Banca de QUALIFICAÇÃO: Márcio Paulo de Aráujo Mafra

Uma banca de QUALIFICAÇÃO de DOUTORADO foi cadastrada pelo programa.
STUDENT : Márcio Paulo de Aráujo Mafra
DATE: 14/03/2024
TIME: 14:00
LOCAL: Microsoft TEAMS
TITLE:

Influence of itria on the fatigue limit obtained by the modified step method in dental zirconia after hydrothermal degradation

KEY WORDS:

Fatigue strength limit, dental Zirconia, yttria, modified staircase method, Hydrothermal Degradation.

PAGES: 113
BIG AREA: Engenharias
AREA: Engenharia Mecânica
SUMMARY:

A significant evolution in dental restoration techniques has been established by the use of
ceramic materials. These materials offer excellent advantages, mainly related to aesthetics,
biocompatibility, and chemical resistance. The trend in dental ceramic techniques has been the
elimination of the metallic substructure of restorations, including those on implants, to achieve
better aesthetics by using ceramics with higher fracture toughness, thus minimizing their fragility. This research aims to investigate the effect of yttria content on the fatigue strength limit
of dental zirconias after hydrothermal degradation using the modified staircase method. In this
regard, evaluations of mechanical properties and four-point bending fatigue were conducted
on zirconia with different additions of Y2O3. The methodology involved uniaxial compaction,
sintering, mechanical strength tests, four-point bending fatigue determination, and characterizations including scanning electron microscopy, X-ray diffraction, relative density, surface
roughness, elasticity modulus, Vickers hardness, and fracture toughness. The Zr-Y2O3 ceramics showed relative densities close to 99%. Surface roughness increased after hydrothermal
degradation, with values not exceeding a 31% relative increase for both Ra and Rz parameters. SEM micrographs revealed differences in grain size and porosity among the compositions,
where the 5Y-PSZ composition showed a 26% increase in average grain size compared to the
3Y-TZP and 4Y-PSZ compositions, which had very similar average grain sizes. The diffractograms of the raw materials indicated mainly two distinct polymorphs, with 90% t-ZrO2 and
10% m-ZrO2. The 4Y-PSZ powder presents a predominantly tetragonal phase (t-ZrO2), while
in the 5Y-PSZ powder, 94% t-ZrO2 and 6% m-ZrO2 were observed. The three studied ceramic compositions exhibited a predominance of the tetragonal phase. The flexural strength
decreased as the yttria content increased, the result from the 3Y-TZP to 4Y-PSZ composition
showed a 30% reduction, and from the 4Y-PSZ to 5Y-PSZ composition, the reduction was 34%
in flexural strength. Vickers hardness increased slightly with yttria content, not exceeding a
12.5% relative increase from one composition to another, while fracture toughness varied with
composition, decreasing significantly as the yttria content increased, with a 38% decrease from 3Y-TZP to 4Y-PSZ and a 48.5% decrease from 3Y-TZP to 5Y-PSZ, and a 17.5% difference
from 4Y-TZP to 5Y-PSZ, reflecting toughening mechanisms and the impact of porosity. The
four-point bending fatigue showed that the fatigue strength limit under cyclic stress decreases
with variations among the compositions, as the yttria content increased, the fatigue resistance
decreased consequently. As a result, the composition from 5Y-TZP to 4Y-PSZ experienced a
reduction of 27.5% in fatigue resistance. The study indicated the efficacy of the processing
and sintering parameters used, resulting in high densifications in the samples. The ceramics’
microstructure was refined and complex, notably with the absence of open porosity in certain
compositions. Fracture toughness was influenced by toughening mechanisms, with better results in samples with lower yttria content. The study also demonstrated that microstructure
significantly affects the fatigue strength limit, providing valuable insights for the development
of dental ceramics with improved performance.

COMMITTEE MEMBERS:
Presidente - 1523261 - COSME ROBERTO MOREIRA DA SILVA
Interno - 1249764 - JORGE LUIZ DE ALMEIDA FERREIRA
Interno - 1345908 - JOSE ALEXANDER ARAUJO
Externo à Instituição - Maycol Moreira Coutinho - UNICEPLAC
Externo ao Programa - 2934459 - RODRIGO ARBEY MUNOZ MENESES - null