title: Effect of multistep processing technique on the formation of micro-defects and residual stresses in zirconia dental restorations.
abstract: OBJECTIVE
  The clinical failures of zirconia dental restorations are often caused by extrinsic artifacts introduced by processing. The aim of this study was to investigate the micro-defects and residual stresses generated during the multistep process of zirconia dental restorations.
  
  
  METHODS
  Thermal spray granulated 3Y-TZP powders were dry pressed by two tools exhibiting distinctly different Young's moduli, cold isostatic pressed (CIP-ed), and pressure-less fully sintered. The green bodies pressed by a stiff tool were treated with different procedures: direct milling (green milling) followed by fully sintering; half-sintering and milling (raw milling) with or without fully sintering; and fully sintering followed by grinding. The fully sintered 3Y-TZP crowns were clinically adjusted using both a diamond bur and SiC bur, respectively. Phase composition and microstructure of the pressed, milled, and ground surfaces were studied by XRD and SEM.
  
  
  RESULTS
  Tetragonal phase was the main phase of all detected 3Y-TZP specimens. Excessive residual stresses introduced by raw milling and grinding were confirmed by a strained T (111) peak, monoclinic phase, and obviously changed I(002)t /I(200)t ratio. The residual stresses would form a compressive stress layer, while it was too shallow to inhibit crack propagation even for ground specimens. Large voids with high-coordination numbers were the common packing micro-defects. Once formed, they were barely healed by CIP-ing and sintering. A stiff pressing tool was confirmed to be useful for reducing the surface packing voids. Milling removed the surface voids, but was no help for the interior ones. Raw milling introduced more serious chippings, most originating from the existing packing voids, than green milling due to its brittle failure and was less recommended for production. Grinding dense 3Y-TZP caused surface grain refinement and much more severe micro-defects, especially when clinical adjustment was applied by diamond bur compared to SiC bur.
  
  
  CONCLUSIONS
  Micro-defects and residual stresses are introduced and accumulated through the entire production chain and determine the final microstructure of zirconia dental restorations. Several procedural improvements are offered and expected to reduce processing micro-defects.
authors.affiliation: Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, P.R. China; Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm, Sweden.
authors.forename: Zhao
authors.initials: Z
authors.lastname: Jing
authors.forename: Zhang
authors.initials: Z
authors.lastname: Ke
authors.forename: Liu
authors.initials: L
authors.lastname: Yihong
authors.forename: Shen
authors.initials: S
authors.lastname: Zhijian
mesh_terms: D017554:Carbon Compounds, Inorganic / Q000737:chemistry; D003764:Dental Materials / Q000737:chemistry*; D003775:Dental Polishing / Q000295:instrumentation; D017267:Dental Prosthesis Design; D018130:Diamond / Q000737:chemistry; D055119:Elastic Modulus; D004548:Elasticity; D006358:Hot Temperature; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D044367:Phase Transition; D016062:Porosity; D011312:Pressure; D017655:Silicon Compounds / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D014961:X-Ray Diffraction; D015019:Yttrium / Q000737:chemistry*; D015040:Zirconium / Q000737:chemistry*
chemical_list: D017554:Carbon Compounds, Inorganic; D003764:Dental Materials; D017655:Silicon Compounds; C499362:yttria stabilized tetragonal zirconia; D015019:Yttrium; D018130:Diamond; D015040:Zirconium; C022088:silicon carbide
keywords: CAD/CAM; Dental ceramics; grinding; reliability; slow crack growth; tetragonal-to-monoclinic transformation