Heat treatment and additive effects on microstructure and properties of tetragonal-(2-3 mol%) yttria-stabilized-zirconia
Baskaran, Suresh
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https://hdl.handle.net/2142/23739
Description
Title
Heat treatment and additive effects on microstructure and properties of tetragonal-(2-3 mol%) yttria-stabilized-zirconia
Author(s)
Baskaran, Suresh
Issue Date
1989
Doctoral Committee Chair(s)
Buchanan, Relva C.
Department of Study
Engineering, Materials Science
Discipline
Engineering, Materials Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
The role of selected additives (Al$\sb2$O$\sb3$, yttrium aluminum garnet (3Y$\sb2$O$\sb3$ $\cdot$ Al$\sb2$O$\sb3$) and Zr) in microstructural development, phase stability and mechanical behavior of predominantly-tetragonal (2-3 mol%) yttria-stabilized-zirconia (YSZ) was investigated. Preliminary studies on compositional effects were studied in 2-3 mol% YSZ prepared from a mixed oxide powder. Microstructure-property relationships were then investigated as a function of heat treatment conditions in 3 mol% YSZ prepared from a commercially available co-precipitated powder. Additive effects on microstructure and mechanical properties were studied for optimized heat treatment schedules in the 3 mol% YSZ system. For the YSZ-Al$\sb2$O$\sb3$ system, microstructures developed from mechanical mixing of YSZ powder were compared with microstructures obtained from nitrate-synthesized powders.
The strength-toughness relationships observed in the 3 mol% YSZ system were correlated with the grain size dependence of the mechanical properties. Grain sizes were optimized for either high toughness ($\approx$10 MPa$\surd$m) or high strengths ($\approx$820 MPa). Results of this study also showed that, in these materials, phase-separation to the equilibrium tetragonal and cubic-related phases on prolonged heat treatment was accompanied by grain growth inhibition.
The role of Al$\sb2$O$\sb3$ in microstructural control was found to depend strongly on the mode of dispersion during processing. Although the final distributions of Al$\sb2$O$\sb3$ in YSZ microstructures were similar, grain growth was inhibited only in Al$\sb2$O$\sb3$-containing YSZ prepared from chemically-synthesized powders. Additions of up to 6.5 wt% Al$\sb2$O$\sb3$ resulted in less than $\pm$15% variation in strength and toughness for all heat treatments studied. The intergranular dispersion of Al$\sb2$O$\sb3$ was accompanied by a decrease in conductivity in 3 mol% YSZ prepared from the commercially available powder. Addition of yttrium aluminum garnet (YAG) resulted in the formation of cubic-related phases, improved low-temperature phase stability and low strengths ($<$600 MPa). Optimum additions of Zr (2 wt%) to 3 mol% YSZ resulted in improvements in fracture toughness.
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