Abstract
This study aimed to investigate the effect of grinding and multi-stimuli aging on the fatigue strength, surface topography and the phase transformation of Y-TZP ceramic. Discs were manufactured according to ISO-6872:2008 for biaxial flexure testing (diameter: 15 mm; thickness: 1.2 mm) and randomly assigned considering two factors "grinding" and "aging": C- control (as-sintered); CA- control + aging; G- ground; GA- ground + aging. Grinding was carried out with coarse diamond burs under water-cooling. Aging protocols consisted of: autoclave (134°C, 2 bars pressure, 20 hours), followed by storage for 365 days (samples were kept untouched at room temperature), and by mechanical cycling (106 cycles by 20 Hz under a load of 50% from the biaxial flexure monotonic tests). Flexural fatigue strengths (20,000 cycles; 6 Hz) were determined under sinusoidal cyclic loading using staircase approach. Additionally, surface topography analysis by FE-SEM and phase transformation analysis by X-ray Diffractometry were performed. Dixon and Mood methodology was used to analyze the fatigue strength data. Grinding promotes alterations of topographical pattern, while aging apparently did not alter it. Grinding triggered t-m phase transformation without impacting the fatigue strength of the Y-TZP ceramic; and aging promoted an intense t-m transformation that resulted in a toughening mechanism leading to higher fatigue strength for as-sintered condition, and a tendency of increase for ground condition (C < CA; G = GA). It concludes that grinding and aging procedures did not affect deleteriously the fatigue strength of the evaluated Y-TZP ceramic, although, it promotes surface topography alterations, except to aging, and t-m phase transformation.
Highlights
In Prosthetic Dentistry, yttrium-stabilized tetragonal zirconia polycrystal ceramics (Y-TZP) has been used for frameworks of single-/multi-unit fixed dental prosthesis, which are veneered with feldspathic ceramic, and for monolithic full-contour restorations (1)
The surface topography analyses (FE-SEM) shows that grinding promotes an alteration of topographical pattern, deforming the surface and introducing scratches parallel to the movement of the grinding tool; while, aging apparently did not lead to any alteration (Fig. 1)
Fractography analysis (Fig. 3) shows that all fractures originated from the surface defects at the center of the side under tensile stress, which is the region with highest stress, as stated by ISO-6872:2008 (18), and that the fracture propagated into the opposite side where compression stress is concentrated
Summary
In Prosthetic Dentistry, yttrium-stabilized tetragonal zirconia polycrystal ceramics (Y-TZP) has been used for frameworks of single-/multi-unit fixed dental prosthesis, which are veneered with feldspathic ceramic, and for monolithic full-contour restorations (1). The major advantages of monolithic restorations are that this assembly allows a substantial ceramic thickness reduction, by eliminating the veneer layer, without compromising the final strength of the system. This assembly results in a more conservative tooth preparation and by that it decreases tooth removal (1). It eliminates the fracture/chipping of the veneer porcelain, which it has been noticing by clinical studies as the main failure of veneered zirconia restorations (2). Kobayashi and collaborators (6) described this mechanism as low-temperature degradation (LTD) and showed that it is accelerated under presence of water and temperature changes
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