Influence of the aging time on the mechanical properti̇es of Alumi̇na-YSZ dental materials

Recently, yttria stabilized zirconia and zirconia-toughened-alumina ceramics have received considerable attention as dental materials, due to their attractive properties, including high-temperature mechanical strength, good thermal shock resistance, wear and oxidation resistance and biocompatibility. The toughening mechanisms of zirconia is attributed to the stress induced phase transformation of metastable tetragonal grains towards the monoclinic symmetry ahead of a propagating crack, leading to an increase in the work of fracture. However, studies on the application of these materials in humid environment at low temperature have shown that the tetragonal-monoclinic transformation can also be induced at the surface of ZrO2 grains, leading to the so-called hydrothermal transformation which produces a slow degradation of the composite mechanical properties. The aim of this study is, investigating the effects of two different stabilizer (yttrium oxide and yttrium nitrate) on stabilization of monoclinic zirconia and producing alumina-zirconia composites by the use of this stabilized tetragonal zirconia powder. Powder mixtures of % 100 ZrO2, % 30 ZrO2+ % 70 Al2O3 ve % 20 ZrO2+ % 80 Al2O3 were prepared by using experimental tetragonal zirconia powders and commercial 3 mol % yttria stabilized zirconia powder, shaped by slip casting and dry pressing, and sintered at 1400, 1450 ve 1500 °C. After sintering, aging process were performed on the samples for different periods and the effects of aging time on physical-mechanical properties and tetragonal-monoclinic transformation were investigated. As a consequence; partially stabilized zirconia powders were obtained by using yttrium oxide and yttrium nitrate and yttrium oxide gave better results than yttrium nitrate. Increasing the aging time resulted in a decraese in tetragonal phase content and mechanical strength in all samples.