Influência do teor de ítria no limite de resistência à fadiga obtido pelo método da escada modificado em zircônias estabilizadas com Y2O3

Abstract

Ceramic materials have been widely used in dentistry due to their biocompatibility, wear resistance and aesthetic appearance with natural teeth. Among them, yttria-stabilized zirconia (Y-TZP) stands out for its promising mechanical properties, making it a superior alternative for problematic prosthetics and implants. hydrothermal and fatigue, which can compromise its durability in clinical applications. This work aimed to carry out fatigue tests on 4 points of zirconia dental ceramics with different Yttria contents and to evaluate the relationship of tetragonal, monoclinic, and cubic phases with fatigue behavior, possible transformation mechanisms, and the fatigue resistance limits of these materials. The current ISO 6872 standards do not include fatigue resistance values as limits on the use of prostheses and implants, being restricted to determining flexural resistance and fracture toughness. In this sense, evaluations of the mechanical properties and flexural fatigue at four points were carried out in the zirconia samples with additions of 3mol.%, 4mol.% and 5mol.% Y2O3. The methodology used involved uniaxial compaction, sintering at 1550 °C with a 2-hour isotherm, mechanical tests, and determination of flexural fatigue at 4 points under a tension condition in sine waves with a frequency of 15 Hz and ratio R=0.2, with the tests being interrupted at 2 x 106 cycles, a value defined as run-out. Characterizations included scanning electron microscopy, X-ray diffractometry, relative density, surface roughness, modulus of elasticity, Vickers hardness, and fracture toughness. Regarding the fractional percent of the tetragonal phase, the 3Y-TZP composition presented 81.3%, 4Y-PSZ had 47.2%, and 5Y-PSZ 20.7%. The other phases observed were monoclinic, cubic, and pseudo-cubic t". Four-point bending fatigue using the modified staircase method showed that the fatigue strength limit under cyclic stress differs with variations in yttria content between compositions. Toughening mechanisms influenced fracture toughness, with better results in samples with lower yttria content. In these cases, during fatigue tests, the tetragonalmonoclinic transformation causes an increase in volume and creates compression zones at the tip of the crack, making it difficult to propagate. With the increase in yttria content, cubic phases prevail, reducing the occurrence of these toughening mechanisms. This study made it possible, through the use of the modified staircase method, to test the limit of flexural fatigue resistance at four points with a limited set of specimens. Their microstructure considerably affected the fatigue resistance limit for the compositions studied since the tensile surface shielding is effective in zirconia with contents of the tetragonal phase higher than that of cubic and pseudo-cubic phases. Tetragonal-monoclinic transformations at the triple points of grain boundaries during cyclic loading produce a restricted elastic zone (shielding). It is inferred that this phenomenon reduces the stress intensity factor at the crack’s tip, slowing its propagation in materials with lower yttria content and a higher percentage of tetragonal phase, producing greater fatigue resistance. Evaluating the cyclic fatigue resistance of the analyzed formulations, the results obtained were of great importance for analyzing the mechanical behavior of ceramics stabilized with yttria. The study also demonstrated that microstructure significantly affects the fatigue strength limit, providing valuable insights for developing better-performing yttria-stabilized zirconia ceramics.