Heat treatment and additive effects on microstructure and properties of tetragonal-(2-3 mol%) yttria-stabilized-zirconia

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.