title: Microstructural evolution and physical behavior of a lithium disilicate glass-ceramic.
abstract: BACKGROUND
  Elucidating the microstructural responses of the lithium disilicate system like the popular IPS e.max® CAD (LS2), made specifically for computer-aided design and computer-aided manufacturing (CAD-CAM), as a temperature-dependent system unravels new ways to enhance material properties and performance.
  
  
  OBJECTIVE
  To study the effect of various thermal processing on the crystallization kinetics, crystallite microstructure, and strength of LS2.
  
  
  METHODS
  The control group of the LS2 samples was heated using the standard manufacturer heating-schedule. Two experimental groups were tested: (1) an extended temperature range (750-840°C vs. 820-840°C) at the segment of 30°C/min heating rate, and (2) a protracted holding time (14min vs. 7min) at the isothermal temperature of 840°C. Five other groups of different heating schedules with lower-targeted temperatures were evaluated to investigate the microstructural changes. For each group, the crystalline phases and morphologies were measured by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Differential scanning calorimeter (DSC) was used to determine the activation energy of LS2 under non-isothermal conditions. A universal testing machine was used to measure 3-point flexural strength and fracture toughness, and elastic modulus and hardness were measured by a nanoindenter. A one-way ANOVA/Tukey was performed per property (alpha=0.05).
  
  
  RESULTS
  DSC, XRD, and SEM revealed three distinct microstructures during LS2 crystallization. Significant differences were found between the control group, the two aforementioned experimental groups, and the five lower-targeted-temperature groups per property (p<0.05). The activation energy for lithium disilicate growth was 667 (±29.0)kJ/mol.
  
  
  CONCLUSIONS
  Groups with the extended temperature range (750-840°C) and protracted holding time (820-840°C H14) produced significantly higher elastic-modulus and hardness properties than the control group but showed similar flexural-strength and fracture-toughness properties with the control group. In general, rapid growth of lithium disilicates occurred only when maximum formation of lithium metasilicates had ended.
authors.affiliation: United States Air Force Institute of Technology, Wright-Patterson Air Force Base, OH, USA; Indiana University School of Dentistry, Indianapolis, Indiana, USA. Electronic address: wenlien2003@yahoo.com.
authors.forename: Wen
authors.initials: W
authors.lastname: Lien
authors.affiliation: United States Air Force, Keesler Air Force Base, MS, USA.
authors.forename: Howard W
authors.initials: HW
authors.lastname: Roberts
authors.affiliation: Indiana University School of Dentistry, Indianapolis, Indiana, USA.
authors.forename: Jeffrey A
authors.initials: JA
authors.lastname: Platt
authors.affiliation: United States Air Force, Joint Base San Antonio, TX, USA.
authors.forename: Kraig S
authors.initials: KS
authors.lastname: Vandewalle
authors.affiliation: Ivoclar Vivadent, Amherst, NY, USA.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Hill
authors.affiliation: Indiana University School of Dentistry, Indianapolis, Indiana, USA.
authors.forename: Tien-Min G
authors.initials: TM
authors.lastname: Chu
mesh_terms: D002152:Calorimetry, Differential Scanning; D017076:Computer-Aided Design; D003460:Crystallization; D003776:Dental Porcelain / Q000737:chemistry*; D003799:Dental Stress Analysis; D055119:Elastic Modulus; D006244:Hardness; D006358:Hot Temperature; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D013499:Surface Properties; D014961:X-Ray Diffraction
chemical_list: C493120:lithia disilicate; D003776:Dental Porcelain
keywords: Crystallization; Differential scanning calorimetry; Glass–ceramic; Heating schedule; IPS e.max® CAD; Lithium disilicate; Lithium metasilicate; Microstructure; Nanoindentation; Nucleation; Phase transformation; Temperature threshold