title: Descriptions of crack growth behaviors in glass-ZrO2 bilayers under thermal residual stresses.
abstract: This study was intended to separate residual stresses arising from the mismatch in coefficients of thermal expansion between glass and zirconia (ZrO2) from those stresses arising solely from the cooling process. Slow crack growth experimentes were undertaken to demonstrate how cracks grow in different residual stress fields.
  
  
  
  Aluminosilicate glass discs were sintered onto ZrO2 to form glass-ZrO2 bilayers. Glass discs were allowed to bond to the ZrO2 substrate during sintering or prevented from bonding by means of coating the ZrO2 with a thin boron nitrade coating. Residual stress gradients on "bonded" and "unbonded" bilayers were assessed using birefringence measurements. Unbonded glass discs were further tested under biaxial flexure in dynamic fatigue conditions in order to evaluate the effect of residual stress on the slow crack growth behavior.
  
  
  
  When fast-ccoling was induced, residual tensile stresses on the glass increased significantly on the side toward the ZrO2 substrate. By allowing the bond between glass and ZrO2, those tensile stresses observed in unbonded specimens are overwhelmed by the contraction mismatch stresses between the ZrO2 substrate and the glassy overlayer. Specimens containing residual tensile stresses on the bending surface showed a time-dependent strength increase in relation to stress-free annealed samples in the dynamic biaxial bending test, with this effect being dependent on the magnitude of the residual tensile stress. The phenomenon observed is explained here on the basis of the water toughening effect, in which water diffuses into the glass promoting local swelling. An additional residual tensile stress at the crack tip adds an applied-stress-independent (Kres) term to the total tip stress intensity factor (Ktip), increasing the stress-enhanced diffusion and the shielding of the crack tip through swelling of the crack faces.
  
  
  
  Residual stresses in the glass influence the crack growth behavior of veneered-ZrO2 bilayered dental prostheses. The role of water in crack growth might be of higher complexity when residual stresses are present in the glass layer.
authors.affiliation: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Medizinische Fakultät, Universitätsklnikum Erlangen, Zahnklinik 1 - Zahnerhaltung und Parodontologie, Forschungslabor für dentale Biomaterialien, Glückstrasse 11, D-91054 Erlangen, Germany. Electronic address: rbelli@dent.uni-erlangen.de.
authors.forename: Renan
authors.initials: R
authors.lastname: Belli
authors.affiliation: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Medizinische Fakultät, Universitätsklnikum Erlangen, Zahnklinik 1 - Zahnerhaltung und Parodontologie, Forschungslabor für dentale Biomaterialien, Glückstrasse 11, D-91054 Erlangen, Germany; Department of Restorative Dentistry, Faculty of Dentistry, University of Concepción, Concepción, Chile.
authors.forename: Michael
authors.initials: M
authors.lastname: Wendler
authors.affiliation: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Medizinische Fakultät, Universitätsklnikum Erlangen, Zahnklinik 1 - Zahnerhaltung und Parodontologie, Forschungslabor für dentale Biomaterialien, Glückstrasse 11, D-91054 Erlangen, Germany.
authors.forename: José I
authors.initials: JI
authors.lastname: Zorzin
authors.affiliation: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Medizinische Fakultät, Universitätsklnikum Erlangen, Zahnklinik 1 - Zahnerhaltung und Parodontologie, Forschungslabor für dentale Biomaterialien, Glückstrasse 11, D-91054 Erlangen, Germany.
authors.forename: Anselm
authors.initials: A
authors.lastname: Petschelt
authors.affiliation: Department of Biomaterials and Oral Biology, University of São Paulo, Brazil.
authors.forename: Carina B
authors.initials: CB
authors.lastname: Tanaka
authors.affiliation: Department of Biomaterials and Oral Biology, University of São Paulo, Brazil.
authors.forename: Josete
authors.initials: J
authors.lastname: Meira
authors.affiliation: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Medizinische Fakultät, Universitätsklnikum Erlangen, Zahnklinik 1 - Zahnerhaltung und Parodontologie, Forschungslabor für dentale Biomaterialien, Glückstrasse 11, D-91054 Erlangen, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Lohbauer
mesh_terms: D003776:Dental Porcelain; D003799:Dental Stress Analysis; D005898:Glass; D008422:Materials Testing; D013314:Stress, Mechanical; D015040:Zirconium
chemical_list: D003776:Dental Porcelain; D015040:Zirconium; C028541:zirconium oxide
keywords: Birefringence; Cooling rate; Crack shielding; Dental zirconia; Residual stresses; Slow crack growth; Thermal incompatibility