title: Influence of low-temperature environmental exposure on the mechanical properties and structural stability of dental zirconia.
abstract: OBJECTIVE
  The effect of dental fabrication procedures of zirconia monolithic restorations and changes in properties during low-temperature exposure in the oral environment is not completely understood. The purpose of this study was to investigate the effect of procedures for fabrication of dental restorations by low-temperature simulation and relative changes of flexural strength, nanoindentation hardness, Young's modulus, surface roughness, and structural stability of yttria-stabilized zirconia.
  
  
  METHODS
  A total of 64 zirconia specimens were prepared to simulate dental practice. The specimens were divided into the control group and the accelerated aging group. The simulated group followed the same procedure as the control group except for the aging treatment. Atomic force microscopy was used to measure surface roughness. The degree of tetragonal-to-monoclinic transformation was determined using X-ray diffraction. Nanoindentation hardness and modulus measurements were carried out on the surface of the zirconia specimens using a nanoindenter XP/G200 system. The yttria levels for nonaged and aged specimens were measured using energy dispersive spectroscopy. Flexural strength was determined using the piston-on-three-ball test. The t-test was used to determine statistical significance.
  
  
  RESULTS
  Means and standard deviations were calculated using all observations for each condition and evaluated using a group t-test (p < 0.05). The LTD treatment resulted in increased surface roughness (from 12.23 nm to 21.56 nm for Ra and 15.06 nm to 27.45 nm for RMS) and monoclinic phase fractions (from 2% to 21%), with a concomitant decrease in hardness (from 16.56 GPa to 15.14 GPa) and modulus (from 275.68 GPa to 256.56 GPa). Yttria content (from 4.43% to 4.46%) and flexural strength (from 586 MPa to 578 MPa) were not significantly altered, supporting longer term in vivo function without biomechanical fracture.
  
  
  CONCLUSIONS
  The LTD treatment induced the tetragonal-to-monoclinic transformation with surface roughening in zirconia prepared using dental procedures.
authors.affiliation: Department of Prosthetic Dental Sciences, College of Dentistry, Taibah University Madina, Saudi Arabia. tariq@uab.edu
authors.forename: Tariq F
authors.initials: TF
authors.lastname: Alghazzawi
authors.forename: Jack
authors.initials: J
authors.lastname: Lemons
authors.forename: Perng-Ru
authors.initials: PR
authors.lastname: Liu
authors.forename: Milton E
authors.initials: ME
authors.lastname: Essig
authors.forename: Alfred A
authors.initials: AA
authors.lastname: Bartolucci
authors.forename: Gregg M
authors.initials: GM
authors.lastname: Janowski
mesh_terms: D003764:Dental Materials / Q000737:chemistry*; D017267:Dental Prosthesis Design; D003799:Dental Stress Analysis / Q000295:instrumentation; D055119:Elastic Modulus; D006244:Hardness; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D055595:Mechanical Phenomena; D018625:Microscopy, Atomic Force; D008855:Microscopy, Electron, Scanning; D044367:Phase Transition; D018583:Pliability; D012464:Saliva, Artificial / Q000737:chemistry; D013052:Spectrometry, X-Ray Emission; D013499:Surface Properties; D013696:Temperature; D013816:Thermodynamics; D014961:X-Ray Diffraction; D015019:Yttrium / Q000737:chemistry*; D015040:Zirconium / Q000737:chemistry*
chemical_list: D003764:Dental Materials; D012464:Saliva, Artificial; C499362:yttria stabilized tetragonal zirconia; D015019:Yttrium; D015040:Zirconium