title: Influence of thermal expansion mismatch on residual stress profile in veneering ceramic layered on zirconia: Measurement by hole-drilling.
abstract: OBJECTIVE
  Mismatch in thermal expansion coefficient between core and veneering ceramic (Δα=αcore-αveneer, ppm/°C) is reported as a crucial parameter influencing veneer fractures with Yttria-tetragonal-zirconia-polycrystal (Y-TZP) prostheses, which still constitutes a misunderstood problem. However, the common positive Δα concept remains empirical. The objective of this study is to investigate the Δα dependence of residual stress profiles in veneering ceramic layered on Y-TZP frameworks.
  
  
  METHODS
  The stress profile was measured with the hole-drilling method in bilayered disc samples of 20mm diameter with a 0.7mm thick Y-TZP framework and a 1.5mm thick veneer layer. 3 commercial and 4 experimental veneering ceramics (n=3 per group) were used to obtain different Δα varying from -1.3ppm/°C to +3.2ppm/°C, which were determined by dilatometric analyses.
  
  
  RESULTS
  Veneer fractures were observed in samples with Δα≥+2.3 or ≤-0.3ppm/°C. Residual stress profiles measured in other groups showed compressive stresses in the surface, these stresses decreasing with depth and then becoming more compressive again near the interface. Small Δα variations were shown to induce significant changes in residual stress profiles. Compressive stress near the framework was found to decrease inversely to Δα.
  
  
  CONCLUSIONS
  Veneer CTE close to Y-TZP (+0.2ppm/°C Δα) gived the most favorable stress profile. Yet, near the framework, Δα-induced residual stress varied inversely to predictions. This could be explained by the hypothesis of structural changes occurrence within the Y-TZP surface. Consequently, the optimum Δα value cannot be determined before understanding Y-TZP's particular behavior when veneered.
authors.affiliation: Dental Biomaterials Research Unit (d-BRU), Department of Fixed Prosthodontics, Institute of Dentistry, University of Liège (ULg, CHU), 45 Quai G. Kurth, Liège, 4020, Belgium; Unité de Recherches Biomatériaux Innovants et Interfaces (URB2I-EA4462), Université Paris Descartes, Sorbonne Paris Cité, Faculté de Chirurgie Dentaire, Paris, France. Electronic address: a.mainjot@chu.ulg.ac.be.
authors.forename: Amélie K
authors.initials: AK
authors.lastname: Mainjot
authors.affiliation: Unité de Recherches Biomatériaux Innovants et Interfaces (URB2I-EA4462), Université Paris Descartes, Sorbonne Paris Cité, Faculté de Chirurgie Dentaire, Paris, France.
authors.forename: Achref
authors.initials: A
authors.lastname: Najjar
authors.affiliation: Unité de Recherches Biomatériaux Innovants et Interfaces (URB2I-EA4462), Université Paris Descartes, Sorbonne Paris Cité, Faculté de Chirurgie Dentaire, Paris, France.
authors.forename: Boris D
authors.initials: BD
authors.lastname: Jakubowicz-Kohen
authors.affiliation: Unité de Recherches Biomatériaux Innovants et Interfaces (URB2I-EA4462), Université Paris Descartes, Sorbonne Paris Cité, Faculté de Chirurgie Dentaire, Paris, France.
authors.forename: Michaël J
authors.initials: MJ
authors.lastname: Sadoun
mesh_terms: D055598:Chemical Phenomena; D019245:Compressive Strength; D003793:Dental Restoration, Permanent; D003799:Dental Stress Analysis / Q000295:instrumentation*; D003801:Dental Veneers; D006801:Humans; D013314:Stress, Mechanical; D013696:Temperature; D013718:Tensile Strength; D015019:Yttrium / Q000737:chemistry*; D015040:Zirconium / Q000737:chemistry*
chemical_list: C499362:yttria stabilized tetragonal zirconia; D015019:Yttrium; D015040:Zirconium
keywords: Chipping; Hole-drilling; Residual stress; Thermal expansion coefficient; Thermal expansion mismatch; Zirconia; veneering ceramic