title: The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio.
abstract: OBJECTIVE
  This study investigated the effect of sintering temperatures on flexural strength, contrast ratio, and grain size of zirconia.
  
  
  METHODS
  Zirconia specimens (Ceramill ZI, Amann Girrbach) were prepared in partially sintered state. Subsequently, the specimens were randomly divided into nine groups and sintered with different final sintering temperatures: 1,300°C, 1,350°C, 1,400°C, 1,450°C, 1,500°C, 1,550°C, 1,600°C, 1,650°C, or 1,700°C with 120 min holding time. Three-point flexural strength (N = 198; n = 22 per group) was measured according to ISO 6872: 2008. The contrast ratio (N = 90; n = 10 per group) was measured according to ISO 2471: 2008. Grain sizes and microstructure of different groups were investigated (N = 9, n = 1 per group) with scanning electron microscope. Data were analyzed using one-way ANOVA with Scheffé test and Weibull statistics (p < 0.05). Pearson correlation coefficient was calculated between either flexural strength or contrast ratio and sintering temperatures.
  
  
  RESULTS
  The highest flexural strength was observed in groups sintered between 1,400°C and 1,550°C. The highest Weibull moduli were obtained for zirconia sintered at 1,400°C and the lowest at 1,700°C. The contrast ratio and the grain size were higher with the higher sintering temperature. The microstructure of the specimens sintered above 1,650°C exhibited defects. Sintering temperatures showed a significant negative correlation with both the flexural strength (r = -0.313, p < 0.001) and the contrast ratio values (r = -0.96, p < 0.001).
  
  
  CONCLUSIONS
  The results of this study showed that the increase in sintering temperature increased the contrast ratio, but led to a negative impact on the flexural strength.
  
  
  CONCLUSIONS
  Considering the flexural strength values and Weibull moduli, the sintering temperature for the zirconia tested in this study should not exceed 1,550°C.
authors.affiliation: Clinic of Fixed and Removable Prosthodontics and Dental Material Science, Center of Dental Medicine, University of Zurich, Plattenstrasse 11, 8032, Zurich, Switzerland. bogna.stawarczyk@zzm.uzh.ch
authors.forename: Bogna
authors.initials: B
authors.lastname: Stawarczyk
authors.forename: Mutlu
authors.initials: M
authors.lastname: Ozcan
authors.forename: Lubica
authors.initials: L
authors.lastname: Hallmann
authors.forename: Andreas
authors.initials: A
authors.lastname: Ender
authors.forename: Albert
authors.initials: A
authors.lastname: Mehl
authors.forename: Christoph H F
authors.initials: CH
authors.lastname: Hämmerlet
mesh_terms: D002516:Ceramics / Q000737:chemistry*; D003116:Color; D003764:Dental Materials / Q000737:chemistry*; D055119:Elastic Modulus; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D010316:Particle Size; D018583:Pliability; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors; D015019:Yttrium / Q000737:chemistry*; D015040:Zirconium / Q000737:chemistry*
chemical_list: D003764:Dental Materials; C499362:yttria stabilized tetragonal zirconia; D015019:Yttrium; D015040:Zirconium; C028541:zirconium oxide