title: Microstructure of veneered zirconia after surface treatments: a TEM study.
abstract: OBJECTIVE
  Clinical studies reveal that veneer chipping is one major problem associated with zirconia based dental restorations, the underlying mechanisms being still investigated. We semi-quantitatively analyzed the effects of different surface treatments (thermal etching, 35/105 μm sandblasting and coarse bur drilling (150 μm)) on the microstructure of a zirconia veneered dental ceramic.
  
  
  METHODS
  The relative monoclinic content on zirconia surfaces was determined using X-ray diffraction (XRD). The microstructure at the zirconia-veneer interface has thereafter been investigated using transmission electron microscopy (TEM). Selected area electron diffraction (SAED) was used to qualitatively assess the depth of the stress-induced phase transformation.
  
  
  RESULTS
  Sandblasting or bur drilling significantly roughened the zirconia surface. A reverse transformation of already transformed monoclinic zirconia grains back into the tetragonal polymorph has been observed after thermal veneering treatment. In TEM, the mechanically treated samples revealed a highly damaged area of 1-3 μm from the interface. The presence of monoclinic phase in veneered zirconia samples has been observed in SAED up to depths of 4 μm (35 μm sandblasted), 11 μm (105 μm sandblasted) and 9 μm (150 μm diamond drilled) below the interface.
  
  
  CONCLUSIONS
  Regardless of the treatment protocol and produced roughness, the veneering ceramic perfectly sealed the zirconia surface. XRD showed an increased amount of monoclinic phase on the surface treated zirconia. However after thermal treatment, the monoclinic phase was re-transformed into the tetragonal polymorph. TEM/SAED analysis has found indication for a greater extend of the monoclinic transformation into the bulk zirconia compared to the treatment related defective zone depth.
authors.affiliation: Center for Nanoanalysis and Electron Microscopy (CENEM), Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Germany; Laboratory for Biomaterials Research, Dental Clinic 1 - Operative Dentistry and Periodontology, University of Erlangen-Nuremberg, Germany.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Grigore
authors.forename: Stefanie
authors.initials: S
authors.lastname: Spallek
authors.forename: Anselm
authors.initials: A
authors.lastname: Petschelt
authors.forename: Benjamin
authors.initials: B
authors.lastname: Butz
authors.forename: Erdmann
authors.initials: E
authors.lastname: Spiecker
authors.forename: Ulrich
authors.initials: U
authors.lastname: Lohbauer
mesh_terms: D003801:Dental Veneers; D046529:Microscopy, Electron, Transmission / Q000379:methods*; D015394:Molecular Structure; D013499:Surface Properties; D014961:X-Ray Diffraction; D015040:Zirconium / Q000737:chemistry*
chemical_list: D015040:Zirconium
keywords: Annealing; Bur drilling; Martensitic transformation; Sandblasting; X-ray diffraction; Y-TZP