title: Post-hot isostatic pressing: a healing treatment for process related defects and laboratory grinding damage of dental zirconia?
abstract: OBJECTIVE
  Processing parameters (powder granulation, compaction, debinding, greenbody shaping, sintering) and post-sinter rough, even fine grinding are influencing the final mechanical properties of 3Y-TZP. The hypothesis of this study was that post-sinter hot isostatic pressing (post-HIP) would be beneficial for improving reliability and strength of both sintered and coarse ground sintered zirconia by closing or reducing surface and/or small volume defects.
  
  
  METHODS
  75 sintered bars of an experimental 3Y-TZP (3mm×4mm×45mm) with chamfered edges and 15μm diamond surface finish were provided by the manufacturer (Ivoclar Vivadent) and randomly distributed in five groups of N=15 each. G1 served as control (as received); G2 was post-HIPed at 1400°C and G3 at 1350°C, both using a pressure of 195MPa in Ar for 1h; G4 was coarse ground with 120μm diamond disk grain size; G5 was ground 120μm and post-HIPed at 1350°C at 195MPa, 1h in Ar. The specimens were fractured in air in 4 point-bending. Weibull characteristic strength (σ0) in MPa, m parameter (reliability) and confidence intervals (CI) at 90% confidence level are reported. Identification of the critical flaw was performed by SEM on the fractured surface of all specimens and XRD performed in all groups.
  
  
  RESULTS
  G1: σ0=973 (932-1016), m=10.6 (7.45-15.1); G2: σ0=930 (871-995), m=6.9 (4.87-9.9); G3: σ0=898 (848-952), m=7.94 (5.6-11.4); G4: σ0=921 (857-991), m=6.35 (4.48-9.11); G5: σ0=881 (847-918), m=11.4 (8.03-16.3). G5 had a significantly lower σ0 than G1. No significant differences were seen in the reliability (m) among the groups. Fractography revealed critical intrinsic subsurface flaws of 10-60μm present in all groups resulting from the processing parameters. No "healing" (i.e. closing of defects by densification) resulted after post-HIP. Grinding sintered zirconia with 120μm diamond disks induced radial cracks of 10-20μm and an important pseudo-cubic phase transformation (56wt%) that was not completely removed after post-HIP. Post-HIP increased slightly the relative density by 0.1% but without improving the strength and reliability.
  
  
  CONCLUSIONS
  Post-HIP was not efficient in closing large (10-60μm) subsurface (volume) processing defects.
authors.affiliation: University of Geneva, School of Dental Medicine, Department of Prosthodontics-Biomaterials, 1205 Geneva, Switzerland. susanne.scherrer@unige.ch
authors.forename: S S
authors.initials: SS
authors.lastname: Scherrer
authors.forename: M
authors.initials: M
authors.lastname: Cattani-Lorente
authors.forename: S
authors.initials: S
authors.lastname: Yoon
authors.forename: L
authors.initials: L
authors.lastname: Karvonen
authors.forename: S
authors.initials: S
authors.lastname: Pokrant
authors.forename: F
authors.initials: F
authors.lastname: Rothbrust
authors.forename: J
authors.initials: J
authors.lastname: Kuebler
mesh_terms: D002516:Ceramics / Q000737:chemistry*; D003461:Crystallography; D003764:Dental Materials / Q000737:chemistry*; D003775:Dental Polishing / Q000295:instrumentation; D018130:Diamond / Q000737:chemistry; D006358:Hot Temperature; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D018583:Pliability; D011312:Pressure; D013314:Stress, Mechanical; D013499:Surface Properties; D014961:X-Ray Diffraction; D015019:Yttrium / Q000737:chemistry*; D015040:Zirconium / Q000737:chemistry*
chemical_list: D003764:Dental Materials; C499362:yttria stabilized tetragonal zirconia; D015019:Yttrium; D018130:Diamond; D015040:Zirconium
keywords: CAD-CAM; Fractography; Grinding; Hot isostatic pressing; Reliability; Zirconia