Influence of thermal expansion mismatch and fatigue loading on phase changes in porcelain veneered Y-TZP zirconia discs.

There is a rapidly growing interest for the use of Y-TZP zirconia as core material in veneered all-ceramic prostheses. It was hypothesized that a mismatch in coefficient of thermal expansion between the veneering porcelain and the Y-TZP zirconia core of these prostheses causes transformation of the tetragonal to the monoclinic structure in Y-TZP zirconia at the interface boundary when exposed to fatigue loading, resulting in fracture at the interface boundary. Y-TZP zirconia discs were veneered with three porcelains differing in coefficient of thermal expansion. Finite element analysis was used to investigate the stress distribution in the bi-layered discs because of the mismatch in thermal expansion. Two of these three groups were fatigued with the veneering ceramic in tension. X-ray diffraction was used to measure the intensity of monoclinic and tetragonal zirconia phase present at the zirconia core surface after sintering, airborne abrasion, veneering, and fatigue loading. It was found that the sintered tetragonal structure was converted to monoclinic up to a depth of 27 mum after airborne abrasion, and reversed back to tetragonal after porcelain veneering with porcelain. Fatigue loading of veneered discs did not, even with the highest possible thermal mismatch stress, cause any conversion from tetragonal to monoclinic phase.