title: Extended glaze firing on ceramics for hard machining: Crack healing, residual stresses, optical and microstructural aspects.
abstract: To evaluate the effect of extended and conventional (manufacturer-recommended) glaze firings on crack healing, residual stresses, optical characteristics and crystalline structure of four ceramics for hard machining.
  
  
  
  Rectangular specimens were obtained by sectioning densely sintered feldspathic (FEL), leucite- (LEU), lithium disilicate- (DIS), and zirconia-reinforced lithium silicate-based (ZLS) prefabricated ceramic blocks and divided into groups according to the applied glaze firing (n=5): conventional glaze/manufacturer-recommended (G), extended glaze (EG) and control/no heat treatment (C). Defects generated by indentation were analyzed by scanning electron microscopy before and after firing (n=1) to evaluate crack healing. Residual stresses were determined by the indentation technique. Color differences (ΔE) after firing were measured by CIEDE2000 formula, and translucency variations were quantified by contrast ratio. Stability of crystalline microstructure was analyzed by X-ray diffraction.
  
  
  
  Regardless of the material, EG had greater ability than G to heal defects, and produced compressive residual stresses, while G generated tensile stresses. Color differences produced by EG were: imperceptible for FEL and LEU ceramics; perceptible, but still clinically acceptable for DIS; clinically unacceptable for ZLS. G produced no perceptible color change. The DIS and ZLS ceramics became ≈1% more opaque after G, ≈4% and ≈15%, respectively, after EG. The crystalline phase of all the ceramics remained stable after G and EG.
  
  
  
  Extended glaze firing could be an alternative to finish feldspathic, leucite-, and lithium disilicate-based ceramic restorations, since it provides greater crack healing than the conventional glaze firing. It develops tolerable residual stresses, and produces clinically acceptable color alterations, without altering the microstructure of these materials.
authors.affiliation: Oral Science Post Graduate Program, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil. Electronic address: ianaprotese.87@gmail.com.
authors.forename: Iana L
authors.initials: IL
authors.lastname: Aurélio
authors.affiliation: Department of Physics, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil. Electronic address: lucio.dorneles@ufsm.br.
authors.forename: Lucio S
authors.initials: LS
authors.lastname: Dorneles
authors.affiliation: Department of Restorative Dentistry, Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil. Electronic address: liligmay@gmail.com.
authors.forename: Liliana G
authors.initials: LG
authors.lastname: May
mesh_terms: D002516:Ceramics; D003776:Dental Porcelain; D006244:Hardness; D008422:Materials Testing; D013499:Surface Properties; D014961:X-Ray Diffraction
chemical_list: D003776:Dental Porcelain
keywords: CAD/CAM; Glass ceramic; Heat treatment; Microstructure analysis; Opacity; Stress state