Title

Low temperature environmental degradation of zirconia ceramics.

Date of Award

2005

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Electrical and Computer Engineering

Keywords

Engineering, Materials Science.

Rights

CC BY-NC-ND 4.0

Abstract

The low temperature environmental degradation (LTED) of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) has been prevented, or at least retarded, by using both bulk doping and surface doping methods with either cation, or anion, stabilizers. The introduction of both mullite and alumina into 3Y-TZP by a bulk-doping method was found to be effective in suppressing the tetragonal-->monoclinic transformation induced by water during hydrothermal treatment thus giving rise to better mechanical properties. The beneficial effects of alumina on the phase stability of the 3Y-TZP ceramic are considered to be due to the increase in the elastic modulus of the constraining matrix, as well as to the segregation of A12O3 at grain boundaries. The LTED transformation kinetics as determined by x-ray diffraction (XRD) and White Light Interferometer (WLI) analysis showed that the isothermal tetragonal-to-monoclinic transformation starts from the surface and has an incubation-nucleation-growth mechanism which can be described by the Johnson-Mehl-Avrami equation. The degradation of Y-TZP ceramic after hydrothermal treatment can be effectively overcome by surface doping by a solid diffusion method with tetravalent dopants: CeO2 and GeO2; with trivalent dopants: La2O 3 and Fe2O3; and with divalent dopants: CuO and MgO. For surface CeO2-, GeO2- and Fe2O 3-doping, this degradation inhibition behaviour is attributed to a localized increase in cation stabilizer content which satisfies the requirements for stabilization of the tetragonal phase. However, in each case, the stability mechanisms are different. For surface La2O3doping, surface doping overcomes the formation of La2O3 and La 2Zr2O7 since the extra La2O3 can further diffuse to the center of the 3Y-TZP ceramic. For CuO-doping, small amounts of CuO form a liquid that can act as a conduit for the re-distribution of yttria. In the case of surface MgO modification, the stabilization results from the isolated nature of the tetragonal phase which is precipitated in a stable cubic zirconia matrix phase. The degradation of Y-TZP/Al2 O3 ceramic can also be effectively prevented by cation stabilizer surface doping using a liquid infiltration method. It was found that the introduction of mullite as a second phase in the surface of the composite is very effective in inhibiting LTED of Y-TZP/Al2O3 composite when the average volume fraction of infiltrated mullite within the infiltration zone is more than 10wt%, and the depth of infiltrated mullite is more than 500 mum.Dept. of Electrical and Computer Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2004 .Z544. Source: Dissertation Abstracts International, Volume: 66-11, Section: B, page: 6224. Thesis (Ph.D.)--University of Windsor (Canada), 2005.