Interfacial toughness of bilayer dental ceramics based on a short-bar, chevron-notch test

Objective: The objective of this study was to test the null hypothesis that the interfacial toughness of each of two types of bonded core-veneer bilayer ceramics is not significantly different from the apparent fracture toughness of the control monolithic glass veneer. Methods: T-shaped short-bars of a lithia-disilicate glass-ceramic core (LC) and yttria-stabilized polycrystalline zirconia core ceramic (ZC) were prepared according to the manufacturer's recommendations. V-shaped notches were prepared by using 25-μm-thick palladium foil, leaving the chevron-notch area exposed, and the bars were veneered with a thermally compatible glass veneer (LC/GV and ZC/GV). Additionally, we also bonded the glass veneer to itself as a control group (GV/GV). Specimens were kept in distilled water for 30 days before testing in tension. Eight glass veneer bars were prepared for the analysis of fracture toughness test using the indentation-strength technique. Results: The mean interfacial toughness of the LC/GV group was 0.69 MPa m^1/2 (0.11), and did not significantly differ from that of the GV/GV control group, 0.74 MPa m^1/2 (0.17) (p > 0.05). However, the difference between the mean interfacial toughness of the ZC/GV group, 0.13 MPa m^1/2 (0.07), and the LC/GV and the GV/GV groups was statistically significant (p < 0.05). Significance: For bilayer all-ceramic restorations with high-strength core materials, the veneering ceramics are the weakest link in the design of the structure. Since all-ceramic restorations often fail from chipping of veneer layers or crack initiation at the interface, the protective effects of thermal mismatch stresses oral prosthesis design should be investigated.