title: Effects of multiple firings on the mechanical properties and microstructure of veneering ceramics for zirconia frameworks.
abstract: OBJECTIVE
  The purpose of this study is to evaluate the effects of multiple firings on the mechanical properties and microstructure of veneering ceramics used with zirconia frameworks.
  
  
  METHODS
  Five different veneering ceramics for zirconia frameworks were used: Vintage ZR (ZR), Cerabien ZR (CZR), Vita VM9 (VM9), Cercon ceram KISS (KISS), IPS e.max ceram (e.max), and one veneering ceramic used for PFM frameworks: Vintage MP (MP). Twenty specimens were fabricated of each veneering ceramic. Ten specimens were fired twice and another ten specimens were fired ten times. Three-point flexural strength following the ISO 6872 and Vickers hardness were measured, and fracture toughness (K(IC)) was calculated. Density and porosity were determined. Specimens were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM).
  
  
  RESULTS
  For all materials, density increased and porosity decreased after 10 firings. Significant differences in density and porosity were observed between 2 and 10 firings, with the exception of VM9 (P<0.05). There were no significant differences in flexural strength between 2 and 10 firings except for MP. The Vickers hardness of ZR, VM9, KISS and MP increased significantly after 10 firings (P<0.001). There were no significant differences in fracture toughness for ZR, CZR, VM9 and MP between 2 and 10 firings. However, e.max underwent a significant increase in fracture toughness (P=0.000), and there was a significant decrease in the toughness of KISS after 10 firings (P=0.007).
  
  
  CONCLUSIONS
  Multiple firings could be effective for improving the densification and the hardness of veneering ceramics for zirconia restorations.
  
  
  CONCLUSIONS
  By 10 firings, the density and hardness of the veneering ceramics used with zirconia frameworks were raised, and porosity was reduced. However, no significant changes occurred in flexural strength, fracture toughness or microstructure.
authors.affiliation: Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, Suita, Osaka, Japan.
authors.forename: Xuehua
authors.initials: X
authors.lastname: Tang
authors.forename: Takashi
authors.initials: T
authors.lastname: Nakamura
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Usami
authors.forename: Kazumichi
authors.initials: K
authors.lastname: Wakabayashi
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Yatani
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry; D000538:Aluminum Silicates / Q000737:chemistry; D001031:Apatites / Q000737:chemistry; D002516:Ceramics / Q000737:chemistry; D003460:Crystallization; D003735:Dental Casting Technique; D003764:Dental Materials / Q000737:chemistry*; D003776:Dental Porcelain / Q000737:chemistry*; D003801:Dental Veneers; D055119:Elastic Modulus; D006244:Hardness; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D055595:Mechanical Phenomena; D008855:Microscopy, Electron, Scanning; D018583:Pliability; D016062:Porosity; D017680:Potassium Compounds / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D013816:Thermodynamics; D014961:X-Ray Diffraction; D015040:Zirconium / Q000737:chemistry*
chemical_list: D000538:Aluminum Silicates; D001031:Apatites; D003764:Dental Materials; D017680:Potassium Compounds; D003776:Dental Porcelain; C016447:feldspar; C078519:leucite; D012822:Silicon Dioxide; C023506:Glass ceramics; D015040:Zirconium; D000537:Aluminum Oxide; C025105:fluorapatite; C028541:zirconium oxide