groups B and C, the patterns were similar to those found in group A for σ1 ("radial") and σ3 ("hoop-arch"). For groups D-F, stress distribution changed significantly, with σ1 forming a "hoop-arch" pattern while σ3 developed a "radial" pattern.
  
  
  CONCLUSIONS
  Hoop tensile stresses generated in the veneering layer during fast cooling protocols due to porcelain high Δα/αsolid ratio will facilitate flaw propagation from the surface toward the core, which negatively affects the potential clinical longevity of a crown.
authors.affiliation: Biomaterials and Oral Biology Department, School of Dentistry, University of São Paulo, São Paulo, Brazil. jo@usp.br
authors.forename: Josete B C
authors.initials: JB
authors.lastname: Meira
authors.forename: Bruno R
authors.initials: BR
authors.lastname: Reis
authors.forename: Carina B
authors.initials: CB
authors.lastname: Tanaka
authors.forename: Rafael Y
authors.initials: RY
authors.lastname: Ballester
authors.forename: Paulo F
authors.initials: PF
authors.lastname: Cesar
authors.forename: Antheunis
authors.initials: A
authors.lastname: Versluis
authors.forename: Michael V
authors.initials: MV
authors.lastname: Swain
mesh_terms: D003442:Crowns; D003776:Dental Porcelain / Q000737:chemistry*; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D003801:Dental Veneers; D020342:Finite Element Analysis; D008422:Materials Testing; D008962:Models, Theoretical; D013696:Temperature; D013718:Tensile Strength; D015019:Yttrium / Q000737:chemistry*; D015040:Zirconium / Q000737:chemistry*
chemical_list: C499362:yttria stabilized tetragonal zirconia; D003776:Dental Porcelain; D015019:Yttrium; D015040:Zirconium