Improvement of the ageing resistance of 2YSZ ceramic

• Autor(a) principal: Macedo, Tânia Sofia Oliveira
• Data de Publicação: 2016
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/21896
• topic: Materiais e dispositivos biomédicos; Biocerâmica - Propriedades mecânicas; Óxido de zircónio - Resistência à degradação; Materiais biocompatíveis; Próteses

Zirconia is a widely studied ceramic and it is applied in several areas such as engineering (solid oxide fuel cells, thermal barrier coatings) and biomedicine, in areas such as orthopedic and odontology. This material can be applied with the purpose to restore a body function that was compromised by a degenerative disease (orthopedic implants), in dental area and sensors due to its excellent mechanical properties, aesthetic characteristics and biocompatibility. However, this ceramic is sensitive to ageing and has a low hardness. To overcome these limitations, samples of Yttria Stabilized Zirconia doped with different oxides were developed and the goal was to improve the ageing resistance and mechanical properties of un-doped 2 mol% Yttria Stabilized Zirconia (2YSZ). A careful selection of the dopants and respective concentrations was performed. According to the literature, ceria (CeO2), silica (SiO2), alumina (Al2O3), lanthana (La2O3), ytterbia (Yb2O3) and manganese oxide (MnO2) improve the ageing resistance and/or the mechanical properties of Yttria Stabilized Zirconia (YSZ). In this study, thirteen different doped 2YSZ compositions were developed. For each selected dopant, three to four compositions were performed with different dopant concentrations. After a wet milling stage, the doped stabilized zirconia powders were obtained by spray-drying, from stabilized suspensions with a controlled nanometric particle size distribution. The obtained doped sprayed powders were characterized through several techniques, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), density and specific surface area (SSA). Green bodies of all doped compositions were obtained by uniaxial pressing (70 MPa). High density ceramics (with relative density between 90% and approximately 99%) were achieved at low sintering temperatures (1350 °C and 1400 °C). The grain size of the sintered samples was measured by SEM images, and a dispersed microstructure with a nanometric grain size was observed for all 2YSZ doped samples. The crystallographic phases present in the doped samples were identified by XRD. In order to assess their thermal ageing resistance, the samples were subjected to a thermal treatment at low temperatures (200 °C) during a period of 36 hours. After this treatment, the fracture toughness of the aged samples was evaluated. From all the starting samples, those with better resistance to thermal ageing and fracture toughness were afterwards selected: samples of 2YSZ doped with CeO2 (0.50 wt%) and with SiO2 (0.25 wt%) and La2O3 (1.07 wt%). Cylindrical bodies of the selected doped 2YSZ compositions were obtained by two pressing stages - uniaxial and cold isostatic pressing - and sintered at 1350 ºC for 3 hours. The behaviour of the sintered samples was investigated in terms of hydrothermal ageing resistance, according to the specifications mentioned in ISO 13356:2008 (134 ± 2 ºC and 0.2 MPa), and mechanical properties: Vickers’ hardness along with fracture toughness and flexural strength. The amount of monoclinic zirconia, indicative of degradation, was determined by XRD after 5 hours of ageing test. Afterwards the mechanical behaviour (Vickers hardness, fracture toughness and flexural strength) of the aged samples was assessed. The results obtained in the present study demonstrate that adequate doped 2YSZ samples, with improved hydrothermal degradation resistance (9.87 % of monoclinic zirconia for SiO2 with La2O3 doped 2YSZ sample while un-doped 2YSZ presented a monoclinic phase higher than 15 %) can be obtained. The mechanical properties of the sample of 2YSZ doped with both oxides (SiO2 and La2O3), before and after the hydrothermal ageing, were practically maintained (1159 vs 1141 HV (hardness), 9.68 vs 9.15 MPa.m1/2 (fracture toughness) and 700 vs 698 MPa (flexural strength)). Nevertheless, a decrease of the mechanical properties was observed for both selected doped 2YSZ samples in relation to un-doped sample.