title: The use of slow heating and slow cooling regimens to strengthen porcelain fused to zirconia.
abstract: BACKGROUND
  Porcelain fused to zirconia prostheses are widely used. However, porcelain chipping, spalling, fracture, and delamination are common clinical problems. Residual stresses of thermal origin have received attention, but clear data and firing guidelines remain absent.
  
  
  OBJECTIVE
  The purpose of this study was to measure the influence of heating and cooling protocols on the strength of porcelain fused to zirconia.
  
  
  METHODS
  A modified 4-point flexural testing technique was used to measure strength, and porcelain buttons were bonded to the beam between the 2 central loading points. Beams (n=54) were made of a tetragonal polycrystalline zirconium dioxide that was partially stabilized with an yttria core and a feldspathic dental porcelain. Three different heating rates and 3 different cooling regimens were used during firing. Two-way analysis of variance (ANOVA) was used to evaluate the 2 main effects of the heating and cooling regimens and their interaction with the delamination force (α=.05). The Tukey multiple comparisons test was used to identify differences among heating or cooling regimens.
  
  
  RESULTS
  During loading, the porcelain buttons separated from the zirconia beams because of delamination within the porcelain, which was close to the porcelain to zirconia interface. ANOVA revealed that the effects of the cooling regimen and heating rate had statistically significant effects on failure load (P<.05). The effect of the cooling regimen was greater than that of the heating regimen.
  
  
  CONCLUSIONS
  Slow cooling and slow heating regimens should be used when firing porcelain to zirconia. Cooling regimens were found to be more influential than heating rates. Failure was localized to the porcelain adjacent to the porcelain-zirconia interface, not to the interface itself, indicating that the residual stresses of thermal origin within the porcelain dominated. The preparation of zirconia with 50 μm aluminum oxide at a pressure of 0.34 MPa was sufficient to prevent interfacial failure.
authors.affiliation: UCLA School of Dentistry, Los Angeles, CA 90095-1668, USA.
authors.forename: John P
authors.initials: JP
authors.lastname: Tan
authors.forename: Dalene
authors.initials: D
authors.lastname: Sederstrom
authors.forename: Joshua R
authors.initials: JR
authors.lastname: Polansky
authors.forename: Edward A
authors.initials: EA
authors.lastname: McLaren
authors.forename: Shane N
authors.initials: SN
authors.lastname: White
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry; D000538:Aluminum Silicates / Q000737:chemistry; D003080:Cold Temperature; D001840:Dental Bonding / Q000379:methods*; D003764:Dental Materials / Q000737:chemistry*; D003776:Dental Porcelain / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000295:instrumentation; D055119:Elastic Modulus; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D018583:Pliability; D017680:Potassium Compounds / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D013718:Tensile Strength; D013997:Time Factors; D015019:Yttrium / Q000737:chemistry; D015040:Zirconium / Q000737:chemistry*
chemical_list: D000538:Aluminum Silicates; D003764:Dental Materials; D017680:Potassium Compounds; C499362:yttria stabilized tetragonal zirconia; D003776:Dental Porcelain; C016447:feldspar; D015019:Yttrium; D015040:Zirconium; D000537:Aluminum Oxide; C028541:zirconium oxide