Fracture and interfacial delamination origins of bilayer ceramic composites for dental restorations

Alumina and zirconia (Y-TZP) based bilayer ceramic dental composites with core to veneer thickness ratio (R-value) of 1:1 and 2:1 were fabricated through an established dental laboratory multi-steps-firing procedure. Their flexural strengths were determined by three-point bending test. A combinational approach of numerical simulations by finite element analysis associated with direct fractography investigation was applied to elucidate the origins of fracture and interfacial delamination and the influence of physical properties mismatch between core ceramic and veneer porcelain. A newly developed argon ion beam cross-section polishing technique was used to conduct fine polishing required for close investigating of the core–veneer interface under scanning electron microscope. For the same core ceramic no significant difference was observed in determined flexural strength of two groups of bilayer composites. The flexural strength of the bilayer composites is ∼55% and ∼35% of the core ceramics and achieved ∼90% and 70–77% of the predicated value respectively in case of Y-TZP and alumina based composites. Numerical simulations by finite element analysis indicate that the often observed interfacial delamination in Y-TZP based bilayer composites has a clear origin of the severe physical properties mismatch between veneer porcelain and core ceramics, particularly the flexural strength, which may be prevented by increasing the flexural strength of veneer porcelain to above 300 MPa. The observation of the formation of microcracks in alumina core immediately one grain-thick under the veneer–core interface warns the possible thermal damages initiated during the veneering operation.