Diffusion and Creep in Silica‐Doped Tetragonal Zirconia

Abstract

Silica segregation at two grain junctions or in amorphous triple junction pockets can influence creep by altering the grain‐boundary diffusion coefficient. Although the addition of silica to superplastic yttria‐stabilized tetragonal zirconia enhances ductility, differences in reported creep parameters have limited critical identification of rate controlling mechanisms. The present study on a pure 3 mol% yttria‐stabilized tetragonal zirconia (3YTZ) and 3YTZ with 0.39 or 3.9 wt% silica involved a detailed characterization of creep over a wide range of experimental conditions and also tracer diffusion measurements. The data broadly show transitions in creep stress exponents from n∼1 to ∼2 to ∼3 with a decrease in the stress. The data at high stresses are consistent with Coble diffusion creep, and creep at lower stresses is attributed to interface‐controlled diffusion creep. Measurements indicated that silica does not have any significant influence on grain boundary or lattice diffusion, and this is consistent with the observation that 3YTZ and 3YTZ with 0.39% or 3.9% silica exhibit essentially identical creep behavior in the Coble creep regime. Silica influences the interface control process so that the transitions in stress exponents are pushed to lower stresses with an increase in silica content.

Micromechanics of fracturing in nanoceramics

“…The new toughening micromechanisms are often related to specific deformation modes that operate in nanomaterials (e.g. experimental data [13,16,44,45]), but typically are not effective enough in conventional coarse-grained polycrystalline ceramics. In particular, these specific deformation modes represent deformation by nanoscale twinning [13], grain boundary sliding [44] (which is the dominant mechanism of superplastic deformation occurring in nanocrystalline ceramics at lower temperatures [2], when compared with microcrystalline ceramics), and Coble creep controlled by grain boundary diffusion [45].…”

Superplasticity and superplastic-like flow in cubic zirconia with silica

“…This latter process has been developed by considering that the grain boundaries are no longer perfect sources and sinks for vacancies, as in conventional diffusion creep, but rather grain boundary dislocations act as discrete sources and sinks for vacancies through their non-conservative motion along the grain boundaries [75]. This introduces an interface-controlled behavior so that the creep rates may be significantly lower, and the stress exponent higher, than in the conventional continuum calculations [76]. Unfortunately, the present results are insufficient to provide a rigorous comparison with the predictions for interface-controlled diffusion creep because no data are available in these experiments covering different initial grain sizes.…”

Influence of grain boundary sliding on diffusion in yttria doped zirconia

“…13,14 . Based on an earlier measurement of Hf lattice diffusion, it was concluded that heating at a rate of 22 K/min in a creep furnace environment and allowing 15 min for stabilization will not have a significant effect on diffusion measurements at testing temperatures of up to 1623 K.…”