Abstract
This study aimed to characterize and compare the effect of different aging regimens on surface characteristic (topography and roughness), structural stability (phase transformation) and mechanical performance (Weibull analysis) of a Y-TZP ceramic. Discs (15 × 1.2 mm; VITA In-Ceram YZ) were prepared according to ISO 6872-2015 for biaxial flexural strength testing and randomly assigned into five groups (n = 30): as-sintered, no aging treatment (CTRL); 20 h in autoclave at 134°C, 2 bar pressure (AUT); intermittent mechanical loading at 20 Hz/106 load pulses (MechLoad); AUT followed by MechLoad (AUT+MechLoad); and storage in distilled water at 37°C, for 1 year (STO). The following analyses were performed: roughness (n = 30), surface topography (n = 2), phase transformation (n = 2) and biaxial flexure strength (n = 30). Phase transformation (increase of m-phase content) was shown to be a spontaneous, unavoidable and time-dependent process, occurring even under ambient conditions (dry storage after 1 year = 6.0% increase), and is considerably accelerated in the presence of moisture (STO = 17.6%; AUT= 63.1%; and AUT+MechLoad = 59.9%). For roughness parameters, only Ra was affected by aging, and the highest values were observed for AUT+MechLoad (0.25 ± 0.07 µm). For Weibull analysis, structural reliability (Weibull moduli) and characteristic strength were not impaired after aging, and some aging conditions led to increased values (highest weibull moduli in AUT, and highest characteristic strength in STO). Phase transformation proves to be a time-dependent spontaneous mechanism that is accelerated in the presence of different stimuli. However, none of the aging regimens had a negative effect on the characteristic strength and structural reliability of Y-TZP ceramic.
Highlights
Submitted: July 6, 2017 Accepted for publication: Sep 11, 2017 Last revision: Oct 02, 2017Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic has been widely used in prosthetic dentistry as frameworks for single or multi-unit fixed dental prostheses (FDPs) and as monolithic fullcontour restorations, being an alternative to the traditional metal-ceramic restorations.[1]
Phase transformation The autoclave aging led to the highest m-phase content values (AUT – 63.1%, AUT+MechLoad 59.9%), while mechanical cycling (MechLoad – 3.8%) promoted the lowest m-phase content (Table)
It should be highlighted that even the storage under ambient conditions led to m-phase content increase (0% to 6.0%, for 1 year), and the specimens submitted to storage in water during 1 year presented 17.61% of m-phase content (Figure 1)
Summary
Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic has been widely used in prosthetic dentistry as frameworks for single or multi-unit fixed dental prostheses (FDPs) and as monolithic fullcontour restorations, being an alternative to the traditional metal-ceramic restorations.[1] Y-TZP is a metastable material with a high crystalline content, Braz. Different stimuli (mechanical, physical and/or chemical) can trigger this transformation.[3] The t-m phase transformation results in a volumetric increase of about 4% in grain size that creates a local compressive stress capable of hindering crack propagation. This mechanism, known as “transformation toughening mechanism”, increases the material toughness and fatigue threshold.[4]
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