Veneer cracking on zirconia core dental material is presenting as a clinical concern. There is no standard method for calculating the magnitude of residual stress that causes these crack patterns.
Objective : Determine the thermal incompatibility stresses ( σ R ) for veneers on zirconia specimens, using the previously established relationship between failure stress and crack branching coefficients (CBC).
Materials and methods : Discs (12 mm dia) were fabricated using nano-composite Ce-TZP/Al 2 O 3 cores, liner and veneer (Matsushita Elec Works Ltd., Japan). The ceramic core/veneer thicknesses were: 0.6 c /0.6 v ; 0.4 c /0.8 v ; 0.8 c /0.4 v . Specimens were placed on an epoxy substrate (3.6 mm) and stored in DI water at 37 °C for ∼1 year. After aging, multiple cracks in specimens were identified visually. Delaminated veneer-from-core specimens were examined under SEM; crack origins identified; and the failure stress calculated ( σ R = K IC /[( Y ) ( c ) 0.5 ]). The non-delaminated cracked veneers underwent fiber optic illumination, microscopic examination, image capture and stress analysis using a grid dimension technique. The number of cracked veneer segments per specimen was recorded. A log/log graph of box counts ( N ) in 8 sizes ( L ), and grid areas ( L max ) was used to establish the crack branching coefficient: CBC = log[ N ( L )]/log( L max / L ) (Sakai, 1991; Mecholsky, 1998). The CBC was related to the calculated σ R of like-specimens.
Results : The internal σ R values calculated from fracture origins of the veneer/core delaminated specimens for 0.4 c /0.8 v and 0.6 c /0.6 v were 40 and 61 MPa, with 3 and 7 veneer segments, respectively, and CBCs of 0.54 ( R 2 = 0.97) and 0.65 ( R 2 = 0.99), respectively. Mean CBCs were: 0.56 for 0.6 c /0.6 v and 0.60 for 0.8 c /0.4 v . Based on the CBC values, the highest internal failure stress was in the thinnest veneer (0.8 c /0.4 v ), with less veneer internal failure stress in the 1:1 ratio (0.6 c /0.6 v ).
Conclusions : The residual stresses (CBC) tended to increase with an increase in veneer/core thickness ratio. The indication is that further investigation with a larger sampling will demonstrate that this technique compliments other methods for thermal incompatibility residual stress estimation in bilayer dental ceramic systems.