title: Accelerated aging characteristics of three yttria-stabilized tetragonal zirconia polycrystalline dental materials.
abstract: BACKGROUND
  Concerns have been expressed about the effect of aging on the mechanical properties of zirconia.
  
  
  OBJECTIVE
  The purpose of this study was to assess the accelerated aging characteristics of 3 commercially available yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) materials by exposing specimens to hydrothermal treatments at 134°C, 0.2 MPa and 180°C, 1.0 MPa in steam.
  
  
  METHODS
  Thin bars of Y-TZP from 3 manufacturers, Lava, Zirkonzahn, and Zirprime, n=30 for each brand (22 × 3 × 0.2 mm), were cut and ground from blocks and sintered according to the manufacturer's specifications. Control specimens (n=10) for each brand were evaluated in the as-received condition. Experimental specimens were artificially aged at standard autoclave sterilization conditions,134°C at 0.2 MPa (n=5 per group at 50, 100, 150, and 200 hours) and standard industrial ceramic aging conditions, 180°C at 1.0 MPa (n=5 per group at 8, 16, 24, 28, and 48 hours). Tetragonal to monoclinic transformation was measured by using X-ray diffraction (XRD) for all groups. Flexural strength was measured in 4-point bending (ASTM1161-B) for all groups. Fracture surfaces were examined by scanning electron microscopy (SEM). Data were analyzed as a function of aging time. The statistical comparisons were based on the log value and 2-way ANOVA with heteroscedasticity-consistent standard errors used to compare mean strength among conditions (α=.05).
  
  
  RESULTS
  After 200 hours at 134°C and 0.2 MPa, flexural strength (SD) decreased significantly from 1156 (87.6) MPa to 829.5 (71) MPa for Lava; 1406 (243) MPa to 882.7 (91) MPa for Zirkonzahn; and 1126 (92.4) MPa to 976 (36.4) MPa for Zirprime with P<.001 for all 3 comparisons. After 200 hours at 134°C and 0.2 MPa, some tetragonal crystals transformed to the monoclinic phase. The relative XRD peak intensity of the monoclinic to tetragonal crystal phases increased from 0.07 to 1.82 for Lava, from 0.06 to 2.43 for Zirkonzahn, and from 0.05 to 0.53 for Zirprime. After 28 hours at 180°C and 1.0 MPa, all Lava and Zirkonzahn specimens spontaneously fractured during aging. The Noritake specimens were intact after 48 hours, and the flexural strength showed no significant change, 1156 (87.6) MPa to 1122 (108) MPa. The flexural strength decreased with an increase in the monoclinic phase. SEM micrographs revealed a transformed layer on the fracture surfaces.
  
  
  CONCLUSIONS
  Hydrothermal aging of Y-TZP can cause significant transformation from tetragonal to monoclinic crystal structure, which results in a statistically significant decrease in the flexural strength of thin bars. Although the strengths of all 3 Y-TZP materials are higher than other materials used for ceramic restorations, there are notable differences among them.
authors.affiliation: Department of Material Science and Engineering, University of Washington, Seattle, WA, USA. bflinn@u.washington.edu
authors.forename: Brian D
authors.initials: BD
authors.lastname: Flinn
authors.forename: Dirk A
authors.initials: DA
authors.lastname: deGroot
authors.forename: Lloyd A
authors.initials: LA
authors.lastname: Mancl
authors.forename: Ariel J
authors.initials: AJ
authors.lastname: Raigrodski
mesh_terms: D002516:Ceramics / Q000737:chemistry*; D003461:Crystallography; D003764:Dental Materials / Q000737:chemistry*; D003799:Dental Stress Analysis / Q000295:instrumentation; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D018583:Pliability; D011312:Pressure; D013227:Steam; D013242:Sterilization / Q000295:instrumentation; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors; D014867:Water; D014961:X-Ray Diffraction; D015019:Yttrium / Q000737:chemistry*; D015040:Zirconium / Q000737:chemistry*
chemical_list: D003764:Dental Materials; D013227:Steam; C499362:yttria stabilized tetragonal zirconia; D014867:Water; D015019:Yttrium; D015040:Zirconium